Ultrastructural analysis of synapses involving tyrosine hydroxylase-containing neurons in the ventral periventricular hypothalamus of the macaque.

Immunocytochemical staining for tyrosine hydroxylase (TH) in the adult macaque brain revealed a network of catecholaminergic (CA) cell bodies and fibers in the arcuate (ARC), anterior ventral periventricular (APV) and lateral suprachiasmatic nuclei (SCN). Coronal Vibratome sections immunostained with PAP or colloidal gold (15 nm) were thin sectioned and examined by electron microscopy. We examined 280 TH-immunopositive processes in individual or in serial thin sections. Of these, 190 engaged in a total of 270 synapses identified as Gray Type I asymmetrical synapses (AS) with distinct postsynaptic densities or Gray Type II symmetrical synapses (SS) without such specializations. The majority (80%) of all synapses were axodendritic, 63% of which exhibited SS and 37% AS, representing almost all of the AS observed. In nearly every case, unlabeled axon terminals containing round, 45 nm, clear vesicles and occasional small dense core vesicles contacted TH-labeled dendrites. About 15% of the synapses were dendrodendritic, all of which were symmetrical. Rare contacts involving other elements (axosomatic, dendrosomatic) constituted only 5% of the total, and occurred predominantly as SS. The predominance of AS and the prevalence of SS almost exclusively on TH-containing dendrites indicates that these CA neurons receive extensive afferent input from other neurotransmitters. TH-labeling of both neural elements in most dendrodendritic, and in some axodendritic SS, also suggests that they modulate one another within the ARC, APV and SCN. The results suggest that these CA neurons perform an important role in local integration, and may act elsewhere to affect the common final pathway of the neuroendocrine system in primates.