Chymopapain-induced reduction of proinflammatory phospholipase A2 activity and amelioration of neuropathic behavioral changes in an in vivo model of acute sciatica.

The mechanism of action underlying chymopapain (Chymodiactin) chemonucleolysis remains obscure. Radiographic studies suggest that chymopapain does not alter disc fragment size acutely; nonetheless, patients often report symptom resolution within a few days, even hours, of treatment. The authors postulate that, in addition to its chemonucleolytic action, chymopapain may possess antiinflammatory properties. To test this hypothesis, the authors assessed the ability of chymopapain to modulate the activity of the proinflammatory enzyme phospholipase A2 (PLA2) and to ameliorate behavioral changes associated with inflammatory neuropathy in an in vivo model of sciatica. Thirty-nine male Fischer rats were randomly assigned to one of three treatment groups: 1) saline, 2) betamethasone, or 3) chymopapain. All of the rats underwent unilateral sciatic nerve ligation with loose chromic gut suture to induce inflammatory mononeuropathy. The animals were tested for thermal and mechanical hyperalgesia on Days 0 (preoperation), 7 (pretreatment), and 14 (prior to death). Three animals were killed on Day 0 to determine the baseline PLA2 activity within unmanipulated rat sciatic nerves. On Day 7, three animals from each group were killed to assess PLA2 activity prior to treatment. The remainder were given a single infusion of saline, betamethasone (0.3 mg/kg), or chymopapain (100 pKat U) around the inflamed nerve. On Day 14, the remaining animals were killed and their sciatic nerves were removed. The tissue was homogenized and the PLA2 activity was determined using [14C]arachidonate-labeled Escherichia coli phospholipid membrane as a substrate. Lipids were extracted and separated by thin-layer chromatography. All animals developed behavioral changes consistent with inflammatory mononeuropathy 24 to 72 hours postoperatively; these included gait disturbance, flexion deformity, and hyperalgesia of the involved hindlimb. The degree of mechanical and thermal hyperalgesia was comparable between groups at Day 7. By Day 14, the thermal hyperalgesia had resolved; the mechanical hyperalgesia was less evident in the betamethasone- and chymopapain-treated groups than in the saline-treated controls (p = 0.003; saline- vs. chymopapain-treated groups p = 0.004; saline- vs. betamethasone-treated groups p = 0.008). The mean PLA2 activity at baseline (Day 0) was 11.6 +/- 4.9 nmol phospholipid hydrolyzed per minute per milligram of protein. The PLA2 activity at Day 7 was 74.4 +/- 18.2 (ligated side) and 21.2 +/- 11.7 (nonligated side). At Day 14, PLA2 activity was reduced in the chymopapain- (47.8 +/- 12.3) and betamethasone- (39.7 +/- 9.5) treated groups compared with the saline control group (62.3 +/- 11.2), (saline- vs. chymopapain-treated groups p < 0.05; saline- vs. betamethasone-treated groups p < 0.01). The PLA2 activity in nonligated specimens was 18.6 +/- 10.1. These data indicate that chymopapain exhibits antiinflammatory properties in vivo, reducing PLA2 activity and ameliorating mechanical hyperalgesia in this model of inflammatory sciatic neuropathy.

Continuous infusion of acidified saline around the rat sciatic nerve produces thermal hyperalgesia.

Recent observations using both clinical and animal models have suggested that acidosis may initiate pain and sensitization. In the present study, we examined if changing the acidic environment around the rat sciatic nerve resulted in thermal hyperalgesia. Fresh solutions of preservative-free saline (PFS) and unbuffered PFS acidified to pH 3.5 were continuously infused around the left rat sciatic nerve for 7 days. Rats receiving unbuffered, acidified PFS developed a progressive thermal hyperalgesia that was maximal on infusion day 6. Unbuffered, acidified PFS significantly decreased the perineural pH (pH 6.9 +/- 0.15, P < 0.05), and decreasing perineural pH values were significantly correlated with increasing thermal hyperalgesia (r = 0.91) for individual rats. While it is likely that multiple factors play a role in the development of neuropathic pain, these data demonstrate that an acidic environment around the sciatic nerve will produce thermal hyperalgesia.

Ketorolac potentiates morphine antinociception during visceral nociception in the rat.

BACKGROUND: Combinations of opioids and nonsteroidal antiinflammatory drugs (NSAIDs) are commonly used to control pain in the perioperative period, yet there are no quantitative evaluations of the interaction between opioids and nonsteroidal antiinflammatory drugs during visceral nociception. This study evaluated the interaction between morphine and ketorolac during visceral nociception in the rat. METHODS: The pressor response to noxious colorectal distention (80 mmHg, 20 s) was evaluated in 29 male Sprague-Dawley rats and dose-response curves were determined for intravenous morphine, ketorolac and the mixture of morphine and ketorolac. The data were interpreted using an isobolographic analysis to establish the nature of the interaction. RESULTS: Intravenous ketorolac alone (8-32 mg/kg) did not have a significant antinociceptive effect, whereas morphine alone (1-4 mg/kg) produced significant antinociception during noxious colorectal distention (dose yielding a 50% reduction in nociceptive response relative to baseline pressor response = 1.7 +/- 0.6 mg/kg). Isobolographic analysis of the antinociceptive interaction demonstrated a highly significant, naloxone-reversible potentiation of intravenous morphine by ketorolac in the rat during visceral nociception (P < 0.001). CONCLUSIONS: Ketorolac is a powerful potentiator of morphine antinociception during visceral nociception in the rat. However, intravenous ketorolac alone did not demonstrate antinociceptive properties during colorectal distention, a model of acute visceral nociception without a major inflammatory component. These data suggest that ketorolac may have a central modulatory effect on opioid pharmacology and the synergistic effect may be separate from its peripheral antiinflammatory properties. This study encourages further basic as well as clinical evaluations of the improved antinociception provided by combination therapy of opioids and nonsteroidal antiinflammatory drugs.

Ethanol dose-dependently attenuates NMDA-mediated thermal hyperalgesia in the rat.

Recent observations using acute and persistent pain models have suggested that activation of the N-methyl-D-aspartate (NMDA) receptor is required for mechanisms that underly the development and maintenance of thermal hyperalgesia. The present results document that both NMDA-mediated thermal hyperalgesia produced after acute intrathecal NMDA administration and NMDA-mediated thermal hyperalgesia produced in a model of neuropathic pain are dose-dependently and reversibly attenuated by intrathecal administration of ethanol (0.5-1.0%; total dose, 106-213 nmol, i.t.). This is consistent with recent reports that ethanol may function as a selective NMDA receptor antagonist at low concentrations and further extends the evidence that thermal hyperalgesia is mediated by NMDA receptors.

Neonatal capsaicin treatment prevents the development of the thermal hyperalgesia produced in a model of neuropathic pain in the rat.

Loose ligation of the sciatic nerve with 4-0 chromic gut sutures in rats produces behavioral evidence of neuropathic pain. In the present experiments we examined the involvement of capsaicin-sensitive afferents in mediating the thermal hyperalgesia produced by this model. Male Sprague-Dawley rats, treated as neonates (within 48 h of birth) with capsaicin (50 mg/kg, s.c.) or vehicle, were used at 16-18 weeks of age. Chromic gut sutures (4-0) were tied around the left sciatic nerve and withdrawal latencies of both hind paws to radiant heat were determined on postoperative days 3, 5, 10 and 20. Whereas there was a pronounced thermal hyperalgesia which lasted for up to 20 days in vehicle-treated rats, there was no evidence of thermal hyperalgesia in capsaicin-treated rats. There was no difference in baseline (pre-surgery) withdrawal latencies between the two groups. Radioimmunoassay revealed that there was a significant depletion of substance P (43.8%) and calcitonin-gene-related peptide (72.6%) in the lumbar spinal cord of neonatal capsaicin-treated rats compared to vehicle-treated rats. These results demonstrate that the chromic gut-induced thermal hyperalgesia is mediated by capsaicin-sensitive afferents and suggest that central mechanisms which process and control the reflex response to heat are different than mechanisms involved in thermal hyperalgesia.

Nitric oxide mediates the thermal hyperalgesia produced in a model of neuropathic pain in the rat.

Recent evidence has shown that activation of the N-methyl-D-aspartate receptor mediates the thermal hyperalgesia produced in a model of neuropathic pain. As the acute nociceptive effects of N-methyl-D-aspartate have been reported to be mediated through production of nitric oxide and activation of soluble guanylate cyclase, these experiments were designed to determine whether the thermal hyperalgesia produced in a rat model of neuropathic pain is also mediated through the production of nitric oxide and activation of soluble guanylate cyclase. Loose ligation of the sciatic nerve with chromic gut sutures, but not bilateral sham rats, demonstrated evidence of a marked thermal hyperalgesia on day 3 post-surgery. In bilateral sham rats, intrathecal administration of either an alternate substrate for nitric oxide synthase, NW-nitro-L-arginine methyl ester, or the soluble guanylate cyclase inhibitor, Methylene Blue, did not produce any change in thermal nociceptive withdrawal latencies. These same treatments blocked the thermal hyperalgesia in rats with chromic gut ligatures for a period of 2 and 4 h, respectively. These results suggest that a sustained production of nitric oxide and subsequent activation of soluble guanylate cyclase in the lumbar spinal cord mediate the thermal hyperalgesia produced in a model of neuropathic pain in the rat.

Management of cancer pain with transdermal fentanyl: phase IV trial, University of Iowa.

A multicenter study was conducted to determine the patient and physician acceptability of transdermal fentanyl in the management of cancer-related pain. In this study, 10 cancer patients at the University of Iowa received transdermal fentanyl after discontinuing their prior opioid analgesic; 7 patients completed questionnaires before and at 2 and 4 wk following transdermal fentanyl application. There was no significant difference in visual analogue scale scores for pain or mood. Verbal pain descriptor scores improved at 2 wk (P less than .05). There was a nonsignificant tendency toward increased depression and nausea; however, patients spent less time thinking about their illness and felt their cancer was less disruptive to their closest friends/relatives. Constipation, appetite, drowsiness, and concentration were not statistically different. Patients reported improved sleep habits at 2 wk (P less than .05) and tended to require less help with eating, dressing, washing, and using the bathroom. All patients completing the study chose to continue transdermal fentanyl for their cancer pain management. In summary, these data demonstrate the analgesic efficacy of the transdermal fentanyl system and suggest that some patients with cancer-related pain could benefit from its use.

Antinociceptive synergy between intrathecal morphine and lidocaine during visceral and somatic nociception in the rat.

Clinical investigations have suggested a synergistic interaction between the analgesic effects of intrathecal opioids and local anesthetics; however, basic pharmacologic evidence for this observation has not been reported. Therefore, the authors have used models of visceral and somatic nociception to quantify the interaction between intrathecal morphine and lidocaine in a crossover study of 24 rats in four equal groups. Combinations of morphine and lidocaine were administered separately, corresponding to time of peak effect for each drug. Colorectal distention, as a noxious visceral stimulus, was applied to two groups while cardiovascular and visceromotor responses, respectively, were recorded. A third group received hot plate testing as a somatic nociceptive stimulus. Intrathecal morphine and lidocaine both attenuated the cardiovascular and visceromotor responses to colorectal distention and increased hot plate latencies in a dose- and time-dependent manner. With the use of isobolographic analysis, the coadministration of morphine and lidocaine demonstrated a synergistic, supraadditive interaction during visceral nociception (P less than 0.001) and somatic nociception (P less than 0.005). In a fourth group, motor function was evaluated by an inclined screen method. Intrathecal lidocaine in the dosage range tested during isobolographic analysis revealed no motor deficits. These data clearly demonstrate antinociceptive synergy between intrathecal morphine and lidocaine during visceral and somatic nociception at dosages that do not impair motor function.