ALPHA-2-ADRENOCEPTOR SUBTYPES AND IMIDAZOLINE-LIKE BINDING-SITES IN THE RAT-BRAIN

1. The binding of [3H]-yohimbine and [3H]-idazoxan to rat cortex and hippocampus is rapid, reversible and of high affinity. Saturation data indicate that a single population of binding sites exist for [3H]-yohimbine in the cortex (B(max) 121 ± 10 fmol mg-1, protein; K(d) 5.2 ± 0.9 nM) and hippocampus (B(max) 72 ± 6 fmol mg-1 protein; K(d) 5.8 ± 0.7 nM). [3H]-idazoxan labels one site in the cortex (B(max) 87 ± 8 fmol mg-1 protein; K(d) 4.1 ± 0.9 nM) and hippocampus (B(max) 30 ± 6 fmol mg-1 protein; K(d) 3.5 ± 0.5 nM), when 3 μM phentolamine is used to define non-specific binding. A second distinct [3H]-idazoxan binding site (B(max) 110 ± 21 fmol mg-1 protein; K(d) 3.6 ± 0.07 nM) is identified in rat cortex if 0.3 μM cirazoline is used to define non-specific binding and 3 μM yohimbine is included to prevent binding to α2-adrenoceptors. 2. Displacement studies indicate that the α1-adrenoceptor antagonist prazosin and the 5-HT1 ligands 8-OH-DPAT, RU 24969 and methysergide differentiate [3H]-yohimbine binding into two compounds; a high and low affinity site. In contrast the displacement of [3H]-idazoxan by each ligand was monophasic. 3. The affinities of 8-OH-DPAT, RU 24969 and methysergide against [3H]-idazoxan binding to the cortex and hippocampus correlate significantly with the binding site displaying low affinity for prazosin and previously designated α(2A). In contrast, a poor correlation exists for the high affinity site for prazosin designated α(2B). 4. [3H]-idazoxan, in the presence of 3 μM yohimbine, labels a site that displays high affinity towards cirazoline, naphazoline and guanabenz, but low affinity towards clonidine, p-aminoclonidine, adrenaline, noradrenaline and the α2-adrenoceptor antagonists yohimbine, rauwolscine, WY 26703 and BDF 6143. 5. The results of this study indicate that [3H]-yohimbine labels two sites; the α(2A)- and α(2B)-adrenoceptors whereas [3H]-idazoxan labels an α2-adrenoceptor with a profile consistent with the α(2A)-adrenoceptor subtype. In addition, [3H]-idazoxan labels an imidazoline binding site in the rat cortex that is pharmacologically distinct from α2-adrenoceptors. The low affinity of clonidine and p-aminoclonidine indicates that the imidazoline-like binding site in rat cortex is different from the site labelled by [3H]-clonidine and [3H]-p-aminoclonidine in human, rat and bovine brain stem, providing evidence of potential heterogeneity within the class of binding sites.