Acute effects of arginine vasotocin on plasma and pituitary levels of prolactin in the male rat: Influence of urethane anesthesia

The acute effects of arginine vasotocin (AVT), a putative pineal peptide, on plasma levels of prolactin were investigated in the unanesthetized, estrogen-progesterone-treated male rat. A 10-µg s.c. injection resulted in significantly increased levels of prolactin while a 1-µg injection depressed plasma levels of this hormone; an inhibition of release was further suggested by increased levels of pituitary prolactin in rats treated with the lower dose. The same dose administered subcutaneously to urethane-anesthetized rats had no significant effect on plasma prolactin titers while a 10-µg dose was still stimulatory. In anesthetized rats, no dose of the AVT administered (1 ng to 10 µg, s.c, or 1 fg to 1 µg, i.v.) resulted in inhibition of prolactin release. In the unanesthetized normal male rat, 5 µg AVT first increased and then decreased levels of prolactin. Urethane anesthesia appears to mask an inhibitory effect of AVT on plasma prolactin levels in these animal preparations. Since AVT is capable of decreasing plasma prolactin levels, this peptide might be involved in the depression of plasma prolactin levels which appears to (at least partially) mediate pineal-induced gonadal degeneration.