Autoradiographic comparison of [3H]-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)AR), but also has high affinity (K(D) = 51 nM) in homogenate binding assays for non-adrenergic imidazoline-binding sites (I-sites; imidazoline receptors). Herein, an autoradiographic comparison of [3H]-clonidine binding to I-sites and alpha(2)AR in sections of human brain is reported. For I-sites, the adrenergic component of 50 nM [3H]-clonidine binding was masked with either 60 microM norepinephrine (NE; alpha(2)AR agonist) or 12.5 microM methoxy-idazoxan (MIDX; selective alpha(2)AR antagonist), whereas the remaining non-adrenergic sites were studied by displacement with 20 microM cirazoline. Levels of [3H]-clonidine binding to alpha(2)AR and I-sites, determined in adjacent tissue sections, were positively correlated across 27 brain regions (p = 0.0003; r(2) = 0.385). The principal olivary nucleus and the rostral portion of the ventrolateral medulla had highest ratios of I-sites: alpha(2)AR (>4:1). Quantitative transepts drawn across hippocampal images revealed alpha(2)AR enrichments in the CA-1 and inner molecular layers of the dentate gyrus-areas not enriched in I-sites. Competition curves were generated for I-sites in caudate sections using 10 ligands known to distinguish between I(1) and I(2) subtypes. The rank-order of affinities were cirazoline > harmane > BDF6143 > idazoxan = tizanidine (affinities of agmatine, efaroxan, moxonidine, NE, and oxymetazoline were too low to be reliable). Only the endogenous I-site ligand, harmane, had a monophasic displacement curve at the non-adrenergic sites (Ki = 521 +/- 12 nM).


CONCLUSIONS
1) the distribution of non-adrenergic [3H]-clonidine binding sites in human brain sections was correlated with, but distinct from alpha(2)AR; and 2) the affinities of these sites was distinct from alpha(1)AR, alpha(2)AR, I(1) or I(2) sites as previously defined in membrane binding assays. The properties of this non-adrenergic [3H]-clonidine binding site are consistent with I-sites previously labeled by [3H]-cirazoline in rat brain.
Piletz
J E
JE
Departments of Psychiatry and Human Behavior, University of Mississippi Medical Center, Jackson, MS 39216-4505, USA.
Ordway
G A
GA
Zhu
H
H
Duncan
B J
BJ
Halaris
A
A
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D001921:Brain / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D003000:Clonidine / Q000493:pharmacokinetics*; D005260:Female; D006801:Humans; D054755:Imidazoline Receptors; D008297:Male; D008875:Middle Aged; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D011869:Radioligand Assay; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects* / Q000378:metabolism; D011955:Receptors, Drug / Q000187:drug effects* / Q000378:metabolism; D014316:Tritium
D054755:Imidazoline Receptors; D018341:Receptors, Adrenergic, alpha-2; D011955:Receptors, Drug; D014316:Tritium; D003000:Clonidine