CONSTIPATION EVOKED BY 5-HT3-RECEPTOR ANTAGONISM - EVIDENCE FOR HETEROGENEOUS EFFICACY AMONG DIFFERENT ANTAGONISTS IN GUINEA-PIGS

The abilities of selective 5-HT3-receptor antagonists to evoke constip ation were examined in conscious guinea-pigs and in preparations of gu inea-pig isolated colon. Compared with vehicle-treated guinea-pigs, ac ute doses of granisetron (0.1, 1 and 10 mg kg(-1), i.p.) and tropisetr on (10 mg kg(-1), i.p., but not 1 and 0.1 mg kg(-1), i.p.) significant ly (P < 0.05) reduced the total number of faecal pellets excreted duri ng a 12-h observation period. By contrast, BRL 46470 (0.1-10 mg kg(-1) , i.p.) had no significant effect on the incidence of defecation. Mid- to-distal lengths of guinea-pig isolated colon spontaneously expelled faecal pellets. Granisetron (0.1 and 1 mu M) and tropisetron (1 mu M) reduced or prevented the rate at which they were spontaneously expelle d. Morphine (0.1 mu M) and clonidine (10 nM) also slowed faecal pellet transit time. Naloxone (0.1 mu M) had no effects alone, but reversed the actions of granisetron, morphine and clonidine. BRL 46470 (1 mu M) had no significant effect on the transit of faecal pellets in guinea- pig isolated colon. In segments of guinea-pig isolated colon which did not contain faecal pellets, granisetron, tropisetron and BRL 46470 an tagonized the ability of 5-HT to evoke cholinergically-mediated contra ctions of the longitudinal muscle. The respective pA(2) values and slo pes of the Schild plots were 8.5 +/- 0.05, slope 1.06 +/- 0.03; 8.5 +/ - 0.1, slope 0.91 +/- 0.04; and 7.9 +/- 0.1, slope 0.93 +/- 0.05. Our experiments suggest that not all 5-HT3-receptor antagonists are the sa me. In particular, BRL 46470 does not prevent defecation or faecal pel let expulsion in guinea-pig colon, even though this compound is an eff ective 5-HT3-receptor antagonist in the same tissue. For the 5-HT3-rec eptor antagonists which did cause constipation, the effects can be at least partly attributed to an indirect opioid-dependent action within the colonic enteric nervous system.