Contribution of nitric oxide-dependent guanylate cyclase and reactive oxygen species signaling pathways to desensitization of μ-opioid receptors in the rat locus coeruleus

Nitric oxide (NO) is involved in desensitization of mu-opioid receptors (MOR). We used extracellular recordings in vitro to unmask the NO-dependent pathways involved in MOR desensitization in the rat locus coeruleus (LC). Perfusion with ME (3 and 10 mu M) concentration-dependently reduced subsequent ME effect, indicative of MOR desensitization. ME (3 gM)-induced desensitization was enhanced by a NO donor (DEA/NO 100 mu M), two soluble guanylate cyclase (sGC) activators (A 350619 30 mu M and BAY 418543 1 mu M) or a cGMP-dependent protein kinase (PKG) activator (8-pCPT-cGMP 30 mu M). DEA/NO-induced enhancement was blocked by the sGC inhibitor NS 2028 (10 mu M). A 350619 effect was also blocked by NS 2028, but not by the antioxidant Trolox. ME (10 mu M)-induced desensitization was blocked by the neuronal NO synthase inhibitor 7-NI (100 mu M) and restored by the PKG activator 8-Br-cGMP (100 -300 mu M). Paradoxically, ME (10 mu M)-induced desensitization was not modified by sGC inhibitors (NS 2028 and ODQ), PKG inhibitors (H8 and Rp-8-Br-PET-cGMP) or antioxidant agents (Trolox, U-74389G and melatonin), but it was attenuated by a combination of NS 2028 and Trolox. In conclusion, MOR desensitization in the LC may be mediated or regulated by NO through sGC and reactive oxygen species signaling pathways. (C) 2015 Elsevier Ltd. All rights reserved.