SELECTIVE-INHIBITION OF MAGNOCELLULAR VASOPRESSIN NEURONS BY HYPOOSMOLALITY - EFFECT ON HISTAMINE-INDUCED AND STRESS-INDUCED SECRETION OF ADRENOCORTICOTROPIN AND PROLACTIN

We investigated the effect of selective inhibition of magnocellular ar ginine vasopressin (AVP) and oxytocin neurons on histamine (HA)- and r estraint-stress-induced adrenocorticotropin (ACTH) and prolactin (PRL) secretion in conscious male rats. The inhibition of magnocellular neu rons was obtained by inducing chronic hypoosmolality via continuous ex posure of the rats to the AVP V-2 receptor agonist 1-deamino(8-D-argin ine)vasopressin (DDAVP) which was released from osmotic pumps implante d subcutaneously. In DDAVP-treated rats, plasma osmolality and sodium concentration were 273 mosm/l and 130 mmol/l, respectively. In control rats, the corresponding values were 291 mosm/l and 139 mmol/l. HA (27 0 nmol) administered intracerebroventricularly or 5 min of restraint s tress stimulated ACTH and PRL secretion 4- to 11-fold in normoosmolar rats. In hypoosmolar rats, the HA-induced ACTH response was inhibited more than 40% whereas the restraint-stress-induced ACTH response was u naffected. Conversely, the PRL response to HA in hypoosmolar rats was unaffected whereas the PRL response to restraint stress was inhibited by 40%. In summary, chronic hypoosmolality inhibits HA-induced ACTH an d restraint-stress-induced PRL secretion indicating involvement of mag nocellular AVP in these responses.