Drug-induced defaecation in rats: role of central 5-HT1A receptors.

1. We investigated the acute effects of 5-hydroxytryptamine (5-HT), and of the 5-HT1A receptor agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), buspirone and SR 57746A, on rat faecal pellet output and water content. 2. 5-HT, 8-OH-DPAT, buspirone and SR 57746A, a new selective 5-HT1A receptor agonist, displaced [3H]-8-OH-DPAT from specific binding sites in rat hippocampus membranes (Ki, nM; 1.8, 1.2, 15, 3.1 respectively) and stimulated rat defaecation dose-dependently. SR 57746A and buspirone induced 1 g dry weight of faeces at 1.3 and 6.1 mg kg-1, p.o. (AD1) respectively. 8-OH-DPAT and 5-HT stimulated defaecation after s.c. injection (AD1, 0.07 and 7.5 mg kg-1, respectively). All these agents increased faecal water content. 3. The putative 5-HT1A receptor antagonist, pindolol, injected s.c. or i.c.v., significantly reduced the defaecation induced by systemically administered 8-OH-DPAT, buspirone or SR 57746A, but not 5-HT. 4. Pretreatment with p-chlorophenylalanine (i.p.) or 5,7-dihydroxytryptamine (i.c.v.), according to protocols designed to cause either generalized or CNS-limited 5-HT depletion respectively, also reduced the defaecation induced by buspirone or SR 57746A. 5. No specific 5-HT1A binding sites could be labelled by incubating rat colon membranes with [3H]-8-OH-DPAT, and in vitro preparations of rat colon segments showed no response to 8-OH-DPAT or SR 57746A up to 5 microM. 6. After eight days' repeated daily treatment, complete tolerance developed to the stimulant effects of SR 57746A and buspirone on faecal water content, but not on faecal pellet output. This suggests that faecal mass excretion and water exchange through the gut wall are affected by independent mechanisms.7. The present findings support the involvement of central 5-HTIA receptors in intestinal propulsion and regulation of luminal fluid content, presumably accounting for the drug-induced defaecation in rats.