INTERACTIONS BETWEEN TRANSIENT RECEPTOR POTENTIAL (TRP) CHANNELS AND THE MU OPIOID RECEPTOR

The sensation of high temperature, thermal pain is substantially mediated by transient receptor potential cation channel subfamily V member 1 (TRPV1), while the sensation of cool temperature is transduced via transient receptor potential cation channel subfamily M member 8 (TRPM8). Pharmacologic relief from pain has long been accomplished using opiates, which principally act via the mu opioid receptor (MOR-1). We found important functional interactions between MOR-1 and both of these thermoreceptors. When activated prior to the addition of opiate, TRPV1 can block opiate-induced phosphorylation of MOR-1 at a functionally important, cytoplasmic serine (S377). This effect is blocked by TRPV1 antagonists and is dependent on extracellular calcium. Phosphorylation inhibition is selective; we demonstrate via incorporation of 35S GTPγS that TRPV1 does not alter G-protein activation. We further show that inhibition of phosphorylation blocks downstream effects of phosphorylation, namely clathrin-mediated endocytosis. Calcium flux through TRPV1 works by removing G-protein receptor kinase 5 (GRK5), the kinase responsible for phosphorylating MOR-1 at serine 377, from its physiologic localization at the plasma membrane and causes it to translocate to the nucleus. Further, we show that GRK5 binds calmodulin in a calcium-dependent manner and is the only GRK family member to do so. MOR-1 and TRPM8 interact in a different manner. We found that genetic deletion of TRPM8 significantly reduces symptoms of morphine withdrawal in opiate-dependent mice. Moreover, we show that MOR-1 physiologically inhibits TRPM8 activity in opiate-naïve mice by demonstrating that the opiate antagonist, naloxone, can strongly potentiate the effect of the TRPM8 agonist, icilin. We confirm the specificity of our drug effects using genetically modified mice lacking TRPM8 and MOR-1. Taken together, these data demonstrate specific and functionally important interactions between MOR-1 and both TRPV1 and TRPM8