Neurotransmission in the rat amygdala related to fear and anxiety

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1 Neurotransmission in the rat amygdala related to fear and anxiety Michael Davis, Don Rainnie and Martin Cassell Michael Davis isat the Dept of Psychiatry, Yale University, Connecticut Mental Health Center, 34 Park Street New Haven, CT0650B, USA, Don Rainnie is atthe Dept of Psychiatry, Harvard University, Brockton Veterans AdmimstratlOn Medical Center, Brockton, MA 02401, USA, and Martin CassellIS at the Dept of Anatomy, University of 10wa, Iowa City, IA 52242, USA. An impressive amount of evidence from many different laboratories using a variety of experimental techniques indicates that the amygdala Plays a crucial role in the acquisition, consolidation and retention or expression of conditioned fear. E lectrophysiological data are beginning to detail the transmitters and inter-amygdala connections that transmit information to, within, and out of the amygdala. In general, treatments that increase the excitability of amygdala output neurons in the basolateral nucleus (for example, by decreasing opiate and GABA transmission, and increasing noradrenergic transmission) improve aversive conditioning, whereas treatments that decrease excitability of these neurons (by increasing opiate and GABA transmission, and decreasing NMDA and noradrenergic transmission) retard aversive conditioning as well as producing anxiolytic effects in appropriate animal tests. A better understanding of brain systems that inhibit the amygdala, as well as the role of its very high levels of peptides, might eventually lead to the development of more effective Pharmacological strategies for treating clinical anxiety and memory disorders. Despite its name, the rat amygdala bears only a passing resemblance to the 'almond' that Karl Burdach saw on dissections of the human brain nearly 200 years ago. Nonetheless, it contains almost all the nuclear groups present in the primate amygdala, its extrinsic and intrinsic connections and neurochemistry are also remarkably similar, and the evidence for its role in complex behavior, learning and memory is generally consistent with the functions attributed to the human and non-human primate amygdala. In the past decade, the development of the in vitro slice preparation and the availability of a wide range of neurotransmitter agonists and antagonists, as well as the availability of detailed anatomical studies on the intrinsic microcircuitry of individual amygdaloid nuclei, have led to a considerable increase in the understanding of the synaptic events underlying some of the behaviors mediated by the rat amygdala. This review will integrate the current knowledge of synaptic transmission within the rat amygdala with the effects of local infusion of drugs into the amygdala on a specific form of aversive learning: fear conditioning. Emphasis will be placed on the role of GAB A and glutamate, because the actions of these neurotransmitters in the amygdala have been strongly implicated in fear conditioning. A large and consistent literature indicates that the amygdala is critically involved in the acquisition and expression of conditioned fear (for review see Ref. 1) Elsevier SCience Ltd /94/$07.00

2 Electrical stimulation of the amygdala produces a pattern of behavioral changes that closely resembles that produced by stressful or fearful stimuli, and lesions of the amygdala block innate or conditioned reactions to stress. Aversive stimuli readily activate the amygdala, and results obtained after local infusion of compounds into the amygdala indicate that it is especially important in the formation, consolidation and expression of memories of events paired with these aversive stimuli. A better understanding of the chemical neuroanatomy of synaptic transmission within the amygdala will eventually lead to novel and superior stratebries for treating clinical anxiety disorders such as post-traumatic stress syndrome and panic, as well as having general relevance to the study of learning and memory. However, it should be emphasized that restricting this review to fear conditioning does not mean that this is the only function of the amygdala. On the contrary, this complex structure is also involved critically in attention, secondary reinforcement, reward magnitude and social behavior (for review see Ref. 2). Intrinsic and extrinsic connections of the amygdala Information from all sensory modalities reaches the amygdala via projections from the cortex and a variety of subcortical structures (most notably the thalamus and parabrachial complex) that converge on the basolateral amygdaloid complex, in particular the lateral nucleus'. The cortical projections, which arise from secondary and polymodal association cortices 1, probably relay cognitive, but otherwise affectively neutral, information pertaining to sensory stimuli. Information concerning the aversive properties of stimuli are probably relayed separately via projections from the external lateral part of the para brachial complex-', the dysgranular insular cortex 1i, and midline and intralaminar thalamic nuclei', each of which receives nociceptive inputs and projects to the basolateral complex. Direct projections from the para brachial nucleus and insular cortex to the central nucleus might also be especially important for relaying aversive information to the amygdala:>. The basolateral complex (consisting of the lateral, basolateral and basomedial nuclei) contains two basic types of neurons x : large, spine-dense pyramidal neurons containing glutamate, and providing the major extrinsic projections of the complex; and spine-sparse non-pyramidal cells that contain GABA, choline acetyltransferase (CAT) and neuropeptides~j, and most of which are presumed to be local circuit neurons, although some also project to the thalamus. Intracellular electrophysiolobrical recordings from morphologically identified neurons of the basolateral complex suggest that the spine-dense and spinesparse neurons (Fig. 1) have characteristic firing properties and membrane input resistance (Table lj. Importantly, the high spine density of projection neurons in the basolateral complex suggests that they are capable of integrating a vast array of synaptic contacts from extrinsic afferent inputs. There are four main output projections from the basolateral complex that arise from the pyramidal neurons of the basolateral nucleus: reciprocal projections back to the cortex including the frontal cortex. and unidirectional projections to ventral B Fig. 1. (A) Photomicrograph of a class I pyramidal neuron of the basolateral nucleus of the amygdala (BLA). Class I neurons comprise >75% of the neurons of the BLA, and are characterized by the spine-laden appearance of the dendrites. This class I neuron has a prominent apical dendrite and pyriform cell soma, and was filled with 2% biocytin during electrophysiological recording, and subsequently visualized with a peroxidase-antiperoxidase complex using diaminobenzedine as the chromogen. (8) Photomicrograph of a non-spiny neuron. Scale bars, m. caudate~putamen, the nucleus accumbens, and the central amygdaloid nucleus 111. The reciprocal cortical projections might be involved in the conscious perception of fear or anxiety, but this awaits verification in humans. Projections to the two striatal areas might relay motivationally significant information to motor areas necessary for the avoidance of harmful stimuli or approach to stimuli associated with primary reinforcers. Projections to the central nucleus of the amygdala, the major intra-amygdaloid target of the basolateral complex, are critical for autonomic and somatic responses produced by stimuli that were previously paired with aversive events. The central nucleus is organized in a manner similar to both the dorsal and ventral striatopallidal systems 11. The lateral central nucleus contains mostly medium sized, spine-dense neurons, many of which have GABA as their neurotransmitterl~ and contain a variety of neuropeptides u. Further similarities with the caudate-putamen and nucleus accumbens include the heavy dopaminergic innervation of the lateral central nucleus 11, direct cortical input to GABA neunms ]C" and the projections of these GABA neurons onto the large, spine-sparse output neurons 209

3 TABLE I. Mean membrane input resistance for several subtypes of amygdaloid neurons Cortex-like Basolateral Lateral Noncortex-like Central Spine-dense MO SOMO 113 MO Spine-sparse 5SMO 90MO 111 MO Cortex-like neurons have the lowest resistance and can be further subdivided into spine-laden putative projection neurons and spine-sparse putative interneurons. Neurons of the lateral nucleus generally have a higher input resistance than the neighboring basolateral nucleus indicating the possibility of greater spatial and temporal summation of synaptic inputs onto these neurons. Neurons of the non-cortex-like central nucleus have the highest input resistance, suggesting an electronically more compact neuron. These neurons are both spine-laden and spine-sparse. However, they have been tentatively subdivided according to the presence (Type A) or absence (Type B) of an afterhyperpolanzation following repetitive burst firing. of the medial central nucleus!5. These medial central nucleus neurons contain a variety of neuropeptides Hi and, in some cases, glutamate!!, and project to a variety of brainstem regions capable of influencing or initiating autonomic and somatic components of the fear reaction (Fig. 2). Electrophysiological studies suggest that the principal neurons of the two central nucleus subdivisions have high input resistances (Table 1) and can be further differentiated according to the presence (type A, medial neurons) or absence (type B, lateral neurons) of a slow after hyperpolarizing potential following repetitive firing!k. The basic intrinsic amygdaloid circuitry potentially involved in conditioned associations between neutral and aver'sive stimuli, based on both anatomical and electrophysiological recording studies, is shown in Fig. 3. For simplification, a number of other intrinsic and other extrinsic connections, principally subcortical ones, are not included. Excitatory amino acid transmission Electrophysiology. Intra- and extracellular recordings from anesthetized rats have demonstrated both EPSPs and IPSPs in the amygdala!~, and have confirmed that individual amygdaloid neurons can receive and integrate synaptic inputs from diverse sources 20. Intracellular recordings from brain slice preparations have shown that stimulation of afferent pathways to the lateral and basolateral amygdala elicit EPSPs. Stimulation of the stria terminalis, the lateral amygdala, the external capsule, or the ventral endopyriform nucleus evokes a glutamate-mediated EPSP in the basolateral amygdala:2!-n. These EPSPs consist of a fast component mediated by D, L-rx-amino-3-hydroxy-5-methyl-4- isoxazole propionic acid (AMP A)lkainate receptors (that is, the fast component is blocked by the antagonist 6-cyano-2, 3-dihydroxy-7 -nitroquinoxaline, CNQX), and a slower component mediated by NMDA receptors (that is, blocked by the antagonist 2-aminophosphonopentanoic acid:2!, APV). Similarly, focal stimulation of the basolateral amygdala evokes a glutamate-mediated EPSP in the lateral nucleus:21.2c,. Thus, most afferents from the projection neurons of the lateral and basolateral nuclei to the central nucleus are glutamatergic, and stimulation of the basolateral amygdala evokes an EPSP in the central nucleus which has both AMPA/kainate and NMDA receptormediated components:2li. Behavior. It has been suggested that fear conditioning might be mediated by associative long-term potentiation2!-2~ (LTP). In pavlovian fear conditioning, a neutral stimulus, which has little behavioral effect by itself, is consistently paired with a strong aversive stimulus. Following a small number of pairings, the neutral stimulus produces effects formerly only produced by the strong, aversive stimulus. This change is not seen when the stimuli are presented in an unpaired fashion. In associative LTP, activation of a weak input to a given postsynaptic cell is paired with activation of a second, strong input Anatomical target Effect of amygdala stimulation Behavioral test or sign of fear or anxiety Conditioned fear stimulus \ Lateral hypothalamus ---- Sympathetic activation Increased respiration ~ Panting, respiratory distress Ventral tegmental area Activation of dopamine, Behavioral and Locus coeruleus noradrenaline ---- EEG arousal, Dorsal lateral tegmental n. and ACh neurons increased vigilance _ Tachycardia, galvanic skin response, paleness, pupil dilation, blood pressure elevation Dorsal motor n. of vagus. Parasympathetic._ Ulcers, urination, Nucleus ambiguus activation defecation, bradycardia Parabrachial nucleus ---- Pontine reticular formation -- Increased reflexes - Increased startle Unconditioned fear stimulus Cessation of Freezing, conflict test. CER. Central grey behavior social interaction analgesia conditioned analgesia Trigeminal, facial motor n. Mouth open, Facial expressions of fear jaw movements Corticosteroid release Paraventricular n. (hypothal.) --- ACTH release ('stress response') Fig. 2. Schematic diagram of selected outputs of the central nucleus of the amygdala to hypothalamic and brainstem targets, and possible relationship of these connections to specific signs of fear and anxiety. Abbreviation. CER, conditioned emotion response. Adapted, with permission, from Ref

4 Insular cortex Secondary sensory cortex, perirhinal and entorhinal cortex Secondary sensory cortex, perirhinal cortex Central nucleus Basolateral nucleus Lateral nucleus Fig. 3. Schematic diagram of pnnclpal connections within the central, basolateral and lateral amygdaloid nuclei based on in vitro electrophysiological recording and anatomical data. For simplicity, many other connections, notably subcortical projections, are not included. projecting to the same cell. Following a small number of pairings, the initially weak synaptic input is potentiated. This potentiation is not seen when an equal number of the weak and strong inputs are presented in an unpaired fashion. In the CAI region of the hippocampus, activation of the weak input releases excitatory amino acids, such as glutamate, which bind to both NMDA and AMPA! kainate receptors on the postsynaptic neuron (for review see Ref. 30). Binding to the NMDA receptor has little effect because the highly Ca~+ -permeable NMDA channel is normally blocked by Mg~+. However, if the postsynaptic neuron is depolarized by a strong synaptic input activating a sufficient number of AMPA/kainate receptors, the Mg L + block is removed and glutamate binding to the NMDA receptor allows Ca~ + to enter the cell, triggering a series of events that leads to a lasting potentiation of synaptic efficacy in the formerly weak input. If glutamate is prevented from binding to the NMDA receptor by administration of a competitive NMDA-receptor antagonist, such as APY, shortly before pairing the weak and strong stimuli, associative LTP does not occur. In classical fear conditioning, a neutral stimulus (conditioned stimulus) could elicit release of glutamate onto neurons in the amygdala, and this glutamate could bind to both NMDA and AMPA/kainate receptors. However, this might not produce much of a behavioral response because at resting membrane potentials only weak activation of AMPA/kainate receptors, and partial blockade of NMDA-channel permeability by Mg~ occur. However, presentation of a strong aversive stimulus at about the same time could further depolarize the neuron, relieve the remaining :.vlg~ + blockade, and enable Ca~ to enter the cell. This could trigger events that would increase the ability of the conditioned stimulus to activate that neuron, enabling it then to produce effects similar to those previously produced only by the strong aversive stimulus. That Mg~+ appears to only partially block NMDA currents in the amygdala might explain why fear conditioning occurs so readily. Like the hippocampus, both NMDA-dependen(D and NMDA-independent3~ forms of LTP have been observed in neurons of the basolateral complex, depending on the amygdaloid afferent stimulated (Fig. 4). Therefore, would local infusion of antagonists of NMDA receptors into the amygdala block the acquisition of aversive conditioning? Using the fearpotentiated startle paradigm, Miserendino and colleagues~9 found that infusion of APY into the basolateral nucleus caused a dose-dependent blockade of the acquisition, but not the expression, of conditioned fear. This effect did not seem to result from a decrease in sensitivity to footshock, a local anesthetic effect, blockade of visual transmission or permanent damage to the amygdala. Using similar doses, Fanselow and colleagues: u found that local infusion of APY into the basolateral nucleus before training blocked conditioned freezing measured 24 h later. Infusion of APY into the immediately adjacent central nucleus had no effect, implying that the effect was highly localized. Using a multiple-trial step through avoidance paradigm, Kim and McGaugh found that intra-amygdala infusion of IJI.-APY, IJ-APY or (±)-2- (carboxypiperazine-4-yl)propyl-i-phosphonic acid (CPP) before training, caused a dose-dependent impairment of retention measured 48 h later: 1!. The potency of the drugs was consistent with their relative affinities to the NMDA receptor. This effect was not seen when APY was infused into the striatum, immediately above the amygdala. Intra-amygdala 211

5 infusion of DL-APV did not affect footshock sensitivity or locomotor activity, and the blockade of memory formation could not be attributed to state-dependent effects. Using step-down inhibitory avoidance, Izquierdo and colieagues: J '> found that immediate posttraining infusion of APV into either the amygdala, medial septum, or hippocampus, blocked memory measured 18 h after training. lj-2-amino-5-phosphonovalerate caused amnesia when infused into either the hippocampus or amygdala immediately after training, but not thereafter:~h. However, the AMP A/ kainate antagonist CNQX caused amnesia if infused into the hippocampus or amygdala either immediately, 90 or 180 min after training. By contrast, APV infused into the entorhinal cortex caused amnesia when given either 90 or 180, but not immediately or 360, min after training:jrd7. These data suggest that a process sensitive first to APV and CNQX and then only to CNQX in the amygdala and hippocampus, is critical for post-training memory processing of this step-down inhibitory task. Later, an APV -sensitive process in the entorhinal cortex might come into play. As mentioned earlier, once learning has occurred, local infusion of APV into the amygdala does not block the expression of either fear -potentiated startle or inhibitory avoidance. In contrast, local infusion of CNQX into the amygdala dose-dependently blocks the expression of fear-potentiated startle:jk or retention in a step-down inhibitory avoidance tes(j9. Once again, this pattern is similar to LTP where APV does not reverse LTP once it is established, whereas CNQX blocks fast synaptic transmission either before or after LTP. Thus, excitatory amino acids mediate synaptic transmission in the amygdala originating from both cortical inputs and intrinsic amygdaloid connections. In the amygdala, NMDA receptors seem to be involved in the formation of conditioned fear, whereas AMPA/kainate receptors appear to be involved in the expression of conditioned fear. As such, these data are consistent with, but by no means prove, the idea that an NMDA-sensitive form of LTP in the amygdala might mediate fear conditioning. Inhibitory amino acids ElectroPhysiology. In the basolateral nucleus, stimulation-induced EPSPs are followed by both fast and slow IPSPs. The fast, GABA-mediated IPSP results from activation of GABAq receptors, and the slower IPSP is mediated by activation of GABAg receptors w (Fig. 4). The fast IPSP is associated with a large decrease in resistance, and its possible somatic or dendritic origin ensures that neurons of the basolateral complex will be effectively inhibited from firing action potentials, even in the face of strong excitatory stimuli. The fast onset and short duration of the IPSP also enables precise moment-to-moment adjustment of the cellular response to excitatory input. In contrast, the small conductance increase and long duration associated with the slow IPSP might only ret-,rulate low-frequency excitatory input during times of normal neuronal function. Blockade of the fast IPSP with the GABA; -receptor antagonist bicuculline results in epileptiform burst firing of norn1ally quiescent basolateral neurons, suggesting that the fast IPSP probably dictates the primary state of excitability in the nucleus. It is possible that both GABAq and GABAg responses might be mecll,ikcl by activation of a heterogeneous population I,f (,'\BA interneurons lo.n. In addition. both b'j,'j"kll. a GABAg-receptor agonist, and tr<1ns-l-,mll:1ii,'\dopentane-l,;3-dicarboxylic acid (tr<1ns-.'\ll']). a metabotropic glutamate-receptor a,l!;oil!st. rl cluce glutamatergic transmission via stimul<1tionii! thl :'tnaterminalis by an action at presynaptic rl'l'lptl Ir' - A reduction in presynaptic GABAw or 111l't;jbl1tl11jJIC glutamate-receptor activation might contribute' ti! the induction of LTP in the basolateral complex. Incb c!. kindling results in a decreased sensitivity of prl'- synaptic GABA B receptors on terminals of the stri;j terminalis that are presumed to be glutamatergic L'. In the central nucleus, focal, low-intensity stimulation of the lateral central nucleus elicits a GABA;- mediated inhibition (blocked by bicuculline methiodide) of medial central nucleus neurons, whereas higher intensity stimulation elicits a GABAg-mediated inhibition (blocked by phaclofen)2fi. Repetitive stimulation also reveals a slow GABAg-mediated IPSP. In addition, glutamatergic transmission was blocked by presynaptic activation of GABA B receptors and A 1 adenosine receptors. Anatomical studies I'> indicate some terminals of GABA neurons appear to be presynaptic to cortical terminals in the lateral part of the central nucleus. The presence of presynaptic A 1 adenosine receptors further suggests that, during periods of high metabolic activity in the adjacent basolateral nucleus, glutamatergic transmission in the central nucleus will be restricted. In addition, output neurons in the central nucleus are heavilv innervated by GABA terminals of intrinsic origin l '>. ~ose and coworkers~(i also demonstrated a strychnine-sensitive long-lasting IPSP following repetitive stimulation of the dorsal lateral subdivision of the central nucleus. This presumed glycine-mediated IPSP probably results from an input arising in the brainstem. In a nucleus that has such a strong output to the brainstem cardiovascular regulatory centers, this tight control of excitability is probably essential. In fact, local infusion of GABA or chlordiazepoxide into the central nucleus reduces the severity of stress-induced ulcers, whereas antagonists of GABA receptors increase severity (for review see Ref. 44). Behavior, The amygdala has a high density of benzodiazepine receptors L', which are known to facilitate GABA transmission. Local infusion of benzodiazepines into the amygdala (for review see Ref. 1) has anxiolytic effects in the operant conflict test, social interaction test, measures of conditioned freezing and hypoalgesia, the light-dark box test in mice, and antagonizes the discriminative stimulus properties of pentylenetetrazol. The anticonflict effect can be reversed by systemic administration of the benzodiazepine antagonist flumazenil or co-administration into the amygdala of the GABA A antagonist bicuculline, and mimicked by local infusion, into the amygdala, of GABA or the GAB.'\,\ agonist muscimol. In general, anticonflict effects of benzodiazepines occur after local infusion into the lateral and basolateral nuclei, which have the highest densities of benzodiazepine receptors in the amygdala, and not after local infusion into the central nucleus. More recently, it has been shown that the anterior part of the basolateral and central nucleus is especially important for conflict performance based on both lesion and local 212 TINS, Vol. 17, No

6 ~ ~ A a Control EPSP I b APV 50 11M " slpsp ~5mV 50 ms c CNOX 10 11M --r d APV 50 11M + CNOX 10 11M e CN~X 10 11M ~ _ 8MI 30 11M B post tetanus I OJ o 11M APV Q en 250 OJ.~ 200 Qj 150 (J) ru ~ M APV 0 C OJ 50 u Q; Time (min) C 35 d a+b c+d :;-.S co OJ o,00 ~o 000CO 00 ~ ~ "0 20 a b c Q. 20 mv E 15 ru oo:po~ fq:x9o CJ) t t 0-5 HFS HFS w APV 0 20 ms Time (min) Fig. 4. Synaptic transmission in the basolateral amygdaloid nucleus (BLA). (A, left) Stimulation of the stria terminalis evokes a multiphasic postsynaptic potential (PSP) in neurons of the BLA. Typically, stimulation (arrow head) elicits an EPSP followed by a fast IPSP (fipsp), and subsequent slow IPSP (sipsp). (A, right) The PSP results from a dual component glutamate-mediated EPSPand activation of a feedforward inhibitory input. a: A typical response to stria stimulation. b: 2-Amino-5-phosphonovalerate (APV) (50IIM) application caused a reduction in the amplitude of both the fast and slow IPSP. c: CNQX (10 11M)abolished both the EPSPand the flpsp and slpsp to reveal a slow EPSP.d: The slow EPSPrevealed by CNQX is blocked by subsequent addition of APV. e: Antagonism of GABA A receptors with bicuculline (BM/) enhanced the expression of the slow EPSP.(B) Long-term potentiation (LTP) induced in BLA neurons following stimulation of the external capsule (EC) is resistant to APV (50,11M).(B, left) High-frequency stimulation of the EC resulted in an increase in the EPSPamplitude (upper trace), the increase was unaffected by 50,uM APV (middle trace), and blocked by looilm APV (lower trace). (B, right) The 50 11MAPV-resistant LTP persisted for several minutes following high-frequency stimulation, suggesting that in this pathway NMDA-receptor activation is not needed for synaptic plasticity. (C) LTP induced in BLA neurons following stimulation of the endopyriform nucleus is sensitive to APV (50,11M).(C, right) In the presence of APV (50 11M),EPSPsevoked before (a), and after (b) high-frequency stimulation of the endopyriform nucleus are of similar amplitude. Following washout of APV, a similar stimulation paradigm resulted in an enhanced EPSP.(C, left) The APV-sensitive LTP persisted for several minutes following high-frequency stimulation, suggesting that NMDA-receptor activation is required for synaptic plasticity in this pathway. infusion of benzodiazepines. Therefore, taken together, these results might be sufficient to explain both fearreducing and anxiety-reducing effects of various drugs given systemically. Local infusion of the benzodiazepine antagonist flumazenil into the amygdala significantly attenuates the anticonflict effect of the benzodiazepine agonist chlordiazepoxide given systemicallyhi. This strongly implicates the amygdala in mediating the anxiolytic effects of benzodiazepines in normal animals. However, benzodiazepines can still have anxiolvtic effects in animals with lesions of the amygdala 17:10'. In these studies, the lesions produced anxiolytic effects by themselves. Nonetheless, some fearful behavior could still be obtained, presumably because other brain areas can mediate conditioned fear following damage to the amygdala, and these areas also seem to be affected by benzodiazepines. Using inhibitory avoidance, intra-amygdala infusion of the GABA antagonists bicuculline methiodide FI, picrotoxin or Ro54864 (4' -purodiazepine) (Ref. 50) 213

7 Acknowledgements Thiswork was supportedby NIMH GrantMH-47840, ResearchScientist DevelopmentA ward MH-00004, and a grant from theair ForceOffice of ScientificResearchto MD., Alzheimer's Assoc./Harold Wand GeorgianaSpaght Memorial Pilot ResearchGrantNo. PRG to D.R. and NIH Grant NS25139to Me. immediately after training, produced a dose-dependent enhancement of retention measured h later. Conversely, infusion of the GABA A agonist muscimol or the GABAI) agonist baclofen produced retention deficits. Infusions into the caltdate nucleus, dorsal to the amygdala, had no effect. Thus, it is clear that GABA can potently regulate cellular excitability in the lateral and basolateral amygdaloid nuclei by decreasing the release of glutamate or by direct inhibitory actions. These direct decreases in excitability might explain the depressant effects of GAB A on aversive conditioning. However, they might also result from an alteration in the release of noradrenaline which is known to modulate learning within the amygdala (for review see Ref. 51). For example, the facilitatory effect of bicuculline can be prevented by the f3-adrenergic antagonist propranolol at a dose which has no significant effect by itself. In addition, output neurons of the central nucleus, which project to brainstem targets areas known to be involved in the autonomic and somatic aspects of conditioned fear, are tightly regulated by GABA and other inhibitory transmitters, disruption of which might greatly amplify fear and stress. Selected references 1 DavIs, M. (1992) in The Amygdala. Neurobiological Aspects of Emotion, Memory, and Mental Dysfunction (Aggleton, J. P., ed.), pp , John Wiley-liss and Sons 2 Aggleton, J. P, ed. (1992) The Amygdala. Neurobiological Aspects of Emotion, Memory, and Mental Dysfunction, John Wiley-Liss and Sons 3 LeDoux, J. 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