Altered nociception in mice with genetically induced hypoglutamatergic tone.

Extensive pharmacological evidence supports the idea that glutamate plays a key role in both acute and chronic pain. In the present study, we investigated the implication of the excitatory amino acid in physiological nociception by using mutant mice deficient in phosphate-activated glutaminase type 1 (GLS1), the enzyme that synthesizes glutamate in central glutamatergic neurons. Because homozygous GLS1-/- mutants die shortly after birth, assays for assessing mechanical, thermal and chemical (formalin) nociception were performed on heterozygous GLS1+/- mutants, which present a clear-cut decrease in glutamate synthesis in central neurons. As compared to paired wild-type mice, adult male GLS1+/- mutants showed decreased responsiveness to mechanical (von Frey filament and tail-pressure, but not tail-clip, tests) and thermal (Hargreaves' plantar, tail-immersion and hot-plate tests) nociceptive stimuli. Genotype-related differences were also found in the formalin test for which GLS1+/- mice exhibited marked decreases in the nociceptive responses (hindlimb lift, lick and flinch) during both phase 1 (0-5 min) and phase 2 (16-45 min) after formalin injection. On the other hand, acute treatment with memantine (1mg/kg i.p.), an uncompetitive antagonist at NMDA glutamate receptors, reduced nociception responses in wild-type but not GLS1+/- mice. Conversely, antinociceptive response to acute administration of a low dose (1mg/kg s.c.) of morphine was significantly larger in GLS1+/- mutants versus wild-type mice. Our findings indicate that genetically driven hypoactivity of central glutamatergic neurotransmission renders mice hyposensitive to nociceptive stimulations, and promotes morphine antinociception, further emphasizing the critical role of glutamate in physiological nociception and its opioid-mediated control.

CGRP receptor blockade by MK-8825 alleviates allodynia in infraorbital nerve-ligated rats.

BACKGROUND: Previous data showed that, in rats, anti-migraine drugs (triptans, olcegepant) significantly reduced mechanical allodynia induced by infraorbital nerve (ION) ligation but not that evoked by sciatic nerve (SN) ligation. Whether this also occurs with MK-8825, a novel anti-migraine drug also acting through CGRP receptor blockade (but chemically unrelated to olcegepant) was tested in the present study, which also investigated possible anti-neuroinflammatory effects of this drug. METHODS: Adult male Sprague-Dawley rats underwent unilateral chronic constriction injury (CCI) to either the ION or the SN, and mechanical allodynia was assessed 2 weeks later within the ipsilateral vibrissae territory or hindpaw, respectively. Transcripts of neuroinflammatory markers were quantified by real-time quantitative RT-PCR in ipsilateral trigeminal ganglion and spinal trigeminal nucleus in CCI-ION rats. RESULTS: Acute as well as repeated (for 4 days) administration of MK-8825 (30-100 mg/kg, i.p.) significantly reduced CCI-ION-induced mechanical allodynia but was ineffective in CCI-SN rats. CCI-ION was associated with marked up-regulation of neuronal and glial inflammatory markers (ATF3, IL6, iNOS, COX2) in ipsilateral trigeminal ganglion but not spinal trigeminal nucleus. MK-8825-induced inhibition of iNOS mRNA up-regulation probably underlay its anti-allodynic effect because pharmacological blockade of iNOS by AMT (6 mg/kg, s.c.) mimicked this effect. CONCLUSIONS: These data further support the idea that CGRP receptor blockade might be a valuable approach to alleviate trigeminal, but not spinal, neuropathic pain through, at least partly, an inhibitory effect on neuropathic pain-associated increase in NO production in trigeminal ganglion.

Effects of tapentadol on mechanical hypersensitivity in rats with ligatures of the infraorbital nerve versus the sciatic nerve.

BACKGROUND: Convergent data showed that neuropathic pain has specific characteristics at cephalic versus extra-cephalic level, where single-targeted drugs differentially alleviate pain. Because the novel analgesic drug, tapentadol, is acting at two targets, µ-opioid receptors (as agonist) and noradrenaline reuptake (as inhibitor), we tested its effects on neuropathic pain at both cephalic and extra-cephalic levels. METHODS: Sprague-Dawley rats underwent unilateral constriction injury (CCI) to the infraorbital nerve (ION; cephalic territory) or the sciatic nerve (SN; extra-cephalic territory), and alleviation of nerve lesion-induced mechanical allodynia/hyperalgesia was assessed after acute or repeated (for 4 days) treatment with tapentadol compared with morphine and/or reboxetine (noradrenaline reuptake inhibitor) 2 weeks after surgery. Possible changes in the expression of the neuroinflammatory markers activating transcription factor 3 (ATF3), interleukin-6 (IL-6) and brain-derived neurotrophic factor (BDNF) by repeated tapentadol treatment were quantified by real-time reverse transcription polymerase chain reaction in ganglia and central tissues. RESULTS: Acute administration of tapentadol (1-10 mg/kg, i.p.) significantly reduced allodynia in both CCI-SN and CCI-ION rats. Although morphine (3 mg/kg, s.c.) or reboxetine (10 mg/kg, i.p.) alone was only marginally active, the combination of both drugs produced supra-additive effects like those observed with tapentadol. In contrast to repeated morphine whose effects vanished, the anti-allodynic effects of tapentadol remained unchanged after a 4-day treatment. However, the latter treatment with tapentadol did not affect nerve lesion-evoked overexpression of ATF3, IL-6 and BDNF transcripts. CONCLUSIONS: The dual synergistic pharmacological properties of tapentadol, which result in clear-cut anti-neuropathic pain effects at both cephalic and extra-cephalic levels, probably involve mechanisms downstream of nerve injury-induced neuroinflammatory reaction.

Differential behavioral effects of peripheral and systemic morphine and naloxone in a rat model of repeated acute inflammation.

BACKGROUND: It has been reported that opioid antinociceptive effects are enhanced in animal models of inflammation, but it remains unclear whether this sensitization to morphine is related to predominant central or peripheral increased effects. METHODS: The authors compared the behavioral effects of intraplantar and intravenous morphine and naloxone in a rat model of repeated acute carrageenan-induced inflammation in which enhanced responses to noxious stimuli result from sensitization in peripheral tissues or central sensitization. The antinociceptive effects of intraplantar morphine (50, 75, 100, 150, and 200 microg), intravenous morphine (0.3, 0.6, and 1 mg/kg), and the pronociceptive effects of intraplantar naloxone methiodide (150 microg) and intravenous naloxone (1 mg/kg) against noxious pressure (vocalization thresholds to paw pressure) in rats were assessed 3 h after one or two carrageenan plantar injections performed 7 days apart. RESULTS: After the first carrageenan injection, intraplantar and intravenous morphine produced significant increase of vocalization thresholds to paw pressure in inflamed but not in noninflamed paws. After the second carrageenan injection, the antinociceptive effects of intraplantar morphine were significantly reduced compared with those obtained after the first carrageenan injection, whereas effects of intravenous morphine were significantly enhanced and present in both hind paws. Intravenous naloxone demonstrated similar pronociceptive patterns after the first and second carrageenan injection. Intraplantar naloxone methiodide produced pronociceptive effects in inflamed hind paw that were significantly enhanced after the second carrageenan injection. CONCLUSIONS: When inflammation is enhanced by recurrent stimulations, the antinociceptive effects of systemic morphine are enhanced. This increase is more likely related to central than peripheral sites of action, beyond endogenous opioid system activation.

The effect of the glycine/NMDA receptor antagonist, (+)-HA966, on morphine dependence in neuropathic rats.

We have previously shown that rats with a painful peripheral neuropathy develop dependence without tolerance after repetitive doses [3mg/kg subcutaneously (s.c.)] of morphine. After injections of a higher dose (10mg/kg s.c.) the animals develop tolerance that can be prevented by the glycine/N-methyl-D-aspartate (NMDA) receptor antagonist, (+)-HA966. This study examined whether (1) dependence develops also after repetitive doses of 10mg/kg of morphine and, if so, (2) whether (+)-HA966 prevents the development of dependence after both the low and the higher morphine pretreatment doses. A 4day pretreatment regimen (post-operative days 12-16) with two daily s.c. injections of saline+saline, saline+morphine (3 or 10mg/kg), (+)-HA966 (2.5 or 5mg/kg)+morphine or (+)-HA966 (5mg/kg)+saline was used, and withdrawal was precipitated by an injection of naloxone [2mg/kg intravenously (i.v.)] at 17h after the last pretreatment injection. Three signs of withdrawal (exploring, writhing, ptosis) appeared after pretreatment with both doses of morphine alone, while other signs (teeth chattering, pilo-erection) developed only after injections at the 3mg/kg dose. One sign (penile grooming/erection) appeared only after the higher morphine dose. Pretreatment with the combination of (+)-HA966 and morphine at 3mg/kg prevented the development of all withdrawal signs. By contrast, except for exploring, (+)-HA966 did not modify the incidence of the withdrawal signs observed after pretreatment with doses of 10mg/kg of morphine. The results suggest that prevention of the development of morphine dependence by glycine/NMDA receptor antagonism depends on the degree of morphine dependence.

Complete prevention but stimulus-dependent reversion of morphine tolerance by the glycine/NMDA receptor antagonist (+)-HA966 in neuropathic rats.

BACKGROUND: Tolerance to the analgesic effect of morphine complicates the management of chronic pain states. The authors studied the ability of the glycine/N-methyl-D-aspartate receptor antagonist (+)-HA966 to modify morphine tolerance in a rat model of neuropathic pain. METHODS: Mononeuropathy was induced by placing four ligatures around the common sciatic nerve. The 4-day pretreatment regimens with two daily subcutaneous injections of saline and saline, saline and morphine (10 mg/kg), (+)-HA966 (2.5 mg/kg) and morphine, or (+)-HA966 and saline were begun on post-operative day 12 to test the ability of (+)-HA966 to prevent the development of tolerance. Behavioral experiments were performed on postoperative day 16, when the pain-related behavior reached a stable maximum. The effect of an acute dose of morphine (1 mg/kg intravenously) was tested against both mechanical (vocalization threshold to paw pressure) and thermal (struggle latency to hind paw immersion into a 46 degrees C hot-water bath) stimuli. In addition, to test the ability of a single injection of (+)-HA966 to reverse established morphine tolerance, groups of morphine-pretreated rats received injections of either (+)-HA966 (2.5 mg/kg subcutaneously) and morphine (1 mg/kg intravenously), saline and morphine, or (+)-HA966 and saline. RESULTS: Baseline vocalization thresholds and struggle latencies did not differ in the various pretreatment groups, indicating that the pretreatments had no effect on pain-related behavior. Coadministration of (+)-HA966 prevented the reduction of the effect observed with morphine alone in both the mechanical test and the thermal test. (+)-HA966 reversed morphine tolerance in the thermal but not in the mechanical test. CONCLUSION: (+)-HA966 prevented morphine tolerance in both mechanical and thermal tests but reversed established morphine tolerance in the thermal test only.

Antinociceptive effect of (S)-N-desmethyl trimebutine against a mechanical stimulus in a rat model of peripheral neuropathy.

Trimebutine (2-dimethylamino-2-phenylbutyl 3,4,5-trimethoxybenzoate, hydrogen maleate) relieves abdominal pain in humans. In the present study, the antinociceptive action of systemic (S)-N-desmethyl trimebutine, a stereoisomer of N-monodesmethyl trimebutine, the main metabolite of trimebutine in humans, was studied in a rat model of neuropathic pain produced by chronic constriction injury to the sciatic nerve. Mechanical (vocalization threshold to hindpaw pressure) stimulus was used. Experiments were performed two weeks after surgery when the pain-related behaviour has fully developed. (S)-N-desmethyl trimebutine (1, 3, 10 mg/kg s.c.) produced dose-dependent antinociceptive effects on the nerve-injured and the contralateral hindpaw. The effect of the lowest dose (1 mg/kg s.c.) of (S)-N-desmethyl trimebutine on the nerve-injured paw was equal to that seen after a ten time stronger dose on the contralateral paw. The effect of (S)-N-desmethyl trimebutine (1 mg/kg) was not naloxone reversible. The results suggest that systemic (S)-N-desmethyl trimebutine may be useful in the treatment of some aspects of neuropathic pain.

Effects of intraplantar morphine on paw edema and pain-related behaviour in a rat model of repeated acute inflammation.

Recent studies suggest that peripheral morphine may represent a valuable treatment in inflammatory painful diseases. This study examined effects of intraplantar morphine against noxious pressure and paw edema in rats with repeated acute inflammation induced by two carrageenin injections 7 days apart. This model mimics at least partly some aspects of recurrent inflammatory pain encountered in the clinical situation. In the first part of the experiment, the effect of intraplantar morphine into the inflamed hindpaw was determined 3 h after carrageenin injection. Intraplantar morphine (50-200 microg) produced significant elevations of vocalization thresholds to paw pressure in inflamed but not in non-inflamed paws after both carrageenin injection; these effects were reversible by intraplantar naloxone methiodide (40 microg). The effects of intraplantar morphine (150 microg) were similar in magnitude to that of intravenous morphine (1 mg/kg) after first carrageenin injection. In contrast, at doses of 150-200 microg, they were significantly lower after second ipsilateral carrageenin injection 7 days later, than first injection. Intraplantar morphine (100-200 microg) had no effect on paw edema associated with both carrageenin injections. In the second part of the experiment, intraplantar morphine was injected 10 min before the first injection of carrageenin. Intraplantar morphine (50 microg) was ineffective, whereas morphine (100-200 microg) prevented reduction of vocalization thresholds to paw pressure of inflamed hindpaw for 3 h. The intraplantar injection of morphine (100 and 150 microg) produced a transient increase in the volume of inflamed hindpaw, not reversible by intraplantar naloxone methiodide (40 microg). Pretreatment with intraplantar morphine had no effect on reduction of vocalization thresholds to paw pressure and edema related to a second ipsilateral injection of carrageenin 7 days later. These findings suggest that peripheral morphine may be useful for the clinical management of acute inflammatory pain rather than in recurrent inflammatory painful situations.

Peripheral component in the enhanced antinociceptive effect of systemic U-69,593, a kappa-opioid receptor agonist in mononeuropathic rats.

The contribution of a peripheral action of the kappa-opioid receptor agonist U-69,593 (trans-3,4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl) cycloexil] benzene-acetamide methanesulfonate) in the augmented antinociceptive effect of this substance was investigated in a well-established rat model of peripheral unilateral neuropathy (chronic constriction of the common sciatic nerve). Relatively low dose of systemic U-69,593 (0.75 mg/kg intravenous (i.v.)) and intraplantar (i.pl.) low doses of specific antagonists of kappa-(nor-binaltorphimine) or mu-(D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2: CTOP) opioid receptors were used. Vocalization thresholds to paw pressure were used as a nociceptive test. The i.pl. injection of nor-binaltorphimine (10-15 microg injected into the nerve-injured hind paw) had no effect on the antinociceptive effect of U-69,593. Higher doses (20-30 microg i.pl. nor-binaltorphimine) significantly reduced the effect of U-69,593 on this paw but not on the contralateral paw, an effect which plateaued at 30 microg. By contrast, the i.pl. injection of CTOP (1 microg into the nerve-injured paw) had no effect on U-69,593 antinociception, whereas it reduced the effect of systemic morphine in these animals. The doses of nor-binaltorphimine used, injected into the contralateral paw or i.v., failed to modify the antinociceptive effects of U-69,593 on either paw. These results provide evidence for a peripheral component in the enhanced antinociceptive effect of systemic U-69,593 in this model of neuropathic pain.

Sensitization of the nervous system, induced by two successive hindpaw inflammations, is suppressed by a local anesthetic.

Two hindpaw injections of carrageenin were performed 7 days apart and the time-course of the vocalization thresholds to pressure (VTPP) of all the 4 paws of the rat were measured after both injections. The first injection of carrageenin induced an increase in the circumference of the injected paw and a significant reduction of the VTPPs of all the 4 paws. The pain-related behavior of both hindpaws was enhanced, when carrageenin was injected for the second time not only into the previously inflamed, but also into the contralateral hindpaw. However, the abnormal responses of the forepaws were not increased by this second hindpaw inflammation. Both after the first and the second injection of carrageenin, the decrease of the VTPPs of all 4 paws was antagonized by lidocaine with epinephrine (LE) injected into the inflamed paw. This anesthetic effect was of a shorter duration after the second than after the first injection of carrageenin. Presumably, the imprint that is left in the nervous system after the first hindpaw inflammation, takes place at a relatively limited segmental level. In addition, it is possible to influence the established sensitization of the nervous system by treating the peripheral process itself, even when the first injury has primed the nervous system to the second injury. However, there still seems to exist some excitatory influences that cannot be suppressed by the local anesthetic.

The enhancement of morphine antinociception by a CCKB receptor antagonist in the rat depends on the phase of inflammation and the intensity of carrageenin-induced hyperalgesia.

The ability of the cholecystokinin B (CCKB) receptor antagonist L-365,260 to modulate the antinociceptive action of systemic morphine was investigated using the well established rat model of localized inflammation induced by intraplantar injection of carrageenin. The effects of morphine (0.1-1 mg/kg i.v.) alone or in combination with the CCKB receptor antagonist (0.2 mg/kg s.c.) were determined at different time-points (at 1, 3 and 24 h) after the injection of carrageenin by measuring the vocalization threshold to paw pressure. L-365,260 was found to be ineffective in modulating the responses to all doses of morphine at 1 and 24 h after carrageenin. By contrast, at 3 h, the CCKB receptor antagonist reversed the ineffectiveness of the low dose (0.1 mg/kg i.v.) of morphine on the inflamed paw. Further, in the L-365,260-pretreated rats, a significant correlation between the antinociceptive effect of the low dose (0.1 mg/kg) of morphine and the intensity of the mechanical hyperalgesia was observed, indicating that the CCK control of the degree of sensitivity to opioids can vary among-the animals. Our data illustrate a differential and limited effect of L-365,260 on opioid antinociception in carrageenin-injected rats, depending on the dose of morphine, the phase of inflammation and the intensity of hyperalgesia.

The selective cholecystokininB receptor antagonist L-365,260 diminishes the expression of naloxone-induced morphine withdrawal symptoms in normal and neuropathic rats.

The ability of a pretreatment with the cholecystokininB-receptor (CCK[B]) antagonist L-365,260 to prevent the development of morphine dependence was studied in normal and neuropathic (unilateral peripheral neuropathy) rats. A 4-day pretreatment regimen with two daily s.c. injections of either saline+saline, saline+morphine (3.0 mg/kg) or L-365,260 (0.2 mg/kg)+morphine was used, and withdrawal was precipitated by an injection of naloxone (1.0 or 2.0 mg/kg i.v.) at 24 h after the last pretreatment injection. After pretreatment with morphine alone, physical dependence developed in both normal and neuropathic rats. However, the incidence of teeth chattering and ptosis was higher in neuropathic rats. Pretreatment with the combination of L-365,260 and morphine prevented the expression of teeth chattering, ptosis, diarrhea, writhing and piloerection, but was devoid of effects on the exploratory activity among both groups of rats. These results suggest that endogenous CCK acting on CCK(B)-receptors may be involved in the development of morphine dependence both in normal and neuropathic rats.

The antinociceptive effect of combined systemic administration of morphine and the glycine/NMDA receptor antagonist, (+)-HA966 in a rat model of peripheral neuropathy.

1. We evaluated the ability of the functional antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex, (+)-(1-Hydroxy-3-aminopyrrolodine-2-one) ((+)-HA966), to modulate the antinociceptive action of systemic morphine in a rat model of neuropathic pain produced by chronic constriction injury to the sciatic nerve. Mechanical (vocalization threshold to hindpaw pressure) and thermal (struggle latency to hindpaw immersion into a water bath) stimuli were used. 2. In the mechanical test, morphine (0.05, 0.1 and 0.3 mg kg(-1), i.v.) alone produced dose-dependent effects in both neuropathic and uninjured rats. Likewise, morphine (0.1, 0.3 and 1 mg kg(-1), i.v.) dose-dependently increased struggle latencies of the nerve-injured hindpaw in the hot noxious (46 degrees C) test but was ineffective in the non-noxious warm (44 degrees C) and cold (10 degrees C) test. 3. Pretreatment with (+)-HA966 (2.5 mg kg(-1), s.c.) dose-dependently enhanced the effect of morphine in the mechanical test with the relative potency being nerve-injured hindpaw > contralateral hindpaw > uninjured rat. 4. Likewise, (+)-HA966 dose-dependently enhanced the effect of morphine against a hot (46 degrees C) stimulus and produced, in combination with morphine, a dose-dependent effect against a warm (44 degrees C) stimulus. In the cold (10 degrees C) test, (+)-HA966 reversed the ineffectiveness of the highest dose of morphine. 5. Naloxone blocked the effect of the combination of (+)-HA966 with morphine in all tests. The drug combination produced no motor deficits in animals using the rotarod test. 6. These findings suggest that combined administration of antagonists, acting at the glycine site of the NMDA receptor complex and morphine may be a promising approach in the treatment of neuropathic and acute pain.

Prevention of tolerance to the antinociceptive effects of systemic morphine by a selective cholecystokinin-B receptor antagonist in a rat model of peripheral neuropathy.

The ability of pretreatment by the selective cholecystokinin-B (CCK(B)) receptor antagonist L-365,260 (0.2 mg/kg s.c.) to prevent the development of tolerance to the antinociceptive action of morphine was evaluated by a well-established rat model of unilateral peripheral mononeuropathy. The 4-day pretreatment regimens (saline, L-365,260 or morphine alone, or with the combination of L-365,260 and morphine) were begun on postoperative day 12. The experiments were performed on day 16, when the pain-related behavior reached a stable maximum. Behavioral test based on a mechanical stimulus (vocalization threshold to paw pressure) and relatively low acute doses of systemic morphine (0.1, 0.3 and 1.0 mg/kg i.v.) were used. On day 16, the base-line vocalization threshold to paw pressure values of the groups pretreated with one of the four regimens were similar, which suggests that the pretreatments had no effect on the development of mechanical allodynia. Pretreatment with morphine alone (10 mg/kg s.c., two times a day during 4 days) induced a complete tolerance to the antinociceptive effect of acute morphine (0.1-1.0 mg/kg i.v.). However, pretreatment with the combination of L-365,260 with morphine completely prevented the development of tolerance to the antinociceptive effect of acute i.v. morphine. The effect of acute morphine in this latter pretreatment group was dose dependent, naloxone reversible and similar to the effect of acute morphine seen in the saline-pretreated group. Our results suggest that in this well-characterized model of neuropathic pain, the development of tolerance to the antinociceptive effect of systemic morphine can be prevented by systemic coadministration of the CCK(B) antagonist L-365,260. We further show, that in contrast to a tonic activity of the endogenous opioidergic system, a tonic activity of the endogenous CCK system cannot be revealed in this rat model of neuropathic pain.

In mononeuropathic rats, the enhancement of morphine antinociception by L-365,260, a selective CCK(B) receptor antagonist, depends on the dose of systemic morphine and stimulus characteristics.

The ability of the selective cholecystokinin(B) (CCK(B)) receptor antagonist L-365,260 (0.2 mg/kg s.c.) to modulate the antinociceptive action of relatively low doses of systemic morphine (0.1, 0.3 and 1.0 mg/kg i.v.) was evaluated using a well established rat model of peripheral unilateral mononeuropathy. Behavioural tests based on both mechanical (vocalization threshold to paw pressure) and thermal (struggle latency after immersion of the paw into a cold (10 degrees C), warm (44 degrees C) or hot (46 degrees C) water bath) stimuli were used. Experiments were performed 2 weeks after the surgery when the pain-related behaviour has fully developed. We demonstrated a differential effect of L-365,260 depending both on the dose of morphine and the test used. Pretreatment with the CCK(B) receptor antagonist (0.2 mg/kg) inverted the ineffectiveness of the lowest dose (0.1 mg/kg i.v.) of morphine against the noxious (46 degrees C) thermal stimulus, and the effect of the combination was equal to that seen after the dose 0.3 mg/kg of morphine alone. Likewise, in the mechanical test, the already enhanced effect of this dose (0.1 mg/kg) of morphine on the nerve-injured paw was further increased (by 4-fold) after L-365,260 pretreatment. These effects were abolished by naloxone (0.01 mg/kg i.v.). However, the effects of the higher doses (0.3 and 1.0 mg/kg i.v.) of morphine against the mechanical or noxious thermal stimuli were not significantly enhanced by pretreatment with the CCK(B) receptor antagonist. Further, L-365,260 was found to be completely ineffective in modulating the responses to morphine at 10 degrees C and at 44 degrees C.

The influence of the timing of bupivacaine infiltration on the time course of inflammation induced by two carrageenin injections seven days apart.

The mechanical allodynia and edema related to a subcutaneous carrageenin injection are increased by a conditioning carrageenin injection 7 days before (Guilbaud et al., 1992). In the present study, the possibility of preventing this by bupivacaine infiltration was tested. In the first part of the experiment, the time course of a carrageenin induced inflammation of the right hind paw was assessed in animals receiving local anesthetic injection (0.2 ml of bupivacaine 0.5% solution with epinephrine) either 5 min before (BUPI PRE group) or 60 min after (BUPI POST group) the carrageenin injection (0.2 ml of 1% solution). Control groups received saline (0.2 ml) with the same timing. In the second part of the experiment, 7 days later, a carrageenin injection was performed either in the right or the left hind paw. Mechanical allodynia and edema were evaluated by the vocalization threshold to paw pressure (VTPP) and paw circumference (PC) in both hind paws at 1, 2, 4, 24 h and 7 days after both carrageenin injections. The first carrageenin injection induced mechanical allodynia and edema maximal at 240 min (42% reduction of VTPP; 23% increase in PC) and the influence of bupivacaine on the VTPP and PC was similar to previous results (Fletcher et al., 1996). The second ipsilateral carrageenin injection induced a more pronounced inflammation in the control groups and BUPI POST group than the first injection (P < 0.001). In contrast, the increase in allodynia and edema was less intense in the BUPI PRE group than in the other groups (P < 0.0001 and P < 0.02 respectively). Bupivacaine injections had no effect on allodynia and edema related to a second contra-lateral carrageenin injection. These results suggest that bupivacaine infiltration, when administered before the first conditioning injection of carrageenin, can prevent the reinforcement of mechanical allodynia and edema related to a second ipsilateral injection of carrageenin 7 days later.

Estrous and sex variations in vocalization thresholds to hindpaw and tail pressure stimulation in the rat.

This study examined sex and estrous differences in vocalization thresholds of rats to hindpaw and tail pressure stimulation tested daily throughout at least 3 weeks. When all the measures were pooled, compared to males, female rats had higher thresholds for tail pressure (499 +/- 6 g, n = 188 measures vs. 466 +/- 2 g, n = 144 measures, respectively), but equal thresholds for hindpaw pressure (321 +/- 6 g, n = 188 measures vs. 319 +/- 2 g, n = 144 measures, respectively). Thresholds of female rats in proestrus and estrus were lower than those of rats in metestrus and diestrus for both tail and hindpaw stimulation, whereas those of males did not vary systematically. Thresholds at the two stimulation sites covaried in females but not in males. These results add to the growing list of important interacting factors that underly behavioral sensitivity to noxious somatic stimulation.

The antinociceptive activity of kappa- but not delta-opioid receptor agonists is maintained in morphine-tolerant neuropathic rats.

The antinociceptive effect of the preferential mu-opioid receptor agonist morphine (1 mg/kg i.v.), the delta-opioid receptor agonists, DTLET ([D-Thr2,Leu5]enkephalin-Thr) (3 and 6 mg/kg i.v.) and BUBUC ([D-Cys(StBu)2,Leu5]enkephalin-Thr(OtBu) (3 mg/kg i.v.), and the kappa-opioid receptor agonist U-69,593 (trans-3,4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl)cyclohexil]benze neacetamide methanesulfonate) (0.25, 0.5 and 0.75 mg/kg i.v.) was evaluated in mononeuropathic (chronic constriction of the common sciatic nerve) rats. The rats were pretreated s.c. with 10 mg/kg of morphine, or saline, twice daily from day 12 to day 16 after the surgery. In morphine-pretreated rats, the antinociceptive effect of morphine on the vocalization threshold to paw pressure was greatly reduced, as compared to the saline-pretreated group. The antinociceptive effect of DTLET and BUBUC had also disappeared in the morphine-pretreated rats. By contrast, the potent antinociceptive effect of U-69,593 was not affected by the morphine pretreatment. Furthermore, the effect of U-69,593 was reversed by the specific kappa-opioid receptor antagonist nor-binaltorphimine (1 and 2 mg/kg i.v.). These results suggest that in mononeuropathic rats, morphine pretreatment results in cross-tolerance to delta- but not to kappa-opioid receptor agonists.

Further evidence for a peripheral component in the enhanced antinociceptive effect of systemic morphine in mononeuropathic rats: involvement of kappa-, but not delta-opioid receptors.

The contribution of a peripheral action of morphine in the augmented antinociceptive effect of this substance was re-evaluated in a well established rat model of peripheral unilateral mononeuropathy (chronic constriction of the common sciatic nerve), using a relatively low dose of systemic morphine (1 mg/kg i.v.) and local low doses of specific antagonists of kappa- (nor-binaltorphimine) or delta-(naltrindole) opioid receptors. Vocalization thresholds to paw pressure were used as a nociceptive test. Escalating doses of nor-binaltorphimine (10-30 micrograms injected locally into the nerve injured paw) significantly and dose dependently reduced the effect of morphine on this paw but not on the contralateral paw, an effect which plateaued at 30 micrograms. By contrast, the local injection of naltrindole (30-40 micrograms into the nerve injured paw) had no effect on morphine analgesia. The doses of opioid receptor antagonists used, injected i.v., in the contralateral paw, or alone in the nerve injured paw had no significant effect. These results suggest that the peripheral effect of systemic morphine in this model of neuropathic pain could be mediated not only by mu- but also by kappa-opioid receptors.

Influence of timing of administration on the analgesic effect of bupivacaine infiltration in carrageenin-injected rats.

BACKGROUND: Recent evidence has suggested that the timing of administration of analgesic drugs could influence their efficacy by reducing the sensitization of the nervous system induced by the nociceptive inputs, but this concept of preemptive analgesia is still debated in both clinical and basic research. METHODS: The model of acute inflammatory pain induced by carrageenin was used to study the influence of timing of administration of bupivacaine (0.2 ml of a 0.5% solution with 0.005 mg/ml epinephrine) on the development of hyperalgesia, edema, and increase in temperature. The animals received bupivacaine 5 min before (BUPI PRE group, n = 20) or 60 min after (BUPI POST group, n = 20) carrageenin (1 ml/kg of 1% solution) was injected into the left hind paw. Two control groups (n = 15 in each) received saline 5 min before or 60 min after administration of carrageenin. Hyperalgesia of the injected paw was evaluated by the vocalization threshold to paw pressure, edema by measuring paw circumference with a thread, and plantar temperature with a thermocouple thermometer. All measurements were done before carrageenin injection then every 30 min thereafter for 240 min. Another series (n = 24), with the same four groups was also evaluated at 24 h. RESULTS: Local injection of bupivacaine 60 min after carrageenin partially reduced the edema and hyperalgesia. The injection of bupivacaine 5 min before carrageenin was more efficient than the delayed injection and reduced hyperalgesia, edema and the increase in temperature temporarily, but did not totally prevent their development. All groups were similar at 240 min and 24 h. CONCLUSIONS: These results show that a slight advantage of infiltration with bupivacaine before injury exists in this carrageenin model of acute inflammatory pain. However, this benefit is limited in time and bupivacaine did not have any preemptive analgesic effect.