Autoradiographic comparison of [H-3]-clonidine binding to non-adrenergic sites and alpha(2)-adrenergic receptors in human brain

Clonidine is a partial agonist at brain alpha (2)-adrenoceptors (alpha (2)A R), but also has high affinity (K-D = 51nM) in homogenate binding assays fo r non-adrenergic imidazoline-binding sites (1-sites; imidazoline receptors) . Herein, an autoradiographic comparison of [H-3]-clonidine binding to I-si tes and alpha (2)AR in sections of human brain is reported. For I-sites, th e adrenergic component of 50 nM [H-3]-clonidine binding was masked with eit her 60 muM norepinephrine (NE; alpha (2)AR agoinst) or alpha (2)AR antagoni st), whereas the remaining non-adrenergic sites were studied by displacemen t with 20 muM cirazoline. Levels of [H-3]-clonidine binding to alpha (2)AR and I-sites: determined in adjacent tissue sections, were positively correl ated across 27 brain regions (p = 0.0003;r(2) = 0.385). The principal oliva ry nucleus and the rostral portion of the ventrolateral medulla had highest ratios of I-sites: alpha (2)AR (>4:1). Quantitative transepts drawn adross hippocampal images revealed alpha (2)AR enriched in the CA-1 and inner mol ecular layers of the dentate gyrus-areas Clonidine is a partial agonist at brain not enriched in I-sites. Competition curves were generated for I-site s in caudate sections using 10 ligands known to distinguish between I-1 and I-2 subtypes. The rank-order of affinities were cirazoline > harmane > BDF 6143 > idazoxan = tizanidine (affinities of agmatine, efaroxan, moxonidine, NE, and oxymerazoline were too low to be reliable). Only the endogenous I- site ligand, harmane, had a monophasic displacement curve at the non-adrene rgic sites (Ki = 521 +/- 12 nM). In conclusion: In the distribution of non- adrenergic [H-3]-clonidine binding sites in human brain sections was correl ated with, but distinct from cr,AR; and 2) the affinities of these sites wa s distinct from alpha (1)AR, alpha (2)AR, I-1 or I-2 sites Its previously d efined in membrane binding assays. The properties of this non-adrenergic [H -3]-clonidine binding site are consistent with I-sites previously labeled b y [H-3]-cirazoline in rat brain.