In vitro analysis of the role of alpha2- and imidazoline receptors in the regulation of gastric motility in mice

Introduction: According to the imidazoline hypothesis some effects of clonidine and structurally related α2-adrenoceptor agonists is mediated by imidazoline receptors, and not by α2-adrenoceptors. Newer, more selective imidazoline receptor ligands (moxonidine, rilmenidine) have been introduced to the antihypertensive therapy, and they may induce less α2-adrenoceptor-mediated side effects (like sedation). The role of imidazoline receptors has also been proposed in the regulation of gastrointestinal functions, however, the lack of sufficiently selective imidazoline receptor agonists raises difficulties in the analysis. The present study aimed to analyze in genetically engineered α2-adrenoceptor-KO mice, whether moxonidine and rilmenidine inhibit gastric motor activity and gastrointestinal transit via α2-adrenoceptors or imidazoline receptors. Methods: Wild type, α2A-, α2B- and α2C-KO C57BL/6 mice were used. For analysis of gastric motor activity, mice were killed by cervical dislocation, their stomachs were removed and fundus stripes were suspended between two electrodes in 5ml organ baths containing 37°C Krebs solution, then EFS was applied. Drugs were added in a cumulative manner. For in vivo analysis of gastrointestinal transit the charcoal meal method was used. Results: 1) Both moxonidine and rilmenidine (1–10000 nmol) inhibited the EFS-induced contractions in a concentration dependent manner in wild type mice. 2) The effect of both drugs was antagonized by the non selective α2-adrenoceptor and imidazoline antagonist idazoxan (10000 nmol) and by the selective α2A-adrenoceptor antagonist BRL 44408 (10000 nmol). 3) Moxonidine inhibited the EFS-induced contractions in α2B- and α2C-KO mice as well, but did not have any effect in α2A-KO mice. 4) Moxonidine (3.6µmol/kg i.p.) significantly reduced the gastrointestinal transit in wild type mice, but failed to alter it in α2A-KO mice. Conclusions: Our results obtained from genetically engineered mice do not support the imidazoline hypothesis, and strongly suggest that moxonidine and rilmenidine induce their gastrointestinal effects via activation of α2-adrenoceptors. The work was supported by ETT 341/2009 from the Scientific Health Council and National Office for Research and Technology (NKTH).