Coupling of I-1 imidazoline receptors to the cAMP pathway: Studies with a highly selective ligand, benazoline

Clonidine and benazoline are two structurally related imidazolines. Whereas clonidine binds both to alpha(2)-adrenoceptors (alpha(2)R) and to I-1 imid azoline receptors (I1R), benazoline showed a high selectivity for imidazoli ne receptors. Although the alpha(2)R are negatively coupled to adenylate cy clase, no effect on cAMP level by activation of I1R has been reported so fa r. We therefore aimed to compare the effects of clonidine and benazoline on forskolin-stimulated cAMP levels in cell lines expressing either I1R only (PC12 cells), alpha(2)R only (HT29 cells), or I1R and alpha(2)R together (N G10815 cells). Clonidine proved able to decrease the forskolin-stimulated c AMP level in the cells expressing alpha(2)R and this effect could be blocke d by rauwolscine. In contrast, in cells lacking these adrenoceptors, clonid ine had no effect. On the other hand, benazoline and other I-1 receptor-sel ective imidazolines decreased forskolin-stimulated cAMP level in the cells expressing I1R, in a rauwolscine- and pertussis toxin-insensitive manner. T hese effects were antagonized by clonidine. According to these results, we demonstrated that 1) alpha(2)R and I1R are definitely different entities be cause they are expressed independently in different cell lines; 2) alpha(2) R and I1R are both implicated in the cAMP pathway in cells (one is sensitiv e to pertussis toxin and the other is not); and 3) I1R might be coupled to more then one transduction pathway. These new data will be essential to fur ther understand the physiological implications of the I1R and the functiona l interactions between I-1 receptors and alpha(2)-adrenoceptors.