Interactions between vasopressin- and gonadotropin-releasing-hormone-containing neuroendocrine neurons in the monkey...

Vasopressin (VP) is a hypophysiotropic hormone which is also implicated in the control of gonadotropin-releasing hormone (GnRH) secretion. We examined whether VP- and GnRH-immunoreactive (-IR) elements interact directly in the supraoptic nucleus (SON) of cynomolgus monkeys. Neuroendocrine (NEU) neurons in 4 juveniles were retrogradely labeled from the median eminence with wheat germ agglutinin apohorseradish peroxidase conjugated to gold before aldehyde perfusion. Frontal vibratome sections were immunostained for GnRH with peroxidase-antiperoxidase (PAP) and for VP with 5- or 15-nm gold. Many of the GnRH-IR and more than half the VP-IR cell bodies in the SON were NEU. VP-IR elements formed axodendritic and axosomatic symmetrical synapses with one another. In addition, VP-IR boutons also synapsed with NEU GnRH-IR neurons. Although GnRH axon terminals and dendrites contacted VP-IR dendrites and NEU cell bodies, we were unable to find convincing examples of GnRH/VP synapses through serial sections, perhaps due to the use of PAP-diaminobenzidine as the GnRH (afferent) immunolabel. In summary, our study demonstrates anatomical synapses between VP-IR and other VP and GnRH-IR neurons in the SON, in which postsynaptic VP or GnRH cell bodies were NEU. On the other hand, reciprocal GnRH/VP contacts but no true synapses were seen. However, the results suggest coordinated roles for VP and GnRH in NEU control of gonadotropin secretion. Whether VP itself and/or coexistent neuroeffectors act directly on NEU GnRH secretion remains to be determined. As such, VP neurons could help coordinate suppression of gonadotropins and augmentation of glucocorticoids during the stress response in primates.