G protein-coupled receptor kinase 2 mediates mu-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 mu-opioid receptor agonists including morphine partially results from activating the NRM-spinal cord pathway. It is generally believed that mu-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 mu-opioid receptor agonist, selectively hyperpolarizes NRM neurons expressing mRNA of glutamate decarboxylase (GAD(67)). Homologous desensitization of mu-opioid receptors in NRM neurons could result in the development of morphine-induced tolerance. G protein-coupled receptor kinase (GRK) is believed to mediate mu-opioid receptor desensitization in vivo. Therefore, we also investigated the involvement of GRK in mediating homologous desensitization of DAMAMGO-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(betagamma)-binding domain of GRK and inhibits G(betagamma) 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 mu-opioid receptors in NRM GABAergic neurons.
Li
A H
AH
Department of Anesthesiology, Chang Gung Memorial Hospital, Taiwan, China Department of Physiology, Chang Gung University School of Medicine, Kwei-San, Tao-Yuan, Taiwan, China.
Wang
H L
HL
D000595:Amino Acid Sequence; D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D001665:Binding Sites; D017868:Cyclic AMP-Dependent Protein Kinases / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D004361:Drug Tolerance / Q000502:physiology; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D054769:G-Protein-Coupled Receptor Kinase 2; D054770:G-Protein-Coupled Receptor Kinase 3; D005968:Glutamate Decarboxylase / Q000096:biosynthesis; D017136:Ion Transport / Q000187:drug effects; D008564:Membrane Potentials / Q000187:drug effects; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000502:physiology; D009842:Oligopeptides / Q000138:chemical synthesis / Q000494:pharmacology; D010146:Pain / Q000503:physiopathology; D018408:Patch-Clamp Techniques; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000378:metabolism; D024661:Potassium Channels, Inwardly Rectifying; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis / Q000235:genetics; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000187:drug effects; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D011903:Raphe Nuclei / Q000187:drug effects / Q000201:enzymology / Q000502:physiology* / Q000503:physiopathology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects; D013116:Spinal Cord / Q000503:physiopathology; D051552:beta-Adrenergic Receptor Kinases; D005680:gamma-Aminobutyric Acid / Q000502:physiology*
D000701:Analgesics, Opioid; D009419:Nerve Tissue Proteins; D009842:Oligopeptides; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D012333:RNA, Messenger; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D005680:gamma-Aminobutyric Acid; D017346:Protein Serine-Threonine Kinases; D017868:Cyclic AMP-Dependent Protein Kinases; D054770:G-Protein-Coupled Receptor Kinase 3; C496569:Grk2 protein, rat; C517851:Grk3 protein, rat; D051552:beta-Adrenergic Receptor Kinases; D054769:G-Protein-Coupled Receptor Kinase 2; D005968:Glutamate Decarboxylase; D011188:Potassium