L-Arginine supplementation prevents allodynia and hyperalgesia in painful diabetic neuropathic rats by normalizing plasma nitric oxide concentration and increasing plasma agmatine concentration.

PURPOSE: Neuropathic pain is a common diabetic complication. It is characterized by symptoms of spontaneous and stimulus-evoked pain including hyperalgesia and allodynia. L-Arginine is a common precursor of many metabolites of biological interest, in particular, nitric oxide (NO), ornithine, and hence polyamines. In central nervous system, NO, glutamate, and polyamines share an N-methyl-D-aspartate (NMDA) receptor-mediated effect. We hypothesized that a variation in arginine metabolism caused by diabetes may contribute to development and maintenance of neuropathic pain and to the worsening of clinical and biological signs of diabetes. METHODS: We examined whether oral L-arginine supplementation (2.58 ± 0.13 g/l in drinking water for 3 weeks) could improve the development of neuropathic pain and the clinical, biological, and metabolic complications of diabetes in streptozocin (STZ)-induced diabetic (D) rats. RESULTS: STZ administration induced classical symptoms of type 1 diabetes. Diabetic rats also displayed mechanical hypersensitivity, tactile, and thermal allodynia. Plasma citrulline and NO levels were increased in diabetic hyperalgesic/allodynic rats. L-Arginine supplementation failed to reduce hyperglycaemia, polyphagia, and weight loss. Moreover, it abolished hyperalgesia and allodynia by normalizing NO plasma concentration and increasing plasma agmatine concentration. CONCLUSIONS: L-Arginine supplementation prevented the development of mechanical hyperalgesia, tactile, and thermal allodynia in painful diabetic neuropathy with concomitant reduction of NO and increased agmatine production, offering new therapeutic opportunities for the management of diabetic neuropathic pain.

Evidence for a differential opioidergic involvement in the analgesic effect of antidepressants: prediction for efficacy in animal models of neuropathic pain?

BACKGROUND AND PURPOSE: Antidepressants are one of the recommended treatments for neuropathic pain. However, their analgesic action remains unpredictable, and there are no selection criteria for clinical use. Better knowledge of their mechanism of action could help highlight differences underlying their unequal efficacy. EXPERIMENTAL APPROACH: We compared the activity of a tricyclic antidepressant (clomipramine) with selective 5-HT and noradrenaline reuptake inhibitors (milnacipran and duloxetine) in streptozocin-induced diabetic and chronic constriction nerve injury-induced neuropathic rats, after repeated injections. We looked for an opioidergic mechanism in their action. KEY RESULTS: Abolition of mechanical hyperalgesia was observed in mononeuropathic rats after five injections of clomipramine (5 mg·kg(-1) , s.c.) and milnacipran (10 or 20 mg·kg(-1) , i.p.) and in diabetic rats after clomipramine. An additional antinociceptive effect was obtained with five injections of duloxetine (3 mg·kg(-1) , i.p.) in both models and milnacipran (10 mg·kg(-1) , i.p.) in diabetic rats. These effects were observed with plasma antidepressant concentrations similar to those found in patients treated for neuropathic pain. Naloxone (1 mg·kg(-1) , i.v.) only suppressed the anti-hyperalgesic effects of clomipramine in both models of pain and of milnacipran in the traumatic model. CONCLUSIONS AND IMPLICATIONS: The opioid system appears to be involved in the mechanism of action of antidepressants that only have an anti-hyperalgesic effect but not in those that have a stronger (i.e. antinociceptive) effect. These differences between the antidepressants occurred whatever the aetiology of the neuropathy and, if confirmed in clinical trials, could be used to decide which antidepressant is administered to a patient with neuropathic pain.

Magnesium attenuates chronic hypersensitivity and spinal cord NMDA receptor phosphorylation in a rat model of diabetic neuropathic pain.

Neuropathic pain is a common diabetic complication affecting 8-16% of diabetic patients. It is characterized by aberrant symptoms of spontaneous and stimulus-evoked pain including hyperalgesia and allodynia. Magnesium (Mg) deficiency has been proposed as a factor in the pathogenesis of diabetes-related complications, including neuropathy. In the central nervous system, Mg is also a voltage-dependent blocker of the N-methyl-d-aspartate receptor channels involved in abnormal processing of sensory information. We hypothesized that Mg deficiency might contribute to the development of neuropathic pain and the worsening of clinical and biological signs of diabetes and consequently, that Mg administration could prevent or improve its complications. We examined the effects of oral Mg supplementation (296 mg l(-1) in drinking water for 3 weeks) on the development of neuropathic pain and on biological and clinical parameters of diabetes in streptozocin (STZ)-induced diabetic rats. STZ administration induced typical symptoms of type 1 diabetes. The diabetic rats also displayed mechanical hypersensitivity and tactile and thermal allodynia. The level of phosphorylated NMDA receptor NR1 subunit (pNR1) was higher in the spinal dorsal horn of diabetic hyperalgesic/allodynic rats. Magnesium supplementation failed to reduce hyperglycaemia, polyphagia and hypermagnesiuria, or to restore intracellular Mg levels and body growth, but increased insulinaemia and reduced polydipsia. Moreover, it abolished thermal and tactile allodynia, delayed the development of mechanical hypersensitivity, and prevented the increase in spinal cord dorsal horn pNR1. Thus, neuropathic pain symptoms can be attenuated by targeting the Mg-mediated blockade of NMDA receptors, offering new therapeutic opportunities for the management of chronic neuropathic pain.

Evidence for an involvement of tachykinins in allodynia in streptozocin-induced diabetic rats.

A better knowledge of the pathophysiology of chronic pain could help to improve the treatment of patients with such syndrome. The aim of the present work was to elucidate the possible involvement of spinal substance P and neurokinin A in the mechanical and thermal allodynia observed in streptozocin-induced diabetic rats. A tachykinin NK(1) receptor antagonist, RP-67,580 ((3aR,7aR) -7, 7-diphenyl-2-(1-imino-2(2-methoxy phenyl)-ethyl) perhydroisoindol-4-one hydrochloride), a tachykinin NK(2) receptor antagonist, SR-48,968 ((S)-N-methyl (4-(acetylamino-4phenylpiperidino)-2-(3, 4-dichlorophenyl) butyl) benzamide) and their respective enantiomers were intrathecally administered 4 weeks after the induction of diabetes. Mechanical and thermal allodynia were evaluated before and up to 60 min after injection. The tachykinin receptor antagonists at the highest doses (10 and 25 microgram) significantly reduced allodynia, their enantiomers being inactive. Both of these data suggest the involvement of substance P and neurokinin A in the neuropathy-induced allodynia and offer a novel hypothesis to treat chronic pain due to diabetes.

Potentiation of morphine and clomipramine analgesia by cholecystokinin -B antagonist CI-988 in diabetic rats.

The aim of this study was to determine the influence of an intrathecally injected cholecystokinin-B (CCK-B) receptor antagonist, CI-988, on the analgesic effect of morphine and clomipramine in diabetic rats. Administered alone, morphine (0.1 mg/kg, i.v.) and clomipramine (3 mg/kg, i.v.) have respectively no effect and only a slight effect on vocalization thresholds to paw pressure in diabetic rats, but, when coadministered with CI-988 (0.1 microg/rat, i.t.), an appreciable antinociceptive effect was observed. This suggests that a spinal blockade of cholecystokininergic system increases the analgesia induced by morphine or clomipramine. A CCK-B receptor antagonist could thus be used to lower dosages of morphine or antidepressant drugs in the management of neuropathic pain in humans, and thereby reduce their side effects.

Synthesis and analgesic effects of 5-[4-(arylpiperazin-1-yl)alkylamino]-4-benzyl-3-methyl-1,2-oxa zin-6-one s.

A series of 5-[4-(arylpiperazin-1-yl)alkylamino]-4-benzyl-3-methyl-1,2-oxaz in-6-ones was synthesized and evaluated for analgesic activity. The structures of these new oxazine derivatives were confirmed by IR, 1H-NMR spectra and by elemental analysis. The three most active compounds, 3c, 3e and 3g possessed significant antinociceptive effects in the phenylbenzoquinone-induced wrigthing test (PBQ-test) in mice, with ED50 values ranging from 19.7 to 68.0 mg/kg i.p. In addition these compounds presented a low toxicity (LD50 > 800 mg/kg i.p.) and did not significantly reduce the spontaneous locomotor activity of mice. They interacted in a synergistic manner with morphine but nevertheless each compound presented its own profile. Thus the analgesic activity of 3c and 3e was naloxone sensitive, suggesting in mu opioidergic mechanism. Otherwise 3c and 3d analgesia was attenuated by oral administration of yohimbine and therefore seemed to be mediated via noradrenergic pathway. Finally, 5-hydroxytryptophan associated to carbidopa only potentiated 3e analgesia, demonstrating an involvement of a serotoninergic mechanism.

Spinal effect of a neuropeptide FF analogue on hyperalgesia and morphine-induced analgesia in mononeuropathic and diabetic rats.

1DMe, a neuropeptide FF (NPFF) analogue, has been shown to produce antinociception and to enhance morphine analgesia in rats after intrathecal administration. To determine whether 1DMe could correct hyperalgesia and restore morphine efficacy in mononeuropathic (MN) and diabetic (D) rats we examined the spinal effect of 1DMe in MN and D rats without and after spinal blockade of mu- and delta-opioid receptors with CTOP and naltrindole, respectively. The influence of 1DMe on morphine-induced antinociception was assessed in the two models using isobolographic analysis. Whereas 1DMe intrathecally injected (0.1, 1, 7.5 microg rat(-1)) was ineffective in normal (N) rats, it suppressed mechanical hyperalgesia (decrease in paw pressure-induced vocalisation thresholds) in both MN and D rats. This effect was completely cancelled by CTOP (10 microg rat(-1)) and naltrindole (1 microg rat(-1)) suggesting that it requires the simultaneous availability of mu- and delta-opioid receptors. The combinations of morphine: 1DMe (80.6:19.4% and 99.8:0.2%, in MN and D rats, respectively) followed by isobolographic analysis, showed a superadditive interaction, relative to the antinociceptive effect of single doses, in D rats only. In N rats, the combination of morphine: 1DMe (0.5 mg kg(-1), i.v.: 1 microg rat(-1), i.t., ineffective doses) resulted in a weak short-lasting antinociceptive effect. These results show a different efficacy of 1DMe according to the pain model used, suggesting that the pro-opioid effects of the NPFF in neuropathic pain are only weak, which should contribute to hyperalgesia and to the impaired efficacy of morphine.

Effect of tachykinin receptor antagonists in experimental neuropathic pain.

The intrathecal effect of 0.1 to 10 microg of RP-67,580 (3aR,7aR)-7,7-diphenyl-2[1-imino-2(2-methoxyphenyl)-ethyl]++ +perhydroisoindol-4-one hydrochloride, CP-96,345 (2S,3S)-cis-(2(diphenylmethyl)-N-[(2-methoxyphenyl) methyl]-1-azabicyclo[2.2.2]octan-3-amine), SR-140,333 (S)-(1-¿2-[3-(3,4-dichlorophenyl)- 1-(3-isopropoxyphenylacetyl)piperidin-3-yl]ethyl¿-4-phenyl-1 -azonia-bicyclo[2.2.2.]-octane,chloride), all neurokinin (NK)1-receptor antagonists, SR-48,968 (S)-N-methyl-N[4-(4-acetylamino-4-[phenylpiperidino)-2-(3,4-dichlorophen yl)-butyl]benzamide, a tachykinin NK2 receptor antagonist and SR-142,801 (S)-(N)-(1-(3-(1-benzoyl-3-(3,4-dichlorophenyl) piperidin-3-yl)propyl)-4-phenylpiperidin-4-yl)-N-methyl acetamide, a tachykinin NK3 receptor antagonist, and of their respective inactive enantiomers on thresholds of vocalization due to a mechanical stimulus in mononeuropathic (sciatic nerve ligature) and diabetic rats, was examined. The tachykinin NK1 and the NK2 receptor antagonists were antinociceptive in both models, with a higher effect of the former in diabetic rats. The tachykinin NK3 receptor antagonist was weakly effective in diabetic rats only. This indicates a differential involvement of the tachykinins according to the model of neuropathic pain, suggesting a potential role for tachykinin receptor antagonists in the treatment of neuropathic pain.

Is the reduced efficacy of morphine in diabetic rats caused by alterations of opiate receptors or of morphine pharmacokinetics?

Because it generally is admitted that neuropathic pain is resistant to opioid analgesia, we investigated the effect of morphine on hyperalgesia in streptozocin-induced diabetes in rats. The antinociceptive effect of morphine (0.5-4 mg/kg i.v.) on mechanical (paw pressure test), thermal (tail immersion test) and chemical (formalin test) hyperalgesia was reduced. To clarify the mechanisms involved in the alteration of morphine analgesia, the binding characteristics of mu and delta receptor agonists and the pharmacokinetics of morphine and its glucuronide metabolites morphine 3-glucuronide and morphine 6-glucuronide were determined. KD and Bmax values for [3H][D-Ala2,(Me)Phe4, Gly(ol)5]enkephalin and [3H][D-Pen2,D-Pen5]enkephalin to cerebral mu and delta opiate receptors were not altered by diabetes. Likewise, the plasma maximal concentration of morphine and metabolites, as well as the area under the curve, did not differ between diabetic and normal rats. Only the total clearance and the apparent volume of distribution of morphine were increased in diabetic rats, which suggests that the diabetes-induced glycosylation of proteins might increase the distribution of morphine in the aqueous compartment. These data indicate that the reduced analgesic effect of morphine caused by diabetes cannot be explained by a decrease in opiate-receptor affinity or density but rather by kinetic alteration of morphine (increase of total clearance and of volume of distribution in comparison with healthy animals).

Daily insulin treatment relieves long-term hyperalgesia in streptozocin diabetic rats.

Painful neuropathy is common in human diabetes. In rats, experimental diabetes results in altered pain sensitivity. We examined the effect of chronic insulin treatment on diabetes-induced hyperalgesia in streptozocin diabetic rats. A 20-week period of diabetes resulted in a 62% decrease in paw withdrawal thresholds compared with age-matched normal rats. Daily injections of insulin progressively reversed mechanical hyperalgesia to normal values parallel to the correction of hyperglycaemia. When the treatment was stopped, mechanical hyperalgesia reappeared, but never reached the degree of hyperalgesia observed before insulin treatment, suggesting that indirect mechanisms underlie the effect of normoglycaemia on nociception. The present data suggest that appropriate blood glucose control can help relieve pain in long-term diabetes through indirect mechanisms.

Pharmacokinetics of amitriptyline and its demethylated and hydroxylated metabolites in streptozocin-induced diabetic rats.

Plasma and brain levels of amitriptyline (AMI), its demethylated and hydroxylated metabolites were determined after acute IP administration of AMI (20 mg/kg) in streptozocin-induced diabetic Sprague-Dawley rats. Results showed 1. in plasma: rapid AMI absorption, but slow elimination; the proportion of AMI similar to those of the rest of compounds; the proportion of its demethylated metabolite, nortriptyline, 1.8-fold higher than that of 10-hydroxy-nortriptyline. 2. in brain: the proportions of AMI and nortriptyline were 9.5- and 2.6-fold higher respectively, than those of whole hydroxylated metabolites, which represented 7.4% of the total amount.

[Drugs for relief of pain]. / Les médicaments de la douleur.

Currently, the pharmacology of analgesics can be sum up to three main compounds: morphine, paracetamol and aspirin. The use of antidepressants and anticonvulsants will also be mentioned. Morphine remains the reference compound among centrally acting analgesics both for acute and chronic pain. Its mechanism of action is relatively well known as well as the role of opiate receptors. Paracetamol is largely used as analgesic for relief of slight to moderate pain. Its mechanism of action still remains unclear. Usually the tolerability of this compound is considered as being excellent. However overdosage can induce severe and possible lethal liver necrosis. Aspirin is also a very old and a largely used compound. Its inhibiting effect on cyclooxygenase acts mainly peripherally but it could also act centrally. According to the current knowledge of its mechanism of action, it seems that the main pharmacological properties as well as adverse events of aspirin are all related to its enzyme inhibiting effect. Slight to moderate pain, related or not to inflammatory disease responds well to aspirin and other NSAIDs.

Study of the sensitivity of the diabetes-induced pain model in rats to a range of analgesics.

The streptozocin-induced diabetic rat has been put forward as a model of chronic pain with signs of hyperalgesia and allodynia that may reflect signs observed in diabetic humans. The aim of this work was to assess, in streptozocin-induced diabetic rats, the pharmacological activity to several analgesic drugs known to be effective (clomipramine, amitriptyline, desipramine, clonidine, lidocaine), ineffective (aspirin), or with a doubtful effectiveness (morphine) in human painful diabetic neuropathy. The animals were submitted to a mechanical pain test (paw pressure) and the ability of the drugs to reverse diabetes-induced hyperalgesia was tested. The tested antidepressants (0.125-8 mg/kg, i.v.) were slightly effective in diabetic rats; amitriptyline and clomipramine induced a weak effect, whereas desipramine was more active, suggesting noradrenergic specificity. This was confirmed by the effectiveness of clonidine (50, 100, 150 micrograms/kg, s.c.). Lidocaine (1-9 mg/kg, i.v.) had prolonged efficacy on mechanical hyperalgesia. Aspirin (100 mg/kg, i.v.) was without effect and morphine (0.5-4 mg/kg, i.v.) induced a dose-dependent antinociceptive effect but at doses twice as high as those used in normal rats. These results demonstrate the high pharmacological predictivity of this model of painful diabetes and suggest that in this pathological condition, among the drugs acting on monoaminergic transmission, noradrenergic drugs seem the most active.

RP-67580, a specific tachykinin NK1 receptor antagonist, relieves chronic hyperalgesia in diabetic rats.

The effect of a non-peptide NK1 receptor antagonist, RP-67580, and of its enantiomer, RP-68651, on nociceptive thresholds in normal and streptozocin-induced diabetic rats submitted to paw pressure is reported. RP-67580 (1, 3, 9 mg/kg s.c.) dose dependently reduced the diabetes-induced mechanical hyperalgesia observed 4 weeks after induction of diabetes. In normal rats, RP-67580 failed to increase nociceptive thresholds. RP-68651 was inactive in both diabetic and normal rats. These results suggest that (i) substance P is involved in the diabetes-induced chronic hyperalgesia, and (ii) NK1 receptor antagonists merit to be studied more extensively in relation with this pathological condition.

Streptozocin-induced diabetic rats: behavioural evidence for a model of chronic pain.

Painful diabetic neuropathy is one of the most common complications of insulin-dependent diabetes in man. Conflicting results have been obtained in experimentally diabetic animals subjected to pain stimuli. This work aimed to systematically study the response of rats made diabetic (hyperglycemia > or = 14 mM) by injection of streptozocin (STZ) (75 mg/kg, i.p.), to various pain stimuli: mechanical, thermal (warm and cold) and chemical. The time course of the scores was followed for 4 weeks simultaneously with the clinical symptoms (weight, body and skin temperature, motility) and hyperglycemia. A decrease in reaction thresholds to noxious heat stimuli (44 degrees C and 46 degrees C) and to non-painful thermal (cold: 10 degrees C, and warm: 38-42 degrees C) and mechanical stimulation (paw pressure) was observed. This can be considered as evidence for hyperalgesia and allodynia, respectively. These troubles appeared gradually and required at least 2 weeks of diabetes to reach statistical significance. Four weeks after the induction of diabetes, the scores obtained in diabetic rats injected with formalin were greater than those in normal rats, indicating hyperalgesia. Variation in sensitivity to pain occurred at the same time as arrested weight increase, fall in skin temperature, some amyotrophy measured in terms of hind-paw volume, and the usual polyuria-polydipsia syndrome. Spontaneous motor activity of the rats was lowered. This model is thus of interest as the observed reactions to noxious and non-noxious stimuli correspond to hyperalgesia and allodynia, symptoms encountered in painful diabetic neuropathy in man. Operating conditions for this model are discussed.