delta-Opioid modulation of social interactions in juvenile mice weaned at different ages.

The environmental stimulus of weaning has been shown to affect both the developmental expression of social behavior and the maturation of the opioid delta-receptors' subpopulation in altricial rodents. The aim of this study was to address both these issues by using the social interaction paradigm. Separate groups of male and female mice were randomly assigned to three different weaning ages -- early (Wean-15), regular (Wean-20), and delayed (Wean-25) -- and assessed when 30 days old under intraperitoneal administration of the selective delta-opioid agonist SNC80 (0, 0.1, or 0.3 mg/kg). Wean-15 male and female subjects were much more involved in investigating the partner as well as the cage environment compared to the regular Wean-20 group. An increased social investigation was also found as a consequence of delayed weaning in the female group. The neurobehavioral changes induced by the manipulation of weaning age were also reflected in an altered responsivity to the effects of SNC80 administration. The drug-induced increase in the expression of investigative and affiliative social interactions was further magnified by early weaning. A delayed weaning time was instead associated with reduced sensitivity to the drug, which suggests a delayed maturation of the system. As a whole, the present results indicate that the time of weaning is able to markedly affect the expression of social interactions of adolescent mice by possibly exerting a direct modulatory role on the development of the still plastic delta-opioid system.