The osmotically and histamine-induced enhancement of the plasma vasopressin level is diminished by intracerebroventricularly administered orexin in rats

The effects of the centrally administered neuropeptides orexin-A on water intake and vasopressin (VP) secretion were studied in male Wistar rats (180-250 g). Different doses (10, 30, and 90 mu g/10 mu l) of the orexins and the specific orexin receptor-1 (OX1) antagonist SB 408124 (30 mu g/10 mu l) were administered intracerebroventricularly (i.c.v.) under anaesthesia, and the water consumption was measured during 6 h. A plasma VP level elevation was induced by histamine (10 mg/kg) or 2.5% NaCl (10 ml/kg) administered intraperitoneally (i.p.). The plasma VP levels were measured by radioimmunoassay. Increased water consumption was observed after the administration of 30 mu g/10 mu l orexin-A. There were no changes in basal VP secretion after the administration of different doses of the orexins. A significant increase in plasma VP concentration was detected following histamine administration. After 2.5% NaCl administration, there was a moderate VP level enhancement. Intracerebroventricularly administered orexin-A (30 mu g/10 mu l) blocked the VP level increase induced by either histamine or 2.5% NaCl administration. The inhibitory effects were prevented by the specific OX1 receptor antagonist. In conclusion, the orexins increased water consumption. After 30 mu g/10 mu l orexin-A administration, the polydipsia was more pronounced. The OX1 receptor antagonist significantly decreased the polydipsia. Histamine or hyperosmotic VP release enhancement was blocked by previously administered orexin. This inhibition was not observed following OX1 receptor antagonist administration. Our results suggest that the effects of the orexins on water consumption or blockade of the histamine and osmosis-induced VP level increase are mediated by the OX1 receptor.