Ultrastructural demonstration of oxytocin and vasopressin release sites in the neural lobe and median eminence of the rat by tannic acid and immunogold methods.

We have investigated the localization of oxytocin (OXT) and vasopressin (AVP) release sites in the neural lobe and median eminence (ME) of the rat. Ultrastructural and post-embedding immunocytochemical studies with protein A-gold as a marker were used for the identification of OXT and AVP neurosecretory granules (NSG). Release of the contents of these OXT and AVP NSG by exocytosis has been clearly demonstrated with the tannic acid-Ringer incubation method in axon terminals of the neural lobe and in varicose fibres with OXT and AVP NSG in the internal zone of the ME. As these fibers lack synaptic specializations, and are not located in the direct vicinity of capillaries of the primary portal plexus, these observations suggest that OXT and AVP are released in a paracrine way in the ME of the rat.