PRENATAL NALTREXONE FACILITATES MALE SEXUAL-BEHAVIOR IN THE RAT

The involvement of endogenous opiates in the differentiation of sexual behavior was tested by exposing rat fetuses to continuous naltrexone during the last 9 days of gestation. Time-mated female rats received o ral naltrexone, 40 mg/kg/day, via their drinking water, from gestation al day 13 until parturition. Early motor development, measured by swim ming ability in 7-, 9-, and 11-day-old offspring of the treated dams, was unaffected by prenatal naltrexone. Adult male offspring were given three tests of male sexual behavior, then castrated, primed with ovar ian hormones, and given two tests of feminine receptivity (lordosis qu otient). Prenatal naltrexone facilitated masculine behavior and suppre ssed feminine receptivity: latencies to first mount and to ejaculation were shorter, mount rate was higher, and lordosis quotient was lower in naltrexone-treated rats, compared with control animals. These findi ngs implicate endogenous opiates in prenatal organization of sex-speci fic behavioral dispositions.