guanosine-triphosphate and Pain

Topics: Action Potentials; Animals; Ganglia, Spinal; GTP-Binding Proteins; Guanosine Triphosphate; Mice; Mice, Knockout; NAV1.9 Voltage-Gated Sodium Channel; Neuropeptides; Nociceptors; Pain; Sodium Channels

This paper reports that regulators of G-protein signalling (RGS) proteins modulate the timing and amplitude of opioid signals by a push-pull mechanism. This is achieved without noticeable changes in the binding properties of opioids, e.g. beta-endorphin to mu-opioid receptors. The expression of RGS proteins was reduced by blocking their mRNA with antisense oligodeoxynucleotides (ODN). Knock down of RGS2 or RGS3 diminished morphine and beta-endorphin analgesia, whereas that of RGS9 or RGS12 enhanced this activity. In mice with impaired RGS9, but not impaired RGS2, the potency and, in particular, the duration of opioid antinociception increased. Further, the animals did not exhibit acute tolerance generated by a single and efficacious dose of morphine, nor did they develop tolerance after a daily i.c.v. injection of the opioid for 4 days. In a model of sustained morphine treatment, the impairment of RGS9 proteins facilitated increases in the response to the delivered opioid. This was only effective for 2--3 h after the subcutaneous implantation of an oily morphine pellet; later, tolerance developed. To reduce the impact of the chronic morphine acting on opioid receptors, other RGS proteins presumably substitute the GTPase-activating function of RGS9 on morphine-activated G-alpha-GTP subunits. The desensitization of mu-opioid receptors appears to be a cell membrane-limited process facilitated by RGS9's sequestering of agonist-segregated G alpha subunits.

Topics: Analgesia; Analgesics, Opioid; Animals; beta-Endorphin; Brain Chemistry; Drug Implants; Gene Targeting; GTP Phosphohydrolases; GTP-Binding Proteins; GTPase-Activating Proteins; Guanosine Triphosphate; Heterotrimeric GTP-Binding Proteins; Humans; Injections, Intraventricular; Macromolecular Substances; Male; Mice; Models, Neurological; Morphine; Nerve Tissue Proteins; Oligodeoxyribonucleotides, Antisense; Pain; Protein Binding; Receptors, Opioid, mu; Reverse Transcriptase Polymerase Chain Reaction; RGS Proteins; RNA, Messenger; Signal Transduction; Tachyphylaxis

Previous studies have shown that chronic i.v. treatment with morphine or heroin decreased mu opioid receptor activation of G-proteins in specific brain regions. The present study examined the effect of intrathecal (i.t.) morphine administration on receptor/G-protein coupling in the spinal cord. In spinal cord membranes, [35S]GTP gamma S binding was stimulated by agonists of several G-protein-coupled receptors, including mu opioid (DAMGO), delta opioid (DPDPE), GABA(B) (baclofen), cannabinoid CB(1) (WIN 55,212-2), muscarinic cholinergic (carbachol) and adenosine A(1) (PIA). [35S]GTP gamma S autoradiography revealed that most of this agonist activation of G-proteins was localized to laminae I and II of dorsal horn. To determine the effects of chronic morphine on these receptor activities, rats were treated for 7 days with 0.11 mg/kg/day i.t. morphine, and receptor activation of G-proteins was determined by [35S]GTP gamma S autoradiography of brain and spinal cord. In spinal cord sections, chronic morphine treatment decreased DAMGO-stimulated [35S]GTP gamma S binding in laminae I and II at all levels of spinal cord examined. There were no effects of morphine treatment on [35S]GTP gamma S stimulation in spinal cord by other receptor systems examined (Adenosine A(1) and GABA(B)), and no significant effects of chronic i.t. morphine treatment were observed in brain sections. These data show that homologous desensitization of mu receptor/G-protein coupling occurs specifically in spinal cord following chronic morphine administration.

Topics: Analgesics, Opioid; Animals; Binding Sites; Drug Administration Schedule; Drug Tolerance; GTP-Binding Proteins; Guanosine Triphosphate; Injections, Spinal; Male; Morphine; Pain; Pain Threshold; Posterior Horn Cells; Radioligand Assay; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu; Sulfur Radioisotopes

Topics: Adenosine Triphosphate; Animals; Cells, Cultured; Ethanol; Guanosine Triphosphate; Membrane Potentials; Neurons, Afferent; Pain; Parabens; Patch-Clamp Techniques; PC12 Cells; Propylene Glycols; Rats; Rats, Wistar

The neurochemical and functional correlates of opioid receptor up-regulation after chronic antagonist administration in vivo and of down-regulation after withdrawal of antagonist were examined. Total brain opioid receptors increased 1.9-fold by day 8 of naltrexone administration, after which no further increase was observed; the newly synthesized or unmasked receptors exhibited an enhanced sensitivity to guanyl nucleotide modulation. Withdrawal from chronic naltrexone treatment resulted in a return to nearly control levels of receptor density and guanyl nucleotide sensitivity in a period of 6 days. These results suggest that up-regulation is accompanied by an increased coupling of the receptors to the inhibitory guanyl nucleotide binding protein (Ni) and that down-regulation involves the dissociation of the receptor/Ni complex. In experiments designed to target opiate receptor subtypes, long-term treatment with naltrexone was found to produce a coordinated up-regulation of brain mu and delta receptors, but did not cause a significant change in the density or affinity of kappa or sigma receptors. These findings indicate that the kappa and sigma opiate receptor classes may be subject to independent control mechanisms. Chronic naltrexone treatment also resulted in an enhanced morphine-induced analgesia. This result indicates that a functional supersensitivity occurs as a result of the selective up-regulation of mu and delta receptors. After withdrawal from naltrexone, supersensitivity to morphine-induced analgesia decreased monotonically and, in parallel to opioid receptor density, to prenaltrexone treatment levels within 6 days. Together, these results suggest a functional significance for antagonist-induced mu and delta opiate receptor up-regulation.

Topics: Animals; Brain Chemistry; Guanosine Triphosphate; Male; Morphine; Naloxone; Naltrexone; Pain; Rats; Rats, Inbred Strains; Receptors, Opioid; Time Factors