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1.  Pronociceptive Effect of 5-HT1A Receptor Agonist on Visceral Pain Involves Spinal NMDA Receptor 
Neuroscience  2012;219:243-254.
The functional role of serotonergic 5-HT1A receptors in the modulation of visceral pain is controversial. The objective of this study was to systematically examine the mechanism and site of action of a selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (DPAT) on visceral pain. In the behavioral model of visceral pain, systemic injection (5 to 250µg/kg) of DPAT produced a significant increase in the viscero-motor response (VMR) to colorectal distension (CRD) and this effect was blocked by the selective 5-HT1A receptor antagonist WAY-100135 (5mg/kg, s.c.). Similarly, intrathecal (i.t.) injection (5µmol) of DPAT into the lumbo-sacral (L6-S1) spinal cord produced a significant increase in VMR. The administration of N-methyl D-aspartate (NMDA) receptor antagonist AP5 (50µg/kg) prior to DPAT injection completely blocked the pronociceptive effect of DPAT. Similarly, DPAT failed to increase VMR in rats chronically treated with NR1 subunit targeted antisense oligoneucleotide (ON), whereas the drug increased VMR in rats treated with mismatched-ON. Chronic i.t. injection of allylglycine (AG), a γ-amino decarboxylase (GAD) enzyme inhibitor, produced significant increase in VMRs, suggesting that the inhibition of GABA synthesis produces pronociception. In AG-treated rats, i.t. injection of DPAT failed to further increase in VMR, suggesting that the DPAT action is linked to GABA release. Similarly, WAY-100135 failed to attenuate VMR in AG-treated rats, suggesting that unlike DPAT, AG action is not via the activation of 5-HT1A receptors. In electrophysiology experiments, DPAT (50µg/kg) significantly increased the responses of spinal neurons to CRD, but did not influence the mechanotransduction property of CRD-sensitive pelvic nerve afferent fibers. The effect of DPAT on spinal neurons remained unaffected when tested in spinal transected (C1–C2) rats. These results indicate that the 5-HT1A receptor agonist DPAT produces pronociceptive effects, primarily via the activation of presynaptic 5-HT1A receptors in GABAergic neuron to restrict GABA release and thereby disinhibits the excitatory glutamatergic neurons in the spinal cord.
PMCID: PMC3402596  PMID: 22626644
2.  Colonic Butyrate- algesic or analgesic? 
Irritable bowel syndrome (IBS) is a common health issue that is characterized by abdominal pain, abnormal bowel movements and altered visceral perception. The complexity and variability in symptoms pose serious challenges in treating IBS. Current therapy for IBS is primarily focused on reducing the abdominal pain, thereby improving the quality of life to a significant extent. Although the use of fiber rich diet is widely recommended in treating IBS, some studies have questioned its use. Intracolonic butyrate, a short chain fatty acid, is primarily produced by the fermentation of dietary fibers in the colon. In the existing literature there are conflicting reports about the function of butyrate. In rats it is known to induce visceral hypersensitivity without altered pathology, whereas in humans it has been reported to reduce visceral pain. Understanding the molecular mechanisms responsible for this contrasting effect of butyrate is important before recommending fiber rich diet to IBS patients.
PMCID: PMC3191935  PMID: 21981302
Inflammatory bowel syndrome; colonic hypersensitivity; short chain fatty acids; sodium butyrate
3.  Visceral analgesic effect of 5-HT4 receptor agonist in rats involves the rostroventral medulla (RVM) 
Neuropharmacology  2013;79:345-358.
The 5-HT4 receptor agonist tegaserod (TEG) has been reported to modulate visceral pain. However, the underlying mechanism remains unknown. The objective of the present study was to examine the analgesic mechanism and site of action of TEG. In male rats, visceral pain was assessed by measuring visceromotor response (VMR) to colorectal distension (CRD). Inflammation was induced by intracolonic injection of tri-nitrobenzene sulfonic acid (TNBS). The effect of TEG on the VMR was tested by injecting intraperitoneal (i.p.), intrathecal (i.t.), intracerebroventricular (i.c.v) or in the rostroventral medulla (RVM). The effect of the drug was also tested on responses of CRD-sensitive pelvic nerve afferents (PNA) and lumbo-sacral (LS) spinal neurons. Systemic injection of TEG attenuated VMR in naive and TNBS-treated rats. Similarly, supraspinal, but not spinal, injection of TEG attenuated the VMR. While GR113808, (selective 5-HT4 antagonist) blocked the effect, naloxone (NLX) an opioid receptor antagonist reversed the effect of TEG. Although i.t. NLX did not block the inhibitory effect of TEG in VMR study, i.t. injection of α2-adrenergic receptor antagonist yohimbine blocked the effect of TEG when given systemically. While TEG had no effect on the responses of CRD-sensitive PNA, it inhibited the responses of CRD-sensitive LS neurons in spinal intact condition. This inhibition was blocked by GR113808, NLX and β-funaltrexamine (β-FNA) when injected into the RVM. Results indicate that TEG produces analgesia via activation of supraspinal 5-HT4 receptors which triggers the release of opioids at supraspinal site, which activates descending noradrenergic pathways to the spinal cord to produce analgesia.
PMCID: PMC4321751  PMID: 24334068
5-HT4 receptors; RVM; Visceral pain; Colon; Descending modulation
4.  Neuronal Plasticity in the Cingulate Cortex of Rats following Esophageal Acid Exposure in Early Life 
Gastroenterology  2011;141(2):544-552.
Background & Aims
The cingulate cortex (CC) has been reported to be involved in processing pain of esophageal origin. However, little is known about molecular changes and cortical activation that arise from early-life, esophageal acid reflux. Excitatory neurotransmission via activation of the N-methyl-D-aspartate (NMDA) receptor and its interaction with post-synaptic density protein-95 (PSD-95) at the synapse appears to mediate neuronal development and plasticity. We investigated the effect of early-life esophageal acid exposure on NMDA receptor subunits and PSD-95 expression in the developing CC.
We assessed NMDA receptor subunits and PSD-95 protein expression in rostral CC (rCC) tissues of rats exposed to esophageal acid or saline (control), either during post-natal days 7–14 (P7–P14) and/or acutely, at adult stage (P60), using immunoblot and immunoprecipitation analyses.
Compared with controls, acid exposure from P7 to P14 significantly increased expression of NR1, NR2A, and PSD-95, measured 6 weeks after exposure. However, acute exposure at P60 caused a transient increase in expression of NMDA receptor subunits. These molecular changes were more robust in animals exposed to acid neonatally and rechallenged, acutely, at P60. Esophageal acid exposure induced calcium calmodulin kinase II-mediated phosphorylation of the subunit NR2B at Ser1303.
Esophageal acid exposure during early stages of life has long-term effects, because of phosphorylation of the NMDA receptor and overexpression in the rCC. This molecular alteration in the rCC might mediate sensitization of patients with acid-induced esophageal disorders.
PMCID: PMC3152593  PMID: 21616075
brain; developmental neuroscience; pain processing; CamKII
5.  Neonatal Cystitis-Induced Colonic Hypersensitivity in Adult Rats: A Model of Viscero-Visceral Convergence 
The objective of this study was to determine if neonatal cystitis alters colonic sensitivity later in life and to investigate the role of peripheral mechanisms.
Neonatal rats received intravesical zymosan, normal saline, or anesthesia only for three consecutive days (postnatal days 14th–16th). The estrous cycle phase was determined prior to recording the visceromotor response (VMR) to colorectal distension (CRD) in adult rats. Eosinophils and mast cells were examined from colon and bladder tissue. CRD or urinary bladder distension (UBD)-sensitive pelvic nerve afferents (PNAs) were identified and their responses to distension were examined. The relative expression of N-methyl-D-aspartic acid (NMDA) NR1 subunit in the L6-S1 spinal cord was examined using Western blot.
The VMR to CRD (≥10mmHg) in the neonatal zymosan group was significantly higher than control in both the diestrus, estrus phase and in all phases combined. There was no difference in the total number of eosinophils, mast cells or number of degranulated mast cells between groups. The spontaneous firing of UBD, but not CRD-sensitive PNAs from the zymosan rats was significantly higher than the control. However, the mechanosensitive properties of PNAs to CRD or UBD were no different between groups (p > 0.05). The expression of spinal NR1 subunit was significantly higher in zymosan-treated rats compared to saline treated rats (p <0.05).
Neonatal cystitis results in colonic hypersensitivity in adult rats without changing tissue histology or the mechanosensitive properties of CRD-sensitive PNAs. Neonatal cystitis does results in overexpression of spinal NR1 subunit in adult rats.
PMCID: PMC3117950  PMID: 21592255
cystitis; visceral hyperalgesia; neonatal; viscero-visceral convergence
6.  Visceral Pain: The Neurophysiological Mechanism 
The mechanism of visceral pain is still less understood compared with that of somatic pain. This is primarily due to the diverse nature of visceral pain compounded by multiple factors such as sexual dimorphism, psychological stress, genetic trait, and the nature of predisposed disease. Due to multiple contributing factors there is an enormous challenge to develop animal models that ideally mimic the exact disease condition. In spite of that, it is well recognized that visceral hypersensitivity can occur due to (1) sensitization of primary sensory afferents innervating the viscera, (2) hyperexcitability of spinal ascending neurons (central sensitization) receiving synaptic input from the viscera, and (3) dysregulation of descending pathways that modulate spinal nociceptive transmission. Depending on the type of stimulus condition, different neural pathways are involved in chronic pain. In early-life psychological stress such as maternal separation, chronic pain occurs later in life due to dysregulation of the hypothalamic–pituitary–adrenal axis and significant increase in corticotrophin releasing factor (CRF) secretion. In contrast, in early-life inflammatory conditions such as colitis and cystitis, there is dysregulation of the descending opioidergic system that results excessive pain perception (i.e., visceral hyperalgesia). Functional bowel disorders and chronic pelvic pain represent unexplained pain that is not associated with identifiable organic diseases. Often pain overlaps between two organs and approximately 35% of patients with chronic pelvic pain showed significant improvement when treated for functional bowel disorders. Animal studies have documented that two main components such as (1) dichotomy of primary afferent fibers innervating two pelvic organs and (2) common convergence of two afferent fibers onto a spinal dorsal horn are contributing factors for organ-to-organ pain overlap. With reports emerging about the varieties of peptide molecules involved in the pathological conditions of visceral pain, it is expected that better therapy will be achieved relatively soon to manage chronic visceral pain.
PMCID: PMC3156094  PMID: 19655104
Visceral pain; Visceral afferents; Spinal cord; Pelvic nerve; Splanchnic nerve; Colon; Urinary bladder; Gender difference; Sensitization
7.  Antinociceptive Effects of Melatonin in a Rat Model of Post-Inflammatory Visceral Hyperalgesia: A Centrally Mediated Process 
Pain  2010;149(3):555-564.
Previous reports suggest that melatonin may play an important role in visceral nociception and neurogenic inflammation. We aimed to examine the role melatonin on visceral hypersensitivity and to explore the site of action using a rat model of post-inflammatory visceral hyperalgesia. In all rats, a baseline visceromotor response (VMR) to graded colorectal distension (CRD; 10-60 mmHg) was recorded prior and one week following trinitrobenzenesulfonic acid (TNBS) induced colonic inflammation. Melatonin (30, 45 or 60 mg/kg, ip) was given 20 minutes before testing the VMR in naïve and TNBS-treated rats. Extracellular single-unit recordings were made from CRD-sensitive pelvic nerve afferent (PNA) fibers and lumbosacral (LS) spinal neurons in TNBS-treated animals. The effect of melatonin (60 mg/kg) was examined on responses of PNAs and spinal neurons to graded CRD. In separate experiments, luzindole (non-specific MT1/MT2 receptor antagonist) or naltrexone (non-specific opiod receptor antagonist) was injected prior to melatonin. Following TNBS, there was a significant increase in the VMR to CRD compared to baseline. This increase was attenuated by melatonin (60 mg/kg) at pressures >20 mmHg. The same dose of melatonin had no effect on the VMR in naïve animals. In TNBS-treated rats, melatonin significantly attenuated the responses of CRD-sensitive spinal neurons to CRD, but had no effect in spinal transected rats or PNA fibers. Both luzindole and naltrexone blocked melatonin's effect on the VMR and LS spinal neurons. Results indicate melatonin's antinociceptive effects are not via a peripheral site of action but rather a supraspinal process linked to the central opioidergic system.
PMCID: PMC2884285  PMID: 20413219
Melatonin; colorectal distension; visceral hyperalgesia; MT receptors
8.  Altered Mechanosensitive Properties of Vagal Afferent Fibers Innervating the Stomach Following Gastric Surgery in Rats 
Neuroscience  2009;162(4):1299-1306.
Background and Aims
Several types of gastric surgeries have been associated with early satiety, dyspepsia and food intolerances. We aimed to examine alterations in gastric vagal afferents following gastric surgery-fundus ligation.
Six week old, male Sprague-Dawley rats underwent chronic ligation (CL) of the gastric fundus. Sham rats underwent surgery, but without ligation. Another group of rats underwent acute ligation (AL) immediately prior to experiments. Animals were allowed to grow to age 3–4 months. Food intake and weights were recorded post-operatively. Gastric compliance and gastric wall thickness was measured at baseline and during gastric distension (GD). Extracellular recordings were made to examine response characteristics of vagal afferent fibers to GD and to map the stomach receptive field (RF). The morphological structures of afferent terminals in the stomach were examined with retrograde tracings from the nodose ganglion.
The CL group consumed significantly less food and weighed less than control. The mean compliance of CL group was significantly less than control, but higher than the AL. The spontaneous firing and responses to GD of afferent fibers from the CL rats were significantly higher than AL rats. There was a marked expansion of the gastric RF in the CL rats with significant reorganization and regeneration of intramuscular array (IMA) terminals. There was no difference in total wall or muscle thickness among the groups.
CL results in aberrant remodeling of IMAs with expansion of the gastric RF and alters the mechanotransduction properties of vagal afferent fibers. These changes could contribute to altered sensitivity following gastric surgery.
PMCID: PMC2741133  PMID: 19477237
Vagal Afferents; Stomach; Satiety; Fundus ligation; Gastric surgery
9.  The Role of TRPV1 in Mechanical and Chemical Visceral Hyperalgesia Following Experimental Colitis 
Neuroscience  2007;148(4):1021-1032.
The transient receptor potential vanilloid 1 receptor (TRPV1) is an important nociceptor involved in neurogenic inflammation. We aimed to examine the role of TRPV1 in experimental colitis and in the development of visceral hypersensitivity to mechanical and chemical stimulation. Male Sprague-Dawley rats received a single dose of trinitrobenzenesulfonic acid (TNBS) in the distal colon. In the pre-emptive group, rats received the TRPV1 receptor antagonist JYL1421 (10μmol/kg, iv) or vehicle 15 minutes prior to TNBS followed by daily doses for 7days. In the post-inflammation group, rats received JYL1421 daily for 7 days starting on day 7 following TNBS. The visceromotor response (VMR) to colorectal distension (CRD), intraluminal capsaicin, capsaicin vehicle (pH6.7) or acidic saline (pH5.0) was assessed in all groups and compared to controls and naïve rats. Colon inflammation was evaluated with H&E staining and myeloperoxidase (MPO) activity. TRPV1 immunoreactivity was assessed in the thoraco-lumbar (TL) and lumbo-sacral (LS) DRGs neurons. In the pre-emptive vehicle group, TNBS resulted in a significant increase in the VMR to CRD, intraluminal capsaicin and acidic saline compared the JYL1421 treated group (p<0.05). Absence of microscopic colitis and significantly reduced MPO activity was also evident compared to vehicle treated rats (p<0.05). TRPV1 immunoreactivity in the TL (69.1± 4.6%) and LS (66.4± 4.2%) DRG in vehicle treated rats was increased following TNBS but significantly lower in the pre-emptive JYL treated group (28.6±3.9 and 32.3±2.3 respectively, p<0.05). JYL1421 in the post-inflammation group improved microscopic colitis and significantly decreased the VMR to CRD compared to vehicle (p<0.05, ≥30mmHg) but had no effect on the VMR to chemical stimulation. TRPV1 immunoreactivity in the TL and LS DRG was no different from vehicle or naïve controls. These results suggest an important role for TRPV1 channel in the development of inflammation and subsequent mechanical and chemical visceral hyperalgesia.
PMCID: PMC2128774  PMID: 17719181
TRPV1; TNBS; visceral hyperalgesia; colorectal distension; capsaicin

Results 1-9 (9)