Region-dependent effects of acute and chronic tranylcypromine in vivo on [H-3]2-BFI binding to brain imidazoline I-2 sites

An imidazoline I-2 site has been localised to monoamine oxidase. However, i n vitro studies of the effect of monoamine oxidase inhibitors on imidazolin e I-2-site radioligand binding have produced conflicting findings. Using th e technique of autoradiography, we examined the effect of in vivo administr ation of the irreversible monoamine oxidase inhibitor tranylcypromine on bi nding of the imidazoline I-2 site-specific ligand [H-3]2-(-2-benzofuranyl)- 2-imidazoline ([H-3]2-BFI) in four rat brain nuclei which are known to poss ess a high density of imidazoline I-2 sites, together with cerebral cortex and cerebellum which show weaker binding. A single acute pre-treatment with tranylcypromine significantly increased imidazoline I-2, site-specific bin ding in four regions: arcuate nucleus, interpeduncular nucleus, pineal glan d and area postrema, but effects in cortical areas and cerebellum were not significant. The extent of the increase was proportional to the control bin ding in each region. In contrast, five daily treatments with the same dose of tranylcypromine significantly reduced [H-3]2-BFI binding in these same a reas. The potential role of monoamine oxidase isoforms in these changes is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.