Naloxone stimulates sexual behaviour in lactating rats

ABSTRACT During lactation the display of sexual receptivity in response to treatment with oestradiol benzoate (OB; 2 or 10 μg) and progesterone (0·5 mg) was inhibited, but the behaviour could be activated by i.p. (5 mg) or intracerebroventricular (i.c.v.; 100 μg) but not intrathecal (i.t.; 100 or 500 μg) injections of the opioid peptide receptor antagonist naloxone. The behaviour was also inhibited in ovariectomized rats in which serum progesterone and prolactin levels had been raised by treatment with progesterone implants and the dopamine receptor antagonist domperidone, and the uterine cervix had been stimulated. Intraperitoneal injections of naloxone (1 mg) reactivated the behaviour of cervically stimulated rats. The concentration of β-endorphin-like immunoreactivity in the serum of lactating rats (42·8± 9·2 pmol/l) was not raised above that of ovariectomized rats (35·8 ± 8·4 pmol/l) nor was the concentration of β-endorphin-like immunoreactivity altered in the pituitary gland (22·5 ± 2·5 pmol/l), midbrain central grey (6·3 ± 2·2 pmol/l) or hypothalamus (5·6± 2·6 pmol/l) of lactating rats in comparison with ovariectomized rats (24·8 ± 4·4, 4·0 ± 2·0 and 4·7 ± 1·4 pmol/l respectively). Adrenalectomy facilitated the display of sexual behaviour in lactating rats treated with OB plus progesterone and caused a slight increase in serum β-endorphin-like immunoreactivity (30·5± 2·7 pmol/l) compared with that in non-adrenalectomized lactating rats (26·1 ± 2·1 pmol/l). It is suggested that an opioid peptide, but probably not β-endorphin, inhibits sexual behaviour during lactation and after cervical stimulation. J. Endocr. (1987) 113, 423–427