Role of the serotonergic system in urethral continence reflexes during sneezing in rats

Urethral pharmacological mechanisms incontinence and bladder emptying: An updated review

Background

The urethral wall consists of layers of striated muscle, circular and longitudinal smooth muscles, collagen fibers, and a vascular plexus. However, the relative contributions of these components to urethral pressure in humans remain poorly understood. The circular and longitudinal smooth muscle components can develop a spontaneous contractile activity, generating a basal tone. They can further contract or relax in response to excitatory or inhibitory stimuli. Animal studies suggest that smooth muscle activity in the mid-urethra plays a crucial role in determining maximal urethral closing pressure. Notably, the highest sympathetic activity occurs in the middle segment of the female urethra during increasing smooth muscle tone. This finding is supported by human studies that did not detect any electromyographic activity from striated muscle in this region.

Objectives

This study was conducted to review the contributions of the primary structural components and control mechanisms of urethral.

Conclusion

In females, the external urethral striated sphincter is located at the distal urethra, which is not the segment associated with the highest closing pressure. Rather, the sphincter has been shown to modulate urethral pressure during exercise and physical stress. Basic science research does not support the notion that mid-urethral pressure is caused by the external striated sphincter tone in females. Instead, findings suggest that, at rest and during bladder filling, maximal urethral pressure is primarily determined by the activity of the urethral smooth muscles.

Citalopram improves vasomotor syndrome and urogenital syndrome of menopause in Mexican women: a randomized clinical trial

PURPOSE

This study aimed to determine the efficacy of non-hormonal therapy with citalopram vs fluoxetine for treating vasomotor syndrome (VMS) and urogenital syndrome of menopause (GSM) in Mexican women.

METHODS

A parallel prospective randomized clinical trial was conducted in 91 postmenopausal women with a total score on the Menopause Rating Scale (MRS) ≥ 17 and with the clinical diagnosis of VSM and GSM. Patients were randomly assigned to receive citalopram (n = 49) or fluoxetine (n = 42). Follow-up was carried out at 3 and 6 months.

RESULTS

The citalopram group experienced a significant improvement compared to the fluoxetine group in the MRS total score (p < 0.01), as well as in the psychological (p < 0.001) and somatic (p < 0.0001) domains at 3 and 6 months of follow-up. After 6 months of follow-up, the group that received citalopram decreased the relative risk (RR) to present VMS symptoms (RR = 0.30, CI 0.19-0.5, p = 0.0001), depressed mood (RR = 0.31, CI 0.15-0.6, p = 0.0002), irritability (RR = 0.40, CI 0.22-0.73, p = 0.002), anxiety (RR = 0.30, CI 0.13-0.69, p = 0.003), physical and mental exhaustion (RR = 0.35, CI 0.18-0.67, p = 0.001), sexual problems (RR = 0.18, CI 0.06-0.48, p = 0.0001), vaginal dryness (RR = 0.34, CI 0.14-0.80, p = 0.01), and urinary problems (RR = 0.36, CI 0.14-0.92, p = 0.043).

CONCLUSION

We conclude that citalopram tends to improve VSM and GSM symptoms in postmenopausal Mexican women. Thus, we recommend the daily use of citalopram 20 mg. However, further studies will be required to support the results of the present work. These should include a larger number of patients and a placebo group.

CLINICAL TRIAL REGISTRATION

This clinical trial was retrospectively registered by the United States National Library of Medicine in the www.

CLINICALTRIALS

gov database on 04/20/2022. The given test Registration Number is NCT05346445.

Effect of single doses of citalopram and reboxetine on urethral pressure: A randomized, double-blind, placebo- and active-controlled three-period crossover study in healthy women

AIMS

Urethral closure function is essential for urinary continence in women and decreased urethral pressure is associated with stress urinary incontinence (SUI). For decades, the effects of serotonergic drugs on central neural control of urethral closure have been investigated and discussed. Epidemiological studies suggest that the use of selective serotonin reuptake inhibitors (SSRIs), such as citalopram, is associated with SUI. However, the literature findings are conflicting. This study aimed to evaluate citalopram's effect on opening urethral pressure (OUP) in healthy women.

METHODS

We conducted a randomized, double-blind, placebo- and active-controlled crossover study in 24 healthy women. On three study days, which were separated by 8 days of washout, the subjects received single doses of either 40 mg citalopram (and placebo_reboxetine ), 8 mg reboxetine (and placebo_citalopram ), or two placebos. Study drugs were administered at a 1-h interval due to a difference in estimated time to peak plasma concentration (t_max ). We measured OUP with urethral pressure reflectometry under both resting and squeezing conditions of the pelvic floor at estimated t_max for both study drugs (one timepoint).

RESULTS

Compared to placebo, citalopram increased OUP by 6.6 cmH₂ 0 (95% confidence interval [CI] 0.04-13.1, p = 0.048) in resting condition. In squeezing condition, OUP increased by 7.1 cmH₂ 0 (95% CI: 1.3-12.9, p = 0.01). Reboxetine increased OUP by 30.0 cmH₂ 0 in resting condition compared to placebo (95% CI: 23.5-36.5, p < 0.001), and 27.0 cmH₂ 0 (95% CI: 21.2-32.8, p < 0.001) in squeezing condition.

CONCLUSION

Citalopram increased OUP slightly compared to placebo suggesting that SSRI treatment does not induce or aggravate SUI.

Increasing Effects of Selective 5-Hydroxytryptamine Type 2C Receptor Stimulation on Evoked Momentary Urethral Closure in Female Rats and Humans

Under healthy conditions, more than one urethra-closing reflex, including both bladder afferent-independent and -dependent actions, function during momentary elevation of intravesical (bladder) pressure to prevent urinary incontinence. In the current study, the effects of a novel selective 5-hydroxytryptamine type 2C (5-HT2C) receptor agonist, TAK-233, on evoked momentary urethra-closing functions were investigated in female rats and humans to elucidate 5-HT2C receptor functions. In anesthetized female rats, TAK-233 dose-dependently and significantly increased urethral resistance during sneezing in rats with distended vaginas and bilaterally transected pelvic nerves. The drug also dose-dependently and significantly increased urethral resistance during momentary intravesical pressure elevation by electrical stimulation of abdominal muscles in rats with a transected spinal cord at the T8-T9 level and intact pelvic nerves. The increased effects observed during electrical stimulation were abolished by either an intravenously administered selective 5-HT2C receptor antagonist, SB 242084, or bilateral transection of the pelvic nerves or somatic nerves innervating the external urethral sphincter and pelvic floor muscles. In the spinal cord-transected and pelvic nerve-intact rats, TAK-233 enlarged the urethra-closing responses induced by both passive and abrupt intravesical pressure elevation, measured by a microtip transducer located in the middle urethra. Additionally, the effects of TAK-233 on the stimulus threshold of urethral contractile responses induced by transcranial magnetic stimulation were investigated in healthy female volunteers. The drug dose-dependently and significantly lowered this stimulus threshold, indicating an increased sensitivity of the response. These results demonstrate that 5-HT2C receptor stimulation enhances the evoked momentary urethra-closing functions in both female rats and humans. SIGNIFICANCE STATEMENT: 5-hydroxytryptamine (serotonin) type 2C (5-HT2C) receptor stimulation by TAK-233 enhanced urethral resistance in rats during an evoked momentary event in which the bladder afferent-independent or -dependent reflex functions via striated muscle-mediated mechanisms. The increases in sensitivity of transcranial magnetic stimulation-evoked urethral contractile responses in healthy female subjects indicates that this mechanism also functions in humans. The evoked momentary conditions activating these reflexes provide a suitable model to demonstrate the effects of 5-HT2C receptor stimulation.

[Overview of pharmacological mechanisms controlling micturition in the central nervous system]

The functions of the lower urinary tract, to storage and periodically release urine, are dependent on the activity of smooth and striated muscles in the bladder, urethra and external urethral sphincter. This activity is in turn controlled by neural circuits not only in the periphery, but also in the central nervous system (CNS). During urine storage, the outlet is closed and the bladder smooth muscle is quiescent by the neural control mechanism mainly organized in the spinal cord. When bladder volume reaches the micturition threshold, activation of a micturition center in the dorsolateral pons (the pontine micturition center) induces micturition through activation of sacral parasympathetic (pelvic) nerves. The brain rostral to the pons (diencephalon and cerebral cortex) is also involved in excitatory and inhibitory regulation of the micturition reflex. Various transmitters including dopamine, serotonin, norepenephrine, GABA, excitatory and inhibitory amino acids, opioids and acetylcholine are implicated in the modulation of the micturition reflex in the CNS. Therefore, injury or neurodegenerative diseases of the CNS as well as drugs can produce bladder and urethral dysfunctions such as urinary frequency, urgency and incontinence or inefficient bladder emptying.

[Therapeutic new targets for stress urinary incontinence in the central nervous system]

Stress urinary incontinence (SUI) is a common and bothersome problem among middle-aged women. However, there are few useful drugs for SUI. Urethral hypermobility and intrinsic sphincter deficiency are two main causes of SUI. Various animal models of SUI, such as vaginal distention, pudendal nerve injury, or ovariectomy, have been developed to study the pathophysiology of SUI. In addition, we have previously reported that cerebral infarction rats also induce SUI. Leak point pressure measurements are the most commonly used methods to evaluate the urethral dysfunction in SUI animal models. Originally, we have developed microtransducer-tipped catheter measurements of urethral activity during sneezing. Previous or our basic research has clarified potential strategies for pharmacotherapy of SUI in the central nervous system. Therapeutic targets include adrenergic and serotonergic (5-HT) receptors in the spinal cord, which stimulate pudendal nerve innervating the external urethral sphincter and/or sympathetic nerve innervating urethral smooth muscle. Activation of α₁-adrenoceptors, 5-HT_2C, or 5-HT₇ receptors enhances the reflex at the spinal cord level whereas pre- or postsynaptic α₂-adrenoceptors and/or 5-HT_1A receptors inhibit the reflex. We have recently reported that stimulation of the spinal μ-opioid receptors by tramadol also enhances the reflex. Thus, we review the recent advances in basic SUI research and potential targets for pharmacotherapy of SUI in the central nervous system.

Effect of a 5-HT2c receptor agonist on urethral closure mechanism in healthy women

AIMS

To evaluate the effect of ASP2205, a selective serotonin 5-HT_2c receptor agonist, and Duloxetine on the urethral pressure in healthy female subjects.

METHODS

Healthy females aged 18 to 55 years were recruited for this phase 1, single site, placebo-controlled, randomized, four-period, cross-over study. The interventions were single oral doses of 10 and 60 mg ASP2205, 80 mg duloxetine, and placebo. As a pharmacodynamics endpoint, opening urethral pressure (OUP), corrected for placebo, was measured using urethral pressure reflectometry under both resting and squeezing condition of the pelvic floor at predose and 3, 6, 12, and 24 hours after dosing. Safety and tolerability of ASP2205 were also compared with duloxetine and placebo.

RESULTS

Eighteen healthy women signed informed consent, however, one dropped out before dosing and one dropped out after the first period, therefore, 16 subjects completed the study. Duloxetine significantly increased the OUP during both resting and squeezing condition (maximal increase 18.1 and 16.8 cmH₂ O, respectively). Both doses of ASP2205 did not increase OUP at any time point. During squeezing OUP decreased significantly in the ASP2205 60 mg group from 6 to 24 hours after dosing. All subjects experienced predominantly central nervous system-related side effects (eg, dizziness and nausea) during ASP2205 treatment, which was most pronounced at 60 mg.

CONCLUSIONS

ASP2205, a serotonin 5-HT_2c receptor agonist, does not increase the urethral pressure and it is therefore unlikely that 5-HT _2c receptor agonists can be used as a treatment for stress urinary incontinence. ASP2205 was less well tolerated than the high dose of duloxetine.

Effect of ASP2205 fumarate, a novel 5-HT2C receptor agonist, on urethral closure function in rats

The pharmacological profile of ASP2205 fumarate (ASP2205), a novel 5-HT_2C receptor agonist, was evaluated in vitro and in vivo. ASP2205 showed potent and selective agonistic activity for the human 5-HT_2C receptor, with an EC₅₀ of 0.85 nM in the intracellular Ca²⁺ mobilization assay. Rat 5-HT_2C receptor was also activated by ASP2205 with an EC₅₀ of 2.5 nM. Intraduodenal administration (i.d.) of ASP2205 (0.1-1 mg/kg) significantly elevated the leak point pressure (LPP) in anesthetized rats in a dose-dependent manner. This ASP2205 (0.3 mg/kg i.d.)-induced LPP elevation was inhibited by SB242084 (0.3 mg/kg i.v.), a selective 5-HT_2C receptor antagonist. Urethral closure responses induced by intravesical pressure loading in rats were enhanced by ASP2205 (0.3 mg/kg i.v.), which was abolished by pretreatment with SB242084 (0.3 mg/kg i.v.) and bilateral transection of the pudendal nerve. In contrast, ASP2205 (0.3 mg/kg i.v.) did not change the resting urethral pressure in rats. These results indicate that ASP2205 can enhance the pudendal nerve-mediated urethral closure reflex via the 5-HT_2C receptor, resulting in the prevention of involuntary urine loss.