HUMAN BRAIN IMIDAZOLINE RECEPTORS - FURTHER CHARACTERIZATION WITH [H-3] CLONIDINE

The aim of the present study was to further characterize [H-3]clonidine binding in the ventrolateral medulla of the human brainstem, the region involved in the vasodepressor effect of imidazoline drugs of the clonidine type. Under basal conditions, [H-3]clonidine can bind both to the imidazoline receptors and to the ar-adrenoceptors. The latter represent only a small part of the total [H-3]clonidine binding with a B-max of 61 +/- 13 fmol/mg proteins and a K-D of 4.9 +/- 2.2 nM. Most of the binding was associated with imidazoline receptors with a K-D of 67 +/- 13 nM and a B-max of 677 +/- 136 fmol/mg protein. alpha-Adrenoceptor binding of [H-3]clonidine could be completely prevented when membranes were either treated during preparation with the alkylating agent phenoxybenzamine or incubated in the presence of 30 mu M (-)-noradrenaline or in the presence of the non-hydrolysable analogue of GTP, guanylyl imidodiphosphate (Gpp(NH)p). When the Lu-adrenoceptors binding was prevented, we demonstrated the insensitivity of [H-3]clonidine binding to Gpp(NH)p and showed that the competition between clonidine and idazoxan for imidazoline receptors was insensitive to Gpp(NH)p suggesting that imidazoline receptors are not G protein coupled receptors. The specificity of [H-3]cloniding binding to imidazoline receptors in the human ventrolateral medulla indicates that these receptors are different from imidazole receptors as defined with p-aminoclonidine in the bovine brainstem.