Endogenous opioid and cannabinoid mechanisms are involved in the analgesic effects of celecoxib in the central nervous system.

BACKGROUND/AIMS: In this study we analyzed the mechanisms underlying celecoxib-induced analgesia in a model of inflammatory pain in rats, using the intracerebroventricular (i.c.v.) administration of selective opioid and cannabinoid antagonists. METHODS AND RESULTS: Analgesic effects of celecoxib were prevented by selective µ-(ß-funaltrexamine) and δ-(naltrindole), but not κ-(nor-binaltorphimine) opioid antagonists, given i.c.v. 30 min before celecoxib. Similar pretreatment with AM 251, but not SR 144528, cannabinoid CB(1) and CB(2) receptor antagonists, respectively, prevented celecoxib-induced analgesia. The fatty acid amide hydrolase inhibitor, URB 597, also prevented celecoxib-induced analgesia. CONCLUSIONS: Our data provided further evidence for the involvement of endogenous opioids and revealed a new cannabinoid component of the mechanism(s) underlying celecoxib-induced analgesia.

Proteinase-activated receptor-4 plays a major role in the recruitment of neutrophils induced by trypsin or carrageenan during pleurisy in mice.

BACKGROUND/AIMS: The activation of proteinase-activated receptors (PARs) has been implicated in the development of important hallmarks of inflammation, including in vivo leukocyte recruitment. Here, we examined the effects of aprotinin, a potent inhibitor of trypsin proteinase and the kallikrein-kinin system, and the PAR-4 antagonist YPGKF-NH(2) (tcY-NH(2)) on neutrophil recruitment in response to carrageenan and trypsin in the pleural cavity of mice. METHODS: BALB/c mice were intrapleurally injected with trypsin or PAR-4-activating peptide AY-NH(2), pretreated with aprotinin or tcY-NH(2) (1 µg/cavity) prior to an intrapleural injection of trypsin or carrageenan, or pretreated with leukotriene B(4) antagonist U-75302 (3 µg/cavity) prior to a trypsin injection. The number of infiltrating neutrophils was evaluated after 4 h. RESULTS: PAR-4-activating peptide AY-NH(2) and trypsin-induced neutrophil recruitment was inhibited by aprotinin, tcY-NH(2) or U-75302. Aprotinin and tcY-NH(2) also inhibited neutrophil recruitment induced by carrageenan. CONCLUSION: These data suggest a key role for PAR-4 in mediating neutrophil recruitment in a mouse model of pleurisy induced by the activity of trypsin or trypsin-like enzymes.

Blockade of proteinase-activated receptor-4 inhibits the eosinophil recruitment induced by eotaxin-1 in the pleural cavity of mice.

OBJECTIVE AND DESIGN: Although proteinase-activated receptor (PAR)-4 has been implicated in inflammation, its role in regulating eosinophil recruitment in response to chemoattractants has not yet been demonstrated. To investigate the contribution of proteinases and PAR-4 activation to eosinophil migration in response to eotaxin-1 or leukotriene B(4) (LTB(4)), the effects of aprotinin or PAR-4 antagonist trans-cinnamoyl-YPGKF-NH(2) (tcY-NH(2)) on eosinophil migration induced by these chemoattractants were investigated. METHODS: BALB/c mice were pretreated with aprotinin or tcY-NH(2) (30 µg/mouse) prior to intrapleural injection of LTB(4) or eotaxin-1 and the number of infiltrating eosinophils was determined 48 h later. RESULTS: Aprotinin (1 mg/kg) inhibited eosinophil recruitment induced by eotaxin-1 (p < 0.01), but not that induced by LTB(4). Moreover, tcY-NH(2) treatment inhibited eosinophil recruitment in response to eotaxin-1 (p < 0.01 by ANOVA/Tukey post-test). CONCLUSION: These data suggest that aprotinin-inhibited proteinases participate in eosinophil migration induced by eotaxin-1 and that PAR-4 activation plays an important role in regulating this migration.

Role of systemic and local administration of selective inhibitors of cyclo-oxygenase 1 and 2 in an experimental model of periodontal disease in rats.

BACKGROUND AND OBJECTIVE: Periodontal disease is an inflammatory condition of tooth-supporting tissues. Arachidonic acid metabolites have been implicated in development of periodontal disease, especially those derived from the cyclo-oxygenase (COX) pathway. This study investigated the role of inhibitors of cyclo-oxygenases (COX-1 and COX-2) in a model of periodontal disease in rats. MATERIAL AND METHODS: A ligature was placed around the molar of rats. Losses of fiber attachment and of alveolar bone were measured morphometrically in histologically prepared sections. Infiltration of cells into gingival tissue surrounding the ligated tooth was also determined. RESULTS: Systemic and local administration of non-selective and selective COX-2 inhibitors, preventively, resulted in significant reduction of the losses of fiber attachment and alveolar bone, as well as decreased leukocyte numbers in gingival tissue. Preventive selective inhibition of COX-1 was as effective as COX-2 inhibition in reducing local fiber attachment loss and cell migration, but did not prevent alveolar bone loss. CONCLUSION: Our results provide evidence for participation of COX-1 and COX-2 in early stages of periodontal disease in rats. Furthermore, local administration of COX inhibitors reduced the signs of periodontal disease to the same extent as systemic treatment. Therapeutic approaches incorporating locally delivered anti-inflammatory drugs could be of benefit for patients suffering from periodontal disease.

The mu-opioid receptor agonist morphine, but not agonists at delta- or kappa-opioid receptors, induces peripheral antinociception mediated by cannabinoid receptors.

BACKGROUND AND PURPOSE: Although participation of opioids in antinociception induced by cannabinoids has been documented, there is little information regarding the participation of cannabinoids in the antinociceptive mechanisms of opioids. The aim of the present study was to determine whether endocannabinoids could be involved in peripheral antinociception induced by activation of mu-, delta- and kappa-opioid receptors. EXPERIMENTAL APPROACH: Nociceptive thresholds to mechanical stimulation of rat paws treated with intraplantar prostaglandin E2 (PGE2, 2 microg) to induce hyperalgesia were measured 3 h after injection using an algesimetric apparatus. Opioid agonists morphine (200 microg), (+)-4-[(alphaR)-alpha-((2S,5R)-4-Allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80) (80 microg), bremazocine (50 microg); cannabinoid receptor antagonists N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) (20-80 microg), 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl(4-methoxyphenyl) methanone (AM630) (12.5-100 microg); and an inhibitor of methyl arachidonyl fluorophosphonate (MAFP) (1-4 microg) were also injected in the paw. KEY RESULTS: The CB1-selective cannabinoid receptor antagonist AM251 completely reversed the peripheral antinociception induced by morphine in a dose-dependent manner. In contrast, the CB2-selective cannabinoid receptor antagonist AM630 elicited partial antagonism of this effect. In addition, the administration of the fatty acid amide hydrolase inhibitor, MAFP, enhanced the antinociception induced by morphine. The cannabinoid receptor antagonists AM251 and AM630 did not modify the antinociceptive effect of SNC80 or bremazocine. The antagonists alone did not cause any hyperalgesic or antinociceptive effect. CONCLUSIONS AND IMPLICATIONS: Our results provide evidence for the involvement of endocannabinoids, in the peripheral antinociception induced by the mu-opioid receptor agonist morphine. The release of cannabinoids appears not to be involved in the peripheral antinociceptive effect induced by kappa- and delta-opioid receptor agonists.

Different mechanisms underlie the analgesic actions of paracetamol and dipyrone in a rat model of inflammatory pain.

BACKGROUND AND PURPOSE: The analgesics, paracetamol and dipyrone are weak inhibitors of the cyclooxygenase isoforms 1 or 2 (COX-1, COX-2) but more potent on COX-3. Both are also weak anti-inflammatory agents, relative to their analgesic and antipyretic activities. In a model of inflammatory pain mediated by prostaglandins, both compounds were analgesic. We have analysed this shared effect further in order to elucidate the underlying mechanisms. EXPERIMENTAL APPROACH: Inflammation was induced in one hind paw of rats by intraplantar injection of 250 microg lambda-carrageenan (CG) and the contralateral paw injected with saline. Nociceptive thresholds to mechanical stimulation were measured immediately before and for 6 h after, injection of CG. The analgesics were s.c. or locally (intraplantar) injected either 30 min before or 2 h after CG. In some groups, naltrexone was injected (s.c. or intraplantar), 1 h before CG. KEY RESULTS: Pretreatment with paracetamol or dipyrone (60-360 mg kg(-1)) reversed hyperalgesia induced by CG and increased nociceptive threshold in the inflamed paw above the basal level (hypoalgesia). Paracetamol, but not dipyrone, also raised nociceptive thresholds in the non-inflamed paw. Subcutaneous, but not local, administration of naltrexone, a specific opioid antagonist, reversed the hypoalgesia induced by paracetamol, but similar naltrexone treatment had no effect on dipyrone-induced analgesia. CONCLUSIONS AND IMPLICATIONS: Although both paracetamol and dipyrone are inhibitors of COX isoforms and thus of prostaglandin biosynthesis and were analgesic in our model, their analgesic actions were functionally and mechanistically different. Satisfactory mechanisms of action for these analgesics still remain to be established.

Crucial role of peripheral kappa-opioid receptors in a model of periodontal disease in rats.

BACKGROUND AND OBJECTIVE: Periodontal disease is a chronic inflammatory condition of the tooth supporting tissues, the periodontium. Opioids have been shown to account for the relief of various chronic and acute inflammatory conditions. The aim of the present study was to investigate the participation of peripheral opioid receptors in development of periodontal disease. MATERIAL AND METHODS: Morphine and selective agonists and antagonists of opioid receptors were used in an experimental model of ligature-induced periodontal disease in rats. To evaluate the development of disease, the loss of fiber attachment, alveolar bone and number of cells in periodontal tissues were assessed. Measurements of these indicators were obtained by morphometric analysis of histological sections of periodontal-diseased tissues stained with hematoxylin and eosin. RESULTS: Local administration of either morphine or a selective kappa-opioid agonist for three consecutive days from the onset of periodontal disease reduced the loss of periodontal tissues, without changing the number of leukocytes in inflamed periodontium. Nor-binaltorphimine, a selective kappa-antagonist, reversed the beneficial effects of both morphine and the compound U-50,488 in this model. The use of either an agonist or an antagonist of delta-opioid receptors, however, did not affect disease progression. CONCLUSION: Our results showed that the beneficial effect of opioids in periodontal disease depended mainly on the activation of specific kappa-opioid receptors located in the periphery. Activation of such receptors could be considered in the management of periodontal disease, since it would not present the classical central side-effects associated with opioid use.

Participation of ATP-sensitive K+ channels in the peripheral antinociceptive effect of fentanyl in rats.

We examined the effect of several K+ channel blockers such as glibenclamide, tolbutamide, charybdotoxin (ChTX), apamin, tetraethylammonium chloride (TEA), 4-aminopyridine (4-AP), and cesium on the ability of fentanyl, a clinically used selective micro-opioid receptor agonist, to promote peripheral antinociception. Antinociception was measured by the paw pressure test in male Wistar rats weighing 180-250 g (N = 5 animals per group). Carrageenan (250 microg/paw) decreased the threshold of responsiveness to noxious pressure (delta = 188.1 +/- 5.3 g). This mechanical hyperalgesia was reduced by fentanyl (0.5, 1.5 and 3 microg/paw) in a peripherally mediated and dose-dependent fashion (17.3, 45.3 and 62.6%, respectively). The selective blockers of ATP-sensitive K+ channels glibenclamide (40, 80 and 160 microg/paw) and tolbutamide (80, 160 and 240 microg/paw) dose dependently antagonized the antinociception induced by fentanyl (1.5 microg/paw). In contrast, the effect of fentanyl was unaffected by the large conductance Ca2+-activated K+ channel blocker ChTX (2 microg/paw), the small conductance Ca2+-activated K+ channel blocker apamin (10 microg/paw), or the non-specific K+ channel blocker TEA (150 microg/paw), 4-AP (50 microg/paw), and cesium (250 microg/paw). These results extend previously reported data on the peripheral analgesic effect of morphine and fentanyl, suggesting for the first time that the peripheral micro-opioid receptor-mediated antinociceptive effect of fentanyl depends on activation of ATP-sensitive, but not other, K+ channels.

Selective inhibitors of cyclo-oxygenase-2 (COX-2) induce hypoalgesia in a rat paw model of inflammation.

1. It is well-established that inhibitors of cyclo-oxygenase (COX) and hence of prostaglandin (PG) biosynthesis reverse inflammatory hyperalgesia and oedema in both human and animal models of inflammatory pain. 2. Paw oedema and hyperalgesia in rats were induced by injecting carrageenan (250 micro g paw(-1)) into a hindpaw. Both inflammatory responses were followed for 24 h after the injection, measuring hyperalgesia by decreased pain threshold in the paws and oedema by plethysmography. 3. Three selective inhibitors of cyclo-oxygenase-2 (COX-2), celecoxib, rofecoxib and SC 236, given systemically in a range of doses, before the inflammatory stimulus, abolished carrageenan-induced hyperalgesia with little reduction of oedema. These inhibitors also induced hypoalgesia, increasing nociceptive thresholds in the inflamed paw above normal, non-inflamed levels. This hypoalgesia was lost at the higher doses of the selective inhibitors, although hyperalgesia was still prevented. 4. In paws injected with saline only, celecoxib, given at the dose inducing the maximum hypoalgesia after carrageenan, did not alter the nociceptive thresholds. 5. Two non-selective inhibitors of COX-2, indomethacin and piroxicam, abolished hyperalgesia and reduced oedema but did not induce hypoalgesia. 6. Celecoxib given locally into the paw also abolished inflammatory hyperalgesia and induced hypoalgesia without reducing oedema. 7. We conclude that hypoalgesia is expressed only over a critical range of COX-2 inhibition and that concomitant inhibition of COX-1 prevents expression of hypoalgesia, although hyperalgesia is still prevented. 8 Our results suggest a novel anti-nociceptive pathway mediating hypoalgesia, involving COX-2 selectively and having a clear peripheral component. This peripheral component can be further explored for therapeutic purposes.

Midazolam-induced hyperalgesia in rats: modulation via GABA(A) receptors at supraspinal level.

The effect of benzodiazepines on the nociceptive threshold was studied in rats using the tail-flick and the formalin tests. Systemic injection of midazolam (10 mg/kg, i.p.) induced a significant decrease of the tail-flick latency and produced a long-lasting nociceptive effect in the formalin test, thus characterising a hyperalgesic state. The hyperalgesia induced by midazolam in the tail-flick test was blocked by flumazenil, a specific antagonist for benzodiazepine sites associated with GABA(A) receptors. Picrotoxin, a Cl- channel blocker, inhibited midazolam-induced hyperalgesia in both tests. Midazolam caused hyperalgesia when administered intracerebroventricularly (i.c.v.; 25 microg) but not intrathecally (i.t.; 75 microg). I.c.v. but not i.t. (5 microg) injection of flumazenil suppressed the hyperalgesia induced by midazolam (10 mg/kg, i.p.). Combination of non-hyperalgesic doses of diazepam (10 mg/kg, i.p.) or ethanol (0.48 g/kg, oral) with midazolam (5 mg/kg, i.p.) also induced hyperalgesia. Our results demonstrate that midazolam and diazepam alone or in combination with ethanol can produce hyperalgesia by interacting with GABA(A) receptors at the supraspinal level in rats. The risk of hyperalgesia should be taken in account when these drugs are used in combination in humans.

Anti-inflammatory and analgesic effects of the phosphodiesterase 4 inhibitor rolipram in a rat model of arthritis.

There has been much interest in strategies which modulate tumour necrosis factor-alpha (TNF-alpha) levels and/or function in rheumatoid arthritis. The elevation of intracellular levels of cyclic AMP in leukocytes by phosphodiesterase 4 inhibitors is accompanied by significant inhibition of the production of TNF-alpha. Nevertheless, these drugs may enhance the hyperalgesia induced by a range of inflammatory mediators, including TNF-alpha. In the present study, we examined the effects of the phosphodiesterase 4 inhibitor rolipram on the local inflammatory infiltrate and hyperalgesia in a rat model of adjuvant-induced arthritis. Rolipram (3 mg/kg) was administered by oral gavage from day 10 to 14 after disease induction. Pretreatment with rolipram abrogated oedema formation and significantly inhibited hyperalgesia. Histopathological analysis revealed a marked inhibition of cellular influx as well as bone and cartilage destruction. Serum and local TNF-alpha levels were suppressed in treated animals whereas there were little changes in interleukin-1beta levels. Although cyclic AMP elevating agents may affect nociceptor threshold to increase the hyperalgesic responses acutely, they also possess significant anti-inflammatory activity, which may hinder local mediator release and/or action. The anti-inflammatory effects of rolipram predominate during this chronic arthritis model in the rat.

Effect of tumor necrosis factor receptor binding protein on cell infiltration induced by lipopolysaccharide and Sephadex beads in guinea pig lung.

In the present study, the effect of a tumor necrosis factor receptor binding protein (TNFbp) on the cell infiltration induced by lipopolysaccharide (LPS) and Sephadex beads in guinea pig lung was examined. The intratracheal injection of LPS (2.5 micrograms) induced a six-fold increase in total cell number recovered in bronchoalveolar lavage (BAL) fluid at 24 hr. This increase in bronchopulmonary inflammation was mainly due to a neutrophil and macrophage infiltration, representing 60% and 35% of the total cells, respectively. The intravenous or intratracheal injection of Sephadex beads to guinea pigs induced a three-fold increase in total cell number recovered in BAL at 24 h and was characterized by a prominent eosinophil, macrophage, and neutrophil infiltration representing 36%, 42%, and 16% of the total cells, respectively. In addition, bronchial tissues isolated from Sephadex-treated guinea pigs showed an increased in vitro reactivity to both histamine and acetylcholine. TNFbp (1-50 micrograms) induced a dose-dependent inhibition of cell infiltration induced by LPS. In contrast TNFbp neither attenuated the bronchopulmonary cell infiltration observed 24 h following intravenous or intratracheal administration of Sephadex beads nor inhibited the increase in bronchial reactivity. These results show that TNF plays an important role in cell infiltration induced by LPS, but not that induced by Sephadex, in the guinea pig lung.

Analgesic and antiinflammatory effects of dipyrone in rat adjuvant arthritis model.

Dipyrone, a pirazolone derivative, is a known analgesic drug with minor toxic effects associated with its administration. The aim of the present study was to determine the analgesic and antiinflammatory effects of dipyrone in a model of chronic inflammation (adjuvant-induced arthritis in rats). Hind-paw hyperalgesia was detected in arthritic rats from the 10th to the 16th day of observation. Edema development was maximum (twofold increase) at the 14th day of observation compared to control animals and reduced at the 16th day of observation. Dipyrone (1-50 mg/kg) dose-dependently reduced both hind-paw hyperalgesia and edema from arthritic rats. However, it was shown to be more potent as analgesic than antiinflammatory in the present model. In contrast, indomethacin (2 mg/kg) and dexamethasone (0.4 mg/kg) completely inhibited hind-paw hyperalgesia and edema development. Our results indicate that dipyrone reduced the hyperalgesia and edema in arthritic rats by a mechanism not involving release of prostaglandin-like substances. The possibility of dipyrone inducing analgesia in arthritic rats through a peripheral action supports the use of dipyrone as an alternative choice drug for the treatment of pain associated with arthritislike diseases in selected cases.

Interleukin-1 mimics the hyperalgesia induced by a factor obtained by macrophage lysis.

1. Lysis of rat thioglycolate-stimulated peritoneal macrophages releases a low molecular weight factor (0.5 less than MW less than 10 kD) into the supernatant. Bilateral hyperalgesia was observed when this factor, denoted macrophage hyperalgesic factor (MHF), was injected into one hind paw or into the peritoneal cavity of the rat. 2. Similar activity was detected in stimulated peritoneal and tumoral mouse macrophages (J774G8) but not in lysates of rat exudate neutrophils or in peritoneal resident (non-stimulated) macrophages. 3. The hyperalgesia induced by MHF was abolished by local intraplantar injection of indomethacin, thus suggesting a peripheral release of cyclo-oxygenase products. This suggestion was supported by the ability of MHF to release prostaglandin-like material when added to a guinea pig lung perfusate. 4. Peritonitis induced by the administration of carrageenin caused concomitant bilateral rat paw hyperalgesia and an MHF-like activity was demonstrable in peritoneal exudate 30 min after the carrageenin insult. 5. Purified human interleukin-1 (IL-1) given locally or systemically also produced bilateral hind paw hyperalgesia which was abolished by local administration of indomethacin. The possibility that MHF may be a fragment of IL-1 is discussed.

Reduction of Sephadex-induced lung inflammation and bronchial hyperreactivity by rapamycin.

Rapamycin is a macrolide antibiotic whose potent immunosuppressor activity was recently described in vivo and in vitro. The aim of the present work was to determine if rapamycin could affect an established inflammatory response. Conscious pathogen-free Dunkin-Hartley guinea pigs (300-400 g) were injected intravenously with Sephadex beads (G50, superfine, 10 to 40 microns, 24 mg/kg) to induce lung inflammation and bronchial hyperreactivity. Bronchoalveolar lavage (BAL) fluid was collected 2, 12 and 24 h after Sephadex administration and the cells were counted. Bronchial tissue was used to construct dose-response (contraction, g) curves to histamine and acetylcholine 24 h after the Sephadex injection, using a cascade system. Results are presented as area under the log dose-response curves. Test animals were injected with rapamycin (5 mg/kg) or its vehicle by the intramuscular route either 2 or 12 h after Sephadex injection and BAL fluid collected 24 h after Sephadex administration. Rapamycin administration 2 h after Sephadex reduced eosinophil and lymphocyte numbers in BAL by 52 and 55%, respectively, but not ex vivo bronchial hyperreactivity induced by Sephadex injection. However, rapamycin administration 12 h after Sephadex reduced BAL eosinophil and lymphocyte numbers (55 and 62%, respectively) and bronchial hyperreactivity. The increase in neutrophil numbers in BAL induced by Sephadex injection was not modified by rapamycin. Since lymphocyte numbers in BAL were significantly increased in Sephadex-treated animals at 12 h but not at 2 h after Sephadex injection, the present results suggest that the inhibition of bronchial hyperreactivity by rapamycin may be dependent on the presence of lymphocytes elicited into the airways by Sephadex injection.

Cyclosporin inhibits hyperalgesia and edema in arthritic rats: role of the central nervous system.

Since arthritis induced by Mycobacterium products (adjuvant) in rats is considered to be immunologically driven, the objective of the present study was to determine if the immunosuppressor drug cyclosporin could affect hindpaw edema and joint hyperalgesia simultaneously. Female Holtzman rats (140-170 g) presented hyperalgesia and edema on the 8th and 12th day following adjuvant injection. Daily systemic (oral or intramuscular) administration of cyclosporin (0.5-5.0 mg kg (-1) day (-1)) or dexamethasone (0.01-0.1 mg kg (-1) day (-1)) for 15 days starting on day zero dose-dependently inhibited the hindpaw edema and hyperalgesia in arthritic rats. However, hyperalgesia but not edema could be detected two days after cyclosporin withdrawal. We concluded that a) the continuous presence of cyclosporin is essential to reduce the development of joint hyperalgesia and that b) different mechanisms underlie the appearance of hyperalgesia and edema in this model. The intracerebroventricular (i.c.v.) administration of 5-50-fold smaller doses of cyclosporin (1.5-150 micrograms/day) or dexamethasone (15 micrograms/day) also reduced the arthritic hindpaw edema and hyperalgesia. Peripheral blood from animals injected with effective systemic cyclosporin doses showed detectable levels of the drug, whereas peripheral blood from those injected with i.c.v. cyclosporin did not, as measured by specific RIA. Our results indicate that cyclosporin administered by the central route is as effective as by the systemic route to reduce joint hyperalgesia and hindpaw edema in arthritic rats. The antiarthritic effect induced by low doses of cyclosporin in the central nervous system (CNS) could be explored to avoid it often associated systemic side effects during chronic therapy. However, the mechanism(s) involved in the antiarthritic response to cyclosporin in the CNS remain to be elucidated.

Acute phenobarbital administration induces hyperalgesia: pharmacological evidence for the involvement of supraspinal GABA-A receptors.

The aim of the present study was to determine if phenobarbital affects the nociception threshold. Systemic (1-20 mg/kg) phenobarbital administration dose dependently induced hyperalgesia in the tail-flick, hot-plate and formalin tests in rats and in the abdominal constriction test in mice. Formalin and abdominal constriction tests were the most sensitive procedures for the detection of hyperalgesia in response to phenobarbital compared with the tail-flick and hot-plate tests. The hyperalgesia induced by systemic phenobarbital was blocked by previous administration of 1 mg/kg ip picrotoxin or either 1-2 mg/kg sc or 10 ng icv bicuculline. Intracerebroventricular phenobarbital administration (5 microg) induced hyperalgesia in the tail-flick test. In contrast, intrathecal phenobarbital administration (5 microg) induced antinociception and blocked systemic-induced hyperalgesia in this test. We suggest that phenobarbital may mediate hyperalgesia through GABA-A receptors at supraspinal levels and antinociception through the same kind of receptors at spinal levels.

Role of endogenous glucocorticoids in hyperalgesia and edema in old arthritic rats.

We compared the intensity and frequency of arthritis in old (8-12 months, N = 12) and juvenile (2 months, N = 10) rats and determined the role played by adrenal glands in this disorder. Arthritis was induced by subcutaneous injection of Mycobacterium butyricum at the base of the tail of female Holtzman rats at day zero. Paw edema and hyperalgesia were monitored from day zero to day 21 after induction as signs of arthritis development. Some (N = 11) old animals were adrenalectomized bilaterally and treated with dexamethasone or celecoxib immediately following surgery. All bilaterally adrenalectomized old animals became susceptible to arthritis and the onset of disease was shortened from the 10th to the 5th day. Hyperalgesia and paw edema responses were less frequent in older animals (50 and 25% compared to control juvenile rats, respectively), although old responder animals showed responses of similar intensity to those of their juvenile counterparts: by the 14th day the data for hyperalgesia were juvenile = 0.8 +/- 0.07/old = 0.8 +/- 0.09, and for paw edema juvenile = 56.6 +/- 6.04/old = 32.24 +/- 12.7, reported as delta% increase in paw edema. Chronic treatment of adrenalectomized old animals with dexamethasone (0.01 or 0.1 mg/kg) but not celecoxib (3 mg/kg), once daily for 21 days by gavage, abolished the effects of adrenalectomy, in particular those related to the hyperalgesia response (old = 0.95 +/- 0.03/dexamethasone = 0 +/- 0; 14th day), thus suggesting a specific participation of circulating corticosteroids in the modulation of pain in old arthritic rats.

Hyperalgesic effect induced by barbiturates, midazolam and ethanol: pharmacological evidence for GABA-A receptor involvement.

The involvement of GABA-A receptors in the control of nociception was studied using the tail-flick test in rats. Non-hypnotic doses of the barbiturates phenobarbital (5-50 mg/kg), pentobarbital (17-33 mg/kg), and thiopental (7.5-30 mg/kg), of the benzodiazepine midazolam (10 mg/kg) or of ethanol (0.4-1.6 g/kg) administered by the systemic route reduced the latency for the tail-flick response, thus inducing a 'hyperalgesic' state in the animals. In contrast, non-convulsant doses of the GABA-A antagonist picrotoxin (0.12-1.0 mg/kg) administered systemically induced an increase in the latency for the tail-flick response, therefore characterizing an 'antinociceptive' state. Previous picrotoxin (0.12 mg/kg) treatment abolished the hyperalgesic state induced by effective doses of the barbiturates, midazolam or ethanol. Since phenobarbital, midazolam and ethanol reproduced the described hyperalgesic effect of GABA-A-specific agonists (muscimol, THIP), which is specifically antagonized by the GABA-A antagonist picrotoxin, our results suggest that GABA-A receptors are tonically involved in the modulation of nociception in the rat central nervous system.

Opioidergic orofacial antinociception induced by electroacupuncture at acupoint St36.

The participation of opioids in the antinociceptive effect of electroacupuncture was evaluated in terms of nociception produced by thermal stimuli applied to the face of male Wistar rats, weighing 180-230 g. Electrical stimulation (bipolar and asymmetric square wave with 0.5 mA intensity for 20 min) of acupoint St36, located in the anterior tibial muscle 10 mm distal to the knee joint, induced antinociception in the present model, which was maintained for 150 min. Acupoint LI4, located in the junction of the first and second metacarpal bones, did not achieve antinociception at any frequency studied (5 Hz: 1.7 +/- 0.1; 30 Hz: 1.8 +/- 0.1; 100 Hz: 1.7 +/- 0.1 vs 1.4 +/- 0.2). The antinociception obtained by stimulation of acupoint St36 was only achieved when high frequency 100 Hz (3.0 +/- 0.2 vs 1.0 +/- 0.1) was used, and not with 5 or 30 Hz (1.2 +/- 0.2 and 0.7 +/- 0.1 vs 1.0 +/- 0.1). The antinociceptive effect of acupuncture occurred by opioid pathway activation, since naloxone (1 and 2 mg/kg, subcutaneously) antagonized it (1.8 +/- 0.2 and 1.7 +/- 0.2 vs 3.0 +/- 0.1).