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 alpha2-adrenoceptors (alpha2R) and to I1 imidazoline receptors (I1R), benazoline showed a high selectivity for imidazoline receptors. Although the alpha2R are negatively coupled to adenylate cyclase, no effect on cAMP level by activation of I1R has been reported so far. 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), alpha2R only (HT29 cells), or I1R and alpha2R together (NG10815 cells). Clonidine proved able to decrease the forskolin-stimulated cAMP level in the cells expressing alpha2R and this effect could be blocked by rauwolscine. In contrast, in cells lacking these adrenoceptors, clonidine had no effect. On the other hand, benazoline and other I1 receptor-selective imidazolines decreased forskolin-stimulated cAMP level in the cells expressing I1R, in a rauwolscine- and pertussis toxininsensitive manner. These effects were antagonized by clonidine. According to these results, we demonstrated that 1) alpha2R and I1R are definitely different entities because they are expressed independently in different cell lines; 2) alpha2R and I1R are both implicated in the cAMP pathway in cells (one is sensitive 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 further understand the physiological implications of the I1R and the functional interactions between I1 receptors and alpha2-adrenoceptors.