The effects of gabapentin on different ligand- and voltage-gated currents in isolated cortical neurons

HCN channels and absence seizures

Hyperpolarization-activation cyclic nucleotide-gated (HCN) channels were for the first time implicated in absence seizures (ASs) when an abnormal Ih (the current generated by these channels) was reported in neocortical layer 5 neurons of a mouse model. Genetic studies of large cohorts of children with Childhood Absence Epilepsy (where ASs are the only clinical symptom) have identified only 3 variants in HCN1 (one of the genes that code for the 4 HCN channel isoforms, HCN1-4), with one (R590Q) mutation leading to loss-of-function. Due to the multi-faceted effects that HCN channels exert on cellular excitability and neuronal network dynamics as well as their modulation by environmental factors, it has been difficult to identify the detailed mechanism by which different HCN isoforms modulate ASs. In this review, we systematically and critically analyze evidence from established AS models and normal non-epileptic animals with area- and time-selective ablation of HCN1, HCN2 and HCN4. Notably, whereas knockout of rat HCN1 and mouse HCN2 leads to the expression of ASs, the pharmacological block of all HCN channel isoforms abolishes genetically determined ASs. These seemingly contradictory results could be reconciled by taking into account the well-known opposite effects of Ih on cellular excitability and network function. Whereas existing evidence from mouse and rat AS models indicates that pan-HCN blockers may provide a novel approach for the treatment of human ASs, the development of HCN isoform-selective drugs would greatly contribute to current research on the role for these channels in ASs generation and maintenance as well as offer new potential clinical applications.

Use of anticonvulsants and antidepressants for treatment of complex regional pain syndrome: a literature review

Complex regional pain syndrome (CRPS) is characterized by pain accompanied by symptoms including skin changes, sensory, motor, trophic changes and autonomic dysfunction. Anticonvulsants and antidepressants are commonly prescribed for neuropathic pain conditions; however, evidence is sparse whether these drugs are effective in reducing CRPS-related pain. As such, Pubmed was searched for studies published from January 1990 through March 2020; 13 studies were included in this review. Overall, evidence is considered insufficient for use of gabapentinoids for CRPS-related pain. However, three randomized controlled trials (RCTs) did find gabapentin to result in significant improvement in pain whereas one RCT reported use of amitriptyline to be equally as effective as gabapentin. Multiple case reports discussing the efficacy of pregabalin in pediatric CRPS patients, with relatively short duration of disease and underlying psychiatric illness, have been reported, but these findings need to be validated with RCTs.

Gabapentin Is a Potent Activator of KCNQ3 and KCNQ5 Potassium Channels

Synthetic gabapentinoids, exemplified by gapapentin and pregabalin, are in extensive clinical use for indications including epilepsy, neuropathic pain, anxiety, and alcohol withdrawal. Their mechanisms of action are incompletely understood, but are thought to involve inhibition of α₂δ subunit-containing voltage-gated calcium channels. Here, we report that gabapentin is a potent activator of the heteromeric KCNQ2/3 voltage-gated potassium channel, the primary molecular correlate of the neuronal M-current, and also homomeric KCNQ3 and KCNQ5 channels. In contrast, the structurally related gabapentinoid, pregabalin, does not activate KCNQ2/3, and at higher concentrations (≥10 µM) is inhibitory. Gabapentin activation of KCNQ2/3 (EC₅₀ = 4.2 nM) or homomeric KCNQ3* (EC₅₀ = 5.3 nM) channels requires KCNQ3-W265, a conserved tryptophan in KCNQ3 transmembrane segment 5. Homomeric KCNQ2 or KCNQ4 channels are insensitive to gabapentin, whereas KCNQ5 is highly sensitive (EC₅₀ = 1.9 nM). Given the potent effects and the known anticonvulsant, antinociceptive, and anxiolytic effects of M-channel activation, our findings suggest the possibility of an unexpected role for M-channel activation in the mechanism of action of gabapentin.

Preventive Treatment of Migraine: Effect on Weight

Weight gain is a side-effect commonly associated with drugs used for headache prophylaxis. Weight gain can adversely affect patient health, exacerbate comorbid metabolic disorders and encourage noncompliance. Few studies have been conducted specifically on the effect of headache medications on weight, and it is important for physicians to have accurate information about weight-gain side-effects when identifying appropriate pharmacological regimens. This review discusses the potential effects on weight of the more common headache medications.

Involvement of NO/cGMP pathway in the antidepressant-like effect of gabapentin in mouse forced swimming test

Based on clinical studies regarding the beneficial effect of gabapentin in depression, we aimed to evaluate the antidepressant-like properties of gabapentin in mice and also the participation of nitric oxide (NO)/cyclic guanosine monophosphate pathway in this effect. The following drugs were used in this study: gabapentin; N(G)-nitro-L-arginine methyl ester (L-NAME), a non-specific NO synthase (NOS) inhibitor; 7-nitroindazole, a specific neuronal NOS inhibitor; aminoguanidine, a specific inducible NOS inhibitor; L-arginine, a NO precursor; and sildenafil, a phosphodiestrase inhibitor. Finally, we studied the behavioral effects through the forced swimming test (FST) and the changes of the hippocampus NO level through nitrite assay. The immobility time was significantly reduced after gabapentin administration. Co-administration of non-effective doses of gabapentin and L-NAME or 7-nitroindazole (7-NI) resulted in antidepressant-like effect in FST, while aminoguanidine did not affect the immobility time of gabapentin-treated mice. Furthermore, the antidepressant-like property of gabapentin was prevented by L-arginine or sildenafil. Also, the hippocampal nitrite level was significantly lower in gabapentin-treated mice relative to saline-injected mice, and co-administration of 7-NI with sub-effective gabapentin caused a significant decrease in hippocampal nitrite levels. Our results indicate that the antidepressant-like effect of gabapentin in the mice FST model is mediated at least in part through nitric oxide/cyclic guanosine monophosphate (cGMP) pathway.

Modulation of voltage-gated Ca2+ channels in rat retinal ganglion cells by gabapentin

The α2δ auxiliary subunits of voltage-gated Ca2+ channels (VGCCs) are important modulators of VGCC function. Gabapentin interacts with α2δ1 and α2δ2 subunits and is reported to reduce Ca2+ channel current amplitude (ICa). This study aimed to determine the effects of gabapentin on VGCCs in retinal ganglion cells (RGCs). Whole cell patch clamp was used to record ICa in isolated RGCs, and calcium imaging was used to measure Ca2+ transients from RGCs in situ. Immunohistochemistry was used to detect the presence of α2δ1-containing VGCCs in isolated RGCs in the absence and presence of gabapentin pretreatment. Acute administration of gabapentin reduced ICa and Ca2+ transients compared to control conditions. In isolated RGCs, pretreatment with gabapentin (4-18 h) reduced ICa, and cell surface α2δ1 staining was reduced compared to nonpretreated cells. Acute administration of gabapentin to isolated RGCs that had been pretreated further reduced ICa. These results show that gabapentin has both short-term and long-term mechanisms to reduce ICa in isolated RGCs. Some Ca2+ channel blockers have been shown to protect RGCs in retinal trauma suggesting that modulation of VGCCs by gabapentin may prevent the deleterious effects of elevated Ca2+ levels in RGCs in trauma and disease.

Gabapentin alleviates facet-mediated pain in the rat through reduced neuronal hyperexcitability and astrocytic activation in the spinal cord

Although joint pain is common, its mechanisms remain undefined, with little known about the spinal neuronal responses that contribute to this type of pain. Afferent activity and sustained spinal neuronal hyperexcitability correlate to facet joint loading and the extent of behavioral sensitivity induced after painful facet injury, suggesting that spinal neuronal plasticity is induced in association with facet-mediated pain. This study used a rat model of painful C6-C7 facet joint stretch, together with intrathecal administration of gabapentin, to investigate the effects of one aspect of spinal neuronal function on joint pain. Gabapentin or saline vehicle was given via lumbar puncture prior to and at 1 day after painful joint distraction. Mechanical hyperalgesia was measured in the forepaw for 7 days. Extracellular recordings of neuronal activity and astrocytic and microglial activation in the cervical spinal cord were evaluated at day 7. Gabapentin significantly (P = .0001) attenuated mechanical hyperalgesia, and the frequency of evoked neuronal firing also significantly decreased (P < .047) with gabapentin treatment. Gabapentin also decreased (P < .04) spinal glial fibrillary acidic protein expression. Although spinal Iba1 expression was doubled over sham, gabapentin did not reduce it. Facet joint-mediated pain appears to be sustained through spinal neuronal modifications that are also associated with astrocytic activation.

PERSPECTIVE

Intrathecal gabapentin treatment was used to investigate behavioral, neuronal, and glial response in a rat model of painful C6-C7 facet joint stretch. Gabapentin attenuated mechanical hyperalgesia, reduced evoked neuronal firing, and decreased spinal astrocytic activation. This study supports that facet joint pain is sustained through spinal neuronal and astrocytic activation.

De Novo mutations in GNAO1, encoding a Gαo subunit of heterotrimeric G proteins, cause epileptic encephalopathy

Heterotrimeric G proteins, composed of α, β, and γ subunits, can transduce a variety of signals from seven-transmembrane-type receptors to intracellular effectors. By whole-exome sequencing and subsequent mutation screening, we identified de novo heterozygous mutations in GNAO1, which encodes a Gαo subunit of heterotrimeric G proteins, in four individuals with epileptic encephalopathy. Two of the affected individuals also showed involuntary movements. Somatic mosaicism (approximately 35% to 50% of cells, distributed across multiple cell types, harbored the mutation) was shown in one individual. By mapping the mutation onto three-dimensional models of the Gα subunit in three different complexed states, we found that the three mutants (c.521A>G [p.Asp174Gly], c.836T>A [p.Ile279Asn], and c.572_592del [p.Thr191_Phe197del]) are predicted to destabilize the Gα subunit fold. A fourth mutant (c.607G>A), in which the Gly203 residue located within the highly conserved switch II region is substituted to Arg, is predicted to impair GTP binding and/or activation of downstream effectors, although the p.Gly203Arg substitution might not interfere with Gα binding to G-protein-coupled receptors. Transient-expression experiments suggested that localization to the plasma membrane was variably impaired in the three putatively destabilized mutants. Electrophysiological analysis showed that Gαo-mediated inhibition of calcium currents by norepinephrine tended to be lower in three of the four Gαo mutants. These data suggest that aberrant Gαo signaling can cause multiple neurodevelopmental phenotypes, including epileptic encephalopathy and involuntary movements.

Astrocyte-derived thrombospondins mediate the development of hippocampal presynaptic plasticity in vitro

Astrocytes contribute to many neuronal functions, including synaptogenesis, but their role in the development of synaptic plasticity remains unclear. Presynaptic muting of hippocampal glutamatergic terminals defends against excitotoxicity. Here we studied the role of astrocytes in the development of presynaptic muting at glutamatergic synapses in rat hippocampal neurons. We found that astrocytes were critical for the development of depolarization-dependent and G(i/o)-dependent presynaptic muting. The ability of cAMP analogues to modulate presynaptic function was also impaired by astrocyte deficiency. Although astrocyte deprivation resulted in postsynaptic glutamate receptor deficits, this effect appeared independent of astrocytes' role in presynaptic muting. Muting was restored with chronic, but not acute, treatment with astrocyte-conditioned medium, indicating that a soluble factor is permissive for muting. Astrocyte-derived thrombospondins (TSPs) are likely responsible because TSP1 mimicked the effect of conditioned medium, and gabapentin, a high-affinity antagonist of TSP binding to the α2δ-1 calcium channel subunit, mimicked astrocyte deprivation. We found evidence that protein kinase A activity is abnormal in astrocyte-deprived neurons but restored by TSP1, so protein kinase A dysfunction may provide a mechanism by which muting is disrupted during astrocyte deficiency. In summary our results suggest an important role for astrocyte-derived TSPs, acting through α2δ-1, in maturation of a potentially important form of presynaptic plasticity.

Effect of L-methionine on hot flashes in postmenopausal women: a randomized controlled trial

OBJECTIVE

Based on a common mechanism of action with gabapentin, we investigated the effects of L-methionine on hot flashes in postmenopausal women.

METHODS

After a 1-week baseline period, 51 postmenopausal women experiencing at least five moderate-severe hot flashes per day were randomized to one of three groups in a 13:13:25 ratio: placebo/placebo, placebo/L-methionine, or L-methionine/L-methionine, respectively (phase 1/phase 2). Phase 1 was 12 weeks long, and phase 2 was 8 weeks long. Participants took 1 g PO BID in phase 1 and 2 g PO BID in phase 2 of either L-methionine or placebo. All participants also took folate 1 mg and methylcobalamin 0.5 mg a day to help minimize the L-methionine-induced elevation in serum homocysteine. The primary outcome variable was the percent change in hot flash composite score from baseline to week 12 obtained from participants' daily hot flash diaries.

RESULTS

In phases 1 and 2, there were no significant differences between the L-methionine and placebo groups for any of the hot flash outcome measures. At week 12, there was a mean of 37.4% decrease in hot flash composite score compared with baseline in the L-methionine group and a mean of 33.4% decrease in the placebo group (P = 0.60). There were expected significant increases in fasting serum homocysteine (1.7 and 5.8 micromol/L) and fasting serum methionine (13.9 and 22.3 micromol/L) at weeks 12 and 20, respectively, associated with the two doses of L-methionine therapy relative to placebo therapy.

CONCLUSIONS

L-methionine therapy seems to be ineffective in the treatment of hot flashes in postmenopausal women.

Pruritus in burns: review article

Pruritus represents a common and distressing feature of burn wounds. Over the last decades, significant advances in neuroanatomical and neurophysiological knowledge have resulted in the elucidation of the mediators and pathways involved in the transmission of pruritic impulses. A plethora of therapeutic approaches have been evaluated mostly in small-scale studies involving burns patients targeting both the peripheral and the central components of the neurologic pathway. Antihistamines, doxepin, massage therapy, and transcutaneous electrical nerve stimulation are effective strategies to combat pruritus in burns patients. Recent studies have provided preliminary evidence regarding the effectiveness of gabapentin and ondansetron. The area of burns pruritus is under-researched and large-scale studies are required to reinforce the armamentarium of specialists with evidence-based regimens for the treatment of this highly distressing symptom.

The antinociceptive effect of systemic gabapentin is related to the type of sensitization-induced hyperalgesia

Background

Gabapentin is a structural analogue of gamma-aminobutyric acid with strong anticonvulsant and analgesic activities. Important discrepancies are observed on the effectiveness and potency of gabapentin in acute nociception and sensitization due to inflammation and neuropathy. There is also some controversy in the literature on whether gabapentin is only active in central areas of the nervous system or is also effective in the periphery. This is probably due to the use of different experimental models, routes of administration and types of sensitization. The aim of the present study was to investigate the influence of the spinal cord sensitization on the antinociceptive activity of gabapentin in the absence and in the presence of monoarthritis and neuropathy, using the same experimental protocol of stimulation and the same technique of evaluation of antinociception.

Methods

We studied the antinociceptive effects of iv. gabapentin in spinal cord neuronal responses from adult male Wistar rats using the recording of single motor units technique. Gabapentin was studied in the absence and in the presence of sensitization due to arthritis and neuropathy, combining noxious mechanical and repetitive electrical stimulation (wind-up).

Results

The experiments showed that gabapentin was effective in arthritic (max. effect of 41 ± 15% of control and ID50 of 1,145 ± 14 micromol/kg; 200 mg/kg) and neuropathic rats (max. effect of 20 ± 8% of control and ID50 of 414 ± 27 micromol/kg; 73 mg/kg) but not in normal rats. The phenomenon of wind-up was dose-dependently reduced by gabapentin in neuropathy but not in normal and arthritic rats.

Conclusion

We conclude that systemic gabapentin is a potent and effective antinociceptive agent in sensitization caused by arthritis and neuropathy but not in the absence of sensitization. The potency of the antinociception was directly related to the intensity of sensitization in the present experimental conditions. The effect is mainly located in central areas in neuropathy since wind-up was significantly reduced, however, an action on inflammation-induced sensitized nociceptors is also likely.

Gabapentin completely attenuated the acute morphine induced c-Fos expression in the rat striatum

The neuro-anatomical sites and molecular mechanism of action of gabapentin (GBP)-morphine interaction to prevent and reverse morphine side effects as well as enhancement of the analgesic effect of morphine is not known. Therefore, we examined the combined effects of GBP-Morphine on acute morphine induced c-Fos expression in rat striatum. The combined effect of GBP-Morphine was examined by means of c-Fos immunohistochemistry. A single intraperitoneal injection (i.p.) of morphine (10 mg/kg), saline (control), co-injection of GBP (150 mg/kg) with morphine (10 mg/kg) was administered under anaesthesia. Ninety minutes after drugs administration the deeply anesthetized rats were perfused transcardially with 4% paraformaldehyde. Serial 40 mum thick sections of brain were cut and processed by immunohistochemistry to locate and quantify the sites and number of neurons with c-Fos immunoreactivity. Detection of c-Fos protein was performed using the peroxidase-antiperoxidase (PAP) detection protocol. Our present study demonstrated that, administration of GBP (150 mg/kg, i.p.) in combination with morphine (10 mg/kg, i.p.) significantly (p < 0.01) attenuated the acute morphine (10 mg/kg, i.p.) induced c-Fos expression in the rat striatum. Present results showed that GBP-morphine combination action prevented the acute morphine induced c-Fos expression in rat striatum. Moreover, this study provides first evidence of neuro-anatomical site and that GBP neutralized the morphine induced activation of rat striatum.

Inhibition of the rat brain sodium channel Nav1.2 after prolonged exposure to gabapentin

Prolonged exposure of neurons to gabapentin inhibits repetitive firing of Na(+)-dependent action potentials. Here, we studied the effect of such prolonged exposure to gabapentin on a rat sodium channel, Nav1.2. After 3 days of continuous incubation with gabapentin (10-1000 microM), Nav1.2 current density was decreased dose-dependently relative to untreated cells. The reduction was 57% at 30 microM gabapentin, while higher concentrations (100-1000 microM) did not result in greater effects. Prolonged treatment with gabapentin also caused the channel to inactivate at more hyperpolarized potentials. These effects provide a mechanistic basis for the inhibition of Na(+)-dependent repetitive firing upon prolonged exposure to gabapentin and may contribute to its anticonvulsant activity.

The acute effects of gabapentin in combination with alcohol in heavy drinkers

BACKGROUND

Alcohol effects in humans involve gamma-amino butyric acid (GABA) neurotransmission. It has been proposed that GABAergic medications may be effective in the treatment of alcohol dependence. This study evaluated the acute effects of gabapentin, an anticonvulsant that increases extracellular GABA, on the subjective, physiological, and performance effects of alcohol in heavy (mean 34 drinks per week) alcohol drinkers.

METHODS

Seventeen volunteers without alcohol dependence were tested using a double-blind design with three 3-day long inpatient phases, each separated by at least a 1-week wash-out period. Each phase, gabapentin (0, 1000, or 2000mg) was administered 4h before alcohol (0.75g/kg), which was given in four divided doses every 20min.

RESULTS

Gabapentin impaired the ability to balance without producing changes in subjective, physiological or other performance measures. Pretreatment with gabapentin did not significantly alter subjective and performance effects of alcohol and did not alter alcohol craving. Gabapentin, dose-dependently enhanced alcohol-induced tachycardia.

CONCLUSIONS

Acute gabapentin administration was well tolerated in combination with alcohol, but did not alter the effects of alcohol.

Targeting N-type and T-type calcium channels for the treatment of pain

Severe chronic pain afflicts a large number of people worldwide but satisfactory relief from such pain is difficult to achieve with drugs that are currently available, and so there is a great need for the development of new, efficacious and safe analgesics. Voltage-gated calcium-permeable ion channels are multi-subunit complexes that regulate neuronal excitability, action-potential firing patterns and neurotransmission in nociceptive pathways. Although multiple subtypes of voltage-gated calcium channels exist, pharmacological and ion-channel gene knockdown approaches in animals have revealed N-type and T-type calcium channels to be particularly attractive molecular targets for the discovery and development of new analgesic drugs. The recent approval of Prialt (Elan Pharmaceuticals) provides the ultimate target validation for N-type calcium channels, namely proof that they are key regulators of nociceptive signaling in humans.

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.

The nitric oxide-cyclic GMP-protein kinase G-K+ channel pathway participates in the antiallodynic effect of spinal gabapentin

The possible participation of the nitric oxide (NO)-cyclic GMP-protein kinase G (PKG) pathway on gabapentin-induced spinal antiallodynic activity was assessed in spinal nerve injured rats. Intrathecal gabapentin, diazoxide or pinacidil reduced tactile allodynia in a dose-dependent manner. Pretreatment with NG-L-nitro-arginine methyl ester (L-NAME, non-specific inhibitor of NO synthase NOS), 7-nitroindazole (neuronal NO synthase inhibitor), 1H-[1,2,4] -oxadiazolo [4,3-a] quinoxalin-1-one (ODQ, guanylyl cyclase inhibitor) or (9S, 10R, 12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo-[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT-5823, specific PKG inhibitor), but not NG-D-nitro-arginine methyl ester (D-NAME) or okadaic acid (protein phosphatase 1 and 2 inhibitor) prevented gabapentin-induced antiallodynia. Pinacidil activity was not blocked by L-NAME, D-NAME, 7-nitroindazole, ODQ, KT-5823 or okadaic acid. Moreover, KT-5823, glibenclamide (ATP-sensitive K+ channel blocker), apamin and charybdotoxin (small- and large-conductance Ca2+-activated K+ channel blockers, respectively), but not margatoxin (voltage-gated K+ channel blocker), L-NAME, 7-nitroindazole, ODQ or okadaic acid, reduced diazoxide-induced antiallodynia. Data suggest that gabapentin-induced spinal antiallodynia could be due to activation of the NO-cyclic GMP-PKG-K+ channel pathway.

The antiallodynic action target of intrathecal gabapentin: Ca2+ channels, KATP channels or N-methyl-d-aspartic acid receptors?

Gabapentin is a novel analgesic whose mechanism of action is not known. We investigated in a postoperative pain model whether adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channels, N-methyl-d-aspartic acid (NMDA) receptors, and Ca2+ channels are involved in the antiallodynic effect of intrathecal gabapentin. Mechanical allodynia was induced by a paw incision in isoflurane-anesthetized rats. Withdrawal thresholds to von Frey filament stimulation near the incision site were measured before and after incision and after intrathecal drug administration. The antiallodynic effect of gabapentin (100 mug) was not affected by intrathecal pretreatment with antagonists of K(ATP) channels, NMDA receptors or gamma-aminobutyric acid (GABA)(A) receptors. K(ATP) channel openers and GABA(A) receptor agonist, per se, had little effect on the postincision allodynic response. The Ca2+ channel blocker of N-type (omega-conotoxin GVIA, 0.1-3 microg), but not of P/Q-type (omega-agatoxin IVA), L-type (verapamil, diltiazem or nimodipine), or T-type (mibefradil), attenuated the incision-induced allodynia, as did gabapentin. Both the antiallodynic effects of gabapentin and omega-conotoxin GVIA were attenuated by Bay K 8644, an L-type Ca2+ channel activator. These results provide correlative evidence to support the contention that N-type Ca2+ channels, but not K(ATP) channels or NMDA or GABA(A) receptors, might be involved in the antiallodynic effect of intrathecal gabapentin.

Use of oxcarbazepine to treat a pediatric patient with resistant complex regional pain syndrome

We describe a 12-year-old patient with severe, protracted complex regional pain syndrome type I. His pain did not respond to gabapentin, amitriptyline, physical therapy, opioids, or nonsteroidal drugs. Sympathetic or regional block was not attempted because of persistent bacteremia and severe local sepsis. His pain responded dramatically to the addition of oxcarbazepine, with rapid improvement in his symptoms and functional status. We suggest that oxcarbazepine might be a useful adjunct in the treatment of gabapentin-resistant complex regional pain syndrome type I in children and should be considered.

PERSPECTIVE

Oxcarbazepine's antinociceptive effect is mediated via sodium channel inhibition in neuropathic models and by inhibition of substance P and prostaglandins in anti-inflammatory models. The efficacy of oxcarbazepine in this patient might be attributable to these mechanisms or possibly to synergism with either gabapentin or the anti-inflammatory effects produced by amitriptyline.

Ion Channel Targets and Treatment Efficacy in Neuropathic Pain

Chronic neuropathic pain due to injury or dysfunction of the nervous system remains a formidable treatment challenge in spite of a growing range of medication choices. We review current clinical research supporting the use of ion channel modulators for neuropathic pain states. New modes of local drug delivery, novel Ca2+ channel targets, and increased choices for drugs with activity at Na+channels are transforming this longstanding therapeutic strategy. Clinical decision making is increasingly informed by a more nuanced understanding of the role of voltage-gated Na+channels (VGSCs) and Ca2+ channels (VGCCs) in the pathophysiology of nerve injury. Although holding great promise for the future, mechanism-based approaches to treatment will require greater understanding of the analgesic mechanisms of drug action and of the relationships between pathophysiologic mechanisms and clinical presentation.

PERSPECTIVE

Treatment options for neuropathic pain targeting ion channels have grown rapidly in the past decade. An evolving body of clinical research supports the widespread use of this longstanding therapeutic strategy. Improved efficacy of ion channel modulators hinges upon further elucidation of the relationship between signs and symptoms of pain and underlying pathophysiology.

Gabapentin for the Symptomatic Treatment of Chronic Neuropathic Pain in Patients with Late-Stage Lyme Borreliosis: A Pilot Study

BACKGROUND

Chronic neuropathic pain occurs in 10-15% of patients with neuroborreliosis and is difficult to treat.

OBJECTIVE

We evaluated the effect of gabapentin monotherapy on residual pain in patients with neuroborreliosis after intravenous ceftriaxone treatment.

METHODS

Ten patients with neuroborreliosis and a long-lasting history of neurologic symptoms were treated with gabapentin, starting with 300 mg/day. Doses were raised over a period of 4-12 weeks to the individually effective and tolerated maximum dose (500-1,200 mg). Treatment was maintained until pain disappeared and then gradually reduced in dose over weeks. If symptoms recurred, the doses were raised again. Therapy was maintained over an average of 1-2 years.

RESULTS

Pain quality and pain quantity were evaluated using the McGill pain questionnaire and a visual analogue scale. There was an improvement of 'crawling' and 'burning' pain sensations, neck and radiating lumbar pain in 9/10 (90%) patients as well as a positive effect on mood, general feeling of health and quality of sleep in 5/10 (50%) patients. The average dose leading to a clear-cut pain reduction was 700 mg.

CONCLUSIONS

In an open pilot study (10 patients), gabapentin monotherapy which has to our knowledge not been published as treatment of chronic neuropathic pain in patients with late Lyme borreliosis is efficacious in treating pain associated with neuroborreliosis and can thus improve quality of life in these patients.

Migraine prophylactic drugs work via ion channels

In recent decades, the concept of a vascular origin of migraine has been replaced by theories based on a neuronal pathophysiology. These theories all involve rapid changes in the functioning of the brain, particularly the brain stem, and the trigeminal nerves. While such paroxysmal changes in function could be the result of altered synaptic transmission, or other physiological changes, they could also be due to changes in the function of voltage-regulated sodium and calcium ion channels. Support for this view of migraine as a channelopathy comes from an examination of the likely mechanism of action of migraine prophylactic drugs. It is the present hypothesis that most of the widely used drugs for migraine prevention work by inhibiting the function of one or both of these ion channels. A review of the laboratory research done on most of the commonly used migraine prophylactic drugs, divided into five classes, reveals that they all may work on sodium channels, calcium channels, or both. If this is the common mechanism of action of migraine prophylactics, it should lead toward the development of more effective prophylactic drugs.

Inhibition of Persistent Sodium Current Fraction and Voltage-gated L-type Calcium Current by Propofol in Cortical Neurons: Implications for Its Antiepileptic Activity

PURPOSE

Although it is widely used in clinical practice, the mechanisms of action of 2,6-di-isopropylphenol (propofol) are not completely understood. We examined the electrophysiologic effects of propofol on an in vitro model of epileptic activity obtained from a slice preparation.

METHODS

The effects of propofol were tested both on membrane properties and on epileptiform events consisting of long-lasting, paroxysmal depolarization shifts (PDSs) induced by reducing the magnesium concentration from the solution and by adding bicuculline and 4-aminopyridine. These results were integrated with a patch-clamp analysis of Na(+) and high-voltage activated (HVA) calcium (Ca(2+)) currents from isolated cortical neurons.

RESULTS

In bicuculline, to avoid any interference by gamma-aminobutyric acid (GABA)-A receptors, propofol (3-100 microM) did not cause significant changes in the current-evoked, sodium (Na(+))-dependent action-potential discharge. However, propofol reduced both the duration and the number of spikes of PDSs recorded from cortical neurons. Interestingly, relatively low concentrations of propofol [half-maximal inhibitory concentration (IC(50)), 3.9 microM) consistently inhibited the "persistent" fraction of Na(+) currents, whereas even high doses (< or =300 microM) had negligible effects on the "fast" component of Na(+) currents. HVA Ca(2+) currents were significantly reduced by propofol, and the pharmacologic analysis of this effect showed that propofol selectively reduced L-type HVA Ca(2+) currents, without affecting N or P/Q-type channels.

CONCLUSIONS

These results suggest that propofol modulates neuronal excitability by selectively suppressing persistent Na(+) currents and L-type HVA Ca(2+) conductances in cortical neurons. These effects might cooperate with the opening of GABA-A-gated chloride channels, to achieve depression of cortical activity during both anesthesia and status epilepticus.

N-salicyloyltryptamine, a new anticonvulsant drug, acts on voltage-dependent Na+, Ca2+, and K+ ion channels

1. The aim of this work was to study the effects of N-salicyloyltryptamine (STP), a novel anticonvulsant agent, on voltage-gated ion channels in GH3 cells. 2. In this study, we show that STP at 17 microM inhibited up to 59.2+/-10.4% of the Ito and 73.1+/-8.56% of the IKD K+ currents in GH3 cells. Moreover, the inhibitory activity of the drug STP on K+ currents was dose-dependent (IC50=34.6+/-8.14 microM for Ito) and partially reversible after washing off. 3. Repeated stimulation at 1 Hz (STP at 17 microM) led to the total disappearance of Ito current, and an enhancement of IKD. 4. In the cell-attached configuration, application of STP to the bath increased the open probability of large-conductance Ca2+-activated K+ channels. 5. STP at 17 microM inhibited the L-type Ca2+ current by 54.9+/-7.50% without any significant changes in the voltage dependence. 6. STP at 170 microM inhibited the TTX-sensitive Na+ current by 22.1+/-2.41%. At a lower concentration (17 microM), no effect on INa was observed. 7. The pharmacological profile described here might contribute to the neuroprotective effect exerted by this compound in experimental 'in vivo' models.

Glycosylation of asparagines 136 and 184 is necessary for the α2δ subunit-mediated regulation of voltage-gated Ca2+channels

The CaValpha2delta auxiliary subunit is a glycosylated protein that regulates the trafficking and function of voltage-gated Ca2+ channels. One of the most prominent roles of CaValpha2delta is to increase whole-cell Ca2+ current amplitude. Using N-glycosidase F and truncated forms of CaValpha2delta, earlier studies suggested an important role for N-linked glycosylation in current stimulation. Here, we used site-directed mutagenesis and heterologous expression in HEK-293 cells to examine the impact of individual glycosylation sites within the CaValpha2delta subunit on the regulation of Ba2+ currents through recombinant Ca2+ channels. We found two N-glycosylation consensus sites (NX(S/T)) in the extracellular alpha2 domain of the protein that are functional. Substitution of asparagines for glutamines at amino acid positions 136 and 184 rendered these sites non-functional as shown by patch-clamp experiments. These results corroborate that N-glycosylation is required for the CaValpha2delta subunit-induced current stimulation and suggest that sites N136 and N184 are directly involved in this action. Likewise, N136Q and N184Q mutations prevented whole-cell current stimulation without altering its kinetic properties, suggesting a regulation on the number of functional channels at the plasma membrane.

Dual effects of gabapentin and pregabalin on glutamate release at rat entorhinal synapses in vitro

We have recently shown that the anticonvulsant drugs phenytoin, lamotrigine and sodium valproate all reduce the release of glutamate at synapses in the entorhinal cortex in vitro. In the present investigation we determined whether this property was shared by gabapentin and pregabalin, using whole-cell patch-clamp recordings of excitatory postsynaptic currents (EPSCs) in layer V neurons in slices of rat entorhinal cortex. Both drugs reduced the amplitude and increased the paired-pulse ratio of EPSCs evoked by electrical stimulation of afferent inputs, suggesting a presynaptic effect to reduce glutamate release. The frequency of spontaneous EPSCs (sEPSCs) was concurrently reduced by GBP, further supporting a presynaptic action. There was no significant change in amplitude although a slight reduction was seen, particularly with gabapentin, which may reflect a reduction in the number of larger amplitude sEPSCs. When activity-independent miniature EPSCs were recorded in the presence of tetrodotoxin, both drugs continued to reduce the frequency of events with no change in amplitude. The reduction in frequency induced by gabapentin or pregabalin was blocked by application of the l-amino acid transporter substrate l-isoleucine. The results show that gabapentin and pregabalin, like other anticonvulsants, reduce glutamate release at cortical synapses. It is possible that this reduction is a combination of two effects: a reduction of activity-dependent release possibly via interaction with P/Q-type voltage-gated Ca channels, and a second action, as yet unidentified, occurring downstream of Ca influx into the presynaptic terminals.

A study comparing the actions of gabapentin and pregabalin on the electrophysiological properties of cultured DRG neurones from neonatal rats

BACKGROUND

Gabapentin and pregabalin have wide-ranging therapeutic actions, and are structurally related to the inhibitory neurotransmitter GABA. Gabapentin, pregablin and GABA can all modulate voltage-activated Ca2+ channels. In this study we have used whole cell patch clamp recording and fura-2 Ca2+ imaging to characterise the actions of pregabalin on the electrophysiological properties of cultured dorsal root ganglion (DRG) neurones from neonatal rats. The aims of this study were to determine whether pregabalin and gabapentin had additive inhibitory effects on high voltage-activated Ca2+ channels, evaluate whether the actions of pregabalin were dependent on GABA receptors and characterise the actions of pregabalin on voltage-activated potassium currents.

RESULTS

Pregabalin (25 nM - 2.5 microM) inhibited 20-30% of the high voltage-activated Ca2+ current in cultured DRG neurones. The residual Ca2+ current recorded in the presence of pregabalin was sensitive to the L-type Ca2+ channel modulator, Bay K8644. Saturating concentrations of gabapentin failed to have additive effects when applied with pregabalin, indicating that these two compounds act on the same type(s) of voltage-activated Ca2+ channels but the majority of Ca2+ current was resistant to both drugs. The continual application of GABA, the GABAB receptor antagonist CGP52432, or intracellular photorelease of GTP-gamma-S had no effect on pregabalin-induced inhibition of Ca2+ currents. Although clear inhibition of Ca2+ influx was produced by pregabalin in a population of small neurones, a significant population of larger neurones showed enhanced Ca2+ influx in response to pregabalin. The enhanced Ca2+ influx evoked by pregabalin was mimicked by partial block of K+ conductances with tetraethylammonium. Pregabalin produced biphasic effects on voltage-activated K+ currents, the inhibitory effect of pregabalin was prevented with apamin. The delayed enhancement of K+ currents was attenuated by pertussis toxin and by intracellular application of a (Rp)-analogue of cAMP.

CONCLUSIONS

Pregabalin reduces excitatory properties of cultured DRG neurones by modulating voltage-activated Ca2+ and K+ channels. The pharmacological activity of pregabalin is similar but not identical to that of gabapentin. The actions of pregabalin may involve both extracellular and intracellular drug target sites and modulation of a variety of neuronal conductances, by direct interactions, and through intracellular signalling involving protein kinase A.

Antiepileptic drugs and cortical excitability: a study with repetitive transcranial stimulation

Repetitive transcranial magnetic stimulation (rTMS) delivered at various intensities and frequencies excites cortical motor areas. Trains of stimuli (at 5 Hz frequency, and suprathreshold intensity) progressively increase the size of muscle evoked potentials (MEPs) and the duration of the cortical silent period (CSP) in normal subjects. The aim of this study was to evaluate the effect of the antiepileptic drugs carbamazepine, gabapentin, and topiramate on cortical excitability variables tested with rTMS. We tested the changes in motor threshold, MEP size and CSP duration evoked by focal rTMS in 23 patients with neuropathic pain before and after a 1-week course of treatment with carbamazepine, gabapentin, topiramate and placebo. None of the three antiepileptic drugs changed the resting or active magnetic and electrical motor threshold. Antiepileptic treatment, but not placebo, abolished the normal rTMS-induced facilitation of MEPs, but left the progressive lengthening of the CSP during the rTMS train unchanged. Our results suggest that carbamazepine, gabapentin and topiramate modulate intracortical excitability by acting selectively on excitatory interneurons.

Tiagabine synergistically interacts with gabapentin in the electroconvulsive threshold test in mice

Polytherapy, based on the rational combining of antiepileptic drugs (AEDs), is required for patients with drug-resistant epilepsy. In such cases, the combinations of AEDs usually offer a significant enhancement of their protective effects against seizures. There has appeared a hypothesis that combining two AEDs, influencing the same neurotransmitter system, results in the potentialization of their anticonvulsant effects. For corroborating this hypothesis, a pharmacological character of interaction between tiagabine (TGB) and gabapentin (GBP)-two novel AEDs affecting the GABA-ergic system, in the maximal electroshock seizure threshold (MEST)-test in mice was evaluated. TGB at the dose of 4 mg/kg and GBP at 75 mg/kg significantly raised the electroconvulsive threshold. Further, using the isobolographic calculations, TGB was coadministered with GBP at three fixed-ratios (1 : 3, 1 : 1, and 3 : 1) of their respective protective drug doses. All examined combinations of TGB with GBP exerted supra-additive (synergistic) interactions against MEST-induced seizures in mice. The interaction index, describing the strength and magnitude of interaction, ranged between 0.25 and 0.50 indicating supra-additivity. Adverse (neurotoxic) effects were evaluated in the chimney (motor performance) and the step-through, light-dark passive avoidance (long-term memory) tests in mice. The examined combinations of TGB with GBP did not affect the motor coordination, except for the fixed-ratio of 1 : 1, at which significant impairment of motor performance was observed. Moreover, all combinations selectively impaired the acquisition of the task in the passive avoidance test, having no impact on consolidation and retrieval in the long-term memory test. The pain threshold test revealed that the observed disturbances in the passive avoidance testing resulted presumably from the antinociceptive activity of these AEDs in combinations. After lengthening the exposing time to the direct current stimulus in the passive avoidance test from 2 to 6 s, the acquisition of the task, in animals receiving the combinations of TGB and GBP was not impaired. Neither the plasma, nor brain concentrations of GBP were affected by TGB application, so pharmacokinetic events that might negatively influence the observed effects are not probable. Results of this study clearly indicate that the activation of the same neurotransmitter system (GABA-ergic) leads to a synergistic interaction. The pain threshold test is a very good paradigm for screening the antinociceptive properties of AEDs, which may disturb the long-term memory testing in animals. Combinations of TGB with GBP (very promising from a preclinical point of view) should be clinically verified for elaborating the most effective treatment regimen in patients with intractable seizures.

Potent analgesic effects of anticonvulsants on peripheral thermal nociception in rats

1. Anticonvulsant agents are commonly used to treat neuropathic pain conditions because of their effects on voltage- and ligand-gated channels in central pain pathways. However, their interaction with ion channels in peripheral pain pathways is poorly understood. Therefore, we studied the potential analgesic effects of commonly used anticonvulsant agents in peripheral nociception. 2. We injected anticonvulsants intradermally into peripheral receptive fields of sensory neurons in the hindpaws of adult rats, and studied pain perception using the model of acute thermal nociception. Commonly used anticonvulsants such as voltage-gated Na+ channel blockers, phenytoin and carbamazepine, and voltage-gated Ca2+ channel blockers, gabapentin and ethosuximide, induced dose-dependent analgesia in the injected paw, with ED50 values of 0.30, 0.32 and 8, 410 microg per 100 microl, respectively. 3. Thermal nociceptive responses were not affected in the contralateral, noninjected paws, indicating a lack of systemic effects with doses of anticonvulsants that elicited local analgesia. 4. Hill slope coefficients for the tested anticonvulsants indicate that the dose-response curve was less steep for gabapentin than for phenytoin, carbamazepine and ethosuximide. 5. Our data strongly suggest that cellular targets like voltage-gated Na+ and Ca2+ channels, similar to those that mediate the effects of anticonvulsant agents in the CNS, may exist in the peripheral nerve endings of rat sensory neurons. Thus, peripherally applied anticonvulsants that block voltage-gated Na+ and Ca2+ channels may be useful analgesics.

Gabapentin reverses mechanical allodynia induced by sciatic nerve ischemia and formalin-induced nociception in mice

The anticonvulsant drug gabapentin has been demonstrated to alleviate symptoms of painful diabetic neuropathy as well as other types of neuropathic pain. The aim of the present study was to investigate the effect of gabapentin in a recently developed mouse model of peripheral neuropathy. This model is based on a photochemical ischemic lesion of the sciatic nerve generated by laser-induced activation of the photosensitizing dye erythrosin B. Following laser irradiation of the sciatic nerve for 2, 5, or 10 min, tactile allodynia was observed during at least 3 weeks. The degree of allodynia was most marked following 10 min of irradiation. Subcutaneous administration of gabapentin [175-300 micromol/kg ( approximately 30-51 mg/kg), cumulative doses, at 1-h intervals] significantly reversed tactile allodynia induced by 10-min laser irradiation. The maximal dose of gabapentin increased the withdrawal threshold from approximately 0.55 to approximately 1.85 g (i.e., about 77% of the threshold in normal animals, approximately 2.4 g). Gabapentin did not affect the tactile withdrawal threshold in intact animals. A dose of gabapentin (100 micromol/kg, sc) that had no effect on allodynia was found to significantly reduce the pain behavior during phase 2 of the formalin test. The present study demonstrates that systemic administration of gabapentin suppresses both allodynia induced by an ischemic lesion of the sciatic nerve and pain behavior in the formalin test.

Gabapentin increases the hyperpolarization-activated cation current Ih in rat CA1 pyramidal cells

PURPOSE

Gabapentin (GBP) is a commonly used drug in the treatment of partial seizures, but its mode of action is still unclear. The genesis of seizures in temporal lobe epilepsy is thought to be crucially influenced by intrinsic membrane properties. Because the Ih substantially contributes to the intrinsic membrane properties of neurons, the effects of GBP on the Ih were investigated in CA1 pyramidal cells of rat hippocampus.

METHODS

CA1 pyramidal cells in hippocampal slices were examined by using the whole-cell patch-clamp technique.

RESULTS

GBP increased the Ih amplitude in a concentration-dependent manner mainly by increasing the conductance, without significant changes in the activation properties or in the time course of Ih. The effects ranged from approximately 20% at 50 microM, approximately 25% at 75 microM, to approximately 35% at 100 microM GBP (at -110 mV). In the presence of intracellular cyclic adenosine monophosphate (cAMP), the effects of GBP on Ih were similar to those obtained in the absence of cAMP.

CONCLUSIONS

These results suggest that GBP increases the Ih through a cAMP-independent mechanism. Because the applied GBP concentrations were in a clinically relevant range, the observed effect may contribute to the anticonvulsant action of GBP in partial seizures and may represent a new concept of how this anticonvulsant drug works.

Gabapentin for the treatment of tremor

OBJECTIVE

To review the use of the antiepileptic drug gabapentin for the treatment of various types of tremor.

DATA SOURCES

A search of biomedical literature was completed through MEDLINE and EMBASE (1993-May 2002) to identify all clinical trials pertaining to the use of gabapentin for the treatment of tremor in humans.

DATA SYNTHESIS

Outcome data from the few published studies have varied widely. Patient groups have been small, and conclusions have been based largely on subjective patient and investigator ratings.

CONCLUSIONS

A trial of gabapentin is warranted in patients who fail therapy with traditional agents. Improvement should be measured by a patient's perceived functional ability.

Gabapentin withdrawal presenting as status epilepticus

A 34-year-old male with lumbar disc disease and surgery was placed on gabapentin daily for chronic back pain. He remained on a steady dose of 8000 mg/day for 9 months, almost doubled what is considered therapeutic. He ran out of medication, was unable to refill his prescription for 2 days and presented to the emergency department in status epilepticus. There was no previous history of seizure disorder and he was on no other medications. A medical evaluation for an alternative etiology of his seizures was negative. Although gabapentin withdrawal has been previously reported and usually consists of anxiety, diaphoresis, and palpitations, this is the first reported patient with generalized seizures and status epilepticus secondary to gabapentin withdrawal.

Gabapentin-- actions on adult superficial dorsal horn neurons

Despite identification of GABA(B) receptors with gb1a-gb2 composition and the alpha2delta calcium channel subunit as putative molecular targets for gabapentin (GBP), its cellular mechanism of action has remained elusive. Therefore, we have used an in vitro spinal cord slice preparation to study the effects of GBP on lamina II neurons. The frequency and amplitude of spontaneous EPSCs and IPSCs were unaffected by GBP, suggesting presynaptic neurotransmitter release is not regulated. Direct modulation of postsynaptic membrane excitability is also unlikely since the level of holding current required to maintain neurons at -70, 0 and +45 mV was unaffected by GBP. Effects on excitatory and inhibitory synaptic transmission were variable across the population. Primary afferent-evoked fast glutamatergic EPSCs were unaffected by GBP, while evoked NMDA receptor-mediated EPSCs and IPSCs were variably affected. In contrast, GBP enhanced responses to bath applied NMDA in 71% of neurons. Thus, in adult rat dorsal horn, synaptic and extrasynaptic NMDA receptors may be differentially regulated by GBP perhaps due to differences in subunit composition.

Long-term regulation of voltage-gated Ca(2+) channels by gabapentin

Gabapentin (GBP) is a gamma-aminobutyric acid analog effective in the treatment of seizures. A high-affinity interaction between GBP and the alpha(2)delta subunit of the voltage-gated Ca(2+) channels has been documented. In this report, we examined the effects of the chronic treatment with GBP on neuronal recombinant P/Q-type Ca(2+) channels expressed in Xenopus oocytes. GBP did not affect significantly the amplitude or the voltage dependence of the currents. Exposure to the drug did, however, slow down the kinetics of inactivation in a dose-dependent fashion. In addition, biochemical analysis showed that the integrity of Ca(2+) channel complex is not apparently affected by GBP binding, suggesting that chronic treatment with the drug might cause the channel kinetic modification through subtle conformational changes of the protein complex.

Gabapentin inhibits presynaptic Ca(2+) influx and synaptic transmission in rat hippocampus and neocortex

Gabapentin is a widely used drug with anticonvulsant, antinociceptive and anxiolytic properties. Although it has been previously shown that Gabapentin binds with high affinity to the alpha(2)delta subunit of voltage-operated Ca(2+) channels (VOCC), little is known about the functional consequences of this interaction. Here, we investigated the effect of Gabapentin on VOCCs and synaptic transmission in rat hippocampus and neocortex using whole-cell patch clamp and confocal imaging techniques. Gabapentin (100-300 microM) did not affect the peak amplitude or voltage-dependency of VOCC currents recorded from either dissociated or in situ neocortical and hippocampal pyramidal cells. In contrast, Gabapentin inhibited K(+)-evoked increases in [Ca(2+)] in a subset of synaptosomes isolated from rat hippocampus and neocortex in a dose-dependent manner, with an apparent half-maximal inhibitory effect at approximately 100 nM. In hippocampal slices, Gabapentin (300 microM) inhibited the amplitude of evoked excitatory- and inhibitory postsynaptic currents recorded from CA1 pyramidal cells by 30-40%. Taken together, the results suggest that Gabapentin selectively inhibits Ca(2+) influx by inhibiting VOCCs in a subset of excitatory and inhibitory presynaptic terminals, thereby attenuating synaptic transmission.

Lamotrigine derivatives and riluzole inhibit INa,P in cortical neurons

The persistent, slowly inactivating fraction of the sodium current is involved in key functions in the CNS such as dendritic integration of synaptic inputs and cellular excitability. We have studied whether established anti-epileptic drugs and neuroprotective agents target the persistent sodium current. Two lamotrigine derivatives (sipatrigine and 202W92) and riluzole inhibited the persistent sodium current at low, therapeutic concentrations. In contrast, lamotrigine and the classical antiepileptic agents phenytoin and valproic acid blocked the fast-inactivating sodium channel but failed to affect the persistent fraction. The ability to influence either mode of channel activity may represent a defining feature of each drug subclass, changing profoundly their clinical indications. Given the damaging role of a sustained influx of sodium in both pharmaco-resistant seizures or excitotoxic insults, we suggest the utilization of drugs that suppress the persistent conductance.

Prevention of oxaliplatin-induced peripheral sensory neuropathy by carbamazepine in patients with advanced colorectal cancer

Oxaliplatin plays a key role in the treatment of advanced colorectal cancer. The dose-limiting side effect of this platinum analogue is neurotoxicity. Significant efforts have been undertaken in an attempt to prevent and/or circumvent the development of neurotoxicity. Sodium channel inactivation kinetics on rat sensory sural nerve preparations are altered after exposure to oxaliplatin. Carbamazepine antagonizes this effect in vitro. Results from preliminary clinical studies indicate that the sodium channel blockers carbamazepine and gabapentin may be effective in preventing neurotoxicity. The role of amifostine is not yet clear. Randomized clinical studies are necessary to confirm the potential benefit of carbamazepine and other sodium channel blockers in preventing and/or overcoming the development of oxaliplatin-induced neurotoxicity.

Gabapentin inhibits high-threshold calcium channel currents in cultured rat dorsal root ganglion neurones

1. This study examined the action of gabapentin (gabapentin,1-(aminomethyl) cyclohexane acetic acid (Neurontin) on voltage-gated calcium (Ca(2+)) channel influx recorded in cultured rat dorsal root ganglion (DRG) neurones. 2. Voltage-gated Ca(2+) influx was monitored using both fura-2 based fluorescence Ca(2+) imaging and the whole-cell patch clamp technique. 3. Imaging of intracellular Ca(2+) transients revealed that gabapentin inhibited KCl (30 mM)-evoked voltage-dependent Ca(2+) influx. Both the duration for 50% of the maximum response (W50) and total Ca(2+) influx were significantly reduced by approximately 25-30% in the presence of gabapentin (25 microM). 4. Gabapentin potently inhibited the peak whole-cell Ca(2+) channel current (I(Ba)) in a dose-dependent manner with an estimated IC(50) value of 167 nM. Block was incomplete and saturated at a maximal concentration of 25 microM. 5. Inhibition was significantly decreased in the presence of the neutral amino acid L-isoleucine (25 microM) but unaffected by application of the GABA(B) antagonist, saclofen (200 microM), suggesting a direct action on the alpha(2)delta subunit of the Ca(2+) channel. 6. Gabapentin inhibition was voltage-dependent, producing an approximately 7 mV hyperpolarizing shift in current voltage properties and reducing a non-inactivating component of whole-cell current activated at relatively depolarized potentials. 7. The use of specific Ca(2+) channel antagonists revealed a mixed pharmacology of the gabapentin-sensitive current (N-, L- and P/Q-type), which is dominated by N-type current. 8. The present study is the first to demonstrate that gabapentin directly mediates inhibition of voltage-gated Ca(2+) influx in DRG neurones, providing a potential means for gabapentin to effectively mediate spinal anti-nociception.