Gabapentin increases GABA-induced depolarization in rat neonatal optic nerve

Efficacy of a topical gabapentin gel in a cisplatin paradigm of chemotherapy-induced peripheral neuropathy

BACKGROUND

Chemotherapy induced peripheral neuropathy (CIPN) has been attributed to chemotherapeutic agents such as cisplatin which adversely affect disease outcome leading to increased cancer related morbidity. The clinical efficacy of systemic gabapentin in neuropathic pain management is limited by central side-effects in addition to a scarceness of conclusive evidence of its efficacy in CIPN management. The topical route therefore may provide a relatively safe alternative for neuropathic pain treatment in general and CIPN in particular.

METHODS

Cisplatin induced neuropathic nociception was established in rats after a single weekly cisplatin injection (3.0 mg/kg, intraperitoneally) for 4 weeks. The evoked neuropathic sensation of allodynia was assessed by plantar application of von Frey monofilaments as the paw withdrawal threshold (PWT), whereas the expression of heat-hypoalgesia was determined on a hot-plate as paw withdrawal latency (PWL). Gabapentin gel (10% w/w) was applied three-times daily on the hind paws while in a concurrent systemic study, gabapentin was administered daily (75 mg/kg, intraperitoneally) for 4 weeks. To assess any evidence of neurological adverse symptoms of cisplatin and the central side-effect propensity of systemic or topical gabapentin, evaluation of motor coordination (rotarod test) and gait (footprint analysis) were performed.

RESULTS

Cisplatin invoked a progressive development of neuropathic hind paw allodynia (decreased PWT, days 7-28) and heat hypoalgesia (increased PWL, days 21-28). Topical gabapentin significantly delayed the expression of both allodynia on protocol days 21 and 28 and heat-hypoalgesia (day 28). Systemic gabapentin displayed a comparative anti-neuropathic predisposition through a sustained suppression of tactile allodynia on days 14 and 21-28 as well as thermal hypoalgesia (days 21 and 28). Systemic gabapentin also impaired motor coordination and gait thus affirming its clinically documented central side effects, but these outcomes were not evident after topical treatment.

CONCLUSIONS

Both topical and systemic gabapentin exhibit a propensity to attenuate CIPN in a cisplatin paradigm. Gabapentin applied topically may therefore provide an adjunctive or alternative route for CIPN management upon cessation of systemic medications due to intolerable side-effects.

Possible involvement of ATP-sensitive potassium channels in the antidepressant-like effects of gabapentin in mouse forced swimming test

Gabapentin as an anticonvulsant drug also has beneficial effects in treatment of depression. Previously, we showed that acute administration of gabapentin produced an antidepressant-like effect in the mouse forced swimming test (FST) by a mechanism that involves the inhibition of nitric oxide (NO). Considering the involvement of NO in adenosine triphosphate (ATP)-sensitive potassium channels (KATP), in the present study we investigated the involvement of KATP channels in antidepressant-like effect of gabapentin. Gabapentin at different doses (5–10 mg/kg) and fluoxetine (20 mg/kg) were administrated by intraperitoneal route, 60 and 30 min, respectively, before the test. To clarify the probable involvement of KATP channels, mice were pretreated with KATP channel inhibitor or opener. Gabapentin at dose 10 mg/kg significantly decreased the immobility behavior of mice similar to fluoxetine (20 mg/kg). Co-administration of subeffective dose (1 mg/kg) of glibenclamide (inhibitor of KATP channels) with gabapentin (3 mg/kg) showed a synergistic antidepressant-like effect. Also, subeffective dose of cromakalim (opener of KATP channels, 0.1 mg/kg) inhibited the antidepressant-like effect of gabapentin (10 mg/kg). None of the treatments had any impact on the locomotor movement. Our study, for the first time, revealed that antidepressant-like effect of gabapentin in mice is mediated by blocking the KATP channels.

A streptozotocin-induced diabetic neuropathic pain model for static or dynamic mechanical allodynia and vulvodynia: validation using topical and systemic gabapentin

Neuropathic vulvodynia is a state of vulval discomfort characterized by a burning sensation, diffuse pain, pruritus or rawness with an acute or chronic onset. Diabetes mellitus may cause this type of vulvar pain in several ways, so this study was conducted to evaluate streptozotocin-induced diabetes as a neuropathic pain model for vulvodynia in female rats. The presence of streptozotocin (50 mg/kg i.p.)-induced diabetes was initially verified by disclosure of pancreatic tissue degeneration, blood glucose elevation and body weight loss 5-29 days after a single treatment. Dynamic (shortened paw withdrawal latency to light brushing) and static (diminished von Frey filament threshold pressure) mechanical allodynia was then confirmed on the plantar foot surface. Subsequently, both static and dynamic vulvodynia was detected by application of the paradigm to the vulval region. Systemic gabapentin (75 mg/kg, i.p.) and topical gabapentin (10 % gel) were finally tested against allodynia and vulvodynia. Topical gabapentin and the control gel vehicle significantly increased paw withdrawal threshold in the case of the static allodynia model and also paw withdrawal latency in the model for dynamic allodynia when compared with the streptozotocin-pretreated group. Likewise, in the case of static and dynamic vulvodynia, there was a significant antivulvodynia effect of systemic and topical gabapentin treatment. These outcomes substantiate the value of this model not only for allodynia but also for vulvodynia, and this was corroborated by the findings not only with systemic but also with topical gabapentin.

The effects of gabapentin in two animal models of co-morbid anxiety and visceral hypersensitivity

Visceral hypersensitivity and an increased response to stress are two of the main symptoms of irritable bowel syndrome. Thus efforts to develop animal models of irritable bowel syndrome have centred on both of these parameters. The anticonvulsant gabapentin, which is widely used as an analgesic agent, also reduces anxiety. No data exists to our knowledge of the effects of gabapentin in animal models of co-morbid exaggerated stress response and visceral pain. Our aim was to assess the effect of gabapentin on stress and visceral hypersensitivity in two different animal models of irritable bowel syndrome. The animal models employed were the genetically susceptible Wistar Kyoto rat and the neonatally stressed maternal separation model. These animals were subjected to the open field paradigm to assess stress-induced defecation rates and colorectal distension to assess the level of visceral sensitivity. Gabapentin (30 mg/kg) prevented the stress-induced increase in faecal pellet output in the maternally separated rat, but not the Wistar Kyoto animals. On the other hand gabapentin (30 mg/kg) reduced the number of pain behaviours in response to colorectal distension in both models. These results show that whilst both models have similar responses to gabapentin in terms of visceral pain they differ in terms of their physiological response to stress. This indicates that the origin of anxiety and perhaps then visceral hypersensitivity differs in these models. Overall, these data suggest that gabapentin may be a useful treatment in disorders of co-morbid pain and an overactive stress system such as irritable bowel syndrome.

Analgesic effects of gabapentin on mechanical hypersensitivity in a rat model of chronic pancreatitis

Gabapentin, an anticonvulsant, is widely accepted as an alternative therapeutic agent for neuropathic pain and has proved to produce analgesic effects in a mouse model of visceral pain. However, it is unknown whether gabapentin is also analgesically effective in chronic pancreatitis. The aim of the present study was to investigate the role and underlying mechanisms of gabapentin in a rat model of chronic pancreatitis. Chronic pancreatitis induced by dibutyltin dichloride (DBTC) produced a marked increase in mechanical sensitivity of the abdomen after the establishment of the model. During the first day to the sixth day in the fourth week, Gabapentin was administered intraperitoneally daily at a dose of 100mg/kg. The behavioral test began 1h after drug administration. The analgesic effect of gabapentin was not evident with a single injection, but gabapentin significantly reduced the responsive frequencies to mechanical stimulation in rats with chronic pancreatitis from the third day to the end of the experiment. To explore the underlying mechanisms, the expression of alpha(2)delta-1 calcium channel subunit was examined in the thoracic spinal cord (T8-11). There was no significant change in alpha(2)delta-1 level of T8-11 following the first injection. But after the sixth injection, the alpha(2)delta-1 level of T8-11 in rats with chronic pancreatitis was declined. Taken together, the present study suggested that repeated administration of gabapentin daily could reduce mechanical hypersensitivity in the upper abdomen and produce an analgesic effect in a rat model of chronic pancreatitis. The down-regulation of alpha(2)delta-1 calcium channel subunit might be one of the mechanisms underlying the analgesic effect of gabapentin.

Cellular and behavioral interactions of gabapentin with alcohol dependence

Gabapentin is a structural analog of GABA that has anticonvulsant properties. Despite the therapeutic efficacy of gabapentin, its molecular and cellular mechanisms of action are unclear. The GABAergic system in the central nucleus of the amygdala (CeA) plays an important role in regulating voluntary ethanol intake. Here, we investigated the effect of gabapentin on GABAergic transmission in CeA slices, on ethanol intake, and on an anxiety measure using animal models of ethanol dependence. Gabapentin increased the amplitudes of evoked GABA receptor-mediated IPSCs (GABA-IPSCs) in CeA neurons from nondependent rats, but decreased their amplitudes in CeA of ethanol-dependent rats. Gabapentin effects were blocked in the presence of a specific GABA(B) receptor antagonist. The sensitivity of the GABA-IPSCs to a GABA(B) receptor antagonist and an agonist was decreased after chronic ethanol, suggesting that ethanol-induced neuroadaptations of GABA(B) receptors associated with ethanol dependence may account for the differential effects of gabapentin after chronic ethanol. Systemic gabapentin reduced ethanol intake in dependent, but not in nondependent, rats and reversed the anxiogenic-like effects of ethanol abstinence using an acute dependence model. Gabapentin infused directly into the CeA also blocked dependence-induced elevation in operant ethanol responding. Collectively, these findings show that gabapentin reverses behavioral measures of ethanol dependence and, in turn, dependence reverses the effects of gabapentin on CeA neurons, and suggest that gabapentin represents a potential medication for treatment of alcoholism.

Mechanisms of action of antiepileptic drugs

The management of seizures in the patient with epilepsy relies heavily on antiepileptic drug (AED) therapy. Fortunately, for a large percentage of patients, AEDs provide excellent seizure control at doses that do not adversely affect normal function. At the molecular level, the majority of AEDs are thought to modify excitatory and inhibitory neurotransmission through effects on voltage-gated ion channels (e.g., sodium and calcium) and gamma-aminobutyric acid (GABA)(A) receptors, respectively. In addition to these effects, two of the "second-generation" AEDs have been found to limit glutamate-mediated excitatory neurotransmission (i.e., felbamate and topiramate). Not surprisingly, those AEDs with broad spectrum clinical activity are often found to exert an action at more than one molecular target. Emerging evidence suggests that receptor and voltage-gated subunits are modified by chronic seizures. Thus, attempts to understand the relationship between target and effect continue to provide important information about the neuropathology of the epileptic network and to facilitate the development of novel therapies for the treatment of refractory epilepsy.

Novel anticonvulsant drugs

Principles of complex mechanisms of action of anticonvulsants including latest reports concerning new antiepileptic drugs (AED) are considered. Different aspects of new anticonvulsant drugs (2nd generation) from preclinical and clinical testing, pharmacokinetics, and mono or combination therapy in children and adults are summarized. In the following condensed synopsis pharmacological and clinical characteristics of gabapentin (GBP), lamotrigine (LTG), levetiracetam (LEV), oxcarbazepine (OXC), pregabalin (PGB) and tiagabine (TGB) as well as topiramate (TPM) and zonisamide (ZNS) are discussed. In addition to the mechanisms of action, pharmacokinetics, interactions, indications and dosages as well as side effects are considered. Important data concerning the effect and tolerability of anticonvulsant drugs can be obtained from controlled studies. In comparison to drugs of the first generation (phenobarbital [PB], primidon [PRD], phenytoin [PHT], carbamazepine [CBZ] and valproic acid [VPA]) the potential for interactions and side effects due to enzyme induction or inhibition is reduced by most of the anticonvulsant drugs of the second generation. New anticonvulsant drugs increase the spectrum of treatment and represent further steps with regard to the optimization of an individual therapy of the epilepsies.

A randomized placebo-controlled trial of gabapentin for cocaine dependence

BACKGROUND

In laboratory animals, augmentation of GABA neurotransmission results in inhibition of cocaine self-administration and inhibition of reinstatement to cocaine-seeking behaviors. If parallel effects were observed in humans, GABA-ergic medication should be effective both in the abstinence-induction as well as in the relapse-prevention phase of cocaine dependence treatment. Gabapentin is an anticonvulsant medication that increases human brain GABA levels. We evaluated the safety and efficacy of gabapentin combined with relapse-prevention therapy in the treatment of cocaine-dependent individuals.

DESIGN

The study involved 129 individuals with cocaine dependence. Of the 99 participants, who were randomized into a double-blind trial 88% were males, 66% were minorities and with an average age of 39 years (range 22-58 years). After 2 weeks of placebo lead-in, participants were randomized to receive either gabapentin 3200 mg (1600 mg bid) or placebo for 12 weeks, followed by 2 weeks of placebo lead-out. Prior to randomization, participants were stratified into four groups based on the principal route of cocaine use (smokers versus intranasal users) and the level of cocaine use during the 2 weeks of lead-in (high level versus low level). Throughout the 16 weeks study, participants received weekly individual relapse-prevention therapy. The outcome measures included: days of cocaine use and a binary indicator of abstinence based on urine toxicology test, self-reported cocaine craving and retention in treatment.

RESULTS

Forty-nine percent of randomized patients completed 12 weeks of the trial. Retention did not differ by treatment group but cocaine-smokers dropped out of treatment at a significantly faster rate than intranasal users. For the entire sample, odds of cocaine use over the course of the study did not differ between gabapentin- and placebo-treated individuals. There was a significant difference in the odds of cocaine use between high and low-use groups, with the odds in high-use groups decreasing over time and odds in the low-use groups gradually increasing over the course of the study, such that by the end of the study low and high users were similarly likely to use cocaine. In the low-use group, there was a non-significant trend suggesting that gabapentin-treated subjects had more favorable outcome compared to placebo-treated individuals. There was no treatment effect on abstinence rates, craving or other substance use. Gabapentin at 3200 mg/day was very well tolerated in this group of cocaine-dependent participants.

CONCLUSIONS

When combined with weekly individual relapse-prevention therapy, gabapentin 1600 mg bid was no more effective than placebo in the treatment of cocaine dependence. When reviewed in conjunction with other published studies, gabapentin and other GABA enhancing anticonvulsant medications may deserve further study as relapse-preventive agents in cocaine-dependent individuals who achieve abstinence early in treatment.

Benefits and risks of pharmacological and surgical treatments for essential tremor: disease mechanisms and current management

Essential tremor (ET) is a neurological disease (and possibly a family of diseases) whose most recognisable feature is an action tremor of the hands and occasionally of the voice and head. Current data support the view that CNS gamma-amino-butyric acid (GABA)-ergic mechanisms may underlie ET and that the tremor may be further modulated by peripheral (muscle) adrenoreceptors. Potential pharmacotherapeutic options, targeted to influence the activity of the neurotransmitter GABA within the CNS and the peripheral adrenergic receptors, are part of the current armamentarium to treat ET. As such, primidone and propranolol remain the mainstays of the therapy for ET. Intramuscular injections of botulinum toxin A may play a role in the treatment of voice and head tremor. Surgical options, which are reserved for patients with severe, medically-refractory tremor, provide adequate tremor control in the majority of patients. As with other progressive neurological disorders of late life, the ability to use neuroprotective medications to intervene in the developing disease to either slow or halt the progression of the pathological process, would involve an understanding of underlying disease mechanisms. The understanding of these mechanisms in ET is limited and further study of these mechanisms is critical for the development of such therapies.

Gabapentin is Effective in the Treatment of Cancer-Related Neuropathic Pain: A Prospective, Open-Label Studya,b

BACKGROUND

Gabapentin has been evaluated in the treatment of nonmalignant neuropathic pain, however, there is little direct evidence evaluating its efficacy in cancer-related neuropathic pain.

OBJECTIVE

This study evaluated the effectiveness of gabapentin to treat cancer-related neuropathic pain.

DESIGN

This was an open-label study. Two parallel groups of patients were recruited with either treatment-related (radiotherapy, surgery, chemotherapy) or tumor-related neuropathic pain. Gabapentin was dose-escalated from 300 mg/d to 1.8 g/d.

MEASUREMENTS

The primary outcome, pain, was assessed using the modified brief pain inventory. In addition patient descriptors of pain and scores of activities of daily living were collated together with demographic data.

RESULTS

We recruited 62 patients with treatment-related (n = 25) or tumor-related (n = 37) neuropathic pain. There was a significant reduction in the worst, average, and current pain scores (p < 0.002), but not the least pain score (p = 0.21). Twenty-eight of 62 (45.2%) of patients achieved at least a one third reduction in pain score (95% confidence interval [CI] 32.5-58.3); the number needed to treat to obtain this benefit is 2.2 (95% CI 1.7-3.1). There was a significant reduction in all scores measuring the impact of pain on daily living (p < 0.003). There was no significant difference in pain scores at day 8 compared to day 15. Analysis of variance suggested that gender, but not etiology, or type of neuropathic pain, was a significant predictor of analgesic response and this warrants further investigation.

CONCLUSION

We conclude that gabapentin is an effective treatment for cancer-related neuropathic pain.

PD-0200347, an alpha2delta ligand of the voltage gated calcium channel, inhibits in vivo activation of the Erk1/2 pathway in osteoarthritic chondrocytes: a PKCalpha dependent effect

OBJECTIVE

To explore the in vivo effects of PD-0200347, an alpha(2)delta ligand of voltage gated Ca(2+) channels, on cell signalling in osteoarthritic (OA) chondrocytes from an experimental dog model, and examine the effect of PD-0200347 on the major signalling pathways involved in OA cartilage degradation.

METHODS

OA was surgically induced in dogs by sectioning the anterior cruciate ligament. OA dogs were divided into three groups and treated orally with (a) placebo; (b) 15 mg/kg/day PD-0200347, or (c) 90 mg/kg/day PD-0200347. The animals were killed 12 weeks after surgery. Cartilage specimens from femoral condyles and tibial plateaus were processed for immunohistochemistry. Specific antibodies against the phosphorylated form of PKCalpha, Ras, c-Raf, the MAP kinases Erk1/2, p38, JNK, and the transcription factors, CREB and Elk-1, were used.

RESULTS

Levels of all the tested signalling mediators were increased in the placebo treated (OA) group compared with the normal group. PD-0200347 treatment significantly reduced the levels of the active forms of PKCalpha, c-Raf, Erk1/2, and Elk-1; however, the levels of the active forms of Ras, p38, JNK, and CREB were not affected by the PD-0200347 treatment.

CONCLUSION

The action of PD-0200347 on OA chondrocytes is probably mediated through the inhibition of Erk1/2 activation via a Ras independent mechanism. This effect is associated with reduction of the activation of transcription factors such as Elk-1, which leads to the inhibition of the induction of the major catabolic factors involved in the degradation process of OA cartilage.

Oral treatment with PD-0200347, an ?2? ligand, reduces the development of experimental osteoarthritis by inhibiting metalloproteinases and inducible nitric oxide synthase gene expression and synthesis in cartilage chondrocytes

OBJECTIVE

To examine the in vivo effects of PD-0200347, an alpha(2)delta ligand of voltage-activated Ca(2+) channels and a compound chemically related to pregabalin and gabapentin, on the development of cartilage structural changes in an experimental dog model of osteoarthritis (OA). The effects of PD-0200347 on the major pathways involved in OA cartilage degradation, including matrix metalloproteinases (MMPs) and the inducible form of nitric oxide synthase (iNOS), were also studied.

METHODS

OA was surgically induced in dogs by sectioning the anterior cruciate ligament. OA dogs were randomly distributed into 3 groups and treated orally with either 1) placebo, 2) 15 mg/kg/day of PD-0200347, or 3) 90 mg/kg/day of PD-0200347. Dogs were killed 12 weeks after surgery. The severity of the lesions was scored macroscopically and histologically. Cartilage specimens from the femoral condyles and tibial plateaus were processed for RNA extraction and quantitative reverse transcription-polymerase chain reaction (RT-PCR) or immunohistochemistry. Specific probes and antibodies were used to study the messenger RNA and protein levels of iNOS, MMP-1, MMP-3, and MMP-13.

RESULTS

No clinical signs of drug toxicity were noted in the treated animals. Treatment with PD-0200347 at both dosages tested (15 and 90 mg/kg/day) reduced the development of cartilage lesions. There was a reduction in the score of lesions, with a statistically significant (P = 0.01) difference when the highest dosage of the drug was administered. The reduction in the score was mainly related to a decrease in the surface size of the lesions. Quantitative RT-PCR showed that PD-0200347 significantly reduced the expression of MMP-13, a key mediator in OA. Immunohistochemical analyses showed that treatment with PD-0200347 significantly reduced the synthesis of all key OA mediators studied.

CONCLUSION

This study demonstrated the efficacy of PD-0200347 in reducing the progression of cartilage structural changes in a dog model of OA. It also showed that this effect is linked to the inhibition of the major pathophysiologic mediators responsible for cartilage degradation.

Gabapentin may inhibit synaptic transmission in the mouse spinal cord dorsal horn through a preferential block of P/Q-type Ca2+ channels

Gabapentin is a lipophilic analog of gamma-amino butyric acid (GABA) with therapeutic activity against certain forms of epilepsy and neuropathic pain. Despite its structural similarity to GABA, it does not bind GABAA or GABAB receptors and the mechanism, especially of its analgesic action, has remained elusive. Here, we have studied its effects on synaptic transmission mediated by the major spinal fast excitatory and inhibitory neurotransmitters, L-glutamate and glycine, in the superficial layers of the spinal cord dorsal horn, a CNS area, which is critically involved in nociception. Gabapentin reversibly reduced evoked excitatory postsynaptic currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA-EPSCs) and inhibitory postsynaptic currents mediated by glycine (gly-IPSCs). Inhibition of AMPA-EPSCs and gly-IPSCs occurred with similar potencies (approximately 10-50 nM) and by about the same degree (approximately 40% at 1 microM). Gabapentin did not affect membrane currents elicited by exogenously applied glutamate or glycine arguing against a postsynaptic site of action. Selective blockade of N-type Ca2+ channels with omega-conotoxin GVIA dramatically increased and blockade of P/Q-type channels with omega-agatoxin IVA strongly attenuated inhibition of evoked synaptic transmission by gabapentin. These results show that gabapentin affects both excitatory and inhibitory spinal neurotransmission via a presynaptic mechanism which preferentially involves P/Q-type Ca2+ channels.

Gabapentin in the treatment of neuropathic pain

This paper reviews the pharmacology and clinical effectiveness of gabapentin in the treatment of neuropathic pain. Gabapentin has antihyperalgesic and antiallodynic properties but does not have significant actions as an anti-nociceptive agent. Its mechanisms of action appear to be a complex synergy between increased GABA synthesis, non-NMDA receptor antagonism and binding to the alpha2delta subunit of voltage dependent calcium channels. The latter action inhibits the release of excitatory neurotransmitters. Clinically, several large randomized controlled trials have demonstrated its effectiveness in the treatment of a variety of neuropathic pain syndromes. Patients with neuropathic pain can expect a mean reduction in pain score of 2.05 points on an 11 point numerical rating scale compared with a reduction of 0.94 points if they had taken the placebo. Around 30% of patients can expect to achieve more than 50% pain relief and a similar number will also experience minor adverse events; the most common of which are somnolence and dizziness. In patients with neuropathic pain due to cancer, higher response rates might be observed with gabapentin when administered with opioids because of a synergistic interaction.

Dynamic equilibrium of neurotransmitter transporters: not just for reuptake anymore

Many electrophysiologists view neurotransmitter transporters as tiny vacuum cleaners, operating continuously to lower extracellular neurotransmitter concentration to zero. However, this is not consistent with their known behavior, instead only reducing extracellular neurotransmitter concentration to a finite, nonzero value at which an equilibrium is reached. In addition, transporters are equally able to go in either the forward or reverse direction, and when they reverse, they release their substrate in a calcium-independent manner. Transporter reversal has long been recognized to occur in response to pathological stimuli, but new data demonstrate that some transporters can also reverse in response to physiologically relevant stimuli. This is consistent with theoretical calculations that indicate that the reversal potentials of GABA and glycine transporters are close to the resting potential of neurons under normal conditions and that the extracellular concentration of GABA is sufficiently high when the GABA transporter is at equilibrium to tonically activate high-affinity extrasynaptic GABAA receptors. The equilibrium for the GABA transporter is not static but instead varies continuously as the driving force for the transporter changes. We propose that the GABA transporter plays a dynamic role in control of brain excitability by modulating the level of tonic inhibition in response to neuronal activity.

Effect of gabapentin and lamotrigine on mechanical allodynia-like behaviour in a rat model of trigeminal neuropathic pain

Injury to the trigeminal nervous system may induce severe pain states. This study examined the antinociceptive effect of the novel anticonvulsants, gabapentin and lamotrigine, in a rat model of trigeminal neuropathic pain produced by chronic constriction of one infraorbital nerve. Responsiveness to von Frey filament stimulation of the vibrissal pad was evaluated 2 weeks post-operation. Hyper-responsive rats received acute and repeated (five injections separated by the half-life of the compound) injections with gabapentin and lamotrigine. 76% of the nerve-injured rats displayed pronounced hyper-responsiveness (median 0.217 g (lower-upper percentiles 0.217-0.217) vs. 12.5 g pre-operative), that was resistant to both single (5-100 mg/kg) and repeated (5-30 mg/kg) injections with i.p. lamotrigine. Repeated (30 and 50 mg/kg), but not single (30-100 mg/kg) injections of i.p. gabapentin partially alleviated the mechanical allodynia-like behaviour. Repeated injections of gabapentin at 50 but not at 30 mg/kg produced motor deficits. The results indicate that gabapentin rather than lamotrigine may be a better therapeutic approach for the clinical management of some trigeminal neuropathic pain disorders.

GABA transaminase inhibition induces spontaneous and enhances depolarization-evoked GABA efflux via reversal of the GABA transporter

The GABA transporter can reverse with depolarization, causing nonvesicular GABA release. However, this is thought to occur only under pathological conditions. Patch-clamp recordings were made from rat hippocampal neurons in primary cell cultures. Inhibition of GABA transaminase with the anticonvulsant gamma-vinyl GABA (vigabatrin; 0.05-100 microm) resulted in a large leak current that was blocked by bicuculline (50 microm). This leak current occurred in the absence of extracellular calcium and was blocked by the GABA transporter antagonist SKF-89976a (5 microm). These results indicate that vigabatrin induces spontaneous GABA efflux from neighboring cells via reversal of GABA transporters, subsequently leading to the stimulation of GABA(A) receptors on the recorded neuron. The leak current increased slowly over 4 d of treatment with 100 microm vigabatrin, at which time it reached an equivalent conductance of 9.0 +/- 4.9 nS. Blockade of glutamic acid decarboxylase with semicarbazide (2 mm) decreased the leak current that was induced by vigabatrin by 47%. In untreated cells, carrier-mediated GABA efflux did not occur spontaneously but was induced by an increase in [K(+)](o) from 3 to as little as 6 mm. Vigabatrin enhanced this depolarization-evoked nonvesicular GABA release and also enhanced the heteroexchange release of GABA induced by nipecotate. Thus, the GABA transporter normally operates near its equilibrium and can be easily induced to reverse by an increase in cytosolic [GABA] or mild depolarization. We propose that this transporter-mediated nonvesicular GABA release plays an important role in neuronal inhibition under both physiological and pathophysiological conditions and is the target of some anticonvulsants.

The effects of GABA(B) agonists and gabapentin on mechanical hyperalgesia in models of neuropathic and inflammatory pain in the rat

We have examined the effects of a novel GABA(B) agonist, CGP35024, in models of chronic neuropathic (partial sciatic ligation) and inflammatory (Freund's complete adjuvant) pain in the rat, and its inhibitory action on spinal transmission in vitro. The effects of CGP35024 were compared with L-baclofen and gabapentin. CGP35024 and L-baclofen reversed neuropathic mechanical hyperalgesia following single subcutaneous or intrathecal administration, but did not affect inflammatory mechanical hyperalgesia. Gabapentin only moderately affected neuropathic hyperalgesia following a single administration by either route, but produced significant reversal following daily administration for 5 days. It was only weakly active against inflammatory hyperalgesia following single or repeated administration. The antihyperalgesic effects of L-baclofen and CGP35024, but not gabapentin, were blocked by the selective GABA(B) receptor antagonist CGP56433A. CGP35024 was seven times more potent against neuropathic hyperalgesia than in the rotarod test for motor co-ordination, whilst L-baclofen was approximately equipotent in the two tests. In the isolated hemisected spinal cord from the rat, CGP35024, L-baclofen and gabapentin all inhibited capsaicin-evoked ventral root potentials (VRPs). CGP35024 and L-baclofen, but not gabapentin, also inhibited the polysynaptic and monosynaptic phases of electrically-evoked VRPs, as well as the 'wind-up' response to repetitive stimulation. These data indicate that CGP35024 and L-baclofen modulate nociceptive transmission in the spinal cord to inhibit neuropathic hyperalgesia, and that CGP35024 has a therapeutic window for antihyperalgesia over spasmolysis.

A placebo-controlled study of lamotrigine and gabapentin monotherapy in refractory mood disorders

There is a pressing need for additional treatment options for refractory mood disorders. This controlled comparative study evaluated the efficacy of lamotrigine (LTG) and gabapentin (GBP) monotherapy versus placebo (PLC). Thirty-one patients with refractory bipolar and unipolar mood disorders participated in a double-blind, randomized, crossover series of three 6-week monotherapy evaluations including LTG, GBP, and PLC. There was a standardized blinded titration to assess clinical efficacy or to determine the maximum tolerated daily dose (LTG 500 mg or GBP 4,800 mg). The primary outcome measure was the Clinical Global Impressions Scale (CGI) for Bipolar Illness as supplemented by other standard rating instruments. The mean doses at week 6 were 274 +/- 128 mg for LTG and 3,987 +/- 856 mg for GBP. Response rates (CGI ratings of much or very much improved) were the following: LTG, 52% (16/31); GBP, 26% (8/31); and PLC, 23% (7/31) (Cochran's Q = 6.952, df = 2, N = 31, p = 0.031). Post hoc Q differences (df = 1, N = 31) were the following: LTG versus GBP (Qdiff = 5.33, p = 0.011); LTG versus PLC (Qdiff = 4.76, p = 0.022); and GBP versus PLC (Qdiff = 0.08, p = 0.70). With respect to anticonvulsant dose and gender, there was no difference between the responders and the nonresponders. The agents were generally well tolerated. This controlled investigation preliminarily suggests the efficacy of LTG in treatment-refractory affectively ill patients. Further definition of responsive subtypes and the role of these medications in the treatment of mood disorders requires additional study.

Antinociceptive effect of systemic gabapentin in mononeuropathic rats, depends on stimulus characteristics and level of test integration

The anticonvulsant gabapentin is effective against neuropathic pain, but the primary site(s) and mechanism(s) of action are unknown. In order to explore the relative contribution of spinal versus supra-spinal mechanisms to the antinociceptive effect of gabapentin, this study used two differentially integrated nociceptive tests. We systematically compared the effects of various doses of gabapentin on the paw withdrawal to pressure (PWTP), a spinally coordinated reflex and the vocalization threshold to paw pressure (VTPP), a supra-spinal integrated test in the sciatic nerve constriction rat model of neuropathic pain. In addition, we evaluated the effect of gabapentin on the struggle latency to paw immersion into a non-noxious cold (10 degrees C) water bath. Similar lower doses (1-30 mg/kg) of gabapentin produced potent antinociception in the VTPP test but were devoid of effects on the PWTP. The effect was observed not only on the nerve-injured side, but also, although less pronounced, on the contralateral side. Only the highest dose (100 mg/kg) of the anticonvulsant was able to induce an increase in the nerve-injured paw threshold in both tests. In the thermal test, gabapentin (3, 10 and 30 mg/kg i.p.) dose-dependently increased the response time to the 10 degrees C stimulus. Gabapentin at 100 mg/kg but not at 30 mg/kg produced motor deficits in animals using the rotarod test. Taken together, our findings suggest that low doses of gabapentin have a preferential action on the more integrated pain-related behaviour in neuropathic rats. The present results confirm that gabapentin may be a useful approach for the clinical management of several aspects of neuropathic pain.

Detection of static and dynamic components of mechanical allodynia in rat models of neuropathic pain: are they signalled by distinct primary sensory neurones?

In the present study, chronic constrictive injury (CCI model) of the sciatic nerve or tight ligation of L5 and L6 spinal nerves (Chung model) produced both dynamic and static components of mechanical allodynia in rats. The two responses were detected, respectively, by lightly stroking the hind paw with cotton wool or application of pressure using von Frey hairs. Animals with spinal nerve ligation developed both types of responses at a faster rate compared to animals with the CCI. Morphine (1-3 mg/kg, s.c.) dose-dependently blocked static but not dynamic allodynia. In contrast, pregabalin (previously S-isobutylgaba and CI-1008) dose-dependently (3-30 mg/kg, p.o.) blocked both types of allodynia. In CCI animals, two administrations of capsaicin (100 microg/50 microl) into the plantar surface of the ipsilateral paw at 1-h intervals blocked the maintenance of thermal hyperalgesia without affecting either static or dynamic allodynia. The similar administration of a further two doses of capsaicin into the same animals blocked the maintenance of static allodynia without affecting the dynamic response. These data indicate that thermal hyperalgesia, static and dynamic allodynia are respectively signalled by C-, Adelta- and Abeta/capsaicin insensitive Adelta- primary sensory neurones. It is suggested that pregabalin possesses a superior antiallodynic profile than morphine and may represent a novel class of therapeutic agents for the treatment of neuropathic pain.

Gabapentin in the management of convulsive disorders

Gabapentin, in clinical use since 1993, is indicated as an adjunctive antiepileptic drug (AED) for treatment of complex partial seizures, with or without secondary generalization, in patients over 12 years of age. Although several cellular actions have been described in the literature, the molecular mechanism(s) of action responsible for the anticonvulsant effect of gabapentin has not been conclusively determined. It is likely that gabapentin has multiple concentration-dependent actions that combine in a unique manner to produce antiepileptic efficacy. The pharmacokinetic properties of this water-soluble, amino-acid AED are generally favorable. Absorption appears to be dependent on transport by the L-system amino acid transporter. Elimination of unmetabolized drug occurs by the renal route. Although its therapeutic range is not well characterized, gabapentin has a broad therapeutic index. This implies that a wide range of doses can be used, based on individual patient needs, without significant limitation due to dose-dependent side effects. Gabapentin has few drug-drug interactions, none of which is clinically limiting. Several studies have demonstrated the long-term efficacy of gabapentin with no systematic evidence of tachyphylaxis. In addition, there is increasing evidence to support the use of gabapentin as monotherapy. Gabapentin is safe and is generally well tolerated. To date, nearly 3 million patients have been treated in studies and in open use without causal relationship to a specific life-threatening organ toxicity. Seizure control superior to that observed in well-controlled trials has been reported at higher doses used in clinical practice and in studies. Therefore, gabapentin dosing must be optimized on an individual basis to achieve an adequate trial of the drug and obtain the best seizure control.

3-substituted GABA analogs with central nervous system activity: a review

Gabapentin and Pregabalin are both 3-alkylated gamma-amino butyric acid (GABA) analogs. Gabapentin was designed as a lipophilic GABA analog and was first synthesized as a potential anticonvulsant and was launched in 1994 as add-on therapy for the treatment of epilepsy. In this review the discovery and development of gabapentin as an anticonvulsant are discussed. During human trials and while in clinical use, it became apparent that gabapentin induced some other potentially useful therapeutic effects in chronic pain states and behavioral disorders. A review of animal and clinical data relating to these other potential therapeutic utilities is presented. Pregabalin was identified after an investigation into other 3-substituted GABA analogs. It has since been shown to have a similar pharmacological profile to gabapentin with greater potency in preclinical models of pain and epilepsy. Studies of the mechanism(s) of action of these compounds are discussed. Work towards identifying new analogs of both gabapentin and pregabalin is also reviewed.

Effects of gabapentin on the different components of peripheral and central neuropathic pain syndromes: a pilot study

Anticonvulsants are widely used in the treatment of neuropathic pain, and are assumed to act preferentially on lancinating, shooting pain. In the present study, the effects of gabapentin, a novel anticonvulsant, were evaluated systematically on both spontaneous and evoked pain in 18 patients with peripheral nerve injuries or central lesions. Gabapentin was administered orally in gradually increasing doses up to a maximum of 2,400 mg/day. Evaluations of spontaneous ongoing and paroxysmal pain, allodynia and hyperalgesia were performed at the beginning of the study ('baseline') and 6 weeks after the steady-state dose had been reached. Quantitative sensory tests were used to measure detection and pain thresholds to mechanical and thermal stimuli and the responses to suprathreshold stimuli. Gabapentin induced a moderate and statistically significant relief of ongoing spontaneous pain and was particularly effective in reducing paroxysmal pain. A striking finding was the significant effect on brush-induced and cold allodynia. In contrast, no effects were observed on detection and pain thresholds to static mechanical and hot stimuli. Side effects were generally minor and did not interfere with everyday activities. The present study suggests that gabapentin has preferential antihyperalgesic and/or antiallodynic effects, and is equally effective in pain due to peripheral nerve injuries and central lesions.

Gabapentin. Antiepileptic mechanism of action

Gabapentin is an antiepileptic drug used in the treatment of partial and generalized tonic-clonic seizures. Its antiepileptic mechanism of action is not known. The transport of gabapentin across membranes and its demonstrated effects on voltage-gated ion channels (sodium, calcium), presynaptic mechanisms that can enhance GABAergic inhibition, and ligand-gated ion channels (GABA receptors and glutamate receptors) are reviewed.

Gabapentin inhibits calcium currents in isolated rat brain neurons

Gabapentin (1(aminomethyl) cyclohexane acetic acid; GBP) is a recently developed anticonvulsant, for which the mechanism of action remains quite elusive. Besides its possible interaction with glutamate synthesis and/or GABA release, in cerebral membranes gabapentin has been shown to bind directly to the alpha2delta subunit of the calcium channel. Therefore, we have tested the possibility that gabapentin affects high threshold calcium currents in central neurons. Calcium currents were recorded in whole-cell patch-clamp mode in neurons isolated from neocortex, striatum and external globus pallidus of the adult rat brain. A large inhibition of calcium currents by gabapentin was observed in pyramidal neocortical cells (up to 34%). Significantly, the gabapentin-mediated inhibition of calcium currents saturated at particularly low concentrations (around 10 microM), at least in neocortical neurons (IC50 about 4 microM). A less significant inhibition was seen in medium spiny neurons isolated from striatum (-12.4%) and in large globus pallidus cells (-10.4%). In all these areas, however, the GBP-induced block was fast and largely voltage-independent. Dihydropyridines (nimodipine, nifedipine) prevented the gabapentin response. Omega-conotoxin GVIA and omega-conotoxin MVIIC, known to interfere with the currents driven by alpha1b and alpha1a calcium channels, did not prevent but partially reduced the response. These findings imply that voltage-gated calcium channels, predominately the L-type channel, are a direct target of gabapentin and may support its use in different clinical conditions, in which intracellular calcium accumulation plays a central role in neuronal excitability and the development of cellular damage.

Gabapentin potentiation of the antiepileptic efficacy of vigabatrin in an in vitro model of epilepsy

1. An enhancement of promoted release of gamma-aminobutyric acid (GABA) and a change in GABA-metabolism have been suggested as mechanisms of action of gabapentin. Vigabatrin is supposed to act mainly via inhibition of GABA-transaminase but it also interferes with GABA-release and GABA-uptake. On the basis of these mechanisms of action, a pharmacodynamic interaction of the two antiepileptic drugs could be supposed which might be of relevance in the sense of a rational polypharmacy. 2. To address the aforementioned hypothesis, experiments were carried out on hippocampal slices (n=107) of guinea-pigs (n=70). Epileptiform field potentials (e.f.p.) were induced by omission of magnesium from the bath solution and recorded in the stratum pyramidale of the CA3 region. Gabapentin (30-600 microM; 5.1-102.72 microg ml(-1)), vigabatrin (50-200 microM, 6.45-25.8 microg ml(-1)) and the GABA(A)-receptor antagonist bicuculline (100 microM) were added to the bath solution for 3 h. 3. Gabapentin, in concentrations up to 600 microM, failed to decrease the repetition rate or duration of e.f.p. (n=19). However, vigabatrin, evoked a dose-dependent reduction of the repetition rate of e.f.p. For a concentration of 100 microM (12.9 microg ml(-1)) there was a reduction down to 48+/-5% (mean+/-s.e.mean) of the initial value within 3 h (n=11). With simultaneous administration of vigabatrin (100 microM) and gabapentin (60 microM) for 3 h (n=15), the repetition rate of e.f.p. decreased down to 8+/-3%, which is significantly different from the values obtained after administration of 100 microM vigabatrin alone (P<0.0001). Both, the antiepileptic effect of vigabatrin alone and the enhancement by gabapentin were blocked by the GABA(A)-receptor antagonist bicuculline (100 microM, n=16). 4. These results demonstrate that gabapentin is able to augment the antiepileptic effects of vigabatrin significantly. It is possible that a change in the GABA-release machinery is induced by vigabatrin which then can be augmented by gabapentin.

GABAergic deafferentation hypothesis of brain aging and Alzheimer's disease revisited

Considering the mechanisms responsible for age- and Alzheimer's disease (AD)-related neuronal degeneration, little attention was paid to the opposing relationships between the energy-rich phosphates, mainly the availability of the adenosine triphosphate (ATP), and the activity of the glutamic acid decarboxylase (GAD), the rate-limiting enzyme synthesizing the gamma-amino butyric acid (GABA). Here, it is postulated that in all neuronal phenotypes the declining ATP-mediated negative control of GABA synthesis gradually declines and results in age- and AD-related increases of GABA synthesis. The Ca2+-independent carrier-mediated GABA release interferes with Ca2+-dependent exocytotic release of all transmitter-modulators, because the interstitial (ambient) GABA acts on axonal preterminal and terminal varicosities endowed with depolarizing GABA(A)-benzodiazepine receptors; this makes GABA the "executor" of virtually all age- and AD-related neurodegenerative processes. Such a role of GABA is diametrically opposite to that in the perinatal phase, when the carrier-mediated GABA release, acting on GABA(A)/chloride ionophore receptors, positively controls chemotactic migration of neuronal precursor cells, has trophic actions and initiates synaptogenesis, thereby enabling retrograde axonal transport of target produced factors that trigger differentiation of neuronal phenotypes. However, with advancing age, and prematurely in AD, the declining mitochondrial ATP synthesis unleashes GABA synthesis, and its carrier-mediated release blocks Ca2+-dependent exocytotic release of all transmitter-modulators, leading to dystrophy of chronically depolarized axon terminals and block of retrograde transport of target-produced trophins, causing "starvation" and death of neuronal somata. The above scenario is consistent with the following observations: 1) a 10-month daily administration to aging rats of the GABA-chloride ionophore antagonist, pentylenetetrazol, or of the BDZ antagonist, flumazenil (FL), each forestalls the age-related decline in cognitive functions and losses of hippocampal neurons; 2) the brains of aging rats, relative to young animals, and the postmortem brains of AD patients, relative to age-matched controls, show up to two-fold increases in GABA synthesis; 3) the aging humans and those showing symptoms of AD, as well as the aging nonhuman primates and rodents--all show in the forebrain dystrophic axonal varicosities, losses of transmitter vesicles, and swollen mitochondria. These markers, currently regarded as the earliest signs of aging and AD, can be reproduced in vitro cell cultures by 1 microM GABA; the development of these markers can be prevented by substituting Cl- with SO4(2-); 4) the extrasynaptic GABA suppresses the membrane Na+, K+-ATPase and ion pumping, while the resulting depolarization of soma-dendrites relieves the "protective" voltage-dependent Mg2+ control of the N-methyl-D-aspartate (NMDA) channels, thereby enabling Ca2+-dependent persistent toxic actions of the excitatory amino acids (EAA); and 5) in whole-cell patch-clamp recording from neurons of aging rats, relative to young rats, the application of 3 microM GABA, causes twofold increases in the whole-cell membrane Cl- conductances and a loss of the physiologically important neuronal ability to desensitize to repeated GABA applications. These age-related alterations in neuronal membrane functions are amplified by 150% in the presence of agonists of BDZ recognition sites located on GABA receptor. The GABA deafferentation hypothesis also accounts for the age- and AD-related degeneration in the forebrain ascending cholinergic, glutamatergic, and the ascending mesencephalic monoaminergic system, despite that the latter, to foster the distribution-utilization of locally produced trophins, evolved syncytium-like connectivities among neuronal somata, axon collaterals, and dendrites, to bidirectionally transport trophins. (ABSTRACT TRUNCATED)

The effect of intrathecal gabapentin and 3-isobutyl gamma-aminobutyric acid on the hyperalgesia observed after thermal injury in the rat

Gabapentin is an anticonvulsant that may represent a novel class of drugs, which has novel spinal antihyperalgesic activity. We sought to characterize this spinal action in a model of hyperalgesia that involves a mild thermal injury to the hind paw of the rat. Rats were prepared with chronic spinal catheters. Under brief halothane anesthesia, a thermal injury was induced by applying the left hind paw to a thermal surface (52.5 degrees C) for 45 s. This exposure results in mild erythema but no blistering. Thermal escape latency of the hind paw was determined using an underglass thermal stimulus with which response latencies of the injured and uninjured (normal) paw could be obtained. Thirty minutes after thermal injury, the response latency in all groups decreased from 10-12 s to 5-7 s. Uninjured paw withdrawal latency was unaltered. The intrathecal injection of gabapentin (30-300 microg) produced a dose-dependent reversal of the hyperalgesia but had no effect on the response latency of the normal hind paw, even at the largest doses. A similar reversal was observed after intrathecal delivery of the structural analog S(+)-3-isobutyl gamma-aminobutyric acid (GABA) (30-300 microg), but not after the largest dose of its stereoisomer R(-)-3-isobutyl GABA (300 microg). The effects of both intrathecal gabapentin and S(+)-3-isobutyl GABA were reversed by intrathecal D-serine, but not L-serine. All effects were observed at doses that had no significant effect on motor function. These observations, in conjunction with the accumulating data on binding and transmitter release, emphasize that these gabapentinoids can selectively modulate the facilitation of spinal nociceptive processing otherwise generated by persistent small afferent input generated by tissue injury.

IMPLICATIONS

Gabapentin and its analog, 3-isobutyl gamma-aminobutyric acid, given spinally, produce a dose-dependent, D-serine-sensitive reversal of the thermal hyperalgesia evoked by mild thermal injury.

Vigabatrin enhances promoted release of GABA in neonatal rat optic nerve

Vigabatrin (gamma-vinyl GABA) is an antiepileptic drug and blocks GABA transaminase activity resulting in elevations in cellular GABA levels in the brain. Nipecotic acid (NPA) promotes release of GABA from neonatal optic nerve astrocytes, resulting in a bicuculline-sensitive depolarization of the optic nerve axons. The NPA-induced depolarization of vigabatrin-treated rats (100 mg/kg, i.p.) more than doubled, suggesting an elevation in free GABA levels; the GABA transporter inhibitor, NO-711 reduced the depolarization. These results are consistent with the known ability of vigabatrin to block the GABA degradation enzyme GABA-transaminase, suggesting that vigabatrin elevates astrocytic GABA levels, thereby favoring greater release of GABA through the GABA transporter.

A summary of mechanistic hypotheses of gabapentin pharmacology

Although the cellular mechanisms of pharmacological actions of gabapentin (Neurontin) remain incompletely described, several hypotheses have been proposed. It is possible that different mechanisms account for anticonvulsant, antinociceptive, anxiolytic and neuroprotective activity in animal models. Gabapentin is an amino acid, with a mechanism that differs from those of other anticonvulsant drugs such as phenytoin, carbamazepine or valproate. Radiotracer studies with [14C]gabapentin suggest that gabapentin is rapidly accessible to brain cell cytosol. Several hypotheses of cellular mechanisms have been proposed to explain the pharmacology of gabapentin: 1. Gabapentin crosses several membrane barriers in the body via a specific amino acid transporter (system L) and competes with leucine, isoleucine, valine and phenylalanine for transport. 2. Gabapentin increases the concentration and probably the rate of synthesis of GABA in brain, which may enhance non-vesicular GABA release during seizures. 3. Gabapentin binds with high affinity to a novel binding site in brain tissues that is associated with an auxiliary subunit of voltage-sensitive Ca2+ channels. Recent electrophysiology results suggest that gabapentin may modulate certain types of Ca2+ current. 4. Gabapentin reduces the release of several monoamine neurotransmitters. 5. Electrophysiology suggests that gabapentin inhibits voltage-activated Na+ channels, but other results contradict these findings. 6. Gabapentin increases serotonin concentrations in human whole blood, which may be relevant to neurobehavioral actions. 7. Gabapentin prevents neuronal death in several models including those designed to mimic amyotrophic lateral sclerosis (ALS). This may occur by inhibition of glutamate synthesis by branched-chain amino acid aminotransferase (BCAA-t).

Measuring human brain GABA in vivo: effects of GABA-transaminase inhibition with vigabatrin

Gamma-aminobutyric acid (GABA) plays a pivotal role in suppressing the origin and spread of seizure activity. Low occipital lobe GABA was associated with poor seizure control in patients with complex partial seizures. Vigabatrin irreversibly inhibits GABA-transaminase, raising brain and cerebrospinal fluid (CSF) GABA concentrations. The effect of vigabatrin on occipital lobe GABA concentrations was measured by in vivo nuclear magnetic-resonance spectroscopy. Using a single oral dose of vigabatrin, the rate of GABA synthesis in human brain was estimated at 17% of the Krebs cycle rate. As the daily dose of vigabatrin was increased to up to 3 g, the fractional elevation of brain GABA was similar to CSF increase. Doubling the daily dose from 3 to 6 g failed to increase brain GABA further. Increased GABA concentrations appear to reduce GABA synthesis in humans as it does in animals. With traditional antiepileptic drugs, remission of the seizure disorder was associated with normal GABA levels. With vigabatrin, elevated CSF and brain GABA was associated with improved seizure control. Vigabatrin enhances the vesicular and nonvesicular release of GABA. The release of GABA during seizures may be mediated in part by transporter reversal that may serve as an important protective mechanism. During a seizure, this mechanism may be critical in stopping the seizure or preventing its spread.

Gabapentin reverses the allodynia produced by the administration of anti-GD2 ganglioside, an immunotherapeutic drug

Systemically administered, the anti-GD2 antibody produces allodynia demonstrated by decreased mechanical withdrawal threshold. Electrophysiologic recordings indicate a probable neuropathic origin, as small-diameter sensory fibers develop continuous high-frequency discharge after antibody administration. Gabapentin (GBP) is a gamma-aminobutyric acid analog originally synthesized for its anticonvulsant actions. Several open-label clinical studies, as well as a wealth of anecdotal evidence, suggest that GBP may be beneficial for the treatment of neuropathic pain. This study examined the effects of GBP given as a posttreatment after induction of an anti-GD2-associated allodynia. Anti-GD2 (1 mg/kg intravenously [i.v.]) administered to Sprague-Dawley rats reduced the mean withdrawal threshold from 14.71 to 4.95 g (P < 0.001), as measured by using von Frey hairs. This was reversed by GBP in a dose-dependent fashion; the minimal effective dose was between 3 and 30 mg/kg i.v. The maximal percent analgesic effect of GBP was 76% and 93% at doses of 30 and 100 mg/kg, respectively (P < 0.001). With these doses, side effects were minimal and were manifested as slightly decreased spontaneous movement and startle response. No changes were seen in reflex responses to corneal or pinna stimulation, and no motor deficits were observed. These data support the use of GBP as an effective therapy for neuropathic pain.

IMPLICATIONS

After the administration of anti-GD2 antibody, rats display an escape reaction to light touch, increased blood pressure, and aberrant firing in nerve fibers associated with pain transmission. Systemic gabapentin reduced or eliminated the escape response and reversed the hypertension with minimal side effects. This suggests that gabapentin blocked the antibody-associated (neuropathic) pain.

The influence of established and new antiepileptic drugs on visual perception. 1. A placebo-controlled, double-blind, single-dose study in healthy volunteers

The influence of single oral dosages of carbamazepine (CBZ), valproic acid, vigabatrin (VGB), lamotrigine (LTG), gabapentin (GBP), and losigamone (LSG) on visual perception was investigated in ten healthy volunteers according to a double-blind, placebo-controlled, cross-over study design. The test battery comprised visual acuity, the Lanthony-D-15-désaturé colour perception test, increment, postadaptation and transient tritanopia threshold measurements, perception threshold assessment for monochromatic and chromatic gaussian dots, monochromatic gratings and gratings of differing spatial frequency, and critical flicker fusion tests with various stimuli. The only consistent and partly significant effects were seen after VGB and GBP. After VGB, increment, postadaptation and transient tritanopia thresholds and the critical flicker fusion increased, whereas GBP led to a somewhat converse profile. The other tests were not influenced consistently by any antiepileptic drug (AED). We conclude that: (i) gamma-amino-butyric acid-(GABA)-related properties as under the prototype drug VGB result in specific alterations of the transient tritanopia phenomenon which is consistent with the physiological hypothesis for this retinal paradigm based on extracellular recordings in primates. The possible mechanisms why VGB improved critical flicker fusion as the only AED in this trial are discussed. The profile of GBP indicates a unique mechanism of action. We have not observed specific influences on visual perception under AEDs which act mainly via alterations of ion membrane conductance. The transient tritanopia and flicker fusion paradigms we used appear to be promising to investigate antiepileptic drugs with hitherto unknown modes of actions in human noninvasively.

The novel anticonvulsant, gabapentin, protects against both convulsant and anxiogenic aspects of the ethanol withdrawal syndrome

The effects of the anticonvulsant, gabapentin, were investigated, in mice, on the withdrawal convulsive behaviour and anxiety-related behaviour that are produced by cessation of prolonged intake of ethanol. When given at 50 or 100 mg/kg, this compound decreased the rise in handling-induced hyperexcitability which occurs during the withdrawal period; the effects were most pronounced for the first 4 hr after administration. Gabapentin also decreased the convulsive response to an audiogenic stimulus during the withdrawal period. The elevated plus-maze, with both traditional and ethological indices of activity was used as a test of anxiety-related behaviour after cessation of chronic ethanol treatment. Gabapentin, at 50 and 100 mg/kg, was found to decrease some, although not all, of the signs of withdrawal-induced anxiety. At doses up to and including 200 mg/kg, gabapentin had no effect on motor co-ordination or spontaneous locomotor activity in control animals. The results demonstrated that gabapentin has a selective action in decreasing both convulsive and anxiety-related aspects of withdrawal behaviour after chronic ethanol treatment. It is possible that further studies with this compound may shed further light on the mechanisms involved in the withdrawal syndrome.

A double-blind controlled study of gabapentin and baclofen as treatment for acquired nystagmus

We conducted a double-blind crossover trial comparing gabapentin (up to 900 mg/day) to baclofen (up to 30 mg/day) as therapy for acquired nystagmus in 21 patients. We measured visual acuity and the nystagmus before, and at the end of, 2 weeks on each medication. For a group of 15 patients with acquired pendular nystagmus (APN), visual acuity improved significantly with gabapentin, but not with baclofen. Gabapentin significantly reduced APN median eye speed in all three planes, but baclofen did so only in the vertical plane. In 10 patients with APN, the reduction of nystagmus with gabapentin was substantial and 8 of these elected to continue taking the drug. In 6 patients with downbeat or torsional downbeat nystagmus, changes in median slow-phase eye speed were less consistent with both drugs, either increasing or decreasing, and being dependent on viewing conditions. Only 1 patient showed consistent reduction of median eye speed, and this was achieved by either drug. Our findings suggest that gabapentin may be an effective treatment for many patients with APN and that occasional patients with downbeat nystagmus will respond to gabapentin or baclofen.

The effect of novel anti-epileptic drugs in rat experimental models of acute and chronic pain

The novel anti-epileptic drugs lamotrigine, felbamate and gabapentin were compared in rat experimental models of acute (tail flick) and chronic pain: the chronic constriction injury and spinal nerve ligation models. Lamotrigine (10-100 mg/kg, s.c.), felbamate (150-600 mg/kg, i.p.) and gabapentin (30-300 mg/kg, i.p.) each reversed cold allodynia (chronic constriction injury model) with ED50 values of 28, 241 and 103 mg/kg, respectively, 1 h post-dose. However, only gabapentin reversed tactile allodynia (spinal nerve ligation model) with an ED50 of 34 mg/kg (i.p.). The established anti-epileptic drugs, carbamazepine (1-30 mg/kg, i.p.) and phenytoin (1-100 mg/kg, s.c.), were ineffective in both models. The anti-allodynic effect of the newer anti-epileptic drugs was observed at doses that were either ineffective or produced only a negligible effect on acute nociceptive function and/or locomotor activity. In conclusion, the data suggest that the newer anti-epileptic drugs appear to have the potential to be effective alternatives to either carbamazepine or phenytoin in the treatment of neuropathic pain. However, only gabapentin ameliorated both cold and touch hyperesthesias.

New antiepileptic drugs in children: present and future

The pharmacologic management of epilepsy has progressed greatly during the last decade. New medications are available for the management of refractory patients, and more are being developed. For some patients, these productions offer added efficacy, better tolerability, and some pharmacodynamic advantages. Since preapproval studies include few pediatric trials, the final role of these medications in the treatment of childhood epilepsy will be dictated by additional studies and postmarketing experience.

Gabapentin for parkinsonism: a double-blind, placebo-controlled, crossover trial

PURPOSE

Gabapentin is a recently available anticonvulsant whose mechanism of action remains unknown. We suspected efficacy from serendipitous observations of gabapentin in patients with parkinsonism. This led us to a double-blind, placebo-controlled, crossover trial.

PATIENTS AND METHODS

We administered gabapentin in a placebo-controlled, double-blind, crossover trial to 19 subjects with advanced parkinsonism. We measured the effect of placebo and gabapentin on subjects' symptoms with the Unified Parkinson's Disease Rating Scale, the Webster Scale, and the Hoehn and Yahr Scale. We assessed tremor with surface-recorded electromyography.

RESULTS

Total Unified Parkinson's Disease Rating Scale improved with gabapentin compared with placebo (P = 0.0005). Likewise, activities of daily living and examination subscore of the Unified Parkinson's Disease Rating Scale improved with gabapentin compared with placebo but did not achieve statistical significance. Webster Scale showed improvement but neither Hoehn and Yahr Scale nor Webster Scale changes reached statistical significance. Tremor as measured by the Unified Parkinson's Disease Rating Scale improved with gabapentin but the use of the root mean square of the rectified electromyography as a measure of tremor activity was not statistically significant.

CONCLUSIONS

This study demonstrates that gabapentin improves rigidity, bradykinesia, and tremor of parkinsonism including both Parkinson's disease and Parkinson's syndrome. The rigidity and bradykinesia of parkinsonism improve on the drug even when the effects of gabapentin on tremor are discounted.

Clinical significance of animal seizure models and mechanism of action studies of potential antiepileptic drugs

More than 50 million persons worldwide suffer from epilepsy, many of whom are refractory to treatment with standard antiepileptic drugs (AEDs). Fortunately, new AEDs commercialized since 1990 are improving the clinical outlook for many patients. Our growing understanding of anticonvulsant mechanisms and the relevance of preclinical animal studies to clinical antiepileptic activity have already contributed to the design of several new AEDs and should be increasingly beneficial to further efforts at drug development. Mechanisms have been identified for older AEDs [phenytoin (PHT), carbamazepine (CBZ), valproate (VPA), barbiturates, benzodiazepines (BZDs), ethosuximide (ESM)] and newer AEDs [vigabatrin (VGB), lamotrigine (LTG), gabapentin (GBP) tiagabine (TGB), felbamate (FBM), topiramate (TPM)]. Several novel anticonvulsant mechanisms have recently been discovered. FBM appears to be active at the strychnine-insensitive glycine binding site of the NMDA receptor. TPM is active on the kainate/AMPA subtype of glutamate receptor and at a potentially novel site on the GABA(A) receptor. For several reasons, availability of a single AED with multiple mechanisms of action may be preferred over availability of multiple AEDs with single mechanisms of action. These reasons include ease of titration, lack of drug-drug interactions, and reduced potential for pharmacodynamic tolerance.

The anticonvulsant gabapentin decreases firing rates of substantia nigra pars reticulata neurons

Gabapentin is a novel anti-epileptic drug which enhances GABA (gamma-aminobutyric acid) turnover in certain brain regions, including substantia nigra. However, the functional consequences of GABA turnover increases in response to gabapentin and their potential involvement in the anticonvulsant action of this drug are not known. In the present study, we examined the effects of gabapentin on the extracellular, single unit activity of nondopaminergic (presumably GABAergic) neurons of the substantia nigra pars reticulata in rats. During the recordings, the animals were infused with the narcotic opioid analgesic fentanyl, associated with a skeletal muscle relaxant and artificial ventilation. The spontaneous firing of substantia nigra pars reticulata neurons was determined up to about 2 h after i.v. or i.p. administration of gabapentin at doses of 15-30 mg/kg. After both routes of administration, gabapentin markedly reduced neuronal firing when administered at a dose of 20-30 mg/kg, while 15 mg/kg were ineffective in this regard. The suppressive effect of gabapentin was rapid in onset (2 min after i.v. and about 20 min after i.p. injection), reached peak values of about 70% below predrug baseline after about 45-60 min, and remained at this level for at least 2 h. Vehicle administration had no effect on substantia nigra pars reticulata neurons. The ability of gabapentin to alter substantia nigra pars reticulata firing does correlate with its known ability to increase nigral GABA turnover. Since a substantial body of evidence suggests that the substantia nigra pars reticulata is a critical site at which decrease of neuronal firing by potentiation of GABAergic influences results in protection against various seizure types, the suppressive effect of gabapentin on substantia nigra pars reticulata activity may contribute to the anticonvulsant action of this drug.