Adolescent forced swim stress increases social anxiety-like behaviors and alters the dynorphin/kappa opioid receptor system in the basolateral amygdala of males

Adolescence is a developmental period marked by robust neural alterations and heightened vulnerability to stress, a factor that is highly associated with increased risk for emotional processing deficits, such as anxiety. Stress-induced upregulation of the dynorphin/kappa opioid receptor (DYN/KOR) system is thought to, in part, underlie the negative affect associated with stress. The basolateral amygdala (BLA) is a key structure involved in anxiety, and neuromodulatory systems, such as the DYN/KOR system, can 1) regulate BLA neural activity in an age-dependent manner in stress-naïve animals and 2) underlie stress-induced anxiety in adults. However, the role of the DYN/KOR system in modulating stress-induced anxiety in adolescents is unknown. To test this, we examined the impact of an acute, 2-day forced swim stress (FSS – 10 min each day) on adolescent (~postnatal day (P) 35) and adult Sprague-Dawley rats (~P70), followed by behavioral, molecular and electrophysiological assessment 24 hours following FSS. Adolescent males, but not adult males or females of either age, demonstrated social anxiety-like behavioral alterations indexed via significantly reduced social investigation and preference when tested 24 hours following FSS. Conversely, adult males exhibited increased social preference. While there were no FSS-induced changes in expression of genes related to the DYN/KOR system in the BLA, these behavioral alterations were associated with a robust switch in BLA KOR function. Specifically, while the KOR agonist, U69593, significantly increased GABA transmission in the BLA of non-stressed adolescent males, U69593 significantly inhibited BLA GABA transmission in stressed adolescent males, consistent with the observed anxiogenic phenotype in stressed adolescent males. This is the first study to demonstrate a KOR-dependent mechanism that may contribute to stress-induced social anxiety in adolescent males. Importantly, these findings provide evidence for potential KOR-dependent mechanisms that may contribute to pathophysiological interactions with subsequent stress challenges.