Naloxone reverses post-ejaculatory inhibition of sexual behaviour in female rats

Sexual receptivity was inhibited in ovariectomized rats treated with oestradiol benzoate (OB: two injections of 2 micrograms) and progesterone (0.5 mg) immediately after ejaculation by the male and restored after the end of the post-ejaculatory refractory period in the male. The post-ejaculatory inhibition of sexual receptivity was reversed by i.p. (5 mg), intracerebroventricular (50 micrograms) or intrathecal (50 micrograms) injection of the opioid peptide receptor antagonist naloxone. The concentration of serum beta-endorphin-like immunoreactivity in ovariectomized rats treated with OB plus progesterone was unaltered by sexual interactions with males (18.3 +/- 6.0 (S.E.M.), 26.4 +/- 2.1 and 21.8 +/- 6.1 pmol/l before sexual activity, after ejaculation and after the end of the post-ejaculatory interval) but reduced to non-detectable by hypophysectomy. Subcutaneous injection of 10 micrograms beta-endorphin raised serum concentrations of beta-endorphin-like immunoreactivity but did not affect the display of sexual behaviour. The behaviour was also unaffected by intracerebroventricular injection of 0.1, 0.2 or 1.0 microgram beta-endorphin or by injections of 0.25 microgram beta-endorphin in the periaqueductal central grey of the mesencephalon. The results show that ejaculation by male rats causes a transient inhibition of sexual receptivity in the female which may be dependent upon opioid peptide receptor mechanisms in the brain and spinal cord. It is unlikely that the peptide is beta-endorphin.