Mechanisms by which the serotonergic system inhibits micturition in rats

AimsSerotonergic neurons and amino acid neurons are involved in the central nervous control of lower urinary tract function. We investigated the role of the serotonergic system in the central regulation of micturition, as well as the relationship between serotonergic neurons and amino acid neurons in the lumbosacral cord of rats.Main methodsUnder urethane anesthesia, bladder and urethral activity were recorded before and after intrathecal injection of serotonin (5-hydroxytryptamine: 5-HT), a 5-HT2A receptor antagonist (ketanserin: KET), or KET + 5-HT by isovolumetric cystometry and measurement of the urethral pressure in intact rats and rats with hypogastric nerve transection (HGNT). Amino acid levels in the lumbosacral cord were also measured after intrathecal injection of 5-HT in intact rats.Key findingsIn intact rats, intrathecal injection of 5-HT transiently abolished rhythmic bladder contractions, decreased the maximal bladder contraction pressure, and increased the intravesical baseline pressure and the urethral baseline pressure. Intrathecal injection of KET + 5-HT also transiently abolished rhythmic bladder contractions. In HGNT rats, intrathecal injection of 5-HT transiently abolished rhythmic bladder contractions and increased the urethral baseline pressure. Intrathecal injection of 5-HT decreased the level of glycine in the lumbosacral cord.SignificanceThe serotonergic system may be involved in blocking the afferent pathway of the micturition reflex, increasing sympathetic activity, and secondary promotion of urethral contraction through inhibition of glycinergic neurons in the lumbosacral cord. 5-HT2A receptors may be involved in these effects on the bladder and urethra. Therefore, the serotonergic system may play a role of the maintenance of urine storage.