Effect of alpha-2 adrenoceptor antagonists on colonic function in rats

We studied the effect of alpha-2 adrenoceptor antagonists on colonic functi on stimulated by water-avoidance stress, 5-hydroxytryptamine (5-HT), bethan echol and castor oil by comparison with the effects of atropine and a 5-hyd roxytryptamine 3 (5-HT3) receptor antagonist, ondansetron. Yohimbine, idazo xan and atropine, but not ondansetron, significantly inhibited water-avoida nce stress-stimulated faecal excretion. Yohimbine and idazoxan inhibited ne ither 5-HT- nor bethanechol-stimulated faecal excretion. In contrast, atrop ine inhibited both 5-HT- and bethanechol-stimulated faecal excretion and on dansetron inhibited 5-HT-stimulated faecal excretion. Yohimbine did not inh ibit the incidence of diarrhoea induced by castor oil, but idazoxan signifi cantly inhibited diarrhoea observed during a 1-h period after the administr ation of castor oil. Both atropine and ondansetron inhibited diarrhoea duri ng a 2-h period after the administration of castor oil. These findings sugg est that alpha-2 adrenoceptor antagonists specifically inhibit colonic moto r function stimulated by stress in rats.