A light and electron microscopic study of oxytocin-containing neurons in the paraventricular nucleus of the rat.

Oxytocin-containing neurons and axon terminals contacting the neurons of the rat paraventricular nucleus were investigated by the peroxidase-antiperoxidase technique for light and electron microscopy. At the light microscopic level the reaction product was seen to fill the somata, dendrites and axons of the neurons. At the ultrastructural level the immunoprecipitate was localized on cytoplasm (including ergastoplasm) and neurosecretory granules (NSG) of the somata; microtubules, ergastoplasm and NSG of the dendrites; and NSG of the axons. Axon terminals synapsing on the surface of the labelled somata and dendrites were exclusively unlabelled. The somata and dendrites were observed to receive both asymmetrical (Gray's type I) and symmetrical (Gray's type II) synapses with clear, mostly spherical and flattened vesicles, respectively. Frequently, pleomorphic vesicles and a few densecore vesicles occurred in both types of synapses. There also were present some unlabelled bridge-like axon terminals making 'double' synapses either on two labelled dendritic processes or on one unlabelled and one labelled dendrite. The present findings demonstrate that the somata and dendrites of the oxytocin-containing neurons receive diverse innervation. At the same time there was no evidence in this study for synaptic input to the labelled axons of the neurons.