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De Souza et al.

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Oxytocin, when administered centrally, has been associated with the modulation of various social initiatives including maternal and sexual behaviors. The nature of these effects depends on gonadal hormone status. In the present experiments, we investigated the effects of centrally administered oxytocin on the behavior of pair-housed male squirrel monkeys during interactions with a familiar female monkey. Pairs of male squirrel monkeys established reliable and persistent dominance relationships with dominant males showing increased sexual and aggressive behavior as well as higher plasma concentrations of testosterone. Oxytocin (0.1, 1.0 micrograms) increased the sexual and aggressive behavior of dominant monkeys without affecting these measures in the subordinate monkeys. In contrast to these effects in the dominant monkeys, oxytocin increased associative and marking behaviors only in subordinate monkeys. Central administration of the oxytocin receptor antagonis d(CH2)5 [Tyr(Me)2, Thr4,Tyr-NH2(9)] OVT (OTA; 0.05 microgram) had no intrinsic effect on behavior but blocked the effects of exogenous oxytocin. To investigate further the specificity of oxytocin's effects on social behavior, we administered the structurally related peptide arginine vasopressin under identical conditions. Vasopressin (0.5, 5.0 micrograms) decreased social behaviors and increased motor activity in both dominant and subordinate monkeys. Previous studies in rodents have demonstrated that oxytocin receptors are induced by gonadal steroids in a regionally specific fashion. The status-related behavioral effects of oxytocin in the squirrel monkey may reflect differences in brain oxytocin receptor density associated with the higher concentrations of testosterone in the dominant animal. Alternatively, the status-related effects may depend on the conditioned behavioral differences associated with social organization.