Mikito Kawamata

Formalin-induced release of excitatory amino acids in the skin of the rat hindpaw

Application of glutamate to skin evokes pain-related behaviors [S.M. Carlton, G.L. Hargett, R.E. Coggeshall, Localization and activation of glutamate receptors in unmyelinated axons of rat glabrous skin, Neurosci. Lett., 197 (1995) 25-28; D.L. Jackson, C.B. Graff, J.D. Richardson, K.M. Hargreaves, Glutamate participates in the peripheral modulation of thermal hyperalgesia in rats, Eur. J. Pharmacol., 284 (1995) 321-325.] and peripherally-administered glutamate antagonists can prevent the nociception produced by inflammation [E.M. Davidson, R.E. Coggeshall, S.M. Carlton, Peripheral NMDA and non-NMDA glutamate receptors contribute to nociceptive behaviors in the rat formalin test, NeuroReport, 8 (1997) 941-946; Jackson et al., 1995.] In this study, the concentrations of glutamate and aspartate in the plantar of the rat hindpaws were measured before and after the subcutaneous administration of formalin. Increases in glutamate and aspartate concentrations were observed on the ipsilateral side, but not on the contralateral side, to the injection. This shows that nociception and inflammation caused by formalin injection induces the release of peripheral glutamate and aspartate, which would contribute to nociception and inflammatory pain.

Comparison between celiac plexus block and morphine treatment on quality of life in patients with pancreatic cancer pain

Twenty-one patients with pancreatic cancer pain were studied to evaluate the effectiveness of celiac plexus block (CPB) on pain relief and quality of life (QOL), compared to the traditional NSAID-morphine treatment. The criteria were morphine consumption, visual analogue pain scale (VAS), performance status (PS) determined by medical and nursing staffs, and answers to QOL questionnaires. Morphine consumption, VAS, PS, and self-assessed QOL scores were taken when the administration of morphine was necessary for pain relief and those scores were used as control. Morphine consumption and the VAS score were recorded at regular weekly intervals and the PS and QOL scores were measured every 2 weeks thereafter. CPB was performed within 2-3 days after the control measurement. The VAS scores of the patients receiving CPB (n = 10) were statistically lower for the first 4 weeks after the procedure than those of the patients receiving the standard NSAID-morphine treatment (n = 11) during the same time period after the control measurement. Morphine consumption was significantly lower in weeks 4-7 (inclusive) following the procedure in the CPB group and continued to be lower thereafter, though not significantly so. Although the PS score slightly improved at the 2nd week after CPB, it was not improved by the start of the NSAID-morphine treatment. Self-assessed QOL scores did not ameliorate statistically after CPB; however, they did deteriorate remarkably in the patients treated only with morphine-NSAID during their survival periods, while they deteriorated only slightly in the CPB group. There were fewer side effects after CPB. These results indicate CPB does not directly improve QOL in patients with pancreatic cancer pain, but it may prevent deterioration in QOL by the long-lasting analgesic effect, limitation of side effects and the reduction of morphine consumption, compared to treatment only with NSAID-morphine.

Involvement of increased excitatory amino acids and intracellular Ca2+ concentration in the spinal dorsal horn in an animal model of neuropathic pain.

&NA; Neuropathic pain following nerve injury is believed to involve excitatory amino acids (EAAs) and Ca2+‐mediated neuronal plastic changes in the central nervous system (CNS). This study was designed to investigate the changes in glutamate and aspartate contents in the dorsal half of the spinal cord following chronic constrictive injury (CCI) of the rat common sciatic nerve. We also examined the changes in intracellular calcium ion concentration ([Ca2+]i) of the spinal dorsal horn in transverse spinal slices in the same animal model. Thermal and mechanical hyperalgesia were observed on day 2 and thereafter following CCI (P < 0.0001). In the CCI rats to which 0.5 mg/kg of i.p. MK‐801 was given 30 min prior to CCI and subsequently three daily treatments with 0.5 mg/kg of i.p. MK‐801, the development of thermal and mechanical hyperalgesia was suppressed for a period of up to 7 days; however, hyperalgesia appeared on day 10 and day 14 (P < 0.001). In CCI rats, significant increases were observed in glutamate and aspartate contents on the ipsilateral side of the dorsal horn to nerve ligation on days 4, 7 and 14 (P < 0.001). Moreover, significant increases in [Ca2+]i in the spinal dorsal horn were also observed in the superficial (lamina I–II) and deep layers (lamina V–VI) on the ipsilateral side to nerve ligation on days 4, 7 and 14 after nerve ligation in the spinal slices (P < 0.0001). The treatment with i.p. MK‐801 suppressed the increases in the contents of glutamate and aspartate and in [Ca2+]i on days 4 and 7. However, the ipsilateral contents of glutamate and aspartate significantly increased on day 14 (P < 0.001 and 0.003, respectively); the increased [Ca2+]i was also observed on day 14 (P < 0.001), and the spatial pattern of the increased regions was similar to untreated CCI rats. We interpret these results to indicate that neuropathic hyperalgesia induced by CCI in the rat is associated with an increase in glutamate and aspartate contents and the subsequent activation of NMDA receptors, followed by an increase in [Ca2+]i within dorsal horn of the spinal cord.

Experimental incision-induced pain in human skin: effects of systemic lidocaine on flare formation and hyperalgesia

&NA; In order to try to gain a better understanding of the mechanisms of post‐operative pain, this study was designed to psychophysically determine physiological and pharmacological characteristics of experimental pain induced by a 4‐mm‐long incision through the skin, fascia and muscle in the volar forearm of humans. In experiment 1, the subjects (n=8) were administered lidocaine systemically (a bolus injection of 2 mg/kg for a period of 5 min followed by an intravenous infusion of 2 mg/kg/h for another 40 min), and then the incision was made. In experiment 2, cumulative doses of lidocaine (0.5–2 mg/kg) were systemically injected in the subjects (n=8) 30 min after the incision had been made, when primary and secondary hyperalgesia had fully developed. Spontaneous pain was assessed using the visual analog scale (VAS). Primary hyperalgesia was defined as mechanical pain thresholds to von Frey hair stimuli (from 7 to 151 mN) in the injured area. The area of secondary hyperalgesia to punctate mechanical stimuli was assessed using a rigid von Frey hair (151 mN). Flare formation was assessed in the first experiment using a laser doppler imager (LDI). Pain perception was maximal when the incision was made and then rapidly disappeared within 30 min after the incision had been made. Primary hyperalgesia was apparent at 15 min after the incision had been made and remained for 2 days. The incision resulted in a relatively large area of flare formation immediately after the incision had been made. The area of flare began to shrink within 15 min and was limited to a small area around the injured area at 30 min after incision. Secondary hyperalgesia was apparent at 30 min after incision and persisted for 3 h after incision and then gradually disappeared over the next 3 h. In experiment 1, pre‐traumatic treatment with systemic lidocaine suppressed primary hyperalgesia only during the first 1 h after the incision had been made. The lidocaine suppressed the development of flare formation without affecting the pain rating when the incision was made. The development of secondary hyperalgesia continued to be suppressed after completion of the lidocaine infusion. In experiment 2, post‐traumatic treatment with lidocaine temporarily suppressed primary as well as secondary hyperalgesia that had fully developed; however, the primary and secondary hyperalgesia again became apparent after completion of the lidocaine administration. These findings suggest that pre‐traumatic treatment with lidocaine reduces the excessive inputs from the injured peripheral nerves, thus suppressing development of flare formation and secondary hyperalgesia through peripheral and central mechanisms, respectively. Pre‐traumatic treatment with lidocaine would temporarily stabilize the sensitized nerves in the injured area, but the nerves would be sensitized after completion of the administration. Post‐traumatic treatment with lidocaine reduced primary and secondary hyperalgesia that had fully developed. However, the finding that the suppressive effect of lidocaine on secondary hyperalgesia was temporary suggests that the development and maintenance of secondary hyperalgesia are caused by different mechanisms.

Different Mechanisms of Development and Maintenance of Experimental Incision-induced Hyperalgesia in Human Skin

BACKGROUND To determine the mechanisms of postoperative pain, the effects of local anesthesia on development and maintenance of surgical incision-induced hyperalgesia were evaluated in a crossover, double-blinded, placebo-controlled human study using 17 subjects. METHODS An experimental 4-mm-long incision through skin, fascia, and muscle was made in the volar forearm of each subject. In experiment 1, 1% lidocaine or saline in a volume of 0.2 ml was subcutaneously injected into the incision site pretraumatically and posttraumatically. In experiment 2, a 5-cm-long strip of skin was subcutaneously injected with 0.2 ml of 1% lidocaine near the incision site pretraumatically and posttraumatically. Flare, spontaneous pain, and primary and secondary hyperalgesia to punctate mechanical stimuli were assessed after the incision had been made. RESULTS Pretraumatic lidocaine injection prevented the occurrence of spontaneous pain and development of flare formation that was found surrounding the incision site immediately (1 min) after the incision had been made. The lidocaine suppressed primary hyperalgesia more effectively than did posttraumatic block, but only for the first 4 h after the incision. The preincision block prevented development of secondary hyperalgesia, whereas posttraumatic block did not significantly affect the fully developed secondary hyperalgesia. The area of flare formation and the area of secondary hyperalgesia did not extend over the strip of the skin that had been pretraumatically anesthetized, whereas the posttraumatic block did not significantly reduce the area of fully developed secondary hyperalgesia. CONCLUSIONS Pretraumatic injection of lidocaine reduces primary hyperalgesia more effectively than does posttraumatic injection, but only for a short period after incision. The spread of secondary hyperalgesia is mediated peripheral nerve fibers, but when secondary hyperalgesia has fully developed, it becomes less dependent on or even independent of peripheral neural activity originating from the injured site.

Peripheral nitric oxide in carrageenan-induced inflammation

Recent studies have suggested that nitric oxide (NO) peripherally produced by different nitric oxide synthase (NOS) isoforms contributes to edema formation and development of hyperalgesia. The present study was designed to examine the effects of NOS isoforms on NO release in carrageenan-induced inflammation at various time points. A microdialysis probe was implanted subcutaneously into the glabrous skin of hindpaws of Sprague-Dawley rats under pentobarbital anesthesia. After sample collection to obtain the basal level of the total amount of nitrite and nitrate (NO2-/NO3-), modified Ringer solution, a non-selective NOS inhibitor, NG monomethyl-L-arginine acetate (L-NMMA), or an iNOS inhibitor, aminoguanidine hemisulfate (AG) was perfused through the microdialysis probe. 2 mg of carrageenan was injected into the plantar surface of the probe-implanted hindpaw. Carrageenan was also injected in rats that had undergone sciatic nerve sectioning. Carrageenan significantly increased the dialysate concentrations of NO2-/NO3- for more than 8 h. L-NMMA suppressed the carrageenan-induced increase in NO2-/NO3- concentration. Although AG did not suppress the increase in NO2-/NO3- for the first 2 h after carrageenan injection, significant suppression of the increase in NO2-/NO3- was observed from 2.5 h after carrageenan injection. In the rats in which the sciatic nerves had been denervated, the increases in concentrations of NO2-/NO3- were completely suppressed up to 3 h and partially suppressed 4.5-8 h after carrageenan injection. The results of the current study show that carrageenan induces peripheral release of NO, the production of which is mediated by nNOS in the early phase and by both nNOS and iNOS in the late phase of carrageenan-induced inflammation.

FORMALIN-INDUCED NOCICEPTION ACTIVATES A MONOAMINERGIC DESCENDING INHIBITORY SYSTEM

Neural plasticity of afferent pain pathways that is induced by prolonged or repeated noxious stimuli may contribute to activate intrinsic inhibitory mechanisms in CNS. In order to clarify the role of the monoaminergic descending inhibitory system in acute nociception and inflammatory pain, we examined if this inhibitory system would modulate the tonic response to formalin-induced nociception. Yohimbine, alpha2 adrenergic antagonist, or methysergide, serotonin antagonist was administered intrathecally before or after subcutaneous 2% formalin injection into the plantar of the hind paw in rats. In another series of the experiment, the tissue of the spinal dorsal half of the untreated rats and post-formalin-treated rats were sampled and analyses of monoamine levels were carried out by HPLC. The subcutaneous formalin evoked biphasic flinching behavior of the injected paw. Intrathecal pretreatment with yohimbine and methysergide produced a significantly greater increase in the number of flinches than in the control in phase 1, intermediate period and phase 2. Posttreatment with yohimbine and methysergide showed a significantly greater increase in the number of flinches in phase 2. Furthermore, formalin injection induced significant increases in noradrenaline, MHPG, serotonin (5-hydroxytryptamine; 5-HT) and 5-HIAA concentrations in both the ipsi- and contralateral dorsal halves. These results suggest that the pain state produced by formalin-induced chemical and/or inflammatory nociception is under the modulation of the monoaminergic (noradrenergic and serotonergic) descending inhibitory system.

Premedication with oral dextromethorphan reduces postoperative pain after tonsillectomy

The aim of the present study was to examine whether premedication with dextromethorphan, a clinically available N-methyl-D-aspartic acid (NMDA) receptor antagonist, could reduce postoperative pain after tonsillectomy. Thirty-six patients scheduled for elective bilateral tonsillectomy were investigated in a double-blinded, randomized study. The patients were randomly assigned to one of three groups: control, dextromethorphan 30 mg (Dex 30), and dextromethorphan 45 mg (Dex 45) groups. In the control group, premedication was with oral placebo and intramuscular (IM) midazolam and atropine. In the Dex 30 and Dex 45 groups, patients were premedicated with IM midazolam and atropine and oral dextromethorphan 30 mg and 45 mg, respectively. Pain was evaluated repeatedly throughout 7 postoperative days, at rest and on swallowing, using a self-rating visual analog scale (VAS). The total doses of analgesics administered postoperatively were also recorded. The Dex 45 group showed significantly lower VAS scores than the control group both at rest and on swallowing throughout the 7 days. The total doses of postoperative analgesics in the Dex 45 group were significantly less than those in the control group. The Dex 30 group showed significantly lower VAS scores than the control group at rest, but not on swallowing. These results indicate that premedication with Dex 45 reduces postoperative pain after tonsillectomy, not only at rest but on swallowing. Implications: Recently, it has been suggested that central sensitization caused by the activation of N-methyl-D-aspartic acid receptors may contribute to the postoperative pain. We found that premedication with 45 mg of dextromethorphan, a clinically available N-methyl-D-aspartic acid receptor antagonist, reduced postoperative pain after tonsillectomy. (Anesth Analg 1998;86:594-7)

Analgesic mechanisms of ketamine in the presence and absence of peripheral inflammation

Background The studies on the mechanisms of ketamine antinociception have led to conflicting results. In this study, the authors investigated the contribution of supraspinal monoaminergic descending inhibitory system to ketamine analgesia for acute nociception and inflammation-induced hyperalgesia. Methods Male Sprague-Dawley rats were used. The paw withdrawal latencies to radiant heat stimuli were measured to assess the thermal nociceptive threshold. The analgesic effects of intrathecal or intraperitoneal ketamine were examined in the rats that received unilateral intraplantar carrageenan and in those that were untreated. In addition, it was examined whether pretreatment with intrathecal yohimbine or methysergide inhibited the analgesic effects of ketamine. Using an intrathecal microdialysis method, noradrenaline and 5-hydroxytryptamine concentrations in lumbar cerebrospinal fluid were measured after intraperitoneal ketamine in both saline- and carrageenan-treated rats. Results In the untreated rats, intraperitoneal but not intrathecal ketamine produced antinociceptive effects in a dose-dependent manner. Pretreatment with intrathecal yohimbine or methysergide inhibited these antinociceptive effects. Intraplantar carrageenan significantly reduced paw withdrawal latencies on the injected paw but not on the contralateral paw. Both intraperitoneal and intrathecal ketamine reversed the shortened paw withdrawal latencies on the injected side in a dose-dependent manner without any effects on the contralateral side. Neither yohimbine nor methysergide inhibited these antihyperalgesic effects. In analyses of monoamines, the magnitude of increase in monoamines after intraperitoneal ketamine was significantly smaller in the carrageenan-treated rats than in the saline-treated rats. Conclusion These results demonstrated that ketamine produced antinociceptive effects through an activation of the monoaminergic descending inhibitory system, whereas, in a unilateral peripheral inflammation-induced hyperalgesic state, the monoaminergic system did not contribute to the antihyperalgesic effects of ketamine. The mechanisms of the antinociceptive and antihyperalgesic properties of ketamine are different.

Potentiation of Antinociceptive Effects of Morphine by Calcium-channel Blockers at the Level of the Spinal Cord

BackgroundOpioids inhibit voltage-dependent calcium-channel conductance, which Is essential for the nervous system to be able to signal a painful event. Accordingly, interference with calcium-channel conductance may enhance opioid analgesia. The current study was designed to investigate the effects of calcium-channel blocking drugs on the antinociception of morphine at the level of the spinal cord. MethodsRats were chronically implanted with catheters in the lumbar intrathecal space. Tall-flick test was used to assess thermal nociception. Intrathecally administered drugs were morphine, calcium-channel blockers (verapamil, diltiazem, and nicardipine), or a combination of morphine and calcium-channel blocker. ResultsIntrathecal administration of morphine produced a significant dose-dependent antinociception in the tail-flick test. In contrast, intrathecal administration of calcium-channel blockers, verapamil, diltiazem, and nicardipine, did not show any antinociception at the employed doses. However, when intrathecally administered calcium-channel blockers, verapamil (50 μg), diltiazem (100 μg), or nicardipine (20 μg), were combined with ineffective (0.25,0.5,1, or 2 μg) or moderately effective (5 μg) doses of intrathecally administered morphine, significant antinociception was produced. These interactions were synergistic. There were no significant changes in MAP or HR after the intrathecal administration of 200 μg verapamil or 2 μg morphine combined with 50 μg verapamil. ConclusionsThe authors interpreted these results to indicate that calcium-channel blocking drugs synergistically potentiate the analgesic effects of morphine at the level of the spinal cord. Before these results can be translated into clinical use, however, adequate toxicity studies must be conducted to examine the effect of the perispinal administration of calcium-channel blocking drugs on spinal cord function.