LNP 906, the first high‐affinity photoaffinity ligand selective for I 1 imidazoline receptors

The hypotensive effect of imidazoline‐like drugs, such as clonidine, was attributed both to α 2 ‐adrenergic receptors and nonadrenergic imidazoline receptors, which are divided into I 1 , I 2 and I 3 subtypes. We have recently synthesized a derivative of (2‐(2‐chloro‐4‐iodo‐phenylamino)‐5‐methyl‐pyrroline (LNP 911), the first high‐affinity and selective ligand for I 1 receptors ( I 1 R ), with a photoactivable function (LNP 906). This work aims to test whether this derivative retained the binding properties of LNP 911 and bound irreversibly to I 1 R. Binding studies showed that LNP 906 exhibited nanomolar affinity for I 1 R and was selective for I 1 R over I 2 receptors and α 2 ‐adrenergic receptors ( α 2 Ars). Upon exposure to u.v. light, LNP 906 irreversibly blocked the binding of [ 125 I]‐paraiodoclonidine (PIC) to I 1 R, time‐ and dose‐dependently, on PC12 cell membranes and interacted with I 1 R in a reversible and competitive manner in the absence of light. Pharmacological studies showed that this blockade was prevented by the concomitant presence of rilmenidine (a well‐known I 1 agonist), but not by rauwolscine (an α 2 antagonist). Finally, LNP 906 clearly antagonized the decrease in forskolin‐stimulated cAMP level induced by rilmenidine, but not by melatonin. These results indicate that LNP 906 is the first high‐affinity and selective photoaffinity ligand for I 1 R and that it behaves as an I 1 R antagonist.British Journal of Pharmacology (2004) 142 , 609–617. doi: 10.1038/sj.bjp.0705784