The imidazol(in)e compounds produce pupillary dilation after systemic application to the rats. This action is assumed to be connected with the stimulation of postsynaptic alpha2-adrenoceptors within the Edinger-Westphal nucleus in the brain. Despite numerous studies, knowledge on the involved receptors structure and on their selective ligands it is still incomplete. In this study the mydriatic effect of clonidine,  rilmenidine  and moxonidine on the pupil size in anaesthesized rats after or without alpha2-adrenergic receptor antagonists pretreatment was examined. The method of testing mydriasis in rats was used as a model system for assessment of selective alpha2-adrenergic activity evoked by potentially pharmacologically active compounds with the imidazol(in)e structure. In the model, experiments were carried out in vivo in the whole animal with testing particular imidazol(in)e agents in a wide range of doses, regarding both the agonistic and the antagonistic properties and assessing the strength of action of the ligands studied.
The aim of this study was to comparatively determine the mydriatic effect in rats of clonidine, rilmenidine and moxonidine. In order to confirm the contribution of alpha2-adrenoceptors (and their subtypes, especially alpha2D), in the observed pharmacological effects, the animals were pretreated with the following alpha2-adrenoceptor antagonists: yohimbine, RS 79948, RX 821002. A selective I1-imidazoline receptor antagonist AGN 192403 was also applied to eliminate the possible involvement of this receptor in mydriatic effects of the agents studied. Additionally the Western blot technique was used to confirm the presence of immunoreactive receptor protein corresponding with alpha2-adrenoceptor type in the brain medulla tissue of the rats. The results obtained confirm that as compared with other in vivo and in vitro pharmacological methods of testing of interactions of ligands with imidazol(in)e structure with central alpha2-adrenergic receptors, this model seems to be especially useful for the preclinical in vivo studies aimed at identification of the potential cardiovascular drugs acting through alpha2-adrenoceptors.

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1. AGN192403 and 2BFI as pharmacological tools for determination of the engagement  of I1/I2-imidazoline receptors in cardiotropic effects of imidazol(in)e compounds on isolated rat heart atria