Spinal opiates affect sexual behaviour in rats

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Abstract
Enkephalin-containing cells in the substantia gelatinosa of the spinal cord1 participate in the neural processing of painful stimuli2,3. Spinal opiates may also be involved in the perception of sensory stimuli related to reproduction, as probing of the uterine cervix stimulates sexual behaviour in rats4 and also induces an analgesia5 which is partially dependent on opiate receptor mechanisms in the spinal cord6–8. We now report that the intrathecal injection of morphine into female ovariectomized rats pretreated with oestradiol benzoate and progesterone inhibits sexual receptivity while injection of the opiate receptor antagonist naloxone enhances it. Similarly, intrathecal injection of morphine increases while injection of naloxone decreases the number of intromissions before ejaculation in male rat. Intraperitoneal injection of morphine or naloxone has no behavioural effect. Recent studies have suggested that brain opiates are involved in the control of sexual behaviour9–11. The results reported here indicate that sexual behaviour may also be influenced by spinal opiates.