Stress alters social behavior and sensitivity to pharmacological activation of kappa opioid receptors in an age-specific manner in Sprague Dawley rats

The dynorphin/kappa opioid receptor (DYN/KOR) system has been identified as a primary target of stress due to behavioral effects, such as dysphoria, aversion, and anxiety-like alterations that result from activation of this system. Numerous adaptations in the DYN/KOR system have also been identified in response to stress. However, whereas most studies examining the function of the DYN/KOR system have been conducted in adult rodents, there is growing evidence suggesting that this system is ontogenetically regulated. Likewise, the outcome of exposure to stress also differs across ontogeny. Based on these developmental similarities, the objective of this study was to systematically test effects of a selective KOR agonist, U-62066, on various aspects of social behavior across ontogeny in non-stressed male and female rats as well as in males and females with a prior history of repeated exposure to restraint (90 min/day, 5 exposures). We found that the social consequences of repeated restraint differed as a function of age: juvenile stress produced substantial increases in play fighting, whereas adolescent and adult stress resulted in decreases in social investigation and social preference. The KOR agonist U-62066 dose-dependently reduced social behaviors in non-stressed adults, producing social avoidance at the highest dose tested, while younger animals displayed reduced sensitivity to this socially suppressing effect of U-62066. Interestingly, in stressed animals, the socially suppressing effects of the KOR agonist were blunted at all ages, with juveniles and adolescents exhibiting increased social preference in response to certain doses of U-62066. Taken together, these findings support the hypothesis that the DYN/KOR system changes with age and differentially responds and adapts to stress across development.