Supraspinal melatonin MT2 receptor agonism alleviates pain via a neural circuit that recruits mu opioid receptors

Melatonin, through its G protein-coupled receptor (GPCR) (MTNR1B gene) MT₂ , is implicated in analgesia, but the relationship between MT₂ receptors and the opioid system remains elusive. In a model of rodent neuropathic pain (spared nerve injured [SNI]), the selective melatonin MT₂ agonist UCM924 reversed the allodynia (a pain response to a non-noxious stimulus), and this effect was nullified by the pharmacological blockade or genetic inactivation of the mu opioid receptor (MOR), but not the delta opioid receptor (DOR). Indeed, SNI MOR, but not DOR knockout mice, did not respond to the antiallodynic effects of the UCM924. Similarly, the nonselective opioid antagonist naloxone and the selective MOR antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP) blocked the effects of UCM924 in SNI rats, but not the DOR antagonist naltrindole (NTI). Electrophysiological recordings in the rostral-ventromedial medulla (RVM) revealed that the typical reduction of the firing activity of pronociceptive ON-cells, and the enhancement of the firing of the antinociceptive OFF-cells, induced by the microinjection of the MT₂ agonist UCM924 into the ventrolateral periaqueductal gray (vlPAG) were blocked by MOR, but not DOR, antagonism. Immunohistochemistry studies showed that MT₂ receptors are expressed in both excitatory (CaMKIIα⁺ ) and inhibitory (GAD65⁺ ) neuronal cell bodies in the vlPAG (~2.16% total), but not RVM. Only 0.20% of vlPAG neurons coexpressed MOR and MT₂ receptors. Finally, UCM924 treatment induced an increase in the enkephalin precursor gene (PENK) in the PAG of SNI mice. Collectively, the melatonin MT₂ receptor agonism requires MORs to exert its antiallodynic effects, mostly through an interneuronal circuit involving MOR and MT₂ receptors.

Biphalin-A Potent Opioid Agonist-As a Panacea for Opioid System-Dependent Pathophysiological Diseases?

Biphalin, one of the opioid agonists, is a dimeric analog of enkephalin with a high affinity for opioid receptors. Opioid receptors are widespread in the central nervous system and in peripheral neuronal and non-neuronal tissues. Hence, these receptors and their agonists, which play an important role in pain blocking, may also be involved in the regulation of other physiological functions. Biphalin was designed and synthesized in 1982 by Lipkowski as an analgesic peptide. Extensive further research in various laboratories on the antinociceptive effects of biphalin has shown its excellent properties. It has been demonstrated that biphalin exhibits an analgesic effect in acute, neuropathic, and chronic animal pain models, and is 1000 times more potent than morphine when administered intrathecally. In the course of the broad conducted research devoted primarily to the antinociceptive effect of this compound, it has been found that biphalin may also potentially participate in the regulation of other opioid system-dependent functions. Nearly 40 years of research on the properties of biphalin have shown that it may play a beneficial role as an antiviral, antiproliferative, anti-inflammatory, and neuroprotective agent, and may also affect many physiological functions. This integral review analyzes the literature on the multidirectional biological effects of biphalin and its potential in the treatment of many opioid system-dependent pathophysiological diseases.

Biphalin analogs containing β(3)-homo-amino acids at the 4,4' positions: Synthesis and opioid activity profiles

Biphalin, a synthetic opioid octapeptide with a palindromic sequence has high analgesic activity. Biphalin displays a strong affinity for μ and δ-opioid receptors, and a significant to κ-receptor. The paper reports the synthesis of novel analogs of biphalin containing β(3)-homo-amino acid residues at the 4,4' positions and a hydrazine or 1,2-phenylenediamine linker. The potency and selectivity of the peptides were evaluated by a competitive receptor-binding assay in rat brain homogenate using [(3)H]DAMGO (a μ ligand) and [(3)H]DELT (a δ ligand). Analogs with β(3)-h-p-NO2Phe in positions 4 and 4' are the most active compounds. Selectivity depends on the degree of freedom between the two pharmacophore moieties. Analogs with a hydrazine linker show noticeable binding selectivity to μ receptors (IC50(μ)=0.72nM; IC50(δ)=4.66nM), while the peptides with a 1,2-phenylenediamine linker show slight δ selectivity (IC50(μ)=10.97nM; IC50(δ)=1.99nM). Tyr-d-Ala-Gly-β(3)-h-p-NO2PheNHNH-β(3)-h-p-NO2Phe (1) and (Tyr-d-Ala-Gly-β(3)-h-p-NO2PheNH)2 (2) produced greater antinociceptive effect compared to morphine after i.t. administration.

Anti-inflammatory and antinociceptive action of the dimeric enkephalin peptide biphalin in the mouse model of colitis: new potential treatment of abdominal pain associated with inflammatory bowel diseases

Biphalin, a mixed MOP/DOP agonist, displays a potent antinociceptive activity in numerous animal models of pain. The aim of the study was to characterize the anti-inflammatory and antinociceptive action of biphalin in the mouse models of colitis. The anti-inflammatory effect of biphalin (5mg/kg, twice daily, i.c. and i.p.) was characterized in a semi-chronic mouse model of colitis, induced by i.c. injection of trinitrobenzenesulfonic acid (TNBS). The antinociceptive action of biphalin (5mg/kg, i.p. and i.c.) in inflamed mice was assessed in mustard oil-induced model of visceral pain and in the hot plate test. In the semi-chronic mouse model of colitis, biphalin i.c. (5mg/kg), but not i.p. improved colitis macroscopic score (2.88±0.19 and 4.99±0.80 units for biphalin and vehicle treated animals, respectively). Biphalin injected i.p. and i.c. (5mg/kg) displayed a potent antinociceptive action in the mustard oil-induced pain test. In the hot plate test, biphalin (5mg/kg, i.p.) produced a potent antinociceptive activity in inflamed mice, suggesting central site of action. Our data suggest that biphalin may become a novel opioid-based analgesic agent in IBD therapy and warrant further investigation of its pharmacological profile.

Molecular assessment of the potential combination therapy of cytokines with biphalin and AZT for Friend leukemia virus infection in vitro

Biphalin, a dimeric enkephalin analog, is under investigation as a potential, long-lasting medication of pain associated with chronic diseases, like cancer or AIDS. The role of cytokines, and splenocytes in anti-Friend leukemia virus (FLV) activity of biphalin, a synthetic opioid, and AZT was investigated in vitro. Mouse splenocytes inhibited FLV replication in Mus dunni (Dunni) cells when they were added to the cell culture. This inhibitory effect of splenocytes also was evident when cells were combined with biphalin and AZT as measured using a focus-forming assay. Under cell-free conditions, recombinant interferon gamma (IFNgamma), interleukin 2 (IL-2) and IL-4 directly inhibited the FLV reverse transcriptase (RT) activity by 27% to 36%. IFNgamma at 0.005 pg to 500 ng inhibited FLVRT activity by 61% to 80%. Acombination of 250 ng IFNgamma and 50 mug biphalin resulted in a 94% reduction of FLVRT activity, as compared with 61% inhibition by IFNgamma alone. The combination of AZT and IFNgamma, IL-2 or IL-4 also induced a stronger suppression of FLV RT activity than either cytokine or AZT used alone. In addition, cloned RT from Moloney murine leukemia virus (MMLV) was directly sensitive to inhibition by biphalin. Thus, the anti-FLV effects of splenocytes in combination with biphalin and AZT in cell culture are likely mediated to a large degree by the direct effect of cytokines. This antiviral activity of splenocytes or cytokines combined with chemotherapy, biphalin, and/or AZT, could be used as a complementary therapy to current approaches for retroviral infection and benefit acquired immunodeficiency syndrome (AIDS) patients. In conclusion, biphalin applied primarily as a new medicine for chronic pain treatment in AIDS patients may play a significant beneficial role as a component of antiviral HIV multidrug therapies.

Bioimmunotherapy of rodent malaria: co-treatment with recombinant mouse granulocyte-macrophage colony-stimulating factor and an enkephalin fragment peptide Tyr-Gly-Gly

We have earlier shown that recombinant mouse granulocyte-macrophage colony-stimulating factor (rmGM-CSF) and methionine-enkephalin co-treatment can protect mice from malaria. We now report the bioimmunotherapeutic effect of rmGM-CSF and a synthetic enkephalin fragment peptide Tyr-Gly-Gly (TGG) co-treatment on blood-induced Plasmodium berghei infection in Swiss mice. Mice were completely aparasitimic following co-treatment with rmGM-CSF (10.0 microg/kg) and TGG (2.0 mg/kg x 3 per day, intraperitoneally (i.p.)) starting from day -1 to day +4; however, in monotherapy, neither of these agents showed any detectable bioimmunotherapeutic effect. Curiously, similar co-treatment with rmGM-CSF (10.0 microg/kg) and higher doses of TGG (10.0 mg/kg) did not protect the mice. The combined bioimmunotherapeutic effect of these agents was abrogated by the separate administration each of rabbit neutralizing anti-rmGM-CSF antibody, non-selective opioid receptor antagonist naltrexone (10.0 mg/kg x 6 per day, i.p.), and silica (3.0 mg per mouse, intravenously (i.v.)). The peritoneal and splenic macrophages from the protected mice showed a significant (P<0.05) increase in their pool-size and the phagocytic activity, ex vivo. Furthermore, the protected mice, as compared to the unprotected ones, showed a significant (P<0.05) maximum increase in their serum nitrate and nitrite, interferon-gamma (IFN-gamma), and tumor necrosis factor-alpha (TNF-alpha) levels in their splenic homogenates, on the day before the beginning of the resolution of parasitaemia. Selective inhibitors of both inducible (aminoguanidine) and all forms (L-N(G)-monomethyl arginine) of nitric oxide (NO) synthase, significantly (P<0.05) augmented the mortality of co-treated mice, suggesting the role of NO in protection. These data show that, in P. berghei-infected mice, co-treatment with rmGM-CSF and conditional doses of TGG can impart protection, apparently through partly NO-dependent and macrophage-mediated mechanism(s).

Antinociceptive structure-activity studies with enkephalin-based opioid glycopeptides

Development of opioid peptides as therapeutic agents has historically been limited due to pharmacokinetic issues including stability and blood-brain barrier (BBB) permeability. Glycosylation of opioid peptides can increase peptide serum stability and BBB penetration. To further define the requirements for optimizing in vivo antinociceptive potency following intravenous administration, we synthesized a series of enkephalin-based glycopeptides using solid phase 9-fluorenylmethyloxy carbamate methods. The compounds differed in the sixth and subsequent amino acid residues (Ser or Thr) and in the attached carbohydrate moiety. In vitro binding and functional smooth muscle bioassays indicated that the addition of mono- or disaccharides did not significantly affect the opioid receptor affinity or agonist activity of the glycopeptides compared with their unglycosylated parent peptides. All of the glycopeptides tested produced potent antinociceptive effects in male ICR mice following intracerebroventricular injection in the 55 degrees C tail-flick test. The calculated A(50) values for the Ser/Thr and monosaccharide combinations were all very similar with values ranging from 0.02 to 0.09 nmol. Selected compounds were administered to mice intravenously and tested for antinociception to indirectly assess serum stability and BBB penetration. All compounds tested produced full antinociceptive effects with calculated A (50) values ranging from 2.2 to 46.4 micromol/kg with the disaccharides having potencies that equaled or exceeded that of morphine on a micromoles per kilogram basis. Substitution of a trisaccharide or bis- and tris-monosaccharides resulted in a decrease in antinociceptive potency. These results provide additional support for the utility of glycosylation to increase central nervous system bioavailability of small peptides and compliment our ongoing stability and blood-brain barrier penetration studies.

[Clinical response to agonists of mu and beta opiate receptors in patients with ischemic heart disease: effects of D-Ala2-Leu5-Arg6-enkephalin on hemodynamics, oxygen balance and lipid spectrum of blood]

Effects of intravenous dalargin daily administered in a dose of 28 micrograms/kg during 7 days on hemodynamics, oxygen balance, lipid spectrum were studied in patients with coronary heart disease. The results of the treatment show that dalargin decreases incidence of anginal attacks, peripheral vascular resistance, venous tonicity, increases exercise tolerance, peripheral blood flow and venous flow. In addition, such treatment reduces oxygen consumption, levels of total cholesterol, triglycerides, low density lipoprotein cholesterol.

Epidural opioid analgesia in infant rats I: mechanical and heat responses

The aim of this study was to investigate the analgesic effects of epidural opioids in neonatal rat pups. The contribution of individual opioid receptor subtypes in the spinal cord to analgesia at different developmental stages was investigated using epidural mu (morphine sulphate), delta (DPDPE) and kappa (U69593) opioid receptor agonists in neonatal rats aged postnatal day (P) 3, 10 and 21. Thresholds for flexion withdrawal reflexes to mechanical stimuli (von Frey hairs) and to noxious heating of the hind paw were low in neonates and increased with postnatal age. The analgesic action of each opioid receptor agonist followed an individual developmental pattern. In mechanical tests, all three opioid agonists were considerably more efficacious analgesics in younger animals and ED50s at P3 were always lower than at P21. In heat tests, the pattern differed. The efficacy of the kappa opioid agonist decreased with postnatal age, morphine efficacy increased over the same period and the effects of the delta agonist remained relatively unchanged. The distribution and concentration of tritiated morphine in the spinal cord following epidural administration did not alter significantly with postnatal age, suggesting that opioid access is not a major determinant of the effects reported here. It is concluded that whereas heat pain is particularly sensitive to spinal kappa opioids in neonates, mechanical sensory thresholds are generally sensitive to all spinal opioids in the newborn. The differing epidural opioid requirements compared to older subjects is likely to be due to developmental changes in spinal cord opioid receptor distribution or pharmacology.

Epidural opioid analgesia in infant rats II: responses to carrageenan and capsaicin

The aim of this study was to investigate the analgesic effects of epidural opioids upon persistent pain sensitivity in neonatal rat pups. Two models of persistent pain were used, subcutaneous injection of carrageenan, and topical application of capsaicin cream, both to the hind paw. The contribution of individual opioid receptor subtypes in the spinal cord to analgesia were tested at different developmental stages using epidural mu (morphine sulphate), delta (DPDPE) and kappa (U69593) opioid receptor agonists in neonatal rats aged P (postnatal day) 3, 10 and 21. Rat pups at all three ages displayed a reduction in mechanical (von Frey hair) threshold following carrageenan-induced inflammation of the hind paw that was evident at 3 h and was still present 5 h after application. This effect was greatest in magnitude at P21. This response was blocked by low doses of all three agonists at all ages, relative effectiveness varying with age. Comparison with potencies in acute tests (Marsh, D., Dickenson, A., Hatch, D. and Fitzgerald, M., Epidural opioid analgesia in infant rats I: mechanical and heat responses, Pain 82 (1999) 23-32) show that opioid potency is significantly greater in the presence of carrageenan inflammation at all ages. Topical capsaicin application to the hind paw produced a significant fall in withdrawal latencies to noxious heat. Generally, epidural opioid agonists did not block this C-fibre induced sensitization except at P3, when morphine and DPDPE did prevent the fall in threshold in a dose dependent manner. The results show that newborn rat pups are capable of displaying both allodynia and hyperalgesia following experimental inflammation that is blocked by epidural mu, delta and kappa opioids. The opioid potency is enhanced compared with antinociception in acute tests. This is not observed following capsaicin hyperalgesia and is therefore not a general consequence of C fibre induced increases in central excitability but relies upon mechanisms special to inflammatory pain.

Activity of tachykinin NK1 and bradykinin B2 receptor antagonists, and an opioid ligand at different stimulation parameters in neurogenic inflammation in the rat

Stimulation of the saphenous nerve in the anaesthetised rat results in cutaneous neurogenic oedema formation. We have examined the effect of a tachykinin NK1 and a bradykinin B2 antagonist, and a mu-opioid agonist on plasma extravasation observed in response to two differing nerve stimulating parameters (10 V, 1 ms, 2 Hz and 25 V, 2 ms, 10 Hz). The NK1 antagonist SR140333 abolished oedema, supporting the theory that an NK1 agonist is a primary mediator of neurogenic oedema. The B2 antagonist HOE 140 had no effect, indicating a lack of involvement of B2 receptors in this response. The pre-junctionally acting mu-opioid agonist DAMGO significantly inhibited oedema formation at the 10 V, 1 ms, 2 Hz (P < 0.001), but not the 25 V, 2 ms, 10 Hz stimulation parameters. Thus a post-junctionally acting NK1 antagonist inhibited neurogenic oedema formation induced by both stimulation parameters, whilst a pre-junctionally acting mu-opioid agonist acted only at 10 V, 1 ms, 2 Hz parameters. These findings could be of interest with respect to therapeutic approaches of pathophysiological conditions which involve a neurogenic component.

Ischemic preconditioning in the intact rat heart is mediated by delta1- but not mu- or kappa-opioid receptors

BACKGROUND

Our laboratory has previously shown that delta-opioid receptors are involved in the cardioprotective effect of ischemic preconditioning in the rat heart. However, this class of receptors consists of two subtypes, delta1, and delta2, and mu- or kappa-opioid receptors may also exist in the heart. Therefore, the purpose of the present study was to test the hypothesis that ischemic preconditioning is mediated through stimulation of one or both delta-opioid receptor subtypes.

METHODS AND RESULTS

Anesthetized, open chest, male Wistar rats were assigned to 1 of 14 groups. All animals were subjected to 30 minutes of occlusion and 2 hours of reperfusion. Ischemic preconditioning was elicited by three 5-minute occlusion periods interspersed with 5 minutes of reperfusion. Two doses of 7-benzylidenenaltrexone (BNTX; 1 and 3 mg/kg i.v.), a selective delta1-opioid receptor antagonist, or naltriben (NTB; 1 and 3 mg/kg i.v.), a selective delta2-opioid receptor antagonist, were given before ischemic preconditioning. To test for a role of mu-opioid receptors, rats were pretreated with beta-funaltrexamine (beta-FNA; 15 mg/kg s.c), an irreversible mu-opioid receptor antagonist, 24 hours before ischemic preconditioning or given the mu-opioid receptor agonist D-Ala,2N-Me-Phe,4glycerol5-enkephalin (DAMGO) as three 5-minute infusions (1, 10, and 100 microg/kg per infusion i.v., respectively) interspersed with 5-minute drug-free periods before the prolonged ischemic and reperfusion periods (lowDAMGO, medDAMGO, and hiDAMGO, respectively). The involvement of kappa-opioid receptors was tested by administering one of two doses of nor-binaltorphimine (nor-BNI; 1 and 5 mg/kg i.v.) before ischemic preconditioning. Infarct size (IS) as a percent of the area at risk (AAR) was measured by triphenyltetrazolium stain. Ischemic preconditioning markedly reduced IS/AAR (14+/-4%, P<.05) compared with control (55+/-4%). NTB, beta-FNA, and nor-BNI were unable to block the cardioprotective effect of ischemic preconditioning. In addition, DAMGO had no effect on IS/AAR. However, the high dose of BNTX (3 mg/kg i.v.) significantly attenuated the cardioprotective effect of ischemic preconditioning (39+/-5%; P<.05 versus control and ischemic preconditioning).

CONCLUSIONS

These results indicate that delta1-opioid receptors play an important role in the cardioprotective effect of ischemic preconditioning in the rat heart.

Ischemic preconditioning in the intact rat heart is mediated by delta1- but not mu- or kappa-opioid receptors

BACKGROUND

Our laboratory has previously shown that delta-opioid receptors are involved in the cardioprotective effect of ischemic preconditioning in the rat heart. However, this class of receptors consists of two subtypes, delta1, and delta2, and mu- or kappa-opioid receptors may also exist in the heart. Therefore, the purpose of the present study was to test the hypothesis that ischemic preconditioning is mediated through stimulation of one or both delta-opioid receptor subtypes.

METHODS AND RESULTS

Anesthetized, open chest, male Wistar rats were assigned to 1 of 14 groups. All animals were subjected to 30 minutes of occlusion and 2 hours of reperfusion. Ischemic preconditioning was elicited by three 5-minute occlusion periods interspersed with 5 minutes of reperfusion. Two doses of 7-benzylidenenaltrexone (BNTX; 1 and 3 mg/kg i.v.), a selective delta1-opioid receptor antagonist, or naltriben (NTB; 1 and 3 mg/kg i.v.), a selective delta2-opioid receptor antagonist, were given before ischemic preconditioning. To test for a role of mu-opioid receptors, rats were pretreated with beta-funaltrexamine (beta-FNA; 15 mg/kg s.c), an irreversible mu-opioid receptor antagonist, 24 hours before ischemic preconditioning or given the mu-opioid receptor agonist D-Ala,2N-Me-Phe,4glycerol5-enkephalin (DAMGO) as three 5-minute infusions (1, 10, and 100 microg/kg per infusion i.v., respectively) interspersed with 5-minute drug-free periods before the prolonged ischemic and reperfusion periods (lowDAMGO, medDAMGO, and hiDAMGO, respectively). The involvement of kappa-opioid receptors was tested by administering one of two doses of nor-binaltorphimine (nor-BNI; 1 and 5 mg/kg i.v.) before ischemic preconditioning. Infarct size (IS) as a percent of the area at risk (AAR) was measured by triphenyltetrazolium stain. Ischemic preconditioning markedly reduced IS/AAR (14+/-4%, P<.05) compared with control (55+/-4%). NTB, beta-FNA, and nor-BNI were unable to block the cardioprotective effect of ischemic preconditioning. In addition, DAMGO had no effect on IS/AAR. However, the high dose of BNTX (3 mg/kg i.v.) significantly attenuated the cardioprotective effect of ischemic preconditioning (39+/-5%; P<.05 versus control and ischemic preconditioning).

CONCLUSIONS

These results indicate that delta1-opioid receptors play an important role in the cardioprotective effect of ischemic preconditioning in the rat heart.

Treatment of chronic allodynia in spinally injured rats: effects of intrathecal selective opioid receptor agonists

We examined the effects of intrathecal (i.t.) selective opioid receptor agonists in alleviating mechanical and cold allodynia in spinally injured rats. Both DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, a mu-opioid receptor agonist) and DPDPE ([D-Phe2,D-Phe5]-enkephalin, a delta-opioid receptor agonist) dose-dependently relieved the chronic allodynia-like behavior at doses selective for their respective receptors. The anti-allodynic effect of DAMGO and DPDPE was reversed by the selective mu- and delta-opioid receptor antagonists CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) and naltrindole, respectively. In contrast, the selective kappa-opioid receptor agonist U50488H did not alleviate the allodynia-like behavior, but rather enhanced it. The anti-nociceptive and anti-allodynic effect of i.t. DAMGO was blocked by U50488H. Thus, activation of spinal mu- and delta-, but not kappa-opioid receptors produced anti-allodynic effect in this model of central pain. Drugs which act selectively on opioid receptor subtypes may be useful in managing chronic central pain of spinal cord origin.

Lumbar but not cervical intrathecal DAMGO suppresses extrasegmental nociception in awake rats

he effect of intrathecally administered [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) on withdrawal latencies evoked by noxious heat applied to either cervical or lumbar dermatomes was studied in awake rats. Administration of DAMGO to the lumbar intrathecal space produces a dose-dependent suppression of withdrawals evoked by noxious thermal stimulation in either lumbar or cervical dermatomes. Administration of the same doses of DAMGO to the cervical spinal cord produces a suppression of withdrawals evoked by stimulation in cervical but not lumbar dermatomes. Control experiments provide evidence that the drugs administered intrathecally to either enlargement do not spread to the other enlargement.

Antinociceptive effects of dynorphin peptides in a model of inflammatory pain

Dynorphin A (DYN) peptides, administered into the central nervous system, have produced inconsistent analgesic actions in tests using thermal stimuli. This study examined antinociceptive effects of intravenous and intraplantar DYN-(2-17) against noxious pressure in rats with Freund's adjuvant-induced unilateral hindpaw inflammation. The effects of DYN-(2-17) were compared to those of the opioid agonists morphine. (D-Ala2,N-Methyl-Phe4,Gly-ol5)-enkephalin (DAMGO) and DYN-(1-17). Intravenous DYN-(2-17) (0.188-10 mg/kg) produced dose-dependent elevations of paw pressure thresholds in inflamed and in non-inflamed paws. These effects were similar in magnitude to those of subcutaneous morphine (2 mg/kg), at doses of 0.375-1.5 mg/kg they were significantly greater on the inflamed (right) than on the non-inflamed (left) paw, and they were not reversible by intravenous naloxone (1-10 mg/kg). Intraplantar Dyn-(2-17)(0.001-0.3 mg) was ineffective, whereas both intraplantar DYN-(1-17)(0.15-0.3 mg) and DAMGO (0.008-0.016 mg) produced dose-dependent and naloxone-reversible elevations of paw pressure thresholds. The intraplantar injection of both Dyn peptides produced a transient increase in the volume of non-inflamed paws. These findings suggest that intravenous DYN-(2-17) produces possibly centrally mediated, non-opioid antinociceptive effects against noxious pressure. At certain doses these effects are more potent in inflamed than in non-inflamed paws. In contrast to the opioid peptides DYN-(1-17) and DAMGO, DYN-(2-17) does not appear to have no peripheral antinociceptive actions.

[The mechanism of the anti-arrhythmia action of mu-opioid-receptor agonists in a model of CaCl2-induced arrhythmias: the role of the autonomic nervous system]

It was established that selective ligands of the mu-opioid receptors DAGO and DALDA exhibit antiarrhythmic activity on a model of CACl2-induced arrhythmias when infused intravenously in a dose of 0.2 mg/kg. The autonomic nervous system does not take part in realization of the antiarrhythmic effect of DAGO and DALDA. It is supposed that this effect of DAGO and DALDA is associated with activation of cardiac mu-opioid receptors.

Analysis of the role of delta opioid receptors in gastroprotection in the rat

The effect of delta opioid agonists - [D-Ala2, D-Leu5]-enkephalin (DADLE), [D-Pen2, D-Pen5]-enkephalin (DPDPE) and deltorphin II - on acidified ethanol induced gastric mucosal lesions was studied in the rat compared with that of morphine. It was found that DADLE, DPDPE, deltorphin II and morphine exerted a dose-dependent inhibition on the mucosal lesions injected subcutaneously, their ID50 values were 0.037, 1.8, 3.5 and 0.35 micromoles/kg, respectively. Naltrindole (10 mg/kg sc.), the selective delta opioid receptor antagonist, inhibited the gastroprotective effect of DADLE, DPDPE and deltorphin II, but it failed to antagonise the effect of morphine. Our results suggest that 1. delta receptors are involved in opioid-mediated gastroprotection, 2. ethanol-induced gastric mucosal damage in the rat may be a quick, simple in vivo model for screening opioid delta receptor agonists and antagonists in the periphery.

Competitive and non-competitive NMDA antagonists block the development of antinociceptive tolerance to morphine, but not to selective mu or delta opioid agonists in mice

N-Methyl-D-aspartate (NMDA) receptor antagonists have been shown to block the development of antinociceptive tolerance to morphine. Assessment of the effects of NMDA antagonists on development of antinociceptive tolerance to selective opioid mu (mu) and delta (delta) agonists, however, has not been reported. In these experiments, selective mu and delta receptor agonists, and morphine, were repeatedly administered to mice either supraspinally (i.c.v.) or systemically (s.c.), alone or after pretreatment with systemic NMDA antagonists. Antinociception was evaluated using a warm-water tail-flick test. Repeated i.c.v. injections of mu agonists including morphine, fentanyl, [D-Ala2, NMePhe4, Gly-ol]enkephalin (DAMGO) and Tyr-Pro-NMePhe-D-Pro-NH2 (PL017) or [D-Ala2, Glu4]deltorphin, a delta agonist, or s.c. injections of morphine or fentanyl, produced antinociceptive tolerance as shown by a significant rightward displacement of the agonist dose-response curves compared to controls. Single injections or repeated administration of MK801 (a non-competitive NMDA antagonist) or LY235959 (a competitive NMDA antagonist) at the doses employed in this study did not produce behavioral toxicity, antinociception or alter the acute antinociceptive effects of the tested opioid agonists. Consistent with previous reports, pretreatment with MK801 or LY235959 (30 min prior to agonist administration throughout the tolerance regimen) prevented the development of antinociceptive tolerance to i.c.v. or s.c. morphine. Neither NMDA antagonist, however, affected the development of antinociceptive tolerance to i.c.v. fentanyl, DAMGO, or [D-Ala2, Glu4]deltorphin. Additionally, MK801 pretreatment did not affect the development of antinociceptive tolerance to i.c.v. PL017 or to s.c. fentanyl. Further, MK801 pretreatment also did not affect the development of tolerance to the antinociception resulting from a cold-water swim-stress episode, previously shown to be a delta-opioid mediated effect. These data lead to the suggestion that the mechanisms of tolerance to receptor selective mu and delta opioids may be regulated differently from those associated with morphine. Additionally, these findings emphasize that conclusions reached with studies employing morphine cannot always be extended to 'opiates' in general.

Beta-endorphin but not metenkephalin counteracts adenosine-provoked angina pectoris-like pain

Whether i.v. infusion of beta-endorphin or metenkephalin can modify adenosine-provoked angina pectoris-like pain was investigated in healthy volunteers with a double-blind controlled design. All seven volunteers experienced dose-related adenosine-provoked chest pain. Metenkephalin did not modulate the dose-effect curve for adenosine while beta-endorphin counteracted (p < 0.01) the development of pain. The results suggest that peripheral p.subtype opioid receptors are involved in the modulation and may play a role in the genesis of silent and painful myocardial ischaemia.

[A pharmacological analysis of the antiedematous action of GABA-ergic and opioidergic agents in craniocerebral trauma]

Experiments on rats with skull inquiry show that biculline (dose 2 mg/kg) removes antiedematous effect of fenibut (50 mg/kg), sodium hydroxybutirate (200 mg/kg), promedol (1 mg/kg), and synthetic analogs of encephalines DAGO and DSLET (100 mg/kg). Naloxon at a dose 1 mg/kg blocks antiedematous effect of ligands of opiate receptors, but does not change the effect of fenibut and sodium hydroxybutirate. The presence of close inter-regulatory interactions between GAMK-ergic and opioidergic systems in forming the traumatic correction is suggested.

Peripheral opioid modulation of pain and inflammation in the formalin test

The effects of local treatment with opioid receptor agonists on the early (0-10 min) and late (20-40 min) behavioural response and extravasation induced by intraplantar injection of 1% formalin in rats were examined. The mu-opioid receptor agonist [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) depressed pain behaviour in the late phase, and extravasation in both phases. The kappa-opioid receptor agonist trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] benzeneacetamide methanesulfonate (U50,488H) suppressed the behavioural response in both phases, but extravasation was enhanced in the early phase and not altered in the late phase. The delta-opioid receptor agonist [D-Pen2,5]enkephalin (DPDPE) enhanced the behavioural response in the late phase, but inhibited extravasation in the both early and late phases. Systemic injection of the agonists had no effects, and pretreatment with s.c. naloxone methiodide reversed the effects of locally administered agonists. These data (1) support the notion that different pathophysiological mechanisms underlie the two phases of the formalin test, and (2) indicate that depending on the receptor specificity, opioid receptor agonists have both pro- and antinociceptive effects, as well as pro- and antiinflammatory activity.

[The mechanism of the anti-arrhythmia action of opioid receptor agonists and antagonists]

Enkephalins were injected intravenously at a dose of 0.1 mg/kg 15 minutes or 6 hours before adrenalin or CaCl2 injection. Enkephalins were reported to prevent adrenal ventricular extrasystoles but not to influence CaCl2-induced dysrhythmias. Maximum antiarrhythmic effect of enkephalins was demonstrated 6 hours later after the intravenous injection. Naloxone at a dose of 2 mg/kg and morphine at a dose of 1.5 mg/kg prevent adrenal arrhythmias 15 min and 6 hours later after injection. We believe that peripheral delta opiate receptors activation by enkephalins as well as the blockade of non-identified opiate receptors by naloxone prevent arrhythmias.

Selective opioid receptor agonists modulate mechanical allodynia in an animal model of neuropathic pain

This study evaluated the antinociceptive effects of systemically administered selective opioid agonists of mu (DAMGO), delta (BUBU) and kappa (U 69593) receptors on the vocalization threshold to paw pressure in a rat model of peripheral unilateral mononeuropathy produced by loose ligatures around the common sciatic nerve. DAMGO (0.5-2 mg/kg), BUBU (1.5-6 mg/kg) and U 69593 (0.75-3 mg/kg) injected intravenously (i.v.) produced a potent long-lasting antinociceptive effect on both hind paws. The effects on the lesioned paw were clearly and statistically more potent than for the non-lesioned paw. The selective antinociceptive effect of 2 mg/kg DAMGO, 3 mg/kg BUBU and 1.5 mg/kg U 69593 were completely prevented by prior administration of the appropriate antagonists: 0.1 mg/kg naloxone, 1 mg/kg naltrindole and 0.4 mg/kg MR 2266. The present data clearly show that an acute i.v. injection of these selective opioid agonists induces potent antinociceptive effects in a rat model of peripheral neuropathy. These data are discussed with regard to the classical view that there is opioid resistance in neuropathic pain.

Prevention of stress-induced gastric ulcers by mu- and delta-opioid agonists in the rat

The effects of intraperitoneal and intracerebroventricular administration of mu- and delta- selective opioid receptor agonists (DAGO and DPDPE, respectively) on gastric lesions, were investigated in cold-restraint-stressed rats. DAGO and DPDPE, peripherally and centrally administered, induced a significant gastric protection. Naloxone prevented the effects of both opioids whereas naltrindole prevented the gastric protection induced by DPDPE but not that by DAGO. The results suggest that mu- and delta-opioid agonists prevent gastric damage induced by stress through an involvement of both central and peripheral mu- and delta-opioid receptor subtypes.

[The significance of the mu- and delta-opiate receptors in realizing enkephalin action on the course of hypoxic hypoxia]

New enkephalins analogues have been synthesized. They are characterized by linear, cyclic and branched peptide chain. A relationship has been established between antihypoxic activity of opioid peptides an their interaction with opiate receptors. Compounds efficiently interacting with mu-receptors irrespective of delta-receptors affinity, promote longer survival of mice in hypoxia. The antihypoxic effect of opioids is proportional to their specificity to mu-receptors.

Enhanced potency of receptor-selective opioids after acute burn injury

Dose-response curves of three receptor-selective opioids were established in a group of nonburned and a group of burned rats. Morphine (mu-agonist), biphalin (mu- and delta-agonist), and U50488H (kappa-agonist) were administered to each group, and analgesia was measured by tail flick latency testing. Each opioid had a significant increase in potency (i.e., a decrease in ED50 values) in the burned (15% body surface area) compared with the nonburned groups. Moderate doses of each drug (i.e., ED50 doses estimated from nonburned group data) in each case augmented stress-induced analgesia in the burned group. Analgesic doses failed to prevent a significant increase in plasma beta-endorphin and corticosterone after larger surface area (25%) burns. Regardless of receptor specificity, opioid analgesic potency is increased acutely after burn injuries.

[Participation of central and peripheral mu- and delta opiate receptors in anti-arrhythmia action of enkephalins]

It was found, that injection of delta-receptors agonist dalargin before the occlusion of left anterior coronary artery in rats prevented the decrease of ventricular fibrillation threshold (VFT). An injection of naloxone in dose 0.5 mg/kg (for the blockade of mu-receptors only) had no influence on the VFT. Naloxone in dose 1 mg/kg (for the blockade peripheric mu- and delta-receptors) decreased VFT. An intraventricular infusion of dalargin (10 mkg) induced bradycardia and an increase of VFT. It was assumed that anti-arrhythmic effects of enkephalins in acute myocardial ischemia could be realized by an activation of peripheric delta-receptors and central mu-receptors.

[The effect of opiate receptor agonists on the blood circulation in rats with hemorrhagic shock]

The effect of intravenous infusion of agonists of mu-(DAGO) and delta-(DADL, DAAE) opiate receptors on mean arterial pressure and heart rate was studied in experiments on rats with dosed blood loss of up to 30% of calculated blood volume. Stimulation of mu-opiate receptors inhibited the drop of mean arterial pressure in hemorrhagic shock, whereas stimulation of delta-opiate receptors failed to produce such an effect. Preliminary injection of naloxone blocked the effects of mu-receptor stimulation.

[Effect of opiate receptor agonists on the course of hemorrhagic shock in rats]

The experiments have been performed on 93 male rats, weighing 200-250 g. In acute blood loss various arterial pressure (AP) changes have been demonstrated--the marked hypertension is being changed by gradual AP increase. The injection of m-receptors' agonist DAGO prevents systolic and diastolic AP increase, agonist DADL prevents diastolic AP increase in acute momentary blood loss. In gradual blood loss DAGO (more than DADL) slows down both the decrease and the subsequent AP increase in rats. DAGO is determined to decrease, and DADL--to increase the minute blood volume. The mechanisms of opioids' action and their significance in pathogenesis of hemodynamic disturbances in shock are being discussed.

[Antihypoxic properties of endorphins, enkephalins and their analogs]

The models of hypoxic hypoxia have been created in the experiments on mice by two ways: placing them into hermetic chamber or "lifting" them to 10.500-10.700 metres in the altitude chamber. The influence of enkephalins and their 12 analogs on the resistance of mice to hypoxia was tested. Enkephalin analogs with antihypoxic activities were detected using both models. It was shown that the mechanism of antihypoxic influence of opioids involves stimulation of their mu- and sigma-receptors and that other neurochemical systems of the body also take part in the realization of antihypoxic effects of the peptides. It is suggested that leu-enkephalin and des-tyr1-gamma-endorphin play, most likely, a role of endogenous antihypoxic agents.

[Role of the peripheral catecholaminergic systems in the antistress action of neuropeptides]

The mediator activity of the peripheral catecholaminergic systems (the adrenergic nerves of dura mater and the concentrations of noradrenaline and adrenaline in the adrenals of rats) during dynamic and immobilization stress was investigated with the help of fluorescent microscopy and spectrofluorometry. Neuropeptides--dalargin and another enkephalin analog--were injected intraperitoneally, 150 mg/kg. A visible antistress action of these neuropeptides has been demonstrated, it was more marked after treatment with dalargin. The role of peripheral catecholaminergic system in the mechanism of stress-protective effects of neuropeptides is discussed.

The antimetastatic activity of enkephalin-like peptides

[Met5]Enkephalin and [Leu5]enkephalin, two naturally occurring opioid-like peptides were found to decrease the number of spontaneous pulmonary metastases in C57BL/6J mice implanted in the foot pad with the B16-BL6 melanoma. The number of such metastatic nodules were reduced more than 2-fold. This effect was noted after several different injection schedules. In addition to their effect on metastasis the growth rate of the primary tumor was also decreased by these compounds.

[Effect of synthetic analogs of enkephalins, morphine and their antagonists on the course of experimental traumatic shock]

Effects of naloxone, nalorphine, thyroliberin, morphine and two analogues of enkephalins (FK 33-824 and Tyr-D-Ala-Gly-Phe(NO2)-NH2) on the course of traumatic shock were studied in experiments on rabbits. It was found that antagonists of opioid peptides aggravated the course of traumatic shock and morphine and synthetic analogues of enkephalins exerted positive effects during its treatment. Endogenous opioid peptides are suggested to play the protective role in experimental traumatic shock.

[Protective action of endogenous opioid-like peptides of different origins in duodenal ulcer in rats]

A study was made of the effects of some endogenous opioids (beta-endorphin, gamma-endorphin, met-enkephalin, leu-enkephalin and dinorphin) formed in the body from different high-molecular precursors (pro-opiomelanocortin, proenkephalins A and B) on the development in rats of the cysteamine-induced duodenal ulcers. All the peptides under study, gamma-endorphin, in particular, had an anti-ulcerous activity which was mediated by specific opiate receptors. The majority of the opioids was characterized by reduction of the anti-ulcerous effect as the dose was raised. It is assumed that protection of the duodenal mucosa under ulcerogenic exposures is an essential property of endogenous peptides. It is concluded that opioid peptides derived from different precursors are arranged in a complex synergic system responsible for cytoprotection of the duodenum.

Antinociceptive profiles of opioid peptide agonists in a rat tooth pulp stimulation procedure

The analgesic activity of the prototypic opioid peptides for the mu (D-Ala2-Me-Phen4-Gly-ol5-enkephalin [DAGO]) kappa (Dynorphin 1-13), delta (D-Ala2-D-Leu5-enkephalin [DADLE]), or epsilon (beta-endorphin) receptor was assessed in a rat tooth pulp stimulation procedure. All opioid peptides tested and the opioid alkaloid U50, 488H (kappa receptor agonist) significantly elevated response thresholds. The rank order of potency based on the Minimum Effective Dose values was beta-endorphin greater than DAGO = dynorphin A (1-13) amide greater than DADLE greater than dynorphin A (1-13) greater than U50,488H. Based on absolute magnitude, the rank order of dose response slopes was DAGO greater than U50,488H greater than dynorphin A (1-13) amide greater than beta-endorphin greater than DADLE. Dynorphin A (1-13) produced the shallowest dose response slope and the magnitude of response threshold was the lowest for all compounds tested. Finally, the general conclusion that mu agonists are effective against noxious stimuli derived from thermal, chemical, and mechanical is extended by our data to include electrical sources derived from tooth pulp stimulation; kappa agonists are effective against noxious stimuli derived from chemical, mechanical, and electrical sources (tooth pulp stimulation) and delta agonists are effective analgesics against thermal, chemical and electrical stimuli (tooth pulp stimulation).

Analgesic efficacy of parenteral metkephamid acetate in treatment of postoperative pain

The analgesic efficacy and side-effects of a single parenteral dose of metkephamid acetate 70 mg were compared with those of pethidine (meperidine) hydrochloride 100 mg and placebo in a double-blind, randomised, controlled clinical trial. 30 out of 32 postoperative patients completed the study--10 in the metkephamid group, 11 in the pethidine group, and 9 in the placebo group. The time-effect curves of summated pain measures and analyses of derived measures all indicated that the analgesic activity of metkephamid 70 mg was significantly greater than that of placebo and not less than that of pethidine 100 mg. The metkephamid group had a greater incidence of side-effects than the other two treatment groups. Some side-effects, such as sensations of heaviness of the extremities and nasal congestion, were peculiar to metkephamid but not distressing.

[Effect of enkephalin and cimetidine on the occurrence and course of duodenal ulcers in rats]

A study was made of the effect of a synthetic enkephalin analog and cimetidinee on experimental duodenal ulcer in rats induced by cysteamine administration. The ulcers were demonstrated to heal within 4 weeks. It was noted that ulcer occurrence was preceded by the increased serum gastrin level. Administration to rats of the enkephalin analog accelerated ulcer healing to a greater extent than that of cimetidin. It is suggested that one of the mechanisms of enkephalin analog protective action might involve the prevention of the gastrin increased level.