Water intake and activity of hypothalamoneurohypophysial system during osmotic and sodium stimulation in rats

Intrajugular infusion (200 microliters/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, decreased with 1.7 M mannitol, and was not significantly altered by the combined solution. Irrespective of changes in plasma Na+ concentration, plasma vasopressin and oxytocin concentrations were elevated to a similar degree after independent infusion of 0.85 M NaCl or 1.7 M mannitol. With the combined infusion, the change in plasma vasopressin was additive but the change in oxytocin tended to be greater. Accordingly, glucose utilization increased throughout the hypothalamoneurohypophysial system after infusion of 0.85 M NaCl and 1.7 M mannitol. With the combined infusion, however, the change in glucose utilization in the paraventricular nucleus was additive but a synergistic effect occurred in the supraoptic nucleus and neural lobe. Drinking responses were similar in all groups receiving hypertonic solutions, with no additive effect after the combined stimulus. Although our results do not completely rule out the participation of cerebrospinal fluid sodium receptors, it is more likely that osmoreceptors regulate the activity of the hypothalamoneurohypophysial system and drinking behavior. Unlike the magnocellular system, however, drinking behavior seems to be negatively influenced by a stress component of the osmotic stimulation.