Vol. 57, Issue 6, 1142-1151, June 2000

Coupling of I₁ Imidazoline Receptors to the cAMP Pathway: Studies with a Highly Selective Ligand, Benazoline

Hugues Greney, Philippe Ronde, Céline Magnier, Françoise Maranca, Carla Rascente, Wilma Quaglia, Mario Giannella, Maria Pigini, Livio Brasili, Claire Lugnier, Pascal Bousquet, and Monique Dontenwill

Laboratoire de Neurobiologie et Pharmacologie Cardiovasculaire, Faculté de Medecine, Université Louis Pasteur, Strasbourg, France (H.G., C.M., F.M., C.R., P.B., M.D.); Dipartimento di Scienze Chimiche, Universita di Camerino, Camerino, Italy (W.Q., M.G., M.P.); Dipartimento di Scienze Farmaceutiche, Universita di Modena, Modena, Italy (L.B.); and Faculté de Pharmacie, Illkirch-Graffenstaden, France (P.R., C.L.)

Clonidine and benazoline are two structurally related imidazolines. Whereas clonidine binds both to ₂-adrenoceptors (₂R) and to I₁ imidazoline receptors (I₁R), benazoline showed a high selectivity for imidazoline receptors. Although the ₂R 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), ₂R only (HT29 cells), or I₁R and ₂R together (NG10815 cells). Clonidine proved able to decrease the forskolin-stimulated cAMP level in the cells expressing ₂R 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 toxin-insensitive manner. These effects were antagonized by clonidine. According to these results, we demonstrated that 1) ₂R and I₁R are definitely different entities because they are expressed independently in different cell lines; 2) ₂R 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 ₂-adrenoceptors.