The contribution of the median preoptic nucleus to renal sympathetic nerve activity increased by intracerebroventricular injection of hypertonic saline in the rat.

The median preoptic nucleus (MnPO) of the hypothalamus is involved in the osmotic control of neurohypophysial hormone release and drinking behavior. At the same time, renal sympathetic nerves exert multiple effects on renal functions such as regulating renal blood flow and urinary sodium excretion. We made the hypothesis that the MnPO may also regulate body fluid balance by exerting an influence on renal sympathetic nerve activity (RSNA). In this study we examined the effect of electrical stimulation of the MnPO on RSNA and the contribution of the MnPO to the change of RSNA induced by intracerebroventricular injection of hypertonic saline in the male Wistar rat. Electrical stimulation of the MnPO and the paraventricular nucleus of the hypothalamus (PVN) elicited an increase in RSNA. This increase of RSNA elicited by electrical stimulation of the MnPO was reduced by microinjections (100 nl) of 10% lidocaine or 4 mM cobaltous chloride (a synaptic transmission blocking agent) bilaterally into the PVN. Both RSNA and the mean arterial pressure (MAP) were increased by the injection of 1.5 M NaCl into the third ventricle, although heart rate (HR) was not significantly changed. These responses of RSNA and MAP were diminished by microinjection of 10% lidocaine (100 nl) into the MnPO. Our results suggest that the MnPO is involved in body fluid regulation not only by controlling vasopressin secretion and water intake but also by modulating central sympathetic outflow which regulates body fluid balance through an effect on the kidney.