Several studies have shown that the sensation of urine flow through the urethra potentiates bladder contractility. The encoding process of urine flow is still unclear but may involve the rich sensory network of the sub-epithelium of the urethra and the cross-talk between these sensory fibres and ciliated neuronal like cells scattered along the urethral epithelium. These neuronal-like cells may express acetylcholine and serotonin (5HT) while sensory fibres in the sub-epithelium express nicotine receptors and 5-HT2 and 5-HT3 receptor subtypes.
Recent observations showed that topical application of 5HT to the urethra enhances the amplitude of bladder contractions and decreases the volume threshold to reflex bladder contractions. 
Therefore, in the present study we hypothesized that the aging process involves a decrease in the number of 5HT-expressing urethral cells and 5HT stimulation of urethral afferents can improve the efficiency of bladder voiding contractions in the aged animals.

A group of female Wistar rats with 18-24 months (aged group) were perfused fixed and the urethra removed for immunohistochemistry analysis against 5HT. The number of 5HT immunoreactive (IR) cells were counted in 10 sections. The same procedure was repeated with a group of female Wistar rats with 5-6 months (control group).
A second group of old female Wistar rats, 18-24 months, were anaesthetised and cystometries, with saline infusion through the bladder dome, were performed to evaluate the bladder voiding efficiency. Whenever a reduced frequency was observed, the efficacy of myogenic contractions was confirmed upon serosa topical application of acetylcholine (1 µM) or ATP (5 mg/kg). The same procedure was repeated in young, 5-6 months, female Wistar rats (control group).
 Aged Wistar rats, 18-24 months, were anaesthetised and cystometries performed with infusion of saline or 5HT solution (100 microM) through the bladder dome to evaluate the frequency of reflex contractions and the bladder voiding efficiency (voided volume/voided volume+post-void residual urine).
A fourth group of aged female Wistar rats,18-24 months, were anaesthetised and isovolumetric cystometries were performed after gentle clamping of the bladder neck, with the bladders filled at 75% of their maximal bladder capacity. During cystometries, the urethra was irrigated first with a saline solution and then with a 5HT solution at 100 microM. The same procedure was repeated with a group of young, 5-6 months, female Wistar rats (control group).
Values are presented as mean ± standard deviation. Two tailed unpaired t-test was used to compare mean values of two groups.

The aged group presented 90.40 ± 10.92 5HT-IR cells while the young control group presented 182.5 ± 25.47 5HT-IR cells (P=0,0087). 
The aged females had 0.26 ± 0.14 bladder contractions/minute while females of control group had 0.55 ± 0.11 bladder contractions/minute (P<0.0004). 
Bladder voiding efficiency in aged rats was 49 ± 3%.  while in young controls was 78 ± 2% (p<0,0001). The infusion of 5HT did not change the frequency of bladder contractions of the aged group (0.38 ± 0.15 bladder contractions/minute, P=0.15). However, 5HT infusion improved bladder voiding efficiency of the aged group from 49 ± 3% to 77 ± 7% (P<0.0001).
In aged animals with the bladder filled at 75% of maximal bladder capacity, isovolumetric cystometries measured a constant pressure of 19.45 ± 5.15 cm H2O. Urethral irrigation with saline had no detectable effect on bladder pressure. The infusion of the 5HT solution, in contrast, immediately triggered strong bladder contractions with a peak pressure of 46.85 ± 11.95 cm H2O. The same experiment carried out in young animals gave the same results. Saline infusion of the urethra did not cause any change in the isovolumetric recorded bladder pressure (20±2.5 cmH2O). Urethral irrigation with 5HT immediately evoked bladder contractions at a mean pressure of 35 ±8.6 cmH2O.

In female rats, ageing is associated with a decreased in the number of 5HT-IR cells. Aged female rats had a low bladder voiding efficiency that was corrected to the values of young animals by the urethral perfusion with 5HT solution. Moreover, 5HT irrigation of the urethra triggered strong bladder contractions, indicating the existence of a urethral-vesical reflex mediated be 5HT.

If a decrease in 5HT associated with aging is confirmed in the urethras of humans with idiopathic underactive bladder, 5HT administration to the urethra could improve bladder voiding efficiency and improve LUTS associated with underactive bladder.