Neonatal exposure to naltrexone affects morphine sensitivity and facilitates sexual behaviour in female rats.

The female copulatory behaviour (lordotic response) is inhibited by morphine. Female rats treated neonatally with naltrexone displayed enhanced copulatory behaviour as adults, and the morphine-induced lordosis inhibitory effect was diminished. The present data suggest that in the female rat the opioid mechanisms involved in the copulatory behaviour are established neonatally and can be influenced by early manipulation of the opioid system.