Are Alpha-2D Adrenoceptor Subtypes Involved in Rat Mydriasis Evoked by New Imidazoline Derivatives: Marsanidine and 7-Methylmarsanidine?

Abstract

The imidazoline compounds may produce mydriasis after systemic administration to some species (rats, cats, and mice). In mydriatic activity of imidazolines, α2D-adrenoceptors subtype(s) seems to be involved. In this study, the pupil dilatory effect evoked by 2 newly synthesized imidazoline derivatives—α2-adrenoceptor agonists: marsanidine and 7-methylmarsanidine—was compared. The compounds were tested alone as well as in the presence of α2-adrenoceptor antagonists (nonselective, yohimbine, and selective toward the following α2-adrenoceptor subtypes—α2A-2-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-2,3-dihydro-1-methyl-1H-isoindole maleate (BRL44408), α2B-2-[2-(4-(2-methoxyphenyl)piperazin-1-yl)ethyl]-4,4-dimethyl-1,3-(2H,4H)-isoquinolindione dihydrochloride (ARC239), α2C-JP1302, α2D-2-(2,3-dihydro-2-methoxy-1,4-benzodioxin-2-yl)-4,5-dihydro-1H-imidazole hydrochloride [RX821002]). The agonists were studied in male Wistar rats and were administered intravenously in cumulative doses. The antagonistic compounds were given in a single dose before the experiment with marsanidine or 7-methylmarsanidine. Pupil diameter was measured with stereoscopic microscope equipped in green light filter. Marsanidine and 7-methylmarsanidine exerted marked mydriatic effects. BRL44408, JP1302, and ARC239 did not cause significant parallel shift to the right of the dose–effect curves obtained for both imidazolines. In case of yohimbine and RX821002, the marked parallel shifts of dose–response curves were observed, with the antagonistic effects of RX821002 more pronounced. In vivo pharmacodynamics experiment suggests that α2D-adrenoceptor subtype is mainly engaged in mydriatic effects evoked in rats by imidazoline derivatives, in particular by clonidine.