Supersensitivity to mu-opioid receptor-mediated inhibition of the adenylyl cyclase pathway involves pertussis toxin-resistant Galpha protein subunits.
Sustained administration of opioids leads to antinociceptive tolerance, while prolonged association of L-type Ca2+ channel blockers (e.g. nimodipine) with opioids results in increased antinociceptive response. Herein, we investigated the changes in mu-opioid receptor signalling underlying this shift from analgesic tolerance to supersensitivity. Thus, the interaction of mu-opioid receptors with G proteins and adenylyl cyclase was examined in lumbar spinal cord segments of rats. In control animals, the mu-opioid selective agonists, sufentanil and DAMGO, stimulated [35S]5'-(gamma-thio)-triphosphate ([35S]GTP gamma S) binding and inhibited forskolin-stimulated adenylyl cyclase activity, through a mechanism involving pertussis toxin (PTX) sensitive G alpha(i/o) subunits. Seven days of chronic sufentanil treatment developed antinociceptive tolerance associated with a reduction in mu-agonist-induced [35S]GTP gamma S binding, mu-agonist-induced adenylyl cyclase inhibition, and co-precipitation of G alpha o, G alpha i2 G alpha z and G alpha q11 subunits with mu-opioid receptors. In contrast, combined nimodipine treatment with sufentanil over the same period increased the sufentanil analgesic response. This antinociceptive supersensitivity was accompanied by a significant increase of mu-agonist-induced inhibition of adenylyl cyclase that was resistant to the antagonism by PTX. In good agreement, co-precipitation of the PTX-resistant, G alpha z and G alpha q/11 subunits with mu-opioid receptors was not lowered. On the other hand, the PTX-sensitive subunits, G alpha i2 and G alpha o, as well as agonist-stimulated [35S]GTP gamma S binding were still reduced. Our results demonstrate that mu-opioid analgesic tolerance follows uncoupling of spinal mu-opioid receptors from their G proteins and linked effector pathways. Conversely, the enhanced analgesic response following combined nimodipine treatment with sufentanil is associated with adenylyl cyclase supersensitivity to the opioid inhibitory effect through a mechanism involving PTX-resistant G protein subunits.
Mostany
Ricardo
R
Departamento de Fisiología y Farmacología, Facultad de Medicina, Universidad de Cantabria, Av. Herrera Oria s/n, E-39011 Santander, Spain.
Díaz
Alvaro
A
Valdizán
Elsa M
EM
Rodríguez-Muñoz
María
M
Garzón
Javier
J
Hurlé
María A
MA
D000067956:Adenylyl Cyclase Inhibitors; D000701:Analgesics, Opioid / Q000008:administration & dosage / Q000494:pharmacology; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D003627:Data Interpretation, Statistical; D004361:Drug Tolerance; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D047468:Immunoprecipitation; D007262:Infusions, Intravenous; D008297:Male; D009553:Nimodipine / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects; D017288:Pain Threshold / Q000187:drug effects; D037342:Pertussis Toxin / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D011930:Reaction Time / Q000187:drug effects; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000037:antagonists & inhibitors*; D015398:Signal Transduction / Q000187:drug effects*; D017409:Sufentanil / Q000008:administration & dosage / Q000494:pharmacology
D000067956:Adenylyl Cyclase Inhibitors; D000701:Analgesics, Opioid; D002121:Calcium Channel Blockers; D017450:Receptors, Opioid, mu; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D009553:Nimodipine; D017409:Sufentanil; D000242:Cyclic AMP; D037342:Pertussis Toxin; D019206:GTP-Binding Protein alpha Subunits, Gi-Go