Naloxone increases vasopressin secretion from the neurointermediate lobe of the hypophysis of the rat: search for the endogenous agonist.

Naloxone increases the electrically induced vasopressin release from rat pituitary neurointermediate lobes under appropriate stimulation conditions. In order to examine a possible role of hypophyseal opioid peptides we studied in vitro the effect of opioid peptides and of naloxone on the electrically induced vasopressin release from the rat neurointermediate lobe or isolated neural lobe of the hypophysis. (D-Ala2,D-Leu5)-enkephalin (5 microM), dynorphin-(1-13) (Dyn; 0.2 microM), beta-endorphin (beta-End; 0.02 and 0.2 microM) and also naloxone (1 or 10 microM) increased the evoked vasopressin release from the neurointermediate lobe, but in higher concentrations (2 microM) Dyn or beta-End had no effect. After removal of the intermediate lobe, beta-End 2 microM inhibited, while naloxone 10 microM did not change the evoked vasopressin release from the isolated neural lobe. These results demonstrate that hypophyseal opioid peptides can influence vasopressin release in several ways and suggest that endogenous opioids predominantly provide inhibitory influences which depend on the presence of the intermediate lobe.