Effects of haloperidol on morphine-induced antinociception morphine tolerance and withdrawal in hyperprolactinaemic rats.

Male rats with hyperprolactinaemia, induced by adenohypophyseal homografts under the kidney capsule, were injected with haloperidol for 3 days and then subjected to treatment with morphine, administered twice in a day at increasing doses for 12 days. At the first treatment, morphine induced an antinociceptive effect that was more prolonged in homografted rats than in sham-operated controls. The pretreatment with haloperidol did not change the prolongation of morphine-induced antinociception in homografted rats. After 12 days of treatment with morphine, all animals were tested for the development of tolerance to the antinociceptive effect of morphine. Pituitary homografts resulted in an inhibition of the development of tolerance, while this effect was absent in homografted rats pretreated with haloperidol. At the end of the treatment with morphine, the naloxone-precipitated abstinence syndrome was studied. Homografted rats shown an attenuation of the withdrawal syndrome and pretreatment with haloperidol did not change this response. These results suggest an involvement of dopaminergic transmission in prolactin-induced changes in the development of tolerance to morphine, but not in the antinociceptive effect following an acute injection of morphine and in naloxone-precipitated withdrawal syndrome of the rat.