Dynorphin activation of kappa opioid receptor reduces neuronal excitability in the paraventricular nucleus of mouse thalamus

It has been reported that kappa opioid receptor (KOR) is expressed in the paraventricular nucleus of thalamus (PVT), a brain region associated with arousal, drug reward and stress. Although intra-PVT infusion of KOR agonist was found to inhibit drug-seeking behavior, it is still unclear whether endogenous KOR agonists directly regulate PVT neuron activity. Here, we investigated the effect of the endogenous KOR agonist dynorphin-A (Dyn-A) on the excitability of mouse PVT neurons at different developmental ages. We found Dyn-A strongly inhibited PVT neurons through a direct postsynaptic hyperpolarization. Under voltage-clamp configuration, Dyn-A evoked an obvious outward current in majority of neurons tested in anterior PVT (aPVT) but only in minority of neurons in posterior PVT (pPVT). The Dyn-A current was abolished by KOR antagonist nor-BNI, Ba2+ and non-hydrolyzable GDP analogue GDP-β-s, indicating that Dyn-A activates KOR and opens G-protein-coupled inwardly rectifying potassium channels in PVT neurons. More interestingly, by comparing Dyn-A currents in aPVT neurons of mice at various ages, we found Dyn-A evoked significant larger current in aPVT neurons from mice around prepuberty and early puberty stage. In addition, KOR activation by Dyn-A didn't produce obvious desensitization, while mu opioid receptor (MOR) activation induced obvious desensitization of mu receptor itself and also heterologous desensitization of KOR in PVT neurons. Together, our findings indicate that Dyn-A activates KOR and inhibits aPVT neurons in mice at various ages especially around puberty, suggesting a possible role of KOR in regulating aPVT-related brain function including stress response and drug-seeking behavior during adolescence.