RELEVANCE OF THE USE OF [(3)H]-CLONIDINE TO IDENTIFY IMIDAZOLINE RECEPTORS IN THE RABBIT BRAIN-STEM

1 [H-3]-clonidine binding was investigated in membranes isolated from the ventral medulla oblongata of the rabbit where clonidine produced a hypotensive effect which was not mediated by adrenoceptors. [H-3]-clonidine specific binding, as defined by the difference between the binding of [H-3]-clonidine in the presence and in the absence of 10 muM cirazoline, occurred at two sites: a high affinity site with a K(D) = 2.9 +/- 0.7 nm and a B(max) of 40 +/- 8 fmol mg-1 protein and a low affinity site with a K(D) = 18.2 +/- 0.4 nm and a B(max) of 66 +/- 14 fmol mg-1 protein. 2 The high affinity sites being catecholamine-sensitive were identified as alpha2-adrenoceptors. The low affinity binding of [H-3]-clonidine was insensitive to catecholamines, as well as to other alpha2-adrenoceptor specific probes, and could be inhibited with high affinity only by compounds which lowered blood pressure when directly injected in the nucleus reticularis lateralis of the ventral brainstem, or by antagonists. 3 It was concluded that in the ventral medulla of the rabbit, [H-3]-clonidine labelled alpha2-adrenoceptors and imidazoline receptors (IRs). Only the latter were related to the hypotensive effects of. clonidine and rilmenidine directly injected into the rostroventrolateral medulla oblongata (RVLM) of the rabbit. The methodological problems regarding the study of IRs with [H-3]-clonidine are discussed.