Local inhibition of inflammatory pain by naloxone and its N-methyl quaternary analogue

A Behavioral Study of Promethazine Interaction with Analgesic Effect of Diclofenac: Pain Combination Therapy

OBJECTIVES

Pain is considered as a cause of sickness and the most prevalent symptom which makes people visit a physician. Nowadays, combination therapy is becoming useful to relieve chronic and postsurgical pain. The aim of this study was to study the promethazine (as an antihistamine) interactions with antinociceptive effect of diclofenac (as a non-steroidal anti-inflammatory drugs).

METHODS

In initial part of the study, we investigate the analgesic effect of diclofenac. Using writhing test, we demonstrate that diclofenac significantly reduces writhe response induced by acetic acid in a dose-dependent manner. In this study, we evaluate the combination effect of promethazine on diclofenac analgesic effect.

RESULTS

We observed that diclofenac inhibited pain in the dose dependent manner which means that by increasing dose of diclofenac a significant decrease in pain was observed. This experimental setup allowed calculation of the dose that caused 50% antinociception (ED50) for diclofenac. The ED50 for diclofenac in this study was determined to be 9.1 mg/kg according our previous study. Additionally, promethazine was showed a dose-dependent inhibition of writhes. The combination of different doses of promethazine (2, 4, 6 mg / kg) with diclofenac ED50 (9.1 mg / kg) was injected to mice. Promethazine 4 and 6 mg / kg in combination with diclofenac had significantly led to increase analgesic effect of diclofenac.

CONCLUSION

In conclusion, these results add important information to the existing knowledge on combination of diclofenac and antihistamine in pain therapies to be used in clinical practice and maybe helpful in designing the future guidelines.

Preliminary study on the effect of parenteral naloxone, alone and in association with calcium gluconate, on bone healing in an ovine "drill hole" model system

Background

Several diseases affect bone healing and physiology. Many drugs that are commonly used in orthopaedics as "analgesics" or anti-inflammatory agents impair bone healing. Stressful conditions are associated with decreased serum osteocalcin concentration. High endorphin levels alter calcium metabolism, blocking the membrane channels by which calcium normally enters cells. The consequent decrease of intracellular calcium impairs the activities of calcium-related enzymes. Naloxone is a pure opioid antagonist. Morphine-induced osteocalcin inhibition was abolished when osteoblasts were incubated with naloxone. Naloxone restored the altered cellular and tissue physiology by removing β-endorphins from specific receptors. However, this is only possible if the circulating Ca concentration is adequate. The aim of the present study was to evaluate the efficacy of parenteral naloxone administration in inducing fast mineralization and callus remodelling in a group of sheep with a standardised bone lesion.

Methods

Twenty ewes were randomly assigned to 4 treatment groups. Group A acted as control, group B received a solution of calcium gluconate, group C a solution of naloxone, and group D a solution of calcium gluconate and naloxone. A transverse hole was drilled in the left metacarpus, including both cortices, then parenteral treatment was administered intramuscularly, daily for four weeks. Healing was evaluated by weekly radiographic examination for eight weeks. For quantitative evaluation, the ratio of the radiographic bone density between the drill area and the adjacent cortical bone was calculated. After eight weeks the sheep were slaughtered and a sample of bone was collected for histopathology

Results

Group D showed a higher radiographic ratio than the other groups. Sheep not treated with naloxone showed a persistently lower ratio in the lateral than the medial cortex (P < 0.01). Histopathology of bone samples showed more caverns and fewer osteoblasts in group D than in the other groups (P ≤ 0.001).

Conclusion

A low-dose parenteral regimen of naloxone enhances mineralization and remodelling of the callus in healing cortical defects in sheep, especially if associated with calcium gluconate.

Modifications in avoidance reactions of mice, on a second exposure to the hot plate, resist to various amnesia-inducing treatments

The avoidance responses of mice exposed to the hot plate (55 degrees C) were found to be modified when tested a second time. In fact, when forepaws licking was no longer observed, the rearing was clearly anticipated (7 s instead of 15 s) as well as jumping (24 s instead of 55 s). These modifications of avoidance strategies as well as their latencies were still observed even 24 days after the first exposure. Avoidance responses were prevented by morphine or haloperidol injected prior to the first exposure, but not with scopolamine or diazepam. These modifications were not affected in mice injected with morphine or submitted to either a supramaximal electroshock or to ether anesthesia delivered immediately after the first hot plate exposure. Among the various known types of memory, these modifications could be linked to procedural memory.

The L-arginine/nitric oxide/cyclic-GMP pathway apparently mediates the peripheral antihyperalgesic action of fentanyl in rats

There are only a few studies on the molecular mechanisms underlying the peripheral antihyperalgesic effect of opioids. The aim of this study was to investigate the molecular bases of the peripheral antihyperalgesic effect of fentanyl in a model of prostaglandin-induced chemical hyperalgesia. Prostaglandin E2 (1.4 nmol) injected into one hind paw of male Wistar rats (200-250 g, N = 6 in each experimental or control group) pretreated with indomethacin (2.5 mg/kg) potentiated the nocifensive response to formalin (1%) injection made 60 min later. Drugs applied locally 30 min after prostaglandin E2 induced the following effects: fentanyl (0.1-1.0 nmol) caused a dose-dependent reversal of the hyperalgesic state, naloxone (2 nmol) co-injected with fentanyl (1 nmol) completely reversed the antihyperalgesic effect, Nomega-nitro-L-arginine (NOARG, 0.05-0.2 mol) in combination with fentanyl (1.0 nmol) caused a dose-dependent inhibition of the antihyperalgesic effect of fentanyl, co-administration of L-arginine (0.5 mol) with NOARG (0.2 mol) plus fentanyl (1.0 nmol) fully restored the antihyperalgesic effect, and the cyclic-GMP phosphodiesterase inhibitor UK-114,542-27 (5-[2-ethoxy-5-(morpholinylacetyl) phenyl]-1,6-dihydro-1-methyl-3-propyl-7H-pyrazolo [4,3-d]-pyrimidin-7-one methanesulfonate monohydrate; 0.5-2.0 mol) potentiated a subeffective dose of fentanyl (0.1 nmol) in a dose-dependent manner. However, UK-114,542-27 (2.0 mol) injected alone did not produce this antihyperalgesic effect. Systemically administered fentanyl (1.0 nmol, sc) did not cause antinociception. Taken together, these results support the view that fentanyl reverses prostaglandin E2-induced hyperalgesia, probably by activating an opioid receptor at the periphery, and furthermore the L-arginine/nitric oxide/cyclic-GMP pathway may mediate this peripheral effect of fentanyl.

Analgesia induced by local plantar injections of opiates in the formalin test in infant rats

Morphine injected locally to the paw of an adult or an infant rat is analgesic. Opiates specific to micro and kappa opioid receptors, and less consistently to delta opioid receptors, given locally to the site of injury in adult animals are also analgesic in a variety of models of inflammatory pain. To determine which opioid receptor(s) are involved in local analgesia in the immature animal, agonists specific for micro, kappa, and delta opioid receptors were injected into the intraplantar pad in infant rats and the resultant nociceptive behavior and Fos expression assayed in the formalin test. The kappa opioid receptor agonist U50,488 reduced nociceptive behavior in both phases of the formalin test and reduced Fos expression in the dorsal horn of the lumbar spinal cord, at 3 and 21 days of age. Morphiceptin (micro opioid agonist) was analgesic in the 21-day-old pups, but not the 3-day-old pups, measured behaviorally or by Fos expression. DPDPE (delta opioid agonist) was not analgesic at either age. We also tested the effects of opioid receptor antagonists on morphine's local analgesic action. Naltrexone, and to a lesser extent the micro opioid antagonist CTOP, antagonized morphine's analgesic effect. Kappa and delta opioid receptor blockers were inactive. The results demonstrate the ability of the kappa opioid system to mediate analgesia in the neonate at the site of injury in acute and chronic pain models, that the micro opioid agonists are active later in development, but that morphine is analgesic in part through micro opioid receptors.

Different effects of methionine-enkephalin on paw edema in two inbred rat strains

The effect of intraplantarly (i.pl.)-injected methionine-enkephalin (ME) on Concanavalin A (Con A)-induced paw edema in Dark Agouti (DA) and Albino Oxford (AO) rats was investigated. ME suppressed edema in DA rats, which was antagonized with naloxone (non-selective opioid receptor antagonist) and naltrindole (delta opioid receptors antagonist). Potentiating effect of ME in AO rats was blocked by naloxone, nor-binaltorphimine (kappa opioid receptors antagonist) and beta-funaltrexamine (mu opioid receptors antagonist). Dexamethasone suppressed edema in both rat strains. These findings suggest that strain-dependent differences in the effects of ME on inflammation in DA and AO rats could be related to diversity in opioid receptors expression in these strains.

Tonic control of peripheral cutaneous nociceptors by somatostatin receptors

The peptide somatostatin [somatotropin release-inhibiting factor (SRIF)] is widely distributed in the body and exerts a variety of hormonal and neural actions. Several lines of evidence indicate that SRIF is important in nociceptive processing: (1) it is localized in a subset of small-diameter dorsal root ganglion cells; (2) activation of SRIF receptors results in inhibition of both nociceptive behaviors in animals and acute and chronic pain in humans; (3) SRIF inhibits dorsal horn neuronal activity; and (4) SRIF reduces responses of joint mechanoreceptors to noxious rotation of the knee joint. The goal of the present study is to show that cutaneous nociceptors are under the tonic inhibitory control of SRIF. This is accomplished using behavioral and electrophysiological paradigms. In a dose-dependent manner, intraplantar injection of the SRIF receptor antagonist cyclo-somatostatin (c-SOM) results in nociceptive behaviors in normal animals and enhancement of nociceptive behaviors in formalin-injected animals, and these actions can be blocked when c-SOM is coapplied with three different SRIF agonists. Furthermore, intraplantar injection of SRIF antiserum also results in nociceptive behaviors. Electrophysiological recordings using an in vitro glabrous skin-nerve preparation show increased nociceptor activity in response to c-SOM, and this increase is blocked by the same three SRIF agonists. Parallel behavioral and electrophysiological studies using the opioid antagonist naloxone demonstrate that endogenous opioids do not maintain a tonic inhibitory control over peripheral nociceptors, nor does opioid receptor antagonism influence peripheral SRIF effects on nociceptors. These findings demonstrate that SRIF receptors maintain a tonic inhibitory control over peripheral nociceptors, and this may contribute to mechanisms that control the excitability of these terminals.

Antinociceptive effect of amikacin and its interaction with morphine and naloxone

Amikacin sulphate (30 mg kg(-1)) administered either intraperitoneally (i.p.) or subcutaneously (s.c.) produced antinociceptive effect in BALB/c mice in the acetic acid writhing test which is employed as an inflammatory pain model. The lack of difference between two routes with regard to antinociceptive potency was taken as evidence for the absence of a local effect. Amikacin sulphate-induced antinociception seems unlikely to be due to non-specific behaviour alteration, since this drug, at a dose range of 15-100 mg kg(-1)did not affect motor coordination of mice in rot-a-rod test. Morphine (1 mg kg(-1)) also caused antinociception when administered i.p. or s.c. but the effect was greater with the latter route. At the i.p. site; the concurrent use of amikacin and morphine produced more remarkable antinociception compared to their individual usages. Besides, naloxone (2 mg kg(-1)) significantly decreased antinociceptive effect of amikacin but itself also exerted antinociception. At present, we have no plausible explanation for these findings at the i.p. site.

Lack of tolerance in peripheral opioid analgesia in mice

We recently developed a sensitive peripheral analgesic test in mice. Bradykinin, a representative pain-producing substance, when given subcutaneously through a polyethylene tube into the plantar of the limb connected to a transducer, induced a flexor reflex response, in a dose dependent manner. When morphine, a mu-opioid receptor agonist, was added to the plantar through another polyethylene tube, bradykinin-induced responses were completely abolished in a naloxone-reversible manner. These peripheral analgesic effects were also observed with DAMGO, another mu-opioid receptor agonist, and U-69,593, a kappa-opioid receptor agonist, but not DSLET, a delta-opioid receptor agonist. When morphine was given subcutaneously to the back, a potent analgesia in the tail pinch test was observed. Repeated administrations of morphine once per day for 5 days showed a marked tolerance or reduction in morphine analgesia on the 6th day, while there was no significant reduction in the peripheral analgesia of morphine. These findings suggest that tolerance to morphine analgesia is mediated through synaptic plasticity in the central nervous system, but not through a receptor desensitization at the level of the single cell.

Effects of yohimbine on naloxone-induced antinociception in a rat model of inflammatory hyperalgesia

Effects of the alpha2-adrenoceptor antagonist yohimbine on the antinociception produced by a low dose of naloxone were examined in a rat model of carrageenan-induced inflammation. In rats receiving saline prior to naloxone injection, the low dose of naloxone (5 microg/kg, i.p.) significantly prolonged paw withdrawal latency in response to noxious thermal stimuli for both the inflamed and the non-inflamed paws 4 h after carrageenan injection (6.0 mg in 0.15 ml saline). In rats receiving yohimbine, the low dose of naloxone failed to produce prolongation of paw withdrawal latencies 4 h after carrageenan, whereas naloxone produced antinociception 7 days after carrageenan. The results suggest that noradrenergic mechanisms are involved in naloxone-induced antinociception only in the early phase of carrageenan-induced inflammation.

The subnucleus reticularis dorsalis is involved in antinociception produced by a low dose of naloxone during carrageenan-induced inflammation

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 microg/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).

The peripheral antinociceptive effect of morphine in a rat model of facial pain

The present study compared the peripheral and systemic antinociceptive effect of morphine on formalin-induced facial pain behavior in the rat. Formalin (5%, 50 microliters) was injected subcutaneously into the vibrissal pad of adult rats (250-300 g). Morphine sulfate at doses of 100-1000 micrograms was subcutaneously injected locally (same area) or systemically (in the neck), 30 min before, or simultaneously with, formalin. The typical biphasic face grooming response, consisting of an early phasic phase (0-6 min) and a delayed tonic phase (12-42 min), displayed by control animals, was suppressed by both local and systemic administration of morphine; this effect was dose dependent. However, the suppression of the early phase with local morphine administration 30 min before formalin could be significantly greater (49-52%) than with systemic administration, depending on the dose used. Administration of local morphine simultaneously with formalin produced up to 34% reduction in the early and an additional 32% reduction of the late phases of face grooming, compared to systemic injections. Local injection of naloxone (10 micrograms) almost completely reversed the antinociceptive effect of 1000 micrograms of morphine (early phase 85 +/- 7%, late phase 100 +/- 26% reduction), whereas the same dose of naloxone applied systemically (i.p.) produced only partial reversal (early phase 29 +/- 16%, late phase 36 +/- 1% reduction). This study further indicates that locally administered morphine can exert an analgesic effect superior to systemic administration in the case of inflammatory and non-inflammatory pain through a peripheral site of action. These results support the clinical use of peripheral opioid administration in the treatment of human painful conditions.

Pharmacological evidence for the involvement of the endogenous opioid system in the response to local inflammation in the rat paw

We have investigated the role of the endogenous opioid system (EOS) on the inflammatory response induced by subplantar (s.p.) injection of saline (SS) and carrageenan (CA) in the hindpaw of the rat. The administration of intraperitoneal (i.p.) naloxone was used in order to unmask the effects of endogenous opiates released during peripheral inflammation. Three groups of rats received one of the following s.p. treatments: SS, CA, or no injection (NI). Pain pressure threshold (PPT), paw volume (edema) and local temperature were evaluated in baseline conditions and 3 h after treatment. In each group, the effects of i.p. vehicle, naloxone and (+)-naloxone (0.1 mg/kg) were also investigated. Both SS and CA induced a significant inflammatory response with hyperalgesia, edema and local hyperthermia. The i.p. administration of naloxone but not that of (+)-naloxone 15 min prior to testing, significantly increased edema in all groups of treatment (P < 0.05), without altering PPT or local temperature. Two-way ANOVA revealed that treatment and drugs, as well as their interaction, had a significant impact on edema which was related to the effects of CA and naloxone. Our findings illustrate the involvement of the EOS in the physiological response to local injury, regulating microvascular leakage in the inflamed tissues.

Mechanism of diclofenac analgesia: direct blockade of inflammatory sensitization

Indomethacin, a typical cyclo-oxygenase inhibitor, acts as an analgesic by preventing the hyperalgesia induced by prostaglandins during inflammation. Analgesics of the dipyrone type directly block the sensitization of nociceptors. In the present investigation, the analgesic effect of diclofenac was compared with that of indomethacin in two algesimetric tests which permit discrimination between the two types of analgesic: the rat knee joint incapacitation and the rat paw hyperalgesia tests. The analgesics were given either pre- or posttreatment relative to the induction of hyperalgesia with carrageenin or prostaglandin E2. In both tests intraperitoneal pretreatment with indomethacin was equally or slightly more potent than diclofenac. Posttreatment with diclofenac was more effective than posttreatment with indomethacin. This was particularly evident in the paw hyperalgesia test in which posttreatment with indomethacin was not effective while diclofenac caused dose-dependent analgesia. When nociception was induced by PGE2 in both tests, the administration of indomethacin directly into the knee joint or rat paw had no effect while diclofenac continued to cause dose-dependent analgesia. Thus, diclofenac has a direct effect on ongoing hyperalgesia in addition to its ability to block cyclo-oxygenase. Naloxone and N-methyl-nalorphine did not affect diclofenac analgesia, thus indicating that the analgesic effect of the latter is independent of a central or peripheral opioid effect. Local administration of agents which inhibit the formation of nitric oxide (NG-monomethyl-L-arginine) or inhibit the activation of guanylate cyclase by nitric oxide (methylene blue) abolished diclofenac-induced analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)

Paradoxical analgesia induced by low doses of naloxone is not potentiated by complete inhibition of enkephalin degradation

The involvement of a presynaptic autoinhibition of endogenous enkephalin release has been suggested as a possible explanation for the paradoxical analgesia induced by low doses of naloxone. This hypothesis was investigated by using the systemically active mixed inhibitor of enkephalin degrading enzymes, RB 101. As already described, in both hot plate (55 +/- 0.5 degrees C) and acetic acid (0.6%) abdominal constriction tests in mice, subcutaneous administration of naloxone produced biphasic effects, with antinociceptive responses at very low doses (microgram range) and hyperalgesia at higher dose (mg range). However at concentrations producing an extracellular increase in enkephalin levels and subsequent analgesia, the mixed inhibitor prodrug of the enkephalin-metabolizing enzymes RB 101 (20 or 100 mg/kg i.v. and 5 or 10 mg/kg i.v., in the hot plate test and in the abdominal constriction test, respectively) did not potentiate the paradoxical analgesia induced by naloxone. These results are inconsistent with a negative autoregulation of endogenous enkephalin release and could suggest the involvement of the diffuse noxious inhibitory controls (DNIC). Indeed, the finding that low doses of RB 101 (1 mg/kg i.v. in the hot plate test, and 250 micrograms/kg i.v. in the abdominal constriction test) were able to induce hyperalgesic responses could indicate that the DNIC are tonically activated by endogenous enkephalins accounting for the antinociceptive responses elicited by low doses of naloxone.

Opioid peptide release evoked by noxious stimulation of the hind instep of rats

Previous studies showed that opioids and opioid peptides modulate vascular reactions such as plasma extravasation and vasodilatation through inhibition of substance P release from peripheral nerve endings of primary afferent fibers, and suggested the existence of endogenous opioid peptide activity related to regulation of inflammatory responses. Here we examined the effect of heat stimulation and antidromic stimulation of primary afferent fibers on the release of immunoreactive opioid peptides into the perfusate of the subcutaneous space in the rat instep. Antidromic stimulation of sectioned sciatic and saphenous nerves did not have any significant effect on the release of Met-enkephalin (Met-EK), while immersion of the hind paw in hot water (47 degrees C) for 30 min caused an increase in Met-EK release into the perfusate. High-pressure liquid chromatography of the perfusate revealed that noxious heat stimulation induced increase in release of Leu-enkephalin (Leu-EK) as well as Met-EK, although the maximal concentration of Leu-EK was less than 16% of that of Met-EK. No beta-endorphin was detected in the perfusate before, during or after heating. We conclude that noxious heat stimulation mainly leads to increase in Met-EK, and that this peptide originates mainly, from peripheral cells containing opioid peptides such as immune cells and/or Merkel cells, not from primary afferent fibers.

Narcotic receptor blockade and its effect on the analgesic response to placebo and ibuprofen after oral surgery

The purpose of this study was to evaluate the contribution of endogenous opiates to the analgesic response after treatment with placebo, codeine, and ibuprofen after oral surgery. Eighty-one patients undergoing complicated dental extractions were pretreated with either a placebo or the narcotic antagonist naltrexone 50 mg, 30 minutes before surgery. After surgery, patients self administered one of three possible postsurgical medications, which included placebo, codeine 60 mg, and ibuprofen 400 mg, when their pain reached a moderate or severe intensity. The study was double-blind with the three postsurgical treatments being randomly allocated within each presurgical treatment block. Pain intensity, pain relief, pain half gone, and overall evaluations were assessed for up to 6 hours. Ibuprofen was significantly more efficacious (p < .05) than codeine or placebo for most analgesic measures. The administration of naltrexone before surgery reduced the analgesic response to both placebo and codeine. Pretreatment with naltrexone did not diminish the peak analgesic response to ibuprofen, but surprisingly prolonged (p < .05) the duration of its action. The results suggest that a blockade of endogenous opiates by naltrexone diminished the placebo response, but that naltrexone may prolong ibuprofen analgesia by some unknown mechanism.

Morphine and naloxone act similarly on glutamate-caused guinea pig ileum contraction

Both morphine (M) and naloxone (NL) have been reported to have NMDA receptor blocking effects, regarded as the reason of opiate physical dependence development. On the other hand, glutamate (GLU) has been known to induce the contraction of isolated guinea pig ileum via acetylcholine release. Therefore, different concentrations of M or NL were investigated on the 1 mM GLU-induced contraction of isolated guinea pig ileum fixed at a resting tension of 1 g in isolated organ bath. The mean value (359.3 +/- 20 mg) of the GLU-elicited contraction force was significantly reduced (318.4 +/- 19.4) by 25 nM M concentration in the medium. Consequently, 500 and 750 nM M caused further decreases in a rather dose-dependent manner (270.8 +/- 17.4 and 167.8 +/- 16.5 mg, respectively). One micromolar M contraction nearly abolished (8.0 +/- 8.2 mg) the GLU-induced contraction. A similar effect was obtained with the naloxone concentrations of 10, 20, 40, and 50 microM. In addition, NL has been shown to elicit the contraction of the isolated M-dependent guinea pig ileum. In the present study, 20- and 30-microM NL concentrations in the bathing medium caused the contraction of the ileum made M-dependent by preincubation with M (333.0 +/- 32.4 and 309.5 +/- 17.7 mg, respectively). These contraction forces were significantly reduced when the NL concentration was increased to 40 microM. And, 50 microM NL concentration not only failed to induce contraction but caused a relaxation (-10.6 +/- 2.3) as well. The results were considered supporting evidence for the fact that both M and NL are NMDA receptor blockers.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential influence of naloxone on the responses of nociceptive neurons in the superficial versus the deeper dorsal horn of the medulla in the rat

Naloxone (200 micrograms/kg, i.v.) reduced the noxious thermal stimuli-evoked responses of 16/25 nociceptive neurons in the superficial laminae whereas it enhanced the responses of 6/10 nociceptive neurons in the deeper dorsal horn. However, a different picture emerged when selectivity of neuronal responsivity (nocireceptive or multireceptive) was considered. In the superficial dorsal horn, naloxone reduced the responses of the majority of (15/18) selectively nocireceptive neurons. The reduction in responses became apparent within 60 sec following naloxone administration and returned to control level within 48 min. In contrast, the responses of the majority of multireceptive neurons in the superficial (6/7), or the deeper (6/10) dorsal horn, were enhanced. The excitatory action in the superficial dorsal horn persisted for only 6-15 min, whereas it persisted for 40-70 min in the deeper dorsal horn. The firing of the majority of cold-receptive neurons (6/8) in the superficial dorsal horn was not altered. These effects were stereoselective since (+)-naloxone, the inactive isomer of naloxone, did not affect the responses of 14/16 nociceptive neurons. It is concluded that naloxone differentially, and selectively, affects the firing of nociceptive neurons in the superficial versus the deeper dorsal horn, and the firing of selectively nocireceptive versus multireceptive neurons. The relevance of these findings to the behavioral effects of naloxone, hyperalgesia and analgesia, is discussed.

Rat knee-joint carrageenin incapacitation test: an objective screen for central and peripheral analgesics

A new behavioral test is described in which quantitation is independent of the observer and is sensitive to all classes of analgesics. A computer-assisted device measures the period during which a rat hind paw fails to touch the surface of a rotating cylinder for 1 min (paw elevation time). Intra-articular injection of carrageenin induces a progressive and dose-dependent incapacitation of the limb. The maximum paw elevation time is attained 3-4 h after carrageenin challenge. The model showed dose-dependent sensitivity to (a) a central acting opiate (morphine, ID50 = 1.5 mg/kg, i.p.), (b) cyclooxygenase inhibitors (indomethacin, ID50 = 0.8 mg/kg, i.p.; diclofenac, ID50 = 0.22 mg/kg, i.p.), and (c) peripheral analgesics which directly antagonize nociceptor hypersensitivity: dipyrone (ID50 = 21 mg/kg, i.p.), N-methyl-nalorphine (ID50 = 14 mg/kg, i.p.) and BW443C (ID50 = 17.5 mg/kg, i.p.). The knee-joint carrageenin incapacitation was also blocked by the sympatholytics, propranolol and guanethidine. After the blockade by either indomethacin or guanethidine, intra-articular injections of prostaglandin E2 or dopamine, respective, reversed carrageenin-induced incapacitation. These results suggest that during inflammatory articular incapacitation cyclooxygenase and sympathomimetic mediators are involved, as has been suggested for the rat paw carrageenin hyperalgesia test and formalin test.

Does failed natural opioid modulation in regional sympathetic ganglia cause reflex sympathetic dystrophy?

Early features of reflex sympathetic dystrophy in a limb resemble the general effects of autonomic arousal associated with opioid withdrawal. It is suggested that natural opioid peptide modulation in regional sympathetic ganglia normally rises to prevent excessive autonomic activity in an injured limb. After an injury (especially a trivial one) to a susceptible subject, this localised increase in opioid modulation may fail or may rapidly wane so that autonomic features of opioid withdrawal appear in the affected limb. Florid dystrophic changes may then occur because of passive complications of disuse associated with pain, or as a result of more active processes. Preservation of an appropriate opioid bias in the regional sympathetic ganglia might be an important factor in the successful management and prevention of reflex sympathetic dystrophy.

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 response 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 response and decreased threshold to noxious pressure. Four hours post-carrageenin, the injection of 1 mg/kg i.v. morphine induced a significantly enhanced antinociceptive effect on the inflamed compared to the non-inflamed paws. Intrapantar injection of extremely low doses of naloxone (0.5 and 1 micrograms in a volume of 0.1 ml) significantly 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 antinociceptive effect of extremely low doses of i.v. naloxone described in several studies is due to a central action.

Peripheral analgesic actions of opioids

This paper will review experimental and clinical studies investigating analgesic effects of opioids outside the central nervous system. The experimental data indicate that 1) opioids can produce antinociceptive effects in the periphery mediated by local opioid receptors, 2) three different receptor types (mu, delta, kappa) can become active in peripheral tissue, 3) inflammatory hyperalgesic conditions are especially amenable to peripheral opioid actions and 4) the activation of opioid receptors located on primary afferent neurons is the most likely mechanism of action. The evidence from clinical studies is equivocal so far, partly because of methodological shortcomings. More extensive controlled studies investigating the effects of local application of opioids in the vicinity of peripheral nerve terminals or of compounds unable to cross the blood-brain barrier are warranted in view of the potential avoidance of central side effects such as respiratory depression, dependence, nausea or sedation.

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 mu- and/or delta-opioid activity were shifted to the left for inflamed hindpaws when compared to contralateral non-inflamed paws. The selective kappa-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 alpha 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 effect of morphine on inflamed hindpaws was blocked by the opioid antagonist naloxone or the alpha 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.

Facilitatory effects of opioids on the discharges of visceral nociceptors

Effects of opioids on the activity of visceral nociceptors were tested using the in vitro testis-spermatic nerve preparations excised from deeply anesthetized dogs. Morphine, DADLE and dynorphin (10 microM) elicited discharges of polymodal receptors in approximately 1/3 of the tested cases. The incidence of the excitatory response of morphine increased at higher concentrations. The excitatory responses were quite variable among preparations and showed a strong tendency for tachyphylaxis. Similar increases in discharges were elicited when morphine was applied during the steady state of the response evoked by bradykinin (BK) or BK mixed with prostaglandin E2. Pretreatment of morphine for 5 min significantly augmented the subsequent BK responses for 30 min or more. Naloxone per se induced neither excitation nor augmentation of the subsequent BK response, however it reversed the augmenting effect of morphine on BK response. In contrast with previous reports proposing peripheral analgesic effects of opioids, suppressive effects on nociceptors were never observed in the present experiment. Peripheral effects of opioids were discussed.

Substance P and peripheral inflammatory hyperalgesia

The hyperalgesic effect of substance P (SP) is usually described as presenting short latency. We now report that multiple injections of sub-threshold doses of SP into the foot pad of a hind paw of rats pre-treated with indomethacin induced a long-lasting hyperalgesia, sensitizing the paw to further challenges with small doses of SP, dopamine or prostacyclin. The sensitizing process also occurred after multiple injections of prostacyclin or prostaglandin E2. The sensitizing effect induced by SP, prostaglandin E2 or prostacyclin is inhibited by pre-treatment with the SP antagonist (D-Arg, D-Pro, D-Trp, Leu)-SP. We suggest that SP has an important role as a modulator in peripheral inflammatory pain by sensitizing nociceptors to its own action and to the action of different mediators. This sensitizing process could also be associated with chronic inflammatory pain.

Antinociceptive effects of the novel opioid peptide BW443C compared with classical opiates; peripheral versus central actions

1. To investigate peripherally mediated antinociceptive effects of opioids, the activity of a novel polar enkephalin analogue H-Tyr-D-Arg-Gly-Phe (4-NO2)-Pro-NH2 (BW443C) has been compared with those of classical tertiary opiates against different nociceptive stimuli in the mouse. 2. In chemically-induced writhing models BW443C, administered subcutaneously, demonstrated dose-related antinociceptive effects less potent than morphine and of a similar order to pethidine and D-propoxyphene. In assays using heat as the noxious stimulus BW443C was markedly less potent than any of the opiates tested. 3. In heat-induced assays, but not in chemically-induced writhing assays, BW443C demonstrated a 'U'-shaped dose-time response relationship. Morphine, pethidine and D-propoxyphene showed simple, approximately linear, dose-time effects in all assays. 4. When given subcutaneously, the inhibitory effects of BW443C and morphine in chemically-induced writhing were antagonized by naloxone given intraperitoneally. The inhibitory effects on writhing of BW443C, but not those of morphine, were also antagonized by prior intraperitoneal administration of the quaternary opioid antagonist N-methyl nalorphine. When this antagonist was administered intracerebroventricularly, the antinociceptive effects in writhing of both BW443C and morphine were antagonized. 5. It is concluded that BW443C, being only poorly able to cross the blood brain barrier, demonstrates peripherally mediated opioid antinociceptive effects in chemically-induced writhing models. In heat-induced models, that detect centrally acting opioids, BW443C is effective only at high doses and at time intervals after dosing sufficient to allow slow penetration of drug into the CNS. It is suggested that the peripheral antinociceptive actions of BW443C are mediated by inhibition of sensory neurones.

Corticotropin-releasing factor (CRF) produces analgesia in humans and rats

The analgesic activity of corticotropin releasing factor (CRF) was determined in a clinical model and in the rat hot plate test. Patients administered CRF reported significantly less postoperative pain than patients pretreated with placebo. In rats, injection of CRF resulted in a significant analgesia which was comparable in both intensity and duration to a 300 times greater molar dose of morphine. These findings suggest that endogenous CRF may play a physiologic role in modulating pain when released under conditions of stress.

Peripheral effects of opioid drugs on capsaicin-sensitive neurones of the guinea-pig bronchus and rabbit ear

The effect of a potent opioid agonist, [D-Met2, Pro5]-enkephalinamide was investigated on two responses involving capsaicin-sensitive afferent neurones, namely, atropine-resistant contractions of the guinea-pig bronchus evoked by electrical field stimulation and the nociceptor stimulation to intraarterial injections of acetylcholine or capsaicin into the vascularly isolated rabbit ear. The hypotheses to be tested were whether (a) opioid receptor activation may inhibit mediator release from primary afferent neurones and (b) the opioid could exert an analgesic effect at a peripheral site of action. Non-cholinergic contractions of the guinea-pig isolated main bronchi due to electrical stimulation were concentration-dependently inhibited by [D-Met2, Pro5]-enkephalinamide (10 nM-1 microM). This effect was abolished by naloxone (1 microM). Naloxone alone induced no change in the stimulation-evoked contractions of the bronchus, indicating that no endogenous opioid control was present. Substance P and neurokinin A induced bronchial contractions that were not influenced by [D-Met2, Pro5]-enkephalinamide. This indicates that [D-Met2, Pro5]-enkephalinamide inhibits electrically-evoked bronchial contractions by reduced mediator release from capsaicin-sensitive sensory nerve endings, since these contractions are most probably brought about by tachykinins, released from afferent neurones. Capsaicin-induced bronchial contractions were in contrast to electrical stimulation not influenced by [D-Met2, Pro5]-enkephalinamide which suggests a different site of action. The activation of sensory neurones in the rabbit ear by i.a. injection of acetylcholine and capsaicin was not reduced under infusion of [D-Met2, Pro5]-enkephalinamide (1 and 10 microM) or lofentanil (1 and 10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Analgesia produced by low doses of the opiate antagonist naloxone in arthritic rats is reduced in morphine-tolerant animals

In a model of experimental chronic pain (adjuvant-induced arthritic rats), low doses of the opiate antagonist naloxone produced a profound analgesia. Maximum analgesia was seen with 3 micrograms/kg (i.v.). In contrast, hyperalgesia was obtained with much higher doses (1-3 mg/kg, i.v.). The hyperalgesic effects were not affected in arthritic animals rendered tolerant to morphine, but the paradoxical analgesic effects were significantly reduced. This decrease suggests that naloxone analgesia involves interaction with opiate receptors and that the operation of endorphinergic systems differs in normal animals and animals which experience persistent pain.

The use of quaternary narcotic antagonists in opiate research

Quaternary ammonium derivatives of narcotic antagonists are commonly used in determining sites of action of opiates in the central nervous system and the periphery because it is widely assumed that they do not readily cross the blood-brain barrier, in contrast to their relatively non-polar tertiary counterparts. However, these compounds possess several unique pharmacological properties which have not been taken into consideration in the design of numerous investigations. This article reviews the current state of knowledge concerning the pharmacology of the quaternary narcotic antagonists, examines their use in physiological and behavioral studies of action of opiates, and proposes guidelines for the design of experiments involving these compounds.

Peripheral opioid receptors located on the rat saphenous nerve

Morphine and ethylketocyclazocine inhibited neurogenic plasma extravasation (NPE) elicited by antidromic stimulation of the saphenous nerve of the rat. The inhibitory effects of the opiate agonists were antagonised by naloxone and N-methyl nalorphine and independent of any local anaesthetic actions since inhibition of NPE was observed on superfusion of the saphenous nerve with lignocaine, but not with morphine or ethylketocyclozocine. The opiate agonists also failed to inhibit extravasation induced by intradermal administration of substance P. The findings support the contention that specific opioid receptors are located on the peripheral part of primary afferent neurones and these receptors may contribute significantly to peripherally mediated antinociceptive effects of opioids.