Effects of 8-OH-DPAT on sexual behavior of male rats castrated at different ages.

The effects of the 5-hydroxytryptamine (5-HT) 1A receptor agonist, 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), on the sexual behavior of male rats castrated at adulthood or during the early postnatal critical period for brain differentiation are presented. In adult castrated males, two distinct groups of animals, divided according to their copulatory activity impairment following castration, were identified: animals that reached a low intromission frequency around Day 60 after testes removal (early reduction group) and males that retained the behavior until approximately Postcastration Day 180 (late reduction group). 8-OH-DPAT (0.10 mg/kg) increased intromission frequency in both groups, but the effect was more pronounced in rats that had a late reduction of mating behavior after castration. In males castrated at 6 hr or 12 days after birth, an increase in intromission frequency, but not in ejaculatory behavior, was also detected following the injection of 8-OH-DPAT (0.10 or 0.25 mg/kg) at adulthood. In conclusion, 8-OH-DPAT was able to enhance intromission behavior, bypassing the lack of testosterone either at the neonatal critical period or at adult age. The 5-HT1A system would exert a parallel modulatory action, in addition to testosterone, on male sexual behavior. Alternatively, since neonatal castrated males showed no ejaculatory behavior after 8-OH-DPAT injection, 5-HT and testosterone would act in an interactive way for the full expression of copulatory behavior.