Antinociception and (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid antagonism by gabapentin in the rat spinal cord in vivo

Gabapentin reduces preoperative anxiety and pain catastrophizing in highly anxious patients prior to major surgery: a blinded randomized placebo-controlled trial

INTRODUCTION

Gabapentin is increasingly being used for the treatment of postoperative pain and a variety of psychiatric diseases, including chronic anxiety disorders. Trials have reported mixed results when gabapentin has been administered for the treatment of preoperative anxiety. We tested the hypothesis that gabapentin 1,200 mg vs placebo would reduce preoperative anxiety in patients who exhibit moderate to high preoperative anxiety.

METHODS

A blinded randomized controlled trial was conducted from September 2009 to June 2011 at the Toronto General Hospital. Following ethics approval and informed consent, 50 female patients with a 0-10 numeric rating scale (NRS) anxiety score of greater than or equal to 5/10 consented to receive either gabapentin 1,200 mg (n = 25) or placebo (n = 25) prior to surgery. Randomization was computer generated, and the Investigational Pharmacy was responsible for the blinding and dispensing of medication. All patients and care providers, including physicians, nurses, and study personnel, were blinded to group allocation. Before administering the study medication, baseline anxiety levels were measured using a NRS, the Spielberger State-Trait Anxiety Inventories, the Pain Catastrophizing Scale, and the Pain Anxiety Symptoms Scale-20. Baseline pain intensity (0-10 NRS) and level of sedation (0-10 NRS and Richmond Agitation-Sedation Scale [RASS]) were also measured. Two hours after the administration of gabapentin or placebo (prior to surgery), patients again rated their anxiety, pain, and sedation levels using the same measurement tools as at baseline. The main outcome was a reduction in preoperative anxiety.

RESULTS

Forty-four patients (22 treated with gabapentin 1,200 mg and 22 treated with placebo) were included in the analysis of the primary outcome. Analysis of covariance in which pre-drug NRS anxiety scores were used as the covariate showed that post-drug preoperative NRS anxiety (Effect size, 1.44; confidence interval [CI] 0.19 to 2.70) and pain catastrophizing (Effect size, 0.43; CI 0.12 to 0.74) scores were significantly lower in the gabapentin group than in the placebo control group, respectively. Post-drug sedation (Effect size, -3.02; CI -4.28 to -1.77) and RASS (Effect size, 0.41; CI 0.12 to 0.71) scores were significantly higher in the gabapentin group than in the placebo group, respectively.

CONCLUSIONS

Administration of gabapentin 1,200 mg prior to surgery reduces preoperative NRS anxiety scores and pain catastrophizing scores and increases sedation prior to entering the operating room. These results suggest that gabapentin 1,200 mg may be a treatment option for patients who exhibit high levels of preoperative anxiety and pain catastrophizing; however, the sedative properties of the medication and the possibility of delayed postoperative discharge in the elective ambulatory population need to be considered.

Effects of gabapentin on brain hyperactivity related to pain and sleep disturbance under a neuropathic pain-like state using fMRI and brain wave analysis

Neuropathic pain is the most difficult pain to manage in the pain clinic, and sleep problems are common among patients with chronic pain including neuropathic pain. In the present study, we tried to visualize the intensity of pain by assessing neuronal activity and investigated sleep disturbance under a neuropathic pain-like state in mice using functional magnetic resonance imaging (fMRI) and electroencephalogram (EEG)/electromyogram (EMG), respectively. Furthermore, we investigated the effect of gabapentin (GBP) on these phenomena. In a model of neuropathic pain, sciatic nerve ligation caused a marked decrease in the latency of paw withdrawal in response to a thermal stimulus only on the ipsilateral side. Under this condition, fMRI showed that sciatic nerve ligation produced a significant increase in the blood oxygenation level-dependent (BOLD) signal intensity in the pain matrix, which was significantly decreased 2 h after the i.p. injection of GBP. Based on the results of an EEG/EMG analysis, sciatic nerve-ligated animals showed a statistically significant increase in wakefulness and a decrease in non-rapid eye movement (NREM) sleep during the light phase, and the sleep disturbance was almost completely alleviated by a higher dose of GBP in nerve-ligated mice. These findings suggest that neuropathic pain associated with sleep disturbance can be objectively assessed by fMRI and EEG/EMG analysis in animal models. Furthermore, GBP may improve the quality of sleep as well as control pain in patients with neuropathic pain.

Neuroprotective effects of gabapentin in experimental spinal cord injury

BACKGROUND

Extensive research has focused on neuroprotection after spinal cord trauma to alleviate the effects of secondary injury. This study aims to investigate the neuroprotective effects of gabapentin in experimental spinal cord injury.

METHODS

Thirty-six adult, male Wistar rats received spinal cord injury using the clip compression method. Animals were divided into five groups. High (200 mg/kg) and low doses (30 mg/kg) of gabapentin were administered to the animals in the treatment groups after spinal cord trauma and ultrastructural findings and lipid peroxidation levels of these two groups were compared with the animals that received only laminectomy, only trauma, and trauma and 30 mg/kg methylprednisolone.

RESULTS

Regarding tissue lipid peroxidation levels after trauma, animals in gabapentin groups demonstrated better results than the trauma group. However, these results were no better than the methylprednisolone group. The results regarding the ultrastructural findings were similar. Treatment groups demonstrated better ultrastructural findings than the trauma group. In addition, the results of the high dose gabapentin group were significantly better than the low dose gabapentin group.

CONCLUSIONS

Gabapentin demonstrated similar neuroprotective effects as methylprednisolone in early phase of spinal cord injury. Further studies with different experimental settings including neurological outcome are required to achieve conclusive results.

Gabapentin evoked changes in functional activity in nociceptive regions in the brain of the anaesthetized rat: an fMRI study

BACKGROUND AND PURPOSE

Gabapentin (GBP; 1-(aminomethyl)cyclohexane acetic acid) is used clinically in the treatment of pain. Nevertheless, the sites and mechanisms of action of GBP are poorly defined. Herein, the effects of GBP on brain activation have been studied.

EXPERIMENTAL APPROACH

Changes in blood oxygen level dependent (BOLD) haemodynamic signal following intravenous infusion of GBP (equivalent to 30 mg kg(-1) p.o., followed by 100 mg kg(-1) p.o.), compared to saline control, were studied in isofluorane anaesthetized rats (n=8 per group). Effects of GBP on mean arterial blood pressure (MAP) were also recorded.

RESULTS

Random effect analysis revealed that the lower dose of GBP produced significant (P<0.001) increases in BOLD signal intensity in several brain regions, including the thalamus and periaqueductal grey (PAG), compared to basal. This dose of GBP also produced significant (P<0.001) decreases in BOLD signal intensity in the amygdala and the entorhinal cortex. Increasing the dose of GBP (100 mg kg(-1)) produced significantly greater changes in BOLD signal intensity in several brain regions including the thalamus and PAG. MAP was not significantly altered by GBP, compared to saline.

CONCLUSIONS AND IMPLICATIONS

GBP had marked positive and negative effects on BOLD signal intensity in a number of brain regions in naïve rats. The activation of key areas involved in nociceptive processing indicate a supraspinal site of action of GBP and this may contribute to its well-described analgesic effects in animal models of pain and clinical studies.

Low dose ketamine: a therapeutic and research tool to explore N-methyl-D-aspartate (NMDA) receptor-mediated plasticity in pain pathways

Ketamine is a dissociative anaesthetic that has been used in the clinic for many years. At low, sub-anaesthetic doses, it is a relatively selective and potent antagonist of the N-methyl-D-aspartate (NMDA) receptor. It belongs to the class of uncompetitive antagonists and blocks the receptor by binding to a specific site within the NMDA receptor channel when it is open. Like other compounds of this class, ketamine can cause hallucinations or other untoward central effects which limit its use in the clinic. Nevertheless, because of the evidence on the importance of NMDA receptor-mediated pLasticity in chronic pain, low doses of ketamine have been explored in a wide range of pain conditions. The majority of studies with ketamine have shown efficacy; however, it has not been possible to separate safely the pain relief from the side effects of the drug. Hence, clinical use of ketamine as a pain treatment is very limited. Nevertheless, ketamine has served as a useful tool to provide a compelling rationale for developing other NMDA antagonists. Some of the new compounds of this class, particularly those acting at the NR2B subtype of the NMDA receptor, have shown promise in preclinical and clinical studies.

A comparison of the glutamate release inhibition and anti-allodynic effects of gabapentin, lamotrigine, and riluzole in a model of neuropathic pain

The effects of treatment with the anti-convulsant agents, lamotrigine and riluzole were compared with gabapentin in a rat experimental model of neuropathic pain. Rats were treated intraperitoneally, with gabapentin (30, 100 and 300 mg/kg), lamotrigine (2, 10 and 50 mg/kg) or riluzole (6 and 12 mg/kg) prior to, and every 12 h for 4 days following chronic constriction injury (CCI) of the sciatic nerve. Mechanical and cold sensitivity were assessed prior to surgery (baseline) and then at 4, 8 and 12 days following CCI. The four-day treatment with each of the agents was effective at producing reductions in the development of mechanical and cold hypersensitivity for periods ranging from the fourth to 12th day. The highest doses of each of the agents were also assessed on formalin-induced nociceptive behaviors and on formalin-induced increases in extracellular glutamate (Glu) and aspartate (Asp) in the spinal cord dorsal horn (SCDH) of awake behaving rats using in vivo microdialysis. Nociceptive scores in formalin test were significantly decreased by gabapentin (300 mg/kg i.p.) and riluzole (12 mg/kg i.p.), but not by lamotrigine (50 mg/kg i.p.). Formalin-induced increases in glutamate levels in SCDH were lowered significantly, as compared with the controls, with all drugs both in the first phase and second phases, with the greatest effects for riluzole and gabapentin. Similar suppressive effects of the drugs were observed on formalin-induced increases in spinal aspartate, except that gabapentin and lamotrigine produced effects only during the second phase. Riluzole produced profound and prolonged reductions in the spinal levels of glutamate and aspartate both for basal and formalin-stimulated release. In conclusion, the results suggest that the anti-convulsant agents gabapentin, lamotrigine and riluzole may reduce the development of hyperalgesia in a rat model of neuropathic pain by reducing the spinal release of glutamate. Riluzole's pronounced suppressive effects on spinal EAA levels is attributed to its established role as a glutamate release inhibitor and an enhancer of glutamate transporter activity.

Mechanisms of the antinociceptive action of gabapentin

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

Evidence that gabapentin reduces neuropathic pain by inhibiting the spinal release of glutamate

Gabapentin is an anticonvulsant that successfully treats many neuropathic pain syndromes, although the mechanism of its antihyperalgesic action remains elusive. This study aims to help delineate gabapentin's antihyperalgesic mechanisms. We assessed the effectiveness of gabapentin at decreasing mechanical and cold hypersensitivity induced in a rat model of neuropathic pain. Thus, we compared the effectiveness of pre- or post-treatment with systemic or intrathecal (i.t.) gabapentin at reducing the development and maintenance of the neuropathic pain symptoms. Gabapentin successfully decreased mechanical and cold hypersensitivity, both as a pretreatment and post-treatment. Furthermore, both i.t. and systemic administration of gabapentin were effective in reducing the behavioral hypersensitivity; however, the i.t. administration was superior to the systemic. We also examined gabapentin's effects at inhibiting hindpaw formalin-induced release of excitatory amino acids (EAAs) in the spinal cord dorsal horn (SCDH) both in naïve rats and in rats with neuropathic pain. We present the first evidence that gabapentin reduces the formalin-induced release of both glutamate and aspartate in SCDH. Furthermore, i.t. gabapentin reduces the enhanced noxious stimulus-induced spinal release of glutamate seen in neuropathic rats. These data suggest that gabapentin reduces neuropathic pain symptoms by inhibiting the release of glutamate in the SCDH.

Efectividad de la gabapentina en el tratamiento de la espasticidad: estudio aleatorizado, a doble ciego y controlado con placebo

BACKGROUND AND OBJECTIVE

We aimed to demonstrate the efficacy and safety of gabapentin for the treatment of spasticity in patients with upper motor neuron syndrome.

PATIENTS AND METHOD

Thirty patients with pyramidal-tract lesions were randomized to up to 3,600 mg/day of gabapentin (n = 15) or placebo (n = 15) in a double-blind, pilot 10-week trial. The primary efficacy variable was the Ashworth Scale score. Secondary variables included the Spasm Frequency Scale, maximal range of movement, H/M Amplitude Ratio, and the Barthel Index for quality of life. Adverse events were recorded. All data were analyzed on an intent-to-treat basis.

RESULTS

Demographic and baseline characteristics were similar between the 2 treatment groups. The gabapentin group showed significant improvement in Ashworth Scale total scores, and scores for individual affected muscle groups. Fifteen of the randomized patients had spasms; the total Spasm Frequency Scale score was not significantly different between groups. Nevertheless, when affected individual muscle groups were analyzed, a significant effect of gabapentin vs placebo was observed at all visits. Gabapentin did not modify the scores of the range of movement, the H/M Amplitude Ratio, or the Barthel Index tests. No gait or displacement impairment were seen during treatment with gabapentin. Related adverse events occurred less frequently in the gabapentin group, and fewer gabapentin patients withdrew because of adverse events.

CONCLUSIONS

Gabapentin demonstrated efficacy and safety at doses between 2,700 and 3,600 mg/day as a therapy for the spasticity associated with the upper motor neuron syndrome.

Does gabapentin act as an agonist at native GABAB receptors?

Gabapentin, a novel anticonvulsant and analgesic, is a gamma-aminobutyric acid (GABA) analogue but was shown initially to have little affinity at GABA(A) or GABA(B) receptors. It was recently reported to be a selective agonist at GABA(B) receptors containing GABA(B1a)-GABA(B2) heterodimers, although several subsequent studies disproved that conclusion. In the present study, we examined whether gabapentin is an agonist at native GABA(B) receptors using a rat model of postoperative pain in vivo and periaqueductal gray (PAG) slices in vitro; PAG contains GABA(B) receptors, and their activation results in antinociception. An intrathecal injection of gabapentin or baclofen, a GABA(B) receptor agonist, induced antiallodynia in this postoperative pain model. Intrathecal injection of GABA(B) receptor antagonists CGP 35348 and CGP 55845 antagonized baclofen- but not gabapentin-induced antiallodynia. In ventrolateral PAG neurons, baclofen activated G-protein-coupled inwardly rectifying K(+) (GIRK) channels in a manner blocked by CGP 35348 or CGP 55845. However, gabapentin displayed no effect on the membrane current. In neurons unaffected by gabapentin, baclofen activated GIRK channels through GABA(B) receptors. It is concluded that gabapentin is not an agonist at GABA(B) receptors that are functional in baclofen-induced antiallodynia in the postoperative pain model in vivo and in GIRK channel activation in ventrolateral PAG neurons in vitro.

Preferential action of gabapentin and pregabalin at P/Q-type voltage-sensitive calcium channels: inhibition of K+-evoked [3H]-norepinephrine release from rat neocortical slices

Gabapentin (GBP; Neurontin) and pregabalin (PGB; CI-1008), efficacious drugs in several neurological and psychiatric disorders, inhibit neurotransmitter release from mammalian brain slices at therapeutically relevant concentrations. A detailed investigation, exploring the basis for this in vitro phenomenon, focused on norepinephrine (NE) and rat neocortical tissue in complementary assays of neurotransmitter release and radioligand binding. The results are consistent with the hypothesis that GBP, PGB, and related substances decrease neocortical NE release by acting at the alpha2delta subunit of presynaptic P/Q-type voltage-sensitive Ca2+ channels (VSCC) subserving Ca2+ influx in noradrenergic terminals. The inhibitory action appears competitive with [Ca2+]o and preferential to those neurons undergoing prolonged depolarization. Other results indicate that the reduction of exocytotic NE release is independent of L- and N-type VSCC, classical drug/peptide binding sites on VSCC, Na+ channels, alpha2-adrenoceptors, NE transporter, and system L amino acid transporter. These findings suggest a selective modulation of P/Q-type VSCC that are implicated in neurotransmission and several GBP-responsive pathologies.

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

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

Voltage-dependent calcium channels--beyond dihydropyridine antagonists

The blockade of L-type calcium channels by dihydropyridines, phenylalkylamines and benzothiazepines has been well described and forms the basis of a multibillion dollar market for the treatment of cardiovascular disease and migraine. More recently, neuron-specific calcium channels have become the subject of intense interest regarding their potential as therapeutic targets for the treatment of chronic and neuropathic pain. A number of recently described agents that selectively target neuronal calcium channels have been described and appear promising for a variety of pain conditions.