Effect of different 5-hydroxytryptamine receptor subtype antagonists on the micturition reflex in rats

OBJECTIVE: To evaluate the effects of antagonists of different subfamilies of 5-hydroxytryptamine (5-HT) receptors on bladder function in anaesthetized and conscious rats. MATEERIALS AND METHODS: The urinary bladder of female anaesthetized rats was catheterized urethrally and filled with physiological saline until spontaneous bladder contractions occurred. Infravesical pressure was measured by a pressure transducer and displayed continuously on a chart recorder. The time of bladder quiescence (to the disappearance of rhythmic contractions) after injection with different compounds tested was recorded. Conscious rats underwent cystometry with chronically (infravesical) implanted catheters to continuously record bladder capacity (evaluated as amount of saline infused between voiding cycles) and maximal voiding pressure. The affinity for the human recombinant serotoninergic 5-HT1A subtype (inhibition of specific binding of [3H]8-hydroxy-2-(di-n-propylamino) tetralin) and the effects on the [35S]guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding in HeLa cells was also evaluated. RESULTS: Among the compounds tested, only 4-(2'-methoxy-phenyl)-1-[2'-(n-2"-pyridinyl)-p-iodobenzamido]-ethyl-piperazine (p-MPPI) and methiothepin showed nanomolar affinity for the 5-HT1A receptors, the former being a neutral antagonist and the latter an inverse agonist in the [35S]GTPgammaS binding model. Intravenous injection of low doses of p-MPPI and methiothepin induced a dose-dependent disappearance of isovolumic bladder contractions in anaesthetized rats (> 10 min). At the highest doses, the dose-response curves were bell-shaped. The amplitude of bladder contractions was not markedly altered. The tested antagonists of 5-HT2, 5-HT3, 5-HT4, and 5-HT6 serotoninergic subtypes were poorly active or inactive in the model. Similarly, these compounds were inactive on cystometry in conscious rats, whereas p-MPPI and methiothepin induced a consistent increase in bladder capacity. Methiothepin also decreased the voiding pressure, whereas p-MPPI did not affect this variable. CONCLUSIONS: These findings confirm that only selective 5-HT1A receptor antagonists have favourable effects on the bladder, inducing an increase in bladder capacity with no derangement of bladder contractility.