Oxytocin: an extremely potent inducer of penile erection and yawning in male rats.

The intracerebroventricular (i.c.v.) injection of oxytocin, in doses ranging from 5 to 90 ng (5-90 pmol) induced penile erection and yawning in male rats. Such response was not induced by doses of the peptide higher than 100 ng, nor by equimolar doses of i.c.v. [Arg8]vasopressin, ACTH-(1-24), alpha-MSH, rat corticotropin-releasing factor (rCRF), delta sleep-inducing peptide, neurotensin or substance P. Oxytocin-induced penile erection and yawning were prevented by atropine and morphine, but not by methylatropine or the opiate antagonist naloxone. Haloperidol, a dopamine receptor antagonist, was ineffective at low doses; it partially prevented penile erection but not yawning at high doses. Since oxytocin is present not only in the neurohypophysis but also in other brain areas, our results suggest that oxytocin is implicated in the regulation of penile erection and yawning, and provide further evidence that oxytocin acts as a neuropeptide in the central nervous system.