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 (Bmax 121 +/- 10 fmol mg-1, protein; Kd 5.2 +/- 0.9 nM) and hippocampus (Bmax 72 +/- 6 fmol mg-1 protein; Kd 5.8 +/- 0.7 nM). [3H]-idazoxan labels one site in the cortex (Bmax 87 +/- 8 fmol mg-1 protein; Kd 4.1 +/- 0.9 nM) and hippocampus (Bmax 30 +/- 6 fmol mg-1 protein; Kd 3.5 +/- 0.5 nM), when 3 microM phentolamine is used to define non-specific binding. A second distinct [3H]-idazoxan binding site (Bmax 110 +/- 21 fmol mg-1 protein; Kd 3.6 +/- 0.07 nM) is identified in rat cortex if 0.3 microM cirazoline is used to define non-specific binding and 3 microM yohimbine is included to prevent binding to alpha 2-adrenoceptors. 2. Displacement studies indicate that the alpha 1-adrenoceptor antagonist prazosin and the 5-HT1 ligands 8-OH-DPAT, RU 24969 and methysergide differentiate [3H]-yohimbine binding into two components; 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 determined against [3H]-idazoxan binding to the cortex and hippocampus correlate significantly with the binding site displaying low affinity for prazosin and previously designated alpha 2A. In contrast, a poor correlation exists for the high affinity site for prazosin designated alpha 2B. 4. [3H]-idazoxan, in the presence of 3 microM yohimbine, labels a site that displays high affinity towards cirazoline, naphazoline and guanabenz, but low affinity towards clonidine, p-aminoclonidine, adrenaline, noradrenaline and the alpha 2-adrenoceptor antagonists yohimbine, rauwolscine, WY 26703 and BDF 6143. 5. The results of this study indicate that [3H]-yohimbine labels two sites; the alpha 2A- and alpha 2B-adrenoceptors whereas [3H]-idazoxan labels an alpha 2-adrenoceptor with a profile consistent with the alpha 2A-adrenoceptor subtype. In addition, [3H]-idazoxan labels an imidazoline binding site in the rat cortex that is pharmacologically distinct from alpha 2-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 this class of binding sites.
Brown
C M
CM
Department of Pharmacology, Syntex Research Centre, Edinburgh.
MacKinnon
A C
AC
McGrath
J C
JC
Spedding
M
M
Kilpatrick
A T
AT
D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D001923:Brain Chemistry / Q000187:drug effects*; D002417:Cattle; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism; D004146:Dioxanes / Q000378:metabolism; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D019329:Idazoxan; D007093:Imidazoles / Q000494:pharmacology*; D054755:Imidazoline Receptors; D066298:In Vitro Techniques; D007211:Indoles / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008784:Methysergide / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects* / Q000378:metabolism; D011955:Receptors, Drug / Q000187:drug effects* / Q000378:metabolism; D013764:Tetrahydronaphthalenes / Q000378:metabolism; D015016:Yohimbine / Q000378:metabolism
D000316:Adrenergic alpha-Agonists; D004146:Dioxanes; D007093:Imidazoles; D054755:Imidazoline Receptors; D007211:Indoles; D011942:Receptors, Adrenergic, alpha; D011955:Receptors, Drug; D013764:Tetrahydronaphthalenes; C025145:5-methoxy 3-(1,2,3,6-tetrahydro-4-pyridinyl)1H indole; D015016:Yohimbine; D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D008784:Methysergide; D019329:Idazoxan