Girk Channel-mediated Inhibition of Melanin-

36 Targeting the melanin-concentrating hormone (MCH) system has been suggested as a 37 potential treatment for obesity, anxiety disorders as well as addiction. Despite the therapeutic 38 potential of MCH agonists and antagonists, the endogenous factors regulating MCH activity, in 39 particular those implicated in anxiety and reward, are ill-defined. The present study investigated 40 the cellular effects of nociceptin/orphanin FQ (N/OFQ), an endogenous opioid with anxiolytic 41 and anti-reward properties, on MCH neurons. We found that N/OFQ induced a concentration-42 dependent reversible outward current in MCH neurons (EC 50 = 50.7 nM), an effect that was 43 blocked by the competitive antagonist of the nociceptin opioid peptide (NOP) receptor UFP-101. 44 N/OFQ-induced outward currents persisted in TTX, reversed near the potassium equilibrium 45 potential and displayed inward rectification, suggesting direct postsynaptic potassium channel 46 activation. Tertiapin Q completely abolished the N/OFQ effect while glibenclamide did not, 47 implicating G-protein dependent inwardly rectifying potassium (GIRK) and not ATP-sensitive 48 potassium (KATP) channels as the effector ion channel. The N/OFQ-induced outward current 49 desensitized during repeated applications and occluded the inhibitory effect of dynorphin, 50 suggesting that dynorphin and N/OFQ activate the same pathway. N/OFQ also reversibly 51 inhibited voltage-gated calcium currents in MCH neurons. In conclusion, our study indicates 52 N/OFQ as a robust endogenous regulator of MCH neurons, which may play a role in anxiety and 53 drug addiction. 54 55