G protein-coupled receptor kinase 2 mediates μ-opioid receptor desensitization in GABAergic neurons of the nucleus raphe magnus

Nucleus raphe magnus (NRM) sends the projection to spinal dorsal horn and inhibits nociceptive transmission. Analgesic effect produced by μ-opioid receptor agonists including morphine partially results from activating the NRM-spinal cord pathway. It is generally believed that μ-opioid receptor agonists disinhibit spinally projecting neurons of the NRM and produce analgesia by hyperpolarizing GABAergic interneurons. In the present study, whole-cell patch-clamp recordings combined with single-cell RT-PCR analysis were used to test the hypothesis that DAMGO ([D-Ala ²,N-methyl-Phe ⁴, Gly-ol ⁵]enkephalin), a specific μ-opioid receptor agonist, selectively hyperpolarizes NRM neurons expressing mRNA of glutamate decarboxylase (GAD ₆₇). Homologous desensitization of μ-opioid receptors in NRM neurons could result in the development of morphine-induced tolerance. G protein-coupled receptor kinase (GRK) is believed to mediate μ-opioid receptor desensitization in vivo. Therefore, we also investigated the involvement of GRK in mediating homologous desensitization of DAMGO-induced electrophysiological effects on NRM neurons by using two experimental strategies. First, single-cell RT-PCR assay was used to study the expression of GRK2 and GRK3 mRNAs in individual DAMGO-responsive NRM neurons. Whole-cell recording was also performed with an internal solution containing the synthetic peptide, which corresponds to G _βγ-binding domain of GRK and inhibits G _βγ activation of GRK. Our results suggest that DAMGO selectively hyperpolarizes NRM GABAergic neurons by opening inwardly rectifying K ⁺ channels and that GRK2 mediates short-term homologous desensitization of μ-opioid receptors in NRM GABAergic neurons.