Prostaglandin hyperalgesia, V: a peripheral analgesic receptor for opiates

Sérgio Ferreira beyond Pharmacology: His Role as a Science Communicator

Historically, toxins from animal venoms have contributed significantly to the discovery of new drugs, as illustrated by captopril, the first drug developed from an animal toxin approved for human use [...].

Endogenous morphine: opening new doors for the treatment of pain and addiction

Nitric oxide (NO) signalling is at the forefront of intense research interest because its many effects remain controversial and seemingly contradictory. This paper examines its role as a potential mediator of pain and tolerance. Within this context discussion covers endogenous morphine, documenting its ability to be made in animal tissues, including nervous tissue, and in diverse animal phyla. Supporting morphine as an endogenous signalling molecule is the presence of the newly cloned mu3 opiate receptor subtype found in animal (including human) immune, vascular and neural tissues, which is coupled to NO release. Importantly, this mu opiate receptor subtype is morphine-selective and opioid peptide-insensitive, further highlighting the presence of morphinergic signalling coupled to NO release. These findings provide novel insights into pain and tolerance as morphinergic signalling exhibits many similarities with NO actions. Taken together, a select morphinergic signalling system utilising NO opens the gate for the development of novel pharmaceuticals and/or the use of old pharmaceuticals in new ways.

Probable activation of the opioid receptor-nitric oxide-cyclic GMP-K+ channels pathway by codeine

There is evidence that local peripheral administration of morphine produces antinociception through the activation of the nitric oxide (NO)-cyclic GMP-K(+) channels pathway. Therefore we evaluated the possible participation of this pathway in the antinociceptive action produced by codeine in the rat 5% formalin test. Local peripheral injection of codeine produced a dose-dependent antinociception during the first and second phases of the test. Local pretreatment of the paws with the NO synthase inhibitor N(G)-L-nitro-arginine methyl ester (L-NAME), the soluble guanylyl cyclase inhibitor methylene blue, the ATP-sensitive K(+) channel inhibitors glibenclamide and tolbutamide, the non-selective voltage-gated K(+) channel inhibitors 4-aminopyridine (4-AP) and tetraethylammonium (TEA) and the opioid receptor blocker naloxone prevented codeine-induced antinociception in both phases of the test. L-NAME, methylene blue, K(+) channel blockers and naloxone by themselves did not modify formalin-induced nociceptive behavior. Our data suggest that codeine could activate the opioid receptor-NO-cyclic GMP-K(+) channels pathway in order to produce its peripheral antinociceptive effect in the formalin test.

Peripheral analgesic blockade of hypernociception: activation of arginine/NO/cGMP/protein kinase G/ATP-sensitive K+ channel pathway

The final step in the direct restoration of the nociceptor threshold by peripheral administration of morphine and dipyrone was recently suggested to result from the opening of ATP-sensitive K(+) channels (K(ATP)(+)). This channel is known to be open either directly by cGMP or indirectly via protein kinase G (PKG) stimulation. In the present study, it was shown that the blockade was caused by a specific PKG inhibitor (KT5823) of the antinociceptive effect of morphine and dipyrone on acute hypernociception and of dipyrone on persistent hypernociception. It was also shown that, in both models, KT5823 prevented the peripheral antinociceptive effect of an analogue of cGMP, the nitric oxide (NO) donor (S-nitroso-n-acetyl-d,l-penicilamine). However, in acute hypernociception, KT5823 did not prevent the peripheral antinociceptive effect of diazoxide (a direct K(ATP)(+) opener). In persistent hypernociception, the sensitization plateau was induced by daily injections of prostaglandin E(2) (PGE(2), 100 ng) into the rat paw for 14 days. After cessation of PGE(2) injections, the pharmacological blockade of persistent hypernociception led to a quiescent phase in which a rather small stimulus restored the hypernociceptive plateau. In this phase, glibenclamide (which specifically closes K(ATP)(+)) fully restored persistent hypernociception, as did injection of PGE(2). Thus, the activation of the arginine/NO/cGMP pathway causes direct blockade of acute and persistent hypernociception by opening K(ATP)(+) via the stimulation of PKG. Analgesic stimulators of the neuronal arginine/NO/cGMP/PKG/K(ATP)(+) pathway constitute a previously undescribed well defined class of peripheral analgesics with a mechanism of action different from either glucocorticoids or inhibitors of cyclooxygenases.

Reduction in [D-Ala2, NMePhe4, Gly-ol5]enkephalin-induced peripheral antinociception in diabetic rats: the role of the L-arginine/nitric oxide/cyclic guanosine monophosphate pathway

To test our hypothesis that the abnormally small efficacy of mu-opioid agonists in diabetic rats may be due to functional changes in the L-arginine/nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway, we evaluated the effects of N-iminoethyl-L-ornithine, methylene blue, and 3-morpholino-sydnonimine on [D-Ala(2), NMePhe(4), Gly-ol(5)]enkephalin (DAMGO)-induced antinociception in both streptozotocin (STZ)-diabetic and nondiabetic rats. Animals were rendered diabetic by an injection of STZ (60 mg/kg intraperitoneally). Antinociception was evaluated by the formalin test. The mu-opioid receptor agonist DAMGO (1 microg per paw) suppressed the agitation response in the second phase. The antinociceptive effect of DAMGO in STZ-diabetic rats was significantly less than in nondiabetic rats. N-Iminoethyl-L-ornithine (100 microg per paw), an NO synthase inhibitor, or methylene blue (500 microg per paw), a guanylyl cyclase inhibitor, significantly decreased DAMGO-induced antinociception in both diabetic and nondiabetic rats. Furthermore, 3-morpholino-sydnonimine (200 microg per paw), an NO donor, enhanced the antinociceptive effect of DAMGO in nondiabetic rats but did not change in diabetic rats. These results suggest that the peripheral antinociceptive effect of DAMGO may result from activation of the L-arginine/NO/cGMP pathway and dysfunction of this pathway; also, events that are followed by cGMP activation may have contributed to the demonstrated poor antinociceptive response of diabetic rats to mu-opioid agonists.

IMPLICATIONS

This is the first study on the role of the nitric oxide (NO)/cyclic guanosine monophosphate pathway on [D-Ala(2), NMePhe(4), Gly-ol(5)]enkephalin (DAMGO)-induced peripheral antinociception and the effect of diabetes on this pathway. The study suggests a possible role of DAMGO as a peripherally-acting analgesic drug.

Effect of morphine sulphate eye drops on hyperalgesia in the rat cornea

In addition to their traditional role in centrally mediated analgesia, opiate compounds produce significant effects when administered peripherally. Using a recently characterized model of acute chemical injury to the rat cornea, we assessed the effects of morphine sulphate eye drops on corneal inflammation and hyperalgesia. Topical application of a 5 microM morphine sulphate eye drop preparation attenuated capsaicin-induced blinking in a concentration-dependent manner. However, morphine had no effect on capsaicin-induced blinking when applied to healthy, non-inflamed rat cornea. In addition, 5 microM morphine given every 2 h following cauterization retarded the development of both stromal edema and the infiltration of immune cells. Both the analgesic and anti-inflammatory effects of morphine were prevented by the opioid receptor antagonists naloxone, CTAP, and naltrindole. We conclude that morphine acts on mu and delta opioid receptors located in the rat cornea to attenuate inflammation and hyperalgesia.

Corticotropin-releasing factor in antinociception and inflammation

Corticotropin-releasing factor (CRF) plays a major role at the level of the hypothalamus and pituitary to control the body's response mechanisms to stressful stimuli. The recent discovery of CRF outside the central nervous system suggests that CRF may well play a similar role in peripheral tissues, most likely in a paracrine manner. While its effects in many other peripheral tissues is not known yet, CRF and its receptors are upregulated in inflammatory pain states pointing to a key role under these circumstances. Indeed, locally expressed CRF seems to act on CRF receptors on immune cells which have migrated into the area of the inflamed tissue, and induce the release of opioid peptides synthesized within these immune cells. These opioids subsequently act on peripheral opioid receptors located on peripheral sensory nerves to inhibit the transmission of painful stimuli. CRF may also affect the inflammatory response; however, these data are still controversial. The peripheral paracrine effects of CRF may be similar to those of hypothalamic CRF, i.e., to counterbalance local stressful events, such as inflammation and pain, so that they do not threaten the homeostasis of the body. Interestingly, CRF-like peptides have been identified not only in mammalians, but also in species such as the frog (Stenzel-Poore et al., 1992, Mol. Endocrinol. 6, 1716) and the teleost fish (Okawara et al., 1988, Proc. Natl. Acad. Sci. USA 85, 8439) indicating that this is a peptide that has been conserved over a long period (200 million years) across species (Lederis et al., 1990, Prog. Clin. Biol. Res. 342, 467) and that the release of ACTH-like peptides by peptides of the CRF family may represent an ancestral type of stress response (Ottaviani et al., 1992, Gen. Comp. Endocrinol. 87, 354; Tran et al., 1990, Gen. Comp. Endocrinol. 78, 351).

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

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

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.

The opioid antagonist naloxone does not alter discharges of nociceptive afferents from the acutely inflamed knee joint of the cat

Recent studies have shown peripheral antinociceptive effects of opiates in inflamed tissue. To test whether the afferent activity during an acute inflammation may also be suppressed by endogenous opioids, we studied whether the application of the opioid antagonist naloxone would alter the afferent discharges from the cat knee joint inflamed by kaolin and carrageenin. After i.a. bolus administration of naloxone (3 micrograms/kg and 1 mg/kg) close to the joint, neither the ongoing activity nor the responses to noxious and innocuous movements significantly changed in group III or group IV units. Since naloxone did not unmask opioidergic activity under these conditions, we conclude that the development of increased activity in joint afferents during an acute kaolin/carrageenin-induced inflammation is not tonically suppressed by endogenous opioids.

Opioid inhibition of Ih via adenylyl cyclase

Opioids are coupled through G proteins to both ion channels and adenylyl cyclase. This study describes opioid modulation of the voltage-dependent cation channel, Ih, in cultured guinea pig nodose ganglion neurons. Forskolin, PGE2, and cAMP analogs shifted the voltage dependence of activation of Ih to more depolarized potentials and increased the inward current at -60 mV. Opioids had no effect on Ih alone, but reversed the effect of forskolin on Ih. This action of opioids was blocked by naloxone. Opioids had no effect on Ih in the presence of cAMP analogs, suggesting that modulation occurs at the level of adenylyl cyclase. The shift in the voltage dependence of Ih by agents that induce inflammation (i.e., PGE2) is one potential mechanism to mediate an increased excitability. Opioid inhibition of adenylyl cyclase and subsequent inhibition of Ih may be a mechanism by which opioids inhibit primary afferent excitability and relieve pain.

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)

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.

Role of opioids in peripheral analgesia

Several pharmacological, neurophysiological and immunohistological studies indicate that exogenous or endogenous opioids can have antinociceptive effects by acting at peripheral sites. Although modulation of mu, delta and kappa receptors can mediate these effects, the nature of the noxious stimulus and the underlying pathological condition may affect the types of opioid receptors involved. Thus, it would be appropriate to develop peripherally-acting opioid analgesics that do not have the untoward central side effects often associated with conventional analgesic drugs. This paper reviews the evidence supportive of a peripheral mechanism of action for opioids.

Sensory pharmacology

During the last decade, evidence has been accumulated to demonstrate that a subpopulation of peptide-containing primary afferent neurones serve a dual sensory-efferent function. Considerable effort has been put into the development of pharmacological tools to modulate the release and/or the postjunctional effects of transmitters of primary afferents. This commentary summarizes the different approaches designated to achieve this goal, which, if successful, will lead to true 'sensory pharmacology'.

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.

Opiates suppress carrageenan-induced edema and hyperthermia at doses that inhibit hyperalgesia

This study determined whether opiates alter vascular components of inflammation (hyperthermia, edema and plasma extravasation) in addition to the suppression of hyperalgesia. Rats were administered carrageenan into one hind paw and saline into the other hind paw, followed by i.p. injection of morphine (0.2-5.0 mg/kg) or saline at 60 min, and testing at 90 min after hind paw injections. Morphine produced a dose-dependent reduction in carrageenan-induced hyperalgesia (17-53%), hyperthermia (39-53%) and edema (24-36%). Morphine treatment did not alter the temperatures of the contralateral saline-injected paws, indicating that opiate suppression of hyperthermia was not confounded by alterations in systemic body temperature or blood flow. The opiate effects on inflammation were stereospecific since levorphanol (1 mg/kg), but not dextrorphan (1 mg/kg), suppressed carrageenan-evoked hyperalgesia, hyperthermia and edema. Pre-treatment with naltrexone (1.5 mg/kg) blocked the effects of a 5 mg/kg dose of morphine sulfate on hyperalgesia, hyperthermia and edema. In a separate study, i.v. injection of morphine sulfate (2 mg/kg) reduced plasma extravasation by 41% (P less than 0.01). Morphine administration resulted in significantly greater increases in paw withdrawal latencies in the inflamed (38-139%) than the contralateral, saline-treated paws (4-19%). The results indicate that opiates exert a moderate, though significant, reduction in the vascular signs of inflammation in addition to their reduction of hyperalgesia. The mechanisms for this vascular effect involve inhibition of both vasodilation (as indicated by a decrease in hyperthermia) and inhibition of vascular permeability. In addition, opiates exhibit enhanced antinociceptive effects in inflamed paws, even when compared to uninjured paws in the same animal.

Modulation of a peripheral inflammatory response to substance P by locally administered opioid receptor agonists

Using a blister model of inflammation in the rat hind footpad, the present study was undertaken to examine possible peripheral effects of specific mu (DAGO) and delta (DSLET) opioid receptor agonists on an inflammatory response induced by substance P, the putative mediator of neurogenic inflammation. When perfused over the blister base, SP induced both plasma extravasation and vasodilatation responses. These responses were significantly inhibited in the presence of either opioid receptor agonist in a naloxone reversible manner. DSLET inhibited SP responses in a dose dependent manner and was 100 times more potent than DAGO. The role of primary afferent sensory nerve terminals in these modulatory effects was investigated in rats pretreated as neonates with capsaicin. The ability of DAGO and DSLET to inhibit the inflammatory response in these rats was significantly less than that in controls. The data raises the possibility that the inhibitory effect of the opioid receptor agonists on the inflammatory response might reflect a role for opioids in modulating tachyphylaxis to SP.

Potent and long lasting antinociceptive effects after injection of low doses of a mu-opioid receptor agonist, fentanyl, into the brachial plexus sheath of the rat

The effect of administering low doses (0.5-1.5 micrograms) of the mu-opioid receptor agonist fentanyl into the right brachial plexus sheath of the rat was examined using the vocalization threshold to paw pressure test. Both forepaws were tested in each rat. Fentanyl injected into the right brachial plexus sheath at 0.5-1.5 micrograms/kg produced a localized, dose-dependent, potent and long lasting antinociceptive effect, as gauged on the right forepaw. At the lower dose used (0.5 microgram/kg of fentanyl), the antinociceptive effect was restricted to the right forepaw and lasted for more than 2 h. Increasing doses of fentanyl (1 and 1.5 micrograms/kg) induced potent effects, lasting up to 5-6 h or even longer. In complete contrast, fentanyl administered i.v. at the dose of 1 microgram/kg had a very transient effect, only lasting up to 25 min. The results of injection of low doses of the opioid antagonist naloxone when administered either i.v. or locally into the paw, on the effect of fentanyl suggest the involvement of a peripheral site of action of the opioid. The present findings suggest that, as already observed in patients in clinical situations, low doses of opiates delivered using this administration route may provide prolonged regional analgesia, with the potential of avoiding centrally mediated side effects.

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.

Purification, characterization, and localization of neuropeptides in the cornea

The immunologically detected neuropeptides methionine enkephalin (ME), substance P (SP), beta-endorphin (beta-End), and alpha-melanocyte stimulating hormone (alpha-MSH) were purified from bovine corneal extracts by gradient, followed by isocratic, reversed phase-high performance liquid chromatography (RP-HPLC) and characterized, after both chromatographic steps, by radioimmunoassay (RIA). Immunologically detected ME and SP were purified from canine corneal extracts by gradient RP-HPLC and characterized by RIA. An anatomical study of the bovine cornea separated the cornea into an epithelium-enriched and a stroma-enriched portion. After gradient RP-HPLC, RIA demonstrated that all the ME-like immunoreactivity was located in the corneal epithelium, whereas the SP-like immunoreactivity was distributed between the stroma and epithelium in an approximate two-to-one ratio.

Peripheral analgesic action of clonidine: mediation by release of endogenous enkephalin-like substances

Clonidine analgesia was tested on the hyperalgesia induced by intraplantar injection of prostaglandin E2 or carrageenin. The antinociceptive effect of clonidine was dose-dependent and was abolished by local administration of the selective alpha 2-adrenoceptor blocker, yohimbine or of the opioid antagonists naloxone or quaternary nalorphine. St-91, a clonidine analog which does not cross the blood-brain barrier also promoted significant antinociception. Repeated administration of drugs possessing a central mechanism of analgesic action leads to the development of tolerance in this test. Significant analgesic tolerance was observed following repeated (5 days) morphine (8 mg/kg) or high doses of clonidine (0.5 mg/kg). In contrast, no tolerance was detected to the analgesic effect of low doses of clonidine (0.15 mg/kg) or of St-91 (0.5 mg/kg). These results suggest that, in addition to its central analgesic action, clonidine can induce peripheral antinociception by an alpha 2-adrenoceptor-mediated local release of enkephalin-like substances.

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.

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)

Lack of opiate effects on cat C polymodal nociceptive fibers

The direct application of preservative-free morphine sulfate (1.5%, 1 ml, 19.8 mumol) or fentanyl (0.06%, 1 ml, 1.07 mumol) on the superficial radial or saphenous nerve of cats did not alter the response of single C polymodal nociceptive fibers (PMNs) to noxious radiant heat stimulation of their peripheral receptive fields. Intravenous administration of fentanyl (100 or 200 micrograms/kg, 0.179 or 0.358 mumol/kg) also showed a similar lack of effect on the radiant heat evoked responses of single PMNs. Slight changes in the mean latencies following drug administration were recognized, which were not statistically significant. The use of morphine (1.5%, 1 ml, 19.8 mumol) with preservatives (chlorbutanol 0.5% and sodium bisulfite less than 0.1%) caused conduction block of PMNs within 6-15 min. Subsequent washout of the drug resulted in the return of the unitary discharges within 8 min. Lidocaine (0.25 and 0.5%, 10.7 mumol and 21.4 mumol) caused conduction block within 5-18 min. These data support the classically held concept that opiates, in clinically useful concentrations, do not alter peripheral nerve function.

Morphine inhibits the carrageenan-induced oedema and the chemoluminescence of leucocytes stimulated by zymosan

Morphine inhibited the oedema formation induced by carrageenan. The anti-inflammatory activity developed 120 min after carrageenan injection, suggesting that inhibition of the kinin phase might be partly responsible. This assumption is supported by the findings that morphine inhibited bradykinin oedema but did not influence oedema formation induced by histamine, 5-HT or PGE2. The anti-inflammatory activity of morphine was partially inhibited by naloxone (0.5-1 mg kg-1) in the carrageenan oedema test. Zymosan-stimulated chemoluminescence of neutrophils of the rat was inhibited both by morphine (0.1-10 microM) and naloxone (1-100 microM). When morphine and naloxone were administered simultaneously (10 microM) their inhibitory effects were additive. Naloxone also failed to antagonize the inhibitory action of morphine in lower dose (0.1 microM). These results suggest that the effect of morphine in inflammation might be mediated either by one of the opiate receptor subtypes insensitive to naloxone or a non-opiate mechanism might be involved.

Intracerebroventricular saline treatment elevates the pain threshold. Is this phenomenon mediated by peripheral opiate receptors?

Intracerebroventricular (i.c.v.) saline treatment of unanaesthetized mice resulted in moderate but well detectable analgesia. This elevation of pain threshold (by 60-90%) could be observed already 2 min after i.c.v. injection and lasted 15-30 min. Surprisingly, the analgesic action of i.c.v. treatment was not attenuated if naloxone was administered i.c.v. instead of saline. In fact if applying it in higher doses, the opiate antagonist potentiated the antinociceptive action. However, giving naloxone subcutaneously (s.c.) before i.c.v. saline treatment, the resulting analgesia was significantly reduced. It is tentatively put forward that i.c.v. treatment is stressful and in itself may induce mobilization of endogenous opioids and the latters elevate the pain threshold via peripheral opiate mechanisms.

Leukotrienes reduce nociceptive responses to bradykinin

The biotransformation of arachidonic acid leads to two important groups of inflammatory mediators, the leukotrienes and the prostaglandins. Hyperalgesic effects have been demonstrated for prostaglandins in a variety of animal models but the effects of leukotrienes on inflammatory pain are less well documented. Using the isolated rabbit ear model of algesia we have shown that perfusion of the ear with the leukotrienes B4, C4 and D4 (10(-8)-10(-7) M) causes a reversible, dose- and time-dependent reduction of the reflex fall in systemic blood pressure and the "head flick" response induced by injection of bradykinin (400 ng), without affecting similar responses induced by the neurotransmitter acetylcholine. The antagonistic effect of leukotrienes on the algesic action of bradykinin could be reversed by the leukotriene antagonist FPL55712 (2 micrograms/ml). This result implies that the leukotrienes may have a desensitizing effect on the nociceptor during an inflammatory response in contrast to the pain threshold-lowering action of the E- and I-type prostaglandins.

Is methylnalorphinium the prototype of an ideal peripheral analgesic?

Oral methylnalorphine ( methylnalorphinium ) caused a dose-dependent selective inhibition of inflammatory hyperalgesia (measured in the rat by a modified version of the Randall- Selitto test) without affecting the oedema. When subcutaneously injected, repeated doses of morphine for 5 days caused progressive analgesic tolerance. Tolerance was not observed after similar treatment with methylnalorphinium or methylmorphinium . Animals displaying analgesic tolerance to systemic morphine did not exhibit tolerance to the local ( intraplantar ) injection of morphine, methylnalorphinium or methylmorphinium . In contrast with nalorphine and other opiates, methylnalorphinium did not reduce intestinal transit in mice. Methylnalorphinium , a mixed opiate agonist-antagonist devoid of central effects, might be considered the prototype of an ideal peripheral analgesic since it was orally active, did not affect intestinal transit and did not cause analgesic tolerance.

Peripheral antinociceptive effects of N-methyl morphine

Morphine and N-methyl morphine were compared in two antinociceptive tests in mice, the hotplate and acetic acid-induced writhing. Whereas morphine was active in both models, N-methyl morphine was only active in the writhing model. In this model, the antinociceptive effects of N-methyl morphine were antagonized by both naloxone and N-methyl nalorphine. In separate experiments the two quaternary analogues were both [14C]-labelled and shown not to penetrate the blood brain barrier. These results indicate a peripheral site of action for the opioid antinociceptive effects of N-methyl morphine in the writhing model.