Shachar Peles

Acute nociceptive somatic stimulus sensitizes neurones in the spinal cord to colonic distension in the rat

The common co‐existence of fibromyalgia and chronic abdominal pain could be due to sensitization of spinal neurones (SNs), as a result of viscero‐somatic convergence. The objective of this study is to explore the influence of acute nociceptive somatic stimulation in the form of acid injections, into the ipsilateral somatic receptive field of neurones responsive to colorectal distension (CRD), and the potential role of ionotropic glutamate receptors on sensitization. Action potentials of CRD‐sensitive SNs were recorded extracellularly from the lumbar (L2–L5) spinal cord. Stimulus–response functions (SRFs) to graded CRD (10–80 mmHg, 30 s) were constructed before and 30 min after ipsilateral injection of low pH (4.0, 100 μl) saline into the somatic receptive fields. In some experiments, cervical (C1–C2) spinalization was performed to eliminate supraspinal influence. The selective NMDA receptor antagonist CGS 19755 and AMPA receptor antagonist NBQX were injected (25 μmol kg−1, i.v.) to examine their influence on sensitization. Three types of neurones were characterized as short‐latency abrupt (SLA, n= 24), short latency sustained (SLS, n= 12), and long‐latency (LL, n= 6) to CRD. Ipsilateral injection of low pH (4.0) in the somatic receptive field, but not the contralateral gastrocnemius (GN) or front leg muscles, sensitized responses of these neurones to CRD. Spinalization had no influence on the development of low pH‐induced sensitization. Both CGS 19755 and NBQX significantly attenuated the sensitized response to CRD in intact and spinalized animals. Acute nociceptive somatic stimulus sensitizes CRD‐sensitive SNs receiving viscero‐somatic convergence. The sensitization occurs at the spinal level and is independent of supraspinal influence. Ionotropic glutamate receptors in the spinal cord are involved in sensitization.

Effects of the 5-HT3 receptor antagonist, alosetron, in a rat model of somatic and visceral hyperalgesia.

&NA; Conflicting results exist regarding the role of 5‐HT3 receptors in somatic and visceral nociceptive processing. We aimed to investigate the effects of the 5‐HT3 receptor antagonist, alosetron, in a rat model of somatic and visceral hyperalgesia. Two injections (100 &mgr;l) of either pH 4.0 or 7.2 saline were given unilaterally in the gastrocnemius (GN) muscle. In all groups, the paw withdrawal thresholds (PWT) to von Frey filaments and the visceromotor responses (VMR) to colorectal distension (CRD) were recorded before the saline injections and 72 h, and 1 week after the second injection. Intrathecal (i.t.) (25 nmol) or intravenous (i.v.) (100 &mgr;g/kg/day) alosetron was given daily following the second injection and compared to either i.v. or i.t. saline (vehicle). There was a significant decrease in the mean PWT bilaterally in all groups following pH 4.0 injections (p < 0.05). Intravenous alosetron resulted in a significant increase in the PWT bilaterally on days 2 and 3. Intrathecal alosetron resulted in significant increase in the PWT starting at day 3 and was significantly higher than baseline on days 4–7 (p < 0.05). At CRD pressures ≥30 mmHg, the VMR of pH 4.0 injected rats was significantly increased at 72 h and 1 week (p < 0.05). Both i.v. and i.t. alosetron treated rats failed to demonstrate any alteration in the VMR. Control rats (pH 7.2) failed to show any alteration in the VMR and were unaffected by alosetron. Both, systemically and centrally administered alosetron, reversed the mechanical somatic hypersensitivity and prevented the development of visceral hyperalgesia, suggesting a centrally mediated effect.

Response properties of antral mechanosensitive afferent fibers and effects of ionotropic glutamate receptor antagonists

The ionotropic glutamate receptors N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are present peripherally in the primary sensory afferent neurons innervating the viscera. Multiple studies have reported roles of glutamate receptors in gastric functions. However, no study has previously shown the direct influence of ionotropic glutamate receptor antagonist on vagal sensory neurons. The objective of this study was to investigate the effects of NMDA and AMPA receptor antagonists on mechanotransduction properties of vagal afferent fibers innervating the rat stomach. Action potentials were recorded from the hyponodal vagus nerve innervating the antrum of the Long-Evans rats. For antral distension (AD), a small latex balloon was inserted into the stomach and positioned in the antrum. The antral contractions were recorded with solid-state probe inserted into the water-filled balloon. Antral units were identified to isovolumic (0.2-1 ml) or isobaric AD (5-60 mm Hg). NMDA and AMPA receptor antagonists were injected in a cumulative fashion (1-100 micromol/kg, i.v.). After the conclusion of experiment, the abdomen was opened and receptive field was mapped by probing the serosa of the stomach. Thirty-two fibers were identified to AD. The receptive fields of 26 fibers were located in the posterior part of the antrum. All fibers exhibited spontaneous firing (mean: 7.00+/-0.97 impulses/s). Twenty fibers exhibited a rhythmic firing that was in phase with antral contractions, whereas 12 fibers exhibited non-rhythmic spontaneous firing unrelated to spontaneous antral contraction. Both groups of fibers exhibited a linear increase in responses to graded isovolumic or isobaric distensions. NMDA (memantine HCl and dizocilpine (MK-801)) and AMPA/kainate (6-cyano-7-nitroquinoxaline 2,3-dione; CNQX) receptor antagonists dose-dependently attenuated the mechanotransduction properties of these fibers to AD. However, competitive NMDA antagonist dl-2-amino-5 phosphopentanoic acid (AP-5) had no effect. The study documents that glutamate receptor antagonists can attenuate responses of gastric vagal sensory afferent fibers innervating the distal stomach, offering insight to potential pharmacological agents in the treatment of gastric disorders.

Differential effects of transient receptor vanilloid one (TRPV1) antagonists in acid-induced excitation of esophageal vagal afferent fibers of rats.

Gastro-esophageal acid reflux can stimulate esophageal vagal sensory afferents by activating proton-sensitive ion channel transient receptor vanilloid one (TRPV1). The objective of this study was to investigate the response characteristics of vagal afferent fibers of rats to acid (0.1 N HCl) and capsaicin (CAP) following esophagitis and differential effects of two classes of TRPV1 antagonists on responses of vagal afferent fibers. The chronic reflux was induced by ligating the fundus of the stomach and partial constriction of pylorus. Extracellular single fiber recordings were made from the cervical vagal afferent fibers from naive control and fundus-ligated (FL) esophagitis rats. Innervations of fibers were identified to esophageal distension (ED) and subsequently tested to CAP and acid before and after injection of TRPV1 antagonist JYL1421 or AMG9810 (10 micromol/kg i.v.). Seventy-five vagal afferent fibers from 70 rats were identified to ED. Intra-esophageal CAP (0.1 ml of 1 mg/ml) excited 39.5% (17/43, 5/22 from naive and 12/21 from FL rats) fibers. In contrast, i.v. injection of CAP (0.03-0.3 micromol/kg) dose-dependently excited 72% (42/58) fibers. Responses to CAP were significantly greater for fibers from FL rats (n=32) than naive rats (n=25). TRPV1 antagonists JYL1421 and AMG9810 (10 micromol/kg) significantly blocked response to CAP. Intra-esophageal acid infusion stimulated 5/17 (29.4%) fibers from naive rats and 12/28 (42%) from FL rats. Effect of acid was significantly blocked by AMG9810, but not by JYL1421. Results indicate that following esophagitis the number of fibers responsive to CAP and acid is greater than noninflamed esophagus, which may contribute to esophageal hypersensitivity. Acid-induced excitation of vagal sensory afferents can be differentially attenuated by different classes of TRPV1 antagonists. Therefore, TRPV1 antagonists play a key role in attenuation of hypersensitivity following reflux-induced esophagitis. The use of TRPV1 antagonists could be an alternative to the traditional symptoms-based treatment of chronic acid reflux and esophageal hypersensitivity.

Neonatal nociceptive somatic stimulation differentially modifies the activity of spinal neurons in rats and results in altered somatic and visceral sensation

The role of intramuscular, low pH saline injections during the neonatal period in the development and maintenance of visceral hyperalgesia has not been systematically studied. We aimed to investigate alterations in visceral sensation and neural circuitry that result from noxious stimuli in early life. Neonatal male Sprague–Dawley rats received sterile saline injections of pH 4.0 or 7.4 in the gastrocnemius muscle starting at postnatal day 8. Injections were given unilaterally every other day for 12 days ending on postnatal day 20. A third group received needle prick only on the same shedule as the second group, while a fourth group was left naïve. At 2 months of age, rats underwent assessment of cutaneous and deep somatic sensitivity using von Frey filaments and gastrocnemius muscle pinch, respectively. A visceromotor response (VMR) to graded colorectal distension (CRD; 10–80 mmHg for 30 s with 180 s interstimulus intervals) was recorded. Extracellular single‐unit recordings from the thoracolumbar spinal neurons (T13–L1) were performed in adult pH 4.0 injected and naïve controls. There was no difference in the threshold for response to mechanical stimulation of the paw in rats injected with pH 4.0 saline compared to all other groups. Conversely, rats treated with pH 4.0 saline showed a significant bilateral reduction in withdrawal threshold to muscle pinch as adults (P < 0.05). At colorectal distensions ≥ 20 mmHg, an increase in the VMR was observed in the pH 4.0 injected group compared to all other groups (P < 0.05). Spinal neurons were classified as short latency abrupt (SL‐A) or short latency sustained (SL‐S). Spontaneous firing of SL‐S (20.6 ± 2.2 impulses s−1), but not SL‐A neurons (5.3 ± 0.9 impulses s−1) in the pH 4.0 treated rats was significantly higher than in control rats (SL‐S, 2.6 ± 0.8 impulses s−1; SL‐A, 3.1 ± 0.7 impulses s−1). The response of SL‐S neurons to CRD in the pH 4.0 group was significantly higher at distension pressures ≥ 20 mmHg. Nociceptive somatic stimulation in neonatal rats results in chronic deep somatic and visceral hyperalgesia in adulthood. Colorectal distension‐sensitive SL‐S neurons are primarily sensitized to neonatal somatic stimulation.