Fear conditioning, a behavioral model of fear learning and cue‐related anxiety, causes enhanced neuronal transmission in the thalamic to lateral amygdala pathway.1,2 In the expression phase of learned fear, this increased transmission recorded in vitro is revealed in increased amplitudes of excitatory postsynaptic currents (EPSCs) and occlusion of paired‐pulse facilitation (PPF) implicating a presynaptic increase in transmitter release. Here we examined the contribution of L‐type calcium channels in fear conditioning. We measured the effect of nimodipine (Nim, 1.5–20 mg/kg), an L‐type calcium channel antagonist, on fear‐potentiated startle in which startle was assessed in animals receiving paired or unpaired tone and foot shock. Nim administered intraperitoneally blocked fear‐potentiated startle but not baseline startle in a dose‐dependent manner. We also analyzed the effect of Nim (10 μM) in vitro on synaptic facilitation of EPSCs and PPF in slices from naïve control, unpaired control, and fear‐conditioned animals. In neurons from naïve control animals, Nim had no effect on EPSC amplitude or PPF, but in slices from fear‐conditioned rats, Nim reduced EPSC amplitude, suggesting the recruitment of L‐type calcium channels within the fear‐conditioning pathway. Nim increased PPF in slices from fear‐conditioned animals, suggesting that L‐type calcium channels may contribute to increased probability of release in fear conditioning. In slices from unpaired animals, Nim decreased synaptic transmission but had little effect on PPF, suggesting that stress or contextual fear learning may induce L‐type channel activity in fear‐conditioned and unpaired control animal groups. We also analyzed protein expression of the α1C and α1D L‐type calcium channel subunits isolated from the amygdala and found that α1C protein was significantly increased in fear‐conditioned animals. These findings suggest that L‐type calcium channels play a role in the amygdala in cued fear conditioning and have important implications in the treatment of anxiety and in emotional learning and plasticity.

G-protein-coupled metabotropic glutamate receptors (mGluRs) are being implicated in various forms of neuroplasticity and CNS disorders. This study examined whether the sensitivities of mGluR agonists are modulated in a distinct fashion in different models of synaptic plasticity, specifically, kindling and chronic cocaine treatment. The influence of kindling and chronic cocaine exposure in vivo was examined in vitro on the modulation of synaptic transmission by group II and III metabotropic glutamate receptors using whole cell voltage-clamp recordings of central amygdala (CeA) neurons. Synaptic transmission was evoked by electrical stimulation of the basolateral amygdala (BLA) and ventral amygdaloid pathway (VAP) afferents in brain slices from control rats and from rats treated with cocaine or exposed to three to five stage-five kindled seizures. This study shows that after chemical stimulation with chronic cocaine exposure or after electrical stimulation with kindling the receptor sensitivities for mGluR agonists are altered in opposite ways. In slices from control rats, group II agonists, (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (LCCG1) and (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740), depressed neurotransmission more potently at the BLA-CeA than at the VAP-CeA synapse while group III agonist, L(+)-2-amino-4-phosphonobutyrate (LAP4), depressed neurotransmission more potently at the VAP-CeA synapse than at the BLA-CeA. These agonist actions were not seen (were absent) in amygdala neurons from chronic cocaine-treated animals. In contrast, after kindling, concentration response relationships for LCCG1 and LAP4 were shifted to the left, suggesting that sensitivity to these agonists is increased. Except at high concentrations, LCCG1, LY354740, and LAP4 neither induced membrane currents nor changed current-voltage relationships. Loss of mGluR inhibition with chronic cocaine treatment may contribute to counter-adaptive changes including anxiety and depression in cocaine withdrawal. Drugs that restore the inhibitory effects of group II and III mGluRs may be novel tools in the treatment of cocaine dependence. The enhanced sensitivity to group II and III mGluR agonists in kindling is similar to that recorded at the lateral to BLA synapse in the amygdala where they reduce epileptiform bursting. These findings suggest that drugs modifying mGluRs may prove useful in the treatment of cocaine withdrawal or epilepsy.

The amygdala plays a critical role in fear conditioning, a model of emotional learning and cue-induced anxiety. In the lateral amygdala, fear conditioning is associated with an enduring increase in synaptic strength mediated through AMPA receptors and with a reduction in paired-pulse facilitation, reflecting an increased probability of neurotransmitter release. Here we show that NMDA-mediated transmission in the thalamic-to-lateral amygdala pathway is not facilitated after fear conditioning, although probability of transmitter release is enhanced. Rather, the EC 50 for NMDA receptor (NR)-mediated current is shifted threefold to fourfold to the right in fear-conditioned animals, suggesting a postsynaptic alteration in NMDA receptors in the maintenance phase of fear memory. Furthermore, the ability of nonselective and subunit-selective antagonists of NMDA receptors to block NMDA receptor-mediated EPSCs is reduced in lateral amygdala neurons from fear-conditioned animals, suggesting a reduction in NMDA receptors at thalamolateral amygdala synapses. In addition, Western blots show a reduction in phosphorylated-NR1, NR2A, and NR2B subunit protein expression in amygdalas from fearconditioned animals. These data indicate that postsynaptic mechanisms are involved in synaptic plasticity in the thalamoamygdala pathway in fear conditioning and raise the possibility that: (1) downregulation of the NMDA receptor may protect against excitotoxicity of unchecked NMDA receptor recruitment during induction and consolidation of fear memories, (2) reduced NMDA current and protein may allow persistence of the "capacity to reactivate" amygdala pathways in NMDA receptor-dependent fear memories, or (3) a persistent long-term depression of NMDA transmission may occur after fear learning.

High pressures of helium affect the physiology of the central nervous system in animals and humans. We examined these effects in rat hippocampal slices. The in vitro preparation displayed a reversible reduction in postsynaptic and antidromic field potentials of CA1 pyramidal cells, but no significant change in the amplitude of the afferent volley. Although the subliminal synaptic response of CA1 neurons was depressed, the ability of these cells to produce population spikes was enhanced. These changes resembled those previously found in vivo in the rat hippocampus. The present results support the hypothesis of a helium pressure-induced depolarization of hippocampal neurons. Other possible mechanisms are discussed.

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Stimulating thalamic fibers exiting from the internal capsule evokes a glutamatergic excitatory postsynaptic current (EPSC) recorded in vitro with patch electrodes in neurons of the rat lateral amygdala (LA). The purpose of this study is to compare paired-pulse facilitation (PPF), a form of short-term synaptic plasticity, of AMPA and NMDA receptor-mediated EPSCs. Analysis of PPF at this synapse is important since, in fear-conditioned animals, PPF reflects an enhanced transmitter release but the amplitude of only AMPA EPSCs is facilitated. PPF magnitude of the composite EPSC is a result of both AMPA and NMDA receptor activation; however, the characteristics of AMPA and NMDA PPF are dissimilar. Specifically, the NMDA EPSC shows greater PPF (NMDA PPF) than does the AMPA EPSC whether measuring the NMDA PPF magnitude in an AMPA antagonist/Mg(2+)-free solution or by subtracting the AMPA EPSC from the composite EPSC in normal Mg(2+). Presynaptic NMDA receptors neither influence AMPA PPF nor account for the difference between the NMDA and AMPA PPF. Another difference was that removal of inhibitory tone enhanced AMPA PPF, while it had mixed effects on NMDA PPF. Furthermore, AMPA PPF was independent of stimulus intensity and postsynaptic voltage, unlike the NMDA PPF. Another dissimilarity was that the amplitudes of pairs of AMPA EPSCs were not correlated, suggesting presynaptic mechanisms. In contrast, NMDA PPF was dependent on stimulus intensity and postsynaptic voltage and the amplitudes of paired NMDA EPSCs had a positive correlation, suggesting a postsynaptic influence. Both AMPA and NMDA PPF were influenced by GABA inhibition and this could be a factor in the magnitude disparity. These data show that AMPA and NMDA PPF have different characteristics and contribute to the composite PPF in the thalamic to lateral amygdala pathway.

Postsynaptic metabotropic glutamate (mGlu) receptor-activated inward current mediated by Na(+)-Ca(2+) exchange was compared in basolateral amygdala (BLA) neurons from brain slices of control (naïve and sham-operated) and amygdala-kindled rats. In control neurons, the mGlu agonist, quisqualate (QUIS; 1-100 microM), evoked an inward current not associated with a significant change in membrane slope conductance, measured from current-voltage relationships between -110 and -60 mV, consistent with activation of the Na(+)-Ca(2+) exchanger. Application of the group I selective mGlu receptor agonist (S)-3,5-dihydroxyphenylglycine [(S)-DHPG; 10-1000 microM] or the endogenous agonist, glutamate (10-1000 microM), elicited the exchange current. QUIS was more potent than either (S)-DHPG or glutamate (apparent EC(50) = 19 microM, 57 microM, and 0.6 mM, respectively) in activating the Na(+)-Ca(2+) exchange current. The selective mGlu5 agonist, (R, S)-2-chloro-5-hydroxyphenylglycine [(R,S)-CHPG; apparent EC(50) = 2. 6 mM] also induced the exchange current. The maximum response to (R, S)-DHPG was about half of that of the other agonists suggesting partial agonist action. Concentration-response relationships of agonist-evoked inward currents were compared in control neurons and in neurons from kindled animals. The maximum value for the concentration-response relationship of the partial agonist (S)-DHPG- (but not the full agonist- [QUIS or (R,S)-CHPG]) induced inward current was shifted upward suggesting enhanced efficacy of this agonist in kindled neurons. Altogether, these data are consistent with a kindling-induced up-regulation of a group I mGlu-, possibly mGlu5-, mediated responses coupled to Na(+)-Ca(2+) exchange in BLA neurons.

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