Platelet alpha 2-adrenoceptors in heroin addicts during withdrawal and after treatment with clonidine.

The density of platelet alpha 2-adrenoceptors, quantitated by means of the binding of [3H]clonidine and [3H]yohimbine, and the aggregation response induced by adrenaline were investigated in thirty-two heroin addicts during spontaneous withdrawal. The number of binding sites for [3H]clonidine, but not for [3H]yohimbine, was significantly increased during withdrawal and the increase followed a time course related to the severity of the abstinence syndrome. There was a positive and significant correlation between the severity of withdrawal and the density of platelet alpha 2-adrenoceptors. Concomitantly, the adrenaline-induced platelet aggregation was potentiated. Treatment with clonidine led to significant decreases in receptor densities as well as in functional responses. These results suggest that only alpha 2-adrenoceptors in the agonist state (i.e. number of binding sites for [3H]clonidine) are modulated during the development of the heroin withdrawal syndrome.