Effects of endogenous hyperprolactinaemia on opiate-induced behavioral changes in rats

Endogenous hyperprolactinaemia, as induced by pituitary homografts under the kidney capsule, was followed by an inhibition of heroin self-administration in rats. This effect was mimicked by intracerebroventricular (i.c.v.) injection of rat prolactin (PRL) or peripheral administration of a hyperprolactinaemia-inducing drug, domperidone. I.c.v. injection of anti-PRL serum totally abolished the effect of hyperprolactinaemia on heroin self-administration behavior and facilitated it in rats with normal plasma PRL levels. The development of tolerance to morphine was facilitated in hyperprolactinaemic animals or in rats injected i.c.v. with rat PRL. This effect was abolished in hyperprolactinaemic animals by i.c.v. injection of anti-PRL serum. Furthermore, rats with normal plasma PRL levels showed an inhibited development of tolerance to morphine after i.c.v. injection of anti-PRL serum. Morphine-induced analgesia appeared to be potentiated in hyperprolactinaemic rats tested in the hot-plate test. It is concluded that endogenous hyperprolactinaemia can affect opiate-induced behavioral changes in rats, possibly through central mechanisms involving opioid transmission.