AMYGDALA EFFERENTS FORM INHIBITORY-TYPE SYNAPSES WITH A SUBPOPULATIONOF CATECHOLAMINERGIC NEURONS IN THE RAT NUCLEUS-TRACTUS-SOLITARIUS

The central nucleus of the amygdala (CNA) integrates visceral response s to stress partially through efferent projections to portions of the medial nuclei of the solitary tracts (mNTS) containing catecholaminerg ic neurons. To determine anatomical sites for CNA modulation of these neurons, immunoperoxidase detection of anterogradely transported Phase olus vulgaris-leucoagglutinin (PHA-L) or biotinylated dextran amine (B DA) was combined with immunogold-silver labeling of the catecholamine- synthesizing enzyme, tyrosine hydroxylase, in adult rat mNTS. From 350 anterogradely labeled terminals identified within the intermediate mN TS, 30% formed symmetric, inhibitory-type synapses and the remainder l acked recognized junctions as seen within a single plane of section. O f the terminals forming symmetric synapses, 16% were presynaptic to ty rosine hydroxylase immunoreactive dendrites and the remainder to unlab eled dendrites. The level of tyrosine hydroxylase immunoreactivity as assessed by density of gold-silver particles was significantly lower i n dendrites receiving synaptic input from CNA efferents as compared wi th dendrites of the same sizes (2.0 mu m(2) in mean area) which receiv ed synapses from unlabeled terminals or lacked recognizable synaptic i nputs. When separately examined without regard to afferent input, the medium- and larger-sized dendrites having mean cross-sectional areas o f 1-3 mu m(2) also contained significantly less tyrosine hydroxylase i mmunoreactivity than small (<1 mu m(2)) dendrites. These results sugge st that CNA efferents to the mNTS inhibit non-catecholamine-containing neurons and a subpopulation of catecholaminergic neurons distinguishe d by their low levels of tyrosine hydroxylase. The findings also indic ate that small, presumably more distal, dendrites in the intermediate mNTS may synthesize and/or release catecholamines. (C) 1995 Wiley-Liss , Inc.