Relationship of mu opioid receptor binding to activation of G-proteins in specific rat brain regions

Abstract

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 [ 35S]guanosine-5′- O-(γ-thio)-triphosphate ([ 35S]GTPγS) and [ 3H]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 [ 3H]naloxone binding and stimulation of [ 35S]GTPγS 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 [ 35S]GTPγS binding was much lower, <30% of the total, and in most regions significant stimulation of [ 35S]GTPγS 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 [ 35S]GTPγS 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.