Synaptic responsiveness of interneurons of the cat lateral amygdaloid nucleus

Previous work in our laboratory has revealed that the excitability of lateral amygdaloid projection neurons is tightly regulated by GABA-mediated inhibitory postsynaptic potentials and intrinsic conductances that can be activated by synaptic inputs. Here, we studied the synaptic responsiveness of lateral amygdaloid interneurons recorded intracellularly in vivo, in the cat, to investigate their role in regulating the activity of projection cells. Interneurons were identified morphologically by their aspiny dendritic trees and physiologically by their ability to generate high frequency, non-adapting spike trains in response to depolarizing current pulses. Cortical shocks of increasing intensity generated opposite response profiles in interneurons and projection cells, with interneurons becoming progressively more excited and projection cells more inhibited. These cortically-evoked response profiles paralleled the activity of interneurons and projection cells in relation to spontaneous electroencephalographic events of differing amplitudes. Only at the lowest intensities were predominantly excitatory responses elicited in both cell types. As a result, only a narrow range of low stimulus intensities could trigger spikes in projection cells. In both cell types, the initial cortically-evoked excitatory postsynaptic potential was followed by a hyperpolarization, which was of markedly lower amplitude and duration in interneurons. In interneurons, the hyperpolarization reversed at approximately -72 mV with potassium acetate pipettes and approximately -55 mV with potassium chloride pipettes, suggesting that this inhibitory postsynaptic potential is primarily mediated by a chloride conductance. In light of previous findings indicating that inhibition in the lateral amygdaloid nucleus arises mostly from local inhibitory neurons, these results suggest that interneurons are synaptically coupled via GABAA receptors. Moreover, the opposite response profiles of interneurons and projection cells to cortical shocks indicate that interneurons play a critical role in regulating the activity of projection cells. The cellular interactions evidenced in the present study suggest that the lateral amygdaloid nucleus is endowed with an inhibitory gating mechanism that regulates information flow through the amygdala.