Non-inflamed Paw Research Articles

Comparison of Tebufelone Distribution in Rat Blood Plasma and Inflamed and Noninflamed Tissues following Peroral and Intravenous Administration

AbstractIt is recognized that some acidic nonsteroidal antiinflammatory drugs (NSAIDs) accumulate in the synovial fluids of inflamed joints. The distribution of tebufelone, a member of the di-fert-butylphenol class of NSAIDs, between rat plasma and paw tissues (inflamed and noninflamed) was determined. Tebufelone was administered (doses providing 80% maximal effectiveness) perorally (po, 10 mg/kg) or intravenously (iv, 0.5 mg/kg) to Sprague-Dawley rats just prior to injection of an inflammatory adjuvant (carrageenan) into one of the hind paws. Levels of the drug were measured in plasma, inflamed paws, and noninflamed paws at prescribed times postdosing by capillary gas chromatography/stable isotope dilution mass spectrometry. Tebufelone levels, as determined from areas under the curves of concentration versus time were sevenfold (po) and 11-fold (iv) higher in paw tissue than in plasma. Inflamed and noninflamed tissues have equal affinity for the drug. Calculated terminalphase half-lives of tebufelone in paw tissue were similar following iv and po dosing (~14 h) and equivalent to the plasma half-life of the drug after po dosing (~10 h). The plasma half-life determined following iv dosing was considerably lower (2.8 h). The anti-inflammatory activity of tebufelone appears to correlate more closely with tissue distribution profiles than plasma drug levels at the dose levels examined.

Inflammation of the rat paw enhances axonal transport of opioid receptors in the sciatic nerve and increases their density in the inflamed tissue

The effect of inflammation, induced by unilateral intraplantar injection of Freund's adjuvant, on opioid receptors transported in the sciatic nerve and on opioid receptors present in the paw of the rat was studied by means of in vitro receptor autoradiography using [ 125I]β-endorphin (human) as ligand. In the absence of inflammation, human β-endorphin binding sites accumulated proximally and distally to a ligature placed on the sciatic nerve in a time-dependent manner, indicating bidirectional axonal transport. Some human β-endorphin binding was also visible in non-inflamed paw tissue. Inflammation of the paw tissue massively increased human β-endorphin binding on both sides of the sciatic nerve ligature and in the ipsilateral paw tissue. In inflamed paw tissue, β-endorphin binding accumulated in the cutaneous nerve fibers as well as in the immune cells infiltrating the surrounding tissue. In the sciatic nerve and paw tissue, β-endorphin binding was displaced by ( d-Ala 2, N-methyl-Phe 4,Gly-ol 5)enkephalin and ( d-Pen 2, d-Pen 5)enkephalin, selective μ- and δ-opioid receptor agonists, respectively, and by the universal opioid antagonist naloxone, but not by U-50,488H, a k-selective receptor agonist. Taken together, these data provide neuroanatomical evidence for local inflammation-induced enhanced axonal transport of opioid receptors in rat sciatic nerve and accumulation in paw tissue.

Further evidence for the involvement of SmI cortical neurons in nociception: their responsiveness at 24 hr after carrageenin-induced hyperalgesic inflammation in the rat.

In this electrophysiological study, the responsiveness of neurons in the primary somatosensory cortex (SmI) was analyzed in rats with carrageenin-induced hyperalgesia for 24 hr. The functional implication of some changes in neuronal activity was improved in a few cases by a pharmacological test with Xylocaine injection in or close to the neuronal receptive field (RF), or with systemic aspirin. Unit recordings were performed alternately in the SmI cortex contralateral (Cc) or ipsilateral (Ci) to the inflamed hindpaw. In 29 rats with hyperalgesia tested prior to the recording session, 218 cells (128 in the Cc, 90 in the Ci) were tested with mechanical stimuli. In each SmI cortex, about 50% of them were driven by the stimulus. The "nonresponsive" neurons exhibited a higher firing rate in the Cc than in the Ci. The "responsive" (i.e., the somatosensory) neurons were classified according to their response to light touch, pinch, or joint movement. There was a highly significant difference between the two cortices, essentially because of the high proportion of "joint" Cc neurons (27 of 73 [37%] of the somatosensory neurons in the Cc, vs. only 8 of 47 [17%] in the Ci). "Light touch" neurons (41 of 73 [56%] in the Cc, vs. 35 of 47 [74.5%] in the Ci) had small RFs contralateral to the recording site. Of the 41 Cc cells of this type, 23 did not exhibit the classical characteristics of "light touch" cells; in particular, they exhibited striking discharges triggered by the stimulus but outlasting the stimulus duration, or occurring without intentional stimulation. These abnormal discharges were depressed or suppressed by injection of a local anesthetic (Xylocaine) in or close to the neuronal RF. "Pinch" neurons were very rare (5 of 73 [7%] in the Cc, vs. 4 of 47 [8.5%] in the Ci). Responses elicited from the inflamed paw were more pronounced than those from the noninflamed paw. "Joint" neurons were more numerous in the Cc than in the Ci. In addition, their responses obtained from contralateral RFs, and therefore from the inflamed paw, were more sustained than Ci responses elicited from the noninflamed paw. Afterdischarges of Ce neuronal responses and spontaneous paroxysmal activity were common on this side and were depressed by local anesthetic (Xylocaine) in their RFs or by systemic aspirin. These electrophysiological data emphasize the implication of SmI cortex in inflammatory hyperalgesia and more generally in pain processing.(ABSTRACT TRUNCATED AT 400 WORDS)

GR94839, a kappa-opioid agonist with limited access to the central nervous system, has antinociceptive activity.

1. The pharmacological profile of GR94839, a kappa-opioid agonist with limited access to the central nervous system, has been investigated. Its antinociceptive activity has been compared with that of GR103545, a centrally-penetrating kappa-agonist and ICI204448, the previously described peripherally-selective kappa-agonist. 2. GR94839 was a potent agonist in the rabbit vas deferens in vitro assay for kappa-opioid receptors (IC50: 1.4 +/- 0.3 nM; n = 6), but had limited activity at mu- or delta-opioid receptors. 3. In the mouse abdominal constriction test, GR94839 was 238 fold more potent when given i.c.v. (ED50: 0.008 (0.004-0.029) mg kg-1; n = 18) than when s.c. (ED50: 1.9 (0.7-3.1) mg kg-1; n = 30). In comparison, GR103545 was equipotent when given i.c.v. or s.c. 4. After intravenous administration, the maximum plasma to brain concentration-ratio attained by GR94839 was 18 compared with 2 for GR85571, a structurally-related kappa-agonist that is centrally-penetrating. 5. GR94839 inhibited the 2nd phase of the rat formalin response at doses 7 fold lower than those required to inhibit the 1st phase (ED50 vs 1st phase: 10.2 (6.7-17.1) mg kg-1, s.c.; ED50 vs 2nd phase: 1.4 (1.0-1.8) mg kg-1, s.c.; n = 18). GR103545 was equipotent against the two phases. 6. Intraplantar administration of the opioid antagonists, norbinaltorphimine (100 micrograms) or naltrexone (1 microgram), reversed the antinociceptive effect of systemic GR94839 (3 mg kg-1, s.c.) against the 2nd phase of the formalin response and intraplantar injection of GR94839 (30-100 micrograms) selectively inhibited the 2nd phase.7. GR94839 and IC1204448 reversed the hyperalgesia in the zymosan-inflamed rat paw at doses (ED50 GR94839: 2.0 (1.1-3.2) mg kg-', s.c.; ED50 IC1204448: 1.2 (0.8-1.7) mg kg-', s.c.), lower than those required to raise the noxious pressure threshold in the non-inflamed paw (EDSO GR94839: 16.4 (8.6-46.7) mg kg', s.c.; ED50 IC1204448: 68.0 (22.1-32000) mg kg', s.c.). GR103545 raised the noxious presure threshold in the inflamed and non-inflamed paws at the same doses.8. GR94839 was sedative in the rat rotarod test (ED50: 35 (12-245) mg kg-', s.c.) at doses higher than those required to inhibit the 2nd phase of the formalin response or reverse hyperalgesia in the zymosan-inflamed rat paw. The doses were comparable to those that inhibited the 1st phase of the formalin response and raised the noxious pressure threshold in the non-inflamed paw.9. The results suggest that GR94839 is a selective kappa-agonist which has antinociceptive activity against inflammatory pain at doses that produce limited central effects. These antinociceptive effects are probably mediated at peripheral opioid receptors.

Increased content and transport of substance P and calcitonin gene-related peptide in sensory nerves innervating inflamed tissue: Evidence for a regulatory function of nerve growth factor in vivo

The responses of sensory neuropeptides during unilateral, Freund's adjuvant-induced, paw inflammation in the rat were examined. After five days of inflammation, the substance P and calcitonin gene-related peptide content in the sciatic nerve supplying the inflamed paw were increased by 60–75% when compared with the contralateral side. At this time-point, there was also a 30–40% increase in the substance P and calcitonin gene-related peptide content of the dorsal root ganglia (L 4–L 6), and a 40% increase in the calcitonin gene-related peptide content of the L 4–L 6 segments of the dorsal spinal cord on the inflammation side. In the dorsal root ganglia, calcitonin gene-related peptide content was also increased as early as 12 h and 48 h after induction of paw inflammation. On day 5 of inflammation, the axonal transport of both sensory neuropeptides towards the inflamed paw, as determined after sciatic nerve ligation, was also markedly increased as compared with the control side. Despite this increased transport, the amount of substance P and calcitonin gene-related peptide present in the inflamed paw itself was either reduced or remained unchanged from day 1 through to day 5 of inflammation pointing towards reduced storage and increased release of the peptides in the inflamed tissue. Nerve growth factor content was markedly increased in the sciatic nerve of the inflamed paw with a peak of +136% at time-point 24 h after induction of inflammation. When rats were systemically treated with anti-nerve growth factor serum, the increase in neuropeptide content in the sciatic nerve of the inflamed paw (day 5) was prevented. On the other hand, local injections of nerve growth factor for 5 days into a noninflamed paw were able to induce an increase in substance P and calcitonin gene-related peptide content in the supplying sciatic nerve. These findings point towards a regulatory function for nerve growth factor in vivo in the stimulation of sensory neuropeptide synthesis during prolonged inflammatory processes.

Antinociceptive activity of eel calcitonin, injected into the inflamed paw in rats

This study examined the possible peripheral activity of eel calcitonin in the modulation of the response to noxious pressure on inflamed paws in rats (Randall and Selitto test). The intraplantar injection of eel calcitonin (20–200 ng/rat) but not the subcutaneous administration (200 ng and 2 μg/rat, s.c.), was able to significantly inhibit hyperalgesia induced by intraplantar injection of carrageenin. The development of oedema on the other hand was not inhibited. The intraplantar administration of eel calcitonin (200 ng/rat) in a non-inflamed paw did not modify paw pressure thresholds. Eel calcitonin (200 ng/rat, intraplantar, i.pl.) was also able to elicit an antinociceptive effect on formalin-induced hyperalgesia, both when the peptide was injected before or after (60 min) formalin. This effect, at difference with morphine (80 μg/rat, i.pl.), was not blocked by naloxone (10 μg/rat, i.pl.). These results demonstrate the local antinociceptive effect of eel calcitonin in inflammatory pain and might indicate a new way of using calcitonin in the control of pain.

Physiological relevance and time course of a tonic endogenous opioid modulation of nociceptive messages, based on the effects of naloxone in a rat model of localized hyperalgesic inflammation

In a rat model of localized hyperalgesic inflammation induced by intraplantar injection of carrageenin, the effect of a relatively high dose of naloxone (1 mg/kg i.v.) was investigated using the measure of the vocalization threshold as a nociceptive test, on both the inflamed and non-inflamed paws. The effects of the drug were determined at two different periods after the intraplantar injection of carrageenin, in the same group of rats. We showed that 4 h after carrageenin (a few hours after the onset of the inflammatory process), naloxone induced a significant further decrease in the vocalization threshold induced by pressure on either paw, suggesting that naloxone had reduced a tonically active inhibitory system involving endogenous opioid peptides. Twenty-four hours after carrageenin, a consistent hyperalgesic effect of naloxone was observable only in rats which had recovered from their carrageenin-induced hyperalgesia. A significant negative correlation between the behavioral effect of naloxone and the degree of hyperalgesia determined for each animal was observed. This suggests that the tonic inhibition exerted by the endogenous opioids was particularly effective in rats which recovered from their initial hyperalgesia. By contrast, these opioid controls could have been weaker in those rats which remained hyperalgesic.

Behavioural evidence for a peripheral component in the enhanced antinociceptive effect of a low dose of systemic morphine in carrageenin-induced hyperalgesic rats

This study reinvestigated the possible contribution of a peripheral action of systemic morphine in the modulation of the responce to noxious pressure on inflamed paws, using a supraspinally integrated test and various low doses of naloxone. Rats received an injection of carrageenin into the right hindpaw which resulted in an ipsilateral inflammatory responce and decreased threshold to noxious pressure. Four hours post-carrageenin, the injection of 1 mg/kg i.v. morphine induced a significantly enhanced antinociceotive effect on the inflamed compared to the non-inflamed paws. Intrapantar injection to extremely low doses of naloxone (0.5 and 1 μh in a volume of 0.1 ml) signifantly reduced this effect (naloxone being more effective when administered at the same time as morphine, compared to 15 min later), while equal doses of naloxone given systemically were inactive. These data confirm that synergism of peripheral and central actions may result in the augmented analgesic potency of morphine in rats subjected to inflammatory conditions. In addition, they provide further evidence for the complexity of opioid actions in inflammatory processes. In particular, the results are in line with the hypothesis that the paradoxical antinoceptive effect of extremely low doses of i.v. naloxone described in several studies is due to a central action.

Periaqueductal gray stimulation produces a spinally mediated, opioid antinociception for the inflamed hindpaw of the rat

The objective of the present study was to characterize stimulation-produced antinociception from the periaqueductal gray matter (PAG) in rats with unilateral hindlimb inflammation induced by an intraplantar injection of Freund's complete adjuvant. Rats were chronically implanted with a bipolar stimulating electrode in the PAG. Nociception was assessed using a paw pressure test. Prior to inflammation, PAG stimulation significantly increased paw pressure threshold in both paws compared to non-stimulated controls. Following inflammation, PAG stimulation inhibited nociception in the inflamed, but not the non-inflamed paw. Systemic administration of naloxone blocked antinociception from ventral, but not dorsal PAG stimulation sites. Intrathecal, but not subcutaneous, administration of quaternary naltrexone completely blocked stimulation-produced antinociception from the PAG. The known increased levels of endogenous opioids occurring in the spinal cord ipsilateral to the site of inflammation suggest a mechanism for the selective antinociceptive effect of ventral PAG stimulation seen for the inflamed paw.

Spinal opioid analgesic effects are enhanced in a model of unilateral inflammation/hyperalgesia: possible involvement of noradrenergic mechanisms

We have examined the spinal analgesic activity of opioid agonists and antagonists in a model of short term, unilateral, carrageenan-induced inflammation/hyperalgesia. Rats received a single s.c. injection of carrageenan (2–6 mg in saline) 3–24 h prior to testing hindpaw withdrawal latencies to noxious thermal stimuli. Dose-response curves for intrathecally administered agonists with μ- and/or δ-opioid activity were shifted to the left for inflamed hindpaws when compared to contralateral non-inflamed paws. The selective κ-receptor agonist U-50,488H had no activity in this analgesic assay on either inflamed or non-inflamed paws when administered intrathecally. However, systemic administration of U-50,488H did produce significant elevations of paw withdrawal latencies in inflamed paws. The α 2-adrenoceptor agonist clonidine also produced dose-dependent antinociception in the paw withdrawal assay after systemic or intrathecal administration. Inflamed hindpaws were significantly more sensitive to the antinociceptive effects of clonidine than were non-inflamed paws. The antinociceptive effect of morphine on inflamed hindpaws was blocked by the opioid antagonist naloxone or the α 2-adrenoceptor antagonist idazoxan. The effect of clonidine was only blocked by idazoxan. Antagonists alone had no significant effect on withdrawal latencies. The data indicate that the analgesic action of opioids during conditions of inflammation may depend on an interaction with spinal noradrenergic pathways.

Intrathecal injections of galanin and its antiserum affect nociceptive response of rat to mechanical, but not thermal, stimuli

To ascertain the involvement of galanin in nociceptive transmission in the spinal dorsal horn, we examined the effects of intrathecal injections of porcine galanin and its antiserum on behavioral nociceptive responses of the rat, using the paw-pressure and paw-radiant heat tests. An intrathecal injection of antiserum against porcine galanin reversed the decreased nociceptive threshold for mechanical, but not thermal, stimulation of the carrageenintreated hind paw of rat, with no effect on the nociceptive threshold of the non-inflamed hind paw. Rat galanin as well as porcine galanin suppressed the binding of [ 125I]porcine galanin to the antiserum, a finding suggesting that the antiserum can bind rat galanin. An intrathecal injection of porcine galanin (0.1 and 1 nmol, but not 0.01 nmol) decreased the mechanical nociceptive threshold of the rat hind paw with no effect on thermal nociception. These results suggest that galanin present in the dorsal horn is involved in the facilitation of mechanical, but not thermal, nociceptive transmission.

Involvement of capsaicin-sensitive neurones in hyperalgesia and enhanced opioid antinociception in inflammation

The effects of capsaicin pretreatment of adult rats was investigated on consequences of unilateral paw inflammation induced by inoculation with Freund's adjuvant. Decrease in mechanical nociceptive threshold in the inflamed paw, as measured by the paw pressure test, was dose-dependently inhibited by capsaicin (20-150 mg/kg s.c.). In control rats, the antinociceptive action of morphine (0.8-1.9 mg/kg s.c.) was greater in the inflamed than in the non-inflamed paw; this difference was absent in capsaicin-treated animals. Increased volume or skin temperature of the inflamed paw was not influenced by capsaicin. It is concluded that capsaicin-sensitive, presumably C-fibre neurones, but not an alteration of the inflammation itself by capsaicin, mediate hyperalgesia and increased morphine antinociception in the rat paw with adjuvant-induced inflammation.

Peripheral opioid receptors mediating antinociception in inflammation. Activation by endogenous opioids and role of the pituitary-adrenal axis

This study investigated the involvement of endogenous opioid peptides in mediating cold water swim (CWS) stress-induced antinociception (SIA) in rats with unilateral hind paw inflammation induced by Freund's complete adjuvant (FCA). Following 0.5, 1 and 2 min of CWS, there was a duration-dependent elevation of paw pressure threshold (PPT) in both inflamed and non-inflamed paws which was maximal immediately after CWS and returned to control values within 15 min. The antinociception elicited in the inflamed paw was significantly greater than that elicited in the non-inflamed paw. The antinociception induced by a 1 min CWS was dose dependently antagonized by tertiary naloxone (0.125–1 mg/kg s.c.) and completely reversed by tertiary naltrexone (0.5 mg/kg). Quaternary naltrexone (5–40 mg/kg s.c.) was similarly effective in reversing the elevation of inflamed PPT induced by a 1 min CWS stress. In contrast, similar doses of quaternary naltrexone had no effect against centrally mediated morphine antinociception in non-inoculated rats. Adrenalectomy was without effect on the pattern of SIA seen in FCA-treated rats. Surgical hypophysectomy completely abolished the differential antinociception induced by 0.5 and l min durations of CWS but had little effect on that following 2 min of CWS stress. Inhibition of hypophysial corticotrophic cell secretion with dexamethasone (300 μg/kg) injected s.c. 120 min prior to CWS completely abolished the differential SIA at all durations of CWS tested. β-Endorphin 12.5 μg/kg administered i.v. in non-stressed rats also caused a greater elevation of PPT in inflamed than in non-inflamed paws. This effect was not reversed by concomitant i.v. administration of (−) tertiary naloxone 5 mg/kg or quaternary naltrexone 20 mg/kg.

Intrinsic mechanisms of antinociception in inflammation: local opioid receptors and beta-endorphin

This study examined antinociception induced through the activation of local opioid receptors in inflammation by endogenous opioids. Rats developed a unilateral localized inflammation upon injection of Freund's adjuvant into one hindpaw. Four to 6 d later they were subjected to cold water swim (CWS), an environmental stimulus known to activate intrinsic opioid systems. Following CWS (1 min) the animals' withdrawal threshold to noxious pressure applied onto the paws increased significantly more on the inflamed paw than on the noninflamed paw. This unilateral antinociceptive effect in inflamed paws was dose-dependently and stereospecifically reversible by intraplantar (i.pl.) but not systemic (i.v. or s.c.) administration of the opioid antagonist naloxone (18 micrograms). This suggested that CWS-induced antinociception in inflamed tissue was brought about by the activation of local opioid receptors. Antiinflammatory or vasoconstrictive events, as measured by paw volume and temperature, did not contribute to this unilateral antinociception. Receptor-selective antagonists indicated the involvement of mu- and delta- but not kappa-receptors. Intravenous application of a universal antibody to endogenous opioid peptides (3-E7) and a specific antibody to beta-endorphin, but not antisera against metenkephalin or dynorphin, abolished the CWS effect. Finally, the i.pl. injection of synthetic beta-endorphin (1-31) produced an antinociceptive effect in inflamed paws which was reversible by i.pl. naloxone and selective mu- and delta-receptor antagonists. These findings suggest that antinociception in inflamed tissue can be induced through the activation of local opioid receptors by endogenous beta-endorphin released during CWS.

Bilateral changes of nociceptive sensitivity in rats with one paw inflamed

Following a significant decrease in pain threshold (PT) in the carrageenin injected paw of rat, the PT of the contralateral uninjected paw was usually increased but might also be significantly decreased or unchanged. These three directions of PT change in the contralateral non-inflamed paws were found to be correlated with the magnitude of decrease in PT in the inflamed sides.

Responses of neurones of the centralis nucleus of the amigdala to peripheral noxious stimuli

The present study was designed to investigate a role of the subnucleus reticularis dorsalis (SRD) in the analgesia produced by a low dose of naloxone during carrageenan-induced inflammation. Male Sprague–Dawley rats were divided into the following two groups: (1) rats with bilateral lesions of the SRD (n=13) and 2) sham-operated rats (n=24). In each group, effects of a low dose of naloxone (5 μg/kg, i.p.) on thermal nociception were examined 4 h, 7 and 28 days after the induction of unilateral inflammation. Carrageenan (6 mg in 0.15 ml saline) was injected subcutaneously into the plantar surface of the left hindpaw. The analgesic effect was assessed by prolongation of the paw withdrawal latency (PWL) to heating. Prior to carrageenan injection, a low dose of naloxone did not prolong PWLs in either group. Four hours after carrageenan, a low dose of naloxone produced a prolongation of PWLs in both sham-operated and SRD-lesioned rats. Seven days after carrageenan, naloxone failed to produce analgesia in the SRD-lesioned rats but did produce analgesia in the sham-operated rats. At 28 days, a low dose of naloxone induced hyperalgesia in the inflamed paw of both groups, whereas naloxone was ineffective in the contralateral non-inflamed paw. These results suggest that the SRD plays a role in naloxone-induced analgesia during the subacute phase of inflammation (e.g. 7 days after induction of inflammation).

A study of the analgesic interaction between intrathecal morphine and subcutaneous nalbuphine in the rat.

Nalbuphine reverses opioid-induced respiratory depression, but the effect on analgesia is unclear. The analgesic interaction between subcutaneous (sc) nalbuphine and intrathecal morphine in conscious, male, Sprague-Dawley rats implanted with chronic intrathecal catheters was investigated. Nalbuphine (10 mg/kg) injected 30 min after intrathecal morphine (4 micrograms) significantly antagonized the effect of morphine in the tail flick test. The antagonism was rapid in onset and persisted beyond the experimental period of 240 min. The magnitude and the duration of the effect were comparable to that observed with sc naloxone (1 mg/kg). In contrast to the results in the tail flick test, nalbuphine enhanced the effect of intrathecal morphine in the noninflamed paw pressure test. Nalbuphine (10 mg/kg) alone had no effect on the time course of tail flick latency but significantly increased paw pressure threshold during the 15-90 min interval after sc injection. Nalbuphine (0.5 mg/kg, sc) alone had no antinociceptive effect in either pain test and did not antagonize the antinociceptive effect of intrathecal morphine (4 micrograms) in the tail flick test. However, sc nalbuphine (0.5 mg/kg), injected 30 min after intrathecal morphine (1.5 micrograms), significantly enhanced the effect of morphine in the paw pressure test compared with intrathecal morphine + sc saline-treated rats. The results indicate a complex analgesic interaction between intrathecal morphine and sc nalbuphine. The net analgesic effect during the interaction was determined by the following: 1) the doses of morphine and nalbuphine; 2) the time after nalbuphine administration; and 3) the nature of the nociceptive stimulus. At lower doses, sc nalbuphine appeared to potentiate the effect of intrathecal morphine in the noninflamed paw pressure test.

Unilateral inflammation of the hindpaw in rats as a model of prolonged noxious stimulation: Alterations in behavior and nociceptive thresholds

Unilateral intraplantar injection of Freund's complete adjuvant (FCA) into one hindpaw of rats led to a localized inflammation that became apparent within 12 hours and reached its peak between 2 and 3 weeks. FCA-treated rats displayed a diminished rate of body weight gain, a reduction of food and water intake and a disruption of circadian temperature regulation, as well as decreased locomotor activity and pronounced scratching behavior in the open field. Paw pressure thresholds were reduced only in inflamed paws. Contralateral, noninflamed paws showed comparable thresholds to those of control animals. Tail-flick and tail-pressure responses were not different from controls. These data suggest that FCA-treated animals experience increased noxious input from the inflamed limb and that changes in thresholds to acutely applied nociceptive stimuli are due to a peripheral hypersensitivity of inflamed tissue. The present condition resembles most closely a state of acute inflammatory pain. The term “chronic pain“ in its strict sense is not appropriate in this model.

Peripheral effect of fentanyl upon nociception in inflamed tissue of the rat

This study examined a possible peripheral site of action of opiates in the modulation of the response to noxious pressure on inflamed paws. Rats received an injection of Mycobacterium butyricum suspension into one hindpaw which resulted in an ipsilateral inflammatory response (swelling) and decreased threshold to noxious pressure within 1–2 days. Four days post-inoculation direct intraplantar injection of fentanyl resulted in a significant increase of paw pressure thresholds in inflamed, but not in non-inflamed paws. Intraplantar injection of (−)-naloxone but not (+)-naloxone reversed this effect. The action of fentanyl was dose dependent at doses of 0.1–1.2 μg. Equal doses of fentanyl given systemically as well as (−)-naloxone or (+)-naloxone given alone were inactive. These data show that fentanyl can modify the response to noxious pressure in inflamed but not non-inflamed paws via a peripheral site of action. Stereospecific reversibility by naloxone as well as dose dependency strongly indicate an opioid mechanism of action. This suggests a possible role for peripherally located opioid receptors in the modulation of nociception in inflamed tissue.

Determination of inflamed paw surface temperature in rats

The surface temperature of a rat's paw is greatly affected by heat transfer because of its contacts with the body, as well as several environmental conditions. To establish a methodology for determining the surface temperature of the inflamed rat's paw, we devised an individual cage with a pair of rolling rods. The rods were set to protect contacts of paws with the body and were devised to roll easily when rats put their feet on the rods. When a rat was transferred to the individual cage with rolling rods from a group cage, the surface temperature of a normal or inflamed paw fell gradually and reached equiliblium within 10 minutes. The application of the rolling rods markedly narrowed the range of fluctuation of paw surface temperatures at the equilibrium, and increased the temperature difference between inflamed and noninflamed paws. We compared local hyperthermia and swelling of inflamed paws, and tested the effects of indomethacin. The results suggest that the local hyperthermia is a useful index of inflammation and a sensitive parameter for the pharmacological evaluation of antiinflammatory drugs.