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1Department of Molecular Physiology and Biophysics, 2Center for Molecular Neuroscience, and 3John F. Kennedy Center for Research on Human Development, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0615
Submitted 10 March 2003; accepted in final form 12 March 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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;
Dong et al. 2001a; Georges and
Aston-Jones 2001,
2002;
Herman and Cullinan 1997;
Herman et al. 2002;
McDonald et al. 1999;
Moga et al. 1989;
Saper and Loewy 1980;
Weller and Smith 1982). The
BNST receives "processive" stressor input from regions of the
brain involved in cognitive information processing, such as the prefrontal
cortex and the hippocampal formation. This information only becomes a stressor
once it has been processed and compared with prior events in one's experience.
The BNST also receives "systemic" stressor information, i.e.,
hypotension or hemorrhage. This input comes directly from visceral efferent
pathways and elicits a stress response without any need of higher order
processing. Evidence suggests that the BNST acts as a key intermediary by
receiving these inputs and then sending its own projections to the
paraventricular nucleus of the hypothalamus (PVN), the key central regulator
of peripheral glucocorticoid levels.
In addition to being interconnected with the stress axis, the BNST is a key
anatomical bridge in an assemblage of brain regions referred to as the
extended amygdala, which also includes the central nucleus of the amygdala and
the shell of the nucleus accumbens (NAc)
(Alheid and Heimer 1988).
Furthermore, the BNST projects to and regulates the firing of dopaminergic
cells within the ventral tegmental area (VTA) (Georges and Aston-Jones
2001,
2002). Thus the BNST is
uniquely positioned to receive stress axis information and integrate it into
reward/motivation circuitry. Consistent with these anatomical
interconnections, recent data demonstrate that the BNST plays a key role in
mediating stress-induced relapse to cocaine-seeking behavior
(Erb et al. 1996;
Shaham et al. 2000;
Sinha et al. 1999), as well as
in stress-induced maintenance and reinstatement of morphine-conditioned place
preference (Wang et al. 2001).
Furthermore, Delfs et al.
(2000) showed that the BNST
plays a key role in morphine withdrawal–induced conditioned place
aversion.
The systemic stress input to the BNST consists primarily of input from the
central nucleus of the amygdala and noradrenergic input from the A1 and A2
cell groups of the caudal medulla. Indeed, the BNST is one of the heaviest
sites of noradrenergic innervation in the CNS. This input is regionally
compartmentalized within the BNST: NE input is much more dense to the ventral
BNST (vBNST) than to the dorsal BNST (dBNST) in the rat and primate brain
(Freedman and Shi 2001;
Georges and Aston-Jones 2001;
Woulfe et al. 1990).
Interestingly, NE levels increase in the BNST as a result of restraint stress
(Pacak et al. 1995), and
inhibiting 
1 and/or 
1-adrenergic receptors
directly in the BNST can reduce anxiety-like behaviors in rats
(Cecchi et al. 2002), as well
as the cocaine- and morphine-related behaviors described above, as does
lesioning the ventral noradrenergic bundle
(Delfs et al. 2000;
Wang et al. 2001). Moreover,
cocaine self-administration in subhuman primates decreases metabolic activity
in BNST, as well as upregulating norepinephrine transporter levels in this
region (Macey et al.
2003).
Currently, little is known of the properties of BNST neurons. The BNST
consists primarily of neurons that stain positively for glutamic acid
decarboxylase (GAD), the enzyme responsible for the conversion of glutamate to
GABA (Sun and Cassell 1993),
and also expresses a variety of neuropeptides
(Ju et al. 1989). Despite a
growing literature on anatomical and neurochemical properties of the BNST,
little is known of the electrophysiological properties of neurons within this
structure. In a recent initial study, Rainnie
(1999) recorded from rat
dorsomedial and dorsolateral BNST cells via whole cell patch methods,
demonstrating that these cells can be recorded from in vitro and that they are
regulated by serotonin.
Here, we have confirmed and extended the findings of this initial report by electrophysiologically characterizing neurons of the anterolateral BNST in vitro, comparing dorsal versus ventral anterolateral BNST. We characterized several properties of excitability in BNST, including a low-threshold spike, an Ih-dependent depolarization, and inward rectification. These properties of excitability are differentially distributed across BNST neurons along a dorsal-ventral gradient, suggesting that common inputs to these two areas may evoke distinct outputs that differentially regulate behavior. Moreover, local fiber stimulation evokes both an AMPA- and N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic potential (EPSP) and a GABAA-receptor mediated inhibitory postsynaptic potential (IPSP). These data allow us to suggest mechanisms by which information flows through stress and drug reward pathways.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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All animals were housed in the Vanderbilt Animal Care Facilities in groups of 2–5. Food and water were available ad libitum. All procedures were approved by the Animal Care and Use Committee at Vanderbilt University.
Brain slice preparation
Male C57Bl/6J mice (5–10 wk old, Jackson Laboratories) were
decapitated under anesthesia (Isoflurane). The brains were quickly removed and
placed in ice-cold artificial cerebrospinal fluid [ACSF: (in mM) 124 NaCl, 4.4
KCl, 2 CaCl2, 1.2 MgSO4, 1
NaH2PO4, 10.0 glucose, and 26.0 NaHCO3].
Slices 350 µm in thickness were prepared using a vibratome (Pelco). Rostral
slices containing anterior portions of BNST (Bregma 0.26–0.02 mm)
(Franklin and Paxinos 1997)
were identified using the internal capsule, anterior commissure, fornix, and
stria terminalis as landmarks (Fig. 1,
A and B). Slices were then transferred to an
interface recording chamber where they were perfused with heated
(approximately 28°C), oxygenated (95% O2-5% CO2)
ACSF at a rate of about 1–1.5 ml/min. Slices were allowed to equilibrate
in normal ACSF for 1 h before experiments began. Increased divalent cation
ACSF consisted of (in mM) 116 NaCl, 2.2 KCl, 7.0 CaCl2, 7.0
MgSO4, 1.0 NaH2PO4, 10.0 glucose, and 26.0
NaHCO3.
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Intracellular recording
Recordings from a total of 121 neurons in the BNST were utilized in this
study. Recording electrodes were pulled on a Flaming-Brown Micropipette Puller
(Sutter Instruments) using thick-walled filament-containing borosilicate glass
capillaries. Electrodes were filled with 2 M potassium acetate and were of
approximately 120–170 M
resistance. A current/voltage
relationship from each cell was obtained by injecting a range of current via
the recording electrode. A bipolar nichrome wire stimulating electrode was
placed locally within the BNST approximately 500 µm from the recording
electrode. An input/output curve of synaptic responses was generated by
stimulating with a range of input voltages with stimulus durations from 50 to
100 µs, starting at 3 V and increasing the stimulus in 2-V increments until
the cell fired an action potential (AP). If the cell did not fire, the maximum
stimulus intensity administered was 35 V. Input resistance (IR) was calculated
by measuring the slope of the line created by plotting current versus voltage
from a series of current injections (–0.25 to +0.25 nA). IR was
monitored throughout the duration of the experiment by injecting a current
pulse at the end of each stimulus test pulse. AP amplitude was calculated by
measuring the difference between the membrane potential at which the AP fired
in an all-or-none manner and the membrane potential at which the AP reversed.
AP amplitude was measured at the beginning and end of each experiment. If APs
did not consistently overshoot zero, the experiment was excluded from final
analysis. AP duration was calculated by measuring the width of the AP at the
membrane potential corresponding to one-third of total AP amplitude. The
membrane time constant (
) was calculated using the standard exponential
curve fitting function provided in pClamp9 (Axon Instruments, Union City, CA).
Synaptic transmission experiments were analyzed by measuring the peak
amplitude of the synaptic response which was normalized and averaged across
experiments for each manipulation using Excel.
Pharmacology
All drugs were bath applied. Picrotoxin, CNQX, D-2-amino-5-phosphono-valeric acid (D-AP5), and nickel chloride hexahydrate were purchased from Sigma (St. Louis, MO). 4-Ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidium chloride (ZD7288) and [R-(R*,S*)]-5-(6,8-dihydro-8-oxofuro[3,4-e]-1,3-benzodioxol-6-yl)-5,6,7,8-tetrahydro-6,6-dimethyl-1,3-dioxolo[4,5-g]isoquinolinium bromide (bicuculline) were purchased from Tocris (Ellisville, MO). DMSO (0.02% vol/vol) was the carrier for picrotoxin and CNQX.
Data analysis
Electrophysiological data were collected using an Axoclamp 2B amplifier and digitized and analyzed using pClamp 8.2 and 9.0 software. Appropriate statistical analyses (indicated within figure legends) were performed using Prizm and InStat software (Graphpad).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Neurons were sampled within the dorsal and ventral lateral portions of the
BNST in an anterior section (Fig.
1). We found that the resting membrane potential (RMP) recorded in
normal ACSF was comparable between dorsal and ventral BNST, as was the
apparent IR (Table 1). AP
amplitudes, durations, and thresholds were also comparable between dBNST and
vBNST. However, we found that the of cells in vBNST was on average
considerably faster than the of cells in dBNST.
Table 1 compares these
properties between dBNST, vBNST, and a neighboring brain region, the NAc core,
since these regions receive overlapping excitatory input. We recorded from 15
NAc core cells for analysis of basic properties. We found that the RMP of NAc
neurons was significantly more hyperpolarized than the RMP of either dBNST or
vBNST neurons, and the IR was significantly lower. AP amplitude was
significantly lower in NAc neurons than in dBNST and vBNST neurons. AP
duration and threshold were comparable between BNST and NAc neurons, but the
for NAc neurons was significantly lower than for either dBNST or vBNST
neurons.
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Depolarization evokes low-threshold spiking in BNST neurons
A substantial proportion of neurons in the BNST displayed a low-threshold
spike (LTS) in response to depolarizing current injections
(Fig. 1, C and
D). This LTS was observed as a relatively rapidly
activating and inactivating envelope lasting 63.5 ± 5.8 ms on which
were frequently superimposed one to five action potentials. The first action
potential on the LTS was similar in amplitude and duration to APs elicited in
the absence of a LTS (Table 2).
Subsequent APs on the LTS became progressively significantly smaller in
amplitude and longer in duration than the first AP on the LTS
(Table 2), in contrast to
groups of multiple APs observed on non-LTS–inducing depolarizing steps.
The LTS was not observable when the cell was held at –60 mV via constant
current injection prior to the depolarizing step
(Fig. 1C, solid
trace). However, on recovery from a hyperpolarizing pulse, the LTS was
observable (Fig. 1C,
dotted trace). The LTS seen on depolarization from a hyperpolarized state
could be reduced by bath application of 200 µM Ni2+
(n = 3; Fig.
1D) at a concentration known to selectively inhibit T-
and R-type calcium channels (Su et al.
2002). Application of NiCl2 had no apparent effect on
the other basic properties of these cells, i.e., membrane potential and input
resistance remained constant during nickel application. The LTS we observed
had an activation threshold of approximately –60 mV, which is consistent
with being driven by T-type Ca2+ channels rather than
R-type Ca2+ channels, which have an activation threshold
of –40 to –25 mV (Randall and
Tsien 1997). The resting state dependence and activation threshold
of the LTS coupled with its Ni2+ sensitivity suggest
that it is mediated by activation of T-type Ca2+
channels. Interestingly, the LTS also occurred on EPSPs (data not shown),
suggesting that cells demonstrating the LTS may be more prone to
synaptically-induced burst firing behavior.
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BNST neurons display a depolarizing sag in response to hyperpolarization as well as inward rectification reminiscent of medium spiny neurons
Another property we observed in BNST neurons was a depolarizing sag in response to a hyperpolarizing current injection (Fig. 2A). This sag was somewhat slowly developed, with the time at which the depolarization began ranging from 13.8 to 120.8 ms, and it had an amplitude ranging from 1 to 15 mV (measured from the peak hyperpolarization after initiation of the current step to the final steady-state hyperpolarization reached before current injection was turned off). Because this type of potential is often mediated by Ih, we tested this possibility via bath application of the Ih channel blocker ZD7288 (n = 3, 100 µM; Fig. 2B). While this drug had no consistent effect on RMP, ZD7288 abolished the depolarizing sag. Inspection of the I-V relationships in the absence and presence of ZD7288 suggests that this conductance contributes to lowering the apparent input resistance in these cells (Fig. 2B, inset).
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Finally, a number of cells displayed moderate inward rectification in
response to current injection (Fig. 2,
C and D). While this property is reminiscent of
medium spiny neurons of the dorsal striatum and NAc, these BNST cells had
substantially higher input resistances (113.3 ± 7.4 M) and more
depolarized resting membrane potentials (–66.3 ± 1.8 mV) than NAc
core medium spiny neurons.
Electrophysiological properties of BNST neurons are differentially distributed within dorsal and ventral BNSTs
Interestingly, many of the basic properties we observed in the BNST were
not homogeneously expressed within the BNST, but rather were regionally
differentiated. The of cells in vBNST was on average considerably faster
than in dBNST neurons (Table
1). The percentage of neurons displaying a LTS was significantly
greater in vBNST than in dBNST—while a LTS could be evoked in 74.5% of
cells in vBNST, it could only be evoked in 22.9% of cells in dBNST (P
< 0.0001, Fig. 3A).
As with the LTS, the Ih-dependent depolarizing sag was
differentially distributed between dorsal and ventral BNST; however, rather
than being found predominantly in vBNST, this property was significantly more
frequently observed in dBNST (15.7 and 48.6%, respectively, P =
0.0002, Fig. 3A).
Cells displaying inward rectification were found only slightly more abundantly
in dBNST (Fig. 3A).
Interestingly, multiple cells from vBNST and dBNST showed two of these three
characteristics, and two cells we recorded from, one in vBNST and one in
dBNST, exhibited all three characteristics
(Fig. 3B).
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Neurons in the dorsal BNST are tonically inhibited by GABA
The BNST receives strong GABAergic input from both intrinsic and extrinsic
sources. To begin to explore the role of GABAergic transmission in BNST
physiology, we examined the properties of dBNST and vBNST neurons in the
presence and absence of the GABAA-receptor antagonist picrotoxin.
RMP and IR of cells in vBNST appeared unaffected by the inhibition of fast
GABAergic transmission. In contrast, the RMP of cells in dBNST depolarized
significantly, from –69.1 ± 2.0 mV in normal ACSF to –64.1
± 1.3 mV in picrotoxin (P = 0.0289;
Fig. 4A), suggesting
tonic inhibition of these cells by GABA. Accordingly, the IR in dBNST cells
also increased significantly in the presence of picrotoxin, from 115.2
± 10.8 M to 143.2 ± 6.2 M (P = 0.0197;
Fig. 4B). It should be
noted that the Ih current was still active in the presence
of picrotoxin; therefore the increase in the apparent IR of dBNST neurons seen
in the presence of picrotoxin is not likely due to an inhibition of
Ih current activation.
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Inhibitory synaptic transmission
Local fiber stimulation in the BNST results in an EPSP and an IPSP in both
dBNST and vBNST. Depending on the membrane potential, the EPSP, the IPSP, or
both are observable (Fig.
4C). The IPSP occurs with a latency of 2.39 ± 0.04
ms (measured in the presence of 10 µM CNQX, an AMPA/kainate-receptor
antagonist). The major component of the IPSP is mediated by the
GABAA-receptor because it can be blocked by application of 25 µM
picrotoxin (Fig. 4D)
or another GABAA-receptor antagonist, 20 µM bicuculline
(Fig. 5E, trace
2). A small late hyperpolarization was occasionally observed after the
initial IPSP that could be a GABAB-mediated IPSP and/or a
glycinergic IPSP (Fig.
4D), but this was inconsistently seen and would need to
be analyzed pharmacologically to definitively identify its origins. To begin
to determine the source of the GABAA-mediated IPSP, we recorded
under conditions of increased concentrations of Mg2+ and
Ca2+ (7 mM each) to increase AP firing threshold. An
increase in AP threshold would be predicted to reduce the probability of
multisynaptic events relative to monosynaptic events. Thus these conditions
allow one to begin to discriminate a monosynaptic response (which should be
unaffected under these conditions) from a polysynaptic response (which should
be reduced). As predicted, this manipulation substantially shifted the
threshold for somatic AP firing to the right, with APs only elicited on
greater amounts of depolarization, i.e., APs fired at a membrane potential of
about –40 mV instead of –55 mV
(Fig. 4E,
insets). Under these conditions of high divalent cation
concentrations, the IPSP was elicited with no failures
(Fig. 4E), suggesting
that at least a component of the IPSP is monosynaptic GABAergic transmission.
Curiously though, this IPSP was partially reduced by application of CNQX (10
µM, Fig. 4F),
suggesting that the IPSP may have both mono- and polysynaptic components that
may be due to both direct GABAergic input stimulation as well as either
indirect low-threshold GABAergic inputs stimulated by AMPA/kainate-receptor
mediated circuits, or conceivably, GABAergic terminals presynaptically
regulated by AMPA/kainate receptors (Braga
et al. 2003).
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Excitatory synaptic transmission
In addition to an IPSP, synaptic stimulation evoked a substantial EPSP, which by raising stimulus strength, was routinely of sufficient size to generate APs (Fig. 5A). This EPSP appeared to be monosynaptic in origin. First, the response was elicited with a very short latency with very little variability (2.56 ± 0.04 ms, measured in the presence of 25 µM picrotoxin). Second, in ACSF containing increased divalent ion concentrations, the EPSP was still reliably elicited and comparable in amplitude to EPSPs recorded in normal ACSF. In addition, there was no increase in failure rate during low frequency stimulation (Fig. 5B). The EPSP recorded in 25 µM picrotoxin ACSF was consistently elicited during relatively high-frequency (400 ms of 25 Hz) stimulation (Fig. 5C) and could be almost entirely blocked by 10 µM CNQX (Fig. 5D). An NMDA-receptor-mediated EPSP could be revealed after blocking an IPSP recorded in the presence of 10 µM CNQX (Fig. 5E, trace 1) with 20 µM bicuculline (Fig. 5E, trace 2). This remaining component of the EPSP could be blocked by the NMDA-receptor antagonist, D-AP5 (100 µM; Fig. 5E, trace 3).
In the presence of picrotoxin, EPSPs in cells in both vBNST and dBNST showed paired pulse facilitation (Fig. 5, F and G) across a range of interpulse intervals, with the most robust facilitation at the shorter time points. The presence of paired pulse facilitation indicates that these cells can undergo at least short-term forms of plasticity.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Excitable properties
From a circuit perspective, it is useful to compare the properties of BNST
neurons to a population of cells receiving heavily overlapping afferent input.
When compared with medium spiny neurons in the core of the NAc, a neighboring
population of neurons that receives many of the same excitatory inputs, we
find that neurons of the BNST are much more excitable, having more depolarized
resting membrane potentials, much higher input resistances, longer membrane
time constants, and slightly larger AP amplitudes. In addition, we report
three voltage-dependent properties present in a large percentage of BNST
neurons. First, we found that these cells express a
Ni2+-sensitive low-threshold spike. The LTS is most
likely mediated by T-type calcium channels, as its activation properties were
consistent with those of a T-type calcium channel
(Randall and Tsien 1997), and
it could be blocked by the T- and R-type calcium channel blocker
Ni2+. Second, we found that many cells exhibited a
depolarizing sag in response to hyperpolarizing current injection that was
sensitive to the Ih blocker ZD7288. The excitable properties we
report here in mouse brain slices with sharp microelectrodes are very similar
to those reported by Rainnie in rat brain slices
(Rainnie 1999). We have here
further extended his results by pharmacologically probing these properties.
Third, we found that many cells exhibited inward rectification in response to
increasing current injection steps that was reminiscent of that observed in
medium spiny neurons of the dorsal striatum and the NAc
(Nisenbaum and Wilson 1995;
O'Donnell and Grace 1993).
Inhibitory transmission
We have found that on synaptic stimulation, both a
GABAA-receptor-mediated IPSP and an AMPA/kainate- and
NMDA-receptor-mediated EPSP could be elicited in BNST neurons. The IPSPs
elicited in these cells via modest distal stimulation (approximately 500 µm
away from the recording electrode) appeared to be at least partially
monosynaptic, because they could be elicited in the presence of high divalent
cation concentrations and had a very short, relatively invariant latency.
These IPSPs likely originated from both a large GABAergic projection from the
central nucleus of the amygdala as well as from intra-BNST projections of
GABAergic BNST neurons. However, even in the presence of high divalent cation
concentrations we found that the IPSP could be partially reduced by
application of 10 µM CNQX. These IPSPs are likely due to highly excitable
local circuit interneurons, although it is possible that GABAergic
transmission in this brain region may be modulated by either axonal or
presynaptic terminal localized ionotropic glutamate receptors
(Braga et al. 2003). In future
studies, analysis of spontaneous transmission will be needed to resolve this
issue.
In addition to activity-dependent fast GABAergic inhibition, we found that
cells in dBNST were tonically inhibited by GABAergic transmission. In the
presence of 25 µM picrotoxin, the RMP of cells in dBNST was significantly
depolarized relative to dBNST cells in normal ACSF. Consistent with relief of
tonic inhibition, the IR of dBNST cells also increased significantly when
picrotoxin was present. This change in IR does not appear to have been driven
by an alteration in Ih activation because
Ih persisted in the presence of picrotoxin. Tonic
inhibition by GABA has been demonstrated in hippocampus
(Yeung et al. 2003) and is
correlated with the presence of specific subunits of the
GABAA-receptor. The 
subunit of the
GABAA-receptor has been clearly demonstrated to have an
extrasynaptic localization, and heteromeric assemblies of this subunit produce
inactivating and nondesensitizing GABAA-receptors
(Bianchi et al. 2002). The
subunit is present in the BNST, particularly in neuronal processes,
although it is not clear whether there is a dorsal versus ventral distribution
(Pirker et al. 2000). It is
also possible that this basal inhibition of cells in dBNST may be the result
of a steady stream of spontaneous inhibitory transmission, as opposed to tonic
presence of GABA. However, we have no evidence that there is an exceptionally
high level of spontaneous neurotransmitter release (either GABA or glutamate)
that could contribute to tonic regulation of membrane potential or input
resistance, although our experiments have not been performed under conditions
which would optimize the detection of these events.
In total, our data suggest at least three different forms of fast GABAergic inhibition in the BNST: 1) glutamatergic transmission-independent forms of GABAergic transmission, possibly arising from the central nucleus of the amygdala and/or local GABAergic transmission, 2) glutamatergic transmission-dependent GABAergic transmission which may reflect local circuit interactions, and/or presynaptic actions of glutamate on GABAergic terminals, and 3) tonic GABAergic tone in dBNST.
Excitatory transmission
In addition to an IPSP, on local stimulation in the BNST, an EPSP could
also be elicited. The ventral subiculum and cortical areas provide the
predominant known glutamatergic input into the BNST
(Herman et al. 2002), and
since the vast majority of cells intrinsic to the BNST are GABAergic, are
likely responsible for the EPSPs recorded here. The EPSP was largely mediated
by non-NMDA ionotropic glutamate receptors, but also had a clear NMDA-receptor
dependent component. The EPSP was most likely monosynaptic, because it could
be elicited in high divalent cation concentrations, followed relatively
high-frequency stimulation with no failures, and had a very short and
invariant latency. EPSPs recorded from cells in dBNST and vBNST exhibited
paired pulse facilitation, a form of short-term plasticity. The amount of
facilitation at longer interpulse intervals was not pronounced, suggesting
that the release probability at these synapses under the present conditions
may be relatively high.
It is also worth noting that many of the glutamatergic inputs to the BNST also project to the NAc core, a neighboring population of neurons also implicated in the drug-reward pathway. Based on our comparison of properties of these cells, it would be predicted that neurons of the BNST would be much more responsive to these inputs than NAc neurons, suggesting that BNST neurons may be activated by glutamatergic stimuli from these regions that would not activate NAc neurons, which may be of functional significance when the stress and drug reward pathways are recruited.
Dorsal and ventral BNST neurons display divergent properties
The differing distribution of excitable properties between dBNST and vBNST
may confer different characteristics of synaptic processing on these two
regions. The low-threshold spike that predominates in the vBNST promotes burst
firing, which may alter transmitter release from these cells. For example, in
dopaminergic cells burst firing results in a more efficient release of DA from
terminals (Suaud-Chagny et al.
1992), and in many systems, is correlated with release of
peptidergic transmitters from dense core vesicles. Efferents from the BNST
contain many different types of neuropeptides, including corticotrophin
releasing factor, neurotensin, galanin, substance P, and cholecystokinin
(Ju et al. 1989), which may be
released in a similar manner from BNST burst firing cells. Moreover, cells in
vBNST also tended to have relatively shorter s compared with cells in
dBNST. The combination of the shorter with the LTS could serve to
increase signal-to-noise ratios by dampening temporal summation of weak inputs
but rewarding larger inputs with burst firing evoked by the LTS.
In contrast to cells of the vBNST, neurons within the dBNST tended not to
express a LTS, were under tonic GABAergic inhibition, and had
Ih activity. The lack of the LTS and the presence of tonic
GABAergic inhibition suggest that these cells may be more quiescent in the
native state and may require greater input in order for efferent connections
to be activated. However, it is interesting to note that the 's for these
neurons were significantly longer than for cells in the vBNST, despite the
presence of Ih activity. This suggests that either these
cells are significantly larger in size than cells of the vBNST, that
compensatory effects with other channel types are employed to compensate for
Ih activity, or a combination of these two.
Potential functional significance
It has been suggested that the BNST acts as an intermediary for regions of
the brain involved in perceiving stress and regions of the brain producing
responses to stress (Herman and Cullinan
1997). The ventral subiculum and cortical areas that transmit
"processive" stressor information and the noradrenergic
projections from the nucleus of the solitary tract that transmit
"systemic" stressor information project to the PVN via a relay in
the BNST, as well as via direct projections to the hypothalamus.
Interestingly, the BNST also projects to brain systems involved in reward
responses, including the VTA and the NAc. The interactions between the BNST,
PVN, and VTA are complex, but one prediction would be that activation of the
BNST by processive stressors (i.e., ventral subiculum and cortical areas)
would result in increased output from BNST to the NAc, VTA, PVN and/or the
hypothalamic area surrounding the PVN (PVN surround). However, it is as yet
unclear whether this would have a net excitatory or inhibitory effect on these
regions. For example, while the majority of the neurons of the BNST appear to
be GABAergic, strong electrophysiological evidence suggests an excitatory
projection between the BNST and the VTA
(Georges and Aston-Jones 2002)
that could result in increased dopamine release in the NAc as a result of BNST
activation.
It is important to note that, although there are similarities in the
organization of dBNST and vBNST, these regions do have anatomically distinct
inputs and outputs. For example, while the dBNST may send a stronger
projection to the NAc (Dong et al.
2001b), vBNST acts as a GABAergic relay for glutamatergic ventral
subicular inputs to the PVN (Cullinan et
al. 1993) and has also been shown to project directly to the VTA
via an excitatory projection (Georges and
Aston-Jones 2002). There also appears to be differential
expression of various neuropeptides in dBNST and vBNST
(Ju et al. 1989;
Moga et al. 1989), which may
also be of functional consequence in integrating their respective inputs and
outputs. Additionally, Rainnie
(1999) described differences
in input resistance and between cells in dorsomedial and dorsolateral
BNST, providing more evidence in support of functional differences between
subregions of the BNST.
In addition to the differences described above, a major distinction between
these regions is the differential noradrenergic innervation of BNST, with the
ventral portions receiving substantially more noradrenergic innervation by the
caudal medulla than the dorsal regions
(Woulfe et al. 1990). This NE
input to vBNST is of key functional relevance given the dependence on
adrenergic receptor activity for many of the roles the BNST plays in drug
related behaviors (Delfs et al.
2000; Wang et al.
2001). Indeed, one possibility to be investigated in future
studies is whether the differential expression of excitable properties we find
reflects differential states of noradrenergic neuromodulation within BNST.
Thus while at present it is difficult to predict the net effects of activation
of BNST cells on target brain regions, these studies coupled with more
detailed understanding of the connectivity of these regions will greatly aid
in our understanding of the integration of the stress and reward axes.
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ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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This work was supported by the National Institute on Alcohol Abuse and Alcoholism and the Whitehall Foundation.
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FOOTNOTES |
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Address for reprint requests: D. G. Winder, Dept. of Molecular Physiology and Biophysics, Vanderbilt Univ. School of Medicine, 724B Robinson Research Bldg., 23rd and Pierce Ave., Nashville, TN 37232-0615 (E-mail: danny.winder{at}vanderbilt.edu).
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