Mechanisms and functional significance of a slow inhibitory potential in neurons of the lateral amygdala

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Abstract
A slow inhibitory potential (sIP) elicited upon synaptic activation in spiny, pyramidal‐like cells with properties indicative of projection neurons was investigated in slices of the rat and guinea‐pig lateral amygdala in vitro. The sIP succeeded the triphasic sequence of excitatory and fast/slow inhibitory postsynaptic potentials mediated via glutamate and GABAA/B receptors, respectively, was readily evoked upon repetitive stimulation of the external capsule and appeared to terminate epileptiform burst discharges during pharmacologically reduced GABAergic influence. The sIP reversed close to the Cl– equilibrium potential, but was not affected by altered transmembrane Cl– gradients and not abolished by antagonists to ligand‐gated Cl– channels. Intracellular injection of QX 314 and resulting blockade of sodium spikes had no effect, whereas the Ca2+ chelator BAPTA blocked the sIP concomitantly with slow hyperpolarizing afterpotentials following intrinsically generated spike firing, thereby indicating the contribution of Ca2+‐dependent mechanisms secondary to synaptic activation. During action of BAPTA and QX 314, an N‐methyl‐d‐aspartate (NMDA) receptor‐mediated potential was unmasked, which contributed to the sIP. The Ca2+‐dependent mechanisms of the sIP involved a membrane K+ conductance, as was indicated by the dependence on the K+ gradient and the shift of the reversal potential towards the K+ equilibrium potential during blocked NMDA receptors. During the presence of GABA receptor antagonists, reduction of the Ca2+‐activated K+ conductance through injection of BAPTA or application of dopamine induced a gradual shift of interictal‐like single bursts of spikes towards the generation of re‐occurring ictal‐like activity. It is concluded that pyramidal‐like projection cells in the AL can generate a sIP upon synaptic activation, which reflects the combined activation of an NMDA receptor‐mediated cation current and a K+ current that is secondary to the rise in intracellular Ca2+ concentration resulting from the preceding depolarizing response. The sIP may play an important role in controlling excitatory activity in the amygdala, particularly in preventing the transformation of interictal‐like activity towards recurrent epileptic discharges during periods of decreased GABAergic influence.