Function of the Cold Receptor (TRPM8) Associated with Voiding Dysfunction in Bladder Outlet Obstruction in Rats

Efficacy of KPR-5714, a selective transient receptor potential melastatin 8 antagonist, on chronic psychological stress-induced bladder overactivity in male rats

AIMS

Chronic psychological stress aggravates lower urinary tract symptoms. Among others, water avoidance stress is a chronic psychological stressor that plays a causal role in the exacerbation and development of bladder dysfunction in rats. In this report, the effects of KPR-5714, which is a selective transient receptor potential melastatin 8 (TRPM8) antagonist, on bladder overactivity induced by water avoidance stress were examined.

METHODS

Male rats were subjected to water avoidance stress for 2 h per day for 10 consecutive days. The effects of water avoidance stress on voiding behavior using metabolic cages and histological bladder changes were investigated in rats. The involvement of bladder C-fiber afferent on voiding frequency in rats exposed to water avoidance stress was assessed using capsaicin. The effects of KPR-5714 on storage dysfunction in rats subjected to water avoidance stress were examined.

RESULTS

In voiding behavior measurements, water avoidance stress-induced storage dysfunction, causing a decrease in the mean voided volume and increasing voiding frequency. A comparison of bladders from normal rats and rats exposed to water avoidance stress showed no histological differences. Water avoidance stress-induced bladder overactivity was completely inhibited by pretreatment with capsaicin. KPR-5714 showed a tendency to increase the mean voided volume and significantly decreased the voiding frequency without affecting the total voided volume in these rats.

CONCLUSION

The results suggest that KPR-5714 is a promising option for treating chronic psychological stress-induced bladder overactivity.

The TRPM8 channel as a potential therapeutic target for bladder hypersensitive disorders

In the lower urinary tract, transient receptor potential (TRP) channels are primarily involved in physiological function, especially in cellular sensors responding to chemical and physical stimuli. Among TRP channels, TRP melastatin 8 (TRPM8) channels, responding to cold temperature and/or chemical agents, such as menthol or icilin, are mainly expressed in the nerve endings of the primary afferent neurons and in the cell bodies of dorsal root ganglia innervating the urinary bladder (via Aδ- and C-fibers); this suggests that TRPM8 channels primarily contribute to bladder sensory (afferent) function. Storage symptoms of overactive bladder, benign prostatic hyperplasia, and interstitial cystitis are commonly related to sensory function (bladder hypersensitivity); thus, TRPM8 channels may also contribute to the pathophysiology of bladder hypersensitivity. Indeed, it has been reported in a pharmacological investigation using rodents that TRPM8 channels contribute to the pathophysiological bladder afferent hypersensitivity of mechanosensitive C-fibers. Similar findings have also been reported in humans. Therefore, a TRPM8 antagonist would be a promising therapeutic target for bladder hypersensitive disorders, including urinary urgency or nociceptive pain. In this review article, the functional role of the TRPM8 channel in the lower urinary tract and the potential of its antagonist for the treatment of bladder disorders was described.

UCP1 and TRPM8 Expression in the Brown Fat Did Not Affect the Restriction of Menthol-Induced Hyperthermia by Estradiol in Ovariectomized Rats

Estradiol (E₂) modulates the central and peripheral thermoregulatory responses to cold. Menthol is an agonist of transient receptor potential melastatin type 8 (TRPM8), which is a peripheral cold receptor. E₂ suppresses menthol-induced elevation of body temperature (T_b) in ovariectomized rats, but the mechanism is unknown. The aim of the present study is to investigate the effect of E₂ on uncoupling protein 1 (UCP1), a thermogenic gene, and TRPM8 mRNA levels in ovariectomized rats applied menthol. A silastic tube was implanted in ovariectomized rats with and without E₂ underneath the dorsal skin (E₂(+) and E₂(-) groups), and data loggers for T_b measurement into peritoneal cavity. After application of 10% L-menthol or vehicle to the skin of the whole trunk of rats, T_b was measured for 2 h. The interscapular brown adipose tissue (BAT) and spinal ganglia of cervical, thoracic, and lumbar parts were obtained for RT-qPCR assay. In the menthol application, T_b in the E₂(+) group was lower than that in the E₂(-) group. The UCP1 mRNA in the BAT, TRPM8 mRNA in the BAT and spinal ganglia in all areas did not differ between the E₂(+) and E₂(-) groups. In conclusion, the UCP1 and TRPM8 expression in the brown fat did not affect the restriction of the menthol-induced hyperthermia by estradiol in ovariectomized rats.

Transient receptor potential channels in sensory mechanisms of the lower urinary tract

Disruptions to sensory pathways in the lower urinary tract commonly occur and can give rise to lower urinary tract symptoms (LUTS). The unmet clinical need for treatment of LUTS has stimulated research into the molecular mechanisms that underlie neuronal control of the bladder and transient receptor potential (TRP) channels have emerged as key regulators of the sensory processes that regulate bladder function. TRP channels function as molecular sensors in urothelial cells and afferent nerve fibres and can be considered the origin of bladder sensations. TRP channels in the lower urinary tract contribute to the generation of normal and abnormal bladder sensations through a variety of mechanisms, and have demonstrated potential as targets for the treatment of LUTS in functional disorders of the lower urinary tract.

Changes in the expression and function of the PDE5 pathway in the obstructed urinary bladder

Our study aims to explore changes in bladder contractility and the phosphodiesterase type 5 (PDE5) signalling pathway in response to partial bladder outlet obstruction (PBOO). A surgically induced male rat PBOO model and human obstructed bladder tissues were used. Histological changes were examined by H&E and Masson's trichrome staining. Bladder strip contractility was measured via organ bath. The expressions of nitric oxide synthase (NOS) isoforms, PDE5, muscarinic cholinergic receptor (CHRM) isoforms and PDE4 isoforms in bladder were detected by RT‐PCR and Western blotting. The immunolocalization of the PDE5 protein and its functional activity were also determined. PBOO bladder tissue exhibited significant SM hypertrophy and elevated responsiveness to KCl depolarization and the muscarinic receptor agonist carbachol. NOS isoforms, PDE5, CHRM2, CHRM3 and PDE4A were up‐regulated in obstructed bladder tissue, whereas no change in PDE4B and PDE4D isoform expression was observed. With regard to PDE5, it was expressed in the SM bundles of bladder. Interestingly, obstructed bladder exhibited less relaxation responsiveness to sodium nitroprusside (SNP), but an exaggerated PDE5 inhibition effect. The up‐regulation of PDE5 could contribute to the lack of effect on Q_max for benign prostatic hyperplasia/lower urinary tract symptom (BPH/LUTS) patients treated with PDE5 inhibitors. Moreover, PDE5 (with presence of NO) and PDE4 may serve as new therapeutic targets for bladder diseases such as BPH‐induced LUTS and overactive bladder (OAB).

The Urothelium: Life in a Liquid Environment

The urothelium, which lines the renal pelvis, ureters, urinary bladder, and proximal urethra, forms a high-resistance but adaptable barrier that surveils its mechanochemical environment and communicates changes to underlying tissues including afferent nerve fibers and the smooth muscle. The goal of this review is to summarize new insights into urothelial biology and function that have occurred in the past decade. After familiarizing the reader with key aspects of urothelial histology, we describe new insights into urothelial development and regeneration. This is followed by an extended discussion of urothelial barrier function, including information about the roles of the glycocalyx, ion and water transport, tight junctions, and the cellular and tissue shape changes and other adaptations that accompany expansion and contraction of the lower urinary tract. We also explore evidence that the urothelium can alter the water and solute composition of urine during normal physiology and in response to overdistension. We complete the review by providing an overview of our current knowledge about the urothelial environment, discussing the sensor and transducer functions of the urothelium, exploring the role of circadian rhythms in urothelial gene expression, and describing novel research tools that are likely to further advance our understanding of urothelial biology.

TRPM8 channels: A review of distribution and clinical role

Ion channels are important therapeutic targets due to their plethoric involvement in physiological and pathological consequences. The transient receptor potential cation channel subfamily M member 8 (TRPM8) is a nonselective cation channel that controls Ca²⁺ homeostasis. It has been proposed to be the predominant thermoreceptor for cellular and behavioral responses to cold stimuli in the transient receptor potential (TRP) channel subfamilies and exploited so far to reach the clinical-stage of drug development. TRPM8 channels can be found in multiple organs and tissues, regulating several important processes such as cell proliferation, migration and apoptosis, inflammatory reactions, immunomodulatory effects, pain, and vascular muscle tension. The related disorders have been expanded to new fields ranging from cancer and migraine to dry eye disease, pruritus, irritable bowel syndrome (IBS), and chronic cough. This review is aimed to summarize the distribution of TRPM8 and disorders related to it from a clinical perspective, so as to broaden the scope of knowledge of researchers to conduct more studies on this subject.

Current Knowledge on the Vascular Effects of Menthol

Menthol is a monoterpene alcohol, widely used in several food and healthcare products for its particular odor and flavor. For some decades, menthol has been known to act on the vasculature directly in the endothelium and vascular smooth muscle, with recent studies showing that it also evokes an indirect vascular response via sensory fibers. The mechanisms underlying menthol's vascular action are complex due to the diversity of cellular targets, to the interplay between signaling pathways and to the variability in terms of response. Menthol can evoke either a perfusion increase or decrease in vivo in different vascular territories, an observation that warrants a critical discussion. Menthol vascular actions in vivo seem to depend on whether the vascular territory under analysis has been directly provoked with menthol or is located deep/distant to the application site. Menthol increases perfusion of directly provoked skin regions due to a complex interplay of increased nitric oxide (NO), endothelium-derived hyperpolarization factors (EDHFs) and sensory nerve responses. In non-provoked vascular beds menthol decreases perfusion which might be attributed to heat-conservation sympathetically-mediated vasoconstriction, although an increase in tissue evaporative heat loss due the formulation ethanol may also play a role. There is increasing evidence that several of menthol's cellular targets are involved in cardiovascular diseases, such as hypertension. Thus menthol and pharmacologically-similar drugs can play important preventive and therapeutic roles, which merits further investigation.

Benefits and limitations of animal models in partial bladder outlet obstruction for translational research

The functions of the lower urinary tract have been investigated for more than a century. Lower urinary tract symptoms, such as incomplete bladder emptying, weak urine stream, daytime urinary frequency, urgency, urge incontinence and nocturia after partial bladder outlet obstruction, is a frequent cause of benign prostatic hyperplasia in aging men. However, the pathophysiological mechanisms have not been fully elucidated. The use of animal models is absolutely imperative for understanding the pathophysiological processes involved in bladder dysfunction. Surgical induction has been used to study lower urinary tract functions of numerous animal species, such as pig, dog, rabbit, guinea pig, rat and mouse, of both sexes. Several morphological and functional modifications under partial bladder outlet obstruction have not only been observed in the bladder, but also in the central nervous system. Understanding the changes of the lower urinary tract functions induced by partial bladder outlet obstruction would also contribute to appropriate drug development for treating these pathophysiological conditions. In the present review, we discuss techniques for creating partial bladder outlet obstruction, the characteristics of several species, as well as issues of each model, and their translational value.

Novel selective, potent naphthyl TRPM8 antagonists identified through a combined ligand- and structure-based virtual screening approach

Transient receptor potential melastatin 8 (TRPM8), a nonselective cation channel, is the predominant mammalian cold temperature thermosensor and it is activated by cold temperatures and cooling compounds, such as menthol and icilin. Because of its role in cold allodynia, cold hyperalgesia and painful syndromes TRPM8 antagonists are currently being pursued as potential therapeutic agents for the treatment of pain hypersensitivity. Recently TRPM8 has been found in subsets of bladder sensory nerve fibres, providing an opportunity to understand and treat chronic hypersensitivity. However, most of the known TRPM8 inhibitors lack selectivity, and only three selective compounds have reached clinical trials to date. Here, we applied two virtual screening strategies to find new, clinics suitable, TRPM8 inhibitors. This strategy enabled us to identify naphthyl derivatives as a novel class of potent and selective TRPM8 inhibitors. Further characterization of the pharmacologic properties of the most potent compound identified, compound 1, confirmed that it is a selective, competitive antagonist inhibitor of TRPM8. Compound 1 also proved itself active in a overreactive bladder model in vivo. Thus, the novel naphthyl derivative compound identified here could be optimized for clinical treatment of pain hypersensitivity in bladder disorders but also in different other pathologies.

TRPM8: a potential target for cancer treatment

Transient receptor potential (TRP) cation channel superfamily plays critical roles in variety of processes, including temperature perception, pain transduction, vasorelaxation, male fertility, and tumorigenesis. One of seven families within the TRP superfamily of ion channels, the melastatin, or TRPM family comprises a group of eight structurally and functionally diverse channels. Of all the members of TRPM subfamily, TRPM8 is the most notable one. A lot of literatures have demonstrated that transient receptor potential melastatin 8 (TRPM8) could perform a myriad of functions in vertebrates and invertebrates alike. In addition to its well-known function in cold sensation, TRPM8 has an emerging role in a variety of biological systems, including thermoregulation, cancer, bladder function, and asthma. Recent studies have shown that TRPM8 is necessary to the initiation and progression of tumors, and the aberrant expression of TRPM8 was found in varieties of tumors, such as prostate tumor, melanoma, breast adenocarcinoma, bladder cancer, and colorectal cancer, making it a novel molecular target potentially useful in the diagnosis and treatment of cancer. This review outlines our current understanding on the role of TRPM8 in occurrence and development of different kinds of tumor and also includes discussion about the regulation of TRPM8 during carcinogenesis as well as therapeutic potential of targeting TRPM8 in tumor, which may be utilized for a potential pharmacological use as a target for anti-cancer therapy.

On the origin of spontaneous activity in the bladder

OBJECTIVES

To characterise separately the pharmacological profiles of spontaneous contractions from the mucosa and detrusor layers of the bladder wall and to describe the relationship in mucosa between adenosine triphosphate (ATP) release and spontaneous contractions.

MATERIALS AND METHODS

Spontaneous contractions were measured (36 °C) from isolated mucosa or detrusor preparations, and intact (mucosa + detrusor) preparations from guinea-pig bladders. Potential modulators were added to the superfusate. The percentage of smooth muscle was measured in haematoxylin and eosin stained sections. ATP release was measured in superfusate samples from a fixed point above the preparation using a luciferin-luciferase assay.

RESULTS

The magnitude of spontaneous contractions was in the order intact >mucosa >detrusor. The percentage of smooth muscle was least in mucosa and greatest in detrusor preparations. The pharmacological profiles of spontaneous contractions were different in mucosa and detrusor in response to P2X or P2Y receptor agonists, adenosine and capsaicin. The intact preparations showed responses intermediate to those from mucosa and detrusor preparations. Low extracellular pH generated large changes in detrusor, but not mucosa preparations. The mucosa preparations released ATP in a cyclical manner, followed by variations in spontaneous contractions. ATP release was greater in mucosa compared with detrusor, augmented by carbachol and reversed by the M2 -selective antagonist, methoctramine.

CONCLUSIONS

The different pharmacological profiles of bladder mucosa and detrusor, implies different pathways for contractile activation. Also, the intermediate responses from intact preparations implies functional interaction. The temporal relationship between cyclical variation of ATP release and amplitude of spontaneous contractions is consistent with ATP release controlling spontaneous activity. Carbachol-mediated ATP release was independent of active contractile force.

Research Findings on Overactive Bladder

Several physiopathologic conditions lead to the manifestation of overactive bladder (OAB). These conditions include ageing, diabetes mellitus, bladder outlet obstruction, spinal cord injury, stroke and brain injury, Parkinson's disease, multiple sclerosis, interstitial cystitis, stress and depression. This review has discussed research findings in human and animal studies conducted on the above conditions. Several structural and functional changes under these conditions have not only been observed in the lower urinary tract, but also in the brain and spinal cord. Significant changes were observed in the following areas: neurotransmitters, prostaglandins, nerve growth factor, Rho-kinase, interstitial cells of Cajal, and ion and transient receptor potential channels. Interestingly, alterations in these areas showed great variation in each of the conditions of the OAB, suggesting that the pathophysiology of the OAB might be different in each condition of the disease. It is anticipated that this review will be helpful for further research on new and specific drug development against OAB.

Caffeine enhances micturition through neuronal activation in micturition centers

Caffeine may promote incontinence through its diuretic effect, particularly in individuals with underlying detrusor overactivity, in addition to increasing muscle contraction of the bladder smooth muscle. Caffeine may also affect bladder function via central micturition centers, including the medial preoptic area, ventrolateral periaqueductal gray, and pontine micturition center. However, the biochemical mechanisms of caffeine in central micturition centers affecting bladder function remain unclear. In the present study, the effects of caffeine on the central micturition reflex were investigated by measuring the degree of neuronal activation, and by quantifying nerve growth factor (NGF) expression in rats. Following caffeine administration for 14 days, a urodynamic study was performed to assess the changes to bladder function. Subsequently, immunohistochemical staining to identify the expression of c‑Fos and NGF in the central micturition areas was performed. Ingestion of caffeine increased bladder smooth muscle contraction pressure and time as determined by cystometry. Expression levels of c‑Fos and NGF in all central micturition areas were significantly increased following the administration of caffeine. The effects on contraction pressure and time were the most potent and expression levels of c‑Fos and NGF were greatest at the lowest dose of caffeine. These results suggest that caffeine facilitates bladder instability through enhancing neuronal activation in the central micturition areas.

The region adjacent to the C-end of the inner gate in transient receptor potential melastatin 8 (TRPM8) channels plays a central role in allosteric channel activation

The ability of transient receptor potential (TRP) channels to sense and respond to environmental and endogenous cues is crucial in animal sensory physiology. The molecular mechanism of channel gating is yet elusive. The TRP box, a conserved region in the N-end of the C terminus domain, has been signaled as pivotal for allosteric activation in TRP channels. Here, we have examined the role of the linker region between the TRPM8 inner gate and the TRP box (referred to as the S6-TRP box linker) to identify structural determinants of channel gating. Stepwise substitutions of segments in the S6-TRP box linker of TRPM8 channel with the cognate TRPV1 channel sequences produced functional chimeric channels, and identified Tyr(981) as a central molecular determinant of channel function. Additionally, mutations in the 986-990 region had a profound impact on channel gating by voltage and menthol, as evidenced by the modulation of the conductance-to-voltage (G-V) relationships. Simulation of G-V curves using an allosteric model for channel activation revealed that these mutations altered the allosteric constants that couple stimuli sensing to pore opening. A molecular model of TRPM8, based on the recently reported TRPV1 structural model, showed that Tyr(981) may lie in a hydrophobic pocket at the end of the S6 transmembrane segment and is involved in inter-subunit interactions with residues from neighbor subunits. The 986-990 region holds intrasubunit interactions between the TRP domain and the S4-S5 linker. These findings substantiate a gating mechanism whereby the TRP domain acts as a coupling domain for efficient channel opening. Furthermore, they imply that protein-protein interactions of the TRP domain may be targets for channel modulation and drug intervention.

TRP channels in lower urinary tract dysfunction

Lower urinary tract dysfunction (LUTd) represents a major healthcare problem. Although it is mostly not lethal, associated social disturbance, medical costs, loss of productivity and especially diminished quality of life should not be underestimated. Although more than 15% of people suffer from a form of LUTd to some extent, pathophysiology often remains obscure. In the past 20 years, transient receptor potential (TRP) channels have become increasingly important in this field of research. These intriguing ion channels are believed to be the main molecular sensors that generate bladder sensation. Therefore, they are intensely pursued as new drug targets for both curative and symptomatic treatment of different forms of LUTd. TRPV1 was the first of its class to be investigated. Actually, even before this channel was cloned, it had already been targeted in the bladder, with clinical trials of intravesical capsaicin instillations. Several other polymodally gated TRP channels, particularly TRPM8, TRPA1 and TRPV4, also appear to play a prominent role in bladder (patho)physiology. With this review, we provide a brief overview of current knowledge on the role of these TRP channels in LUTd and their potential as molecular targets for treatment.

Menthol relaxes rat aortae, mesenteric and coronary arteries by inhibiting calcium influx

Menthol, a naturally occurring compound in mint, is known to give cold sensation. However, previous findings about its pharmacological activity in blood vessels are full of paradox. The present study investigated the action of menthol on vascular reactivity in different arteries isolated from male Sprague-Dawley rats, e.g. aortae, main mesenteric arteries and coronary arteries. The arterial segments were suspended in organ bath or in wire myograph for measurement of isometric force. Menthol concentration-dependently relaxed all three arteries with similar relaxing sensitivity in arteries contracted by different contractors, KCl, U46619 (9,11-dideoxy-9α,11α-methanoepoxy Prostaglandin F2α) and phenylephrine. Menthol-induced relaxations were unaffected by nitric oxide synthase inhibitor L-NAME, soluble guanylyl cyclase inhibitor ODQ, or mechanical removal of endothelium. Menthol also concentration-dependently suppressed 60mM KCl-induced constriction and CaCl2-induced contraction in Ca(2+)-free K(+)-containing solution. Calcium fluorescent imaging using fluo-4 showed that 10 min-incubation with 1mM menthol inhibited 60mM KCl-induced Ca(2+) influx in rat aortic smooth muscle cell line A7r5 and vascular smooth muscle of coronary arteries. To conclude, menthol induces relaxation and inhibits contraction in rat aortae, mesenteric and coronary arteries primarily through inhibiting Ca(2+) influx via nifedipine-sensitive Ca(2+) channels in vascular smooth muscle.