Periaqueductal gray μ and κ opioid receptors determine behavioral selection from maternal to predatory behavior in lactating rats

Every mother must optimize her time between caring for her young and her subsistence. The rostro lateral portion of the periaqueductal grey (rlPAG) is a critical site that modulates the switch between maternal and predatory behavior. Opioids play multiple roles in both maternal behavior and this switching process. The present study used a pharmacological approach to evaluate the functional role of rlPAG mu and kappa opioid receptors in behavioral selection. Rat dams were implanted with a guide cannula in the rlPAG and divided into three experiments in which we tested the role of opioid agonists (Experiment 1), the influence of mu and kappa opioid receptor blockade in the presence of morphine (Experiment 2), and the influence of mu and kappa opioid receptor blockade (Experiment 3). After behavioral test, in Experiment 4, we evaluated rlPAG mu and kappa receptor activation in all Experiments 1-3. The results showed that massive opioidergic activation induced by morphine in the rlPAG inhibited maternal behavior without interfering with predatory hunting. No behavioral changes and no receptor activation were promoted by the specific agonist alone. However, kappa receptor blockade increased hunting behavior and increased the level of mu receptor activation in the rlPAG. Thus, endogenous opioidergic tone might be modulated by a functional interaction between opioid receptor subtypes. Such a compensatory receptor interaction appears to be relevant for behavioral selection among motivated behaviors. These findings indicate a role for multiple opioid receptor interactions in the modulation of behavioral selection between maternal and predatory behaviors in the PAG. (C) 2014 Elsevier B.V. All rights reserved.