Effect of methysergide on pudendal inhibition of micturition reflex in cats

Highlights

• Pudendal neuromodulation inhibits nociceptive bladder overactivity.

• 5-HT₂ receptor mechanism is involved in pudendal neuromodulation.

• 5-HT₂ receptor plays an excitatory role in micturition reflex.

• Opioid mechanism interacts with 5-HT₂ mechanism in micturition reflex.

• 5-HT₂ receptor could be a new target for pharmacotherapy of overactive bladder.

Abstract

The role of 5-HT₂ and opioid receptors in pudendal inhibition of bladder activity induced by intravesical infusion of saline or 0.25% acetic acid (AA) was investigated in anesthetized cats using methysergide (a 5-HT₂ receptor antagonist) and naloxone (an opioid receptor antagonist). AA irritated the bladder and significantly (P < 0.0001) reduced bladder capacity to 27.0 ± 7.4% of saline control capacity. Pudendal nerve stimulation (PNS) at multiples of the threshold (T) intensity for inducing anal sphincter twitching restored bladder capacity to 60.1 ± 8.0% at 1–2T (P < 0.0001) and 92.2 ± 14.1% at 3–4T (P = 0.001) of the saline control capacity. Methysergide (0.03–1 mg/kg, i.v.) suppressed low intensity (1–2T) PNS inhibition but not high intensity (3–4T) inhibition, and also significantly (P < 0.05) increased control bladder capacity at the dosage of 0.3–1 mg/kg. During saline infusion without AA irritation, PNS significantly increased bladder capacity to 150.8 ± 9.9% at 1–2T (P < 0.01) and 180.4 ± 16.6% at 3–4T (P < 0.01) of the saline control capacity. Methysergide (0.1–1 mg/kg) significantly (P < 0.05) increased saline control bladder capacity and suppressed PNS inhibition at the dosage of 0.03–1 mg/kg. After methysergide treatment (1 mg/kg), naloxone significantly (P < 0.05) reduced control bladder capacity during AA infusion but had no effect during saline infusion. Naloxone also had no influence on PNS inhibition. These results suggest that 5-HT₂ receptors play a role in PNS inhibition of reflex bladder activity and interact with opioid mechanisms in micturition reflex pathway. Understanding neurotransmitter mechanisms underlying pudendal neuromodulation is important for the development of new treatments for bladder disorders.

Introduction

Overactive bladder (OAB) is a symptom complex of urinary urgency, frequency and incontinence that can be treated by pudendal nerve or sacral neuromodulation (Peters et al., 2005, Peters et al., 2010, van Kerrebroeck et al., 2007). Pudendal neuromodulation has been reported to be superior to sacral neuromodulation in patients with intractable OAB or painful bladder syndrome (PBS) (Peters, 2002, Peters et al., 2005). However, the mechanisms underlying neuromodulation therapies are currently still unknown. Understanding the mechanisms could potentially improve the efficacy of these therapies or develop new treatments for bladder disorders (Andersson, 2004, Andersson and Wein, 2004).

Our studies in cats (Tai et al., 2012, Zhang et al., 2012) have identified an involvement of opioid receptors in tibial nerve stimulation-induced inhibition of nociceptive bladder overactivity caused by intravesical acetic acid (AA) irritation. Activation of opioid receptors also plays a minor role in pudendal nerve stimulation (PNS)-induced inhibition of non-nociceptive reflex bladder activity in cats caused by saline distention of the bladder (Chen et al., 2010); but does not contribute to PNS inhibition of nociceptive bladder overactivity (Mally et al., 2013). Thus, neurotransmitters other than endogenous opioid peptides must mediate PNS inhibition of reflex bladder activity.

Previous studies in various species (cats, rats, guinea pigs) have raised the possibility that 5-hydroxytryptamine (5-HT) may function as an inhibitory or excitatory transmitter that modulates the micturition reflex pathway in the brain and spinal cord (Cheng and de Groat, 2010, de Groat, 2002, Mbaki et al., 2012, Ramage, 2006) and that it also has an important role in the regulation of nociceptive mechanisms in the central nervous system (Basbaum and Fields, 1984). Thus, we have conducted a pharmacological study to examine the contribution of 5-HT to the inhibition of nociceptive and non-nociceptive bladder reflexes induced by PNS. We have used methysergide, a non-selective 5-HT₂ receptor antagonist, which in a rat model of somatic nociception was effective after intrathecal administration in significantly reducing the antihyperalgesic effect of transcutaneous electrical nerve stimulation or mechanical joint manipulation (Radhakrishnan et al., 2003, Skyba et al., 2003). These results indicated that descending serotonin (5-HT) inhibitory mechanisms might be involved in neuromodulation of somatic nociception in the spinal cord.

In addition, intrathecal administration of methysergide to cats enhanced the axonal firing in the lateral funiculus of T11–T12 spinal segments elicited by electrical stimulation of bladder afferent axons in the pelvic nerve (Espey et al., 1998). These observations suggested that methysergide-sensitive 5-HT receptors generate a tonic inhibitory modulation of the ascending limb of the spinobulbospinal micturition reflex pathway (Espey et al., 1998). This modulatory input arises in the brain stem raphe nuclei where electrical (McMahon and Spillane, 1982, Morrison and Spillane, 1986) or chemical stimulation (Chen et al., 1993) has been shown to inhibit bladder reflexes. Therefore, the present study was undertaken to determine if methysergide-sensitive 5-HT receptors might be involved in PNS inhibition of nociceptive or non-nociceptive bladder reflexes.

Intravesical infusion of diluted (0.25%) AA was used in this study to irritate the bladder, activate the nociceptive bladder C-fiber afferents, and induce bladder overactivity in α-chloralose anesthetized cats, while saline infusion was used to distend the bladder, activate the non-nociceptive bladder Aδ-fiber afferents, and induce normal reflex bladder activity (Fowler et al., 2008, Häbler et al., 1990). PNS was employed as the antinociceptive stimulus to model the clinical use of pudendal neuromodulation in treating OAB or painful bladder syndrome (PBS) (Peters, 2002, Peters et al., 2005). Methysergide and naloxone (an opioid receptor antagonist) were administered intravenously to determine the role of 5-HT and opioid receptor mechanisms in the neuromodulation. WAY100635 (a 5-HT_1A receptor antagonist) was used to exclude the possibility that methysergide affects the bladder activity through activation of 5-HT_1A receptors.