Immunocytochemical and radioautographic procedures were combined at the ultrastructural level to study the noradrenergic synaptic input to vasopressin neurons in selected portions of the paraventricular nucleus of the hypothalamus (PVN) of the rat. Radioactive norepinephrine (NE) was infused into the lateral ventricle or applied topically to the region of the PVN. After appropriate survival times, brain tissues were processed for ultrastructural immunocytochemical demonstration of vasopressin using a monoclonal antibody16. [3H]NE varicosities were detected by electron microscopic radioautogry. In the periventricular zone of the PVN, radioactive varicosities were numerous accounting for 20-30% of all nerve terminals in this zone. These NE terminals primarily innervated dendritic processes of non-vasopressinergic neurons. Although an occasional axosomatic synapse was observed, input to vasopressin positive neurons was exclusively to their dendrities. In the lateral magnocellular sub-nucleus of the PVN (designed pvl2), noradrenergic terminals were fewer in number accounting for only 1-2% of the total. These terminals were found predominately but not exclusively making axodendritic synapses onto non-vasopression processes. In both regions, many of the radiolabeled terminals had well-defined membrane appositions with their post-synaptic partners which includes a synaptic cleft and post-synaptic density of varying thickness. In both the periventricular zone and the lateral magnocellular regions, noradrenergic varicosities were seen in close proximity to numerous blood vessels.
- paraventrcular nucleus