Functional roles of TRPV1 and TRPV4 in control of lower urinary tract activity: dual analysis of behavior and reflex during the micturition cycle

The present study used a dual analysis of voiding behavior and reflex micturition to examine lower urinary tract function in transient receptor potential vanilloid (TRPV)1 knockout (KO) mice and TRPV4 KO mice. In metabolic cage experiments conducted under conscious conditions (i.e., voluntary voiding behavior), TRPV4 KO mice showed a markedly higher voiding frequency (VF; 19.3 ± 1.2 times/day) and a smaller urine volume/voiding (UVV; 114 ± 9 μl) compared with wild-type (WT) littermates (VF: 5.2 ± 0.5 times/day and UVV: 380 ± 34 μl). Meanwhile, TRPV1 KO mice showed a similar VF to WT littermates (6.8 ± 0.5 times/day) with a significantly smaller UVV (276 ± 20 μl). Water intake among these genotypes was the same, but TRPV4 KO mice had a larger urine output than the other two groups. In cystometrogram experiments conducted in decerebrate unanesthetized mice (i.e., reflex micturition response), no differences between the three groups were found in any cystometrogram variables, including voided volume, volume threshold for inducing micturition contraction, maximal voiding pressure, and bladder compliance. However, both TRPV1 KO and TRPV4 KO mice showed a significant number of nonvoiding bladder contractions (NVCs; 3.5 ± 0.9 and 2.8 ± 0.7 contractions, respectively) before each voiding, whereas WT mice showed virtually no NVCs. These results suggest that in the reflex micturition circuit, a lack of either channel is involved in NVCs during bladder filling, whereas in the forebrain, it is involved in the early timing of urine release, possibly in the conscious response to the bladder instability.