The characteristics of adrenoceptors in homogenates of human cerebral cortex labelled by (3H)-rauwolscine.

The alpha-2-adrenoceptor antagonist (3H)-rauwolscine has been used to label adrenoceptors in membranes from human cerebral cortex. The radioligand binds with high affinity (KD 2.08 nM) to a single population of sites with a density of 135 fmoles/mg protein. Adrenoceptor antagonists displaced binding in a simple monomolecular fashion with an order of affinity rauwolscine greater than yohimbine greater than phentolamine greater than corynanthine greater than prazosin indicating binding to alpha-2-adrenoceptors. Agonists displaced with an order of affinity clonidine greater than (-) adrenaline greater than (-) noradrenaline greater than dopamine greater than (-) isoprenaline but all displayed apparent Hill coefficients less than unity indicating heterogeneity of binding. The relatively high affinity of the alpha-1 antagonist prazosin for (3H)-rauwolscine binding sites in rat cerebral cortex was not observed in the human tissue which had pharmacological properties similar to those described previously in human platelet.