Hypothesis / aims of study
Bladder dysfunction is common post-menopause (1), but it is unclear whether bladder changes are evident during the perimenopause transition. Recently a NK3 receptor antagonist was approved for vasomotor symptoms of perimenopause (2). NK3 receptors may also influence the micturition reflex in perimenopause, although this is still not clear. The aim of this study was to use a preclinical model to determine the impact of perimenopause on cystometric parameters and investigate the role of neurokinin 3 receptors.
Study design, materials and methods
Female C57/BL6J mice (12-week-old; n=8 per group) were randomly allocated to a control or perimenopause group. Mice in the perimenopause group were treated with 4-vinylcyclohexene diepoxide (VCD) (160mg/kg i.p. for 10 consecutive days from day 0). Age-matched controls receive daily vehicle injections. Oestrus cycle length was determined using vaginal cytology, while voiding behaviour was recorded before treatment and prior to sacrifice. On day 100 animals were anaesthetised (urethane), the bladder cannulated and intravesical pressure recorded for cystometric analysis. Substance P (SP) and senktide (NK1 and NK3 receptor agonists respectively), were added serosally to the bladder (both at 10 nmol/L). Data (mean ±SEM) were compared via Student’s t-test.
Results
Vaginal cytology confirmed that all mice were cycling normally prior to treatment (4 or 5 days). Animals in the VCD-treated group (but not the control group) developed abnormal cycles characterized by extended days in oestrus. Voiding behaviour was not altered in the perimenopausal animals.
Cystometry - Intercontractile interval, baseline bladder pressure, voiding threshold pressure, voiding peak pressure, frequency and amplitude of non-voiding contractions and bladder compliance were not different in perimenopausal mice. However, pressure responses to KCl (60mM) were significantly reduced (P<0.01) in perimenopausal mice (23.3±0.9 mmHg in controls vs 18.4±1.1mmHg in VCD-treated animals, Figure 1B).
Effect of NK agonists – Senktide (NK3-agonist) reduced baseline intravesical pressure in both groups by 13.6±1.4% (P<0.001) in controls and 24.5±8.4% (P<0.05) in perimenopausal mice (Figure 1C). The threshold pressure at which voiding occurred and the peak voiding pressure were also reduced by senktide similarly in both groups: by 22.0±5.5% (P<0.05) and 4.2±1.2% (P<0.01) respectively (controls) and by 22.3±6.5% (P<0.05) and 4.1±1.5 (P<0.01) respectively (perimenopause) (Figure 1D). Bladder compliance was increased by senktide in controls (by 40.6 ± 8.4%, P<0.01), although not in perimenopause. In terms of non-voiding contractions, senktide reduced the threshold pressure for contractions by 25.6±4.2% (controls, P<0.01) and 26.4±9.3% (perimenopause, P<0.05). Peak pressure for non-voiding contractions was also reduced by senktide (by 24.6±3.7% in controls (P<0.001) and 26.5±5.2% in perimenopause (P<0.01)).
In contrast, SP (NK1-agonist) had minimal effects, reducing only baseline pressure slightly, and only in perimenopause (from 3.2±0.5 mmHg to 3.0±0.5 mmHg, P<0.05).
Interpretation of results
VCD induced a perimenopausal-like state with abnormal oestrus cycles. Overall micturition parameters were unaffected, even though direct detrusor contraction (response to KCl) was reduced, suggesting that compensatory mechanisms may be at play to maintain voiding function in the transition phase of menopause.
The results with the neurokinin agonists suggest that NK1 receptors have little influence on micturition during cystometry, but that NK3 receptor stimulation reduces the threshold and peak pressures for voiding and for initiating non-voiding detrusor contractions. These NK3 receptor-mediated effects were observed in normal cycling mice and were maintained in the perimenopausal state.