The area postrema is not involved in osmotic activation of neurosecretory cells in the supraoptic nucleus.

Experiments were performed to examine whether or not the area postrema (AP) is involved in the osmotic control of neurohypophysial hormone release. In control rats and in rats bearing extensive lesions of AP, extracellular action potentials were recorded from neurosecretory cells in the supraoptic nucleus (SON) and firing rates determined before and after the intraperitoneal (i.p.) injection of 1.5 M-NaCl solution. Lesion of the AP significantly (p less than or equal to 0.05, Mann-Whitney's U-test) lowered firing rate of putative vasopressin (phasic) cells but not that of oxytocin (non-phasic) cells. In lesioned animals, i.p. injection of hypertonic saline, however, caused similar changes in plasma osmotic pressure, plasma oxytocin concentration and mean firing rates of both putative oxytocin and vasopressin cells to those in control rats. The results suggest that the osmotic control of SON neurosecretory cell activity in the rat can take place in the absence of the AP.