TRPM8 is a non-selective cation channel having multiple modes of activation such as cold (<28 degrees celsius) and chemical agents (icilin and menthol), and has been widely expressed in urothelial cells, in sensory nerve fibers within the urothelium and suburothelium of the bladder, and in L6 dorsal root ganglia (DRG) of the rat, and also in the urothelial cells of the human bladder [1]. Because TRPM8 channels have thermal sensitivity, it is possible that drugs acting on TRPM8 can influence body temperature. In fact, hypothermic effects of TRPM8 antagonist have been demonstrated in rats [2], which may be an unfavorable effect for clinical development. RQ-00434739 (RQ) was newly developed as a potent and selective antagonist for TRPM8, and its selectivity for TRPM8 channel is more than 100-fold against other ion channels (TRPA1, TRPV1, TRPM2, Nav1.3, Nav1.5, Nav1.7, Cav2.2 and Cav3.2) [3]. To disclose the physiological role of TRPM8 channel in the bladder and to explore the possibility of RQ as a therapeutic target for bladder sensory disorders, we examined effects of RQ on deep body temperature and on bladder function including the in vivo primary bladder single-unit afferent activities (SAAs) in rats.

Thirty-four female Sprague-Dawley rats were used. The effect of RQ on deep body temperature (intrarectal temperature) was evaluated with intravenous (i.v.) administration of RQ (1 mg/kg) or its vehicle under urethane-anesthesia (1.2 g/kg intraperitoneally). Effects of RQ (1 mg/kg, i.v.) on bladder function were investigated using SAA and cystometry (CMG) measurements during saline-instillation (6 ml/hour). In the SAA measurements, rats were anesthetized with urethane (1.2 g/kg, intraperitoneally) and the SAAs recorded from the left L6 dorsal root were classified by conduction velocity (2.5 m/second) as Aδ- or C-fibers by electrical stimulation of the left pelvic nerve and by bladder distention. In the CMG measurements, 4 days after bladder catheterization, measurements were performed in a conscious free-moving condition, and CMG parameters were analysed before and after drug-administrations for 1 hour in each.

RQ did not affect deep body temperature (Figure 1). In the CMG measurements (N = 6 in each group), mean voided volume was significantly increased after RQ administration (before-administration: 1.39 ± 0.16 ml, after-administration: 1.69 ± 0.10 ml), but such increased response was not seen with vehicle administration. In the SAA measurements, 16 single afferent fibers (n = 8 in each fiber) were isolated from 13 rats. SAAs of C-fibers, but not of Aδ-fibers, were significantly decreased after RQ administration (Figure 2).

RQ (1 mg/kg), of which dose did not influence body temperature, significantly increased mean voided volume and inhibited SAAs of C-fibers during saline-instillation. These results suggest that RQ, a novel TRPM8 antagonist, has an inhibitory effect on the physiological bladder sensory activation via supressing mechanosensitive C-fibers in rats. These results are in line with those of a previous study by measuring ex vivo afferent activities [2].

The present results demonstrated that RQ, a novel TRPM8 antagonist, increases mean voided volume and inhibits activity of bladder mechanosensitive C-fiber without affecting body temperature. RQ may be a promising drug for the treatment of bladder sensory disorders.

Stein, R. J. et al. Cool (TRPM8) and hot (TRPV1) receptors in the bladder and male genital tract. J Urol 172, 1175-1178, doi:10.1097/01.ju.0000134880.55119.cf (2004).
Ito, H. et al. Functional role of the transient receptor potential melastatin 8 (TRPM8) ion channel in the urinary bladder assessed by conscious cystometry and ex vivo measurements of single-unit mechanosensitive bladder afferent activities in the rat. BJU Int 117, 484-494, doi:10.1111/bju.13225 (2016).
Ohshiro, H. et al. In Vitro and In Vivo Charactarization of RQ-00434739: A Novel Orally Active and Selective TRPM8 Antagonist for the Treatment of Oxalipaltin-Induced Peripheral Neuropathic Pain. Poster No. 284 in 16th World Congress on Pain (Sep 26, 2016- Sep 30, 2016, Yokohama, Japan)