partial derivative-opioid modulation of social interactions in juvenile mice weaned at different ages

The environmental stimulus of weaning has been shown to affect both the dev elopmental expression of social behavior and the maturation of the opioid p artial derivative -receptors' subpopulation in altricial rodents. The aim o f this study was to address both these issues by using the social interacti on 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 adm inistration of the selective partial derivative -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 t he regular Wean-20 group. An increased social investigation was also found as a consequence of delayed weaning in the female group. The neurobehaviora l changes induced by the manipulation of weaning age were also reflected in an altered responsivity to the effects of SNC80 administration. The drug-i nduced increase in the expression of investigative and affiliative social i nteractions 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 indica te that the time of weaning is able to markedly affect the expression of so cial interactions of adolescent mice by possibly exerting a direct modulato ry role on the development of the still plastic partial derivative -opioid system. (C) 2001 Elsevier Science Inc. All rights reserved.