Japanese Society for Animal Reproduction: award for outstanding research 2002. Mechanisms controlling neurohypophysial hormone release in the rat.

To explore mechanisms of osmotic control of neurohypophysial hormone release, electrical activity of magnocellular neurons (MCNs) in the hypothalamus, related neurons and efferent renal sympathetic nerve activity (RSNA) were recorded from urethane-anesthetized rats. Local osmotic stimulation applied to organum vasculosum of the lamina terminalis (OVLT) or median preoptic nucleus (MnPO) excited MCNs. Although OVLT neurons projected to MCNs were unresponsive to hyperosmotic stimulation, those projected to MnPO and also receiving excitatory inputs from MCNs, were excited by it. MnPO neurons, which were driven by OVLT stimulation and projected to MCNs, were also osmosensitive. Excitatory connections thus exist from MCNs to OVLT, from OVLT to MnPO and from MnPO to MCNs. Neurons in each of these connections were osmosensitive. This circuit thus appears to constitute an osmoreceptor complex essential for the osmoreception of MCNs. MnPO neurons constituting a part of the osmosensitive circuit were also sensitive to hemodynamic change. Thus this circuit may integrate hemodynamic and osmotic information. Local anesthesia of MnPO diminished activation of RSNA and pressor response induced by third cerebroventricular injection of hypertonic saline. The results suggest that the osmosensitive circuit is involved in body fluid regulation not only by controlling vasopressin secretion but also by modulating sympathetic outflow.