Levodopa acts centrally to induce an antinociceptive action against colonic distension through activation of D2 dopamine receptors and the orexinergic system in the brain in conscious rats

Brain Neuropeptides, Neuroinflammation, and Irritable Bowel Syndrome

BACKGROUND

Irritable bowel syndrome (IBS) is a functional bowel disorder characterized by chronic abdominal symptoms, but its pathogenesis is not fully understood.

SUMMARY

We have recently shown in rats that neuropeptides such as orexin, ghrelin, and oxytocin act in the brain to improve the intestinal barrier dysfunction, which is a major pathophysiology of IBS. We have additionally shown that the neuropeptides injected intracisternally induced a visceral antinociceptive action against colonic distension. Since it has been known that intestinal barrier dysfunction causes visceral hypersensitivity, the other main pathophysiology of IBS, the neuropeptides act centrally to reduce leaky gut, followed by improvement of visceral sensation, leading to therapeutic action on IBS. It has been recently reported that there is a bidirectional relationship between neuroinflammation in the brain and the pathophysiology of IBS. For example, activation of microglia in the brain causes visceral hypersensitivity. Accumulating evidence has suggested that orexin, ghrelin, or oxytocin could improve neuroinflammation in the CNS. All these results suggest that neuropeptides such as orexin, ghrelin, and oxytocin act in the brain to improve intestinal barrier function and visceral sensation and also induce a protective action against neuroinflammation in the brain.

KEY MESSAGES

We therefore speculated that orexin, ghrelin, or oxytocin in the brain possess dual actions, improvement of visceral sensation/leaky gut in the gut, and reduction of neuroinflammation in the brain, thereby inducing a therapeutic effect on IBS in a convergent manner.

Prostaglandin I2 suppresses the development of gut-brain axis disorder in irritable bowel syndrome in rats

In this study, we attempted to clarify a role of prostaglandin (PG) I₂ and its specific receptor, IP in the pathogenesis of irritable bowel syndrome (IBS) using a maternal separation (MS)-induced IBS model. Administration of beraprost (BPS), a specific IP agonist, improved visceral hypersensitivity and depressive state with decreased serum CRF level in the IBS rats. To clarify the mechanism of the effect of BPS, we performed serum metabolome analysis and 1-methylnicotinamide (1-MNA) was identified as a possible candidate for a clue metabolite of pathogenesis of IBS. The serum 1-MNA levels revealed inverse correlation to the level of visceral sensitivity, and positive correlation to a depression marker, immobilizing time. Administration of 1-MNA induced visceral hypersensitivity and depression with increased levels of serum CRF. Since fecal 1-MNA is known for a marker of dysbiosis, we examined the composition of fecal microbiota by T-RFLP analysis. The proportion of clostridium cluster XI, XIVa and XVIII was significantly changed in MS-induced IBS rats treated with BPS. Fecal microbiota transplant of BPS-treated rats improved visceral hypersensitivity and depression in IBS rats. These results suggest for the first time that PGI₂-IP signaling plays an important role in IBS phenotypes such as visceral hypersensitivity and depressive state. BPS modified microbiota, thereby inhibition of 1-MNA-CRF pathway, followed by improvement of MS-induced IBS phenotype. These results suggest that the PGI₂-IP signaling could be considered to be a therapeutic option for IBS.

Pramipexole treatment attenuates mechanical hypersensitivity in male rats experiencing chronic inflammatory pain

Opioids are commonly prescribed for pain despite growing evidence of their low efficacy in the treatment of chronic inflammatory pain and the high potential for misuse. There is a clear need to investigate non-opioid alternatives for the treatment of pain. In the present study, we tested the hypothesis that acute and repeated dopamine agonist treatment would attenuate mechanical hypersensitivity in male Long-Evans rats experiencing chronic inflammatory pain. We used two clinically available therapeutics, l-DOPA (precursor of dopamine biosynthesis) and pramipexole (dopamine D2/3 receptor agonist), to examine the functional role of dopamine signaling on mechanical hypersensitivity using an animal model of chronic inflammatory pain (complete Freund's adjuvant, CFA). We found that both acute and repeated pramipexole treatment attenuated hyperalgesia-like behavior in CFA-treated animals but exhibited no analgesic effects in control animals. In contrast, there was no effect of acute or repeated l-DOPA treatment on mechanical hypersensitivity in either CFA- or saline-treated animals. Notably, we discovered some extended effects of l-DOPA and pramipexole on decreasing pain-like behavior at three days and one week post-drug treatment. We also examined the effects of pramipexole treatment on glutamatergic and presynaptic signaling in pain- and reward-related brain regions including the nucleus accumbens (NAc), dorsal striatum (DS), ventral tegmental area (VTA), cingulate cortex (CC), central amygdala (CeA), and periaqueductal gray (PAG). We found that pramipexole treatment decreased AMPA receptor phosphorylation (pGluR1845) in the NAc and DS but increased pGluR1845 in the CC and CeA. A marker of presynaptic vesicle release, pSynapsin, was also increased in the DS, VTA, CC, CeA, and PAG following pramipexole treatment. Interestingly, pramipexole increased pSynapsin in the NAc of saline-treated animals, but not CFA-treated animals, suggesting blunted presynaptic vesicle release in the NAc of CFA-treated animals following pramipexole treatment. To examine the functional implications of impaired presynaptic signaling in the NAc of CFA animals, we used ex vivo electrophysiology to examine the effects of pramipexole treatment on the intrinsic excitability of NAc neurons in CFA- and saline-treated animals. We found that pramipexole treatment reduced NAc intrinsic excitability in saline-treated animals but produced no change in NAc intrinsic excitability in CFA-treated animals. These findings indicate alterations in dopamine D2/3 receptor signaling in the NAc of animals with a history of chronic pain in association with the anti-hyperalgesic effects of pramipexole treatment.

Central regulatory mechanisms of visceral sensation in response to colonic distension with special reference to brain orexin

Visceral hypersensitivity is a major pathophysiology in irritable bowel syndrome (IBS). Although brain-gut interaction is considered to be involved in the regulation of visceral sensation, little had been known how brain controls visceral sensation. To improve therapeutic strategy in IBS, we should develop a novel approach to control visceral hypersensitivity. Here, we summarized recent data on central control of visceral sensation by neuropeptides in rats. Orexin, ghrelin or oxytocin in the brain is capable of inducing visceral antinociception. Dopamine, cannabinoid, adenosine, serotonin or opioid in the central nervous system (CNS) plays a role in the visceral hyposensitivity. Central ghrelin, levodopa or morphine could induce visceral antinociception via the orexinergic signaling. Orexin induces visceral antinociception through dopamine, cannabinoid, adenosine or oxytocin. Orexin nerve fibers are identified widely throughout the CNS and orexins are implicated in a number of functions. With regard to gastrointestinal functions, in addition to its visceral antinociception, orexin acts centrally to stimulate gastrointestinal motility and improve intestinal barrier function. Brain orexin is also involved in regulation of sleep/awake cycle and anti-depressive action. From these evidence, we would like to make a hypothesis that decreased orexin signaling in the brain may play a role in the pathophysiology in a part of patients with IBS who are frequently accompanied with sleep disturbance, depressive state and disturbed gut functions such as gut motility disturbance, leaky gut and visceral hypersensitivity.

Research progress on the mechanism of orexin in pain regulation in different brain regions

Orexin is a neuropeptide that is primarily synthesized and secreted by the lateral hypothalamus (LH) and includes two substances derived from the same precursor (orexin A [OXA] and orexin B [OXB]). Studies have shown that orexin is not only involved in the regulation of eating, the sleep–wake cycle, and energy metabolism, but also closely associated with various physiological functions, such as cardiovascular control, reproduction, stress, reward, addiction, and the modulation of pain transmission. At present, studies that have been performed both domestically and abroad have confirmed that orexin and its receptors are closely associated with pain regulation. In this article, the research progress on acute pain regulation involving orexin is reviewed.

Synergistic antiallodynic and antihyperalgesic interaction between L-DOPA and celecoxib in parkinsonian rats is mediated by NO-cGMP-ATP-sensitive K+ channel

Pain is a usual and troublesome non-motor symptom of Parkinson's disease, with a prevalence of 29-82%. Therefore, it's vital to find pharmacological treatments for managing PD-associated pain symptoms, to improve patients' quality of life. For this reason, we tested the possible synergy between L-DOPA and celecoxib in decreasing allodynia and hyperalgesia induced by unilateral lesioning with 6-OHDA into the SNpc in rats. We also tested whether the antiallodynic and antihyperalgesic effect induced by combination of L-DOPA and celecoxib is mediated by the NO-cGMP-ATP-sensitive K⁺ channel pathway. Tactile allodynia and mechanical hyperalgesia were evaluated using von Frey filament. Isobolographic analyses were employed to define the nature of the drug interaction using a fixed dose ratio (0.5: 0.5). We found that acute and sub-acute (10-day) treatment with a single dose of L-DOPA (3-25 mg/kg, i. p.) or celecoxib (2.5-20 mg/kg, i. p.) induced a dose-dependent antiallodynic and antihyperalgesic effect in parkinsonian rats. Isobolographic analysis revealed that the ED₅₀ values obtained by L-DOPA + celecoxib combination was significantly less than calculated additive values, indicating that co-administration of L-DOPA with celecoxib produces synergistic interactions in its antiallodynic and antihyperalgesic effect in animals with nigrostriatal lesions. Moreover, the antiallodynic and antihyperalgesic effects induced by L-DOPA + celecoxib combination were blocked by intrathecal pre-treatment with L-NAME, ODQ, and glibenclamide. Taken together, the data suggest that L-DOPA + celecoxib combination produces an antiallodynic and antihyperalgesic synergistic interaction at the systemic level, and these effects are mediated, at the central level, through activation of the NO-cGMP-ATP-sensitive K⁺ channel pathway.

Analgesic Effects of Electroacupuncture at St25 and Cv12 in a Rat Model of Postinflammatory Irritable Bowel Syndrome Visceral Pain

Background

Treatment with electroacupuncture (EA) at ST25 and CV12 has a significant analgesic effect on postinflammatory irritable bowel syndrome (PI-IBS) visceral pain. Enterochromaffin (EC) cells and serotonin (5-hydroxytryptamine (5-HT)) are important in the development of visceral hyperalgesia.

Objective

To investigate the analgesic effect and underlying mechanisms of EA at ST25 and CV12 on the treatment of trinitrobenzene sulfonic acid (TNBS)-induced PI-IBS visceral hyperalgesia in rats.

Methods

After EA at ST25 and CV12, changes in abdominal withdrawal reflex (AWR), electromyography (EMG) recordings, colonic EC cell numbers, and expression of tryptophan hydroxylase (TPH), 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) of TNBS-induced PI-IBS visceral hyperalgesia in rats were examined.

Results

The results of AWR tests and EMG recordings indicated a significant analgesic effect of EA stimulation at ST25 and CV12on PI-IBS visceral hyperalgesia (p<0.05). In addition, the increased EC cell numbers and colonic expression of TPH and 5-HT in rats with TNBS-induced PI-IBS visceral hyperalgesia were significantly reduced by EA (p<0.05).

Conclusions

EA stimulation at ST25 and CV12 can attenuate visceral hyperalgesia. This analgesic effect may be mediated via reduction of both colonic EC cell number and 5-HT concentration.

Ameliorative effect of chlorpromazine hydrochloride on visceral hypersensitivity in rats: possible involvement of 5-HT2A receptor

BACKGROUND AND PURPOSE

Visceral hypersensitivity is responsible for pathogenesis of irritable bowel syndrome (IBS). Therefore, its prevention can help avoid abdominal pain and discomfort in IBS. To find candidate drugs for visceral hypersensitivity, we screened existing medicines for their ability to prevent visceral sensitivity induced by colorectal distension (CRD) in rats and identified chlorpromazine, a typical antipsychotic drug, as a candidate compound. In this study, we investigated the effect of chlorpromazine on visceral hypersensitivity.

EXPERIMENTAL APPROACH

Visceral sensitivity (visceromotor response) was monitored by measuring the electrical activity of the abdominal external oblique muscle contraction in response to CRD using a barostat apparatus. Visceral hypersensitivity was induced by a colonic instillation of sodium butyrate or acetic acid in neonates.

KEY RESULTS

Oral administration of chlorpromazine suppressed butyrate-induced visceral hypersensitivity to CRD. Interestingly, atypical antipsychotic drugs, quetiapine and risperidone, ameliorated butyrate-induced visceral hypersensitivity, whereas the typical antipsychotic drugs, haloperidol and sulpiride, did not. Pharmacological analysis using specific inhibitors showed that a selective 5-HT_2A receptor antagonist, ketanserin, suppressed butyrate-induced visceral hypersensitivity, whereas a selective dopamine D₂ receptor antagonist, L-741626, did not. Furthermore, the 5-HT_2A receptor agonist AL-34662 stimulated visceral sensitivity to CRD in healthy control rats but not in butyrate-treated rats. These findings suggest that increased 5-HT levels in the colon contribute to the induction of visceral hypersensitivity.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that chlorpromazine ameliorates visceral hypersensitivity and that the 5-HT_2A receptor is a potential therapeutic target for treating abdominal pain and discomfort in IBS.