Oxytocin-induced penile erection and yawning: site of action in the brain.

Microinjection of oxytocin into the paraventricular nucleus of the hypothalamus or into the CA1 field of the hippocampus induced a dose-dependent increase in the number of penile erection and yawning episodes in male rats. The minimal effective dose of oxytocin injected into the paraventricular nucleus was 3 ng. This dose induced the above-mentioned behaviors in 60% of the treated rats. Doses of 9 ng or higher induced the symptomatology in more than 85% of the animals. On the other hand, when the peptide was injected into the CA1 field of the hippocampus, 9 ng bilaterally were necessary to elicit penile erection and yawning in 62% of the rats. Arg8-vasopressin, which only differs from oxytocin in two amino acids, induced penile erection and yawning when injected either into the paraventricular nucleus or into the hippocampus, but was 5-10 times less potent than oxytocin. Oxytocin injection into the lateral septum, caudate nucleus, subiculum, preoptic area, ventromedial nucleus and supraoptic nucleus, was ineffective. The powerful effect of oxytocin on the induction of yawning and penile erection, suggests a physiological role of hypothalamic and hippocampal oxytocin in the regulation of such responses.