Evidence for a perisomatic innervation of parvalbumin-containing interneurons by individual pyramidal cells in the basolateral amygdala.

The basolateral amygdala (ABL) contains pyramidal projection neurons (PNs) and several discrete subpopulations of nonpyramidal interneurons. Interneurons containing the calcium-binding protein parvalbumin (PV) constitute about half of all ABL interneurons, and provide a robust innervation of the perisomatic domain of PNs. Although it is known that PNs reciprocate this projection by innervating PV interneurons, little is known about the details of these connections. In the present study, we investigated the innervation of PV interneurons by individual PNs in rat amygdalar slices. PNs in the basolateral nucleus, identified in vitro by their distinctive electrophysiological characteristics in whole cell patch-clamp recordings, were filled with biocytin by diffusion from the patch electrode. PV interneurons and biocytin-labeled PNs were visualized with a two-color immunoperoxidase procedure using nickel-enhanced DAB (black) for biocytin and non-enhanced DAB (brown) for PV. In slices with well-stained PN axons and PV neurons, light microscopy revealed numerous synapse-like contacts between these structures. The main PV+ targets of PN axons were the somata and proximal dendrites of PV neurons, although there were also contacts with more distal PV dendrites. In many cases, the PN axons ran along PV somata and/or proximal dendrites, forming multiple contacts. However, the great majority the PN axon terminals did not contact PV neurons. These observations suggest that there are robust reciprocal perisomatic PN-to-PV connections that may be important for the precise timing of rhythmic activity in the basolateral amygdala.