Yoshito Nakayama

The effects of peripheral administration of a novel selective antagonist for prostaglandin E receptor subtype EP1, ONO-8711, in a rat model of postoperative pain

Mechanically evoked pain, also known as incident pain, induced by coughing or deep breathing after surgery leads to potentially devastating consequences. It is generally thought that the prostaglandin receptor- (especially, the receptor for prostaglandin E2, EP receptor) mediated sensitization of sensory nerve fibers is a key contributor to the generation of hyperalgesia. We examined whether a peripherally administered novel selective EP1 antagonist, ONO-8711, would be a potential analgesic for incision-induced mechanical hyperalgesia. We used a rat model of postoperative pain introduced by Brennan et al. (1). Withdrawal thresholds to punctate stimulation and response frequencies to nonpunctate mechanical stimulation were determined by using von Frey filaments applied adjacent to the wound and directly to the incision site of the hind paw, respectively. Mechanical hyperalgesia to punctate and nonpunctate stimuli was observed 2 and 24 h after the incision. ONO-8711 (2, 10, or 50 &mgr;g) or saline was administered subcutaneously into the hind paw on the ipsilateral side to the incision. ONO-8711 significantly (P < 0.01) increased the withdrawal thresholds to punctate mechanical stimulation and significantly (P < 0.01) decreased the response frequencies to nonpunctate mechanical stimulation in a dose- and time-dependent manner 2 and 24 h after the incision. We conclude that EP1 receptor-mediated sensitization of sensory nerve fibers may contribute to the generation of mechanical hyperalgesia produced by incisional surgery, and that the EP1 receptor antagonist ONO-8711 may be an option for treatment of postoperative pain, especially incident pain. Implications The peripheral administration of an antagonist for EP1 receptor that is a subtype of prostaglandin E receptors can inhibit the mechanical hyperalgesia induced by a surgical incision.

Role of Prostaglandin Receptor Ep1 in the Spinal Dorsal Horn in Carrageenan-induced Inflammatory Pain

Background Prostaglandin E2 (PGE2) and the receptor for PGE2 (EP receptor) are key factors contributing to the generation of hyperalgesia caused by inflammation. The current study was designed to investigate the roles of PGE2 and EP1 receptors in the spinal cord in the development and maintenance of inflammatory pain, using behavioral, microdialysis, and intracellular calcium ion concentration ([Ca2+]i) assays. Methods Inflammation was induced by an injection of carrageenan into the plantar surface of the rat hind paw. The effects of inflammation were evaluated at the time points of 3 h (early phase) and 15 h (late phase) after carrageenan injection. In behavioral assays, withdrawal thresholds to mechanical stimuli were evaluated. The effect of an intrathecally administered selective EP1 antagonist, ONO-8711, on the carrageenan-induced hyperalgesia was examined. Using a spinal microdialysis method, PGE2 concentration in the spinal dorsal horn was measured. In [Ca2+]i assays, we measured [Ca2+]i in the spinal dorsal horn in transverse spinal slices and examined the effects of pretreatment with ONO-8711. Sensitivities of the changes in [Ca2+]i to PGE2 perfusion were also assessed. Results Mechanical hyperalgesia and paw edema were observed in both the early and late phases. The hyperalgesia was inhibited by intrathecal ONO-8711 in the late, but not early, phase. The concentration of PGE2 in the spinal dorsal horn increased in the late phase. The [Ca2+]i in the dorsal horn increased on the ipsilateral side to the inflammation in the late, but not early phase. This increase was suppressed by the pretreatment with ONO-8711. Magnitude of the increase in [Ca2+]i on the ipsilateral side in response to PGE2 perfusion was greater in the late phase than in the early phase. Conclusion The results suggested that activation of spinal EP1 receptors was crucial in the carrageenan-induced mechanical hyperalgesia in the late phase. It seems that some of the mechanisms underlying inflammation-induced plastic changes are mediated by time-dependent increase in PGE2 concentration, activation of EP1 receptors, and increase in [Ca2+]i in the spinal dorsal horn.

The effects of intrathecal administration of an antagonist for prostaglandin E receptor subtype EP(1) on mechanical and thermal hyperalgesia in a rat model of postoperative pain.

Despite substantial advances in understanding acute pain mechanisms and in the treatment of pain, postoperative pain, especially mechanically evoked pain (incident pain), is generally not effectively treated. Tissue injury and inflammation increase the release of prostaglandin E2 in the spinal cord, contributing to the development of hyperalgesia. We designed the present study to determine whether the intrathecal administration of an antagonist for prostaglandin E2 receptor subtype EP1, ONO-8711, has an analgesic effect on incision-induced mechanical and thermal hyperalgesia. A 1-cm longitudinal skin incision was made in the plantar aspect of the rat foot. The withdrawal threshold to mechanical stimulation and the withdrawal latency to thermal stimulation applied adjacent to the wound of the hindpaw were investigated. Both mechanical and thermal hyperalgesia were observed at 2 h and 24 h after the incision had been made. ONO-8711 (50, 80, 100 &mgr;g) or saline was administered intrathecally. ONO-8711 significantly increased the withdrawal thresholds to mechanical stimulation, but not to thermal stimulation, in a dose- and time-dependent manner. We conclude that EP1 receptor-mediated sensitization of the spinal dorsal horn may contribute to the generation of mechanical, but not thermal, hyperalgesia and that an EP1 receptor antagonist administered intrathecally is a potential analgesic for postoperative pain, especially mechanically evoked pain (incident pain).

Effects of a novel selective agonist for prostaglandin receptor subtype EP4 on hyperalgesia and inflammation in monoarthritic model.

Background Cytokines have crucial role in the development and maintenance of inflammation and pain in arthritis. Activation of prostaglandin receptor subtype EP4 suppresses cytokine production in immune cells. The purpose of this study was to evaluate whether a novel EP4 agonist would be able to suppress thermal and mechanical hyperalgesia and paw swelling in acute and chronic phases in rat monoarthritic model. Methods Monoarthritis was induced by an injection of complete Freunds adjuvant (CFA) intracapsularly into the tibiotarsal joint of the rats. Withdrawal latencies to thermal stimulation on the hind paw, withdrawal thresholds to mechanical stimulation, paw volume, and ankle diameter were measured 24 h and 4 weeks after the CFA injection. A novel selective EP4 receptor agonist, ONO-AE1–329 (10, 25, or 50 &mgr;g) or saline was administered intracapsularly into the joint. Results Withdrawal latencies and withdrawal thresholds were significantly (P < 0.05) shortened and decreased, respectively, on the arthritic side but not on the contralateral side 24 h and 4 weeks after the CFA injection. In addition, significant (P < 0.05) increases in paw volume and ankle diameter on the arthritic side were observed. Intracapsularly administered ONO-AE1–329 showed significant (P < 0.05) inhibition of thermal and mechanical hyperalgesia and significant (P < 0.05) decrease in paw volume and ankle diameter in a dose-dependent manner at 24 h and 4 weeks after CFA. Conclusion Intracapsular administration of EP4 receptor agonist effectively inhibited mechanical and thermal hyperalgesia and inflammatory reactions in acute and chronic monoarthritis. An EP4 agonist would be a potential strategy for inflammatory pain in arthritis.

Role of prostaglandin receptor subtype EP1 in prostaglandin E2-induced nociceptive transmission in the rat spinal dorsal horn.

It has been indicated that prostaglandin E2 (PGE2) and the receptor for PGE2 (EP receptor) are key factors contributing to the facilitated generation of nociception. This study was designed to investigate the roles of PGE2 and EP1 receptors in the spinal cord in the nociceptive transmission, using behavioral and intracellular calcium ion concentration ([Ca2+]i) assays and in situ hybridization. Experiments were conducted on Sprague-Dawley rats. In behavioral assays, withdrawal thresholds to mechanical stimuli were evaluated using von Frey filament. The effect of an intrathecally administered selective EP1 antagonist, 6-[(2S,3S)-3-(4-chloro-2-methylphenylsulfonylaminomethyl)-bicyclo[2.2.2]octan-2-yl]-5Z-hexenoic acid (ONO-8711), on the intrathecal PGE2-induced hyperalgesia was examined. In [Ca2+]i assays, we measured [Ca2+]i in the dorsal horn of spinal cord slices and examined the effects of PGE2 and ONO-8711 perfusion on the [Ca2+]i changes. In situ hybridization using EP1 digoxigenin probe was performed on the slice sections of the lumbar spinal cord and bilateral L4 and L5 dorsal root ganglions (DRGs). Mechanical hyperalgesia was observed after intrathecal PGE2 administration. Intrathecal administration of ONO-8711 attenuated the PGE2-induced mechanical hyperalgesia in a dose- and time-dependent manner. Perfusion of ONO-8711 markedly suppressed PGE2-induced [Ca2+]i increment in laminae II-VI in dorsal horn of the spinal cord slice. Moreover, in situ hybridization revealed EP1 hybridization signals in the DRG neurons, but not in the spinal cord. The results of this study suggested that spinal PGE2 activates the EP1 receptors existing on the central terminals of primary afferents, subsequently increasing in [Ca2+]i in the spinal dorsal horn, which are involved in the mechanisms of spinal PGE2-induced nociceptive transmission.

Sepsis attenuates the intensity of the neuromuscular blocking effect of d-tubocurarine and the antagonistic actions of neostigmine and edrophonium accompanying depression of muscle contractility of the diaphragm.

Background: Prolonged effects of non‐depolarizing muscle relaxants in septic patients have been reported, although the influence of sepsis on neuromuscular transmission has not yet been clarified satisfactorily. These studies were intended to elucidate the influence of sepsis on neuromuscular transmission and on the action of drugs being utilized for regulation of muscle tone (a neuromuscular blocker and anti‐cholinesterase (anti‐ChE) drugs).

Activation of peripheral NMDA-nitric oxide cascade in formalin test.

Background It has been suggested that peripheral glutamate and nitric oxide (NO) released by tissue-damaging stimuli play an important role in peripheral nociceptive transmission. This study was conducted to determine whether NO was released in the periphery after subcutaneous injection of formalin and whether the peripheral N-methyl-d-aspartate (NMDA)–NO cascade was activated. Methods During pentobarbital anesthesia, a microdialysis probe was implanted subcutaneously into the glabrous skin of both hind paws of Sprague-Dawley rats. After sample collection to obtain the basal level of NO metabolites (total amount of nitrite [NO2−] and nitrate [NO3−] [NO2−–NO3−]), 5% formalin was injected into the plantar surface of the right hind paw during perfusion of the dialysis catheters with artificial cerebrospinal fluid (ACSF), the NO synthase inhibitor NG-monomethyl-L-arginine acetate, or the NMDA antagonist D,l-2-amino-5-phosphonovaleric acid through a microdialysis probe. Formalin also was injected in the animals that underwent sciatic nerve sectioning. In another series of experiments, NMDA was perfused through one probe. Samples for measurement of NO2−–NO3− were collected and immediately analyzed using high-performance liquid chromatography. Results Subcutaneous formalin significantly increased the dialysate concentrations of NO2−–NO3− (maximum increase 144 ± 12% of baseline value 30 min after formalin administration;P < 0.05) on the side ipsilateral to the injection. Both NG-monomethyl-l-arginine acetate and D,l-2-amino-5-phosphonovaleric acid significantly (P < 0.05) suppressed the formalin-induced increases in NO2−–NO3− concentration. In the rats with denervation of the sensory nerves, formalin did not change the NO2−–NO3− concentration. In addition, NMDA perfusion significantly (P < 0.05) increased the concentrations of NO2−–NO3−. Conclusion The results of the current study show that subcutaneous formalin injection induces peripheral release of NO, the production of which is mediated by activation of NMDA receptors in the peripheral nervous system.

Propofol enhances a d-tubocurarine-induced twitch depression in septic rat diaphragm.

We estimated the effect of d-tubocurarine (dTc) on neuromuscular transmission and the action of propofol on dTc-induced twitch depression by using sham control and septic rat nerve-hemidiaphragm preparations in vitro. Isometric twitch tension elicited by indirect (phrenic nerve) or direct (muscle) stimulation at 0.1 Hz was evaluated. Sepsis induced by panperitonitis attenuated the twitch tension elicited by indirect and direct stimulation (P < 0.01 in each group) in the absence of significant morphological inflammatory damage to the diaphragm. dTc (1 &mgr;M) decreased the twitch tension elicited by indirect stimulation (P < 0.01) less intensely in the septic group than in the sham group (P < 0.01). Propofol accentuated dTc-induced depressed twitch more intensely in the septic group (P < 0.01 or 0.05). These results demonstrate that sepsis attenuates both muscle contractile force and the effect of a neuromuscular blocker and that propofol more intensely enhances dTc-induced twitch depression during sepsis. Implications Propofol and nondepolarizing muscle relaxants are widely used for various clinical cases, including sepsis. Interactions between nondepolarizing muscle relaxants and propofol during sepsis are interesting from a clinical point of view. We demonstrated that propofol significantly enhances d-tubocurarine-induced twitch depression in vitro in the septic rat model compared with that in the nonseptic rat model.