Evidence that apomorphine induces penile erection and yawning by releasing oxytocin in the central nervous system.

Oxytocin (10 and 30 ng) injected into a lateral ventricle (i.c.v.) or the dopamine agonist apomorphine (40 and 80 micrograms/kg) injected subcutaneously induced repeated episodes of penile erection and yawning in male rats. The concomitant administration of the two substances did not produce any further increase in the number of penile erection and yawning episodes. Penile erection and yawning induced by either oxytocin or apomorphine were antagonized in a dose-dependent manner by i.c.v. pretreatment with the oxytocin antagonists [d(CH2)5Tyr(Me)-Orn8]vasotocin, [Pen1,Phe(Me)2,Thr4,Orn8]oxytocin and [d(CH2)5Tyr(Me)-Arg8]vasopressin, with a rank order of potency that follows their antioxytocic activity. (i.e. [d(CH2)5Tyr(Me)-Orn8]vasotocin congruent to [Pen1,Phe(Me)2,Thr4,Orn8]-oxytocin greater than [d(CH2)5Tyr(Me)-Arg8]vasopressin). The results suggest that apomorphine induces penile erection and yawning by releasing oxytocin in the central nervous system.