WATER-INTAKE AND ACTIVITY OF HYPOTHALAMONEUROHYPOPHYSEAL SYSTEM DURING OSMOTIC AND SODIUM STIMULATION IN RATS

Intrajugular infusion (200 mu/min for 10 min) of 0.85 M NaCl or 1.7 M mannitol in conscious adult male Sprague-Dawley rats increased plasma osmolality similarly and had an additive effect when combined. Plasma Na+ concentration, however, increased with infusion of 0.85 M NaCl, de creased with 1.7 M mannitol, and was not significantly altered by the combined solution. Irrespective of changes in plasma Naf concentration , plasma vasopressin and oxytocin concentrations were elevated to a si milar degree after independent infusion of 0.85 M NaCl or 1.7 M mannit ol. With the combined infusion, the change in plasma vasopressin was a dditive but the change in oxytocin tended to be greater. Accordingly, glucose utilization increased throughout the hypothalamoneurohypophysi al system after infusion of 0.85 M NaCl and 1.7 M mannitol. With the c ombined infusion, however, the change in glucose utilization in the pa raventricular nucleus was additive but a synergistic effect occurred i n the supraoptic nucleus and neural lobe. Drinking responses were simi lar in all groups receiving hypertonic solutions, with no additive eff ect after the combined stimulus. Although our results do not completel y rule out the participation of cerebrospinal fluid sodium receptors, it is more likely that osmoreceptors regulate the activity of the hypo thalamoneurohypophysial system and drinking behavior. Unlike the magno cellular system, however, drinking behavior seems to be negatively inf luenced by a stress component of the osmotic stimulation.