Spontaneous and evoked activity of intercalated amygdala neurons

The intercalated cell masses are clusters of GABAergic neurons interposed b etween the basolateral and centromedial nuclear groups of the amygdala. Tra ct-tracing studies have revealed that the main projection sites of intercal ated neurons are the central amygdaloid nucleus and the basal forebrain. Th rough these projections, intercalated neurons could influence the activity of widespread regions of the central nervous system. However, no data are a vailable regarding their physiological properties because of the paramount methodological difficulties raised by the small size of intercalated cell m asses. Here, we have investigated the spontaneous and evoked activity of in tercalated neurons in unanaesthetized, chronically implanted cats. Extracel lular recording sites were identified using stringent histological criteria . The intercalated cell masses were found to contain a population of neuron s firing at much higher rates than commonly observed in neighbouring amygda loid nuclei. Individual intercalated neurons displayed state-dependent chan ges in firing rates, but these varied from cell to cell. Most tested interc alated neurons displayed short-latency orthodromic responses to cortical sh ocks and were responsive to a variety of auditory stimuli. Considering that the vast majority of intercalated neurons use gamma-aminobutyric acid (GAB A) as a transmitter, the presence of neurons with high spontaneous firing r ates within the intercalated cell masses suggests that these cell clusters may provide a tonic inhibitory input to their projection sites. Moreover, t he fact that the firing probability of some intercalated neurons could be a ltered by the presentation of sensory stimuli suggests that this inhibitory input can be modulated as a function of environmental contingencies.