A summary of mechanistic hypotheses of gabapentin pharmacology

Effects of gabapentin on cocaine self-administration, cocaine-triggered relapse and cocaine-enhanced nucleus accumbens dopamine in rats

Gabapentin is a gamma-aminobutyric acid (GABA) analogue, with GABAmimetic pharmacological properties. Gabapentin is used for the treatment of seizures, anxiety and neuropathic pain. It has been proposed that gabapentin may be useful in the treatment of cocaine dependence. However, clinical trials with gabapentin have shown conflicting results, while preclinical studies are sparse. In the present study, we investigated the effects of gabapentin on intravenous cocaine self-administration and cocaine-triggered reinstatement of drug-seeking behavior, as well as on cocaine-enhanced dopamine (DA) in the nucleus accumbens (NAc). We found that gabapentin (25-200 mg/kg, i.p., 30 min or 2 h prior to cocaine) failed to inhibit intravenous cocaine (0.5 mg/kg/infusion) self-administration under a fixed-ratio reinforcement schedule or cocaine-triggered reinstatement of cocaine-seeking behavior. In vivo microdialysis showed that the same doses of gabapentin produced a modest increase (approximately 50%, p<0.05) in extracellular NAc GABA levels, but failed to alter either basal or cocaine-enhanced NAc DA. These data suggest that gabapentin is a weak GABA-mimic drug. At the doses tested, it has no effect in the addiction-related animal behavioral models here tested. This is in striking contrast to positive findings in the same animal models shown by another GABAmimetic--gamma-vinyl GABA (see companion piece to present article).

Clinical pharmacokinetics of XP13512, a novel transported prodrug of gabapentin

Gabapentin absorption occurs in only a limited region of the small intestine and saturates at doses used clinically, resulting in dose-dependent pharmacokinetics, high interpatient variability, and potentially ineffective drug exposure. XP13512/GSK1838262 is a novel transported prodrug of gabapentin that is absorbed throughout the entire length of the intestine by high-capacity nutrient transporters. In 4 studies of healthy volunteers (136 subjects total), the pharmacokinetics of XP13512 immediate- and extended-release formulations were compared with those of oral gabapentin. XP13512 immediate-release (up to 2800 mg single dose and 2100 mg twice daily) was well absorbed (>68%, based on urinary recovery of gabapentin), converted rapidly to gabapentin, and provided dose-proportional exposure, whereas absorption of oral gabapentin declined with increasing doses to <27% at 1200 mg. Compared with 600 mg gabapentin, an equimolar XP13512 extended-release dose provided extended gabapentin exposure (time to maximum concentration, 8.4 vs 2.7 hours) and superior bioavailability (74.5% vs 36.6%). XP13512 may therefore provide more predictable gabapentin exposure and decreased dosing frequency.

The interaction of gabapentin and N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) on mechanical allodynia in rats with a spinal nerve ligation

We examined the antiallodynic interaction between gabapentin and adenosine A1 receptor agonist, N(6)-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), in a rat model of nerve ligation injury. Rats were prepared with ligation of left L5-6 spinal nerves and intrathecal catheter implantation for drug administration. Mechanical allodynia was measured by applying von Frey filaments. Gabapentin and R-PIA were administered to obtain the dose-response curve and the 50% effective dose (ED(50)). Fractions of ED(50)s were administered concurrently to establish the ED(50) of the drug combination. The drug interaction between gabapentin and R-PIA was analyzed using the isobolographic method. Adenosine A1 receptor antagonist was administered intrathecally to examine the reversal of the antiallodynic effect. Locomotor function changes were evaluated by rotarod testing. Intrathecal gabapentin and R-PIA and their combination produced a dose-dependent antagonism against mechanical allodynia without severe side effects. Intrathecal gabapentin synergistically enhanced the antiallodynic effect of R-PIA when coadministered. There were no significant changes in rotarod performance time, except gabapentin 300 microg. In the combination group, the maximal antiallodynic effect was reversed by A1 adenosine receptor antagonist. These results suggest that activation of adenosine A1 receptors at the spinal level is required for the synergistic interaction on the mechanical allodynia.

Isobolographic and behavioral characterizations of interactions between vigabatrin and gabapentin in two experimental models of epilepsy

The aim of this study was to characterize the pharmacodynamic, pharmacokinetic and adverse-effect profiles of vigabatrin and gabapentin. Isobolographic analysis was used in two mouse experimental models of epilepsy: the maximal electroshock seizure threshold test and pentylenetetrazole-induced seizures. In the maximal electroshock seizure threshold test, electroconvulsions were produced by a current with various intensities whilst in the pentylenetetrazole test a CD(97) dose (100 mg/kg) was used. Potential adverse-effect profiles of interactions of vigabatrin with gabapentin at three fixed-ratios of 1:3, 1:1 and 3:1 from both seizure tests were evaluated in the chimney (motor performance) and grip-strength (skeletal muscular strength) tests. Vigabatrin and gabapentin total brain concentrations were determined with high performance liquid chromatography. Vigabatrin and gabapentin administered singly increased the electroconvulsive threshold (TID(20) - 226.2 and 70.0 mg/kg, respectively). With isobolography, the combination of vigabatrin with gabapentin at the fixed-ratio of 1:3 exerted supra-additive (synergistic) interactions whilst at 1:1 and 3:1 additivity occurred. Similarly, vigabatrin and gabapentin administered singly suppressed the pentylenetetrazole-induced seizures (ED(50) values - 622.5 and 201.1 mg/kg, respectively). Isobolography revealed that vigabatrin with gabapentin in combination at the fixed-ratio of 1:1 produced supra-additive (synergistic) interaction whilst at 1:3 and 3:1 additivity occurred. In combination neither motor coordination nor skeletal muscular strength was affected. Total vigabatrin and gabapentin brain concentrations revealed that neither drug affected the pharmacokinetics of the other. Vigabatrin and gabapentin have a favorable pharmacodynamic interaction in animal seizure models in the absence of acute adverse effects or concurrent pharmacokinetic changes.

Subchronic treatment with antiepileptic drugs modifies pentylenetetrazol-induced seizures in mice: Its correlation with benzodiazepine receptor binding

Experiments using male CD1 mice were carried out to investigate the effects of subchronic (daily administration for 8 days) pretreatments with drugs enhancing GABAergic transmission (diazepam, 10 mg/kg, ip; gabapentin, 100 mg/kg, po; or vigabatrin, 500 mg/kg, po) on pentylenetetrazol (PTZ)-induced seizures, 24 h after the last injection. Subchronic administration of diazepam reduced latencies to clonus, tonic extension and death induced by PTZ. Subchronic vigabatrin produced enhanced latency to the first clonus but faster occurrence of tonic extension and death induced by PTZ. Subchronic gabapentin did not modify PTZ-induced seizures. Autoradiography experiments revealed reduced benzodiazepine receptor binding in several brain areas after subchronic treatment with diazepam or gabapentin, whereas subchronic vigabatrin did not induce significant receptor changes. The present results indicate differential effects induced by the subchronic administration of diazepam, vigabatrin, and gabapentin on the susceptibility to PTZ-induced seizures, benzodiazepine receptor binding, or both.

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.

Gabapentin activates ROMK1 channels by a protein kinase A (PKA)-dependent mechanism

BACKGROUND AND PURPOSE

Gabapentin is an effective anticonvulsant. The major physiological function of renal outer medullary potassium (ROMK1) channels is to maintain the resting membrane potential (RMP). We investigated the effect of gabapentin on ROMK1 channels and the mechanism involved.

EXPERIMENTAL APPROACH

Xenopus oocytes were injected with mRNA coding for wild-type or mutant ROMK1 channels and giant inside-out patch-clamp recordings were performed.

KEY RESULTS

Gabapentin increased the activity of ROMK1 channels, concentration-dependently and enhanced the activity of wild-type and an intracellular pH (pH(i))-gating residue mutant (K80M) channels over a range of pH(i). Gabapentin also increased activity of channels mutated at phosphatidylinositol 4,5-bisphosphate (PIP(2))-binding sites (R188Q, R217A and K218A). However, gabapentin failed to enhance channel activity in the presence of protein kinase A (PKA) inhibitors and did not activate phosphorylation site mutants (S44A, S219A or S313A), mutants that mimicked the negative charge carried by a phosphate group bound to a serine (S44D, S219D or S313D), or a mutated channel with a positive charge (S219R). These findings show that gabapentin activates ROMK1 channels independently of the pH(i) and not via a PIP(2)-dependent pathway. The effects of gabapentin on ROMK1 channels may be due to a PKA-mediated phosphorylation-induced conformational change, but not to charge-charge interactions.

CONCLUSIONS AND IMPLICATIONS

ROMK1 channels are the main channels responsible for maintaining the RMP during cellular excitation. Gabapentin increased the activity of ROMK1 channels by a PKA-dependent mechanism, reducing neuronal excitability, and this may play an important role in its antiepileptic effect.

Gabapentin for Intractable Hiccups in Palliative Care

Intractable hiccups are not common in the general population or in the palliative care population but can adversely impact quality of life and cause other complications such as weight loss and sleep disturbance. Many treatments have been proposed for intractable hiccups, but there is little consensus regarding treatment in the medical literature. This is partly because hiccups are relatively uncommon and many of the proposed treatments are unproven or have long-term side effects. Pharmacologic treatments rather than home remedies or surgical treatments are more appropriate for the palliative care patient. Gabapentin is a promising medication for the treatment of intractable hiccups for its safety, lack of serious side effects, and rapid onset of action. Further research is indicated to determine whether gabapentin is consistently effective.

Gabapentin

Several lines of evidence suggest that loss of central inhibition after deprivation of input from the ear (peripheral deafferentation) may be one cause of chronic tinnitus. Aging and acoustic trauma, the two most common causes of peripheral damage to the auditory system, each decrease input to central auditory structures. Loss of input to tonic inhibitory systems would release excitatory structures from inhibitory regulation. The increased activity resulting may be interpreted by more rostral structures in the auditory pathway as tinnitus. Down-regulation of gamma-amino butyric acid (GABA), a major inhibitory neurotransmitter of the central auditory pathway, is a potential mechanism for the loss of inhibition. Both animal studies and human clinical trials implicate loss of inhibition, and specifically loss of GABA function, in the development of acoustic trauma-induced tinnitus.

Going beyond efficacy: strategies for cancer pain management

Despite great advances in the fields of pain management and palliative care, pain directly or indirectly associated with a cancer diagnosis remains significantly undertreated. The present paper reviews the current standard for cancer pain management and highlights new treatments and targeted interventional techniques.

Efficacy of Pregabalin in the Treatment of Trigeminal Neuralgia

This prospective, open-label study aimed to evaluate the efficacy of pregabalin treatment in patients suffering from trigeminal neuralgia with and without concomitant facial pain. Fifty-three patients with trigeminal neuralgia (14 with concomitant chronic facial pain) received pregabalin (PGB) 150-600 mg daily and were prospectively followed for 1 year. The primary outcome was number of patients pain free or with reduction of pain intensity by > 50% and of attack frequency by > 50% after 8 weeks. Secondary outcome was sustained pain relief after 1 year. Thirty-nine patients (74%) improved after 8 weeks with a mean dose of 269.8 mg/day (range 150-600 mg/day) PGB: 13 (25%) experienced complete pain relief and 26 (49%) reported pain reduction > 50%, whereas 14 (26%) did not improve. Patients without concomitant facial pain showed better response rates (32 of 39, 82%) compared with patients with concomitant chronic facial pain (7 of 14, 50%, P = 0.020). Concomitant chronic facial pain appears to be a clinical predictor of poor treatment outcome. PGB appears to be effective in the treatment of trigeminal neuralgia.

Influence of positive allosteric modulators on GABA(B) receptor coupling in rat brain: a scintillation proximity assay characterisation of G protein subtypes

Little is known concerning coupling of cerebral GABA(B) receptors to G protein subtypes, and the influence of positive allosteric modulators (PAMs) has not been evaluated. These questions were addressed by an antibody-capture/scintillation proximity assay strategy. GABA concentration-dependently enhanced the magnitude of [(35)S]GTPgammaS binding to Galphao and, less markedly, Galphai(1/3) in cortex, whereas Gq and Gs/olf were unaffected. (R)-baclofen and SKF97581 likewise activated Galphao and Galphai(1/3), expressing their actions more potently than GABA. Similar findings were acquired in hippocampus and cerebellum, and the GABA(B) antagonist, CGP55845A, abolished agonist-induced activation of Galphao and Galphai(1/3) in all structures. The PAMs, GS39783, CGP7930 and CGP13501, inactive alone, enhanced efficacy and potency of agonist-induced [(35)S]GTPgammaS binding to Galphao in all regions, actions abolished by CGP55845A. In contrast, they did not modify efficacies at Galphai(1/3). Similarly, in human embryonic kidney cells expressing GABA(B(1a+2)) or GABA(B(1b+2)) receptors, allosteric modulators did not detectably enhance efficacy of GABA at Galphai(1/3), though they increased its potency. To summarise, GABA(B) receptors coupled both to Galphao and to Galphai, but not Gq and Gs/olf, in rat brain. PAMs more markedly enhanced efficacy of coupling to Go versus Gi(1/3). It will be of interest to confirm these observations employing complementary techniques and to evaluate their potential therapeutic significance.

Gabapentin and pregabalin for chronic neuropathic and early postsurgical pain: current evidence and future directions

PURPOSE OF REVIEW

Gabapentin and pregabalin bind to the alpha-2-delta calcium channel subunit and represent a novel analgesic drug class. The evidence base supporting their use for chronic neuropathic and early postsurgical pain is reviewed.

RECENT FINDINGS

Multiple, large, high-quality trials have demonstrated the safety and efficacy of gabapentin and pregabalin in neuropathic pain. Treatment-related improvement of pain and sleep positively impact upon quality of life. Sedation, dizziness and ataxia are important and relatively common adverse effects, however. Accumulating evidence indicates that gabapentin, and possibly pregabalin, also exert important effects following surgery. Multiple high-quality trials have demonstrated analgesic and opioid-sparing efficacy with gabapentin following various surgical procedures. Gabapentin and pregabalin reduce movement-evoked pain and this can lead to enhanced functional postoperative recovery. Postoperative opioid sparing is of questionable relevance since few trials have shown reduced opioid-related adverse effects. Sedation, dizziness and ataxia have been reported in only a few trials. Future larger-scale perioperative trials focused on safety assessment are needed, however.

SUMMARY

Gabapentin and pregabalin are efficacious treatments for neuropathic and postsurgical pain. Future research addressing several specific questions would serve to better delineate their optimal roles in treating these and other pain conditions.

The Analgesic Effects of Preemptive Gabapentin in Patients Undergoing Surgery for Brachial Plexus Injury—A Preliminary Study

There are reports indicating that gabapentin may have place in the treatment of postoperative pain. No study has evaluated the effects of gabapentin on acute, postoperative pain in patients undergoing surgery for brachial plexus injuries. In this preliminary study, we evaluated gabapentin as preemptive analgesic for intraoperative period and during the acute postoperative period at rest and during movement. Twenty consecutive adult patients undergoing surgery for brachial plexus injury were enrolled for the study. Patients randomly received either oral gabapentin 800 mg or placebo capsules 2 hours before surgery. General anesthesia was induced and maintained with propofol, at bispectral index value between 40 and 60. Intraoperative fentanyl and propofol requirements were noted. Postoperatively, all patients were alert and pain was assessed using visual analog scale (VAS) for 24 hours, both during rest and movement. Whenever VAS score was more than 50 or on patients' demand, ketorolac 30 mg was given as rescue analgesic. The demographics, duration of surgery, and propofol consumption in both groups were comparable. Intraoperative and postoperative heart rate and mean blood pressure were also comparable. Significant difference was noted in intraoperative fentanyl consumption (P=0.03), total dose of rescue analgesic (P=0.004), and VAS score at rest and movement, between the 2 groups; less in gabapentin group as compared with placebo group (P=0.01 and 0.04, at rest and movement, respectively). A single oral dose of gabapentin 800 mg, as preemptive analgesic in patients undergoing surgery for brachial plexus injury is found to be an effective adjunct to intraoperative and postoperative pain. Pain is reduced not only at rest but also during movement.

Influence of aminophylline on the anticonvulsive action of gabapentin in the mouse maximal electroshock seizure threshold model

Accumulating evidence indicates that aminophylline [theophylline(2) x ethylenediamine] markedly attenuates the anticonvulsant action of conventional antiepileptic drugs in experimental animal models of epilepsy and evokes severe seizure activity in patients treated with this methylxanthine. The objective of this study was to determine the influence of acute (single) and chronic (twice daily for 14 consecutive days) treatments with aminophylline on the anticonvulsant potential of gabapentin (a second-generation antiepileptic drug) in the mouse maximal electroshock seizure threshold model. Additionally, the effects of acute and chronic administration of aminophylline on the adverse effect potential of gabapentin in terms of motor coordination impairment were assessed in the chimney test. To evaluate pharmacokinetic characteristics of interaction between drugs, total brain concentrations of gabapentin and theophylline were estimated with high-pressure liquid chromatography and fluorescence polarization immunoassay, respectively. Results indicated that gabapentin (at doses of 75 and 100 mg/kg, i.p.) increased the threshold for electroconvulsions in mice. Aminophylline in non-convulsive doses of 50 and 100 mg/kg (i.p.), both in acute and chronic experiments, did not attenuate the anticonvulsant potential of gabapentin in the maximal electroshock seizure threshold test in mice. Similarly, aminophylline at a dose of 100 mg/kg had no impact on the adverse effect potential of gabapentin in the chimney test. Pharmacokinetic evaluation of total brain concentrations of gabapentin and theophylline revealed no significant changes in total brain concentrations of the drugs after both, acute and chronic applications of aminophylline in combination with gabapentin. The data show that aminophylline did not alter the ability of gabapentin to protect mice against seizures induced by electroconvulsive shock. The observed interaction between gabapentin and aminophylline in both acute and chronic experiments was pharmacodynamic in nature.

Gabapentin and an opioid combination versus opioid alone for the management of neuropathic cancer pain: a randomized open trial

Neuropathic cancer pain represents a major challenge. Treatment often requires adjuvant analgesics, including gabapentin, to complement the effects of opioids. This study aimed to compare the effectiveness and safety of gabapentin combined with an opioid versus opioid monotherapy for the management of neuropathic cancer pain. Seventy-five cancer patients who were receiving opioid therapy and reported sufficient pain relief of nociceptive, but not neuropathic, pain were enrolled. Sixty-three patients completed the study. Patients were randomized to one of the following treatment protocols: 1) gabapentin adjuvant to ongoing opioid treatment titrated according to pain response while opioid dose was kept constant (group GO), and 2) continuation of opioid monotherapy according to the World Health Organization treatment ladder approach (group OO). Changes in pain intensity, allodynia, and analgesic drug consumption were evaluated at Day 4 and Day 13. Side effects were also recorded. Both treatments resulted in a significant reduction of pain intensity at Day 4 and Day 13 compared to baseline. However, mean pain intensity for burning and shooting pain was significantly higher in the OO group compared to the GO group at both the fourth (P=0.0001) and 13th (P=0.0001) days of the study. An earlier significant decrease (at Day 4, P=0.002) was observed for allodynia in the GO group compared to the OO group. The rate of side effects in the GO group was significantly lower than that in the OO group (P=0.015). These data suggest that gabapentin added to an opioid provides better relief of neuropathic pain in cancer patients than opioid monotherapy; this combination of gabapentin and an opioid may represent a potential first-line regimen for the management of pain in these patients.

Use of gabapentin for perioperative pain control -- a meta-analysis

BACKGROUND

Gabapentin, an anticonvulsant, has recently been suggested as an effective postoperative 'analgesic' agent. The objective of the present study was to examine the analgesic effectiveness, opioid-sparing effects and side effects associated with the use of gabapentin in a perioperative setting.

METHODS

Following the Quality of Reporting of Meta-analyses recommendations, nine electronic databases until February 2006 were searched, without language restriction, for randomized controlled trials comparing gabapentin with control for postoperative pain control. Outcome measures, namely, 24 h cumulative opioid consumption, visual analogue scale pain scores and adverse effects, were expressed as odds ratios, ratio of means or weighted mean differences (as appropriate), which were aggregated under the fixed or random effects models.

RESULTS

Gabapentin caused a 35% reduction in total opioid consumption over the first 24 h following surgery (ratio of means 0.65, 95% CI 0.59 to 0.72), a significant reduction in postoperative pain at rest (in the first 24 h) and with movement (at 2 h, 4 h and 12 h), regardless of whether treatment effects were expressed as ratios of means or weighted mean differences, and a reduction of vomiting (relative risk [RR] 0.73, 95% CI 0.56 to 0.95) and pruritus (RR 0.30, 95% CI 0.13 to 0.70). It was associated with a significant increase in dizziness (RR 1.40, 95% CI 1.06 to 1.84) and an increase in sedation of borderline significance (RR 1.65, 95% CI 1.00 to 2.74).

CONCLUSION

Gabapentin improves the analgesic efficacy of opioids both at rest and with movement, reduces analgesic consumption and opioid-related adverse effects, but is associated with an increased incidence of sedation and dizziness.

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.

The importance of genetic background on pain behaviours and pharmacological sensitivity in the rat spared serve injury model of peripheral neuropathic pain

Neuropathic pain conditions can encompass a diverse constellation of signs and symptoms consisting of sensory deficits, allodynia and hyperalgesia. Animal models of neuropathic pain have enabled the identification of key pathophysiological changes occurring within nociceptive pathways as a result of injury, and serve an invaluable role for preclinical screening of novel analgesic candidates. We have produced the first systematic description of the development and maintenance, and the pharmacological sensitivity of nociceptive behaviours in four rat strains with different genetic background (outbred Sprague-Dawley and inbred Brown Norway, Lewis and Fischer 344 rats), using the spared nerve injury model of peripheral neuropathic pain. Hindpaw mechanical hypersensitivity was evident from 7 to 30 days post-injury in all four strains, developing most quickly and severely in Fischer 344 rats; Lewis rats were least affected. Morphine (6 but not 3 mg/kg, s.c.) and gabapentin (100 but not 50 mg/kg, s.c.) had significant antiallodynic and antihyperalgesic actions in all four strains after spared nerve injury, although marked differences in potency across strains were observed. Two strains (Fischer 344 rats and Lewis) were insensitive to the antihyperalgesic properties of gaboxadol (15 mg/kg) whereas gaboxadol (6 mg/kg) was equipotent to morphine (6 mg/kg) in two other strains (Sprague-Dawley and Brown Norway). The observed pharmacogenetic variations have important implications for the preclinical testing of drugs, and provide a basis for development of pharmacogenomics in neuropathic pain.

Prolonged myotonia and dystonia after general anaesthesia in a patient taking gabapentin

This is the report of a 55-yr-old female who developed severe myotonia and dystonia after general anaesthesia. Before starting on gabapentin therapy for a neuropathic pain condition, she had undergone numerous uneventful general anaesthetics. Since receiving treatment with gabapentin, she has experienced severe movement disorders on emergence from each subsequent general anaesthetic. The events were unrelated to the choice of anaesthetic or anti-emetic. The most recent event that required a protracted stay in hospital after a day-case surgery is presented in detail, and the possible mechanisms to explain the interaction are discussed.

Pharmacotherapy of dual substance abuse and dependence

The US FDA has approved a limited number of treatments for alcohol, nicotine and opioid dependence; however, no treatments for other abused drugs such as marijuana, cocaine or methamphetamine are approved. This review focuses on research into drug pharmacotherapies, particularly single-drug therapies, for substance abuse and dependence contributing to the most important dual substance use disorders (SUDs). Given the implications of poly-substance abuse, it is essential that clinicians and researchers be aware of potential pharmacotherapies for the treatment of dual SUDs.A substantial number of patients abuse more than one drug concurrently, complicating the treatment of SUD and leaving clinicians with few FDA-approved drug options for their patients. In this era of evidence-based medicine, such patients are typically treated with therapeutically proven medications, but in ways that are outside the scope of a drug's original indication by the FDA. Such 'off-label' prescribing has become an important therapeutic strategy for practitioners seeking treatments for other diseases in subpopulations such as paediatrics and gerontology or for medical conditions such as oncology or mental illness. Similarly, the information that most clinicians use to make their decisions for treating patients abusing multiple drugs stems from trials treating a single SUD, anecdotal experiences from their own practice or that of their colleagues, or single-case studies reported in the literature. The existing evidence suggests there are few treatments for SUDs that confer significant reductions in substance use across a broad patient population. Moreover, even fewer clinical efficacy trials have been conducted that provide evidence of therapeutic benefit for these drugs. Recognising the difficulty in making the proper drug choice for facilitating maximum treatment success, this review highlights the single drugs or drug combinations that show some potential for treating dual SUDs. This review finds strongest support for the use of disulfiram for treatment of alcohol and cocaine dependence (with or without concomitant methadone maintenance), baclofen for alcohol and cocaine dependence (but not opioid-dependent cocaine users), tiagabine for cocaine dependence in methadone-maintained patients, and topiramate for alcohol, nicotine and cocaine dependence. While ondansetron and olanzapine show some efficacy in treating alcohol and cocaine dependence, more research is needed to better delineate the subpopulation in which these drugs may provide their maximum effect.

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.

Pharmacological evaluation of opioid and non-opioid analgesics in a murine bone cancer model of pain

The intramedulary injection of osteosarcoma cells in the mouse femur has served as a laboratory model to study bone cancer pain. However, the efficacy of different classes of analgesics has not fully been analyzed in this model. Therefore, the acute antinociceptive properties of different classes of drugs were evaluated on post-inoculation day 15 when the degrees of spontaneous pain and mechanical hypersensitivity in the ipsilateral inoculated hind paw reached almost their maximal effects. At high doses, the opioids fentanyl, morphine, and tramadol had full efficacies for all pain parameters tested. Antagonism experiments with naloxone (10 mg/kg s.c.) or its peripheral analogue methylnaltrexone (10 mg/kg s.c.), suggest that the analgesic effects of fentanyl were predominantly mediated by centrally located mu-opiate receptors. Acetaminophen, the non-steroidal anti-inflammatory drug indomethacin, and the COX-2-inhibitor celecoxib did not significantly improve pain behavior. The tricyclic antidepressants amitriptyline and desipramine significantly reduced spontaneous pain behavior but this only at sedative doses; the serotonin reuptake inhibitor fluoxetine had limited efficacy. Also with the anticonvulsants lamotrigine, topiramate, and gabapentin limited or no efficacies were found. In conclusion, the present study provided integrated information about the tumor-induced bone pain in mice, and clarified acute efficacies of different categories of analgesics for the spontaneous lifting, limb-use impairment, and mechanical hypersensitivity. Moreover, the finding that bone cancer-pain behaviors are attenuated by various established compounds further supports the validity of the murine bone cancer model for the study of bone cancer pain and its use for the identification of novel treatments.

Pain syndromes after missile-caused peripheral nerve lesions: part 2--treatment

OBJECTIVE

To analyze treatment procedures and treatment outcomes of painful missile-caused nerve injuries and factors influencing the outcome.

METHODS

The study included 326 patients with clinically significant pain syndromes, including complex regional pain syndrome Type II, deafferentation pain, reinnervation pain, and neuralgic pain. Treatment modalities included drug therapy, nerve surgery, sympatholysis, and dorsal root entry zone operation. Pain intensity was assessed before and after the treatment using a visual analog scale, and treatment outcome was defined as successful (pain relief >70%), fair (pain relief between 50 and 69%), or poor (pain relief <50%). The outcome was compared between different pain syndromes and different treatment modalities.

RESULTS

A successful outcome was achieved in 28.6% of patients with deafferentation pain, in 76.9% of patients with complex regional pain syndrome Type II, and in 87.9 to 100% of patients with other pain syndromes (P = 0.002). Each type of pain syndrome required a specific treatment algorithm, but average pain relief was similar for all definitive treatment modalities (range, 81-88%; P > 0.05). Ten factors were found to significantly influence the treatment outcome, but only three factors were independent predictors of a successful outcome: type of pain syndrome (P < 0.001), severity of nerve injury (P < 0.001), and absence of pain paroxysms (P = 0.03).

CONCLUSION

The treatment outcome of painful nerve injury depends on several factors, including the type of pain syndrome, severance of nerve injury, and absence of pain paroxysms. Drug therapy (carbamazepine, amitriptyline, or gabapentin) should be recommended, at least as a part of treatment, for patients with reinnervation pain, deafferentation pain, and complex regional pain syndrome Type II. Nerve surgery should be recommended for patients with posttraumatic neuralgia, either as the first treatment choice (acute nerve compression or intraneural foreign particles) or after unsuccessful pharmacological treatment (other causes of neuralgic pain).

Clinical efficacy of gabapentin versus tiagabine for reducing cocaine use among cocaine dependent methadone-treated patients

BACKGROUND

GABAergic medications appear to reduce the reinforcing effects of cocaine by attenuating cocaine-induced dopamine release. This study evaluated gabapentin and tiagabine compared to placebo in reducing cocaine taking behavior.

METHODS

A total of 76 treatment seeking, cocaine dependent, methadone-treated, predominately Caucasian male subjects were randomly assigned to tiagabine 24 mg/day (N=25), gabapentin 2400 mg/day (N=26) or placebo (N=25) in a 10-week double-blind placebo-controlled trial. Study medications were slowly increased to their full dosages by the end of week 5 and maintained through week 10. The primary outcome measure was thrice-weekly drug free urine samples.

RESULTS

Treatment retention was significantly less for the gabapentin group relative to the other groups (log rank=5.29, d.f.=1, p=0.02). The proportion of cocaine-free urine samples during weeks 6-10 was significantly larger in the tiagabine treated group (p<0.05). The longitudinal data showed significant change in thrice-weekly cocaine free urines that reached a greater abstinent rate for the tiagabine treated group (22%) compared to gabapentin (5%) or placebo (13%) treated groups. Mixed-effects ordinal regression models showed a significant tiagabine by time interaction compared to gabapentin (Z=2.48, d.f.=1, p=0.01) and placebo (Z=3.90, d.f.=1, p=0.0001). The gabapentin group did not differ from placebo.

CONCLUSION

Gabapentin showed poor treatment retention and ineffectiveness in reducing cocaine use. Tiagabine significantly reduced cocaine taking behavior compared to placebo or gabapentin among methadone-stabilized cocaine abusers.

A Double-Blind Evaluation of Gabapentin on Alcohol Effects and Drinking in a Clinical Laboratory Paradigm

INTRODUCTION

There has been increasing interest in the use of anticonvulsant agents in the treatment of alcoholism. Anticonvulsant agents have mostly been evaluated as an alternative to benzodiazepines in the treatment of alcohol withdrawal. Among the advantages of using anticonvulsant agents in this capacity is their purported lack of interaction with alcohol (i.e., interactions that could increase psychomotor deficits, cognitive impairment, and increase intoxication). This is particularly important in the treatment of alcohol withdrawal and relapse prevention in outpatients. Unfortunately, these untoward clinical interactions between anticonvulsants and alcohol in alcoholic patients have not been thoroughly assessed. The current clinical laboratory study was conducted to evaluate the safety and tolerability of the anticonvulsant gabapentin in alcoholic subjects. In addition, the ability of gabapentin to reduce alcohol craving and consumption was evaluated.

METHODS

Thirty-five non-treatment-seeking alcoholic subjects were enrolled in a subacute human laboratory study and received double-blind treatment with up to 1,200 mg of gabapentin (n=18) or placebo (n=17) for 8 days. The safety and tolerability of gabapentin were monitored in the natural environment during the first 5 days of medication treatment and during a free-choice limited access consumption paradigm following an initial drink of alcohol in a bar-lab setting on Day 7.

RESULTS

There was no overall effect of gabapentin on drinking or craving; however, it was tolerated (e.g., mood and sedation) as well as placebo over 5 days of natural drinking. During the bar-lab drinking session, there were no differences in subjective high or intoxication between subjects treated with gabapentin or placebo.

DISCUSSION

This study provides initial evidence that the anticonvulsant gabapentin is safe if used in conjunction with alcohol consumption in alcoholic individuals. Further study is needed with this and other lab models to determine the utility and safety of gabapentin in the treatment of alcoholism.

Ca2+ channel alpha2delta ligands: novel modulators of neurotransmission

The term 'Ca2+ channel alpha2delta ligands' has recently been applied to an evolving drug class that includes gabapentin (Neurontin) and pregabalin (Lyrica), and reflects significant progress over the past decade in elucidating the mechanism of action of these drugs: a novel, specific action at one of the subunits constituting voltage-sensitive Ca2+ channels. Binding of these ligands to the alpha2delta subunit is considered to explain their usefulness in treating several clinical disorders, including epilepsy, pain from diabetic neuropathy, postherpetic neuralgia and fibromyalgia, and generalized anxiety disorder. The evidence indicates a relationship between alpha2delta subunit binding and the modulation of processes that subserve neurotransmission. This modulation is characterized by a reduction of the excessive neurotransmitter release that is observed in certain neurological and psychiatric disorders.

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.

Treatment of Tinnitus With Gabapentin

OBJECTIVE

To evaluate the effectiveness of gabapentin (Neurontin) improve the disease-specific quality of life in patients with moderate tinnitus.

STUDY DESIGN

Randomized, double blind, placebo-controlled clinical trial.

SETTING

Single-center academic outpatient otolaryngology practice.

INTERVENTION

Gabapentin 1800 mg daily versus placebo.

MAIN OUTCOME MEASURES

The study design is a randomized, double blind placebo controlled single site trial conducted in an academic medical center. Inclusion criteria included patients between ages 18 and 70 with a complaint of nonpulsatile, subjective tinnitus, bilateral or unilateral, greater than 3 months in duration. The primary outcome measure is the Tinnitus Handicap Inventory; secondary measures include the Profile of Mood States (POMS) rating scale, subjective tinnitus severity. The null hypothesis addressed in this study is that the drug would not result in significant alleviation of the symptom of tinnitus.

RESULTS

Seventy-six patients completed the trial; of these 52 received the drug. No significant differences were found between the two groups after 5 weeks of treatment with gabapentin.

CONCLUSION

There is insufficient evidence to support the effectiveness of gabapentin in the treatment of tinnitus.

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.

Gabapentin in the treatment of fibromyalgia: A randomized, double-blind, placebo-controlled, multicenter trial

OBJECTIVE

To assess the efficacy and safety of gabapentin in patients with fibromyalgia.

METHODS

A 12-week, randomized, double-blind study was designed to compare gabapentin (1,200-2,400 mg/day) (n=75 patients) with placebo (n=75 patients) for efficacy and safety in treating pain associated with fibromyalgia. The primary outcome measure was the Brief Pain Inventory (BPI) average pain severity score (range 0-10, where 0=no pain and 10=pain as bad as you can imagine). Response to treatment was defined as a reduction of >or=30% in this score. The primary analysis of efficacy for continuous variables was a longitudinal analysis of the intent-to-treat sample, with treatment-by-time interaction as the measure of effect.

RESULTS

Gabapentin-treated patients displayed a significantly greater improvement in the BPI average pain severity score (P=0.015; estimated difference between groups at week 12=-0.92 [95% confidence interval -1.75, -0.71]). A significantly greater proportion of gabapentin-treated patients compared with placebo-treated patients achieved response at end point (51% versus 31%; P=0.014). Gabapentin compared with placebo also significantly improved the BPI average pain interference score, the Fibromyalgia Impact Questionnaire total score, the Clinical Global Impression of Severity, the Patient Global Impression of Improvement, the Medical Outcomes Study (MOS) Sleep Problems Index, and the MOS Short Form 36 vitality score, but not the mean tender point pain threshold or the Montgomery Asberg Depression Rating Scale. Gabapentin was generally well tolerated.

CONCLUSION

Gabapentin (1,200-2,400 mg/day) is safe and efficacious for the treatment of pain and other symptoms associated with fibromyalgia.

Identification of the alpha2-delta-1 subunit of voltage-dependent calcium channels as a molecular target for pain mediating the analgesic actions of pregabalin

Neuropathic pain is a debilitating condition affecting millions of people around the world and is defined as pain that follows a lesion or dysfunction of the nervous system. This type of pain is difficult to treat, but the novel compounds pregabalin (Lyrica) and gabapentin (Neurontin) have proven clinical efficacy. Unlike traditional analgesics such as nonsteroidal antiinflammatory drugs or narcotics, these agents have no frank antiinflammatory actions and no effect on physiological pain. Although extensive preclinical studies have led to a number of suggestions, until recently their mechanism of action has not been clearly defined. Here, we describe studies on the analgesic effects of pregabalin in a mutant mouse containing a single-point mutation within the gene encoding a specific auxiliary subunit protein (alpha2-delta-1) of voltage-dependent calcium channels. The mice demonstrate normal pain phenotypes and typical responses to other analgesic drugs. We show that the mutation leads to a significant reduction in the binding affinity of pregabalin in the brain and spinal cord and the loss of its analgesic efficacy. These studies show conclusively that the analgesic actions of pregabalin are mediated through the alpha2-delta-1 subunit of voltage-gated calcium channels and establish this subunit as a therapeutic target for pain control.

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

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