The role of peripheral corticotropin-releasing factor signaling in a rat model of stress-induced gastric hyperalgesia.

BACKGROUND: Functional dyspepsia (FD) is a common gastrointestinal disorder associated with persistent or recurrent upper gastrointestinal tract symptoms such as pain without any obvious pathological changes. Psychological and psychiatric factors might have a pathogenic role in FD. Changes in the sensation of stomach pain were determined after application of stress to adult rats. The involvement of corticotropin-releasing factor (CRF), Type 2 CRF receptor (CRF2) and inflammatory cytokine interleukin-6 (IL-6) was also investigated in the gastric hyperalgesia observed in this model. RESULTS: Repeated water avoidance stress (WA-S) produced gastric hyperalgesia, with no obvious lesions in the gastric mucosa. Gastric hyperalgesia was inhibited by CRF and CRF2 antagonists, suggesting their involvement in gastric hyperalgesia observed after application of stress. Gastric hyperalgesia was inhibited by IL-6 neutralizing antibody. Immunofluorescence staining demonstrated CRF, CRF2, urocortin (Ucn)1, and Ucn2-positive cells in the gastric mucosa. CRF2-positive cells increased after WA-S, compared to sham stress. CRF2 and Ucn2 were expressed in the mast cells in the gastric mucosa. CONCLUSIONS: CRF2 plays an important role in gastric hyperalgesia produced by stress. CRF2 signaling may be a useful therapeutic target for functional dyspepsia.

Thin-fibre receptors expressing acid-sensing ion channel 3 contribute to muscular mechanical hypersensitivity after exercise.

BACKGROUND: Delayed onset muscle soreness (DOMS) is characterized by mechanical hyperalgesia after lengthening contractions (LC). It is relatively common and causes disturbance for many people who require continuous exercise, yet its molecular and peripheral neural mechanisms are poorly understood. METHODS: We examined whether muscular myelinated Aδ-fibres, in addition to unmyelinated C-fibres, are involved in LC-induced mechanical hypersensitivity, and whether acid-sensing ion channel (ASIC)-3 expressed in thin-fibre afferents contributes to this type of pain using a rat model of DOMS. The peripheral contribution of ASIC3 was investigated using single-fibre electrophysiological recordings in extensor digitorum longus muscle-peroneal nerve preparations in vitro. RESULTS: Behavioural tests demonstrated a significant decrease of the muscular mechanical withdrawal threshold following LC to ankle extensor muscles, and it was improved by intramuscular injection of APETx2 (2.2 µM), a selective blocker of ASIC3. The lower concentration of APETx2 (0.22 µM) and its vehicle had no effect on the threshold. Intramuscular injection of APETx2 (2.2 µM) in naïve rats without LC did not affect the withdrawal threshold. In the ankle extensor muscles that underwent LC one day before the electrophysiological recordings, the mechanical response of Aδ- and C-fibres was significantly facilitated (i.e. decreased response threshold and increased magnitude of the response). The facilitated mechanical response of the Aδ- and C-fibres was significantly suppressed by selective blockade of ASIC3 with APETx2, but not by its vehicle. CONCLUSIONS: These results clearly indicate that ASIC3 contributes to the augmented mechanical response of muscle thin-fibre receptors in delayed onset muscular mechanical hypersensitivity after LC. SIGNIFICANCE: Here, we show that not only C- but also Aδ-fibre nociceptors in the muscle are involved in mechanical hypersensitivity after lengthening contractions, and that acid-sensing ion channel (ASIC)-3 expressed in the thin-fibre nociceptors is responsible for the mechanical hypersensitivity. ASIC3 might be a novel pharmacological target for pain after exercise.

Peripheral and spinal mechanisms of nociception in a rat reserpine-induced pain model.

Chronic widespread pain is a serious medical problem, yet the mechanisms of nociception and pain are poorly understood. Using a reserpine-induced pain model originally reported as a putative animal model for fibromyalgia, this study was undertaken to examine the following: (1) expression of several ion channels responsible for pain, mechanotransduction, and generation/propagation of action potentials in the dorsal root ganglion (DRG), (2) activities of peripheral nociceptive afferents, and (3) alterations in spinal microglial cells. A significant increase in mRNA expression of the acid-sensing ion channel (ASIC)-3 was detected in the DRG, and the behavioral mechanical hyperalgesia was significantly reversed by subcutaneous injection of APETx2, a selective blocker of ASIC3. Single-fiber recordings in vitro revealed facilitated mechanical responses of mechanoresponsive C-fibers both in the skin and muscle although the proportion of mechanoresponsive C-nociceptors was paradoxically decreased. In the spinal dorsal horn, microglial cells labeled with Iba1 immunoreactivity was activated, especially in laminae I-II where the nociceptive input is mainly processed compared with the other laminae. The activated microglia and behavioral hyperalgesia were significantly tranquilized by intraperitoneal injection of minocycline. These results suggest that the increase in ASIC3 in the DRG facilitated mechanical response of the remaining C-nociceptors and that activated spinal microglia may direct to intensify pain in this model. Pain may be further amplified by reserpine-induced dysfunction of the descending pain inhibitory system and by the decrease in peripheral drive to this system resulting from a reduced proportion of mechanoresponsive C-nociceptors.

Repeated intramuscular injections of nerve growth factor induced progressive muscle hyperalgesia, facilitated temporal summation, and expanded pain areas.

Intramuscular injection of nerve growth factor (NGF) is known to induce deep-tissue mechanical hyperalgesia. In this study it was hypothesised that daily intramuscular injections of NGF produce a progressive manifestation of soreness, mechanical hyperalgesia, and temporal summation of pain. In a double-blind placebo-controlled design, 12 healthy subjects were injected on 3 days with NGF into the tibialis anterior muscle and with isotonic saline on the contralateral side. Assessments were performed before and after the injections on days 0, 1, and 2, and repeated on days 3, 6, and 10. The self-perceived muscle soreness was assessed on a Likert scale. Computer-controlled pressure algometry was used to assess the pressure pain thresholds (PPTs). Temporal summation of pain after repeated pressure stimulations was assessed by computer-controlled pressure algometry. The pain distribution following painful pressure stimulation was also recorded. Compared with baseline and isotonic saline, the NGF injections caused (P<0.05): (1) progressively increasing soreness scores from 3 hours after the first injection until day 2, after which it remained increased; (2) decreased PPTs at days 1 to 3; (3) facilitated temporal summation of pressure pain at days 1 to 10; and (4) enlarged pressure-induced pain area after the injection on day 1 to day 6. The daily injections of NGF produced a progressive manifestation of muscle soreness, mechanical hyperalgesia, temporal summation of pressure pain, and pressure-induced pain distribution. These data illustrate that the prolonged NGF application affects peripheral and central mechanisms and may reflect process in musculoskeletal pain conditions.

Involvement of NGF in the rat model of persistent muscle pain associated with taut band.

UNLABELLED: Myofascial pain syndrome (MPS) is an important clinical condition characterized by chronic muscle pain and a myofascial trigger point (MTrP) located in a taut band (TB). However, its pathogenic mechanism is still unclear. We developed an animal model relevant to conditions of MPS, and analyzed the mechanism of the muscle pain in this model. We applied eccentric contraction (EC) to a rat's gastrocnemius muscle (GM) for 2 weeks, and examined the mechanical withdrawal thresholds, histological changes, and expressions and contents of nerve growth factor (NGF). The mechanical withdrawal threshold decreased significantly at the next day of first EC and continued up to 9 days after EC. TBs were palpable at 3 to 8 days after initiation of EC. In EC animals, necrotic and regenerating muscle cells were found significantly more than in control animals. In EC animals, NGF expressions in regenerating muscle cells and NGF contents of GM were significantly higher than control animals. Administration of NGF receptor (TrkA) inhibitor K252a showed significant suppression of mechanical hyperalgesia in EC animals. Repeated EC induced persistent mechanical muscle hyperalgesia associated with TB. NGF expressed in regenerating muscle cells may have an important role in persistent mechanical muscle hyperalgesia which might be relevant to pathogenesis of MPS. PERSPECTIVE: The present study shows that NGF expressed in regenerating muscle cells is involved in persistent muscular mechanical hyperalgesia. NGF-TrkA signaling in primary muscle afferent neurons may be one of the most important and promising targets for MPS.

Immunolocalization of the PP family and its receptors in the gastrointestinal tract of house musk shrew, Suncus murinus.

OBJECTIVES: To investigate the immunolocalization of the pancreatic polypeptide (PP)-fold peptide family, important regulatory factors for food intake, in the gastrointestinal tract of Suncus murinus, and to discuss the relation with the obesity-resistance, visceral fat accumulation-resistance phenomenon in Suncus murinus. METHODS: The gastrointestinal tract of adult Suncus murinus, except for the stomach and pylorus, was divided into five sections (S1, corresponding to the duodenum, S2, S3 and S4, corresponding to the jejunum and ileum, and S5, corresponding to the colon and rectum in other mammals), to investigate the PP family and their receptor-producing cells by means of immunohistochemistry. RESULTS: NPY, PYY, Y1 and Y4-immunoreactive cells were distributed widely throughout the gastrointestinal tract, moreover, the PP family and their receptor-immunoreactive cells were predominantly distributed at the end of the gastrointestinal tract, the rectum. CONCLUSION: In this study, we investigated the distribution of the PP family and their receptor-producing cells in the gastrointestinal tract of Suncus murinus in detail for the first time. It was presumed that the wide distribution of Y4 in the gastrointestinal tract may be related to (associated with) the phenomenon of natural obesity-resistance, visceral fat accumulation-resistance in aging Suncus murinus.