Spinal gabapentin and antinociception: mechanisms of action

Efficacy of topical gabapentin in women with primary macular amyloidosis: A side-by-side triple-blinded randomized clinical trial

BACKGROUND

Primary cutaneous macular amyloidosis (PCMA) is a chronic pruritic cutaneous disease characterized by heterogeneous extracellular deposition of amyloid protein in the skin.

AIMS

This study aimed to evaluate the efficacy of topical 6% gabapentin cream for the treatment of patients with PCMA.

MATERIALS AND METHODS

In this triple-blind clinical trial, a total of 34 patients, who were diagnosed with PCMA, treated using two different strategies of topical gabapentin as the active group and vehicle cream as the control group.

RESULTS

Pruritus score reduction in both groups was statistically significant compared with the baseline value (p < 0.001). There was a significant pigmentation score reduction in intervention group compared with control group after 1 month of the study (p < 0.001). The differences of pigmentation score changes between the groups were not significant at month 2 (p = 0.52) and month 3 (p = 0.22).

CONCLUSIONS

The results of this study suggest that topical gabapentin cream may be effective as a topical agent in the treatment of pruritus associated with PCMA without any significant adverse effects. It is recommended to perform similar studies with a larger sample size and longer duration in both sexes.

Pharmacological interactions between intrathecal pregabalin plus tianeptine or clopidogrel in a rat model of neuropathic pain

Background

There is still unmet need in treating neuropathic pain and increasing awareness regarding the use of drug combinations to increase the effectiveness of treatment and reduce adverse effects in patients with neuropathic pain.

Methods

This study was performed to determine the individual and combined effects of pregabalin, tianeptine, and clopidogrel in a rat model of neuropathic pain. The model was created by ligation of the L5-L6 spinal nerve in male Sprague-Dawley rats; mechanical allodynia was confirmed using von Frey filaments. Drugs were administered to the intrathecal space and mechanical allodynia was assessed; drug interactions were estimated by isobolographic or fixed-dose analyses.

Results

Intrathecal pregabalin and tianeptine increased the mechanical withdrawal threshold in a dose-dependent manner, but intrathecal clopidogrel had little effect on the mechanical withdrawal threshold. An additive effect was noted between pregabalin and tianeptine, but not between pregabalin and clopidogrel.

Conclusions

These findings suggest that intrathecal coadministration of pregabalin and tianeptine effectively attenuated mechanical allodynia in the rat model of neuropathic pain. Thus, pregabalin plus tianeptine may be a valid option to enhance the efficacy of neuropathic pain treatment.

Have We Just Scratched the Surface? A Narrative Review of Uremic Pruritus in 2020

Purpose of review:

Uremic pruritus is a highly prevalent and debilitating symptom in patients with chronic kidney disease (CKD) and end-stage kidney disease (ESKD). The purpose of this review is to examine current evidence on the mechanisms and treatments of pruritus in CKD and highlight promising areas for future research.

Sources of information:

Published literature, including randomized controlled trials, cohort studies, case reports, and review articles, was searched for evidence pertaining to the pathophysiology and treatment of uremic pruritus.

Methods:

A comprehensive narrative review was conducted to explore the molecular mechanisms underlying uremic pruritus, as well as the evidence (or lack thereof) supporting pharmacological and nonpharmacological treatments for uremic pruritus. The potential role of patient sex in the pathophysiology and management of uremic pruritus is also discussed.

Key findings:

The pathophysiology of uremic pruritus involves a complex interplay of uremic toxins, systemic inflammation, mast cell activation, and imbalance of opioid receptors. Classic treatment strategies for uremic pruritus include optimization of dialysis parameters, amelioration of CKD-related mineral and bone disease, topical emollients and analgesics, antihistamines, the anticonvulsant medications gabapentin and pregabalin, and ultraviolet light B (UV-B) phototherapy. Strong data to support many of these classical treatments for uremic pruritus are limited. Newly evolving treatment approaches for uremic pruritus include opioid receptor modulators, neurokinin-1 inhibitors, and cannabinoids. Further studies regarding their efficacy, pharmacodynamics, and safety in the CKD and ESKD population are needed before these agents are accepted into widespread use. Additional nonpharmacological strategies aimed at treating uremic pruritus include psychotherapy, acupuncture, omega-3 fatty acids, and exercise. Finally, sex differences may exist regarding uremic pruritus, but studies directly addressing sex-specific mechanisms of uremic pruritus remain absent.

Limitations:

High-quality evidence in the management of uremic pruritus remains lacking. Most recommendations are based on expert opinion or studies involving small numbers of patients. In addition, our understanding of the pathophysiological mechanisms behind uremic pruritus is incomplete and continues to evolve over time.

Implications:

Uremic pruritus is a common symptom which reduces quality of life in CKD and ESKD. The identification of novel targeted treatment approaches may ease the burden of uremic pruritus in the future.

Interventions for chronic pruritus of unknown origin

BACKGROUND

Pruritus is a sensation that leads to the desire to scratch; its origin is unknown in 8% to 15% of affected patients. The prevalence of chronic pruritus of unknown origin (CPUO) in individuals with generalised pruritus ranges from 3.6% to 44.5%, with highest prevalence among the elderly. When the origin of pruritus is known, its management may be straightforward if an effective treatment for the causal disease is available. Treatment of CPUO is particularly difficult due to its unknown pathophysiology.

OBJECTIVES

To assess the effects of interventions for CPUO in adults and children.

SEARCH METHODS

We searched the following up to July 2019: Cochrane Skin Group Specialised Register, CENTRAL, MEDLINE, Embase, and trials registries. We checked the reference lists of included studies for additional references to relevant trials.

SELECTION CRITERIA

We sought to include randomised controlled trials and quasi-randomised controlled trials that assessed interventions for CPUO, as defined in category VI ('Other pruritus of undetermined origin, or chronic pruritus of unknown origin') of the International Forum for the Study of Itch (IFSI) classification, in children and adults. Eligible interventions were non-pharmacological or topical or systemic pharmacological interventions, and eligible comparators were another active treatment, placebo, sham procedures, or no treatment or equivalent (e.g. waiting list).

DATA COLLECTION AND ANALYSIS

We used standard methodological procedures expected by Cochrane. Our primary outcomes were 'Patient- or parent-reported pruritus intensity' and 'Adverse events'. Our secondary outcomes were 'Health-related quality of life', 'Sleep disturbances', 'Depression', and 'Patient satisfaction'. We used GRADE to assess the certainty of evidence.

MAIN RESULTS

We found there was an absence of evidence for the main interventions of interest: emollient creams, cooling lotions, topical corticosteroids, topical antidepressants, systemic antihistamines, systemic antidepressants, systemic anticonvulsants, and phototherapy. We included one study with 257 randomised (253 analysed) participants, aged 18 to 65 years; 60.6% were female. This study investigated the safety and efficacy of three different doses of oral serlopitant (5 mg, 1 mg, and 0.25 mg, once daily for six weeks) compared to placebo for severe chronic pruritus; 25 US centres participated (clinical research centres and universities). All outcomes were measured at the end of treatment (six weeks from baseline), except adverse events, which were monitored throughout. A pharmaceutical company funded this study. Fifty-five per cent of participants suffered from CPUO, and approximately 45% presented a dermatological diagnosis (atopic dermatitis/eczema 37.3%, psoriasis 6.7%, acne 3.6%, among other diagnoses). We unsuccessfully attempted to retrieve outcome data from study authors for the subgroup of participants with CPUO. Participants had pruritus for six weeks or longer. Total study duration was 10 weeks. Participants who received serlopitant 5 mg may have a greater rate of relief of patient-reported pruritus intensity as measured by the visual analogue scale (VAS; a reduction in VAS score indicates improvement) compared to placebo (126 participants, risk ratio (RR) 2.06, 95% confidence interval (CI) 1.27 to 3.35; low-certainty evidence). We are uncertain of the effects of serlopitant 5 mg compared to placebo on the following outcomes due to very low-certainty evidence: adverse events (127 participants; RR 1.48, 95% CI 0.87 to 2.50); health-related quality of life (as measured by the Dermatology Life Quality Index (DLQI); a higher score indicates greater impairment; 127 participants; mean difference (MD) -4.20, 95% CI -11.68 to 3.28); and sleep disturbances (people with insomnia measured by the Pittsburgh Sleep Symptom Questionnaire-Insomnia (PSSQ-I), a dichotomous measure; 128 participants; RR 0.49, 95% CI 0.24 to 1.01). Participants who received serlopitant 1 mg may have a greater rate of relief of patient-reported pruritus intensity as measured by VAS compared to placebo; however, the 95% CI indicates that there may also be little to no difference between groups (126 participants; RR 1.50, 95% CI 0.89 to 2.54; low-certainty evidence). We are uncertain of the effects of serlopitant 1 mg compared to placebo on the following outcomes due to very low-certainty evidence: adverse events (128 participants; RR 1.45, 95% CI 0.86 to 2.47); health-related quality of life (DLQI; 128 participants; MD -6.90, 95% CI -14.38 to 0.58); and sleep disturbances (PSSQ-I; 128 participants; RR 0.38, 95% CI 0.17 to 0.84). Participants who received serlopitant 0.25 mg may have a greater rate of relief of patient-reported pruritus intensity as measured by VAS compared to placebo; however, the 95% CI indicates that there may also be little to no difference between groups (127 participants; RR 1.66, 95% CI 1.00 to 2.77; low-certainty evidence). We are uncertain of the effects of serlopitant 0.25 mg compared to placebo on the following outcomes due to very low-certainty evidence: adverse events (127 participants; RR 1.29, 95% CI 0.75 to 2.24); health-related quality of life (DLQI; 127 participants; MD -5.70, 95% CI -13.18 to 1.78); and sleep disturbances (PSSQ-I; 127 participants; RR 0.60, 95% CI 0.31 to 1.17). The most commonly reported adverse events were somnolence, diarrhoea, headache, and nasopharyngitis, among others. Our included study did not measure depression or patient satisfaction. We downgraded the certainty of evidence for all outcomes due to indirectness (only 55% of study participants had CPUO) and imprecision. We downgraded outcomes other than patient-reported pruritus intensity a further level due to concerns regarding risk of bias in selection of the reported result and some concerns with risk of bias due to missing outcome data (sleep disturbances only). We deemed risk of bias to be generally low.

AUTHORS' CONCLUSIONS

We found lack of evidence to address our review question: for most of our interventions of interest, we found no eligible studies. The neurokinin 1 receptor (NK1R) antagonist serlopitant was the only intervention that we could assess. One study provided low-certainty evidence suggesting that serlopitant may reduce pruritus intensity when compared with placebo. We are uncertain of the effects of serlopitant on other outcomes, as certainty of the evidence is very low. More studies with larger sample sizes, focused on patients with CPUO, are needed. Healthcare professionals, patients, and other stakeholders may have to rely on indirect evidence related to other forms of chronic pruritus when deciding between the main interventions currently used for this condition.

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

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

The role of inversely operating glutamate transporter in the paradoxical analgesia produced by glutamate transporter inhibitors

Controlling extracellular glutamate level in a physiological range is important to maintain normal sensory transmission. Here, we investigated the paradoxical action of glutamate transporters in the rat formalin test to elucidate a possible role of inversely operating transporters in its analgesic mechanism. The effects of glutamate transporter inhibitor on formalin-induced pain behavior were examined. Then we performed a microdialysis study to clarify the differential change in extracellular glutamate concentration by intrathecal administration of transportable and non-transportable blockers. And we further investigated the mechanism pharmacologically via pretreatment with antagonists of various receptors and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL) staining. Intrathecally-injected glutamate transporter inhibitors, non-transportable DL-threo-β-benzyloxyaspartat (TBOA) and transportable trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC), produced paradoxical antinociception in the formalin test. In normal rats, inhibition of the glutamate transporter increased extracellular glutamate. In the formalin model rats, TBOA suppressed while t-PDC enhanced glutamate release. When tPDC was pretreated 30min prior to formalin injection, glutamate release was blocked. Blocking α-2 adrenergic receptors reversed the tPDC analgesia. Increased apoptosis was not apparent in the spinal dorsal horn of tPDC-treated rats compared to the control group. These data suggest that glutamate transporters in a formalin-induced pain state work in a reverse mode and can be blocked from releasing glutamate by TBOA and preloaded tPDC. The analgesic mechanism of TBOA may be related to the blockade of inversely operating transporter, and that of tPDC may be associated with the activation of noradrenergic neurotransmission but not with dorsal horn neurotoxicity.

Does preoperative gabapentin affects the characteristics of post-dural puncture headache in parturients undergoing cesarean section with spinal anesthesia?

Background:

Gabapentin is effective for treating different types of headache including post-dural puncture headache (PDPH), also used for prophylaxis against migraine. We studied the effect of pre-operative administration of gabapentin on the characteristics of PDPH in parturients undergoing cesarean section (CS) under spinal anesthesia.

Materials and Methods:

Women undergoing elective cesarean section under spinal anesthesia were randomized to receive preoperative gabapentin 600 mg or placebo. Spinal anesthesia was achieved with 12.5 mg hyperbaric bupivacaine plus 25 μg fentanyl. Babies were followed up by Apgar scores, umbilical artery blood gases, breastfeeding difficulties, and need for NICU admission. The mothers were followed up for any side-effects of gabapentin for 24 h. Patients with PDPH were re-admitted and onset and duration of the headache were reported and severity was assessed using a visual analog scale (VAS) for 4 days from diagnosis. Paracetamol with caffeine and diclofenac were given for treatment, and the doses were adjusted according to VAS; also number of doses given for each group was recorded.

Results:

Eighty eight patients were randomized, and 2 were excluded. The incidence of headache and co-existing symptoms were similar in both groups. The onset of headache was significantly delayed in gabapentin group (P < 0.05). Also, severity and duration of headache were significantly less in gabapentin group (P < 0.05). The incidence of sedation was more in gabapentin group 11 (26.19%) versus placebo group 3 (6.81%). Neonatal outcomes were statistically insignificant between both groups.

Conclusion:

Pre-operative administration of gabapentin has no effect on incidence of (PDPH) but delays its onset and reduces its severity and duration in parturients undergoing cesarean section with spinal anesthesia without significant adverse effects on the mother or the baby.

Use of pregabalin in the management of chronic uremic pruritus

CONTEXT

Uremic pruritus (UP) affects many patients suffering from chronic kidney disease (CKD) and has a negative impact on quality of life and survival. It has become increasingly evident that central transmission and sensitization processes similar to those observed in chronic pain are important mechanisms of pruritus.

OBJECTIVES

To test the potential role of pregabalin in reducing the intensity of UP in CKD patients.

METHODS

We prospectively collected data on CKD patients who suffered from severe intractable pruritus. Patients were asked to record the intensity of pruritus on a visual analogue scale.

RESULTS

Twelve patients were studied. The average pretreatment pruritus score was 9.7 ± 0.9 and decreased to 3.7 ± 2.35, 3.2 ± 1.75, and 3 ± 1.5 after one, four, and 24 weeks of treatment, respectively (P < 0.05). The positive effect of pregabalin was demonstrated during the first week of therapy in six patients. Most patients required 25mg a day. Pregabalin was well tolerated, with somnolence and dizziness developing in two patients.

CONCLUSION

We demonstrated dramatic improvement of long-standing UP after the initiation of pregabalin. We suggest that pregabalin can be used safely in CKD but careful titration of the dose is required to obtain an optimal response and minimize the possible adverse effects.

Antinociceptive drug interaction between intrathecal vitamin E and gabapentin in the rat formalin test

Background

Gabapentin is thought to exert an effect through the voltage-dependent calcium channel. Vitamin E is a widely known antioxidant which neutralizes the harmful effect of ROS which is considered to play a prominent role in various painful conditions. This study was therefore conducted to assess the antinociceptive effects of gabapentin and vitamin E and the interaction of these drugs in the modulation of pain in rats subjected to a formalin test.

Methods

Sprague-Dawley rats with a lumbar intrathecal catheter were tested for their paw flinches by 5% formalin injection after intrathecal injection of gabapentin or vitamin E. After obtaining dose-response curves for each drug, the effect of the combination was tested by the total dose fraction value and isobolographic analysis.

Results

When a single drug was injected intrathecally, significant dose-dependent decreases in flinches were shown only in the late phase. ED₅₀ values of intrathecal gabapentin and vitamin E in the late phase were 75.3 ± 9.58 µg, and 17.56 ± 1.65 mg/kg respectively. The combination of gabapentin and vitamin E produced dose-dependent decreases in the number of flinches in both phases induced by the formalin test. The ED₅₀ value of the combination was lower than the theoretical additive values in the late phase, but did not show a significant difference with the theoretical additive value.

Conclusions

Gabapentin and vitamin E (by itself) have no antinociceptive effect in the early phase; however their combination has shown an antinociceptive effect. In addition, they show additive effects in the late phase of the formalin test.

Gabapentin for Pruritus in Palliative care

Itch/pruritus can be very distressing in palliative care population and often is difficult to treat. Conventional antihistamines lack efficacy. Cutaneous and central pathogenesis of itch is extremely complex and unclear, making its treatment challenging. Neuronal mechanisms have been identified in the pathophysiology of itch hence providing a myriad of therapeutic options. It has been established that pruritus and pain neuronal pathway interact with each other, hence neuropathic analgesics like gabapentin has shown to be efficacious antipruritic therapeutic option. Gabapentin impedes transmitting nociceptive sensations to brain, thus also suppressing pruritus. Gabapentin is safe and found to be effective in uremic pruritus, cancer/hematologic causes, opiod-induced itch, brachioradial pruritis, burns pruritus, and pruritus of unknown origin. Further research is required in this area to establish whether gabapentin is consistently effective.

Antinociceptive effects of gabapentin & its mechanism of action in experimental animal studies

BACKGROUND & OBJECTIVES

Several studies have shown the possible analgesic effects of gabapentin, widely used as an antiepileptic. Thus, clinical studies have been carried out especially for neuropathic syndroms. This study was undertaken to investigate experimentally whether gabapentin has analgesic effects in mice and rats.

METHODS

The mice were divided into 10 groups (n=7) with various treatments to assess central and peripheral antinociceptive activity of gabapentin. Hot plate, tail clip and tail flick tests were applied for the investigation of central antinociceptive activity and the writhing test was applied for the investigation of peripheral antinociceptive activity. In addition, we also evaluated the levels of PGE 2 and nNOS on perfused hippocampus slices of rats.

RESULTS

Gabapentin showed a peripheral antinociceptive effect at all doses and a central antinociceptive effect at 30mg/kg dose. While the L-NAME and cyproheptadine changed the central and peripheral effects of gabapentin, naloxone did not change these effects. In vitro studies showed that gabapentin significantly increased nNOS level. PGE 2 and nNOS were found to have an important role in the antinociceptive effects of gabapentin at all doses and its combinations with L-NAME, cyproheptadine, indomethacine, and naloxone. As expected, PGE 2 levels decreased in all groups, while nNOS levels increased, which is believed to be an adaptation mechanism.

INTERPRETATION & CONCLUSIONS

Our findings indicate that arachidonate, nitrergic and serotonergic systems play an important role in the antinociceptive activity of gabapentin except for the opioidergic system. Additionally, this effect occured centrally and peripherally. These effects were also mediated by nNOS and PGE2.

Evaluation of the neurological safety of epidurally-administered pregabalin in rats

Background

The primary site of action of pregabalin, i.e. the α-2-δ subunit of the voltage-dependent calcium channel, is located at the dorsal root ganglion and dorsal horn of the spinal cord. Therefore, the epidural administration of pregabalin could have advantages over oral administration. However, the possibility of its neurotoxicity should be excluded before any attempt at epidural administration. We evaluated the neuronal safety of epidurally-administered pregabalin by observing the sensory/motor changes and examining the histopathology of spinal cord in rats.

Methods

Sixty rats of 180-230 g were divided into three groups; 3 mg of pregabalin dissolved in 0.3 ml saline (group P, n = 20), 0.3 ml 40% alcohol (group A, n = 20), or 0.3 ml normal saline (group N, n = 20) was administered epidurally to the rats in each group. Pinch-toe test, motor function evaluation, and histopathologic examination of vacuolation, chromatolysis, meningeal inflammation, and neuritis were performed at the 1st, 3rd, 7th, and 21st day after each epidural administration.

Results

All rats enrolled in group P, like those in group N, showed neither sensory/motor dysfunction nor any histopathological abnormality over the 3-week observation period. In contrast, in group A, 80% of the rats showed abnormal response to the pinch-toe test and all rats showed decreased motor function during the entire evaluation period. In addition, all histopathologic findings of neurotoxicity were observed exclusively in group A.

Conclusions

The epidurally administered pregabalin (about 15 mg/kg) did not cause any neurotoxic evidence, in terms of both sensory/motor function evaluation and histopathological examination in rats.

Roles of serotonergic and adrenergic receptors in the antinociception of selective cyclooxygenase-2 inhibitor in the rat spinal cord

Background

The analgesic mechanisms of cyclooxygenase (COX)-2 inhibitors have been explained mainly on the basis of the inhibition of prostaglandin biosynthesis. However, several lines of evidence suggest that their analgesic effects are mediated through serotonergic or adrenergic transmissions. We investigated the roles of these neurotransmitters in the antinociception of a selective COX-2 inhibitor at the spinal level.

Methods

DUP-697, a selective COX-2 inhibitor, was delivered through an intrathecal catheter to male Sprague-Dawley rats to examine its effect on the flinching responses evoked by formalin injection into the hindpaw. Subsequently, the effects of intrathecal pretreatment with dihydroergocristine, prazosin, and yohimbine, which are serotonergic, α1 adrenergic and α2 adrenergic receptor antagonists, respectively, on the analgesia induced by DUP-697 were assessed.

Results

Intrathecal DUP-697 reduced the flinching response evoked by formalin injection during phase 1 and 2. But, intrathecal dihydroergocristine, prazosin, and yohimbine had little effect on the antinociception of intrathecal DUP-697 during both phases of the formalin test.

Conclusions

Intrathecal DUP-697, a selective COX-2 inhibitor, effectively relieved inflammatory pain in rats. Either the serotonergic or adrenergic transmissions might not be involved in the analgesic activity of COX-2 inhibitors at the spinal level.

Roles of adenosine and serotonin receptors on the antinociception of sildenafil in the spinal cord of rats

PURPOSE

The phosphodiesterase 5 inhibitor sildenafil has antinociceptive effects, mediated by an increase in cGMP. This study examined the role of spinal adenosine and serotonin receptors played in the antinociceptive effects of intrathecal sildenafil.

MATERIALS AND METHODS

Intrathecal catheters were inserted into the subarachnoid space of Sprague-Dawley male rats as a drug delivery device. Pain was induced by injecting formalin into the plantar surface of rats and observing nociceptive behavior (flinching response) for 60 minutes. Then, the effects of intrathecal adenosine and serotonin receptor antagonists on the antinociceptive activity of intrathecal sildenafil were examined.

RESULTS

Intrathecal sildenafil suppressed the flinching response in a dose-dependent manner during phases 1 and 2 in the formalin test. Both CGS 15943 and dihydroergocristine decreased the antinociceptive effects of sildenafil during phases 1 and 2 in the formalin test.

CONCLUSION

Intrathecal sildenafil effectively attenuated the pain evoked by formalin injection. Both adenosine and serotonin receptors may be involved in the antinociceptive action of sildenafil at the spinal level.

Eosinophilic lung diseases: a clinical, radiologic, and pathologic overview

Eosinophilic lung diseases are a diverse group of pulmonary disorders associated with peripheral or tissue eosinophilia. They are classified as eosinophilic lung diseases of unknown cause (simple pulmonary eosinophilia [SPE], acute eosinophilic pneumonia [AEP], chronic eosinophilic pneumonia [CEP], idiopathic hypereosinophilic syndrome [IHS]), eosinophilic lung diseases of known cause (allergic bronchopulmonary aspergillosis [ABPA], bronchocentric granulomatosis [BG], parasitic infections, drug reactions), and eosinophilic vasculitis (allergic angiitis, granulomatosis [Churg-Strauss syndrome]). The percentages of eosinophils in peripheral blood and bronchoalveolar lavage fluid are essential parts of the evaluation. Chest computed tomography (CT) demonstrates a more characteristic pattern and distribution of parenchymal opacities than does conventional chest radiography. At CT, SPE and IHS are characterized by single or multiple nodules with a surrounding ground-glass-opacity halo, AEP mimics radiologically hydrostatic pulmonary edema, and CEP is characterized by nonsegmental airspace consolidations with peripheral predominance. ABPA manifests with bilateral central bronchiectasis with or without mucoid impaction. The CT manifestations of BG are nonspecific and consist of a focal mass or lobar consolidation with atelectasis. The most common CT findings in Churg-Strauss syndrome include sub-pleural consolidation with lobular distribution, centrilobular nodules, bronchial wall thickening, and interlobular septal thickening. The integration of clinical, radiologic, and pathologic findings facilitates the initial and differential diagnoses of various eosinophilic lung diseases.

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.

α2δ and the mechanism of action of gabapentin in the treatment of pain

Gabapentin is a drug that has been widely used in the treatment of chronic pain states. Despite its widespread usage, it is only recently that light has been shed on the mechanism of action of this agent. In the current review, the authors document the pharmacological, biochemical and molecular information that has led to the identification of the alpha2delta1 auxilliary subunit of voltage gated calcium channels as the target for this drug's actions.

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.

Effect of oral gabapentin on postoperative epidural analgesia †

BACKGROUND

Gabapentin has been used successfully as a non-opioid analgesic adjuvant for postoperative pain management. We hypothesized that gabapentin might be a useful adjuvant for postoperative analgesia provided with patient-controlled epidural analgesia (PCEA).

METHODS

Forty patients undergoing lower extremity surgery procedures were randomly assigned to receive (i) placebo capsules (control) or (ii) gabapentin (1.2 g day(-1)) before and for 2 days after surgery. Anaesthetic technique was standardized. Postoperative assessments included verbal rating scale scoring for pain and sedation, PCEA usage, quality of recovery assessment, times of GI function recovery, and patient satisfaction scoring for pain management.

RESULTS

Pain scores at 1, 4, 8, 12, and 16 h (P<0.001), PCEA bolus requirements (n) at 24 [21 (3), 14 (2)], 48 [15 (4), 10 (3)] and 72 [8 (5), 2 (3)] (P<0.05) and paracetamol (mg) consumption [700 (523), 350 (400)]; P<0.05), were significantly lower in the gabapentin-treated patients than in the control group. Patient satisfaction with postoperative pain management at 24 h was better in gabapentin-treated patients [85.5 (7.5), 66.5 (15)]; P<0.001). Gabapentin-treated patients had less motor block when compared with control group. Times of return of bowel function, hospitalization, and resumption of dietary intake were similar in the groups. However, the incidence of dizziness was higher in the gabapentin group (35% vs 5%; P<0.05).

CONCLUSIONS

Oral gabapentin (1.2 g day(-1)) as an adjunct to epidural analgesia decreased pain and analgesic consumption. Despite an increased incidence of dizziness it also increased patient satisfaction.

Attenuation of morphine tolerance by intrathecal gabapentin is associated with suppression of morphine-evoked excitatory amino acid release in the rat spinal cord

This study was designed to investigate the effect of acute and chronic intrathecal (i.t.) injection of gabapentin (GBP) on the antinociceptive effect of morphine and tolerance development using a tail-flick latency test. Levels of excitatory amino acids (EAA) in i.t. CSF dialysates were also measured by high performance liquid chromatography. Male Wistar rats were implanted with either one or two i.t. catheters for drug injection or pump infusion and with a microdialysis probe for CSF dialysate collection. The effect of acute GBP (10 microg i.t.) injection on the morphine dose response was examined in both naïve rats and rats made tolerant by continuous infusion of morphine (15 microg/h i.t.) for 5 days. At such a low dose (10 microg i.t.), GBP did not enhance morphine's antinociception in naïve rats. In morphine-tolerant rats, however, acute GBP (10 microg i.t.) injection potentiated morphine's antinociception and yielded a 14.6-fold shift in morphine's dose-response curve. When GBP (10 microg/h i.t.) was co-infused with morphine (15 microg/h i.t.) to examine its effect on the development of morphine tolerance, GBP attenuated the development of morphine tolerance. The effect of GBP and morphine on CSF glutamate and aspartate levels was examined in naïve rats, and the effect of morphine challenge on CSF glutamate and aspartate levels was examined in rats previously infused for 5 days with morphine alone or morphine plus GBP. Acute injection of GBP (10 microg i.t.), morphine (50 microg i.t.), or GBP (10 microg i.t.) followed by morphine (50 microg i.t.) 30 min later had no significant effect on CSF EAA concentration in naïve rats; however, in tolerant rats, morphine challenge (50 microg i.t.) increased aspartate and glutamate levels to 221 +/- 22% and 296 +/- 43%, respectively, of those before morphine challenge, and this phenomenon was inhibited by GBP co-infusion. Our results show that GBP, at a dose without enhanced effect on morphine's antinociception in naïve rats, not only potentiates morphine's antinociceptive effect in morphine-tolerant rats but also attenuates the development of morphine tolerance. The mechanism of the effect of GBP on morphine tolerance might be via suppression of the EAA concentration in spinal CSF dialysate.

Antinociceptive interactions between intrathecal gabapentin and MK801 or NBQX in rat formalin test

Antagonists for spinal N-methyl-D-aspartate (NMDA) and amino-hydroxy-methtyl-isoxazolepropionate (AMPA) receptors are effective in attenuating acute nociception or injury-induced hyperalgesia. The antinociception of spinal gabapentin is developed in injury-induced hyperalgesia without affecting acute nociception. The authors evaluated the effects of intrathecal gabapentin, NMDA antagonist (MK801) and AMPA antagonist (NBQX) in the formalin test which shows injury-induced hyperalgesia as well as acute pain. We further assessed the interactions between gabapentin and either MK801 or NBQX. Male Sprague-Dawley rats were implanted with intrathecal catheters. To evoke pain, 50 microL of 5% formalin solution was injected into the hindpaw. The interaction was investigated by a fixed dose analysis or an isobolographic analysis. MK801 and NBQX suppressed flinching responses during phase 1 of the formalin test, while gabapentin had little effect on phase 1. All three agents decreased the phase 2 flinching response. A fixed dose analysis in phase 1 showed that gabapentin potentiated the antinociceptive effect of MK801 and NBQX. Isobolographic analysis in phase 2 revealed a synergistic interaction after coadministration of gabapentin-MK801 or gabapentin-NBQX. Correspondingly, spinal gabapentin with NMDA or AMPA antagonist may be useful in managing acute pain and injury-induced hyperalgesia.

The Effects of Intrathecal Gabapentin on Spinal Morphine Tolerance in the Rat Tail-Flick and Paw Pressure Tests

Analgesic tolerance to opioids has been described in both experimental and clinical conditions and may limit the clinical utility of these drugs. We have previously shown that systemic gabapentin (GBP), a non-opioid drug, prevents and reverses tolerance to systemic morphine in the rat. In this study, we investigated the effect of intrathecal GBP on spinal morphine tolerance. Studied rats were given 7 days of intrathecal injections with saline (10 microL), GBP (300 microg), morphine (15 microg), or a GBP-morphine combination, and analgesic testing using tail-flick and paw-pressure tests was conducted before and 30 min after the drug injection. On Day 8, an antinociceptive dose-response curve was constructed and the 50% effective dose (ED(50)) values for morphine (given alone) were calculated for each study group. Coinjection of GBP with morphine blocked the development of tolerance, as shown by the preservation of morphine analgesia over 7 days as well as by a concomitant decrease in ED(50) values on Day 8, as compared with the morphine-alone group. Although additive analgesia over Days 1-7 cannot be ruled out, ED(50) reductions in the GBP-morphine combination group indeed suggest some suppression of tolerance. These data support previous evidence that GBP prevents opioid tolerance and, more specifically, indicate that intrathecal GBP prevents the development of spinal opioid tolerance. Future studies are required to examine the respective roles of supraspinal and peripheral sites of GBP-morphine interaction and to investigate the mechanisms underlying the action of GBP on opioid tolerance.

Interaction between intrathecal gabapentin and adenosine in the formalin test of rats

Spinal gabapentin and adenosine have been known to display an antinociceptive effect. We evaluated the nature of the interaction between gabapentin and adenosine in formalin-induced nociception at the spinal level. Male Sprague-Dawley rats were prepared for intrathecal catheterization. Pain was evoked by injection of formalin solution (5%, 50 microL) into the hindpaw. After examination of the effects of gabapentin and adenosine, the resulting interaction was investigated with isobolographic and fractional analyses. Neither gabapentin nor adenosine affected motor function. Gabapentin or adenosine decreased the sum of the number of flinches during phase 2, but not during phase 1 in the formalin test. Isobolographic analysis, in phase 2, revealed an additive interaction between gabapentin and adenosine. Taken together, intrathecal gabapentin and adenosine attenuated the facilitated state and interacted additively with each other.