μ-opioid receptor and α₂-adrenoceptor agonist stimulation of [³⁵S]GTPγS binding to G-proteins in postmortem brains of opioid addicts

Repeated opioid administration has been associated in human brain with unaltered density of μ-opioid receptors (agonist radioligand binding sites and immunodetected receptor protein). These receptors are coupled to G(i)G(o)-proteins, which are increased in brain of heroin addicts. To assess the activity of G-proteins and their coupling to receptors after chronic opioid abuse, [³⁵S]GTPγS binding was quantified in postmortem prefrontal cortices of 15 opioid-dependent subjects and 15 matched controls. The stimulation of [³⁵S]GTPγS binding by the μ-opioid receptor agonist DAMGO or the α₂-adrenoceptor agonist UK14304 was used as a functional measure of the status of the receptor-G-protein coupling. [³⁵S]GTPγS binding basal values were similar in opioid addicts (819 ± 83 fmol mg^-1 of protein) and controls (918 ± 106 fmol mg^-1 of protein). In opioid addicts, [³⁵S]GTPγS binding stimulation by DAMGO showed a maximal effect (62 ± 8%) and a potency (EC₅₀ = 1.09 ± 0.26 μM) that did not differ from the maximal effect (60 ± 12%) and potency (EC₅₀ = 2.01 ± 0.58 μM) in controls. In opioid addicts, [³⁵S]GTPγS binding stimulation by UK14304 was not different in maximal effect (28 ± 3%) from controls (32 ± 8%), but the potency of the agonist was decreased (EC₅₀ = 4.36 ± 1.81 μM) when compared with controls (EC₅₀ = 0.41 ± 0.15 μM). The results provide a direct evidence of an apparent normal functional activity of brain μ-opioid receptors (G(i)/G(o)-protein coupling) during the opioid dependence process in humans. The data also demonstrate a functional uncoupling of α₂-adrenoceptors from G-proteins, which indicates a heterologous desensitization of these receptors, This finding could represent an adaptive mechanism against the decreased noradrenergic activity induced by the chronic presence of opioid drugs.