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 mu-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, [S-35]GTP gamma S binding was quantified in postmortem prefrontal cortices of 15 opioid-dependent subjects and 15 matched controls. The stimulation of [S-35]GTP gamma S binding by the mu-opioid receptor agonist DAMGO or the alpha(2)-adrenoceptor agonist UK14304 was used as a functional measure of the status of the receptor-G-protein coupling, [S-35]GTP gamma S 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, [S-35]GTP gamma S binding stimulation by DAMGO showed a maximal effect (62 +/- 8%) and a potency (EC50 = 1.09 +/- 0.26 mu M) that did not differ from the maximal effect (60 +/- 12%) and potency (EC50 = 2.01 +/- 0.58 mu M) in controls. In opioid addicts, [S-35]GTP gamma S 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 mu M) when compared with controls (EC50 = 0.41 +/- 0.15 mu M). The results provide a direct evidence of an apparent normal functional activity of brain mu-opioid receptors (G(i)/G(o)-protein coupling) during the opioid dependence process in humans. The data also demonstrate a functional uncoupling of alpha(2)-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.