Optimized Tibial Nerve Stimulation Partially Reduces Visceral Hypersensitivity in Rats Mediated via Autonomic and Opioid Mechanisms

OBJECTIVES

The aim of this study was to investigate the therapeutic effect and possible mechanisms of tibial nerve stimulation (TNS) on visceral hypersensitivity in rats.

MATERIALS AND METHODS

1) The effects of TNS with five sets of parameters on visceral sensitivity in normal rats were evaluated by the assessment of abdominal electromyogram (EMG) and abdominal withdrawal reflex (AWR). 2) The effects and mechanisms of TNS with a special set of parameters (14 Hz, 330 μsec, and 40% motor threshold) were evaluated in acute visceral hypersensitivity rats induced by restraint stress and colonic hypersensitized rats induced by acetic acid during the neonatal stage assessed by the EMG, AWR, and the spectral analysis of heart rate variability derived from the electrocardiogram.

RESULTS

1) In normal rats, TNS did not show any effect on the visceromotor reflex. 2) In rats with restraint stress-induced hypersensitivity, TNS with the special set of parameters reduced AWR scores and EMG responses to rectal distention at a pressure of 20-60 mmHg (p < 0.05, vs. baseline for both AWR and EMG). Concurrently, TNS increased vagal activity and decreased sympathetic activity (p < 0.03 for both). 3) Similar effects were noted on the EMG (p < 0.05, vs. baseline) and AWR (p < 0.05 vs. baseline) with acute and chronic TNS in rats with chronic colonic hypersensitivity and the effects were blocked by naloxone.

CONCLUSIONS

TNS with parameters of 14 Hz, 330 μsec, and 40% motor threshold is effective in improving visceral hypersensitivity in rodent models of colonic hypersensitivity via the modulation of autonomic and opioid mechanisms.

Sacral nerve stimulation with optimized parameters improves visceral hypersensitivity in rats mediated via the autonomic pathway

The purpose of this study was to determine effects and mechanisms of sacral nerve stimulation (SNS) on visceral hypersensitivity in rodent models of colonic hypersensitivity. SNS was performed with different sets of parameters for 30 min in six regular rats. Visceral sensitivity was assessed by the measurement of electromyogram and abdominal withdrawal reflex before and after SNS. Real/sham SNS with optimized parameters was performed in 8 restraint stress-induced visceral hypersensitivity rats and 10 neonatal acetic acid-treated colonic hypersensitivity rats; acute effect of SNS was assessed by comparing electromyogram and heart rate variability. Neonatal acetic acid-treated rats were treated by SNS (n = 10) or sham-SNS (n = 10) daily for seven days for the assessment of the chronic effect of SNS. (1) When the stimulation amplitude was reduced from 90% of motor threshold to 65% or 40% motor threshold, SNS with certain parameters showed an inhibitory effect on abdominal withdrawal reflex. The best stimulation parameters for SNS were “14 Hz, 330 µs, and 40% motor threshold.” (2) SNS significantly reduced visceral hypersensitivity and improved autonomic function in restraint stress-induced rats. The inhibitory effect was blocked by naloxone. (3)Acute and chronic SNS significantly reduced visceral hypersensitivity and improved autonomic function in acetic acid-treated rats. SNS with reduced stimulation strength may be used to treat colonic hypersensitivity and the best stimulation parameters seem to be “14 Hz, 330 µs and 40% motor threshold”. SNS with optimized parameters improved visceral hypersensitivity in rodent models of colonic hypersensitivity mediated via the autonomic and opioid mechanisms.

Roles of noradrenergic transmission within the ventral part of the bed nucleus of the stria terminalis in bidirectional brain-intestine interactions

Aims

The bed nucleus of the stria terminalis (BNST) is a limbic structure mediating autonomic and neuroendocrine responses and negative affective states such as anxiety and fear. We previously demonstrated that noradrenergic transmission via β‐adrenoceptors within the ventral part of BNST (vBNST) is involved in bidirectional interactions between the brain and the upper gastrointestinal (GI) tract. The present study aimed to examine the roles of intra‐vBNST noradrenergic transmission via β‐adrenoceptors in bidirectional interactions between the brain and lower GI tract.

Methods

In vivo microdialysis experiments were performed to examine colorectal distention (CRD)‐induced noradrenaline release within the vBNST of freely moving male Sprague‐Dawley rats. Colonic transit and abdominal pain perception were examined following intra‐vBNST injections of isoproterenol, a β‐adrenoceptor agonist, with and without co‐administration of timolol, a β‐adrenoceptor antagonist.

Results

CRD increased extracellular noradrenaline levels within the vBNST and evoked abdominal contractions in a pressure‐dependent manner (30‐60 mm Hg). Bilateral intra‐vBNST injections of isoproterenol (30 nmol/side) significantly increased CRD (30 mm Hg)‐induced abdominal contractions. Intra‐vBNST injections of isoproterenol (30 nmol/side) significantly increased colonic transit, which was reversed by co‐administration of timolol (30 nmol/side).

Conclusion

The results of this study suggest (a) the existence of a positive feedback loop between intra‐vBNST noradrenaline release and abdominal pain perception, and (b) the modulation of colonic motility by intra‐vBNST noradrenergic transmission via β‐adrenoceptors. Dysfunction of the lower GI tract may increase noradrenaline release within the vBNST, which, in turn, may exacerbate impairment of its motility and pain perception.

In vivo microdialysis experiments demonstrated that colorectal distention (CRD) increased extracellular levels of noradrenaline within the vBNST. Intra‐vBNST injections of isoproterenol, a β‐adrenoceptor agonist, induced visceral hypersensitivity to CRD and increased colonic transit, and the increase in colonic transit was reversed by co‐administration of timolol, a β‐adrenoceptor antagonist. The present findings demonstrated important roles of noradrenergic transmission via β‐adrenoceptors within the vBNST in bidirectional brain‐intestine interactions.

EXPRESS: Histone hyperacetylation modulates spinal type II metabotropic glutamate receptor alleviating stress-induced visceral hypersensitivity in female rats

Stress is often a trigger to exacerbate chronic pain including visceral hypersensitivity associated with irritable bowel syndrome, a female predominant functional bowel disorder. Epigenetic mechanisms that mediate stress responses are a potential target to interfere with visceral pain. The purpose of this study was to examine the effect of a histone deacetylase inhibitor, suberoylanilide hydroxamic acid, on visceral hypersensitivity induced by a subchronic stressor in female rats and to investigate the involvement of spinal glutamate receptors. Three daily sessions of forced swim induced visceral hypersensitivity. Intrathecal suberoylanilide hydroxamic acid prevented or reversed the stress-induced visceral hypersensitivity, increased spinal histone 3 acetylation and increased mGluR2 and mGluR3 expression. Chromatin immunoprecipitation (ChIP) analysis revealed enrichment of H3K9Ac and H3K18Ac at several promoter Grm2 and Grm3 regions. The mGluR2/3 antagonist LY341495 reversed the inhibitory effect of suberoylanilide hydroxamic acid on the stress-induced visceral hypersensitivity. In surprising contrast, stress and/or suberoylanilide hydroxamic acid had no effect on spinal NMDA receptor expression or function. These data reveal histone modification modulates mGluR2/3 expression in the spinal cord to attenuate stressinduced visceral hypersensitivity. HDAC inhibitors may provide a potential approach to relieve visceral hypersensitivity associated with irritable bowel syndrome.

Rifaximin alters intestinal bacteria and prevents stress-induced gut inflammation and visceral hyperalgesia in rats

BACKGROUND & AIMS

Rifaximin is used to treat patients with functional gastrointestinal disorders, but little is known about its therapeutic mechanism. We propose that rifaximin modulates the ileal bacterial community, reduces subclinical inflammation of the intestinal mucosa, and improves gut barrier function to reduce visceral hypersensitivity.

METHODS

We induced visceral hyperalgesia in rats, via chronic water avoidance or repeat restraint stressors, and investigated whether rifaximin altered the gut microbiota, prevented intestinal inflammation, and improved gut barrier function. Quantitative polymerase chain reaction (PCR) and 454 pyrosequencing were used to analyze bacterial 16S ribosomal RNA in ileal contents from the rats. Reverse transcription, immunoblot, and histologic analyses were used to evaluate levels of cytokines, the tight junction protein occludin, and mucosal inflammation, respectively. Intestinal permeability and rectal sensitivity were measured.

RESULTS

Water avoidance and repeat restraint stress each led to visceral hyperalgesia, accompanied by mucosal inflammation and impaired mucosal barrier function. Oral rifaximin altered the composition of bacterial communities in the ileum (Lactobacillus species became the most abundant) and prevented mucosal inflammation, impairment to intestinal barrier function, and visceral hyperalgesia in response to chronic stress. Neomycin also changed the composition of the ileal bacterial community (Proteobacteria became the most abundant species). Neomycin did not prevent intestinal inflammation or induction of visceral hyperalgesia induced by water avoidance stress.

CONCLUSIONS

Rifaximin alters the bacterial population in the ileum of rats, leading to a relative abundance of Lactobacillus. These changes prevent intestinal abnormalities and visceral hyperalgesia in response to chronic psychological stress.

Linaclotide inhibits colonic nociceptors and relieves abdominal pain via guanylate cyclase-C and extracellular cyclic guanosine 3',5'-monophosphate

BACKGROUND & AIMS

Linaclotide is a minimally absorbed agonist of guanylate cyclase-C (GUCY2C or GC-C) that reduces symptoms associated with irritable bowel syndrome with constipation (IBS-C). Little is known about the mechanism by which linaclotide reduces abdominal pain in patients with IBS-C.

METHODS

We determined the effects of linaclotide on colonic sensory afferents in healthy mice and those with chronic visceral hypersensitivity. We assessed pain transmission by measuring activation of dorsal horn neurons in the spinal cord in response to noxious colorectal distention. Levels of Gucy2c messenger RNA were measured in tissues from mice using quantitative reverse transcription polymerase chain reaction and in situ hybridization. We used human intestinal cell lines to measure release of cyclic guanosine-3',5'-monophosphate (cGMP) by linaclotide. We performed a post-hoc analysis of data from a phase III, double-blind, parallel-group study in which 805 patients with IBS-C were randomly assigned to groups given an oral placebo or 290 μg linaclotide once daily for 26 weeks. We quantified changes in IBS-C symptoms, including abdominal pain.

RESULTS

In mice, linaclotide inhibited colonic nociceptors with greater efficacy during chronic visceral hypersensitivity. Intra-colonic administration of linaclotide reduced signaling of noxious colorectal distention to the spinal cord. The colonic mucosa, but not neurons, was found to express linaclotide's target, GC-C. The downstream effector of GC-C, cGMP, was released after administration of linaclotide and also inhibited nociceptors. The effects of linaclotide were lost in Gucy2c(-/-) mice and prevented by inhibiting cGMP transporters or removing the mucosa. During 26 weeks of linaclotide administration, a significantly greater percentage of patients (70%) had at least a 30% reduction in abdominal pain compared with patients given placebo (50%).

CONCLUSIONS

We have identified an analgesic mechanism of linaclotide: it activates GC-C expressed on mucosal epithelial cells, resulting in the production and release of cGMP. This extracellular cGMP acts on and inhibits nociceptors, thereby reducing nociception. We also found that linaclotide reduces chronic abdominal pain in patients with IBS-C.

Peripheral modulation of bladder inhibition by colorectal distention in rats

PURPOSE

Bladder activity can be inhibited by afferent input from the colorectum (inhibitory rectovesical reflex). We evaluated the functional response of the rat bladder to nonnoxious and noxious colorectal distention, and investigated the mechanical and pharmacological peripheral modulation of this response.

MATERIALS AND METHODS

In 70 female Sprague-Dawley® rats we evaluated the effect of nonnoxious (20 mm Hg) and noxious (40 and 60 mm Hg) colorectal distention on the micturition volume threshold and on bladder activity in a filled bladder. We also studied the effect of rectal balloon size (1.5 vs 3.5 cm long), and rectal administration of 2% lidocaine jelly or 1 mM allyl isothiocyanate solution on the inhibitory rectovesical reflex.

RESULTS

Colorectal distention at 60 mm Hg increased the micturition volume threshold (mean ± SE 0.640 ± 0.056 vs 0.448 ± 0.035 ml in controls, p <0.001). Bladder contraction frequency was significantly decreased by 40 and 60 mm Hg colorectal distention vs controls (mean 0.62 ± 0.06 and 0.33 ± 0.05 per minute, respectively, vs 0.77 ± 0.03, each p <0.001). These effects were reversible and pressure dependent (p <0.001), and more pronounced using a large rectal balloon (mean 40 vs 60 mm Hg colorectal distention 0.35 ± 0.12 vs 0.07 ± 0.04 per minute, p = 0.004). We noted no significant graded inhibition of bladder contraction amplitude or duration. The inhibitory rectovesical reflex was reversibly abolished by intrarectal lidocaine administration. Intrarectal allyl isothiocyanate administration significantly increased the effect of noxious colorectal distention on bladder contraction frequency.

CONCLUSIONS

Only noxious levels of colorectal distention initiated the inhibitory rectovesical reflex. The effect increased with rectal balloon size and with intrarectal administration of allyl isothiocyanate. It was reversibly abolished by lidocaine. Results suggest that spinal interneurons are the mechanism behind the inhibitory rectovesical reflex.

Pelvic nerve input mediates descending modulation of homovisceral processing in the thoracolumbar spinal cord of the rat

BACKGROUND & AIMS

Colonic afferents project to the lumbosacral and thoracolumbar spinal cord via the pelvic and hypogastric/lumbar colonic nerves, respectively. Both spinal regions process inflammatory colonic stimuli. The role of thoracolumbar segments in processing acute colorectal pain is questionable, however, because the lumbosacral spinal cord appears sufficient to process reflex responses to acute pain. Here, we show that activity in pelvic nerve colonic afferents actively modulates thoracolumbar dorsal horn neuron processing of the same colonic stimulus through a supraspinal loop: homovisceral descending modulation.

METHODS

Dorsal horn neurons were recorded in the rat thoracolumbar spinal cord after acute or chronic pelvic neurectomy and cervical cold block.

RESULTS

Acute pelvic neurectomy or lidocaine inhibition of lumbosacral dorsal roots facilitated the excitatory response of thoracolumbar dorsal horn neurons to colorectal distention (CRD) and decreased the percentage of neurons inhibited by CRD, suggesting colonic input over the pelvic nerve inhibits thoracolumbar processing of the same stimulus. Ectopic activity developed in the proximal pelvic nerve after chronic neurectomy reactivating the inhibitory circuit, inhibiting thoracolumbar neurons. Cervical cold block alleviated the inhibition in intact or chronic neurectomized rats. However, the facilitated response after acute pelvic neurectomy was inhibited by cervical cold block, exposing an underlying descending facilitation. Inhibiting pelvic nerve input after cervical cold block had minimal effect.

CONCLUSIONS

These data demonstrate that input over the pelvic nerve modulates the response of thoracolumbar spinal neurons to CRD by a supraspinal loop and that increasing thoracolumbar processing increases visceral hyperalgesia.

Is there a pathway in the posterior funiculus that signals visceral pain?

The present report provides evidence that axons in the medial part of the posterior column at T10 convey ascending nociceptive signals from pelvic visceral organs. This evidence was obtained from human surgical case studies and histological verification of the lesion in one of these cases, along with neuroanatomical and neurophysiological findings in animal experiments. A restricted lesion in this area can virtually eliminate pelvic pain due to cancer. The results remain excellent even in cases in which somatic structures of the pelvic body wall are involved. Following this procedure, neurological testing reveals no additional neurological deficit. There is no analgesia to pinprick stimuli applied to the body surface, despite the relief of the visceral pain. Since it is reasonable to attribute the favorable results of limited midline myelotomies to the interruption of axons of visceral nociceptive projection neurons in the posterior column, we have performed experiments in rats to test this hypothesis. The results in rats indicate that the dorsal column does indeed include a nociceptive component that signals pelvic visceral pain. The pathway includes neurons of the postsynaptic dorsal column pathway at the L6-S1 segmental level, axons of these neurons in the fasciculus gracilis, and neurons of the nucleus gracilis and the ventral posterolateral nucleus of the thalamus.