Ž . European Journal of Pharmacology 423 2001 109–114 www.elsevier.nlrlocaterejphar w 3 x w 3 x ž / Differential H idazoxan and H 2- 2-benzofuranyl -2-imidazoline ž / 2-BFI binding to imidazoline I receptors in human postmortem 2 frontal cortex Luis F. Callado a, ) , Ana I. Maeztu a,b , Javier Ballesteros b,c , Miguel Gutierrez b,d , ´ J. Javier Meana a a Department of Pharmacology, UniÕersity of the Basque Country, E-48940 Leioa, Bizkaia, Spain b Department of Neurosciences, UniÕersity of the Basque Country, E-48940 Leioa, Bizkaia, Spain c Institute of Psychiatric Research, A Maria Josefa Recio B Foundation, Simon BoliÕar 1, E-48010 Bilbao, Spain d Department of Psychiatry, Hospital Santiago, Osakidetza, Olaguibel 29, E-01004 Vitoria-Gasteiz, Spain Received 19 February 2001; received in revised form 23 May 2001; accepted 29 May 2001 Abstract w 3 x Ž . Ž . w 3 x H 2- 2-benzofuranyl -2-imidazoline 2-BFI and H idazoxan are the most used tools to characterise imidazoline I receptors. We 2 evaluated the binding of both radioligands to human postmortem frontal cortex membranes. Saturation binding analyses revealed that w 3 x Ž . H idazoxan in the presence of 2 mM efaroxan to avoid radioligand binding to a -adrenoceptors and imidazoline I receptors and 2 1 w 3 x w 3 x Ž H 2-BFI bound with high affinity to an apparent single population of sites. However, in competition studies whereas H idazoxan 10 . w 3 x Ž . nM binding was displaced monophasically by idazoxan and 2-BFI, both drugs displayed biphasic curves for H 2-BFI 1 nM . The w 3 x proportion of the low-affinity binding site increased from 17% to 25% when 10 nM H 2-BFI was displaced by idazoxan. Amiloride w 3 x Ž . w 3 x inhibited H 2-BFI 10 nM binding with low affinity and in a monophasic way. These data indicate that H 2-BFI recognises in human w 3 x postmortem brain membranes a second binding site different from the imidazoline I receptors labelled by H idazoxan. q 2001 Elsevier 2 Science B.V. All rights reserved. w 3 x w 3 x Ž . Ž . Keywords: Imidazoline I receptor; H idazoxan; H 2- 2-benzofuranyl -2-imidazoline 2-BFI ; Brain, human, postmortem 2 1. Introduction In the last few years, the existence of non-adrenoceptor Ž . binding sites for imidazol ine rguanidine drugs has been shown in various tissues of several species, including the Ž central nervous system Regunathan and Reis, 1996; Eglen . et al., 1998 . Based on the order of affinity for different ligands, imidazoline receptors have been classified into Ž two main types: the imidazoline I and I receptors Michel 1 2 . and Insel, 1989 . The imidazoline I receptors are widely 2 distributed in the brain and changes in their density have been described in some pathologies such as glial tumours Ž . Ž Martın-Gomez et al., 1996 , depression Meana et al., ´ . 1993; Sastre and Garcıa-Sevilla, 1997 , Alzheimer’s de- ´ Ž . mentia Ruiz et al., 1993; Garcıa-Sevilla et al., 1998 and ´ Ž . Huntington’s disease Reynolds et al., 1996 . ) Corresponding author. Tel.: q 34-946012762; fax: q 34-944800128. Ž . E-mail address: kfbcahel@lg.ehu.es L.F. Callado . Traditionally, imidazoline I receptors have been char- 2 w 3 x acterised using the a -adrenoceptor antagonist H idazo- 2 xan in the presence of an a -adrenoceptor masking sub- 2 Ž . stance Michel and Insel, 1989 . Recently, new ligands have been developed showing increased selectivity for imidazoline I receptors vs. a -adrenoceptors, as well as 2 2 higher affinity at imidazoline I receptors when compared 2 w 3 x w 3 x Ž to H idazoxan. These new ligands include H 2- 2-ben- . Ž . w 3 x zofuranyl -2-imidazoline 2-BFI . H 2-BFI has been shown to selectively label imidazoline I receptors in rat 2 Ž . Ž . King et al., 1998 , rabbit Lione et al., 1996 and human Ž . brains Wiest and Steinberg, 1997 . However, whereas it w 3 x has been reported that H 2-BFI has similar binding char- w 3 x acteristics as H idazoxan for imidazoline I receptors in 2 Ž . the rat brain Alemany et al., 1997 , other observations w 3 x w 3 x have suggested that H idazoxan and H 2-BFI may dif- fer in their ability to access or bind to a population of Ž imidazoline I receptors in the human brain Wiest and 2 . Steinberg, 1997 . 0014-2999r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0014-2999 01 01097-4