Antinociceptive interactions between intrathecal gabapentin and MK801 or NBQX in rat formalin test

(2R,6R)-hydroxynorketamine (HNK) reverses mechanical hypersensitivity in a model of localized inflammatory pain

The ketamine metabolite (2R,6R)-hydroxynorketamine, or (2R,6R)-HNK, was recently reported to evoke antinociception in response to a noxious thermal stimulus in healthy mice and reverse mechanical hypersensitivity in a murine model of neuropathic pain. This study reports the behavioral effects of (2R,6R)-HNK in male and female C57BL/6J mice exposed to a localized inflammatory pain condition and the broad pharmacological mechanism underlying this effect. Hind paw intraplantar injection of λ-carrageenan (CARR) caused inflammation and mechanical hypersensitivity in mice within 2 hours, lasting at least 48 hours. Intraperitoneal administration of (2R,6R)-HNK (10-30 mg/kg i.p.) 2 hours following CARR injection significantly reversed mechanical hypersensitivity within 1 hour in male and female mice, and the effect persisted for 24 hours following a single dose. The magnitude and timing of the analgesic effect of (2R,6R)-HNK were comparable to the non-steroidal anti-inflammatory drug carprofen. The reversal of hypersensitivity by (2R,6R)-HNK was blocked at 4 and 24 hours after administration by pretreatment with the AMPA receptor antagonist NBQX and was not accompanied by changes in locomotor activity. These findings reinforce the growing evidence supporting (2R,6R)-HNK as a novel analgesic in multiple preclinical pain models and further support an AMPAR-dependent mechanism of action. SIGNIFICANCE: The ketamine metabolite (2R,6R)-HNK reversed mechanical hypersensitivity associated with localized inflammation with onset less than one hour and duration greater than 24 hours in an effect comparable to the NSAID carprofen. Reversal of mechanical hypersensitivity by (2R,6R)-HNK is AMPAR-dependent.

Mammalian target of rapamycin signaling pathway is involved in synaptic plasticity of the spinal dorsal horn and neuropathic pain in rats by regulating autophagy

Unveiling the etiology and the underlying mechanism of neuropathic pain, a poorly treated disease, is essential for the development of effective therapies. This study aimed to explore the role of mammalian target of rapamycin (mTOR) signaling in autophagy-mediated neuropathic pain. We established a spared nerve injury (SNI) model in adult male SD rats by ligating the common peroneal nerve and tibial, with the distal end cutoff. The paw withdrawal threshold (PWT) and C/A-fiber evoked field potentials were determined by electrophysiologic tests at day 0 (before operation), day 7 and day 14 postoperation, and SNI significantly increased field potentials (P < 0.05). Immunohistochemistry and western blots using spinal cord tissues showed that the expressions of GluR1, GluR2, Beclin-1, p62, mTOR and 4EBP1 were significantly increased after SNI (all P < 0.05), whereas the expressions of LC3 and LAMP2 were significantly decreased after SNI (all P < 0.05). Rapamycin efficiently counteracted the effect of SNI and restored the phenotypes to the level comparable to the sham control. In conclusion, rapamycin inhibits C/A-fiber-mediated long-term potentiation in the SNI rat model of neuropathic pain, which might be mediated by activation of autophagy signaling and downregulation of GluRs expression.

Possible Participation of Ionotropic Glutamate Receptors and l-Arginine-Nitric Oxide-Cyclic Guanosine Monophosphate-ATP-Sensitive K+ Channel Pathway in the Antinociceptive Activity of Cardamonin in Acute Pain Animal Models

The perception of pain caused by inflammation serves as a warning sign to avoid further injury. The generation and transmission of pain impulses involves various pathways and receptors. Cardamonin isolated from Boesenbergia rotunda (L.) Mansf. has been reported to exert antinociceptive effects in thermal and mechanical pain models; however, the precise mechanism has yet to be examined. The present study investigated the possible mechanisms involved in the antinociceptive activity of cardamonin on protein kinase C, N-methyl-d-aspartate (NMDA) and non-NMDA glutamate receptors, l-arginine/cyclic guanosine monophosphate (cGMP) mechanism, as well as the ATP-sensitive potassium (K⁺) channel. Cardamonin was administered to the animals intra-peritoneally. Present findings showed that cardamonin significantly inhibited pain elicited by intraplantar injection of phorbol 12-myristate 13-acetate (PMA, a protein kinase C activator) with calculated mean ED₅₀ of 2.0 mg/kg (0.9–4.5 mg/kg). The study presented that pre-treatment with MK-801 (NMDA receptor antagonist) and NBQX (non-NMDA receptor antagonist) significantly modulates the antinociceptive activity of cardamonin at 3 mg/kg when tested with glutamate-induced paw licking test. Pre-treatment with l-arginine (a nitric oxide precursor), ODQ (selective inhibitor of soluble guanylyl cyclase) and glibenclamide (ATP-sensitive K⁺ channel inhibitor) significantly enhanced the antinociception produced by cardamonin. In conclusion, the present findings showed that the antinociceptive activity of cardamonin might involve the modulation of PKC activity, NMDA and non-NMDA glutamate receptors, l-arginine/nitric oxide/cGMP pathway and ATP-sensitive K⁺ channel.

The Contribution of NMDA Receptors in Antinociceptive Effect of Pregabalin: Comparison of Two Models of Pain Assessment

Background

Pregabalin has shown remarkable antinociceptive effects in neuropathic pain; however, its efficacy against acute and visceral pain remained controversial.

Objectives

The present study aimed at investigating the involvement of N-methyl-D-aspartate (NMDA) receptors in the antinociceptive effect of pregabalin in both acute and visceral pain using and comparing hot plate test and writhing test in male mice.

Methods

NMDA (15 and 30 mg/kg), as an agonist or MK801 (0.02 and 0.05 mg/kg) as an NMDA receptor (NMDAR) antagonist, were injected intraperitoneally either alone or 15 minutes before a dose of pregabalin that produced almost 30% antinociception (100 mg/kg in hot plate test and 5 mg/kg in writhing tests). Then, the percentage of maximal possible effect (MPE%) at the 30th and 60th minutes in hot plate test and effect percentage (E%) in writhing test were measured and compared as antinociceptive indexes.

Results

In hot plate test, pretreatment with MK801 (0.05 mg/kg) significantly increased antinociceptive effect of 100 mg/kg pregabalin, but pretreatment with NMDA did not result in any effect. Pretreatment with MK801 in writhing test significantly increased the antinociceptive effect of 5 mg/kg pregabalin (In contrast to 30 mg/kg NMDA that significantly decreased it.). NMDA induced antinociception reduction or MK801 increased antinociception in writhing test were significantly higher than what was observed in hot plate test.

Conclusions

Our results suggested that pregabalin antinociception in acute and visceral pain is mediated through NMDA receptors. Although this effect depends on the dose of NMDAR ligand, it is more pronounced in the behavioral response in the writhing test.

The Role of NMDARs Ligands on Antinociceptive Effects of Pregabalin in the Tail Flick Test

Background:

Pregabalin as a new anticonvulsant has been used in different pain treatments.

Objectives:

The aim of this study was to investigate the role of N-methyl-D-aspartate (NMDA) ligands in antinociceptive effect of pregabalin in mice using tail flick.

Materials and Methods:

NMDA (15 and 30 mg/kg) as an agonist or MK801 (0.02 and 0.05 mg/kg) as an antagonist were injected intraperitoneally either alone or 15 minutes before antinociceptive dose of pregabalin (100 mg/kg). Then the latency times and %MPE were measured in the tail flick assay during 75 minutes.

Results:

NMDA and MK801 had no effects alone. NMDA pretreatment significantly decreased the latency times of pregabalin till 75^th minutes. In NMDA pretreated groups, %MPE₃₀ unlike %MPE₇₅ decreased significantly compared to those of pregabalin. MK801 delayed the latency times in pretreated groups, but %MPE₃₀ and %MPE₇₅ did not change significantly compared to pregabalin alone.

Conclusions:

Our findings support the role of NMDARs in pregabalin antinociception, because the NMDAR agonist, unlike the antagonist, decreased the antinociceptive effect of pregabalin, even if tail flick is not an adequate pain assessment method in this regard.

Interleukin-33 mediates formalin-induced inflammatory pain in mice

Interleukin-33 (IL-33), a member of the IL-1 family, has attracted growing interest since its discovery in 2003. IL-33 has been implicated in many diseases, including arthritis, asthma, allergies, and cardiovascular and infectious diseases. However, few studies have investigated its role in the transmission and modulation of pain. The present study was designed to explore the possible roles of IL-33 and its receptor, ST2, in formalin-induced inflammatory pain in mice. We found that both subcutaneous (s.c., 300 ng) and intrathecal injection (i.t., 3 ng) of recombinant IL-33 (rIL-33) increased paw lifting and licking time not only in normal mice but also in formalin models. Administration of ST2 antibody, which blocked the IL-33/ST2 signaling, alleviated the formalin-induced spontaneous pain behavior. Moreover, the ST2(-/-) mice showed significantly decreased pain behavior, as well as reduced ultrasonic vocalization induced by formalin, compared with the wild-type group. Additionally, ST2 antibody alleviated the potentiating effects of rIL-33 on pain behavior in the formalin mice, indicating that IL-33 plays a role in pain modulation through its ST2 receptor. These data suggest IL-33 and its ST2 receptor mediate formalin-induced inflammatory pain, and as a result this cytokine and its receptor may be new targets for the development of analgesics.

Antinociceptive drug interaction between intrathecal vitamin E and gabapentin in the rat formalin test

Background

Gabapentin is thought to exert an effect through the voltage-dependent calcium channel. Vitamin E is a widely known antioxidant which neutralizes the harmful effect of ROS which is considered to play a prominent role in various painful conditions. This study was therefore conducted to assess the antinociceptive effects of gabapentin and vitamin E and the interaction of these drugs in the modulation of pain in rats subjected to a formalin test.

Methods

Sprague-Dawley rats with a lumbar intrathecal catheter were tested for their paw flinches by 5% formalin injection after intrathecal injection of gabapentin or vitamin E. After obtaining dose-response curves for each drug, the effect of the combination was tested by the total dose fraction value and isobolographic analysis.

Results

When a single drug was injected intrathecally, significant dose-dependent decreases in flinches were shown only in the late phase. ED₅₀ values of intrathecal gabapentin and vitamin E in the late phase were 75.3 ± 9.58 µg, and 17.56 ± 1.65 mg/kg respectively. The combination of gabapentin and vitamin E produced dose-dependent decreases in the number of flinches in both phases induced by the formalin test. The ED₅₀ value of the combination was lower than the theoretical additive values in the late phase, but did not show a significant difference with the theoretical additive value.

Conclusions

Gabapentin and vitamin E (by itself) have no antinociceptive effect in the early phase; however their combination has shown an antinociceptive effect. In addition, they show additive effects in the late phase of the formalin test.

Antinociceptive effects of intraperitoneal and intrathecal vitamin e in the rat formalin test

Background

Vitamin E is widely known to be one of the reactive oxygen species (ROS) scavengers and a drug that can easily be obtained, and it has been shown to attenuate the pain responses induced by various causes in animal pain models. Thus, this experiment was conducted to assess the antinociceptive effects of vitamin E by comparing intraperitoneal and intrathecal injections in rats subjected to the formalin test.

Methods

After the intraperitoneal and intrathecal injections of vitamin E were carried out, respectively (IP: 500 mg/kg, 1 g/kg, and 2 g/kg, IT: 3 mg/kg, 10 mg/kg, and 30 mg/kg), the formalin test was perfumed. As soon as 5% formalin was injected into left hind paw, the number of flinches induced by pain was measured at 5-minute intervals for 1 hour.

Results

Formalin injected into the left hind paw induced biphasic nociceptive behavior in all animals. Intraperitoneal injection of vitamin E diminished the nociceptive behavior in a dose-dependent manner during the early and late phase. Intrathecal vitamin E diminished nociceptive behavior dose dependently during the late phase but showed no significant difference in the early phase.

Conclusions

Vitamin E attenuated acute nociception when it was injected systemically, while both systemic and intrathecal injection produced analgesia in a rat model of formalin-induced hyperalgesia.

Role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit GluR1 in spinal dorsal horn in inflammatory nociception and neuropathic nociception in rat

The present study aims to investigate changes of spinal cord AMPA receptor GluR1 and its phosphorylation in inflammatory and neuropathic pain. Complete Freund's adjuvant (CFA) injection into the hind paw produced inflammatory thermal hyperalgesia that was assessed by decreased response latency to radiant heat; spinal nerve ligation (SNL) was used to induce mechanical allodynia that was evaluated with von Frey hairs. By method of Western blot, expression of GluR1 (the main subunit of the AMPA receptor) and its phosphorylated forms at serine 845 (pGluR1-Ser845) and at serine 831 (pGluR1-Ser831) in the spinal dorsal horn was observed. It was found that the expression of pGluR1-Ser845 and pGluR1-Ser831 increased significantly at 1 h after CFA injection, reached peak at 4 h and returned to the normal control level at 24 h, while no significant change was detected in GluR1 itself. In contrast, neither GluR1 nor pGluR1 showed any significant change in rats following SNL. These results suggest that phosphorylated GluR1 (pGluR1-Ser845 and pGluR1-Ser831) might play a role in the induction of inflammatory but not neuropathic pain.

The effect of combined administration of ethanol and gabapentin on rabbit electroencephalographic activity

The central effect of ethanol is mainly connected with the effect on GABAergic, glutamatergic, serotonergic and opioid transmission. The mechanism of gabapentin effect suggests that it may alleviate the rewarding effect of ethanol, which may be used in the treatment of addiction. We decided to examine the interaction of ethanol with gabapentin by a pharmaco-electroencephalographic (EEG) method. The influence of gabapentin on the effect of ethanol on EEG of rabbits (midbrain reticular formation, hippocampus, frontal cortex) was tested. Gabapentin was administered at a single dose (25 and 100 mg/kg orally) or repeatedly twice a day at a total dose of 25 mg/kg for 14 days. Ethanol was injected at a dose of 0.8 g/kg 60 min. after gabapentin treatment. Ethanol caused an increase in the slow frequencies (0.5-4 Hz) in the recording, as well as a marked decrease of the fastest frequencies (13-30 and 30-45 Hz). Gabapentin lead to changes in rabbit EEG recording suggesting an depressant effect on the CNS (increase of slow and decrease of fast frequencies). The effects were less pronounced after repeated doses, which may indicate adaptative changes in the receptors. Gabapentin administered both in a single dose and for 7 days markedly enhanced the effect of ethanol on EEG recordings in rabbits. Repeated doses of gabapentin decrease the sensitivity of the hippocampus to the effect of ethanol.

Examination of the interaction between peripheral diclofenac and gabapentin on the 5% formalin test in rats

It has been shown that the association of diclofenac with other analgesic agents can increase its antinociceptive activity, allowing the use of lower doses and thus limiting side effects. Therefore, the aim of the present study was to examine the possible pharmacological interaction between diclofenac and gabapentin at the peripheral level in the rat using the 5% formalin test and isobolographic analysis. Diclofenac, gabapentin or a fixed-dose ratio diclofenac-gabapentin combination were administrated locally in the formalin-injured paw and the antinociceptive effect was evaluated using the 5% formalin test. All treatments produced a dose-dependent antinociceptive effect. ED30 values were estimated for the individual drugs and an isobologram was constructed. The derived theoretical ED30 for the diclofenac-gabapentin combination was 597.5+/-87.5 microg/paw, being significantly higher than the actually observed experimental value, 170.9+/-26.07 microg/paw. These results correspond to a synergistic interaction between diclofenac and gabapentin at the peripheral level, potency being about three times higher with regard to that expected from the addition of the effects of the individual drugs. Data suggest that low doses of the diclofenac-gabapentin combination can interact synergistically at the peripheral level and therefore this drug association may represent a therapeutic advantage for the clinical treatment of inflammatory pain.

Mechanisms of the antinociceptive action of gabapentin

Gabapentin, a gamma-aminobutyric acid (GABA) analogue anticonvulsant, is also an effective analgesic agent in neuropathic and inflammatory, but not acute, pain systemically and intrathecally. Other clinical indications such as anxiety, bipolar disorder, and hot flashes have also been proposed. Since gabapentin was developed, several hypotheses had been proposed for its action mechanisms. They include selectively activating the heterodimeric GABA(B) receptors consisting of GABA(B1a) and GABA(B2) subunits, selectively enhancing the NMDA current at GABAergic interneurons, or blocking AMPA-receptor-mediated transmission in the spinal cord, binding to the L-alpha-amino acid transporter, activating ATP-sensitive K(+) channels, activating hyperpolarization-activated cation channels, and modulating Ca(2+) current by selectively binding to the specific binding site of [(3)H]gabapentin, the alpha(2)delta subunit of voltage-dependent Ca(2+) channels. Different mechanisms might be involved in different therapeutic actions of gabapentin. In this review, we summarized the recent progress in the findings proposed for the antinociceptive action mechanisms of gabapentin and suggest that the alpha(2)delta subunit of spinal N-type Ca(2+) channels is very likely the analgesic action target of gabapentin.