Synergistic antiallodynic interaction between gabapentin or carbamazepine and either benfotiamine or cyanocobalamin in neuropathic rats

Anticonvulsants, including gabapentin and carbamazepine, have shown activity against several types of neuropathic pain; however, they have limiting side effects that may minimize their use. In this study the possible synergistic interaction between anticonvulsants and benfotiamine or cyanocobalamin on spinal nerve ligation-induced tactile allodynia was assessed. Oral administration of gabapentin (15-300 mg/kg), carbamazepine (10-300 mg/kg), benfotiamine (30-600 mg/kg) or cyanocobalamin (0.3-6.0 mg/kg) significantly reduced tactile allodynia in rats. Maximal antiallodynic effects were reached with gabapentin 300 mg/kg (approximately 70%), carbamazepine 300 mg/kg (approximately 66%), benfotiamine 600 mg/kg (approximately 51%) and cyanocobalamin 6 mg/kg (approximately 59%). At the highest tested doses, gabapentin, but not carbamazepine, benfotiamine or cyanocobalamin, significantly reduced motor coordination. Coadministration of gabapentin or carbamazepine with benfotiamine or cyanocobalamin in a fixed ratio markedly reduced spinal nerve ligation-induced tactile allodynia, showing a synergistic interaction between anticonvulsants and B vitamins. Data indicate that combinations of anticonvulsants with benfotiamine or cyanocobalamin are able to reduce tactile allodynia without affecting motor coordination in rats, and suggest the possible clinical use of these combinations in the treatment of neuropathic pain in humans.     

Effect of K+ channel modulators on the antiallodynic effect of gabapentin

The effect of K+ channel inhibitors on the antiallodynic activity induced by spinal gabapentin was assessed in rats. Ligation of L5 and L6 spinal nerves made the rats allodynic, whereas that intrathecal administration of gabapentin (25-200 microg) reduced tactile allodynia in a dose-dependent manner. Spinal pretreatment with glibenclamide (12.5-50 microg, ATP-sensitive K+ channel inhibitor), charybdotoxin (0.01-1 ng) or apamin (0.1-3 ng, large-and small-conductance Ca2+-activated K+ channel blockers, respectively), but not margatoxin (0.01-10 ng, voltage-dependent K+ channel inhibitor), significantly prevented gabapentin-induced antiallodynia. Pinacidil (1-30 microg, K+ channel opener) significantly reduced nerve ligation-induced allodynia. Intrathecal glibenclamide (50 microg), charybdotoxin (1 ng) and apamin (3 ng), but not margatoxin (10 ng), significantly reduced pinacidil-induced antiallodynia. K+ channel inhibitors alone did not modify allodynia produced by spinal nerve ligation. Results suggest that gabapentin and pinacidil may activate Ca2+-activated and ATP-sensitive K+ channels in order to produce part of its spinal antiallodynic effect in the Chung model.     

Thiamine and cyanocobalamin relieve neuropathic pain in rats: synergy with dexamethasone

Treatment of neuropathic pain is an area of largely unmet medical need. Therefore, this pain may require the development of novel drug entities. In the search for alternatives, B vitamins have been found to be a clinically useful pharmacological tool for patients with neuropathic pain. However, preclinical studies supporting this use are lacking. In this study, we assessed the possible antiallodynic effects of thiamine, pyridoxine, and cyanocobalamin as well as dexamethasone and their combination on spinal nerve ligation induced allodynia. Sub cutaneous administration of thiamine (75-600 mg/kg), pyridoxine (75-600 mg/kg), cyanocobalamin(0.75-6 mg/kg), and dexamethasone (4-32 mg/kg) significantly reduced tactile allodynia in rats. Maximal antiallodynic effects were reached with 600 mg/kg of thiamine (approximately 58%), 600 mg/kg of pyridoxine (approximately 22%), 6 mg/kg of cyanocobalamin (approximately 73%), and 32 mg/kg of dexamethasone (approximately 68%). Since a small antiallodynic effect was observed with pyridoxine, this drug was not further analyzed in the combinations. Coadministration of thiamine or cyanocobalamin and dexamethasone remarkably reduced spinal nerve ligation induced allodynia (approximately 90%), showing a synergistic interaction between either thiamine or cyanocobalamin and dexamethasone. Our data indicate that thiamine and pyridoxine as well as the combination of B vitamins and dexamethasone are able to reduce tactile allodynia in rats and suggest the possible clinical use of these drugs in the treatment of neuropathic pain in human beings.     

Evaluation of the enantioselective antiallodynic and pharmacokinetic profile of propylisopropylacetamide, a chiral isomer of valproic acid amide

Propylisopropylacetamide (PID) is a chiral CNS-active constitutional isomer of valpromide, the amide derivative of the major antiepileptic drug valproic acid (VPA). The purpose of this work was: a) To evaluate enantiospecific activity of PID on tactile allodynia in the Chung (spinal nerve ligation, SNL) model of neuropathic pain in rats; b) To evaluate possible sedation at effective antiallodynic doses, using the rotorod ataxia test; c) To investigate enantioselectivity in the pharmacokinetics of (R)- and (S)-PID in comparison to (R,S)-PID; and d) To determine electrophysiologically whether PID has the potential to affect tactile allodynia by suppressing ectopic afferent discharge in the peripheral nervous system (PNS). (R)-, (S)- and (R,S)-PID produced dose-related reversal of tactile allodynia with ED(50) values of 46, 48, 42 mg/kg, respectively. The individual PID enantiomers were not enantioselective in their antiallodynic activity. No sedative side-effects were observed at these doses. Following i.p. administration of the individual enantiomers, (S)-PID had lower clearance (CL) and volume of distribution (V) and a shorter half-life (t(1/2)) than (R)-PID. However following administration of (R,S)-PID, both enantiomers had similar CL and V, but (R)-PID had a longer t(1/2). Systemic administration of (R,S)-PID at antiallodynic doses did not suppress spontaneous ectopic afferent discharge generated in the injured peripheral nerve, suggesting that its antiallodynic action is exerted in the CNS rather than the PNS. Both of PID's enantiomers, and the racemate, are more potent antiallodynic agents than VPA and have similar potency to gabapentin. Consequently, they have the potential to become new drugs for treating neuropathic pain.     

Discovery of Novel 1,2,4‐Triazol‐5‐Ones as Tumor Necrosis Factor‐Alpha Inhibitors for the Treatment of Neuropathic Pain

In this work, synthetic integration of substituted semicarbazides and various aliphatic, aryl and heteroaryl acids into 1,2,4‐triazol‐5‐ones was accomplished. Following the assessment of neurotoxicity and peripheral analgesic activity, the compounds were evaluated in two peripheral models of neuropathic pain, the chronic constriction injury and partial sciatic nerve ligation to assess their antihyperalgesic and antiallodynic potential. ED₅₀ studies undertaken for selected compounds exhibiting promising efficacies ( 1c , 3c and 4a ) revealed values ranging from 13.21 to 39.85 mg/kg in four behavioral assays of hyperalgesia and allodynia (spontaneous pain, tactile allodynia, cold allodynia, and mechanical hyperalgesia). Mechanistic studies revealed that the compounds suppressed the inflammatory component of the neuropathic pain inhibiting tumor necrosis factor‐alpha and preventing oxidative and nitrosative stress.     

A spinal mechanism of action for duloxetine in a rat model of painful diabetic neuropathy

BACKGROUND AND PURPOSE

This study was designed to clarify mechanisms responsible for the anti-allodynic effects of duloxetine in diabetes.

EXPERIMENTAL APPROACH

The streptozotocin-induced diabetic rat model was used to compare the efficacy of duloxetine, 5-HT, the 5-HT_2A receptor agonist [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI)] and two antagonists (ketanserin and pruvanserin) on tactile allodynia.

KEY RESULTS

Systemic or intrathecal injection of duloxetine alleviated tactile allodynia in diabetic rats. The effect of systemic duloxetine was reduced by intrathecal administration of ketanserin or pruvanserin, indicating participation of spinal 5-HT_2A receptors in the mechanism of action of duloxetine. In contrast to spinal delivery, systemic and local peripheral injections of ketanserin or pruvanserin alleviated tactile allodynia in diabetic rats. This effect was reversed immediately after systemic or local DOI injection.

CONCLUSIONS AND IMPLICATIONS

These results support the involvement of spinal 5-HT_2A receptors in the ability of duloxetine to ameliorate painful diabetic neuropathy. Our data also suggest that the role of 5-HT_2A receptors depends on the level of the neuraxis at which activation takes place, with peripheral activation contributing to tactile allodynia in diabetic rats, whereas spinal activation of this receptor alleviates tactile allodynia. The development of selective peripheral 5-HT_2A receptor antagonists may offer a novel approach for the treatment of diabetic neuropathic pain.     

Spinal mechanism underlying the antiallodynic effect of gabapentin studied in the mouse spinal nerve ligation model

We studied the antiallodynic effect of gabapentin (GBP) in the mouse model of neuropathic pain, aiming at clarifying the underlying mechanism. The L5 spinal nerve ligation induced tactile allodynia, an increase of CD11b expression, and an increase in the protein expression level of the voltage-dependent Ca(2+) channel α(2)/δ-1 subunit in the spinal dorsal horn on the injured side. The chronic intrathecal administration of GBP (100 μg/body per day) as well as ω-conotoxin MVIIA, an N-type Ca(2+)-channel blocker, completely suppressed allodynia, but did not attenuate the CD11b expression. The antiallodynic effect of GBP lasted for several days after the termination of the drug, while that of ω-conotoxin MVIIA disappeared immediately after the termination. GBP suppressed the elevation of the protein level of the α(2)/δ-1 subunit in the spinal dorsal horn, although it did not affect its mRNA level in the L5 DRG. These results suggest that GBP inhibits the development of allodynia by suppressing the up-regulation of N-type Ca(2+) channels, through normalization of the protein level of the α(2)/δ-1 subunit at the primary afferent nerve terminal via the inhibition of its anterograde transport. In addition, we propose that the nerve injury enhances the expression level of α(2)/δ-1 in the downstream of the activation of microglia.     

Pre-emptive intrathecal quinidine alleviates spinal nerve ligation-induced peripheral neuropathic pain

Objectives Quinidine, a class I anti‐arrhythmic agent, is a sodium channel blocker that is more potent than lidocaine and mexiletine. This study tested pre‐emptive intrathecal quinidine to attenuate neuropathic pain induced by lumbar spinal nerve ligation (SNL). Methods Ninety‐six adult male Sprague–Dawley rats were grouped equally (n = 24 per group) as follows: group S (sham), removal of transverse process only; group L, SNL; group Q₃₅, SNL pretreated with intrathecal quinidine 35 mm (50 µl); group Q₇₀, SNL pretreated with intrathecal quinidine 70 mm (50 µl). Neuropathic pain was measured by thermal hyperalgesia and mechanical allodynia. Other measurements included dys‐regulation of sodium channel Nav_1.3 in dorsal root ganglion (DRG) and spinal microglia activation in spinal dorsal horn. Key findings Spinal nerve ligation induced abnormal mechanical allodynia and thermal hyperalgesia, up‐regulated Nav_1.3 in DRG, and activated microglia in spinal cord. Group Q₇₀ showed attenuated thermal hyperalgesia (P < 0.001) and mechanical allodynia (P < 0.05) on postoperative day 5 (POD₅) but not on POD₇, reversed up‐regulated expression of Nav_1.3 on POD₃ and POD₇ in DRG and significantly attenuated microglia activation on POD₇ (P = 0.032) in spinal cord. Conclusions Pretreatment with intrathecal quinidine 70 mm before SNL attenuates nerve ligation‐induced neuropathic pain. The duration of the effect is 5 days.     

Evaluation of the antiallodynic,teratogenic and pharmacokinetic profile of stereoisomers of valnoctamide,an amide derivative of a chiral isomer of valproic acid

The purpose of this study was to evaluate the stereoselective pain relieving (antiallodynic) activity, antiallodynic–anticonvulsant correlation, teratogenicity and pharmacokinetic profile of two stereoisomers of valnoctamide (VCD), a CNS-active amide derivative of a chiral isomer of valproic acid (VPA). The individual stereoisomers (diastereomers), (2R,3S)-VCD and (2S,3S)-VCD were synthesized and their antiallodynic activity was evaluated in rats using the spinal nerve ligation model of neuropathic pain. The pharmacokinetic profile of the two stereoisomers was evaluated in rats following: 1) i.p. administration of racemic-VCD, 2) i.p. administration of the individual stereoisomers (2R,3S)-VCD and (2S,3S)-VCD. Teratogenicity of racemic-VCD and its two individual stereoisomers was evaluated in a SWV mouse strain known to be highly susceptible to VPA-induced teratogenicity. Racemic-VCD, (2R,3S)-VCD and (2S,3S)-VCD showed a dose-related reversal of tactile allodynia with ED₅₀ values of 52, 61 and 39 mg/kg, respectively. (2S,3S)-VCD was significantly more potent than (2R,3S)-VCD but the opposite is true for its anticonvulsant-effect. In the teratogenicity evaluation racemic-VCD and its two individual stereoisomers showed mild embryotoxicity at doses 7–10 times higher than their antiallodynic-ED₅₀ values, while (2S,3S)-VCD was significantly less embryotoxic than (2R,3S)-VCD and racemic-VCD. Following administration of the racemic-VCD there was an increase in the primary pharmacokinetic parameters of (2S,3S)-VCD but not of (2R,3S)-VCD. This study demonstrates that both racemic-VCD and its stereoisomers show high potency as antiallodynic compounds and possess a wide safety margin. (2S,3S)-VCD is more potent and less embryotoxic than (2R,3S)-VCD and thus, has a potential to become a candidate for development as a new drug for treating neuropathic pain.     

Efficacy of antiepileptic tetramethylcyclopropyl analogues of valproic acid amides in a rat model of neuropathic pain

Antiepileptic drugs (AEDs) are widely utilized in the management of neuropathic pain. The AED valproic acid (VPA) holds out particular promise as it engages a variety of different anticonvulsant mechanisms simultaneously. However, the clinical use of VPA is limited by two rare but potentially life-threatening side effects: teratogenicity and hepatotoxicity. We have synthesized several tetramethylcyclopropyl analogues of VPA amides that are non-teratogenic, and are likely to be non-hepatotoxic, and that exhibit good antiepileptic efficacy. In the present study we have assessed the antiallodynic activity of these compounds in comparison to VPA and gabapentin (GBP) using the rat spinal nerve ligation (SNL) model of neuropathic pain. TMCA (2,2,3,3-tetramethylcyclopropanecarboxylic acid, 100–250 mg/kg), TMCD (2,2,3,3-tetramethylcyclopropanecarboxamide, 40–150 mg/kg), MTMCD (N-methyl-TMCD, 20–100 mg/kg), and TMCU (2,2,3,3-tetramethylcyclopropanecarbonylurea, 40–240 mg/kg) all showed dose-related reversal of tactile allodynia, with ED₅₀ values of 181, 85, 41, and 171 mg/kg i.p., respectively. All were more potent than VPA (ED₅₀ = 269 mg/kg). An antiallodynic effect was obtained for TMCD, MTMCD and TMCU at plasma concentrations as low as 23, 6 and 22 mg/L, respectively. MTMCD was found to be non-toxic, non-sedative and equipotent to gabapentin, currently the leading AED in neuropathic pain treatment. Tetramethylcyclopropyl analogues of VPA amides have potential to become a new series of drugs for neuropathic pain treatment.     

Benfotiamine relieves inflammatory and neuropathic pain in rats

Benfotiamine has shown therapeutic efficacy in the treatment of painful diabetic neuropathy in human beings. However, so far there is no evidence about the efficacy of this drug in preclinical models of pain. The purpose of this study was to assess the possible antinociceptive and antiallodynic effect of benfotiamine in inflammatory and neuropathic pain models in the rat. Inflammatory pain was induced by injection of formalin in non-diabetic and diabetic (2 weeks) rats. Reduction of flinching behavior was considered as antinociception. Neuropathic pain was induced by either ligation of left L5/L6 spinal nerves or administration of streptozotocin (50 mg/kg, i.p.) in Wistar rats. Benfotiamine significantly reduced inflammatory (10-300 mg/kg) and neuropathic (75-300 mg/kg) nociception in non-diabetic and diabetic rats. Results indicate that oral administration of benfotiamine is able to reduce tactile allodynia from different origin in the rat and they suggest the use of this drug to reduce inflammatory and neuropathic pain in humans.     

Venlafaxine compromises the antinociceptive actions of gabapentin in rat models of neuropathic and persistent pain

Rationale Neuropathic pain is associated with a number of disease states of diverse aetiology that can share common pathophysiological mechanisms. Antiepileptic drugs modulate ion channel function and antidepressants increase extracellular monoamine levels, and both drug classes variously attenuate signs and symptoms of neuropathic pain. Thus, coadministration of the antiepileptic gabapentin and the antidepressant venlafaxine may provide superior pain relief to administration of either drug alone.Objectives To systematically establish the pain relieving efficacies of venlafaxine and gabapentin alone and in combination.Materials and methods Gabapentin (50 and 100 mg/kg, s.c.) and venlafaxine (10, 25, 50 mg/kg, s.c.) were tested alone or in combination in the rat spared nerve injury (SNI) model of neuropathic pain and the rat formalin test of persistent pain. Diuresis was measured in a separate experiment after administration of venlafaxine.Results Hindpaw mechanical allodynia was dose-dependently reversed by gabapentin (50 and 100 mg/kg, s.c.), whereas venlafaxine was ineffective (10 and 50 mg/kg, s.c.). Both gabapentin and venlafaxine also attenuated hindpaw mechanical hyperalgesia. Surprisingly, coadministration of venlafaxine (50 mg/kg) significantly lowered the antiallodynic effect of both doses of gabapentin by up to 60% in spared-nerve-injury rats and a negative antinociceptive interaction between gabapentin and venlafaxine was also observed in the rat formalin test. We demonstrated that venlafaxine administration was associated with a dose-dependent increase in urine output over the time course of the nociceptive experiments.Conclusion Venlafaxine compromises the antiallodynic effects of coadministered gabapentin most probably as consequence-increased diuresis.     

Blockade of 5-HT7 receptors reduces tactile allodynia in the rat

This study assessed the role of systemic and spinal 5-HT₇ receptors on rats submitted to spinal nerve injury. In addition, the 5-HT₇ receptors level in dorsal root ganglion and spinal cord was also determined. Tactile allodynia was induced by L5/L6 spinal nerve ligation. Systemic (0.01-10 mg/kg) or spinal (0.3-30 μg) administration of the selective 5-HT₇ receptor antagonist SB-269970 but not vehicle reduced in a dose-dependent manner established tactile allodynia. This effect was maintained for about 6 h. SB-269970 was more potent and effective by the spinal administration route than through systemic injection. Spinal nerve ligation reduced expression of 5-HT₇ receptors in the ipsilateral but not contralateral dorsal root ganglia. Moreover, 5-HT₇ receptor levels were lower in the ipsilateral dorsal spinal cord of neuropathic rats compared to naïve and sham rats. No changes in the receptor levels were observed in the contralateral dorsal spinal cord and in both regions of the ventral spinal cord. Data suggest that spinal 5-HT₇ receptors play a pronociceptive role in neuropathic rats. Results also indicate that spinal nerve injury leads to a reduced 5-HT₇ receptors level in pain processing-related areas which may result from its nociceptive role in this model. Data suggest that selective 5-HT₇ receptor antagonists may function as analgesics in nerve injury pain states.     

The role of delta-opioid receptor subtypes in neuropathic pain

A large body of evidence suggests an important role of delta-opioid receptor agonists in antinociception at the level of the spinal cord. Our study was undertaken to analyse the spinal antinociceptive and antiallodynic effects of delta(1)- and delta(2)-opioid receptor agonists and antagonist after their acute and chronic intrathecal administration in a neuropathic pain model in the rat. In rats with a crushed sciatic nerve, the delta(1)-opioid receptor agonist [D-Pen(2), D-Pen(5)]enkephalin (DPDPE, 5-25 microg i.t.) and the delta(2)-opioid receptor agonist deltorphin II (1.5-25 microg i.t.) dose dependently antagonized the cold-water allodynia which developed after sciatic nerve injury. These effects of DPDPE were antagonized by 7-benzylidenenaltrexon (BNTX, 1 microg i.t.) while the effects of deltorphin II were antagonized by 5'naltrindole izotiocyanate (5'NTII, 25 microg i.t.). Both agonists had a dose-dependent, statistically significant effect on the tail-flick latency in two tests, with focused light and cold water. Chronic administration of DPDPE (25 microg i.t.) and deltorphin II (15 microg i.t.) resulted in significant prolongation of the reaction time determined on days 2, 4 and 6 post-injury. In conclusion, our results show an antiallodynic and antinociceptive action of DPDPE and deltorphin II at the spinal cord level, which suggests that both delta-opioid receptor subtypes play a similar role in neuropathic pain. This indicates that not only delta(1)- but also delta(2)-opioid receptor agonists can be regarded as potential drugs for the therapy of neuropathic pain.     

Fluvoxamine, a selective serotonin reuptake inhibitor, exerts its antiallodynic effects on neuropathic pain in mice via 5-HT2A/2C receptors

There is an association between depression and chronic pain, and some antidepressants exert antinociceptive effects in humans and laboratory animals. We examined the effects of fluvoxamine, a selective serotonin reuptake inhibitor, on mechanical allodynia and its mechanism of action in the mouse chronic pain model, which was prepared by partially ligating the sciatic nerve. The antiallodynic effect was measured using the von Frey test. Fluvoxamine produced antiallodynic effects following both systemic and intrathecal administration. In 5-hydroxytryptamine (5-HT)-depleted mice, prepared by intracerebroventricular injection of 5,7-dihyroxytryptamine, the fluvoxamine-induced antiallodynic effect was significantly attenuated. The antiallodynic effects of systemic fluvoxamine were also reduced by both systemic and intrathecal administration of ketanserin, a 5-HT2A/2C receptor antagonist. In addition, fluvoxamine also induced antinociceptive effect in the acute paw pressure test, and this effect was antagonized by the 5-HT3 receptor antagonist granisetron. These results indicate that fluvoxamine exerts its antiallodynic effects on neuropathic pain via descending 5-HT fibers and spinal 5-HT2A or 5-HT2C receptors, and the antinociception on acute mechanical pain via 5-HT3 receptors.     

Isobolographic analyses of the gabapentin-metamizol combination after local peripheral, intrathecal and oral administration in the rat

This study was designed to evaluate the possible antinociceptive interaction between gabapentin and metamizol on formalin-induced nociception. Gabapentin, metamizol or a fixed dose-ratio combination of both drugs were assessed after local peripheral, intrathecal and oral administration in rats. Isobolographic analyses were employed to define the nature of the interaction between drugs. Gabapentin, metamizol and gabapentin-metamizol combinations yielded a dose-dependent antinociceptive effect when administered by the three different routes. ED30 values were estimated for the individual drugs and isobolograms were constructed. Theoretical ED30 values for the combination estimated from the isobolograms were 21.11 +/- 1.17 microg/paw, 104.6 +/- 5.5 microg/rat and 78.8 +/- 5.5 mg/kg for the local peripheral, intrathecal and oral administration routes, respectively. These values were significantly higher than the experimentally obtained ED30 values which were 11.3 +/- 1.5 microg/paw, 36.8 +/- 3.1 microg/rat and 15 +/- 1.2 mg/kg indicating a synergistic interaction. Systemic administration resulted in the highest synergism. Data confirm that low doses of the gabapentin and metamizol can interact synergistically to reduce formalin-induced nociceptive behavior suggesting that this combination could be useful to treat inflammatory pain in humans.