Several lines of evidence suggest that imidazoline receptors mediate various physiological processes. It is rather difficult, however, to distinguish the imidazoline receptor-mediated effects from the alpha2- adrenoceptor-mediated ones due to the reasonable affinity of most imidazoline ligands for the alpha2-adrenoceptors. In the present study the effects of different imidazoline ligands were tested on the electrical field stimulation (EFS)-induced gastric contractions in wild-type (WT), alpha 2A-, alpha2B- and alpha2C-adrenoceptor knockout (KO) mice in order to analyze, whether imidazoline I1 and I 2 receptors take part in the regulation of gastric motor activity. Clonidine, moxonidine and rilmenidine inhibited the EFS-induced gastric contractions in a concentration dependent manner in WT, alpha2B- and alpha2C-adrenoceptor KO mice, whereas they had no or only weak effect in alpha2A-adrenoceptor KO mice. Their effects in WT mice were inhibited by idazoxan and BRL 44408, but not by ARC 239, AGN 192403 and BU 224. The endogenous imidazoline receptor ligand agmatine failed to affect the EFS-induced contractions, while harmane (an other endogenous imidazoline receptor ligand) and 2-BFI (a selective imidazoline I2 receptor agonist) exerted a slight effect in both WT and alpha2A-adrenoceptor KO mice, but this was not reversible by idazoxan, AGN 192403 and BU 224. It can be concluded, that the inhibitory effect of the tested imidazoline compounds on cholinergic gastric contractions is mediated mainly by alpha2A- adrenoceptors. Although at higher concentrations other receptors may also contribute to their effects, the lack of inhibition by AGN 192403 and BU 224 suggests that these are not imidazoline I1 and I2 receptors.
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