Differential innervation of parvalbumin-immunoreactive interneurons of the basolateral amygdaloid complex by cortical and intrinsic inputs.

In the basolateral (BL) amygdaloid complex, the excitability of projection cells is regulated by intrinsic inhibitory interneurons using gamma-aminobutyric acid (GABA) as a transmitter. A subset of these cells are labeled in a Golgi-like manner by Parvalbumin (PV) immunohistochemistry. Recently, we have shown that the overwhelming majority of axon terminals contacting these PV-immunoreactive neurons form asymmetric synapses. The present study was undertaken to identify the source(s) of these inputs. Since previous work had revealed that thalamic axons form very few synapses on BL interneurons (< 1%), we focused on cortical and intra-amygdaloid inputs. Iontophoretic injections of the anterograde tracers Phaseolus vulgaris-leucoagglutinin or biotinylated dextran amine were performed in various cortical fields in cats (perirhinal, entorhinal, pre/infralimbic cortices) and monkeys (orbitofrontal region) or in the BL amygdaloid nucleus in cats. These injections resulted in a large number of anterogradely labeled terminals forming asymmetric synapses in the BL complex. Following cortical injections, numerous anterogradely labeled terminals were found in the vicinity of PV-immunoreactive interneurons in the BL amygdala. However, only approximately 1% of these terminals formed synaptic contacts with PV-immunoreactive profiles. In contrast, as many as 11% of the terminals contributed by the intranuclear axon collaterals of BL projection cells established synapses with PV-immunoreactive elements. Since the axon terminals of PV-immunoreactive interneurons are enriched in GABA and they exclusively form symmetric synapses, these results suggest that PV-immunoreactive interneurons are predominantly involved in feedback inhibition in the BL amygdaloid complex.