Relationship of mu opioid receptor binding to activation of G-proteins in specific rat brain regions.
This study investigated the relationship between mu receptor binding and mu agonist activation of G-proteins in the rat brain. To directly compare agonist potencies in receptor binding (K(i) values) and G-protein activation (K(s) values), both agonist-stimulated [(35)S]guanosine-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) and [(3)H]naloxone binding assays were conducted under identical conditions, using the full mu agonist [d-Ala(2), N-Me(4), Gly(5)-ol]-enkephalin (DAMGO). DAMGO exhibited biphasic competition of [(3)H]naloxone binding and stimulation of [(35)S]GTPgammaS binding in most regions. Whereas the high-affinity component represented a large percentage (50-80%) of total receptor sites, the high-affinity component of DAMGO-stimulated [(35)S]GTPgammaS binding was much lower, <30% of the total, and in most regions significant stimulation of [(35)S]GTPgammaS binding did not occur until the high-affinity binding sites were completely occupied. Moreover, the low-affinity potencies for DAMGO in receptor binding and G-protein activation were the same across different regions. Receptor-transducer amplification factors were calculated by the ratio of the apparent B(max) of net agonist-stimulated [(35)S]GTPgammaS binding to the B(max) of receptor binding. Amplification factors for the nine regions examined were relatively high and varied significantly across regions, from a ratio of 8 in the thalamus to 38 in the cortex, suggesting that the efficiency of mu opioid receptor coupling to G-proteins varies across brain regions.
Maher
C E
CE
Department of Physiology and Pharmacology, Center for the Neurobiological Investigation of Drug Abuse, Wake Forest University School of Medicine, Winston-Salem, NC 27157, USA.
Selley
D E
DE
Childers
S R
SR
D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000378:metabolism*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D013462:Sulfur Radioisotopes; D014316:Tritium
D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D017450:Receptors, Opioid, mu; D013462:Sulfur Radioisotopes; D014316:Tritium; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D009270:Naloxone; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D019204:GTP-Binding Proteins