Somatostatin-like immunoreactive axon terminals on oxytocin-like immunoreactive neurons in the paraventricular nucleus of the rat hypothalamus.

By double label immunohistochemistry, we studied the synaptic relation between somatostatin (SS) and oxytocin (OT) or vasopressin (VP) neurons in the paraventricular nucleus (PVN) of the rat hypothalamus. In the light microscopic observation, SS-immunoreactive (ir) fibers were seen very frequently in contact with OT-ir neurons, but not with VP-ir neurons. Immunoelectron microscopic analysis revealed that somata and their dendrites of OT-ir magnocellular neurons were in synaptic contacts with SS-ir axon terminals. No SS-ir synaptic terminals were found on VP-ir magnocellular neurons. The results suggest that OT-containing magnocellular neurons in the PVN are under regulatory influences of SS-containing neurons.