APOMORPHINE-INDUCED AND OXYTOCIN-INDUCED PENILE ERECTION AND YAWNING IN INTACT AND CASTRATED MALE-RATS - EFFECT OF SEXUAL STEROIDS

The effect of apomorphine (80 mug/kg s.c.) and oxytocin (30 ng i.c.v.) on penile erection and yawning was studied in intact and castrated ma le rats. In castrated rats both apomorphine and oxytocin responses wer e abolished. In these animals, testosterone (100 mug/kg s.c. once a da y for 3 days), restored penile erection while estradiol benzoate (I 0 mug/kg s.c. once a day for 3 days) restored yawning induced by both co mpounds. 5-Dihydrotestosterone (DHT) or progesterone (each at a dose o f 100 mug/kg s.c. once a day for 3 days) were ineffective. Given toget her, estradiol benzoate and DHT partially restored apomorphine- and ox ytocin-induced yawning and penile erection, whereas estradiol benzoate and progesterone restored only yawning. Estradiol benzoate-induced re covery of yawning was prevented by the antiestrogen tamoxifen (1 mg/kg s.c. once a day for 3 days). In intact rats, progesterone increased a nd estradiol benzoate decreased apomorphine- and oxytocin-induced yawn ing without modifying penile erection, although oxytocin-induced yawni ng was prevented much less by estradiol benzoate than that induced by apomorphine. Testosterone or DHT were ineffective on both responses. E stradiol benzoate inhibition of apomorphine- and oxytocin-induced yawn ing was prevented by tamoxifen, which per se failed to modify apomorph ine and oxytocin responses, as well as by testosterone or progesterone . The present results suggest that apomorphine- and oxytocin-induced p enile erection and yawning are endocrine-dependent and differentially modulated by sexual steroids, suggesting that the mechanisms controlli ng the two behaviors are different even though they are often associat ed.