EFFECTS OF BETA-FUNALTREXAMINE TREATMENT AND SEXUAL ISOLATION IN THE PERINATAL-PERIOD ON THE DEVELOPMENT OF MU-OPIOID RECEPTORS AND NOCICEPTION

Male and female rats were segregated from birth (sexually isolated animals). Additional litters containing both male and female pups were kept as controls (mixed-housed animals). Beta-FNA (5 mg/kg) or water was administered SC at day 0 or day 7 to isolated and mixed-housed animals. The development of mu-opioid receptors and nociceptive responses in the two groups was asssessed at day 7 or 14, respectively. Mu-receptor binding was measured in whole brain using (H-3) DAGO as a binding ligand and nociception assessed using the tail immersion test. Beta-FNA treatment depressed mu-receptors when measured 1 but not 7 days later. However, male and female rats treated at day 0 with beta-FNA had lower brain protein content. Sexual isolation had little effect on mu-receptor number and did not augment the beta-FNA effect. However, isolation increased pain sensitivity in 7-day-old animals and in 14-day-old females. Beta-FNA treatment had little effect on nociceptive threshold but reversed the effects of sexual isolation.