Exogenous serotonin regulates colorectal motility via the 5-HT2 and 5-HT3 receptors in the spinal cord of rats.
BACKGROUND
We previously reported that intrathecal injection of noradrenaline or dopamine causes enhancement of colorectal motility. As these monoamines are neurotransmitters of descending pain inhibitory pathways in the spinal cord, we hypothesized that serotonin, which is one of the neurotransmitters involved in descending pain inhibition, also influences the lumbosacral defecation center. Therefore, we examined whether serotonin acting on the spinal defecation center enhances colorectal motility.


METHODS
Colorectal intraluminal pressure and propelled liquid volume were recorded in vivo in anesthetized rats.


KEY RESULTS
Intrathecal injection of serotonin into the L6-S1 spinal cord elicited periodic increases in colorectal intraluminal pressure, being associated with increases in liquid output. Pharmacological experiments revealed that the effect of serotonin is mediated by both 5-HT2 and 5-HT3 receptors. The serotonin-induced enhancement of colorectal motility was unaffected even after disconnection of the defecation center from supraspinal regions by cutting the T8 spinal cord, while transection of the parasympathetic pelvic nerves prevented the colokinetic effect of serotonin. Finally, we investigated interactions among serotonin, noradrenaline and dopamine. Simultaneous administration of sub-effective doses of these monoamine neurotransmitters into the spinal cord caused propulsive colorectal motility slightly but substantially.


CONCLUSIONS AND INFERENCES
These results demonstrate that exogenous serotonin acts on 5-HT2 and 5-HT3 receptors in the lumbosacral defecation center and activates the parasympathetic nervous system to enhance colorectal motility in cooperation with noradrenaline and dopamine.
['Nakamori|H|H|', 'Naitou|K|K|', 'Sano|Y|Y|', 'Shimaoka|H|H|', 'Shiina|T|T|0000-0002-3425-950X', 'Shimizu|Y|Y|0000-0001-7368-6398']
D000818:Animals D003106:Colon D005769:Gastrointestinal Motility D008297:Male D017207:Rats, Sprague-Dawley D044348:Receptors, Serotonin, 5-HT2 D044406:Receptors, Serotonin, 5-HT3 D012007:Rectum D012701:Serotonin D058826:Serotonin 5-HT2 Receptor Agonists D013116:Spinal Cord
serotonin descending pain inhibitory pathway lumbosacral defecation center defecation reflex gut motility