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 |
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Authors: | R Alemany Gabriel Olmos Jesús A García-Sevilla |
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Institution: | (1) Laboratory of Neuropharmacology, Department of Biology, University of the Balearic Islands, Cra. Valldemossa km 7.5, E-07071 Palma de Mallorca, Spain, ES |
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Abstract: | 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 (K
i= 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 (K
iH= 2.9-78 nM; R
H= 61-83%) and low (K
iL= 4.7-158 μM) 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 pK
i 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 (K
i= 900 nM) further indicated that these brain I2-sites belong to the I2B-subtype. 3H]2-BFI binding sites (B
max= 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 °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.
Received: 10 January 1997 / Accepted: 6 March 1997 |
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Keywords: | [3H]2-(2-benzofuranyl)-2-imidazoline ([3H]2-BFI) I2B-imidazoline receptor MAO isoenzymes Irreversible MAO inhibitors Rat brain and liver |
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