Studies of μ-, κ-, and δ-opioid receptor density and G protein activation in the cortex and thalamus of monkeys

The aim of this study was to investigate the relative density of μ-, κ-, and δ-opioid receptors (MOR, KOR, and DOR) and guanosine 5′-O-(3-[³⁵S]thio)triphosphate ([³⁵S]GTPγS) binding stimulated by full agonists in cortical and thalamic membranes of monkeys. The binding parameters [B_max (femtomoles per milligram)/K_d (nanomolar)] were as follows: [³H][D-Ala², N-Me-Phe⁴,Gly⁵-ol]-enkephalin (DAMGO) (MOR; 80/0.7), [³H]U69593 [(5α,7α,8β)-(-)-N-methyl-N-(7-(1-pyrrolidinyl)-1- oxaspiro(4,5)dec-8-yl) benzeneacetamide] (KOR; 116/1.3), and [³H][D-Pen²,D-Pen⁵]-enkephalin (DPDPE) (DOR; 87/1.3) in the cortex; [³H]DAMGO (147/0.9), [³H]U69593 (75/2.5), and [³H]DPDPE (22/2.0) in the thalamus. The relative proportions of MOR, KOR, and DOR in the cortex were 28, 41, and 31% and in the thalamus were 60, 31, and 9%. Full selective opioid agonists, DAMGO (EC₅₀ = 532-565 nM) and U69593 (EC₅₀ = 80-109 nM) stimulated [³⁵S]GTPγS binding in membranes of cortex and thalamus, whereas SNC80 [(+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)- 3-methoxy-benzyl]-N,N-diethyl-benzamide] (DOR; EC₅₀ = 68 nM) was only active in cortical membranes. The magnitudes of [³⁵S]GTPγS binding stimulated by these agonists were similar in the cortex, ranging from 17 to 25% over basal binding. In the thalamus, DAMGO and U69593 increased [³⁵S] GTPγS binding by 44 and 23% over basal, respectively. Opioid agonist-stimulated [³⁵S]GTPγS binding was blocked selectively by antagonists for MOR, KOR, and DOR. The amount of G protein activated by agonists was highly proportional to the relative receptor densities in both regions. These results distinguish the ability of opioid agonists to activate G proteins and provide a functional correlate of ligand-binding experiments in the monkey brain. In particular, the relative densities of opioid receptor binding sites in the two brain areas reflect their functional roles in the pharmacological actions of opioids in the central nervous system of primates.