On the mechanism of carbamazepine-induced antinociception in the formalin test

The bradycardic agent ivabradine decreases conduction velocity in the AV node and in the ventricles in-vivo

Ivabradine blocks hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels, thereby lowering the heart rate, an action that is used clinically for the treatment of heart failure and angina pectoris. We and others have shown previously that ivabradine, in addition to its HCN channel blocking activity, also inhibits voltage-gated Na channels in vitro at concentrations that may be clinically relevant. Such action may reduce conduction velocity in cardiac atria and ventricles. Here, we explore the effect of administration of ivabradine on parameters of ventricular conduction and repolarization in the surface ECG of anesthetized mice. We found that 5 min after i.p. administration of 10 mg/kg ivabradine spontaneous heart rate had declined by ~13%, which is within the range observed in human clinical studies. At the same time a significant increase in QRS duration by ~18% was observed, suggesting a reduction in ventricular conduction velocity. During transesophageal pacing at heart rates between 100 and 220 beats/min there was no obvious rate-dependence of ivabradine-induced QRS prolongation. On the other hand, ivabradine produced substantial rate-dependent slowing of AV nodal conduction. We conclude that ivabradine prolongs conduction in the AV-node and in the ventricles in vivo.

Anti-nociceptive and anti-inflammatory effect of imperatorin: evidences for involvement of COX-2, iNOS, NFκB and inflammatory cytokines

Aim: The objective of the current investigation was to explore the analgesic effect of naturally occurring furanocoumarin, imperatorin and the involvement of inducible cyclooxygenase (COX-2), inducible nitric oxide synthase (iNOS), NFκB and cytokines in the observed effect.Materials and methods: Anti-nociceptive effect was explored by inducing chemical hyperalgesia using acetic acid and formalin in mice. ED₅₀ of imperatorin was calculated in acetic acid model. Modulation of cyclooxygenase and nitric oxide pathway by imperatorin was examined by stimulator/precursor challenge with substance P and L-arginine, respectively and quantification of COX-2, iNOS and NFκB expression by immunohistochemical analysis in spinal tissues. Involvement of inflammatory cytokines TNF-α and IL-1β was investigated using LPS challenge and subsequent ELISA analysis of these inflammatory mediators in serum. Carrageenan inflicted paw edema was employed to explore the anti-inflammatory activity of imperatorin.Results: A significant reduction in the nociceptive behaviour was observed with imperatorin treatment in acetic acid and formalin test. ED₅₀ of imperatorin was found to be 4.53 mg/kg. Pre-treatment with substance P and L-arginine significantly attenuated the anti-nociceptive activity of imperatorin in formalin test. Immunohistochemical findings revealed marked decrease in spinal COX-2, iNOS and NFκB expression. Imperatorin administration significantly reduced LPS induced rise in level of TNF-α and IL-1β dose dependently. In carrageenan-induced paw edema test, maximum possible anti-inflammatory effect of imperatorin was evident after 240 min of carrageenan administration.Conclusion: Current investigation revealed that anti-nociceptive and anti-inflammatory potential of imperatorin is probably mediated through the attenuation of COX-2, iNOS, NFκB activity and reduction in circulatory cytokines.

Antiepileptic drugs as analgesics/adjuvants in inflammatory pain: current preclinical evidence

Inflammatory pain is the most common type of pain that is treated clinically. The use of currently available treatments (classic analgesics - NSAIDs, paracetamol and opioids) is limited by insufficient efficacy and/or side effects/tolerance development. Antiepileptic drugs (AEDs) are widely used in neuropathic pain treatment, but there is substantial preclinical evidence on their efficacy against inflammatory pain, too. In this review we focus on gabapentinoids (gabapentin and pregabalin) and dibenzazepine AEDs (carbamazepine, oxcarbazepine, and recently introduced eslicarbazepine acetate) and their potential for relieving inflammatory pain. In models of somatic, visceral and trigeminal inflammatory pain, that have a translational value for inflammatory conditions in locomotor system, viscera and head/face, AEDs have demonstrated analgesic activity. This activity was mostly consistent, dependent on the dose and largely independent on the site of inflammation and method of its induction, nociceptive stimuli, species, specific drug used, its route of administration and dosing schedule. AEDs exerted comparable efficacy with classic analgesics. Effective doses of AEDs are lower than toxic doses in animals and, when expressed as equivalent human doses, they are largely overlapping with AEDs doses already used in humans for treating epilepsy/neuropathic pain. The main mechanism of antinociceptive/antihyperalgesic action of gabapentinoids in inflammatory pain models seems to be α₂δ-dependent suppression of voltage-gated calcium channels in primary sensory neurons that leads to reduced release of neurotransmitters in the spinal/medullar dorsal horn. The suppression of NMDA receptors via co-agonist binding site primarily at spinal sites, activation of various types of K⁺ channels at spinal and peripheral sites, and activation of noradrenergic and serotonergic descending pain modulatory pathways may also contribute. Inhibition of voltage-gated sodium channels along the pain pathway is probably the main mechanism of antinociceptive/antihyperalgesic effects of dibenzazepines. The recruitment of peripheral adrenergic and purinergic mechanisms and central GABAergic mechanisms may also contribute. When co-administered with classic/other alternative analgesics, AEDs exerted synergistic/additive interactions. Reviewed data could serve as a basis for clinical studies on the efficacy/safety of AEDs as analgesics/adjuvants in patients with inflammatory pain, and contribute to the improvement of the treatment of various inflammatory pain states.

Analgesic Effect of 17β-Estradiol on Nucleus Paragigantocellularis Lateralis of Male Rats Mediated Via GABAA Receptors

Introduction:

Beside its autonomic functions, the nucleus paragigantocellularis lateralis (LPGi) is involved in the descending pain modulation. 17β-Estradiol is a neuroactive steroid found in several brain areas such as LPGi. Intra-LPGi microinjection of 17β-estradiol can elicit the analgesic responses. 17β-Estradiol modulates nociception by binding to estrogenic receptors as well as allosteric interaction with other membrane-bound receptors like GABA_A receptors. This study aimed to examine the role of GABA_A receptors in the pain modulating effect of intra-LPGi injection of 17β-estradiol.

Methods:

To study the antinociceptive effects of 17β-estradiol, cannulation into the LPGi nucleus of male Wistar rats was performed. About 500 nL of drug was administered 15 minutes prior to formalin injection (50 μL of 4%). Then, formalin-induced flexing and licking behaviors were recorded for 60 minutes. For evaluating the role of GABA_A receptors in the estradiol-induced pain modulation, 17β-estradiol was administered into the LPGi nucleus 15 minutes after the injection of 25 ng/μL bicuculline (the GABA_A receptor antagonist). Then, the formalin-induced responses were recorded.

Results:

The results of the current study showed that intra-LPGi injection of 17β-estradiol decreased the flexing duration in both phases of formalin test (P<0.001); but it only attenuated the second phase of licking behavior (P<0.001). 17β-estradiol attenuated the second phase of formalin test of both behaviors (P<0.001). Bicuculline prevented the antinociceptive effect of intra-LPGi 17β-estradiol in both first and second phases of formalin-induced responses (P<0.001).

Conclusion:

According to the results of this study, the analgesic effect of intra-LPGi 17β-estradiol on the formalin-induced inflammatory pain might be mediated via GABA_A receptors.

Comparative effects of α2δ-1 ligands in mouse models of colonic hypersensitivity

AIM: To investigate anti-hypersensitive effects of α₂δ-1 ligands in non-inflammatory and inflammation-associated colonic hypersensitivity (CHS) mouse models.

METHODS: To induce an inflammation-associated CHS, 1% dextran sulfate sodium (DSS) was administered to C57Bl/6J male mice, in drinking water, for 14 d. Regarding the non-inflammatory neonatal maternal separation (NMS) -induced CHS model, wild-type C57BI/6J pups were isolated from their mother from day 2 to day 14 (P2 to P14), three hours per day (from 9:00 a.m. to 12:00 p.m.). Colorectal distension was performed by inflating distension probe from 20 μL to 100 μL by 20 μL increment step every 10 s. After a first colorectal distension (CRD), drugs were administered subcutaneously, in a cumulative manner, (Gabapentin at 30 mg/kg and 100 mg/kg; Pregabalin at 10 mg/kg and 30 mg/kg; Carbamazepine at 10 mg/kg and 30 mg/kg) and a second CRD was performed one hour after each injection.

RESULTS: The visceromotor response (VMR) to CRD was increased by our NMS paradigm protocol in comparison to non-handled (NH) mice, considering the highest distension volumes (80 μL: 0.783 ± 0.056 mV/s vs 0.531 ± 0.034 mV/s, P < 0.05 and 100 μL: 1.087 ± 0.056 mV/s vs 0.634 ± 0.038 mV/s, P < 0.05 for NMS and NH mice, respectively). In the inflammation-associated CHS, DSS-treated mice showed a dramatic and significant increase in VMR at 60 and 80 μL distension volumes when compared to control mice (60 μL: 0.920 ± 0.079 mV/s vs 0.426 ± 0.100 mV/s P < 0.05 and 80 μL: 1.193 ± 0.097 mV/s vs 0.681 ± 0.094 mV/s P < 0.05 for DSS- and Water-treated mice, respectively). Carbamazepine failed to significantly reduce CHS in both models. Gabapentin significantly reduced CHS in the DSS-induced model for both subcutaneous injections at 30 or 100 mg/kg. Pregabalin significantly reduced VMR to CRD in the non-inflammatory NMS-induced CHS model for the acute subcutaneous administration of the highest cumulative dose (30 mg/kg) and significantly reduced CHS in low-dose DSS-treated mice in a dose-dependent manner. Finally, the percent decrease of AUC induced by acute GBP or Pregabalin treatment were higher in the inflammatory DSS-induced CHS model in comparison to the non-inflammatory NMS-induced CHS model.

CONCLUSION: This preclinical study demonstrates α₂δ-1 ligands efficacy on inflammation-associated CHS, highlighting their potential clinical interest in patients with chronic abdominal pain and moderate intestinal inflammation.

Combination of diacerhein and antiepileptic drugs after local peripheral, and oral administration in the rat formalin test

Preclinical Research The present study was designed to evaluate the possible antinociceptive interaction between diacerhein and some antiepileptic drugs (carbamazepine, topiramate and gabapentin) on formalin-induced nociception. Diacerhein, each of the antiepileptics or a fixed dose-ratio combination of these drugs was assessed after local peripheral and oral administration in rats. lsobolographic analyses were used to define the interaction between drugs. Diacerhein, antiepileptic drugs (carbamazepine, topiramate and gabapentin) or their combinations yielded a dose-dependent antinociceptive effect when administered by both routes. Theoretical ED30 values for the combination estimated from the isobolograms were obtained as follows: diacerhein-carbamazepine (85.99 ± 7.07 μg/paw; 56.53 ± 4.56 mg/kg po), diacerhein-topiramate (197.97 ± 22.90 μg/paw; 13.06 ± 2.44 mg/kg po) and diacerhein-gabapentin (96.87 ± 17.73 μg/paw; 17.90 ± 4.70 mg/kg p.o.) for the local peripheral and oral administration routes, respectively. These values were significantly higher than the experimentally obtained ED30 values: diacerhein-carbamazepine (49.33 ± 3.37 μg/paw; 35.49 ± 7.91 mg/kg po), diacerhein-topiramate (133.00 ± 39.10 μg/paw; 8.87 ± 1.46 mg/kg po) and diacerhein-gabapentin (70.98 ± 14.73 μg/paw; 10.95 ± 3.23 mg/kg po). The combinations produced their antinociceptive effects without motor impairment in the rotarod test indicating synergistic interactions with a good side effect profile.

Cuneus and fusiform cortices thickness is reduced in trigeminal neuralgia

Background

Chronic pain disorders are presumed to induce changes in brain grey and white matters. Few studies have focused CNS alterations in trigeminal neuralgia (TN).

Methods

The aim of this study was to explore changes in white matter microstructure in TN subjects using diffusion tensor images (DTI) with tract-based spatial statistics (TBSS); and cortical thickness changes with surface based morphometry. Twenty-four patients with classical TN (37-67 y-o) and 24 healthy controls, matched for age and sex, were included in the study.

Results

Comparing patients with controls, no diffusivity abnormalities of brain white matter were detected. However, a significant reduction in cortical thickness was observed at the left cuneus and left fusiform cortex in the patients group. The thickness of the fusiform cortex correlated negatively with the carbamazepine dose (p = 0.023).

Conclusions

Since the cuneus and the fusiform gyrus have been related to the multisensory integration area and cognitive processing, as well as the retrieval of shock perception conveyed by Aδ fibers, our results support the role of these areas in TN pathogenesis. Whether such changes occurs as an epiphenomenon secondary to daily stimulation or represent a structural predisposition to TN in the light of peripheral vascular compression is a matter of future studies.

Role of nitric oxide in the rat hippocampal CA1 in morphine antinociception

In the present study, the effects of intra-hippocampal CA1 injections of l-arginine, a nitric oxide (NO) precursor and N(G)-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, on morphine-induced antinociception in rat formalin test were investigated. To induce inflammation pain, formalin (50 microl at 2.5%) was injected into the right hind-paw of male Wistar rats prior to testing. Morphine (3-9 mg/kg) was injected intraperitoneally (i.p.) 10 min before injection of formalin. The present study shows that administration of L-arginine (0.08, 0.15, 0.3, 1.0 and 3.0 microg/rat), but not L-NAME (0.15, 0.3 and 1.0 microg/rat), 5 min before formalin injection reversed morphine-induced antinociception at the early phase of formalin test. However, both drugs blocked morphine antinociception at the late phase of the test, but none of these drugs elicited any response by themselves at the tonic phase when injected alone. Moreover, the response to l-arginine was potentiated by L-NAME pre-treatment. It should be noted that a single injection of both L-arginine and L-NAME showed nociceptive effect at the early phase of the test. The present study reveals an expression of NADPH-diaphorase in the rat brain samples administered by L-arginine. Expression of NADPH-d is decreased in the samples which were pre-injected with L-NAME. This study suggests NO participation in the rat hippocampal CA1 area in morphine-induced antinociception.

Inhibitory effect of lidocaine on pain and itch using formalin-induced nociception and 5'-guanidinonaltrindole-induced scratching models in mice: behavioral and neuroanatomical evidence

The aim of this study was to establish the effect of lidocaine, a local anesthetic, on pain and itch using formalin-induced nociception and kappa opioid antagonist-induced scratching models in mice. We investigated if local intradermal pretreatment (at -10 min) with lidocaine N-ethyl bromide (lidocaine, 2%, 0.1 ml) antagonizes behavioral responses and prevents c-fos expression induced by pain and itch. Male, Swiss Webster mice (25-30 g, n=6-10) were used. Formalin (5%, 20 microl, s.c.) or saline was administered to the right dorsal hindpaw and the time spent licking this paw was recorded at 0-10 min and 20-35 min. For itching, mice were challenged with 5'-guanidinonaltrindole (GNTI, 0.3mg/kg, s.c., behind the neck) or saline and the number of neck-directed scratches with hindpaws was counted for 30 min. C-fos immunohistochemistry was performed in lumbar (for pain) and cervical (for scratching) spinal sections 2h after the respective treatments. We found that lidocaine (a) antagonizes both formalin-induced pain and GNTI-induced scratching and (b) prevents c-fos expression evoked by pain (medial side of the superficial layer and deeper layers of the dorsal horn) and itch (lateral side of the superficial layer of the dorsal horn). Additionally, GNTI caused c-fos activation in mice wearing an Elizabethan collar (to prevent scratching of the neck) suggesting that GNTI provokes c-fos expression by inducing an itch sensation. Our results highlight the antipruritic properties of lidocaine and argue for its comprehensive clinical testing against pruritic states.