Labelling of I2B-imidazoline receptors by [3H]2-(2-benzofuranyl)-2-imidazoline (2-BFI) in rat brain and liver: characterization, regulation and relation to monoamine oxidase enzymes.
The novel selective imidazoline radioligand [3H]2-(2-benzofuranyl)-2-imidazoline (2-BFI) was used to characterize and assess further the nature of I2-imidazoline receptors in rat brain and liver. In the cerebral cortex, 2-BFI displayed high affinity (Ki = 9.8 nM) for a single class of [3H]2-BFI binding sites. Other imidazoline/guanidine compounds (e.g. aganodine, cirazoline and idazoxan) displayed biphasic competition curves, indicating the existence of high (KiH = 2.9-78 nM; R(H) = 61-83%) and low (KiL = 4.7-158 microM) affinity sites. The pharmacological profile for [3H]2-BFI binding (aganodine > cirazoline > 2-BFI >> clonidine > amiloride >> efaroxan) was typical of that for I2-sites. This profile was almost identical to that obtained against [3H]idazoxan (correlation between pKi values, r = 0.97) which indicated that the sites characterized with [3H]2-BFI in brain corresponded to I2-imidazoline receptors. The low affinity of amiloride against [3H]2-BFI (Ki = 900 nM) further indicated that these brain I2-sites belong to the I2B-subtype. [3H]2-BFI binding sites (Bmax = 72 fmol/mg protein) in brain were differentially modulated by treatment (7 days) with cirazoline (up-regulation: 25%) and the MAO inhibitor phenelzine (down-regulation: 31%), indicating that these I2-sites are regulated in vivo, as is the case for those labelled by [3H]idazoxan. Chronic treatment with 2-phenylethylamine, a phenelzine metabolite and endogenous amine, did not alter the density of brain of I2-imidazoline receptors labelled by [3H]idazoxan. Preincubation of liver membranes with the MAO inhibitor clorgyline (10(-7) M) abolished the binding of [3H]Ro 41-1049 (N-(2-aminoethyl)-5-(m-fluorophenyl)-4-thiazole carboxamide) to MAO-A, but it did not alter the binding of [3H]Ro 19-6327 (N-(2-aminoethyl)-5-chloro-2-pyridine carboxamide) to MAO-B or that of [3H]2-BFI to I2-sites. At 10(-4) M it also abolished MAO-B sites, but a substantial proportion of I2-sites (40%) remained intact. Preincubation of liver membranes at 60 degrees C also abolished MAO-A/B sites, whereas still 22% of I2-sites remained. The results indicate that [3H]2-BFI is a good tool for the identification of I2-imidazoline receptors and suggest further that certain I2-sites and MAO are different proteins.
Alemany
R
R
Department of Biology, University of the Balearic Islands, Palma de Mallorca, Spain.
Olmos
G
G
García-Sevilla
J A
JA
D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000345:Affinity Labels / Q000378:metabolism*; D000818:Animals; D001572:Benzofurans / Q000378:metabolism*; D001667:Binding, Competitive / Q000187:drug effects; D001921:Brain / Q000201:enzymology / Q000378:metabolism*; D002540:Cerebral Cortex / Q000201:enzymology / Q000378:metabolism; D019329:Idazoxan / Q000378:metabolism; D007093:Imidazoles / Q000378:metabolism* / Q000494:pharmacology; D054755:Imidazoline Receptors; D066298:In Vitro Techniques; D007527:Isoenzymes / Q000378:metabolism; D008024:Ligands; D008099:Liver / Q000201:enzymology / Q000378:metabolism*; D008297:Male; D008995:Monoamine Oxidase / Q000378:metabolism*; D008996:Monoamine Oxidase Inhibitors / Q000494:pharmacology; D010624:Phenelzine / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D011955:Receptors, Drug / Q000187:drug effects / Q000378:metabolism*
D000319:Adrenergic beta-Antagonists; D000345:Affinity Labels; D001572:Benzofurans; D007093:Imidazoles; D054755:Imidazoline Receptors; D007527:Isoenzymes; D008024:Ligands; D008996:Monoamine Oxidase Inhibitors; D011955:Receptors, Drug; C103723:2-(2-benzofuranyl)-2-imidazoline; D008995:Monoamine Oxidase; D010624:Phenelzine; C014282:cirazoline; D019329:Idazoxan