Hypothesis / aims of study
In previous study, unanesthetized female mice that lack for metabotropic glutamate receptor subtype 1 (mGluR1) gene presented much greater micturition volume, compared with wild type mice . Meanwhile, other study showed that systemic administration of mGluR1 antagonist only has little effects on lower urinary tract activity in conscious male rats . The different results between the two studies may be attributable to the species; however, the sex difference as an influential factor should not be ignored.
Recently, medical research has started to understand the importance of taking the sex into account as the gene-associated phenotypes and response to drug treatment may be very different between sexes. Thus, the aim of this study was to examine if mGluR1 gene-associated phenotype in lower urinary tract activity is affected by the sex.
Study design, materials and methods
We used 12-13 week-old mGulR1-knockout (KO) mice (n=18 for each sex) that were backcrossed on a C57BL/6N background as well as wild-type (WT) littermates. In this study, a dual analysis of voiding behavior and reflex micturition was conducted to examine lower urinary tract function in these mice . For evaluating micturition behavior, conscious mice were individually placed in metabolic cages, and frequency-volume charts (FVC) were measured. For assessing reflex micturition, mice were decerebrated under sevoflurane anesthesia and cystometrogram (CMG) recordings were conducted under unanesthetized conditions by continuously infusing saline (10 μl/min). Evaluated parameters are: water intake (ml/day), urine output (ml/day), number of voiding (i.e., urinary frequency) (times), and urine volume/void (μl) for metabolic cage study; and voiding volume per micturition (VV, μl), volume threshold for inducing micturition (VT, μl), voiding efficiency (VE, %), and maximal voiding pressure (MVP, mmHg) for CMG study. All values are expressed as mean ± S.E.M. Statistical analyses were made using Mann-Whitney test and p < 0.05 was considered significant (*p < 0.05, **p < 0.01, ***p < 0.001).
Analysis of voluntary voiding behavior in the metabolic cage (Fig. 1):
Water intake, urine output, number of voiding, and urine volume/void were evaluated per 24 h, per 12 h in the dark period, and per 12 h in the light period. In females, KO mice presented greater urine volume/void and smaller number of voiding than WT mice. However, no differences in water intake and urine output were found between KO and WT. Meanwhile, in males, no differences were found in any variables evaluated in the study.
Evaluation of reflex activity of the lower urinary tract during CMG (Table 1):
Female KO mice showed larger VV (89.5%) and VT (92.2%), and lower VE (3.5%), compared with female WT mice. Meanwhile, male KO mice presented larger VT (36.2%) and lower VE (4.0%) compared with male WT mice, but no difference in VV.
Interpretation of results
The dual analysis implied that the neural pathway via mGluR1 is involved in excitatory afferent signal transmission from the bladder. FVC revealed significant differences between KO and WT in urine volume/void and urinary frequency in females, but not in males. Likewise, CMG showed significant difference between KO and WT in VV in females but not in males, although it revealed differences between the two groups in other evaluated variables (i.e., VT, VE, MVP) in both males and females. Overall, however, the differences in variables during storage phase between KO and WT are more markedly presented in females than in males. Higher MVP in KO, compared with that in WT, is common in both males and females and can be due to greater bladder contractility, higher urethral resistance, or both.