AL CATECHOLAMINE CELL GROUP - FINE-STRUCTURE AND SYNAPTIC INPUT FROM THE NUCLEUS OF THE SOLITARY TRACT

Preembedding immunoperoxidase staining methods were used to characteri ze tyrosine hydroxylase-immunoreactive (TH-ir) elements in the caudal ventrolateral medulla, and to determine the extent to which neurons of the Al cell group are directly innervated by projections of the nucle us of the solitary tract (NTS). TH-ir neurons in the Al region were me dium-sized and multipolar. They possessed rounded nuclei with infreque nt invaginations, well-developed Golgi apparati, high cytoplasmic dens ities of mitochondria, and a low to moderate tendency for rough endopl asmic reticulum (RER) to align in parallel stacks. Al cell bodies were commonly juxtaposed to TH-positive and TH-negative neurons, myelinate d profiles, glia and/or vascular elements, but close membrane appositi ons were only seen with glial elements. Synaptic input to Al neurons w as predominantly asymmetric, provided virtually exclusively by non-TH- ir terminals, and directed principally to dendritic shafts; Al somata are relatively sparsely innervated. In a second experiment, silver-int ensified immunogold localization of TH-ir was combined with immunopero xidase labeling for anterogradely transported Phaseolus vulgaris-leuco agglutinin (PHA-L), following tracer injections in the caudal aspect o f the medial division of the NTS. These experiments revealed a small p roportion of PHA-L-labeled axon terminals that made asymmetric contact s with dendritic shafts of TH-ir neurons. These results suggest that t he fine structure and synaptic input of A1 neurons are somewhat distin ct from that of rostrally situated C1 catecholamine cells. In addition , while they document a direct NTS-A1 projection that may participate in the interoceptive control of vasopressin secretion, the bulk of ven trolaterally directed projections from the caudomedial NTS contact non catecholaminergic elements in the Al region, some of which may corresp ond to so-called depressor neurons implicated in the baroreflex contro l of sympathetic outflow and vasopressin secretion. (C) 1995 Wiley-Lis s, Inc.