The sensory input generated by the urethra has an important role on bladder function (1). Although the mechanism of urethral sensory fibers excitation is unknown, it might involve specialized cells located along the urethral epithelium, including those expressing serotonin (5-HT) (1). The present work aimed to study the contribution of urethral 5-HT to the activation of local sensory fibers and initiation of an urethro-vesical reflex in the female rat. Deficient mice for typtophan hydroxylase 1 (TPH1), a rate-limiting enzyme in the conversion of tryptophan to 5-HT at the peripheral nervous system, were additionally used to investigate the effect of the absence of urethral 5-HT to bladder activity.  Finally, human tissue was also used to characterize cells expressing 5-HT in the human female bladder and urethra.

Urethane-anesthetized female rats and TPH1-/- mice underwent isovolumetric or urethral-open cystometries during intravesical or intraurethral infusion of saline or serotonin solution (100μM). Human and rat bladders and urethrae were immunoreacted (IR) against 5-HT, TPH1, the neuronal markers beta-3 tubulin, synaptic vesicle 2 (SV2), calcitonin gene-related peptide (CGRP) and vesicular acetylcholine transporter (VAChT).  Levels of serotonin concentration and TPH1 mRNA were determined in rat tissue by HPLC and qPCR, respectively.

In rats, under isovolumetric conditions, intraurethral serotonin infusion, but not saline, evoked strong bladder contractions. This was abolished by urethral anesthesia with 2% lidocaine and by treatment with 5-HT2 (ritanserin, 1 mg/Kg) and 5-HT3 (Y-25130, 1 mg/Kg). serotonin receptor antagonists. Still under isovolumetric conditions serotonin infusion into the bladder had no effect on reflex contractions. Under urethral-open conditions, serotonin infusion reduced the frequency and increased the amplitude of reflex bladder voiding contractions, compared to saline infusion. TPH1-/- mice, under urethral-opened conditions, exhibited increased frequency and reduced amplitude of voiding contractions compared to WT. Serotonin concentration and TPH1 mRNA expression were high in the urethra and very low in the bladder. Cells 5-HT+ were found in the human and rat urethral epithelium, close to a sub-epithelial network of VAChT+ (cholinergic) and  CGRP+ (sensory) fibers. but not in the bladder. The 5HT-IR  co-localized  with the IR for the neuronal markers beta-3 tubulin and SV2.

The production and release of 5-HT by urethral specialized cells, by influencing the underlying neuronal net, initiates an urethro-vesical crosstalk that reinforces bladder voiding contractions. This process is mediated by 5-HT receptors on urethral sensory afferents. The luminal stimuli leading urethral cells to release 5-HT are still unknown.

This study found an urethral-bladdder reflex mediated by serotonin released in the urethra, confirming previous assumptions that the urethra is a complex structure that may modulate bladder voiding activity. The stimulation of this reflex may be important in conditions where bladder emptying is not effective, such as bladder underactivity.

Acta Physiol (Oxf). 2018 Feb;222(2). doi: 10.1111/apha.12919