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1Department of Psychology, University of Pennsylvania; and 2Neuroscience Graduate Group, and 3Department of Psychiatry, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104
Submitted 3 July 2003; accepted in final form 26 September 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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; DiChiara 1998; Everitt et al. 2001; Robbins et al. 1983; Robbins and Everitt 1999). 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. 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 (for review see Everitt et al. 2001).
Neuropharmacological data indicate that the drug-induced facilitation of learning-related mechanisms, and the contribution of this drug-learning interaction to drug addiction, is mediated by drug effects on accumbal neural activity (Everitt et al. 2001; Leshner and Koob 1999; Wise and Bozarth 1987). However, the nature of these drug effects is not yet understood. A simple hypothesis is that drug amplifies the excitatory phasic responses that appear to mediate the accumbal contribution to learning (Lavoie and Mizumori 1994; Schultz et al. 1992; Shidara et al. 1998; Tabuchi et al. 2000; Williams 1989). Based on this proposal one might predict that accumbal neurons would exhibit responses to drug-reward-related events under drug-free conditions. One might also expect that the neural responses would continue to be present, albeit in an amplified form, once drug was in the body. Both of these predictions remain to be tested fully.
In regard to the first prediction, there is substantial evidence that accumbal neurons exhibit phasic, and primarily excitatory, responses to drug-directed instrumental behaviors and drug-associated cues during the maintenance phase of drug self-administration sessions, when drug level is asymptotic (Bowman et al. 1996; Carelli et al. 1993; Chang et al. 1994; Janak et al. 1999; Uzwiak et al. 1997). Nevertheless, accumbal neurons do not exhibit these phasic responses during the initial loading phase of self-administration sessions, when drug level is low. Based on this finding, it has been proposed that accumbal neurons are unable to respond to drug-reward-related events during a period of drug seeking until some minimum level of drug, and presumably dopamine (DA) is attained in the brain (Carelli et al. 1993, 1999). However, there has been no characterization of the pattern of accumbal activity across drug-free and -exposed periods. The prediction that accumbal neurons exhibit responses to drug-reward-related events under drug-free conditions, as well as drug-exposed conditions, thus requires further testing before it can be evaluated.
In addition to the existent uncertainties concerning the responsiveness of accumbal neurons to drug-reward-related events, there are questions regarding the prediction that addictive drugs amplify accumbal responsiveness. In line with the findings of acute recording studies, previous studies of accumbal neural activity during drug self-administration sessions indicate that the primary pharmacological effect of self-administered drug is to decrease average firing rate of a majority of neurons (for review see Nicola et al. 2000; O'Donnell 2003; Peoples 2002). Thus the pharmacological effect of drug is generally opposite the change in firing associated with reward-related accumbal neural signals. The mechanism by which drug might amplify reward-related signals is therefore not readily apparent.
A possible resolution of this issue can be proposed on the basis of studies of DA modulation of accumbal neural transmission. These DA studies indicate that contextually relevant and hence robust neural signals are not as inhibited by DA as are less relevant and weaker signals. Such a differential decrease in firing would enhance the signal-to-background ratio of the most robust signals and thereby amplify, or otherwise facilitate, the influence of those signals (for review see Nicola et al. 2000; O'Donnell 2003; Pennartz et al. 1994). It is possible that a similar mechanism mediates the amplification of drug-reward-related signals by the inhibitory effects of addictive drugs. That is, neurons responsive to drug-reward-related events at the onset of drug exposure may maintain higher firing rates during drug self-administration relative to other neurons such that the impact of the drug-reward-related signals is enhanced relative to all other signals.
In light of the above, the present study focused on two primary questions. First, is there evidence that accumbal neurons respond to drug-reward-related events under drug-free conditions, and if so, is there evidence that neurons responsive under drug-free conditions tend to be the same neurons that are responsive during drug self-administration? Second, do neurons that receive excitatory signals associated with drug-reward-related events under drug-free conditions maintain greater firing rates than do other neurons during the drug self-administration session? To address these questions, we characterized the activity of accumbal neurons during a drug-free period in which well-trained animals were first exposed to cues that signaled drug availability and then subsequently initiated drug seeking. We then characterized the activity of the same neurons during a cocaine self-administration session that ensued immediately thereafter. The results of these analyses were consistent with an affirmative answer to both experimental questions. The findings of the present study demonstrate that accumbal neurons exhibit patterns of firing that may correspond to the accumbal mediated drug-learning interactions hypothesized to contribute to drug addiction.
Portions of the data were reported previously in abstract form (Giampoala et al. 2000)
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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The subjects (24 male Long-Evans rats), neural recording sessions, and neurons described in the present report are the same as those studied by Peoples and Cavanaugh (2003). 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 were approved by the Animal Care and Use Committee of Rutgers, The State University of New Jersey.
Surgery and postoperative maintenance
Animals were anesthetized with sodium pentobarbital (50 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 12-16 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). At least 7 days after surgery and 3 days before self-administration training, subjects were transferred to a Plexiglas chamber where they remained for 24 h/day for the duration of the study. Subjects had free access to water and were fed approximately 15 g of food each day to maintain body weight at 350 g. A more detailed description was provided in another report (Peoples and West 1996).
Behavioral procedures
A DAILY COCAINE SELF-ADMINISTRATION SESSION: SEQUENCE OF EVENTS. 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 the insertion of the response lever into the chamber. Once the lever was inserted, subjects received an intravenous infusion of saline (0.2 ml) that was paired with a 7.5-s tone and the offset of the stimulus light for 40 s. The sequence of events is referred to henceforth as the discriminative stimuli (SD). Following 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 solution (0.7 mg/kg/0.2 ml infusion) that was paired with a 7.5-s tone and the offset of the stimulus light for 40 s. During the light-off period, presses on the lever 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. Self-administration training sessions were conducted 7 days/wk. Prior to the recording study, subjects completed 12-17 days of self-administration training. For additional details see Peoples and Cavanaugh (2003).
CHARACTERISTICS OF THE Sd AND THE FIRST PRESS. Given that subjects were housed in the chamber 24 h/day, the SD was expected to be a good predictor of the onset of drug availability. Additionally, components of the SD closely matched the stimuli that were paired later in the session with each infusion of cocaine. Given these similarities, some stimulus generalization (cf. Kehoe and Gormezano 1980; Pearce et al. 1994) would be expected to occur between the stimuli associated with drug delivery during the drug self-administration session and the SD, which would be expected to strengthen further the association between the SD and the drug reward (i.e., strengthen the drug-reward-related incentive properties of the SD). The first drug infusion of the day was that which followed the first lever press made by an animal post-SD. Behavior associated with the first lever press thus corresponded to drug-directed instrumental behavior (i.e., drug seeking) under drug-free conditions.
Video recording
During each recording session, behavior was videotaped using a JVC HR-78004 SuperVHS 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. After the recording session, in frame-by-frame analysis, time stamps (temporal resolution = 33 ms) associated with the onsets or offsets of particular behaviors were read from the video frames displayed on a video monitor (Peoples et al. 1998a).
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 to either the drug-associated 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 (450-10 kHz) and amplifier (Riverpoint Electronics). Using software and hardware of DataWave Technologies Inc. (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 and Plexon) was used to discriminate neural waveforms. After a population of waveforms was isolated by the discrimination procedures, it was subjected to an interspike interval histogram analysis to confirm that it corresponded to a discriminated single neuron (cf. Peoples 2002).
Data analysis
QUESTION 1: DO NEURONS RESPOND TO DRUG-REWARD-RELATED INFORMATION UNDER DRUG-FREE CONDITIONS? Individual neuron analysis. To evaluate accumbal responses to drug reward-related events under drug-free conditions, we characterized neural responses to the SD and to the first press (1st press). Neural responses to the SD were defined on the basis of a statistical criterion. Specifically, an SD response was defined as a significant change in firing rate between the 30-s pre-SD and the 30-s post-SD. To compare firing pre- and post-SD, firing rate (total number of discharges) was determined as a function of 1-s bins. The number of discharges in the 30 1-s bins pre-SD was then compared with the number of discharges in the 30 1-s bins post-SD using a Mann-Whitney test (Siegel and Castellan 1988). A 1st-press neural response was similarly defined and analyzed. Specifically, a significant 1st-press response was defined as a significant change in firing during the 30 s immediately preceding the 1st press, relative to the 30-s pre-SD. There was no overlap between the SD presentation and the 30 s before the 1st press. The comparisons of the pre-SD period to the post-SD and 1st-press periods thus reflected comparisons of the pre-SD period to two other nonoverlapping intervals. Neurons that showed significant increases in firing to either or both the SD and the 1st press were referred to as excitatory SD/1st-press neurons. All other neurons were referred to as nonexcitatory SD/1st-press neurons.
Is there continuity in accumbal neural responses to drug-reward-related events during drug-free and -exposed periods? We tested for continuity between neural responses to drug-reward-related events during drug-free and -exposed conditions by comparing the average response of excitatory and nonexcitatory SD/1st-press neurons to the cocaine-reinforced lever presses during the maintenance phase of the self-administration session. The average response to the cocaine-reinforced lever press was calculated as follows. For each neuron, we determined the number of discharges during two time points in relation to each of the last 27 lever presses of the session (i.e., all presses excluding the 1st 10.). These time points included -.5 to +1.5 s of the lever press and the 10 to 12 s prepress. These time periods were consistent with intervals used in previous studies to define phasic neural responses to cocaine-reinforced lever presses. The discharges in each interval were summed across the 27 lever presses for each neuron. Comparisons of average firing during these two time points were made between the excitatory and nonexcitatory SD/1st-press neuron groups using Generalized Estimating Equations (GEE) models (described in the next section).
QUESTION 2: DO NEURONS THAT SHOW EXCITATORY RESPONSES TO DRUG-REWARD-RELATED EVENTS UNDER DRUG-FREE CONDITIONS MAINTAIN HIGHER FIRING RATES DURING THE SELF-ADMINISTRATION SESSION THAN DO OTHER NEURONS? To make comparisons of firing rate during the self-administration session between the excitatory and nonexcitatory SD/1st-press neurons, we determined firing rates during two sample periods of the self-administration session, the 30-s prepress and the 30-s postpress for the first 37 cocaine-reinforced lever presses. Behavior during the 30 s before and after the cocaine-reinforced lever press is quite similar across subjects (Peoples et al. 1998a). For example, drug-directed behaviors (e.g., approaches to the response lever) are consistently maximal during the 30-s prepress and minimal during the 30-s postpress. The analysis of firing during these periods was thus expected to yield measures of firing rate that were controlled among neurons in terms of behavioral variables. In addition, the 30-s prepress and the 30-s postpress correspond, respectively, to the minimum and maximum drug levels and the maximum and minimum firing rates for most neurons. Thus analysis of firing during the two periods of time was expected to yield measures of firing rate that were pharmacologically controlled among neurons, as well as measures of firing that delimited the overall firing rate of neurons over the course of the session.
We used GEE models (Diggle et al. 1994; Hosmer and Lameshow, 2000; Lipsitz et al. 1994) to compare the mean firing rates of the neuron groups across time. The GEE analyses are more appropriate than traditional univariate repeated measures analyses for data, such as those of the present study, which do not fit a normal distribution. In comparison with traditional univariate repeated measures models, these GEE models provide valid estimates of time and group main effects, and group by time interaction effects, under less restrictive assumptions about the distribution of the responses (firing rates) and the nature of within-neuron correlations among the repeated measures. In particular, the GEE models provide valid estimates of the effects even when the true correlation structure is different from the one assumed in the models. If the true and assumed structures are very different, then the standard errors associated with the estimated effects may be too large, and it is usual to try to make a reasonable choice from the correlations structure. For our analyses we considered three possible structures for the correlations between the firing rates at different time points for a given neuron: independence, which assumes that the firing rates at any two points are the same; exchangeable (compound symmetry), which assumes that the correlations between any two time points are the same; and autoregressive, which assumes that the firing rates at time points that are close together are higher than those that are further apart. The results were similar for the three choices, by virtue of the robustness of the models to the choice of correlation structure. We report the results for the autoregressive choice, as it seemed to provide a better fit to the observed data.
With respect to the distributional assumptions, the GEE models require only that the relationship between the mean and variance of the responses are specified, rather than the full distribution. Preliminary graphical examination of the firing patterns suggested that the counts should be transformed prior to formal analyses, as the distributions of counts in any trial were heavily skewed toward higher values. We used the log transformation of x + 1 (to the base 10), which reduced the skewness to appropriate levels. After performing the log transform, we found that the variance was comparable across different levels of (log10) firing rate. In our GEE models, we therefore specified that the variance and mean were independent of each other.
We report on models fit separately to the first 10 and the final 27 lever presses. Models that considered all 37 lever presses together required more complicated specification of the effects of time, as they had to adapt to a pattern of relatively large changes in firing rate over the first 10 trials, and a "steady state" behavior of very little change in firing over the final 27 trials. The models for either the separate time frame analyses or the single time frame analyses provided the same conclusions for the group effects. We fit separate models for the rates of firing before and after the lever press. These analyses also provided for the same conclusions. We report the prepress findings.
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.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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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 experiments (e.g., Pettit and Justice 1989; Yokel 1989). Sixty-six percent of the 24 subjects approached and pressed the response lever within 5 min following the SD (see METHODS). Ninety percent of the subjects responded within 25 min. 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. Following these rapidly completed self-infusions, animals slowed responding to a session low for several infusions before they finally stabilized response rate at an intermediate and regular rate that was exhibited during the remainder (i.e., maintenance phase) of the session. During the maintenance phase, the median interinfusion interval ranged between 6.0 and 8.5 min for 86% of the subjects. Estimated drug level rose rapidly in association with the initial presses to a session maximum. As response rate slowed, drug level at the time of the press first decreased slightly from the peak that was attained during loading and then remained within stable narrow limits for the duration of the session (Fig. 1).
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The total neuron sample included 70 accumbal neurons. Examples of waveforms and interspike interval histograms are shown in Fig. 2. Fifty-three percent of the neurons were located in the core, primarily the dorsal portion of the anterior two-thirds of the core. Twenty-eight percent of the neurons were located in the shell. The remaining neurons were located in border regions of the core and shell. Because of the relatively small sample of shell neurons and the distribution of the 70 neurons across the rostral-caudal axis it was not possible to make meaningful comparisons between subterritories of the relative proportions of neurons that exhibited various firing patterns or combinations thereof. However, it is worth noting that all patterns of firing observed in the study were exhibited by both core and shell neurons (e.g., SD/1st-press responses were exhibited by both core and shell neurons). Therefore all findings in the present study are reported for the combined sample of 70 neurons.
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Twenty-five neurons showed a significant change in firing in association with either or both the SD and the 1st press (Figs. 3 and 4). The majority of these neural responses were excitatory: 22 neurons showed a significant increase in firing; 3 showed a significant decrease. Of 22 neurons that showed a significant excitatory SD/1st-press response, 5 showed a response to only the SD, 7 showed a response to only the 1st press, and 10 showed a response to both. These data are consistent with the conclusion that accumbal neurons show responses in relation to drug-reward-related events under drug-free conditions.
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Is there continuity in accumbal neural responses to drug-reward-related events during drug-free and -exposed periods?
Visual inspection of the average maintenance lever-press histograms of the excitatory and nonexcitatory SD/1st-press neurons showed that there was evidence of an average maintenance lever-press response for only the excitatory SD/1st-press neurons (Fig. 5; see Fig. 6 for individual neuron examples of lever-press responses). The results of statistical analyses were consistent with this observation. GEE analyses were used to compare firing at the time of the press (i.e., -0.5 to +1.5 s that bracketed the lever press) relative to a "control" period (i.e., -12 to -10 s prepress). The analysis showed that there was a significant group x time interaction (P = 0.007). The excitatory SD/1st-press group showed a significant increase in firing at the time of the lever press, relative to the prepress control period (difference = 0.42, P < 0.0001). In contrast, the nonexcitatory SD/1st-press group showed no significant change in firing rate between the time of the press and the prepress control period (difference = 0.11, P > 0.05). These observations are consistent with the conclusion that there is significant continuity between the average response of neurons to the SD/1st-press events and the average response of neurons to the occurrence of cocaine-reinforced lever presses.
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FIRING RATE DURING THE 30-S PREPRESS: COMPARISON OF EXCITATORY AND NONEXCITATORY SD/1ST-PRESS NEURONS. On the first trial, the estimated prepress firing rate was significantly greater for the excitatory SD/1st-press neurons (1.69) than for the nonexcitatory SD/1st-press neurons (0.84) (P < 0.0001 for the comparison). Visual inspection of the firing of the two groups of neurons across the first 10 lever presses indicated that firing rate tended to decrease relative to firing at the time of the 1st press (Fig. 7; see Fig. 8 for individual neuron examples). However, the GEE analysis showed that there was a significant group x time interaction with the decrease in firing rate for the excitatory group being significantly greater than it was for the nonexcitatory group (P = 0.005). Despite the greater decrease, across all 10 presses, the excitatory SD/1st-press neurons maintained a higher overall firing rate than did the nonexcitatory SD/1st-press neurons (difference = 0.66, P = 0.0001 for the comparison).
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CONTROL ANALYSES. The pre-SD firing rate for the excitatory group (1.18) was greater than pre-SD firing rate for the nonexcitatory group (0.84) (P = 0.02 for the comparison). It was possible therefore that the difference in firing rate during the self-administration session reflected only the persistence of a presession difference in firing rather than a difference between the responses of the two neuron groups to self-administered drug. To investigate this possibility we matched the two groups for pre-SD firing rate by excluding low-firing neurons, which were present in greater number in the nonexcitatory SD/1st-press group than in the excitatory group and then repeated the between-group comparison of self-administration firing. After exclusion of the low-firing neurons (total of 19), average firing rates pre-SD equaled 1.20 and 1.17 for the excitatory and nonexcitatory neurons, respectively, and were not significantly different (P = 0.80 for the comparison). All other findings of the analysis were identical to those described above (Fig. 7). Most importantly, at the 1st press, estimated firing rates for the excitatory (1.66) and nonexcitatory (1.05) SD/1st-press groups were significantly different (P < 0.0001 for the comparison). Moreover, during the last 27 trials, the excitatory SD/1st-press neurons showed significantly higher firing rates than did the nonexcitatory SD/1st-press neurons: the estimated firing rates were 1.21 for the excitatory group and 0.91 for the nonexcitatory group (P = 0.04 for the comparison). These findings show that the between-group difference in firing reflects something other than the maintenance of a between-group difference in basal firing. Additional analyses showed that calculated drug level (µM/kg) did not differ between the excitatory SD/1st-press neurons (median = 11.9 ± 0.59) and the nonexcitatory SD/1st-press neurons (median = 12.60 ± 0.33) during the first comparison of the neuron groups [t(68) = 1.18, P > 0.05]. The same was true when we repeated the comparison of firing after removing the low-firing neurons from the two groups. The group difference in self-administration firing thus was not related to a between-group difference in drug level. It is thus possible that the group difference in firing was due to an interaction between the responsiveness of neurons to drug-reward-related events and the response of neurons to self-administered cocaine.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Accumbal neurons exhibited changes, predominantly increases, in average firing rate when drug-free animals were exposed to stimuli that signaled drug availability (i.e., SD). Neurons also showed excitatory responses during the seconds before the 1st press. Further group analyses showed that neurons that exhibited excitatory responses in association with either or both the SD and the 1st press (i.e., excitatory SD/1st-press neurons), but not other neurons, showed a significant average phasic response time-locked to cocaine-reinforced lever presses during the self-administration session. Excitatory SD/1st-press neurons maintained higher firing rates than did other neurons during the self-administration session.
Question 1: Do neurons respond to drug-reward-related information under drug-free conditions?
A number of previous studies showed that accumbal neurons do not exhibit phasic firing responses to drug-reward-related events during the loading phase of the self-administration session, when drug level is low (Carelli et al. 1993, 1999; L.L. Peoples and D Cavanaugh, unpublished observations). The original observation was interpreted as indicating that, in well-trained animals, accumbal neurons do not respond to information about drug-directed behaviors during daily sessions of drug seeking until accumbal neurons have been recruited or "tuned" by the acute effects of a minimum drug level in brain (Carelli et al. 1993, 1999). However, the present study showed that, in well-trained animals, accumbal neurons are responsive to the occurrence of drug-reward-related events under drug-free non-extinction conditions (for related observations made during extinction testing see Ghitza et al. 2003; Peoples et al. 1999). Moreover, as a group, neurons that were responsive under the drug-free conditions were also responsive during drug exposure. These findings indicate that accumbal neurons can and do respond to drug-reward-related events under drug-free conditions. These observations also indicate that some factor other than a necessity for recruitment of responsive neurons by an acute drug effect is required to explain the lack of phasic firing during the loading phase of the FR1 self-administration sessions.
Observations in our laboratory indicate that one factor contributing to the absence of phasic firing during the loading phase is the high frequency of drug-seeking behaviors during that phase of the session that confounds attempts to test for differential rates of firing during the presence and absence of drug seeking (cf. Peoples and Cavanaugh 2003). Another factor that is likely to contribute to the differential presence of phasic firing during the loading and maintenance phases is the difference between the two phases in the state of drug reward (i.e., drug level) and the associated state of reward anticipation. During a drug self-administration session, reward anticipation might be expected to be high and sustained during the loading phase, when drug level is low. However, during the maintenance phase, when drug level is asymptotic, reward anticipation is expected to be low, except during brief periods of nonsatiety shortly before each successive lever press (cf. Wise 1999). Based on previous findings, this loading-to-maintenance phase transition in the occurrence of reward expectancy should influence accumbal firing. Evidence suggests that accumbal neurons can show sustained increases in firing during intervals leading up to reward delivery that vary in magnitude with degree of reward expectancy (Bowman et al. 1996; Schultz et al. 1992; Shidara et al. 1998). There is evidence that accumbal neurons respond similarly to anticipation of drug reward (Nicola and Deadywler 2000; Peoples et al. 1998a; Peoples and West 1996). It is thus possible that the loading-to-maintenance phase change in drug reward anticipation from sustained to phasic is associated with a parallel change in the neural response.
Why is the responsiveness of accumbal neurons under drug-free and low-dose conditions important?
The issue of accumbal responsiveness to drug-reward-related events under drug-free conditions is relevant to a number of hypotheses regarding accumbal function and drug addiction. First, the presence versus absence of accumbal neural responses at the onset of drug self-administration has implications for questions regarding the role of the accumbens in controlling the activation and initiation of drug seeking and possibly relapse in drug-addicted individuals. An inability of accumbal neurons to process drug-reward-related information in the absence of drug would preclude a possible contribution of the accumbens to relapse under drug-free conditions. The presence of accumbal responses to drug-reward-related events in drug-free animals, on the other hand, is suggestive of a potential role for that structure in the initiation of drug-related behaviors under drug-free conditions. Such a role for accumbens is consistent with other animal and human evidence that accumbens may contribute to relapse in drug-free addicted individuals (e.g., Breiter et al. 1997; Cornish and Kalivas 2000; DiCiano and Everitt 2001; Grant et al. 1996; Ito et al. 2000; Kilts et al. 2001; Stewart 1983; Weiss et al. 2000; although see See et al. 2002 and Shaler et al. 2002).
Second, whether accumbal responses occur in association with drug-reward-related events under drug-free conditions has implications for how we might think about the possible effects of DA-mediated actions of cocaine on accumbal neurons. For example, as we have already discussed, if a minimum drug level were required before accumbal responses could be observed, it would be reasonable to hypothesize that a minimum drug level, and hence perhaps a minimum DA level, is required to switch-in or recruit accumbal responsiveness to drug-reward-related afferent input. On the other hand, the existence of neural responses during drug-free periods that are significantly related to neural responses during drug-exposed conditions is suggestive of an alternative conclusion, which is that drug (and perhaps DA) may modulate preexisting patterns of activity. Finally, as we have already noted, the responsiveness of accumbal neurons to drug-reward-related events under drug-free conditions is also relevant to evaluating hypotheses about drug effects that contribute to addiction (see INTRODUCTION and last two sections of DISCUSSION).
Question 2: Do neurons that show excitatory responses to drug-reward-related events under drug-free conditions maintain greater firing rates during periods of drug self-administration than do other neurons?
ACUTE DRUG-INDUCED AMPLIFICATION OF ACCUMBAL DRUG-REWARD-RELATED SIGNALS. The different firing rates of the excitatory and nonexcitatory SD/1st-press neurons during the self-administration session is consistent with the hypothesis that the inhibitory effect of addictive drugs has a net facilitative influence on the transmission of the predominantly excitatory drug-reward-related firing relative to other firing. This finding is in line with the proposal that addictive drugs, like DA, may differentially inhibit signals that are related to the current behavioral setting relative to signals less related to that context (for review see Nicola et al. 2000; O'Donnell 2003; Pennartz et al. 1994; Wise and Bozarth 1987). What might be the mechanistic nature of the interaction between inhibitory drug effects and neural responsiveness to drug-reward-related events? There are numerous potential answers to this question.
The excitatory SD/1st-press neurons showed robust decreases in average firing over the course of the self-administration session. It thus does not appear that the differential interaction with cocaine reflected a general insensitivity of the excitatory SD/1st-press neurons to drug. An alternative explanation is that the net decrease in firing of the excitatory SD/1st-press neurons was limited by the additional excitatory input to those neurons during drug exposure. Yet another possibility is that the drug-reward-related signals per se were less sensitive to the inhibitory effects of drug relative to other signals. A lesser inhibition of drug-reward-related signals could contribute to a higher average firing rate of neurons that receive those signals relative to neurons that do not. Evidence consistent with this interpretation was observed in additional analyses of the presently described SD/1st-press neurons (see Peoples and Cavanaugh 2003).
MECHANISMS BY WHICH DRUG-INDUCED AMPLIFICATION OF ACCUMBAL REWARD-RELATED SIGNALS MIGHT AMPLIFY LEARNING AND CONTRIBUTE TO DRUG ADDICTION. A current view of accumbal function is that information is represented by "fine-grained spatiotemporal firing patterns [of ensembles of neurons]..." (Pennartz et al. 1994; p 726). Moreover, DA input to the accumbens is thought to refine the spatiotemporal pattern of neural activity by selectively facilitating or maintaining signals that are most relevant to the behavioral setting and inhibiting all others (for review O'Donnell 2003; Pennartz et al. 1994). The presently observed patterns of firing indicate that self-administered addictive drugs may have similar effects on patterns of neural activity. Given the important role of DA in mediating accumbal contributions to drug self-administration, it is likely that the DA-like effects of self-adminstered drug reflect actions of drug-induced increases in accumbal DA.
The filtering effect of DA could contribute to learning by reducing the number of neurons 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. Such plasticity is thought to lead to a differential strengthening and weakening of synaptic connections and neural responses and to ultimately underpin learning (cf. Berke and Hyman 2000; Pennartz et al. 1994). The drug-induced elevation in DA is likely to be higher than that which occurs in relation to nondrug rewards. The filtering effects of DA and the influence of those effects on learning may therefore be amplified during drug self-administration, relative to that which occurs in relation to nondrug rewards. Additionally, drug-induced elevations in DA are also unlikely to be subject to normal experience-related constraints (cf. DiChiara et al. 1998; Schultz 2000). That is, new additional strengthening and weakening of various synapses and neural responses may occur with each and every self-administration experience. Thus, with repeated drug exposure, there may be a general and unusually strong weakening of cellular and synaptic responses to nondrug-reward-related signals and an unusually selective maintenance or strengthening of drug-reward-related signals. These exaggerated filtering and plasticity effects may contribute to the aberrant learning that is proposed to facilitate the development of compulsive and uncontrollable drug seeking (Peoples and Cavanaugh 2003).
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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GRANTS
This research was supported by National Institute of Drug Abuse DA-06886, DA-13401, and DA-05186.
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FOOTNOTES |
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Address for reprint requests and other correspondence: L. L. Peoples, Department of Psychology, University of Pennsylvania, 3720 Walnut St., Philadelphia PA, 19104 (E-mail: lpeoples{at}psych.upenn.edu).
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REFERENCES |
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25 ms. Number of intervals (counts) is plotted as a function of 0.1-ms increments in interval length. Above each histogram is a sample of the corresponding neural waveform. The sample consists of the first 500 consecutively recorded waveforms of the recording session. Positive voltage is up. Each waveform trace spans 0.64 ms. Vertical calibration bar = 50 µV. The letter in the upper left-hand corner of each histogram corresponds to identifying labels in other figures.
) and after (
) individual cocaine-reinforced lever presses is plotted as a function of lever press number. These data are shown for all neurons that exhibited excitatory SD/1st-press responses (top left) and for all neurons that showed no excitatory SD/1st press response (top right). In each plot, the dashed line shows mean firing rate during the 30-s pre-SD and the solid line shows mean firing rate during the 30-s post-SD. The same data is plotted for the subset of excitatory SD/1st-press (bottom left) and nonexcitatory SD/1st press (bottom right) neurons that remained after the low firing (i.e., low Hz) neurons were excluded.