M3 receptor mRNA expression in the canine urinary bladder is affected by gender and neutering

Victoria Coit, Fiona Dowell, Neil Evans
Glasgow University Veterinary School, Glasgow, United Kingdom

Acquired urinary incontinence in the bitch (AUI) is currently reported to affect 20% of neutered bitches and <1% of intact bitches and males (Holt et al., 1993); while the full aetiology remains unknown, it is likely to be a multifactorial condition. Our previous in vitro study demonstrated that neutering is associated with a decrease in the responsiveness of the canine detrusor muscle to muscarinic stimulation, independent of gender (Coit et al., 2006). As the M3 receptor is thought to be receptor primarily responsible for bladder contraction and emptying in healthy animals and humans (Abrams et al., 2006), the aim of this study was to investigate whether the changes in bladder responsiveness associated with neutering were due to changes in the expression of the mRNA for M3 receptor in the bladder wall.

Sections of bladder wall were dissected post mortem from entire and neutered male and female dogs (ME, MN, FE, FN respectively) with informed owner consent. mRNA was extracted, reverse transcribed and M3 receptor mRNA expression quantified by rtPCR (arbitory units, relative to β-actin). Results are expressed as mean ± s.e.mean (n= No of dogs). To normalise the distribution, data were log transformed prior to statistical analysis (2-way ANOVA with Bonferroni post-tests).

ANOVA indicated significant effects of neutering, gender and an interaction between gender and neutering. In both sexes, neutering was associated with significantly (p<0.005) increased expression of the mRNA for the M3 receptor, FN 144.67±14.81 (n=12) compared to FE 19.48±2.21 (n=19), and MN 31.56±3.66 (n=13) compared to ME 18.60±1.52 (n=31). M3 receptor mRNA was similar between the two entire groups but expression was significantly (p<0.05) higher in the FN compared to the MN group. There were no statistically significant effects of age or weight of the animals on the results.

These results demonstrate that in the canine, neutering is associated with increased M3 mRNA expression in the bladder, this effect being more pronounced in the female than the male. While, based upon our previous study looking at contractility and sensitivity to muscarinic stimulation, it might have been hypothesised that M3 receptor mRNA expression would be decreased after neutering, it is possible that mRNA expression levels are increased following neutering due to a reduction in translation. Alternatively, increased M3 receptor mRNA expression post neutering may reflect a compensatory mechanism by which the body attempts to counteract a decrease in contractility, caused by an as yet undetermined mechanism. It is interesting to note that the changes post neutering in M3 receptor expression were more pronounced in the females than the males, as development of AUI predominantly occurs in the female; this suggests that changes in M3 receptor mRNA expression could be associated with AUI. Unfortunately our attempts to quantify M3 protein expression in the canine bladder have as yet been unsuccessful; however, this is an area that requires further study as it could potentially lead to the development of new targets and therapies for treating AUI in the dog and potentially in other species.

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Coit, V., et al. (2006). Br Pharml Soc, Abstracts, 75th Anniversary Meeting, C007.
Holt, P.E. & Thrusfield, M.V. (1993). Vet Rec, 133, 177-80.