mu-opioid receptor and alpha(2)-adrenoceptor agonist stimulation of [S-35]GTP gamma S binding to G-proteins in postmortem brains of opioid addicts

Repeated opioid administration has been associated in human brain with unal tered 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 ac tivity of G-proteins and their coupling to receptors after chronic opioid a buse, [S-35]GTP gamma S binding was quantified in postmortem prefrontal cor tices of 15 opioid-dependent subjects and 15 matched controls. The stimulat ion 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 bindin g basal values were similar in opioid addicts (819 +/- 83 fmol mg(-1) of pr otein) 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 (6 2 +/- 8%) and a potency (EC50 = 1.09 +/- 0.26 mu M) that did not differ fro m the maximal effect (60 +/- 12%) and potency (EC50 = 2.01 +/- 0.58 mu M) i n controls. In opioid addicts, [S-35]GTP gamma S binding stimulation by UK1 4304 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 pro vide a direct evidence of an apparent normal functional activity of brain m u-opioid receptors (G(i)/G(o)-protein coupling) during the opioid dependenc e process in humans. The data also demonstrate a functional uncoupling of a lpha(2)-adrenoceptors from G-proteins, which indicates a heterologous desen sitization of these receptors. This finding could represent an adaptive mec hanism against the decreased noradrenergic activity induced by the chronic presence of opioid drugs.