Endogenous oxytocin and intermale interactions after oxytocin administrations in Norway rats selected for behavior.

The neuropeptide oxytocin (OT) secreted by specialized neurons in the hypothalamus affects social behavior and aggression in various animal species in a dose-dependent manner. Our earlier studies showed that OT administration by nasal application to adult and adolescent Norway rat males selected for enhanced aggressive response to humans reduced aggression upon the opponent in the resident-intruder test. By contrast, OT administration to rats selected for tame behavior exerted no effect on behavior or even enhanced aggression. It was still unknown how selection for behavior affected the endogenous oxytocinergic system in rats. Here we study the populations of OT-containing cells in the paraventricular and supraoptic nuclei of the hypothalamus in intact tame and aggressive rats with regard to lateralization, as the hypothalamus is known to be functionally asymmetrical. We have also assessed blood OT changes after nasal OT application to rats selected for behavior. As it is known that the effect of OT on rat aggressiveness may depend on the basal level of the latter, we have analyzed the effect of OT administration on behavior in tame and aggressive rats interacting on neutral ground, where the aggressiveness of males manifests itself less than in the defense of territory in the resident-intruder test. The asymmetry in the numbers of OT-containing cells in the left and right halves of the paraventricular and supraoptic nuclei has been observed only in tame rats. The number of such cells in the right half of tame rats is greater than in aggressive. In contrast, the blood OT level in tame rats is significantly lower than in aggressive ones both in the intact animals and after OT administration. Oxytocin administration to aggressive rats shortens aggressive interactions and lateral threats and reduces the number of the latter as compared to animals of the same behavior pattern having received saline. This observation may point to an anti-aggressive effect of OT. In tame rats, though, OT administration increases the number of hind leg kicks and kicking duration. It appears that the differences in the endogenous OTergic system of hypothalamus found in this study are associated with both the behavior formed during selection and different responses to exogenous OT in tame and aggressive animals.