Synaptic input to vasopressin neurons of the paraventricular nucleus (PVN)

Following injections of horseradish peroxidase into the PVN, retrogradely filled cells were found in regions of the limbic system known to contain glucocorticoid concentrating neurons [4, 31, 44]. To determine if these regions which include the lateral septum, medial amygdala and ventral subiculum have a monosynaptic input to vasopressin neurons we developed a double label ultrastructural technique [20] to simultaneously visualize immunoreactive neuropeptide and anterogradely transported HRP. Following injections of tracer into all three of these regions, HRP labeled fibers were seen at the light microscopic level to form a halo in the perinuclear, cell poor zone around the PVN. Ultrastructural examination of this area resulted in the discovery of a small number of limbic system synapses on vasopressin dendrites. These synapses were most numerous in the ventral and medial portion of the cell poor zone. A similar pattern of innervation was seen for the supraoptic and suprachiasmatic nucleic which also contain vasopressin cells whose dendrites extend beyond the nuclear boundaries. In a similar fashion we were interested in determining the distribution of noradrenergic terminals on vasopressin neurons in the various subnuclei of the PVN. We have combined immunocytochemistry for vasopressin with radioautography for 3H-norepinephrine (NE) at the ultrastructural level. NE terminals were numerous in the periventricular zone, innervating both vasopressin containing dendrite and non-immunoreactive dendrites and cell bodies. The vasopressin dendrites could originate from cells either resident in the periventricular zone or from cells situated in more lateral subnuclei. In the main, lateral magnocellular region, noradrenergic terminals were very few in number and innervated almost exclusively non-vasopressin containing structures. These studies demonstrate the need for ultrastructural analysis of synaptic input to neurosecretory cells.