Antinociceptive effects of locally administered morphine in infant rats.

Opiates injected into a site of injury are analgesic in adult animals, but there are no data on the effectiveness of this route of administration in immature organisms. Since the biological processes that regulate the effects of locally administered opiates are in flux during the early postnatal life of the rat, it is not clear whether or not opiates given directly into local tissue would be effective as analgesics. To test this we injected morphine (0.12, 0.60, 3.0 microg/injection) directly into the hindpaw (intraplantar) of infant rats at 3, 10 and 21 days of age, and assessed the behavioral response and the induction of Fos like immunocytochemistry in the dorsal horn of the spinal cord in the formalin test. Controls included saline injections to the paw, or comparable doses of morphine injected subcutaneously. At 3 days of age, the two higher doses were behaviorally analgesic when given into the paw, but there was limited selectivity over the subcutaneous route. At both 10 and 21 days of age, intraplantar injections were effective analgesics, whereas subcutaneous injections were not. The number of Fos stained cells in the dorsal horn of the spinal cord, induced by the formalin treatment, was decreased significantly by the 3.0 mg dose of morphine at all three ages. The results demonstrate that local treatment with morphine is an effective and selective analgesic in the infant rat.     

Characterization of nociceptive responses and spinal releases of nitric oxide metabolites and glutamate evoked by different concentrations of formalin in rats

A comparison was made of spontaneous nociceptive behaviors elicited by subcutaneous injection of formalin (0.5-10.0%) into the plantar or dorsal surface of the right hindpaw in rats. In the present study, we also examined the effect of paw formalin injection on the release of nitric oxide (NO) metabolites (nitrite/nitrate) and glutamate from the spinal cord in anesthetized rats using a dialysis probe placed in the lumbar subarachnoid space. Two distinct quantifiable behaviors indicative of pain were identified by formalin injected into both regions of the paw. There were no significant alterations in the number of flinches during the early and late phases induced by different regions of formalin injection. However, the early phase licking/biting activity evoked by formalin injection into the plantar surface of the paw was significantly higher than that evoked by formalin injected into the dorsal region. The maximum effect in the early and late phases was produced by 5.0% formalin injection into the dorsal and plantar paw. At a higher concentration (10.0%) of formalin, nociceptive behavioral responses were decreased except for the late phase flinching when injected into the dorsal paw. Injections of formalin (5.0%) into both regions of the paw evoked a biphasic spinal release of nitrite/nitrate with a significant increase during the early phase (0-10 min) and the late phase (30-80 or 90 min). A higher concentration of formalin (10.0%) failed to produce a clear-cut release of nitrite/nitrate. A significant increase of glutamate was observed in the 0-10 min samples obtained after injection of formalin (5.0%) into the plantar and dorsal surface of the paw, whereas 0.5 and 10.0% formalin induced no substantial release. These results suggest that 5.0% formalin should be used when studying antinociceptive activity of NO- and N-methyl-D-aspartate-related compounds in the formalin test in rats. Formalin injection into the plantar surface of the paw might prove to be useful for evoking the licking/biting response, particularly in the early phase.     

Long-lasting inflammation and long-term hyperalgesia after subcutaneous formalin injection into the rat hindpaw.

Subcutaneous formalin injection is widely used as a nociceptive stimulus in the rat. This procedure evokes overt behaviors that last about 90 minutes. However, little is known about the duration of paw inflammation and alterations in pain sensitivity to noxious stimuli after 2 hours. We studied the nociceptive responses to thermal and mechanical stimuli 2 hours to 4 weeks after formalin injection into the dorsal or plantar side of the hindpaw. Thirty-two adult male Sprague-Dawley rats were divided into 3 groups: In group I, 50 microL of 5% formalin was injected into the plantar side (n = 12); in group II, 50 microL of 5% formalin was injected into the dorsal side (n = 12); in group III, 50 microL saline was injected into the dorsal or plantar side of the hindpaw (n = 8). Nociceptive responses to thermal and mechanical stimuli applied to the dorsal or plantar surfaces of the injected and the contralateral hindpaws were recorded. The injection of formalin into the rat's hindpaw produced a hypoalgesic region around the injection site. In contrast, hyperalgesic responses to thermal and mechanical stimulation were induced on the opposite surface of the injected hindpaw as well as in the contralateral noninjected hindpaw. The hyperalgesic responses, which were observed 2 hours after formalin administration, were enhanced 1 to 3 days after injection and lasted 3 to 4 weeks. These results suggest that peripheral inflammation after subcutaneous formalin injection produces a long-lasting sensitization. Possible mechanisms for these changes in nociception are discussed.     

The behavioral and neuronal effects induced by repetitive nociceptive stimulation are affected by gonadal hormones in male rats

The role of gonadal hormones in inducing long-term modifications in response to transient nociceptive stimuli was investigated in adult male rats. Three weeks after gonadectomy or sham surgery, animals were randomly divided into groups to be exposed to sham (only a prick in the dorsal hind paw) or formalin treatment (50 microl, 5% s.c. in the dorsal hind paw) once a week for the following 3 weeks. In gonadectomized animals the formalin-induced responses (licking, flexing and jerking of the injected paw) did not differ from those of intact animals after the first formalin injection. However, their levels were higher after the second or third injections. Indeed, in intact animals the formalin-induced responses progressively decreased, being significantly lower after the third injection than after the first; in gonadectomized animals, the formalin-induced responses did not change with repetition of the formalin treatment. In intact rats, c-Fos expression in the paraventricular nucleus of the thalamus and arcuate nucleus of the hypothalamus remained at control levels or decreased in animals injected two or three times with formalin; in gonadectomized rats, c-Fos expression increased with repetition of the noxious stimulation, reaching the highest levels in animals injected three times with formalin. These results show that male gonadal hormones have an inhibitory, adaptive effect on the behavioral and neuronal responses to repeated nociceptive stimulation.     

Antihyperalgesic activity of epibatidine in the formalin model of facial pain

A growing body of evidence supports a nicotinic cholinergic approach to pain management, as neuronal nicotinic receptor agonists have shown efficacy across animal models of both inflammatory and neuropathic pain. However, most of these investigations have focused on the spinal system, and there is to date no report of nicotinic receptor-mediated antinociception in any pain model involving the trigeminal field of innervation. Thus, the purpose of the present studies was to evaluate whether the neuronal nicotinic receptor agonist epibatidine possesses antihyperalgesic activity in the formalin model of facial pain. Adult, male, Sprague--Dawley rats received a 50-microl, subcutaneous injection of 5% formalin into one vibrissal pad and the consequent, facial grooming behavior was monitored. Consistent with previous investigations using the formalin model, animals exhibited two periods of nocifensive grooming: (1) an acute phase that began immediately, peaked at 3 min and almost completely abated by 6 min, and (2) a tonic phase that began between 6 and 9 min, peaked at 21 min and slowly diminished over the ensuing 24 min. The subcutaneous administration of epibatidine (1--5 microg/kg) 5 min prior to the formalin injection led to a significant, dose-dependent reduction of both the acute and tonic phases of hyperalgesia. Separate groups of animals receiving epibatidine either 15, 30 or 60 min prior to the formalin injection exhibited a progressively diminishing antihyperalgesic response that was no longer significant in either phase by 30 min. Finally, pretreatment with the selective neuronal nicotinic receptor antagonist mecamylamine completely abolished the antihyperalgesic effect of epibatidine in both phases. Taken together, these studies demonstrate that in both the acute and tonic phases of the formalin model of facial pain, epibatidine produces a neuronal nicotinic receptor-mediated antihyperalgesia that is both dose- and time-dependent. These results support the rationale for exploring the clinical efficacy of nicotinic agonists as analgesics to treat certain types of trigeminal pain in humans.     

Involvement of cholecystokinin in peripheral nociceptive sensitization during diabetes in rats as revealed by the formalin response

The possible pronociceptive role of peripheral cholecystokinin (CCK-8) as well as CCK(A) and CCK(B) receptors in diabetic rats was assessed. Subcutaneous injection of 0.5% formalin induced a greater nociceptive behavior in diabetic than in non-diabetic rats. Moreover, local peripheral injection of CCK-8 (0.1-100 microg) significantly increased 0.5% formalin-induced nociceptive activity in diabetic, but not in non-diabetic, rats. This effect was restricted to the formalin-injected paw as administration of CCK-8 into the contralateral paw was ineffective. Local peripheral administration of CCK-8, in the absence of formalin injection, produced a low level of, but significant increase in, flinching behavior in diabetic compared to non-diabetic rats. In addition, local peripheral administration of the non-selective CCK receptor antagonist proglumide (1-100 microg), CCK(A) receptor antagonist lorglumide (0.1-100 microg) or CCK(B) receptor antagonist CR-2945 (0.1-100 microg), but not vehicle or contralateral administration of CCK receptor antagonists, significantly reduced 0.5% formalin-induced flinching in diabetic rats. CR-2945 was the most effective drug in this condition. These effects were not observed in non-diabetic rats. The local peripheral pronociceptive effect of CCK-8 (100 microg) was significantly reduced by proglumide (100 microg), lorglumide (100 microg), and CR-2945 (100 microg). Results suggest that diabetes-induced peripheral sensitization could be due to a local peripheral release of CCK-8, which in turn would act on CCK(B), mainly but also in CCK(A), receptors located on the primary afferent neurons.     

The antinociceptive effect of intrathecal administration of epibatidine with clonidine or neostigmine in the formalin test in rats

The analgesic effect of intrathecal injection of epibatidine, clonidine and neostigmine, compounds that elevate ACh, was examined in the formalin test, a model of post-injury central sensitization in the rat. The compounds were injected alone and in combination. Intrathecal injection of epibatidine alone did not alter pain behaviors, compared to vehicle-treated rats. Intrathecal injection of clonidine dose-dependently reduced tonic pain behaviors (ED(50)+/-95% confidence limits=6.7+/-4.8 microg). The combination of clonidine and epibatidine (C:E), in the ratio of 26:1, dose-dependently reduced tonic pain behaviors; and the ED(50) of C:E was 1.1+/-0.98 microg a significant 6-fold leftward shift of the dose response curve, compared with clonidine alone. The antinociceptive effect of C:E (26:1) was attenuated by pre-treatment with the nAChR antagonist mecamylamine. Neostigmine dose-dependently reduced tonic pain behaviors (ED(50)=1.5+/-1.3 microg). The combination of neostigmine and epibatidine, in a ratio of 8:1, significantly shifted the dose response curve 4-fold to the left (ED(50)=0.4+/-0.3 microg). The effect is mediated in part by the activation of the nAChR and possibly by the enhanced release of ACh. These data demonstrate significant enhancement of the antinociceptive effects of spinally delivered analgesics by a nAChR agonist, suggesting that this class of compounds may have utility as adjuvants when combined with conventional therapeutics.     

Effects of pain-attenuating brain stimulation and morphine on electrical activity in the raphe nuclei of the awake rat

Evoked potential and multiple unit responses to noxious shock and pinch as well as to innocuous air puffs were recorded in the dorsal raphe, median raphe and raphe magnus nuclei of awake rats. Concurrent measurements of various behavioral responses to noxious stimuli were also made. Electrical stimulation of midbrain central gray and of medial thalamus, as well as systemic administration of morphine, greatly diminished all behavioral and electrophysiological responses to noxious stimuli without reliably affecting responses to air puffs. At the same time that brain stimulation and morphine attenuated nociceptive responses, a significant elevation was seen in the spontaneous multiple unit activity of these brain areas, particularly nucleus raphe magnus. In another group of animals, a comparison was made of the analgesic effectiveness of stimulation sites in the bulbar raphe (including raphe magnus) and sites dorsal or lateral to this region. More consistently potent effects were obtained from the raphe placements. These findings point to the importance of the bulbar raphe in mechanisms of analgesia. It is suggested that brain stem stimulation and morphine administration activate descending controls of raphe origin which selectively inhibit nociceptive elements in the spinal cord.     

乙状结肠痛对大鼠中缝背核Fos-LI和NADPH-d阳性神经元表达影响的实验研究

目的：探讨中缝背核还原型尼克酰胺腺嘌呤二核苷酸脱氢酶(NADPH-d)阳性神经元是否参与调控大鼠乙状结肠痛。方法：采用Fos免疫组织化学、NADPH-d组织化学及Fos／NADPH-α双标技术，观察乙状结肠痛大鼠中缝背核NADPH—d阳性神经元和Fos样免疫反应蛋白(Fos-LI)的变化。结果：乙状结肠痛刺激后，中缝背核内NADPH-d阳性神经元个数和染色深度增加；中缝背核Fos—LI明显增加；中缝背核内有Fos-LI／NADPH-d双标神经元的表达，约占NADPH-d神经元的11％，与生理盐水对照组相比有显著性差异。结论：中缝背核NADPH-d阳性神经元可能参与对乙状结肠痛刺激的信息传递。     

Role of the paratrigeminal nucleus in nocifensive responses of rats to chemical, thermal and mechanical stimuli applied to the hind paw

Anatomical and immunohystochemical data suggest the paratrigeminal nucleus (Pa5) may play a role in nociceptive processing. The current study examines the influence of unilateral Pa5 lesion on nocifensive responses of conscious rats to noxious thermal (Hargreaves test), mechanical (electronic von Frey and Randall-Selitto tests), and chemical (formalin 2.5%; 50 microl) stimuli applied to the hind paw. Lesion of the Pa5 induced by ibotenic acid did not affect the latency for radiant heat-induced withdrawal of either paw. In contrast, the mean mechanical threshold for withdrawal of the contralateral (but not ipsilateral) paw in Pa5-lesioned rats was reduced by approximately 45% and 20%, in electronic von Frey and Randall-Selitto tests, respectively, when compared to sham-operated animals. Conversely, animals with Pa5 lesions injected with formalin in the contralateral paw spent less time engaged in focused (licking, biting or scratching the injected paw) and total nocifensive behavior (i.e., focused nocifensive behavior plus protection of the injected paw during movements) in both the first and second phases of the test [ approximately 50% inhibition of each parameter during first phase (0-5 min) and at 20, 25, and 30 min of second phase, relative to the sham-operated group], but the number of paw-jerks was unaffected. Pa5 lesion also delayed the onset of second phase focused pain induced by formalin in the ipsilateral paw. The results suggest that the Pa5 integrates the supraspinal pain control system and plays a differential modulatory role in the central processing of mechanical and chemical nociceptive information.     

Time course of the alteration in dorsal horn substance P levels following formalin: blockade by naloxone.

Substance P (SP) found in the dorsal horn of the spinal cord has been proposed as a mediator of nociception. Formalin injected into the hind paw of a rat as a nociceptive stimulus has been shown to increase the amount of immunoreactive SP in the dorsal horn, perhaps by decreasing SP release from primary afferent neurons. These SP changes may be due to the actions of endogenous opiates which can block SP release from primary afferent neurons. In order to determine the time course of SP changes in the dorsal horn and their modulation by naloxone, anesthetized rats pretreated subcutaneously with naloxone or saline were injected in the right hind paw with 0.4 ml of either saline or 5% formalin. After various time intervals, the animals were perfused and the lumbar enlargement of the spinal cord removed. Immunohistochemical staining and manual photometry were used to quantitate SP-like immunoreactivity (SPLI) in the dorsal horn. The results show that saline injection produced an increase in SPLI lasting 20 min, while formalin produced a biphasic effect with early (0-20 min) and late (20-60 min) increases in SPLI. Naloxone pretreatment 30 min prior to hind paw injection partially blocked the initial SPLI increase due to saline or formalin. However, this was not the case if naloxone was injected 2 min following hind paw injection. The formalin-induced late SPLI increase was blocked by naloxone only if it was administered prior to the formalin. This blockade of SPLI increases in the dorsal horn by naloxone implies that endogenous opioid systems play a role in the control of SP levels in the dorsal horn during nociception.     

Stimulus-dependent specificity for annexin 1 inhibition of the inflammatory nociceptive response: the involvement of the receptor for formylated peptides

In this study we investigated how the peptides derived from the glucocorticoid-inducible protein annexin 1 are able to alter the nociceptive threshold of mice. The effects of the annexin1 fragment 2-26 (Anxa1(2-26)) on nociceptive threshold were studied using both chemical (formalin test) and thermal (hot plate and tail flick test) nociceptive stimuli on mice. Subcutaneous administration of Anxa1(2-26) into the dorsal surface of the mouse's hind paw was able to selectively reduce formalin-induced nociceptive behavior in the last phase of the test. The same effect was observed after intracerebroventricular administration, however, this was not the case when performing the hot plate or tail flick tests. Of the shortest Anxa1(2-26)-derived peptides, Anxa1(2-12) reduced the nociceptive response to formalin, however, the Anxa1(2-6) did not. The possible involvement of the receptors for formylated peptide in the anti-nociceptive action of Anxa1(-26) and Anxa1(2-12) was studied, choosing the formalin test. We found that the formyl peptide receptor agonist formyl-MLF (fMLF) induced anti-nociceptive effects in the formalin test both after the peripheral and central administration. The formyl peptide receptor antagonist N-t-butoxycarbonyl-MLP did not alter the response to formalin, but it was able to block the anti-nociceptive effects of Anxa1(2-26,) Anxa1(2-12) and fMLF after peripheral or central administration. These results indicate that exogenously administered Anxa1 can peripherally and centrally inhibit the nociceptive transmission associated with inflammatory processes through a mechanism that involves formyl peptide receptors.     

Local interactions between anandamide, an endocannabinoid, and ibuprofen, a nonsteroidal anti-inflammatory drug, in acute and inflammatory pain

Anandamide, an endocannabinoid, is degraded by the enzyme fatty acid amide hydrolase which can be inhibited by nonsteroidal anti-inflammatory drugs (NSAIDs). The present work was designed to study the peripheral interactions between anandamide and ibuprofen (a non-specific cyclooxygenase inhibitor) in the rat formalin test. We first determined the ED50 for anandamide (0.018 microg +/- 0.009), ibuprofen (0.18 microg +/- 0.09), and their combination (0.006 microg +/- 0.002). Drugs were given 15 min before a 2.5% formalin injection into the dorsal surface of the right hind paw. Results were analyzed using isobolographic analysis. The antinociceptive interaction between anandamide and ibuprofen was synergistic. To further investigate the mechanisms by which the combination of anandamide with ibuprofen produced their antinociceptive effects, we used specific antagonists for the cannabinoid CB1 (AM251; 80 microg) and CB2 (AM630; 25 microg) receptors. We demonstrated that the antinociceptive effects of ibuprofen were not antagonized by either AM251 or AM630 and that those of anandamide were antagonized by AM251 but not by AM630. The synergistic antinociceptive effects of the combination of anandamide with ibuprofen were completely antagonized by AM251 but only partially inhibited by AM630. In conclusion, locally (hind paw) injected anandamide, ibuprofen or combination thereof decreased pain behavior in the formalin test. The combination of anandamide with ibuprofen produced synergistic antinociceptive effects involving both cannabinoid CB1 and CB2 receptors. Comprehension of the mechanisms involved needs further investigation.     

Induction of opioid tolerance by lysine-acetylsalicylate in rats

The analgesic effect of non-steroidal antiinflammatory drugs (NSAIDs) is due to their action upon the peripheral damaged tissues, the spinal cord, and brain stem structures of the 'descending pain-control system' such as the periaqueductal gray matter (PAG) and the nucleus raphe magnus (NRM). The NSAID dipyrone (metamizol) has been shown to engage opioidergic circuits at the PAG, the NRM and the spinal cord, but it is unknown whether this can be generalized to typical NSAIDs and to systemic administration. In the present study lysine-acetylsalicylate (LASA), an injectable form of the prototypical NSAID aspirin, was microinjected into the PAG (100 microg/0.5 microl) in freely moving rats to induce inhibition of tail flick and hot plate responses. This antinociception was reverted by naloxone (1 mg/kg i.p.). PAG microinjection of LASA twice daily for three days induced tolerance to LASA (i.e. a progressive loss of effectiveness) and cross-tolerance to PAG-microinjected morphine (5 microg/0.5 microl). The antinociceptive effect of systemically administered LASA (300 mg/kg i.p., equivalent to the 1000 mg analgesic dose for humans) was also abolished by naloxone. Intraperitoneal injection of LASA twice daily induced tolerance to LASA and cross-tolerance to i.p. morphine (1 or 5 mg/kg). LASA-tolerant rats showed opioid withdrawal signs when injected with naloxone. These findings support the notion that the contribution of the PAG and downstream pain-control structures to the analgesic effect of NSAIDs involves opioidergic mechanisms, and suggest that repeated therapeutic administration of NSAIDs may induce tolerance, cross-tolerance to opiates, and susceptibility to a withdrawal syndrome.     

The Orofacial Formalin Test in the Mouse: A Behavioral Model for Studying Physiology and Modulation of Trigeminal Nociception

The aim of the current study was to adapt the orofacial formalin pain model previously developed in rats for use in mice and to characterize as fully as possible the behavioral changes in this species. The effects of subcutaneous injection of different formalin concentrations (.5%, 1%, 2%, 4%, and 8%) were examined on the face-rubbing response. In mice, formalin injection into the upper lip induced sustained face-rubbing episodes with vigorous face-wash strokes directed to the perinasal area. A positive linear relationship between formalin concentration and amplitude of the rubbing activity was observed during the first and second phase of the test with concentration up to 4%. With the highest concentration used (8%), the amplitude of both phases had plateaued. Systemic administration of morphine and paracetamol induced a dose-dependent inhibition of the rubbing behavior during the second phase. Although both paracetamol and morphine inhibited the first phase, a dose-dependent inhibition was found only for morphine. The ED50 value (95% confidence interval) for suppressing the rubbing response during the first phase was 2.45 mg/kg (1.90-3.08 mg/kg) for morphine. The ED50 values for suppressing the rubbing response during the second phase were 3.52 mg/kg (2.85-4.63 mg/kg) for morphine and 100.66 mg/kg (77.98-139.05 mg/kg) for paracetamol. Heterosegmental nociceptive stimulation evoked by subcutaneous injection of capsaicin into the back of the animal 10 min before the formalin test produced a dose-dependent inhibition of the second phase of the rubbing response. The ED50 values for suppressing the rubbing response during the first and second phases were 9.04 microg (1.36-65.13 microg) and 0.92 microg (0.28-2.99 microg), respectively. In conclusion, the mouse orofacial formalin test appears to be a reliable model for studying the behavioral encoding of the intensity of nociceptive orofacial stimulation and the counter-irritation phenomenon and for testing analgesic drugs. PERSPECTIVE: To further exploit the new opportunities of investigating nociceptive processing at the molecular level with the transgenic "knockout" approach, we require suitable behavioral models in mice. The presented mouse orofacial formalin test appears to be a reliable model for studying the behavioral encoding of the intensity of nociceptive stimulation and the counter-irritation phenomenon and for testing analgesic drugs.     

Naloxone blocks the formalin-induced increase of substance P in the dorsal horn

Substance P (SP) has been proposed as a mediator of nociception in the dorsal horn of the spinal cord. Activation of nociceptive pathways by stimuli such as formalin injected into the hind paw has been shown to produce an increase in the amount of immunoreactive SP in primary afferent neurons. Opiate agonist and antagonist binding in the dorsal horn has been shown to affect the SP levels and release. In order to determine the effects of opiates on SP mediated nociception in the spinal cord, anesthetized rats pretreated subcutaneously with morphine, naloxone, or saline were injected in the right hind paw with 0.4 ml of either saline of 5% formalin. After 1 h, the animals were perfused and the lumbar enlargement of the spinal cord removed. SP-like immunoreactivity (SPLI) in the dorsal horns was quantitated using immunohistochemical staining and manual photometry. The results show that formalin injection increases the SPLI in the dorsal horn after 1 h, as does pretreatment with morphine. Morphine pretreatment combined with formalin injection further increases SPLI, but not significantly higher than either treatment alone. The morphine-induced increases could be blocked by naloxone, which had no effects on saline-treated controls. Most importantly, naloxone was able to block the formalin-induced increase in SPLI, implying that endogenous opioid systems play a role in the SP increases seen during formalin-induced nociception.     

Systemic capsaicin and olvanil reduce the acute algogenic and the late inflammatory phase following formalin injection into rodent paw.

Systemic capsaicin and an analogue, olvanil (NE-19550, 4-hydroxy-3-methoxyphenyl methyl-9Z-octadecenamide), were tested for antinociceptive activity in a model of persistent pain produced by the subcutaneous injection of formalin into the rodent hind paw. Formalin induced a biphasic nociceptive response in mice and rats which was measured (a) by the time spent licking the injected paw in mice and (b) by making electrophysiological recordings of single nociceptive neurone discharges in L1-L3 of the spinal dorsal horn of halothane-anaesthetised rats. In mice the initial phase of the response was reduced by systemic administration of morphine, capsaicin and olvanil but not by indomethacin. The second, more prolonged, inflammatory phase of the response was reduced by each agent. In rats, similar concentrations of capsaicin and olvanil reduced both the first and second components of the formalin response. These data show that capsaicin and a non-pungent analogue, olvanil, are efficacious antinociceptive agents in a model of prolonged chemical nociception induced by formalin. Their activity compares favourably with that of morphine and appears superior to that of indomethacin.     

Analgesia produced by lidocaine microinjection into the dentate gyrus.

The local anesthetic lidocaine was injected into the dentate gyrus (DG) of alert, unrestrained rats 10 min prior to investigation within the formalin test. Regional anesthesia of the DG resulted in a reduction of pain scores when administered contralateral to the site of subcutaneous formalin injection. The analgesic effect was evident 30-50 min after central infusion. These results provide evidence of the involvement of the hippocampal formation (HF) in pain perception.     

Antihyperalgesic effects of intrathecally administered magnesium sulfate in rats

Intrathecal administration of MgSO(4) is reported to cause paralysis. However, the characteristic sensory disturbances have not been thoroughly investigated. We examined the effect of intrathecally administered MgSO(4) on the nociceptive threshold, using three different nociceptive measures, formalin test, hot plate test and paw pressure test in rats. The dose of MgSO(4) was 30, 100 or 300 microg. In acute nociceptive tests, intrathecal MgSO(4) did not cause any significant changes in the pain threshold. However, phase 2 of the formalin test was suppressed dose-dependently. It is known that spinal NMDA receptors are involved in the changes seen during the second (tonic) phase of the formalin test and in vitro studies showed that Mg(2+) can cause voltage-dependent blockade of NMDA receptor channel in the neurons of spinal dorsal horn. Thus, the suppressive effect of intrathecally administered MgSO(4) on the tonic inflammation-evoked behavior is mediated by the spinal NMDA receptors. Our results suggest that intrathecal administration of MgSO(4) may be therapeutically beneficial for patients with tonic pain involving the spinal NMDA receptors.     

Suramin inhibits spinal cord microglia activation and long-term hyperalgesia induced by formalin injection.

Previous studies in our laboratory have shown that long-term (a period of weeks) increases in pain-related behavior were correlated with the activation of spinal microglia after subcutaneous injection of formalin into the dorsal surface of 1 hind paw. The present study examined whether intrathecal delivery of suramin (a P2 receptor antagonist) blocks microglia activation and long-term hyperalgesia induced by formalin injection. Suramin was administered by using an osmotic pump attached to an intrathecal catheter. Suramin delivery (1.25 microg/kg/h) began 1 day before the formalin injection and lasted for 4 days. Rats were observed by using a modified hot plate test before and at different times after formalin injection. The spinal cord was surveyed for changes in microglia labeling as shown by OX-42 staining at different times after formalin injection. Suramin decreased both the hyperalgesic sensitivity to the thermal stimuli and microglial activation induced by formalin injection as compared to the saline-treated group. This suggests that adenosine triphosphate is one potential mediator that activates spinal cord microglia and enhances pain-related behavior in the formalin model. PERSPECTIVE: This report suggests that blocking specific spinal P2 receptors might decrease the central enhancement of pain caused by peripheral injury and inflammation. One mechanism might be by blocking the activation of spinal microglia. Thus, P2 antagonists might have therapeutic usefulness in certain pain conditions.