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1 Department of Psychology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104 2 Neuroscience Graduate Group, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104
Submitted 24 September 2002; accepted in final form 13 May 2003
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ABSTRACT |
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INTRODUCTION |
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,
2002;
Everitt et al. 2001;
Robbins and Everitt 1999;
Robinson and Berridge 1993;
Stewart 1992;
Stewart et al. 1984;
White 1996). These acute drug
effects are proposed to occur with each drug self-administration experience so
that ultimately facilitation of drug-seeking behavior by drug-associated cues
becomes irresistible and drug-seeking behaviors become compulsive and
uncontrollable.
Several lines of evidence are consistent with possible contributions of
drug-learning interactions to addiction and additionally provide clues as to
the neural mechanisms that might mediate them. Consistent with a possible role
for drug-learning interactions, animal studies show that addictive drugs
amplify the influences of conditioned stimuli on instrumental behavior
directed toward natural rewards and may facilitate reward-related learning
(Harmer and Phillips, 1998;
Killcross et al. 1997;
Krivanek and McGaugh 1969;
Robbins 1978; Robbins et al.
1983,
1989;
Taylor and Horger, 1999;
Taylor and Robbins 1986;
Wyvell and Berridge 2000).
Both of these effects of addictive drugs appear to be transduced, at least in
part, by dopamine (DA)-mediated actions in the accumbens. Studies of drug
reward indicate that the accumbens also makes important contributions to the
acquisition and the expression of drug-reward-related behavior (for review,
Koob et al., 1998;
Leshner and Koob 1999;
Wise and Bozarth 1987; also
see Cornish and Kalivas 2000).
Finally, studies of natural rewards show that the nucleus accumbens mediates
aspects of drug reward-related learning and the influence of that learning on
behavior (for review, see Cardinal et al.
2002; Hall et al.
2001; Parkinson et al.
1999; Smith-Roe and Kelley
2000). These data are consistent with the proposal that addictive
drugs may impact mechanisms that are normally involved in reward-related
learning and behavior and may thereby contribute to aberrant
drug-reward-related learning. Moreover, the data indicate that this
drug-learning interaction, and the contribution of this interaction to drug
addiction may be mediated by drug effects on the accumbens.
Extracellular recordings of accumbal firing patterns during intravenous
drug self-administration sessions are consistent, at least to some extent,
with the hypothesis that drug effects on the accumbens may be involved in
mediating a drug-induced amplification of drug-reward-related learning. These
studies show that some accumbal neurons exhibit excitatory phasic firing time
locked to drug-reward-related events, including the drug-directed instrumental
behavior and cues that predict the delivery of drug
(Carelli et al. 1993;
Chang et al. 1994;
Janak et al. 1999;
Peoples and West 1996;
Uzwiak et al. 1997).
Characterization of the functional role of the phasic firing patterns
indicates that they are related to drug-reward-related events
(Bowman et al. 1996; also see
Carelli 2000;
Carelli and Deadwyler 1996a;
Peoples et al. 1997).
Interestingly, most of these neurons, like the majority of accumbal neurons,
also exhibit decreases in average firing during the self-administration
session (Peoples et al. 1998b,
1999; also see
Peoples and West 1996).
Several lines of pharmacological and behavioral evidence indicate that the
decreases in average firing rate reflect the predominant pharmacological
effect of self-administered cocaine
(Nicola and Deadwyler 2000;
Peoples and West 1996; Peoples
et al. 1994,
1998a,b;
also see Nicola et al. 1996;
Qiao et al. 1990;
Rebec and Zimmerman 1980;
Uchimura and North 1990;
White 1990;
White et al. 1993). That
accumbal neurons exhibit responses to both drug-reward-related events and
pharmacological effects of drug is consistent with the hypothesis that
self-administered drug modulates accumbal processing of drug-reward-related
signals (Peoples et al. 1998b,
1999).
A question to be addressed is how might the predominantly inhibitory effects of self-administered cocaine on average firing rate "amplify," or otherwise enhance, the primarily excitatory lever-press firing patterns. One possibility is that drug has a less inhibitory effect on the drug-reward-related signals than on other, background, neural activity. Such differential changes in firing would increase the signal-to-background ratio of the reward-related signals and potentially translate into an increased impact of those signals on accumbal contributions to learning and behavior. In the present study, we investigated whether accumbal neurons exhibit firing patterns during drug self-administration sessions that are consistent with this hypothesis. Animals were trained to intravenously self-administer cocaine. Neurons that exhibited both a lever-press firing pattern and a decrease in average firing during the self-administration session were identified and analyzed for differential changes in signal and background firing over the course of a self-administration session.
Portions of the data were presented at the Society for Neuroscience 32nd
Annual Meeting (Peoples and Cavanaugh
2002).
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METHODS |
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The subjects (24 male Long-Evans rats) and neurons described in the present
report are a subset of those included in a previously described study
(Peoples et al. 1998b).
Inclusion of subjects in the present study was contingent on a procedural
criterion. Specifically, during the recording session, subjects had to
initiate drug seeking under drug-free conditions (i.e., no priming infusion of
drug). All animal care and protocols were in accordance with the Guide for the
Care and Use of Laboratory Animals published by the U.S. Public Health Service
and approved by the Animal Care and Use Committee of Rutgers, The State
University of New Jersey.
Surgery and postoperative maintenance
Animals were anesthetized with pentobarbital sodium (50 mg/kg ip). Before
surgery, subjects received injections of atropine methyl nitrate (10 mg/kg ip)
and penicillin G (75,000 U/0.25 ml ip). Anesthesia was maintained with
periodic injections of pentobarbital sodium (5–10 mg/kg ip) and ketamine
hydrochloride (60 mg/kg ip). A catheter was implanted in the jugular vein and
exited through a j-shaped stainless steel cannula cemented to the skull. An
array of quad-Teflon-coated stainless steel wires was implanted in the
accumbens (between 0.7 and 2.7 mm anterior from bregma; between 0.8 and 2.2 mm
lateral from bregma; and between 6.8 and 7.2 mm ventral from level skull)
(Paxinos and Watson 1996). The
array consisted of 12–16 microwires (diameter of each uninsulated wire
tip, 50 µm) arranged in two parallel rows, which were 
2 mm in length
and separated from one another by 0.45–0.55 mm (wire center to wire
center).
After surgery, subjects were housed in steel-grid chambers. The catheter
was connected to a fluid swivel (Brown et
al. 1976). A motor-driven pump perfused the catheter with 0.2 ml
of heparinized bacteriostatic saline once per hour. Occasionally, outside the
experimental sessions, a brief period of anesthesia was induced by intravenous
administration of methohexital (10 mg/kg) to either confirm catheter patency
or to facilitate attachment of the electrical harness. At least 7 days after
surgery and 3 days before self-administration training, subjects were
transferred to a Plexiglas chamber where they remained 24 h/day for the
duration of the study. Subjects had free access to water and were fed 15
g of food each day to maintain body weight at 350 g.
Procedures
DAILY COCAINE SELF-ADMINISTRATION SESSION. Onset of each self-administration session was signaled by a regular sequence of events that began with the illumination of a stimulus light followed by insertion of the response lever into the chamber. Once the lever was inserted animals received an intravenous infusion of saline (0.2 ml). The saline infusion was paired with a 7.5-s tone that corresponded with the duration of the syringe pump operation, and the offset of the stimulus light for 40 s. These tone and light stimulus events are the same events associated with cocaine infusions later during the self-administration session. The sequence of events that occurred between the first illumination of the stimulus light and the re-illumination of the light, at the end of the 40-s light-off period, is referred to as the discriminative stimuli (SD).
After completion of the SD, animals had the opportunity to self-administer cocaine according to a fixed-ratio 1 (FR 1) schedule of drug reinforcement. Each press of the lever was followed immediately by a 0.2-ml intravenous infusion of cocaine (0.7 mg/kg per 0.2-ml infusion), a 7.5-s tone that corresponded to the duration of the syringe pump operation, and a 40-s time-out, during which the stimulus light was turned off and a lever press had no programmed consequence. Each session was limited in duration to 6 h or 80 infusions. At the end of the self-administration session, the stimulus light was turned off, and the response lever was removed from the chamber.
OVERALL SELF-ADMINISTRATION HISTORY PRIOR TO THE RECORDING SESSION. All self-administration sessions, from the first to the last, were conducted identically and according to the procedures described in the preceding section. Animals typically approached and pressed the lever on the first day of self-administration training and were self-administering drug within the first one to three sessions. Self-administration training sessions were conducted 7 days/wk. Prior to the recording study, subjects completed 12–17 days of self-administration training.
VIDEO RECORDING. During each recording session, behavior was videotaped using a JVC HR-78004 Super VHS recorder. Each video frame (30 frames/s) was sequentially time-stamped by a computer coupled with a video frame counter (Thalner Electronics VC-436). Frames were time-stamped according to the same computer clock that time-stamped each neural discharge. The camera view was oriented perpendicular to the response lever. The entire chamber was visible; thus the rat was always visible. The video system allowed us to document, off-line, the timing of behaviors completed in any area of the chamber with a temporal resolution of 33 ms.
ELECTROPHYSIOLOGICAL RECORDING SESSIONS. Phases of the recording session. A recording session started with a 20-min nondrug baseline-recording period. At the end of the 20-min period, the typical daily self-administration session was conducted. A 40-min nondrug recovery period followed the self-administration session. During the nondrug baseline and recovery periods that bracketed the self-administration session, subjects were not exposed either to the drug cues, the response lever or the drug.
Electrophysiological recording equipment and procedures. Activity
from each recorded microwire was first led into a field effect transistor in
the headstage of the electronic harness (NB Labs, Denison, TX). The neural
signal was then led through a modified fluid and electronic swivel
(CAY-675-24, Airflyte Electronics, Bayonne, NJ) to a preamplifier (Riverpoint
Electronics, Goldsboro, NC) that differentially amplified the signal on the
recording wire against another microwire. The signal was then led through a
band-pass filter (450 Hz to 10 kHz) and amplifier (Riverpoint Electronics,
Goldsboro, NC). Using software and hardware of DataWave Technologies
(Longmont, CO), electrical signals were sampled (50-kHz sampling frequency for
each recording wire), digitized, time-stamped (0.1-ms resolution), and stored
for off-line analysis. After the experiment, cluster analysis software
(DataWave Technologies) was used to discriminate neural waveforms. After a
population of waveforms was isolated by the discrimination procedures, it was
subjected to an inter-spike-interval analysis to confirm that it corresponded
to a discriminated single neuron (see Fig.
1). When more than a single population of neural waveforms
appeared to have been recorded from a given wire, a cross-correlation analysis
was used to confirm that the population corresponded to distinct neurons (cf.,
Peoples 2002).
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Data analysis
CHARACTERIZATION OF THE RESPONSIVENESS OF INDIVIDUAL NEURONS TO THE
OCCURRENCE OF THE LEVER PRESS AND TO THE ADMINISTRATION OF DRUG DURING THE
MAINTENANCE PHASE. The statistical analyses used to test the
significance of changes in firing exhibited by an individual neuron differed
depending on whether firing was evaluated with respect to either an event that
occurred repeatedly during the session (e.g., lever press) or an event that
occurred only once. In the case of an event that occurred repeatedly during
the session, firing rate (number of discharges, also referred to as counts)
was calculated during two intervals per occurrence of the event, one interval
that preceded the onset of the event and one interval that followed the onset
of the event. Discharges during the pre- and postevent intervals were then
compared using a Wilcoxon matched-pairs test
(Peoples and West 1996;
Peoples et al. 1997,
1998b;
Schultz et al. 1992;
Siegel and Castellan 1988).
Analyses of neural responses to events that occurred once per session were
carried out in either of two ways. First, in some cases, the total number of
discharges during an interval before and after the onset of the event were
simply counted and used to calculate a percent change in firing rate. Second,
discharges before and after the event were compared using a Mann-Whitney
test.
Although the nonparametric statistical tests are designed for comparisons
between populations of subjects, they can be appropriately applied to
"single-subject" data that are not autocorrelated
(Kazdin 1984) across the time
periods being employed in the analysis. Additionally, extensive experience in
evaluating the utility of various statistical approaches has shown us that the
use of the tests, in conjunction with visual inspection methods
(Kazdin 1984), is a reliable
and rigorous method for defining responsive neurons.
There are three main categories of firing patterns exhibited by accumbal
neurons during an FR 1 cocaine self-administration session. These firing
patterns have been defined and characterized in a number of previous reports
(e.g., Peoples and West 1996;
Peoples et al. 1997,
1998a,b;
Uzwiak et al. 1997). The
procedures used to analyze the firing patterns are thus only briefly described
herein.
Lever-press firing patterns were defined as a significant increase
or decrease in average firing rate within ±3 s of the lever press,
relative to firing during the –12 to –9-s prepress. To test for an
increase in firing rate, discharges during the 12 s before and after each
lever press (excluding the 1st 8–10 presses and any presses preceded or
followed by an inter-infusion interval of <6 min) were determined using a
sliding window method (i.e., 0.3-s window and 0.1-s step). Maximum firing rate
in a 0.3-s window within the 3-s pre- and postpress was compared with median
firing rate during –12 to –9 s prepress (Wilcoxon matched pairs
test, 1-tailed, 
= 0.01). Comparable methods were used to test for a
decrease in firing rate time-locked to the lever press. Onset of the change
was defined as the first of three successive 0.3-s windows that showed
significantly different firing from the median firing rate during –12 to
–9 s prepress. Offset was defined as the first of three 0.3-s windows
that showed firing rate that did not significantly differ from the same median
firing rate.
An inter-infusion-interval firing pattern consisted of a change in
average firing rate within the 2-min postpress, relative to the 2-min
prepress. To test for this type of change, firing rate (i.e., number of
discharges) during the 4 min before and after each lever press (excluding the
1st 8–10 presses and any presses preceded or followed by an
inter-infusion interval of <6 min) was first calculated using a sliding
window method (i.e., 0.5-min window and 0.1 min step). To test for a postpress
decrease in firing, the minimum firing rate postpress was then compared with
the maximum firing rate prepress (Wilcoxon matched pairs test, 1-tailed test,
= 0.05). A postpress increase in firing was defined as a significant
difference between maximum firing rate postpress and minimum firing rate
prepress.
A session change in firing was defined as a significant change in firing rate during each of two 20-min periods during the maintenance phase of the self-administration session relative to the 20-min nondrug baseline recording period. The two self-administration periods included the first 20 min of the second hour of self-administration behavior and the last 20-min of the self-administration session. Neurons that showed significant unidirectional changes in firing rate during the two 20-min self-administration periods, relative to the predrug period, were defined as showing a session change in firing rate.
ANALYSIS OF SIGNAL AND BACKGROUND. For lever-press neurons, signal period was defined on an individual neuron basis and equaled the interval elapsing between the onset and offset of the phasic increase in firing. The background period equaled –12 to –4 s prepress for all neurons. The background period was generally longer than the signal period; however, the measures of signal and background firing rate were equated by calculating each as discharges per second (i.e., Hz). Calculation of background firing rates over the longer time period provided measures of background firing that were consistent with the overall firing rates observed during the nonsignal portions of the peri-event histograms. Signal and background firing rates were calculated on a trial-by-trial basis for each of the first 36 self-infusions for all neurons that showed the combined profile of a lever-press response and a session decrease in firing. Changes in signal and background firing were evaluated relative to two periods: the 30 s that preceded the onset of the self-administration session (referred to as the pre-SD period) and the 30 s before the first press. The ratio of signal and background firing (S:B) was also determined on a trial-by-trial basis.
As a control for nonspecific determinants of the increases in S:B, we made additional comparisons of average firing during a "signal-control" period and a background period for all neurons that showed a session decrease in firing (i.e., were tonically inhibited) but showed no lever-press response during the maintenance phase. Signal equaled the average signal period for all lever-press neurons (i.e., –0.5 s prepress to +1.5 s postpress). The background period was defined as the same period used for the lever-press neurons.
BEHAVIORAL CONTROL ANALYSIS. The lever-press firing patterns are
closely associated with drug seeking. It was thus possible that differential
changes in percent time spent in drug seeking during signal and background
periods contributed to differential changes in signal and background firing.
We tested for this possibility in two ways. First, we characterized the
percent of time that animals spent in lever-directed locomotion during the
signal and background periods. In this analysis, lever-directed locomotion was
defined as locomotion that brought the animal proximal to the lever. In most
cases, this meant that the animal was close enough so as to press the lever.
Lever-directed locomotion increases in frequency shortly before each
cocaine-reinforced lever press during ongoing drug self-administration
sessions (see Peoples et al.
1998a). The extent to which the pattern of change in the
lever-directed locomotion is consistent with the pattern of change in signal
and background firing would therefore be indicative of whether differential
changes in drug-seeking behaviors during signal and background potentially
explain differential changes in firing.
Second, we conducted a behavioral clamp analysis
(Peoples 2002;
Rank et al. 1983).
Calculations of signal and background firing rate were limited to periods in
which animals were engaged in a specific behavior. For all neurons, the
background firing rate was calculated during the first period within the
–12 to –4 s prepress during which the animal engaged in 0.5 s of
uninterrupted stereotypy (or comparable behavior such as standing in place in
a corner on the non-lever side of the chamber). Signal firing rate was
calculated in either of two ways, depending on the timing of the signal
period. For most neurons, signal firing was calculated during drug-seeking
behavior that occurred within the last 0.5 s before a reinforced lever press.
Drug seeking was defined as an approach to the lever that terminated with a
press of the lever. The lever approach had to be 
0.5 s in duration to be
included in the analysis. For other neurons, limiting calculation of signal
firing to drug seeking was not possible because the signal period was
exclusively postpress (4 neurons). For these neurons, signal firing was
calculated during locomotion away from the lever, immediately after completion
of the lever press. Limiting calculations of firing rate to these behaviorally
defined periods allowed us to evaluate the possibility that differential
changes in behavior, and more specifically drug-reward-related behavior,
during the signal and background periods contributed to differential changes
in the signal and background firing rates over the course of the
self-administration session. Additionally, it allowed us to test further the
hypothesis that drugs may affect differentially activity that encodes
reward-related events during the self-administration session relative to other
neural activity. The videotapes of one experiment were not useable. The data
from that experiment (1 subject and 2 neurons) were therefore not included in
the behavioral control analyses.
Histology
Subjects were injected with a lethal dose of pentobarbital sodium. Anodal
current (50 µA for 4 s) was passed through each microwire. Animals were
perfused with formalin-saline. Coronal sections (50 µm) were mounted on
slides and incubated in a solution of 5% potassium ferricianide and 10% HCl to
stain the iron deposits left by the recording tip. The tissue was
counterstained with 0.2% solution of Neutral Red. The location of each wire
tip was plotted on the coronal plate
(Paxinos and Watson 1996) that
most closely corresponded to its anterior-posterior position. Only those
neurons verified histologically to be located within the nucleus accumbens
were included in the study.
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RESULTS |
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The patterns of self-administration behavior and the associated changes in
drug level observed in the present study were consistent with those described
in many previous studies (e.g., Pettit and
Justice 1989; Yokel
1987). The pattern of drug intake included an initial loading
phase and a subsequent maintenance phase. The initial presses of the loading
phase were executed rapidly relative to all subsequent presses. After these
rapidly completed self-infusions, animals slowed responding to a session low
before stabilizing response rates at an intermediate rate that was maintained
during the remainder (i.e., maintenance phase) of the session
(Fig. 2).
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Estimated drug levels rose rapidly in association with the initial presses to a session maximum ("overshoot" stage of loading phase). As response rate slowed, drug level at the time of the press first decreased from the peak that was attained during loading ("recovery" stage of loading phase) and then remained within stable narrow limits for the duration of the session. In addition to these loading-to-maintenance changes, drug level also showed a regular oscillation between successive lever presses. Specifically, drug rapidly increased to a stable maximum shortly after each self-infusion and then slowly decreased to a stable minimum that was attained shortly before the next self-infusion (Fig. 3).
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Neuron firing patterns
AVERAGE FIRING PATTERNS DURING THE MAINTENANCE PHASE. Phasic changes in firing during the seconds before and after the cocaine-reinforced lever press. Twenty of 68 accumbal neurons showed a phasic change in firing rate during the few seconds that bracketed cocaine-reinforced lever presses (e.g., Fig. 3B). For all but one neuron, the phasic change was an increase in firing rate around the time of the press relative to background (hereafter referred to as lever-press neurons).
Changes in firing associated with delivery of self-administered
drug. The 19 excitatory lever-press neurons exhibited two types of
changes in firing in response to self-administered drug. The most common type
involved a stable change in average discharge rate during the maintenance
phase, relative to the pre- and postdrug recording periods (henceforth
referred to as a session change or a tonic change in firing). For 14/19
neurons, the session change in firing was a decrease (i.e., neurons were
tonically inhibited, Fig.
3A); for 4/19 neurons, the session change was an increase
(i.e., neurons were tonically excited, Fig.
4). Consistent with the predominance of decreases, average firing
rate of all 19 lever-press neurons decreased during the self-administration
session relative to the nondrug pre- and postdrug recording periods. This
could be seen visually when average firing rate during the maintenance phase
was compared with average firing during the pre- and postsession nondrug
periods (Fig. 4A). The
decrease in the average firing rate of the population of lever-press neurons
was also evident visually and statistically when calculation of firing rate
was restricted to the seconds (i.e., ±10 s) that bracketed cocaine
self-infusions and firing was compared across successive self-infusions
[Fig. 5; F(35,595) =
2.21, P < 0.001]. The second type of change in firing that was
associated with drug delivery was exhibited by 15/19 neurons and occurred
during the minutes that elapsed between successive self-infusions. Firing rate
decreased during the first minute postpress and then progressively increased
until the time of the next lever press (inter-infusion-interval firing pattern
referred to previously as decrease + progressive reversal)
(Peoples and West 1996). This
firing pattern (Fig.
3C) closely mirrored the regular oscillation in drug
level that occurred between successive self-infusions
(Fig. 3A).
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The neurons that showed no lever-press firing pattern (i.e., non-lever-press neurons) also showed changes in association with drug delivery. The number of those neurons that showed session decreases and increases were 31/48 and 14/48 (i.e., 64 and 29%) respectively. Sixteen of the 48 neurons showed inter-infusion-interval pattern.
CHANGES IN SIGNAL AND BACKGROUND FIRING. Examples of signal and background periods are shown for two neurons in Fig. 6. In Fig. 7 (left), average signal and background firing are plotted as a function of lever-press number for all 14 of the tonically inhibited lever-press neurons. Comparisons between the 30-s pre-SD and the signal and background for the first lever press [F(2,26) = 3.56, P < 0.05] showed that signal (Tukey test, P < 0.05) but not background (Tukey test, P > 0.05) was significantly greater than average firing pre-SD. Across the first 36 presses, average signal showed no significant change [F(35,455) < 1.0] and thus remained elevated above the pre-SD rate throughout the session. In contrast, average background decreased significantly [F(35,455) = 3.49, P < 0.01] and in a dose-related fashion (Fig. 7B, left). The differential changes in signal and background produced a significant increase in both the average ratio of signal-to-background (S:B) [F(35,455) = 1.61, P < 0.05] and the average difference between signal and background (S-B) [F(35,455) = 1.56, P < 0.05; (Fig. 7, C and D, left].
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A more-detailed analysis of signal and background firing of single neurons showed that the changes in group mean firing reflected an average of two predominant patterns of signal and background change that were exhibited by 11 of the 14 neurons. For one group of six neurons (e.g., Figs. 8 and 9), both background and signal tended to be elevated above pre-SD levels at the time of the first press. Over the course of the self-administration session, the decrease in average signal was smaller in magnitude than was the decrease in average background firing rate. Therefore signal firing rate remained above the pre-SD rate throughout the self-administration session. The S:B ratio thus increased over the course of the session. For a second group of five neurons (e.g., Fig. 10), there was no consistent trend for signal and background firing rate to be either greater than or less than pre-SD at the time of the first press, although in all cases firing was low and close to pre-SD. Over the course of the self-administration session, decreases in background were small as would be expected given that the firing rates of the neurons were already low at the start of the session. The signal firing rate for this group of neurons did not consistently differ from background firing during the loading phase of the session, but subsequently increased to rates that tended to exceeded background and pre-SD. The increase occurred at either of the following times: the time at which drug reached the maximum level at the end of the overshoot stage of the loading phase or around the time at which the recovery phase was ending and the maintenance phase began. After this transition, signal showed trial-to-trial variation but hovered around a stable "mean" rate that was equal to or greater than the pre-SD rate. As a consequence of this pattern of signal and background firing, S:B showed an overall increase during the maintenance phase, relative to the loading phase (Fig. 10). Despite the differences between the two predominant patterns, both were consistent in showing that signal either attained a rate or remained at a rate that exceeded pre-SD, whereas background firing rate generally fell below pre-SD.
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BEHAVIORAL CONTROL. Analysis of videotapes showed that at the beginning of the self-administration session, animals spent time approaching the response lever during the background period as well as during the signal period. As the loading phase progressed, approaches to the response lever decreased in frequency and became restricted primarily to the signal period (Fig. 11). Time spent approaching the lever showed a greater decline during the signal period than during the background period. This pattern of differential change in the frequency of the behavior during signal and background periods is not in line with the differential changes in signal and background firing (e.g., compare Figs. 7B and 11). It is thus difficult to conclude that the changes in firing were due simply to changes in the occurrence of drug-seeking-related behaviors per se.
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Nevertheless, it was possible that the decreases in background firing rate and hence the increase in S:B were due, in part, to the decrease in the frequency of drug seeking during the background period. If this interpretation were correct, eliminating periods of drug seeking from the calculation of background firing would be expected to diminish the decrease in background firing rate over the course of the session and to also reduce the increase in S:B. We tested for this possibility in the six neurons that showed increases in S:B that were attributable only to differential decreases in signal and background firing. Specifically, we limited calculations of signal to periods in which animals were engaged in drug seeking and limited calculation of background firing to periods in which animals were engaged in stereotypy (or a comparable behavior). These recalculations of signal and background did not eliminate either the decrease in background firing rate or the increase in S:B (Figs. 12 and 13). These data show that the increases in S:B did not reflect changes in drug-seeking behavior during the signal and background periods. Moreover, controlling for drug seeking during the background period increased the apparent difference between background and signal firing at trial 1 for 5/6 neurons (e.g., Fig. 13), thus confirming that the increase in S:B for those neurons reflected an amplification of firing that was present at the first press rather than an emergence of a firing pattern that was initially absent.
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NON-LEVER-PRESS NEURONS THAT WERE TONICALLY INHIBITED. As a control for nonspecific determinants of the increase in S:B, we made comparisons of firing during the background period and a "signal control" period for another group of neurons. This second group of neurons included all recorded neurons that were tonically inhibited but showed no excitatory phasic response in relation to the lever press (referred to as non-lever-press neurons). For these neurons, there were no significant differences among average pre-SD firing and average signal and background firing at the time of the first press [F(2,60) = 1.01, P > 0.05]. Across the first 36 self-infusions, firing during the signal and background periods decreased significantly [F(35,1050) = 2.48, P < 0.001] and comparably [F(35,1050) = 1.34, P > 0.05]. There was no significant change in either the average S:B [F(35,1050) = 1.15, P > 0.05] or the average S-B [F(35,1050) = 1.28, P > 0.05; Fig. 7, right].
POPULATION S:B. The analyses of signal and background of the lever-press and non-lever-press neurons showed that the resistance of signal to inhibition during the self-administration session was unique compared with the firing exhibited by other accumbal neurons. It is thus possible that the magnitude of the reward-related signals increased relative to the background firing of all accumbal neurons. Evidence consistent with this hypothesis was observed in the present study. Average signal of the lever-press neurons showed no significant change over the course of the self-administration session [F(35,595) < 1.0]; whereas, average background of all recorded neurons decreased significantly ([F(35,2275) = 3.05, P < 0.001]. These differential changes in signal and background were associated with a doubling of the population S:B (Fig. 14).
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Histology
Three-quarters of the lever-press neurons were in the anterior half of the accumbens core. The other lever-press neurons were in the shell or along the shell core border. The neuron number was too small to make any definitive between-territory comparisons; although, it is interesting to note that none of the lever-press neurons that showed increases in signal were located within the shell.
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DISCUSSION |
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Learning theories of drug addiction propose that drug-induced amplification
of accumbal mechanisms that normally mediate aspects of reward-related
learning and behavior contributes to the etiology of drug addiction.
Neurophysiological mechanisms that might transduce such a drug effect on
accumbal mechanisms have yet to be identified. Based on previous studies of
accumbal firing patterns during drug self-administration sessions and previous
studies of DA modulation of striatal information flow, we hypothesized that
self-administered drug may lead to aberrant amplified drug-reward-related
learning, in part, by enhancing the S:B of drug-reward-related signals during
drug self-administration sessions (for similar and related proposals, see
Carelli et al. 1999;
Nicola et al. 2000;
Peoples et al. 1998a).
Given this hypothesis, in the present study, we tested whether the excitatory phasic firing responses to drug-reward-related events such as the cocaine-reinforced lever press (i.e., signal), would be less sensitive than other accumbal firing (i.e., background) to the inhibitory effects of self-administered cocaine. Trial-by-trial characterizations of signal and background firing of lever-press neurons showed that signal either remained at a rate or attained a rate during the self-administration session that exceeded the pre-SD firing rate, whereas, background firing rate generally fell below pre-SD. Behavioral control analyses showed that the differential changes in firing could not be attributed to differential changes in behavior during signal and background periods. It is thus possible that the lesser inhibition of signal relative to background reflected a differential pharmacological effect of cocaine on signal and background firing.
The differential changes in signal and background firing rate were associated with an increase in the ratio of signal-to-background for the individual neurons. Perhaps more importantly, the unique resistance of signal firing to the inhibitory effects of self-administered cocaine was associated with an increase in signal firing of the lever-press neurons relative to the firing of all the recorded accumbal neurons. These increases in S:B might be expected to increase the influence of the accumbal drug-reward-related signals on the state of accumbal-related neural circuits and hence on accumbal-mediated functions (e.g., learning). It is thus a mechanism by which the predominantly inhibitory effects of self-administered drug could amplify accumbal contributions to reward-related learning during drug self-administration sessions. Consistent with the proposals of learning theories of addiction, such drug effects could potentially contribute to the development of drug addiction.
Contribution of primary drug effects to the increases in S:B
The individual neuron and population increases in S:B were largely
attributable to the differential inhibition of signal and background firing.
It is possible that the differential inhibition reflected pharmacological
mechanisms. As we have already noted, control analyses ruled out the
possibility that the S:B changes were caused by differential changes in
behavior during the signal and background periods. Moreover, numerous analyses
in previous accumbal recordings during drug self-administration sessions show
that for most neurons, the decreases in average firing are likely to be
pharmacological in origin (Chang et al.
1994; Nicola and Deadwyler
2000; Peoples
2002; Peoples and West
1996; Peoples et al.
1994,
1998a,b).
A pharmacological mediation of the S:B increase would be consistent with
the findings of various types of electrophysiological experiments. Acute
electrophysiological recording studies generally show that the primary
pharmacological effect of cocaine on spontaneous firing of accumbal neurons is
inhibition (Nicola et al.
1996; Qiao et al.
1990; Rebec and Zimmerman
1980; Uchimura and North
1990; White et al.
1993). Additionally, in both anesthetized animals and awake,
spontaneously active animals, iontophoretic application of either cocaine or
amphetamine to the accumbens (and elsewhere) inhibits tonic, spontaneous,
firing to a greater degree than it inhibits evoked excitatory signals
(Haracz et al. 1993;
Jimenez-Rivera and Waterhouse
1991; Kiyatkin and Rebec
2000; Wang and Rebec
1993). Given these data, it appears that the contribution of
inhibition to increases in the ratio of signal to background observed in the
present study is consistent with, and potentially mediated by, a primary
intra-accumbal pharmacological effect of cocaine.
Neurochemical mediation of decreases in background firing rate
Cocaine blocks the reuptake of the monoamines norepinephrine (NE),
serotonin (5-HT), and DA (for review, see
Ritz et al. 1987).
Electrophysiological studies suggest that NE
(Unemoto et al. 1985), 5-HT
(White et al. 1993) and DA
(for review, see Nicola et al.
2000; O'Donnell et al.
1999; Pennartz et al.
1994) can influence accumbal neural activity. There is also
evidence that each of the monoamines tends to differentially influence signal
and background firing in at least some regions of the brain
(Foote et al. 1983;
Funke and Eysel 1993;
Kiyatkin and Rebec 1996;
Rolls et al. 1984;
Sawaguchi et al. 1990;
Waterhouse et al. 1986,
1991;
Woodward et al. 1979). Thus a
priori, any of the monoamines potentially contributed to the increases in S:B
during cocaine self-administration. However, there are reasons to expect that
DA might have played a particularly important role. First, the concentrations
of NE and 5-HT are low in the accumbens relative to the concentrations of DA
(Allin et al. 1988). Second,
acute electrophysiological studies show that the inhibitory pharmacological
effects of cocaine in the accumbens are mediated predominantly by DA
(Qiao et al. 1990;
Rebec and Zimmerman 1980;
White 1990;
White et al. 1993; for a
related observation, see Nicola et al.
1996; but see Kiyatkin and
Rebec 2000). Third, cocaine reward depends on DA actions
in the accumbens rather than on NE and 5-HT actions (for review, see
Ritz et al. 1987;
White 1990;
Wise and Bozarth 1987;
Wise et al. 1995). Given the
involvement of accumbal DA in drug reward, it seems likely that changes in
drug-reward-related accumbal firing would be mediated by DA rather than by the
other monoamines.
A role of DA in mediating the S:B changes observed in the present study
would be consistent with the hypothesis that DA modulates accumbal and
striatal activity in a behaviorally dependent and neural activity-dependent
manner. More specifically, increases in DA input are expected to enhance
transmission of strong, contextually relevant, excitatory signals
relative to weaker and less relevant excitatory inputs
(Floresco et al. 2001;
Hernandez-Lopez 1997; Kiyatkin and Rebec
1996; Levine et al.
1996; Mogenson and Yim
1991; O'Donnell and Grace
1996; Pennartz et al.
1994; Pierce and Rebec
1995; Rolls et al.
1984). This hypothesis is based primarily on the effects of
experimenter-applied DA during either acute recording conditions or recordings
in awake animals engaged in spontaneous behavior (although see
Rolls et al. 1984). The
present study shows that accumbal firing patterns consistent with the
differential modulation of signal and background can be observed during a
period in which subject-determined reward-relevant increases in
accumbal DA regulate ongoing instrumental behavior. The data thus suggest that
the S:B mechanism and the hypotheses regarding DA modulation of accumbal
information flow may be applicable to reward-related accumbal DA function.
Relation to other studies
COCAINE SELF-ADMINISTRATION. The present study is the first to
test for trial-to-trial changes in the signal and background firing of
accumbal neurons over the course of the self-administration session. However,
related analyses were conducted in a number of previous studies. Specifically,
Carelli and colleagues compared average firing during the 10-s pre-
and postpress for all lever presses during the loading phase to
average firing pre- and postpress during the maintenance phase
(Carelli and Deadwyler 1996b;
Carelli et al. 1999). These
researchers observed that average firing 10-s pre- and postpress was greater
during the maintenance phase than during the loading phase and that this
increase in average firing was associated with increased evidence of the
phasic lever-press firing patterns. Based on these observations, Carelli and
colleagues (Carelli and Deadwyler 1996;
Carelli et al. 1999) concluded
that accumbal neurons are responsive to drug-reward-related signals only after
drug, and presumably DA, exceeds some threshold level. The results of the
present study also show that phasic firing patterns are more apparent during
the maintenance phase relative to the loading phase. However, we propose that
the findings of the present study, along with other data, are consistent with
alternative conclusions regarding the mechanisms that potentially mediate the
enhancement of phasic firing over the course of the self-administration
session.
For a significant subset of the neurons in the present study, signal firing
was elevated at the time of the first press in relation to firing before the
start of the self-administration session. For these neurons, signal firing
either remained stable or decreased somewhat but nevertheless remained
elevated relative to baseline firing rates throughout the self-administration
session. On the other hand, the background firing decreased over the course of
the self-administration session to rates that were below baseline firing
rates. Indeed, average background firing of all lever-press neurons decreased
over the course of the self-administration session. The same was true for the
non-lever-press neurons. These findings are consistent with the interpretation
that at least some accumbal neurons are responsive to afferent input related
to drug-reward events at the onset of the self-administration session.
Moreover, the present findings are consistent with the further conclusion that
a primary effect of drug is to inhibit or cull non-drug-reward-related
responses rather than to switch-in responsiveness of accumbal neurons to
excitatory afferent input. These conclusions are consistent with evidence that
activation of accumbal afferents can evoke drug seeking and that neurochemical
changes in the accumbens correlate with the initiation of drug seeking (e.g.,
Cornish and Kalivas 2000;
Cornish et al. 1999;
Gratton and Wise 1994; Ito et
al. 2000; Kiyatkin and Stein
1996; Phillips et al.
2003; Vorel et al.
2001; Weiss et al.
2000).
It is the case that there were some neurons in our study that showed
increases in signal firing and average firing pre- and postpress over the
course of the self-administration session. A priori, it is possible that these
increases reflected a drug-induced enhancement in excitatory accumbal
responses. There are a number of DA-mediated facilitative mechanisms that have
been identified in acute electrophysiological studies that could potentially
mediate such a drug-induced enhancement in the absolute magnitude of the
signal (for review, see Nicola et al.
2000; O'Donnell 1999; Pennartz
et al. 1994). However, data from several studies point to
additional possible interpretations of the increases in firing. First,
although there is evidence that in the majority of cases decreases in firing
during drug self-administration are pharmacological in nature, there is little
evidence that the increases in firing exhibited by accumbal neurons are also
(for review, see Peoples
2002). Indeed, when increases in signal were observed in the
present study (also in the studies of
Carelli and Deadwyler 1996b;
Carelli et al. 1999), the
increase most often occurred abruptly at particular points in the session.
This non-dose-dependent step-function change in firing is more likely to
reflect a nonpharmacological neural response to a change in the state of
either the environment or the animal rather than a direct pharmacological
effect of the drug that influences the ability of neurons to respond to
preexisting afferent input. Second, accumbal recordings made in animals
engaged in sequences of behavior directed toward nondrug rewards indicate that
at least some accumbal neurons exhibit phasic responses to reward-related
events contingent on the proximity of the event to reward
(Shidara et al. 1998). Those
data suggest that context-dependent responses of some accumbal neurons to
reward-related events may help to track progress through a reward-related
sequence. It is possible that some of the increases in signal exhibited by
lever-press neurons during a drug self-administration session reflect such
tracking rather than a drug-induced change in the ability of a neuron to
respond to particular afferent input.
OTHER ADDICTIVE DRUGS. Electrophysiological recordings conducted
by other investigators show that accumbal neurons exhibit firing patterns
during sessions of ethanol and heroin self-administration that are comparable
to patterns exhibited during cocaine self-administration
(Chang et al. 1998;
Janak et al. 1999). Moreover,
acute recording studies show that addictive drugs, in general, tend to
decrease average firing of accumbal neurons (Criado et al.
1995,
1997; Hakan and Henricksen
1989; and references cited in the INTRODUCTION). It will thus be of
interest in future studies to explore the possibility that self-administration
of other drugs produce within-neuron increases in S:B and population increases
in S:B that are comparable to those observed in the present study of cocaine
self-administration. Given the common inhibitory effects of addictive drugs,
it is possible that a drug-induced increase in drug-reward-related S:B is a
common neurophysiological mechanism contributing to the reinforcing and
possibly addictive effects of the drugs (discussed further in Accumbal
contributions to reward-related learning and drug addiction).
NATURAL REWARDS. Electrophysiological recordings of striatal and
accumbal firing during instrumental sessions maintained by natural rewards
have not yielded reports of changes in S:B (although, see
Rolls et al. 1984). There are
a number of potential explanations for the absence of such reports. First, the
increases in S:B ratio may be unique to drug self-administration. This
explanation is plausible given that self-administered drug is likely to
produce changes in accumbal neurochemistry that are distinct quantitatively,
if not qualitatively, from neurochemical changes associated with instrumental
behavior directed toward natural rewards. Second, it is possible that studies
of natural rewards have not yet applied analyses appropriate to observing the
phenomena. In regard to this latter possibility, there has been little
characterization of background (tonic) firing in electrophysiological studies
of nondrug rewards. It would be of interest to perform such analyses. There
are many commonalities among the variables and mechanisms that influence
behavior directed toward drug and nondrug rewards. It thus seems possible that
the S:B changes observed in the present study are closely related to the
mechanisms that normally regulate (i.e., gate) the flow of signals through the
accumbens and thereby direct reward-related behavior and influence
reward-related learning (discussed further in the following text).
Accumbal contributions to reward-related learning and drug addiction
QUESTIONS ANSWERED AND QUESTIONS STILL REMAINING. The present
findings are relevant to understanding the involvement of accumbal neurons in
learning and drug addiction. The role(s) of the nucleus accumbens in learning
is not fully understood. However, microinjection and lesion studies show that
dopamine-mediated accumbal mechanisms do contribute to the occurrence and
expression of certain types of reward-related learning. For example, the
accumbens is involved in autoshaping (Pavlovian conditioning) (e.g.,
Cardinal et al. 2002). The
accumbens additionally modulates the impact of previously conditioned stimuli
on instrumental (reward-directed) behavior (for review, see Everitt et al.
1999,
2001). Acutely administered
cocaine and other psychomotor stimulants enhance the influence of previously
conditioned stimuli on reward-related behavior and have been observed to
facilitate some types of reward-related learning (e.g.,
Cardinal et al. 2000;
Harmer and Phillips 1998;
Killcross et al. 1997;
Krivanek and McGaugh 1969;
Robbins 1978;
Robbins et al. 1989;
Taylor and Horger 1999;
Wyvell and Berridge 2000).
The drug effects appear to be mediated, in part, by dopaminergic modulation of
accumbal neural activity (Robbins et al.
1989; Taylor and Horger
1999; Taylor and Robbins
1986; Wyvell and Berridge
2000). These and other observations have led to the proposal that
drug-induced increases in accumbal DA contribute to the development of drug
addiction by amplifying accumbal mechanisms that normally contribute to
reward-related learning and behavior. This facilitation of accumbal mechanisms
is thought to lead to the abnormal strengthening of drug-directed behaviors
and stimulus control thereof, which in turn contributes to compulsive
drug-seeking (for review and related proposals, see
Berke and Hyman 2000; DiChiara
1998,
2002;
Hyman and Malenka 2001;
Robbins et al. 1983;
Robbins 1978;
Robinson and Berridge 1993;
Robinson and Everitt 1999; Stewart
1992; Stewart et al.
1984; White
1996).
Based on this learning view of addiction, one can make a number of predictions. One such prediction is that drug actions in the accumbens would enhance reward-related accumbal signaling during periods of drug self-administration. In contrast to this prediction, and as already described in the preceding text, we have not as of yet observed evidence that pharmacological effects of self-administered drug increase the absolute magnitude of excitatory reward-related accumbal signals. However, the present study demonstrates that self-administered drug is associated with a differential inhibition of signal and background such that there is a net enhancement of drug reward-related signals relative to background firing. It is possible that this increase in S:B corresponds to the predicted drug-induced amplification of drug-reward-related signals.
If the increase in S:B did reflect an amplification of drug-reward-related
signals, one would further predict, on the basis of the learning view, that
the increase in S:B might amplify the contribution of the accumbens to
learning. To evaluate this prediction, it would be helpful to first consider
the firing patterns of accumbal neurons during non-drug-reward-related
learning and to understand how those firing patterns normally contribute to
learning. One could then ask if the drug-induced changes in firing patterns
(i.e., the enhanced S:B) potentially amplify the contribution of the
accumbens. Patterns of phasic activity exhibited by accumbal neurons during
reward-related sequences are consistent with the functional contributions that
DA-mediated accumbal mechanisms are known to make to reward-related behavior
and learning. For example, consistent with an accumbal contribution to
autoshaping and an accumbal modulation of the influence of conditioned stimuli
on instrumental behavior, accumbal neurons show phasic firing in relation to
conditioned stimuli, instrumental behavior, and reward (e.g.,
Apicella et al. 1991;
Bowman et al. 1996;
Carelli and Deadwyler 1994;
Chang et al. 1994; Lavoie and
Mizumori 1994; Peoples et al.
1997; Shidara et al.
1998; Williams et al.1989). Despite this correspondence, very
little is known about the mechanisms by which the reward-related accumbal
signals actually contribute to either the formation of associations or to the
influence of conditioned stimuli on reward-directed behavior. Whether and how
increases in the S:B of reward-related signals might facilitate accumbal
contributions to learning and behavior are therefore a matter of
speculation.
To make progress in addressing these questions, it is necessary to form testable hypotheses. In considering developing views of the neurophysiology of accumbal contributions to learning and behavior, in conjunction with the present findings, we have identified a possible direction for future research. In the next section, we briefly describe current views of accumbal neurophysiology and DA modulation thereof. Familiarity with these views is necessary for understanding our hypotheses, which are described in the final section.
DEVELOPING VIEWS OF THE NEUROPHYSIOLOGY OF ACCUMBAL CONTRIBUTIONS TO
LEARNING AND BEHAVIOR. Electrophysiological recordings in behaving
animals engaged in behavior directed toward reward show that a relatively
small minority of neurons (<30%) actually respond to stimulus and
behavioral events (e.g., Bowman et al.
1996; Carelli and Deadwyler
1994; Chang et al.
1994; Lavoi and Mizumori
1994; Schultz et al.
1992; Uzwiak et al.
1997; Williams
1989; Williams et al.
1993; although see Shidara et
al. 1998). Moreover, these neural responses tend to be
heterogeneous in terms of the particular reward-related events that evoke them
and also in terms of the direction of the change in firing, although
excitatory responses predominate. These data are consistent with the
previously stated conclusion that ".. .behaviorally meaningful
information in the nucleus accumbens is represented by fine-grained
spatiotemporal firing patterns [of ensembles of neurons]... rather than by
massive waves of activity uniformly sweeping from Acb to the ventral pallidum
and related terminal fields"
(Pennartz et al. 1994; p 726).
The make-up of or ensembles of neurons that respond in a particular situation
is unknown; however, consistent with what is understood of striatal circuitry,
it has been hypothesized that an ensemble is a group of neurons that have
similar afferent and efferent relationships and are closely related
functionally (Pennartz et al.
1994). More specifically, ensembles may be groups of neurons
defined by membership within particular
cortico-striato-pallidothalamo-cortical loops
(Pennartz et al. 1994; also
see Alexander et al. 1990;
Groenewegen et al. 1990,
1993;
Haber and McFarland, 2001).
Based on acute electrophysiological studies of individual neurons, activation
of ensembles are proposed to require convergent synchronous input
(Pennartz et al. 1994;
O'Donnell and Grace 1996;
O'Donnell 1999), which is thought to derive from functionally related cortical
and limbic afferents. Activation of particular accumbal ensembles is expected
to lead to the activation of functionally related groups of neurons in
down-stream mesencephalic, cortical (via ventral-pallido-thalamic
projections), and hypothalamic targets. The activation of certain ensembles,
in conjunction possibly with inactivation of others, is thought to mediate
changes in sensory and cognitive processing, the internal milieu, and the
motivation and behavior of the organism (see
Pennartz et al. 1994). Thus
the heterogeneity of accumbal responses during a reward sequence may be
understood as reflecting the sequential and simultaneous activation of
multiple ensembles within distinct cortico-striato-pallido-thalamo-cortical
loops in response to the various components and aspects of a reward-related
sequence.
DA input to the accumbens is thought to refine the spatiotemporal pattern
of neural activity and to thereby modulate accumbal contributions to behavior
and learning (O'Donnell 1999; Pennartz et
al. 1994). Acute electrophysiological evidence suggests that this
DA influence on neural and ensemble activity may be mediated by multiple
mechanisms (see Nicola et al.
2000; O'Donnell 1999; Pennartz
et al. 1994 for review). One mechanism is firing-rate-dependent
(i.e., frequency-dependent) inhibition, which is thought to contribute to the
selective activation of ensembles that are most relevant to the behavioral
setting (Pennartz et al.
1994). The differential inhibition of drug-reward-related firing
and background firing observed in the present study can be viewed as
potentially reflecting such differential modulation of ensemble activity. This
modulatory, "filtering" effect of DA on ensemble activation could
contribute to directing behavior by constraining which neurons and ensembles
are activated and thus able to pass signals on to down-stream targets and
consequently influence behavior.
The filtering effect of DA could similarly contribute to learning by
narrowing the ensembles in accumbens, or down-stream targets, that are
available to participate in processes that mediate the development and
strengthening of associations. It is also possible that a differential
inhibition of reward-related and nonreward-related signals could influence
learning by modulating the susceptibility of neurons to frequency-dependent
cellular and synaptic plasticity (e.g., long-term potentiation and
depression). Such plasticity is thought to lead to a differential
strengthening and weakening of synaptic connections, neuronal responses, and,
perhaps, ensemble responses and to ultimately underpin learning and selective
changes in behavioral responses to environmental stimuli predictive of reward
(cf. Berke and Hyman 2000;
Hyman and Malenka 2001;
Pennartz et al. 1994).
Finally, it should be noted that shifts in DA activation that occur as a
function of repeated behavioral experiences, for example, the shift in DA
responses from the primary reward to stimuli that predict reward (for review,
see Schultz 2000;
Schultz and Dickinson 2000),
may further contribute to selection and constraint of synapses and neurons
that are subject to neuroplasticity and thereby further limit or cap learning
(i.e., limit the extent to which certain patterns of activation are
strengthened and weakened) and facilitate development and maintenance of
adaptive contextually appropriate behavioral responses.
HYPOTHESES REGARDING DRUG-INDUCED AMPLIFICATION OF ACCUMBAL
CONTRIBUTIONS TO LEARNING AND THE PHARMACOLOGICAL MECHANISMS THAT CONTRIBUTE
TO DRUG ADDICTION. All addictive drugs elevate DA in the accumbens (for
review, DiChiara 1995;
DiChiara and Imperato 1988;
Leshner and Koob 1998; Wise and Bozarth
1987). These drug-induced elevations in DA are likely to be
substantially higher than those that occur under non-drug-reward-related
conditions. It is thus likely that during drug self-administration,
drug-reward-related and non-drug-reward-related patterns of accumbal neural
activity are subject to abnormally strong effects of DA on ensemble selection
and plasticity. Additionally, because of the direct effects of addictive drugs
on DA neurons, increases in DA in relation to drug reward may not dissipate as
a function of repeated experience to the same extent as occurs in relation to
non-drug rewards (DiChiara
1998,
2002). It is thus possible
that DA influences on learning are not subject to normal experience-related
constraints. That is, additional strengthening and weakening of various
synapses and neural responses may occur with each and every
self-administration experience.
Given the unusual characteristics of DA input to the accumbens during drug
self-administration sessions, and observations of the present study, we
hypothesize that during drug self-administration sessions, DA-mediated drug
effects may engender an unusually global and powerful weakening of synaptic
connections and neural responses that are involved in transmission of signals
unrelated to drug reward. Conversely, and simultaneously, a relatively unique
sparing of drug-reward-related signals during drug self-administration may
lead to an unusually selective susceptibility of the relevant synapses and
neural responses to maintenance or perhaps strengthening. Thus with repeated
drug self-administration, there may be a general weakening of neural responses
that are unrelated to drug reward and a highly selective sparing of neural
responses related to drug reward. The proposed differential weakening of
synaptic connections and neural excitability could lead to a progressive
decline in the throughput of non-drug-reward-related signals through accumbal
circuits, including cortico-striato-pallido-thalamo-cortical loops, and a
decline in the influence of non-drug-reward-related signals on behavior and
learning. Simultaneously, drug-reward-related signals would exert an
increasingly dominant "lone-voice" role in influencing the same.
Such a narrowing of information flow in addition to contributing to the
progressive and selective increase in drug-reward related behaviors could also
potentially contribute to the progressive narrowing in behavioral repertoire,
anhedonia, decreased reward sensitivity, and cognitive deficits that are
characteristic of drug addiction (cf.
Grant et al. 2000;
Koob and LeMoal 2001;
Martin-Soelch et al.
2001a,b;
Rogers et al. 1999;
Volkow et al. 2000).
A number of observations made in our laboratory, in addition to the present
findings, suggest that this proposal is worth exploring. We observed
previously that accumbal neurons that are inhibited by cocaine self
administration show a progressive decline in average basal firing rate as a
function of repeated drug self-administration sessions
(Peoples et al. 1997). This
decline in average firing is consistent with evidence that repeated exposure
to drug can be associated with weakening of synaptic strength in the accumbens
(Thomas et al. 2001). It is
also consistent with cellular plasticity that has been documented to occur in
animals with a history of repeated drug exposure and that is expected to be
associated with a decrease in neural excitability (e.g.,
Nestler 2001;
White et al. 1995;
Zhang et al. 1998; for a
related observation, see O'Donnell and
Grace 1993). Finally, preliminary analyses in our laboratory
suggest that across repeated drug self-administration sessions, firing related
to drug-reward-related events does not decrease to the same extent to which
background firing decreases (unpublished observations, Peoples and Cavanaugh).
These observations are consistent with the proposal that an increase in S:B is
a neurophysiological mechanism that could mediate the drug-induced
amplification of the accumbal contribution(s) to learning that is hypothesized
to contribute to drug addiction.
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DISCLOSURES |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Address for reprint requests: L. L. Peoples, Department of Psychology, University of Pennsylvania, 3815 Walnut St, Philadelphia PA, 19104. Email: lpeoples{at}psych.upenn.edu
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ABSTRACT
INTRODUCTION
25 ms. Number of intervals (counts, y
axis) is plotted as a function of 0.1-ms increments in interval length. Above
the histogram is a sample of the corresponding neural waveform. The sample
consists of the 1st 500 consecutively recorded waveforms. Positive voltage is
up. Each waveform trace spans 0.64 ms.
) is plotted as a function of
cocaine-reinforced lever presses. Average calculated drug level (
) (cf.
Pan et al. 1991
) firing rates are plotted as a function of lever press. Dashed line
indicates average firing rate during the 30-s pre-SD. C:
S:B plotted as a function of lever press number. Dotted line indicates an S:B
of 1. Left and right: a single neuron.
