Abstract
The ultrastructural relationships between oxytocin (OT) -like immunoreactive neurons and other neurons in the rat paraventricular nucleus (PVN) and supraoptic nucleus (SON) were observed by means of immunocytochemistry using anti-OT serum. Axo-somatic, axo-dendritic or axo-axonic synapses between the same OT-like immunoreactive neurons were often observed. OT-like immunoreactive axon terminals containing both OT-like immunoreactive granules and immunonegative small vesicles were also observed to make synaptic contacts with immunonegative neurons in the PVN and SON. These OT-like immunoreactive granules were smaller than elementary granules in the neurosecretory nerve fibers of the neurohypophysis, but they were similar in size to those in OT neurons projecting to the extrahypothalamic regions. Additionally, applying the autoradiographic technique using 3H-noradrenaline (NA) or 3H-dopamine (DA) to the immunocytochemical sections, axon terminals labeled with silver grains after 3H-NA or 3H-DA injection were often observed near the OT-like immunoreactive neurons in both PVN and SON. In a limited number of cases, axon terminals labeled with 3H-DA were observed to make synaptic contacts with OT-like immunreactive nerve fibers. These findings suggest that catecholamine-containing neurons play important roles in the control of OT secretion in the OT-oroducing neurons.
