Potent analgesic effects of anticonvulsants on peripheral thermal nociception in rats

Efficacy and safety of intravenous fosphenytoin for patients with acute herpes zoster-associated pain: A placebo-controlled randomized trial

Acute zoster-associated pain develops in most patients with herpes zoster. Nonopioid analgesics are usually used to treat acute zoster-associated pain but are frequently ineffective. We administered intravenous fosphenytoin, the prodrug of phenytoin, to patients with acute zoster-associated pain to examine its analgesic efficacy and safety. At 13 medical institutions in Japan, we conducted a phase II, double-blind, placebo-controlled, randomized trial of intravenous fosphenytoin in Japanese inpatients with acute zoster-associated pain for whom nonopioid analgesics had shown an insufficient analgesic effect. The patients were randomly assigned (1:1:1) to receive a single intravenous dose of fosphenytoin at 18 mg/kg (high dose), a single intravenous dose of fosphenytoin at 12 mg/kg (low dose), or placebo. The primary endpoint was the mean change per hour (slope) in the numerical rating scale score from the baseline score until 120 min after dosing. Seventeen patients were randomly assigned to the low-dose fosphenytoin group (n = 6, median age 62.5 years, range 39-75 years), high-dose fosphenytoin group (n = 5, median age 69.0 years, range 22-75 years), and placebo group (n = 5, median age 52.0 years, range 38-72 years). One patient was excluded because of investigational drug dilution failure. This study was discontinued because of the influences of coronavirus disease 2019. The slope was significantly lower in the high- and low-dose fosphenytoin groups than in the placebo group (P < 0.001 and P = 0.016, respectively). Responsiveness to intravenous fosphenytoin (≥2-point reduction in the numerical rating scale score from baseline to 120 min after dosing) was inferred at plasma total phenytoin concentrations of 10-15 μg/mL. Treatment-emergent adverse events caused no safety concerns in the clinical setting and intravenous fosphenytoin was well tolerated. Intravenous fosphenytoin appears to be an effective and promising alternative treatment for acute zoster-associated pain. Trial Registration: ClinicalTrials.gov NCT04139330.

Modulatory role of neurosteroidogenesis in the spinal cord during peripheral nerve injury-induced chronic pain

The brain and spinal cord (SC) are both targeted by various hormones, including steroid hormones. However, investigations of the modulatory role of hormones on neurobiological functions usually focus only on the brain. The SC received little attention although this structure pivotally controls motor and sensory functions. Here, we critically reviewed key data showing that the process of neurosteroid biosynthesis or neurosteroidogenesis occurring in the SC plays a pivotal role in the modulation of peripheral nerve injury-induced chronic pain (PNICP) or neuropathic pain. Indeed, several active steroidogenic enzymes expressed in the SC produce endogenous neurosteroids that interact with receptors of neurotransmitters controlling pain. The spinal neurosteroidogenesis is differentially regulated during PNICP condition and its blockade modifies painful sensations. The paper suggests that future investigations aiming to develop effective strategies against PNICP or neuropathic pain must integrate in a gender or sex dependent manner the regulatory effects exerted by spinal neurosteroidogenesis.

Phenytoin Decreases Pain-like Behaviors and Improves Opioid Analgesia in a Rat Model of Neuropathic Pain

Neuropathic pain remains a clinical challenge due to its complex and not yet fully understood pathomechanism, which result in limited analgesic effectiveness of the management offered, particularly for patients with acute, refractory neuropathic pain states. In addition to the introduction of several modern therapeutic approaches, such as neuromodulation or novel anti-neuropathic drugs, significant efforts have been made in the repurposing of well-known substances such as phenytoin. Although its main mechanism of action occurs at sodium channels in excitable and non-excitable cells and is well documented, how the drug affects the disturbed neuropathic interactions at the spinal cord level and how it influences morphine-induced analgesia have not been clarified, both being crucial from a clinical perspective. We demonstrated that single and repeated systemic administrations of phenytoin decreased tactile and thermal hypersensitivity in an animal model of neuropathic pain. Importantly, we observed an increase in the antinociceptive effect on thermal stimuli with repeated administrations of phenytoin. This is the first study to report that phenytoin improves morphine-induced antinociceptive effects and influences microglia/macrophage activity at the spinal cord and dorsal root ganglion levels in a neuropathic pain model. Our findings support the hypothesis that phenytoin may represent an effective strategy for neuropathic pain management in clinical practice, particularly when combination with opioids is needed.

Novel neuroactive steroid with hypnotic and T-type calcium channel blocking properties exerts effective analgesia in a rodent model of post-surgical pain

BACKGROUND AND PURPOSE

Neuroactive steroid (3β,5β,17β)-3-hydroxyandrostane-17-carbonitrile (3β-OH) is a novel hypnotic and voltage-dependent blocker of T-type calcium channels. Here, we examine its potential analgesic effects and adjuvant anaesthetic properties using a post-surgical pain model in rodents.

EXPERIMENTAL APPROACH

Analgesic properties of 3β-OH were investigated in thermal and mechanical nociceptive tests in sham or surgically incised rats and mice, with drug injected either systemically (intraperitoneal) or locally via intrathecal or intraplantar routes. Hypnotic properties of 3β-OH and its use as an adjuvant anaesthetic in combination with isoflurane were investigated using behavioural experiments and in vivo EEG recordings in adolescent rats.

KEY RESULTS

A combination of 1% isoflurane with 3β-OH (60 mg·kg^-1 , i.p.) induced suppression of cortical EEG and stronger thermal and mechanical anti-hyperalgesia during 3 days post-surgery, when compared to isoflurane alone and isoflurane with morphine. 3β-OH exerted prominent enantioselective thermal and mechanical antinociception in healthy rats and reduced T-channel-dependent excitability of primary sensory neurons. Intrathecal injection of 3β-OH alleviated mechanical hyperalgesia, while repeated intraplantar application alleviated both thermal and mechanical hyperalgesia in the rats after incision. Using mouse genetics, we found that Ca_V 3.2 T-calcium channels are important for anti-hyperalgesic effect of 3β-OH and are contributing to its hypnotic effect.

CONCLUSION AND IMPLICATIONS

Our study identifies 3β-OH as a novel analgesic for surgical procedures. 3β-OH can be used to reduce T-channel-dependent excitability of peripheral sensory neurons as an adjuvant for induction and maintenance of general anaesthesia while improving analgesia and lowering the amount of volatile anaesthetic needed for surgery.

Neurosteroids and neuropathic pain management: Basic evidence and therapeutic perspectives

Complex mechanisms involved in neuropathic pain that represents a major health concern make its management complicated. Because neurosteroids are bioactive steroids endogenously synthesized in the nervous system, including in pain pathways, they appear relevant to develop effective treatments against neuropathic pain. Neurosteroids act in paracrine or autocrine manner through genomic mechanisms and/or via membrane receptors of neurotransmitters that pivotally modulate pain sensation. Basic studies which uncovered a direct link between neuropathic pain symptoms and endogenous neurosteroid production/regulation, paved the way for the investigations of neurosteroid therapeutic potential against pathological pain. Concordantly, antinociceptive properties of synthetic neurosteroids were evidenced in humans and animals. Neurosteroids promote peripheral analgesia mediated by T-type calcium and gamma-aminobutyric acid type A channels, counteract chemotherapy-induced neuropathic pain and ameliorate neuropathic symptoms of injured spinal cord animals by stimulating anti-inflammatory, remyelinating and neuroprotective processes. Together, these data open interesting perspectives for neurosteroid-based strategies to manage/alleviate efficiently neuropathic pain.

Inhibition of multiple voltage-gated calcium channels may contribute to spinally mediated analgesia by epipregnanolone in a rat model of surgical paw incision

Voltage-activated calcium channels play an important role in excitability of sensory nociceptive neurons in acute and chronic pain models. We have previously shown that low-voltage-activated calcium channels, or T-type channels (T-channels), increase excitability of sensory neurons after surgical incision in rats. We have also found that endogenous 5β-reduced neuroactive steroid epipregnanolone [(3β,5β)-3-hydroxypregnan-20-one] blocked isolated T-currents in dorsal root ganglion (DRG) cells in vitro, and reduced nociceptive behavior in vivo, after local intraplantar application into the foot pads of heathy rats and mice. Here, we investigated if epipregnanolone exerts an antinociceptive effect after intrathecal (i.t.) application in healthy rats, as well as an antihyperalgesic effect in a postsurgical pain model. We also studied if this endogenous neurosteroid blocks currents originating from high voltage-activated (HVA) calcium channels in rat sensory neurons. In in vivo studies, we found that epipregnanolone alleviated thermal and mechanical nociception in healthy rats after i.t. administration without affecting their sensory-motor abilities. Furthermore, epipregnanolone effectively reduced mechanical hyperalgesia after i.t application in rats after surgery. In subsequent in vitro studies, we found that epipregnanolone blocked isolated HVA currents in nociceptive sensory neurons with an IC₅₀ of 3.3 μM in a G-protein-dependent fashion. We conclude that neurosteroids that have combined inhibitory effects on T-type and HVA calcium currents may be suitable for development of novel pain therapies during the perioperative period.

Selective inhibition of CaV3.2 channels reverses hyperexcitability of peripheral nociceptors and alleviates postsurgical pain

Pain-sensing sensory neurons of the dorsal root ganglion (DRG) can become sensitized or hyperexcitable in response to surgically induced peripheral tissue injury. We investigated the potential role and molecular mechanisms of nociceptive ion channel dysregulation in acute pain conditions such as those resulting from skin and soft tissue incision. We used selective pharmacology, electrophysiology, and mouse genetics to link increased current densities arising from the CaV3.2 isoform of T-type calcium channels (T-channels) to nociceptive sensitization using a clinically relevant rodent model of skin and deep tissue incision. Furthermore, knockdown of the CaV3.2-targeting deubiquitinating enzyme USP5 or disruption of USP5 binding to CaV3.2 channels in peripheral nociceptors resulted in a robust antihyperalgesic effect in vivo and substantial T-current reduction in vitro. Our study provides mechanistic insight into the role of plasticity in CaV3.2 channel activity after surgical incision and identifies potential targets for perioperative pain that may greatly decrease the need for narcotics and potential for drug abuse.

Synergistic Interaction of a Gabapentin- Mangiferin Combination in Formalin-Induced Secondary Mechanical Allodynia and Hyperalgesia in Rats Is Mediated by Activation of NO-Cyclic GMP-ATP-Sensitive K+ Channel Pathway

Preclinical Research Gabapentin is an anticonvulsant used to treat neuropathic pain. Mangiferin is an antioxidant that has antinociceptive and antiallodynic effects in inflammatory and neuropathic pain models. The purpose of this study was to determine the interaction between mangiferin and gabapentin in the development and maintenance of formalin-induced secondary allodynia and hyperalgesia in rats. Gabapentin, mangiferin, or their fixed-dose ratio combination were administrated peripherally. Isobolographic analyses was used to define the nature of the interaction of antiallodynic and/or antihyperalgesic effects of the two compounds. Theoretical ED₅₀ values for the combination were 74.31 µg/paw and 95.20 µg/paw for pre- and post-treatment, respectively. These values were higher than the experimental ED₅₀ values, 29.45 µg/paw and 37.73 µg/paw respectively, indicating a synergistic interaction in formalin-induced secondary allodynia and hyperalgesia. The antiallodynic and antihyperalgesic effect induced by the gabapentin/mangiferin combination was blocked by administration of L-NAME, the soluble guanylyl cyclase inhibitor, ODQ and glibenclamide. These data suggest that the gabapentin- mangiferin combination produces a synergistic interaction at the peripheral level. Moreover, the antiallodynic and hyperalgesic effect induced by the combination is mediated via the activation of an NO-cyclic GMP-ATP-sensitive K⁺ channel pathway. Drug Dev Res 78 : 390-402, 2017. © 2017 Wiley Periodicals, Inc.

Assessment of the effectiveness and safety of ethosuximide in the treatment of abdominal pain related to irritable bowel syndrome - IBSET: protocol of a randomised, parallel, controlled, double-blind and multicentre trial

INTRODUCTION

Irritable bowel syndrome (IBS) is characterised by the association of abdominal chronic pain with bowel habit disorders in the absence of identifiable organic disease. This is the first reason for consultation in gastroenterology, with an estimated prevalence of 10%-15% in industrialised countries. Although this is a benign gastrointestinal disease, its chronicity profoundly impacts the patient's quality of life and causes considerable health spending. Actual medical treatments are poorly efficient on IBS-related abdominal pain, making it a major public health concern. The mechanisms causing IBS symptoms are unknown. Recent studies have shown the involvement of T-type channel in abdominal pain. We aim to evaluate the therapeutic potential of ethosuximide, a T-type channel blocker, on the abdominal pain of patients presenting an IBS.

METHODS AND ANALYSIS

The IBSET trial is a randomised, controlled, parallel, double-blind and multicentre study. It is the first clinical trial evaluating the efficacy and safety of ethosuximide on abdominal pain related to IBS. Adults with IBS that report significant abdominal pain (≥4/10) at least for 3 months will be included. 290 patients will be randomly assigned to receive either ethosuximide or placebo over 12 weeks after 1 week of run-in period. The primary endpoint is the rate of responders (pain reduction ≥30% and Subject Global Assessment of Relief score ≥4). The intensity of abdominal pain will be assessed by an 11-point Numerical Rating Scale before and after 12 weeks of treatment and the score of the Subject Global Assessment of Relief scale at the end of treatment. The secondary endpoints are the safety of ethosuximide, the intensity and features of IBS and quality of life.

ETHICS AND DISSEMINATION

The study was approved by an independent medical ethics committee (CPP Sud-Est VI, Clermont-Ferrand, France). The results will be published in a peer-review journal and presented at international congresses.

TRIAL REGISTRATION NUMBER

NCT02973542; Pre-results.

Combination of diacerhein and antiepileptic drugs after local peripheral, and oral administration in the rat formalin test

Preclinical Research The present study was designed to evaluate the possible antinociceptive interaction between diacerhein and some antiepileptic drugs (carbamazepine, topiramate and gabapentin) on formalin-induced nociception. Diacerhein, each of the antiepileptics or a fixed dose-ratio combination of these drugs was assessed after local peripheral and oral administration in rats. lsobolographic analyses were used to define the interaction between drugs. Diacerhein, antiepileptic drugs (carbamazepine, topiramate and gabapentin) or their combinations yielded a dose-dependent antinociceptive effect when administered by both routes. Theoretical ED30 values for the combination estimated from the isobolograms were obtained as follows: diacerhein-carbamazepine (85.99 ± 7.07 μg/paw; 56.53 ± 4.56 mg/kg po), diacerhein-topiramate (197.97 ± 22.90 μg/paw; 13.06 ± 2.44 mg/kg po) and diacerhein-gabapentin (96.87 ± 17.73 μg/paw; 17.90 ± 4.70 mg/kg p.o.) for the local peripheral and oral administration routes, respectively. These values were significantly higher than the experimentally obtained ED30 values: diacerhein-carbamazepine (49.33 ± 3.37 μg/paw; 35.49 ± 7.91 mg/kg po), diacerhein-topiramate (133.00 ± 39.10 μg/paw; 8.87 ± 1.46 mg/kg po) and diacerhein-gabapentin (70.98 ± 14.73 μg/paw; 10.95 ± 3.23 mg/kg po). The combinations produced their antinociceptive effects without motor impairment in the rotarod test indicating synergistic interactions with a good side effect profile.

Efficacy of various analgesics on shoulder function and rotator cuff tendon-to-bone healing in a rat (Rattus norvegicus) model

Although relief of postoperative pain is an imperative aspect of animal welfare, analgesics that do not interfere with the scientific goals of the study must be used. Here we compared the efficacy of different analgesic agents by using an established rat model of supraspinatus tendon healing and a novel gait-analysis system. We hypothesized that different analgesic agents would all provide pain relief in this model but would cause differences in tendon-to-bone healing and gait parameters. Buprenorphine, ibuprofen, tramadol-gabapentin, and acetaminophen were compared with a no-analgesia control group. Gait measures (stride length and vertical force) on the operative forelimb differed between the control group and both the buprenorphine (2 and 4 d postsurgery) and ibuprofen (2 d postsurgery) groups. Step length was different in the control group as compared with the tramadol-gabapentin (2 d after surgery), buprenorphine (2 and 4 d after surgery), and ibuprofen (2 d after surgery) groups. Regarding tendon-to-bone healing, the ibuprofen group showed less stiffness at the insertion site; no other differences in tendon-to-bone healing were detected. In summary, the analgesics evaluated were associated with differences in both animal gait and tendon-to-bone healing. This information will be useful for improving the management of postsurgical pain without adversely affecting tissue healing. Given its ability to improve gait without impeding healing, we recommend use of buprenorphine for postsurgical pain management in rats. In addition, our gait-analysis system can be used to evaluate new analgesics.

Positive effect of calcitonin on the seizures induced by pentylenetetrazole in rats

There are many difficulties involved with the treatment of epilepsy, and these problems have driven the search for new agents to control epileptic seizures. Calcitonin is a peptide hormone that has been well studied and shown to have a positive effect on neuropathic and chronic pain. The mechanism by which calcitonin affects these pain syndromes is thought to be similar to the effect of antiepileptic drugs, such as pregabalin, gabapentin and carbamazepine. In this study, we aim to investigate the effects of calcitonin on seizures induced by pentylenetetrazole (PTZ) in rats. The rats were divided into four groups. The first group was the control group, and the rats were given no medications. The second group was given saline+PTZ. The third group was given 50IU/kg calcitonin+PTZ, and the fourth group was given 100IU/kg calcitonin+PTZ. EEG traces, Racine's convulsion stages and the time of onset of the first myoclonic jerk were compared between the groups. Between the groups, there were significant differences in the Racine's convulsion stages, the onset of the 'first myoclonic jerk', and the rate of the spikes in the EEG traces. The differences were more pronounced in the 100IU/kg calcitonin-treated group (p<0.001). It has been stated that calcitonin relieves pain via regulating voltage-gated Ca(2+) and/or Na(+) channels. Calcitonin has a positive effect on convulsions in epileptic rats, possibly using the same mechanisms as is used in the treatment of neuropathic and chronic pain.

Semi-mechanistic modelling of the analgesic effect of gabapentin in the formalin-induced rat model of experimental pain

PURPOSE

The formalin-induced rat model of nociception involves moderate continuous pain. Formalin-induced pain results in a typical repetitive flinching behaviour, which displays a biphasic pattern characterised by peaks of pain. Here we described the time course of pain response and the analgesic effect of gabapentin using a semi-mechanistic modelling approach.

METHODS

Male Sprague-Dawley rats received gabapentin (10-100 mg/kg) or placebo 1 h prior to the formalin injection, as per standard protocol. A reduction in the frequency of the second peak of flinching was used as a behavioural measure of gabapentin-mediated anti-nociception. The flinching response was modelled using a mono-exponential function to characterise the first peak and an indirect response model with a time variant synthesis rate for the second. PKPD modelling was performed using a population approach in NONMEM v.7.1.2.

RESULTS

The time course of the biphasic response was adequately described by the proposed model, which included separate expressions for each phase. Gabapentin was found to reversibly decrease, but not suppress the flinching frequency of the second response peak only. The mean IC50 estimate was 7,510 ng/ml, with relative standard error (RSE%) of 40%.

CONCLUSIONS

A compartmental, semi-mechanistic model provides the basis for further understanding of the formalin-induced flinching response and consequently to better characterisation of the properties of gabapentin, such as the potency in individual animals. Moreover, despite high exposure levels, model predictions show that gabapentin does not completely suppress behavioural response in the formalin-induced pain model.

Influence of calcium channel blockers on anticonvulsant and antinociceptive activities of valproic acid in pentylenetetrazole-kindled mice

BACKGROUND

Comorbidities of epilepsy comprise some pain disorders, including acute nociceptive pain, therefore, antiepileptic drugs can prove efficacy in the management of this kind of pain albeit with several adverse reactions. The current study aimed to evaluate the modulatory effects of calcium channel blockers on the anticonvulsant and antinociceptive effects of valproic acid (VPA) in pentylenetetrazole (PTZ)-kindled mice.

METHODS

Kindled mice were treated with 20 mg/kg (ip) of diltiazem, nifedipine, or verapamil, then VPA(200 mg/kg, ip) at 30 min intervals before PTZ administration (35 mg/kg, ip).

RESULTS

Our data demonstrated that the three calcium channel blockers afforded a protection against sub-convulsive doses of PTZ. Their protective effects were comparable to that exerted by the standard antiepileptic drug, VPA. The anticonvulsant activity of VPA was further enhanced by its combination with diltiazem. Also, PTZ-kindling reduced pain-threshold as evaluated by hot plate analgesimeter and acetic acid-induced writhing test. Although the repeated administration of VPA significantly increased pain-threshold in kindled mice, it was not able to normalize it. Similar results were obtained with diltiazem and nifedipine. Interestingly, combination of diltiazem or nifedipine with VPA elicited the most profound antinociceptive effect in kindled mice.

CONCLUSIONS

These results demonstrate for the first time the beneficial role of some calcium channel blockers in combination with VPA in the management of acute nociceptive pain. Therapeutically, this enhancing profile for calcium channel blockers fosters a safer and more effective drug-combination regimen than valproic acid alone.

Temperature-dependent enhancement of the antinociceptive effects of opioids in combination with gabapentin in mice

Prescription opioids and anticonvulsants such as gabapentin are often used as combination therapeutics for chronic as well as acute post-operative pain conditions although the effectiveness of such combinations may be dependent on the intensity of the pain state. To test the capacity of gabapentin to enhance opioid effectiveness in the presence of different thermal stimulus intensities, morphine, oxycodone and gabapentin were examined alone and in combination for antinociception in Swiss-Webster male mice using a hot-plate set to one of three temperature intensities (48.5°C, 50.5°C, 52.5°C). Morphine and oxycodone produced significant dose- and stimulus intensity-dependent antinociception whereas gabapentin produced only modest antinociception. However, in combination, gabapentin enhanced the effectiveness of sub-antinociceptive doses of morphine and oxycodone and the gabapentin and oxycodone combinations were both dose- and temperature intensity-dependent. These results provide evidence that the effectiveness and magnitude of the interactions between gabapentin and opioids are dependent on the intensity of the pain stimulus in acute thermal pain states.

TTA-P2 is a potent and selective blocker of T-type calcium channels in rat sensory neurons and a novel antinociceptive agent

Several agents that are preferential T-type calcium (T-channel) blockers have shown promise as being effective in alleviating acute and chronic pain, suggesting an urgent need to identify even more selective and potent T-channel antagonists. We used small, acutely dissociated dorsal root ganglion (DRG) cells of adult rats to study the in vitro effects of 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2), a derivative of 4-aminomethyl-4-fluoropiperdine, on T currents, as well as other currents known to modulate pain transmission. We found that TTA-P2 potently and reversibly blocked DRG T currents with an IC(50) of 100 nM and stabilized channel in the inactive state, whereas high-voltage-activated calcium and sodium currents were 100- to 1000-fold less sensitive to channel blocking effects. In in vivo studies, we found that intraperitoneal injections of 5 or 7.5 mg/kg TTA-P2 reduced pain responses in mice in phases 1 and 2 of the formalin test. Furthermore, TTA-P2, at 10 mg/kg i.p., selectively and completely reversed thermal hyperalgesia in diabetic rats treated with streptozocin but had no effect on the nociceptive response of healthy animals. The antihyperalgesic effects of TTA-P2 in diabetic rats were completely abolished by administration of oligonucleotide antisense for Ca(V)3.2 isoform of T channels. Thus, TTA-P2 is not only the most potent and selective blocker of T channels in sensory neurons yet described, but it also demonstrates the potential for the pharmacological effectiveness of this approach in addressing altered nociceptive responses in animal models of both inflammatory and neuropathic pain.

Analgesic effects of tramadol, tramadol-gabapentin, and buprenorphine in an incisional model of pain in rats (Rattus norvegicus)

Postoperative pain management in laboratory animals relies heavily on a limited number of drug classes, such as opioids and nonsteroidal antiinflammatory drugs. Here we evaluated the effects of saline, tramadol, tramadol with gabapentin, and buprenorphine (n = 6 per group) in a rat model of incisional pain by examining thermal hyperalgesia and weight-bearing daily for 6 d after surgery. All drugs were administered preemptively and continued for 2 consecutive days after surgery. Rats treated with saline or with tramadol only showed thermal hyperalgesia on days 1 through 4 and 1 through 3 after surgery, respectively. In contrast, buprenorphine-treated rats showed no thermal hyperalgesia on days 1 and 2 after surgery, and rats given tramadol with gabapentin showed reduced thermal hyperalgesia on days 2 and 4. For tests of weight-bearing, rats treated with saline or with tramadol only showed significantly less ipsilateral weight-bearing on day 1 after surgery, whereas rats given either buprenorphine or tramadol with gabapentin showed no significant change in ipsilateral weight-bearing after surgery. These data suggest that tramadol alone provides insufficient analgesia in this model of incisional pain; buprenorphine and, to a lesser extent, tramadol with gabapentin provide relief of thermal hyperalgesia and normalize weight-bearing.

Analgesic neuropeptide W suppresses seizures in the brain revealed by rational repositioning and peptide engineering

Anticonvulsant neuropeptides play an important role in controlling neuronal excitability that leads to pain or seizures. Based on overlapping inhibitory mechanisms, many anticonvulsant compounds have been found to exhibit both analgesic and antiepileptic activities. An analgesic neuropeptide W (NPW) targets recently deorphanized G-protein coupled receptors. Here, we tested the hypothesis that the analgesic activity of NPW may lead to the discovery of its antiepileptic properties. Indeed, direct administration of NPW into the brain potently reduced seizures in mice. To confirm this discovery, we rationally designed, synthesized, and characterized NPW analogues that exhibited anticonvulsant activities following systemic administration. Our results suggest that the combination of neuropeptide repositioning and engineering NPW analogues that penetrate the blood-brain barrier could provide new drug leads, not only for the treatment of epilepsy and pain but also for studying effects of this peptide on regulating feeding and energy metabolism coupled to leptin levels in the brain.

The pharmacology of voltage-gated sodium channels in sensory neurones

Voltage-gated sodium channels (VGSCs) are vital for the normal functioning of most excitable cells. At least nine distinct functional subtypes of VGSCs are recognized, corresponding to nine genes for their pore-forming alpha-subunits. These have different developmental expression patterns, different tissue distributions in the adult and are differentially regulated at the cellular level by receptor-coupled cell signalling systems. Unsurprisingly, VGSC blockers are found to be useful as drugs in diverse clinical applications where excessive excitability of tissue leads to pathological dysfunction, e.g. epilepsy or cardiac tachyarrhythmias. The effects of most clinically useful VGSC blockers are use-dependent, i.e. their efficacy depends on channel activity. In addition, many natural toxins have been discovered that interact with VGSCs in complex ways and they have been used as experimental probes to study the structure and function of the channels and to better understand how drugs interact with the channels. Here we have attempted to summarize the properties of VGSCs in sensory neurones, discuss how they are regulated by cell signalling systems and we have considered briefly current concepts of their physiological function. We discuss in detail how drugs and toxins interact with archetypal VGSCs and where possible consider how they act on VGSCs in peripheral sensory neurones. Increasingly, drugs that block VGSCs are being used as systemic analgesic agents in chronic pain syndromes, but the full potential for VGSC blockers in this indication is yet to be realized and other applications in sensory dysfunction are also possible. Drugs targeting VGSC subtypes in sensory neurones are likely to provide novel systemic analgesics that are tissue-specific and perhaps even disease-specific, providing much-needed novel therapeutic approaches for the relief of chronic pain.

Post-surgical neuropathic pain

Surgeons and anaesthetists are involved in Pain Medicine, as they have a responsibility to contribute to postoperative pain management and are often consulted about longer-term pain problems as well. A large component of persistent pain after surgery can be defined as neuropathic pain (NP). Nerves are injured during surgery and pain can persist after the surgical wound has healed. NP is because of a primary lesion or dysfunction of the peripheral or central nervous system. Prevalence estimates indicate that 2-3% of the population in the developed world experience NP. Persistent post-surgical NP is a mostly unrecognized clinical problem. The chronicity and persistence of post-surgical NP is often severely debilitating and impinges on the psychosocial, physical, economic and emotional well-being of patients. Options for treatment of any neuropathic factors are based on understanding the pain mechanisms involved. The current understandings of the mechanisms involved are presented. There is reasonable evidence for the efficacy of pharmacological management for NP. The aim of this article was to appraise the prevention, diagnostic work-up, the physical and particularly the pharmacological management of post-surgical NP and to provide a glimpse of advances in the field. It is a practical approach to post-surgical NP for all surgeons and anaesthetists. The take-home message is that prevention is better than waiting for post-surgical NP to become persistent.

Inhibition of osteosarcoma-induced thermal hyperalgesia in mice by the orally active dual enkephalinase inhibitor PL37. Potentiation by gabapentin

We have previously shown that stimulation of peripheral opioid receptors by exogenous opiates counteracts the thermal hyperalgesia elicited by a tibial osteosarcoma due to intraosteal inoculation of NCTC 2472 cells to mice. Aiming to study whether pheripheral endogenous enkephalins could also counteract this painful symptom, we assayed in this model the effects of PL37, an orally active dual inhibitor of enkephalin inactivating enzymes. Oral administration of PL37 (25 mg/kg) completely supressed osteosarcoma-induced thermal hyperalgesia through the activation of micro-opioid receptors, since the administration of cyprodime (1 mg/kg) inhibited its antihyperalgesic effect. Neither naltrindole (0.1 mg/kg) nor nor-binaltorphimine (10 mg/kg) modified this PL37-induced antihyperalgesic effect. Moreover, the inhibition of the antihyperalgesic effect induced by PL37 after the administration of naloxone-methiodide (2 mg/kg), a non selective opioid antagonist that does not cross the blood-brain barrier, demonstrates the involvement of peripheral opioid receptors. In contrast, centrally mediated effects may be detected when assaying a higher dose of PL37 (50 mg/kg). Besides, the administration of gabapentin (6.25-25 mg/kg, i.p.) dose-dependently inhibited osteosarcoma-induced thermal hyperalgesia. Interestingly, the combined administration of subeffective doses of PL37 and gabapentin completely prevented this type of thermal hyperalgesia. An isobolographic analysis of this interaction demonstrated a synergistic interaction between both drugs.

Gabapentin action and interaction on the antinociceptive effect of morphine on visceral pain in mice

BACKGROUND AND OBJECTIVE

Visceral pain is one of the most common forms of pain and for which new drugs would be welcome. The aim of this study was to investigate whether gabapentin inhibits induced abdominal contractions in mice and to examine the effect of its co-administration with morphine.

METHODS

A total of 96 mice received acetic acid intraperitoneally after administration of saline or gabapentin (1, 5, 10, 50 and 100 mg kg(-1)) or morphine (0.25, 0.5, 1, 3 and 5 mg kg(-1)) or a combination of morphine and gabapentin. Other groups also received naloxone. The number of writhes were counted.

RESULTS

Both gabapentin and morphine reduced writhing in a dose-dependent manner. The number of writhes was decreased significantly by gabapentin (50 and 100 mg kg(-1)) and morphine (0.5, 1, 3 and 5 mg kg(-1)) (P < 0.001). Also, the lowest dose of morphine 0.25 mg kg(-1) when combined with low doses of gabapentin significantly decreased the number of writhes (P < 0.005). The combination of a low effective dose of gabapentin (50 mg kg(-1)) with a low dose of morphine decreased the writhing by 94% as compared to the controls. The antinociceptive effect of combined administration was not reversed by naloxone.

CONCLUSION

These data demonstrated the comparable efficacy of gabapentin with morphine in visceral pain. Also, the results showed that the combination of doses of gabapentin and morphine, which were ineffective alone, produced a significant analgesic effect in the writhing model of pain. This may be clinically important in the management of visceral pain.

Ethosuximide: from bench to bedside

Ethosuximide, 2-ethyl-2-methylsuccinimide, has been used extensively for "petit mal" seizures and it is a valuable agent in studies of absence epilepsy. In the treatment of epilepsy, ethosuximide has a narrow therapeutic profile. It is the drug of choice in the monotherapy or combination therapy of children with generalized absence (petit mal) epilepsy. Commonly observed side effects of ethosuximide are dose dependent and involve the gastrointestinal tract and central nervous system. Ethosuximide has been associated with a wide variety of idiosyncratic reactions and with hematopoietic adverse effects. Typical absence seizures are generated as a result of complex interactions between the thalamus and the cerebral cortex. This thalamocortical circuitry is under the control of several specific inhibitory and excitatory systems arising from the forebrain and brainstem. Corticothalamic rhythms are believed to be involved in the generation of spike-and-wave discharges that are the characteristic electroencephalographic signs of absence seizures. The spontaneous pacemaker oscillatory activity of thalamocortical circuitry involves low threshold T-type Ca2+ currents in the thalamus, and ethosuximide is presumed to reduce these low threshold T-type Ca2+ currents in thalamic neurons. Ethosuximide also decreases the persistent Na+ and Ca2+ -activated K+ currents in thalamic and layer V cortical pyramidal neurons. In addition, there is evidence that in a genetic absence epilepsy rat model ethosuximide reduces cortical gamma-aminobutyric acid (GABA) levels. Also, elevated glutamate levels in the primary motor cortex of rats with absence epilepsy (but not in normal animals) are reduced by ethosuximide.

Vasculopatia livedóide: relato de caso com possível evidência de etiopatogenia neurovascular

A etiopatogênese da vasculopatia livedóide ainda hoje permanece indeterminada e, apesar de algumas etiologias aventadas, com freqüência elas não apresentam associação com a vasculopatia em seu curso. Os tratamentos descritos na literatura não apresentam resultados reproduzíveis. Relata-se caso de paciente de 42 anos, com vasculopatia livedóide e boa resposta ao tratamento pioneiro com a carbamazepina. A droga foi instituída diante das discretas evidências de inflamação, da inconstância dos achados relacionados às teorias sugeridas, da forte sintomatologia dolorosa e dos recentes relatos da participação neural na coagulação, que levaram os autores a considerar a possibilidade de associação vasculoneural na patogênese dessa vasculopatia.

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.

Evaluation of anticonvulsant and analgesic effects of benzyl- and benzhydryl ureides

The anticonvulsant effects of benzyl- and benzhydryl ureides in mice models of seizures (maximal electroshock seizure test, pentylenetetrazol test, picrotoxin-induced seizure test) and the influence on spontaneous locomotor activity has been assessed. Furthermore, the analgesic effect of ureide derivatives was studied in the hot-plate test in mice. Selected compounds were investigated for their in vitro interaction with adenosine receptors as well as the benzodiazepine binding site of GABA(A) receptors. This study demonstrated the strong anticonvulsant activity of several ureides in electrically or chemically induced seizure models, and structure-activity relationships were discussed. 1-Benzyl-3-butyrylurea (9) was found to be equipotent to ethosuximide in the pentylenetetrazol test with regard to the number of attacks as well as the time of the onset of seizures. The ureide 9 also revealed the highest protective activity against seizures in the other models, maximal electroshock seizure and picrotoxin test. Moreover, 1-benzyl-3-butyrylurea was not neurotoxic at doses up to 200 mg/kg. Benzylureides 8-10 showed affinity to the adenosine A1 receptors at low micromolar concentrations. However, the apparent anticonvulsant activity in different seizure models does not appear to result from direct activation of adenosine A1 receptors or GABA(A) receptors, respectively. In the hot-plate test, the majority of investigated compounds exhibited analgesic activity. Again, compound 9 was superior to the other substances investigated, suggesting a potential therapeutic value of that ureide derivative.

Examination of the interaction between peripheral diclofenac and gabapentin on the 5% formalin test in rats

It has been shown that the association of diclofenac with other analgesic agents can increase its antinociceptive activity, allowing the use of lower doses and thus limiting side effects. Therefore, the aim of the present study was to examine the possible pharmacological interaction between diclofenac and gabapentin at the peripheral level in the rat using the 5% formalin test and isobolographic analysis. Diclofenac, gabapentin or a fixed-dose ratio diclofenac-gabapentin combination were administrated locally in the formalin-injured paw and the antinociceptive effect was evaluated using the 5% formalin test. All treatments produced a dose-dependent antinociceptive effect. ED30 values were estimated for the individual drugs and an isobologram was constructed. The derived theoretical ED30 for the diclofenac-gabapentin combination was 597.5+/-87.5 microg/paw, being significantly higher than the actually observed experimental value, 170.9+/-26.07 microg/paw. These results correspond to a synergistic interaction between diclofenac and gabapentin at the peripheral level, potency being about three times higher with regard to that expected from the addition of the effects of the individual drugs. Data suggest that low doses of the diclofenac-gabapentin combination can interact synergistically at the peripheral level and therefore this drug association may represent a therapeutic advantage for the clinical treatment of inflammatory pain.

On the mechanism of carbamazepine-induced antinociception in the formalin test

In the present study, the effect of lidocaine (a sodium channel blocker) on carbamazepine-induced antinociception, in formalin test was investigated. Intraperitoneal (i.p.) administration of different doses of carbamazepine (3.5, 7, 15, and 30 mg/kg) induced a dose-dependent antinociception in mice, in the first and second phases of the test. Different doses of lidocaine as a sodium channel blocker (5, 10, and 20 mg/kg, i.p.) also induced antinociception in both phases of the formalin test. It is noted that lidocaine could potentiate the response of carbamazepine in the first, but not in the second, phase of the formalin test. Meanwhile i.p. administration of different doses of Prazosin, alpha1 adrenoceptor antagonist (0.125, 0.25, and 0.5 mg/kg), Yohimbine, alpha2 adrenoceptor antagonist (0.25, 0.5, and 1 mg/kg), Bicuculline, GABAA receptor antagonist (1.5 and 3 mg/kg), and CGP 35348, GABAB receptor antagonist (100 and 200 mg/kg) exert dose-dependent antinociceptive effect in both phases of the formalin test. It should be noted that bicuculline 0.75 mg/kg by itself increased pain score in the second phase of the formalin test, indicating that blockade of GABAA receptor subtype may induce chronic pain. None of the aforementioned drugs could alter the antinociceptive response of carbamazepine in the formalin test. It is concluded that sodium channel mechanisms may be involved partly in the antinociceptive induced by carbamazepine.

Reversal of neuropathic pain by α-hydroxyphenylamide: A novel sodium channel antagonist

Sodium (Na) channel blockers are known to possess antihyperalgesic properties. We have designed and synthesized a novel Na channel antagonist, alpha-hydroxyphenylamide, and determined its ability to inhibit both TTX-sensitive (TTX-s) and TTX-resistant (TTX-r) Na currents from small dorsal root ganglion (DRG) neurons. alpha-Hydroxyphenylamide tonically inhibited both TTX-s and TTX-r Na currents yielding an IC(50) of 8.2+/-2.2 microM (n=7) and 28.9+/-1.8 microM (n=8), respectively. In comparison, phenytoin was less potent inhibiting TTX-s and TTX-r currents by 26.2+/-4.0% (n=8) and 25.5+/-2.0%, respectively, at 100 microM. alpha-Hydroxyphenylamide (10 microM) also shifted equilibrium gating parameters of TTX-s Na channels to greater hyperpolarized potentials, slowed recovery from inactivation, accelerated the development of inactivation and exhibited use-dependent block. In the chronic constriction injury (CCI) rat model of neuropathic pain, intraperitoneal administration of alpha-hydroxyphenylamide attenuated the hyperalgesia by 53% at 100mg/kg, without affecting motor coordination in the Rotorod test. By contrast, the reduction in pain behavior produced by phenytoin (73%; 100mg/kg) was associated with significant motor impairment. In summary, we report that alpha-hydroxyphenylamide, a sodium channel antagonist, exhibits antihyperalgesic properties in a rat model of neuropathic pain, with favorable sedative and ataxic side effects compared with phenytoin.

Antinociceptive, antiedematogenic and antiangiogenic effects of benzaldehyde semicarbazone

Semicarbazones induce an anticonvulsant effect in different experimental models. As some anticonvulsant drugs also have anti-inflammatory activity, the effects of benzaldehyde semicarbazone (BS) on models of nociception, edema and angiogenesis were investigated. BS (10, 25 or 50 mg/kg, i.p.) markedly inhibited the second phase of nociceptive response induced by formaldehyde (0.34%, 20 microl) in mice, but only the highest dose inhibited the first phase of this response. The thermal hyperalgesia and mechanical allodynia induced by carrageenan (1%, 50 microl, i.pl.) in rats were also inhibited by BS (50 mg/kg, i.p.). However, treatment of mice with BS did not induce an antinociceptive effect in the hot-plate model. The paw edema induced by carrageenan (1%, 50 microl, i.pl.) in rats was inhibited by BS (25 or 50 mg/kg, i.p.). Treatment of mice with BS (0.25, 0.5 or 2.5 mg/kg/day, i.p., 7 days) also inhibited angiogenesis induced by subcutaneous implantation of a sponge disc. It is unlikely that the antinociceptive effect induced by BS results from motor incoordination or a muscle relaxing effect, as the mice treated with this drug displayed no behavioral impairment in the rotarod apparatus. In conclusion, we demonstrated that BS presents antinociceptive, antiedematogenic and antiangiogenic activities. An extensive investigation of the pharmacological actions of BS and its derivatives is justified and may lead to the development of new clinically useful drugs.

Possible activation of the NO-cyclic GMP-protein kinase G-K+ channels pathway by gabapentin on the formalin test

The effect of modulators of the nitric oxide-cyclic GMP-protein kinase G-K+ channels pathway on the local peripheral antinociceptive action induced by gabapentin was assessed in the rat 1% formalin test. Local peripheral administration of gabapentin produced a dose-dependent antinociception in the second phase of the test. Gabapentin-induced antinociception was due to a local action as its administration in the contralateral paw was ineffective. Local peripheral pretreatment of the paws with NG-L-nitro-arginine methyl ester (L-NAME, a nitric oxide synthesis inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) and KT-5823 (a protein kinase G inhibitor) dose-dependently reduced gabapentin-induced antinociception. Likewise, glibenclamide or tolbutamide (ATP-sensitive K+ channel inhibitors), 4-aminopyridine or tetraethylammonium (non-selective inward rectifier K+ channel inhibitors) or charybdotoxin (large-conductance Ca2+-activated-K+ channel blocker), but not apamin (small-conductance Ca2+-activated-K+ channel blocker) or naloxone (opioid receptor antagonist), reduced the antinociception induced by gabapentin. Our data suggest that gabapentin could activate the nitric oxide-cyclic GMP-protein kinase G-K+ channels pathway in order to produce its peripheral antinociceptive effect in the rat 1% formalin test.

Peripheral Antinociception by Carbamazepine in an Inflammatory Mechanical Hyperalgesia Model in the Rat: a New Target for Carbamazepine?

This study investigated whether carbamazepine could produce local peripheral antinociception in a rat model of inflammatory mechanical hyperalgesia, and whether adenosine receptors are involved. Carbamazepine (100-1000 nmol/paw) co-administrated with a pro-inflammatory compound, concanavalin A, into the hind paw caused a significant dose- and time-dependent anti-hyperalgesia. Coadministration of caffeine (250-1000 nmol/paw), a nonselective adenosine-receptor antagonist, as well as DPCPX (10-30 nmol/paw), a selective adenosine A(1)-receptor antagonist, with carbamazepine, significantly depressed its anti-hyperalgesic effect. Drugs injected into the contralateral hind paw did not produce significant effects. These results suggest that carbamazepine produces local peripheral anti-hyperalgesia via peripheral adenosine A(1) receptors.

Centrally mediated antihyperalgesic and antiallodynic effects of zonisamide following partial nerve injury in the mouse

Some antiepileptic drugs are used clinically to relieve neuropathic pain. We have evaluated the effects and investigated the possible mechanisms of action of zonisamide, an antiepileptic drug, on thermal hyperalgesia and tactile allodynia in a murine chronic pain model that was prepared by partial ligation of the sciatic nerve. Subcutaneously administered zonisamide (10 and 30 mg/kg) produced antihyperalgesic and antiallodynic effects in a dose-dependent manner; these effects were manifested by elevation of the withdrawal threshold in response to a thermal (plantar test) or mechanical (von Frey) stimulus, respectively. Similar analgesic effects were obtained in both the plantar and von Frey tests when zonisamide was injected either intracerebroventricularly (i.c.v., 10 and 30 microg) or intrathecally (i.t., 10 and 30 microg). It is thought that this elevation of the thermal and mechanical withdrawal thresholds after local injection of zonisamide is not generated secondarily via impaired motor activity, since zonisamide (30 microg, i.c.v. or i.t.) did not affect locomotor activity, as assessed in sciatic-nerve-ligated mice. Moreover, the nitric oxide synthase inhibitor L-NAME, when injected either i.c.v. or i.t., potentiated the analgesic effects of zonisamide. In contrast, neither i.c.v. nor i.t. zonisamide produced antinociceptive effects against acute thermal and mechanical nociception in non-ligated mice. Together, following peripheral nerve injury, it appears that zonisamide produces centrally mediated antihyperalgesic and antiallodynic effects partly via the blockade of nitric oxide synthesis.

Comparison of the effects of anticonvulsant drugs with diverse mechanisms of action in the formalin test in rats

The purpose of the present studies was to compare anticonvulsant drugs with diverse mechanisms of action in a persistent pain model, the formalin test. In addition, the anticonvulsant effects of the compounds were determined in the threshold electroshock tonic seizure test and the 6-Hz limbic seizure test. The effects of the compounds were also determined on locomotor activity. Carbamazepine, oxcarbazepine, lamotrigine, gabapentin and ethosuximide all produced statistically significant analgesic effects in the formalin test whereas phenytoin, topiramate, zonisamide, phenobarbital, tiagabine, valproate and levetiracetam did not. All compounds were anticonvulsant. In addition, morphine and phenobarbital increased locomotor activity while ethosuximide had no effect and all other compounds decreased locomotor activity. For those compounds that were analgesic, the doses required to produce analgesia were larger in magnitude than the anticonvulsant ED50 values in the threshold electroshock and 6-Hz tests, as well as larger than doses that altered locomotor activity. The present results demonstrate that the anticonvulsant and analgesic effects of clinically used antiepileptic drugs do not necessarily correlate and therefore suggest that the anticonvulsant and analgesic efficacy of these drugs may be due to different pharmacologic mechanisms.

The management of neuropathic pain

Although the definitive treatment for neuropathic pain remains elusive, scientific investigation continues to provide the field with better and better therapies. As our understanding of the neurophysiologic mechanisms of pain improves, pharmaceutic therapies have become more effective even as side effects are minimized. Surgical therapies have become more precise and less invasive. Advances in neurophysiology have given rise to new advances in the field of neuro-modulation. As this therapy continues to emerge, ablative procedures recede as therapies offering minimal invasiveness, reversible mechanisms, and long-standing relief emerge to the forefront of treatment for neuropathic pain.

The neurobiology of antiepileptic drugs for the treatment of nonepileptic conditions

Antiepileptic drugs (AEDs) are commonly prescribed for nonepileptic conditions, including migraine headache, chronic neuropathic pain, mood disorders, schizophrenia and various neuromuscular syndromes. In many of these conditions, as in epilepsy, the drugs act by modifying the excitability of nerve (or muscle) through effects on voltage-gated sodium and calcium channels or by promoting inhibition mediated by gamma-aminobutyric acid (GABA) A receptors. In neuropathic pain, chronic nerve injury is associated with the redistribution and altered subunit compositions of sodium and calcium channels that predispose neurons in sensory pathways to fire spontaneously or at inappropriately high frequencies, often from ectopic sites. AEDs may counteract this abnormal activity by selectively affecting pain-specific firing; for example, many AEDs suppress high-frequency action potentials by blocking voltage-activated sodium channels in a use-dependent fashion. Alternatively, AEDs may specifically target pathological channels; for example, gabapentin is a ligand of alpha2delta voltage-activated calcium channel subunits that are overexpressed in sensory neurons after nerve injury. Emerging evidence suggests that effects on signaling pathways that regulate neuronal plasticity and survival may be a factor in the delayed clinical efficacy of AEDs in some neuropsychiatric conditions, including bipolar affective disorder.

5beta-reduced neuroactive steroids are novel voltage-dependent blockers of T-type Ca2+ channels in rat sensory neurons in vitro and potent peripheral analgesics in vivo

T-type Ca(2+) channels are believed to play an important role in pain perception, and anesthetic steroids such as alphaxalone and allopregnanolone, which have a 5alpha-configuration at the steroid A, B ring fusion, are known to inhibit T-type Ca(2+) channels and cause analgesia in a thermal nociceptive model (Soc Neurosci Abstr 29:657.9, 2003). To define further the structure-activity relationships for steroid analgesia, we synthesized and examined a series of 5beta-reduced steroids for their ability to induce thermal antinociception in rats when injected locally into the peripheral receptive fields of the nociceptors and studied their effects on T-type Ca(2+) channel function in vitro. We found that most of the steroids completely blocked T-type Ca(2+) currents in vitro with IC(50) values at a holding potential of -90 mV ranging from 2.8 to 40 microM. T current blockade exhibited mild voltage-dependence, suggesting that 5beta-reduced neuroactive steroids stabilize inactive states of the channel. For the most potent steroids, we found that other voltage-gated currents were not significantly affected at concentrations that produce nearly maximal blockade of T currents. All tested compounds induced dose-dependent analgesia in thermal nociceptive testing; the most potent effect (ED(50), 30 ng/100 microl) obtained with a compound [(3beta,5beta,17beta)-3-hydroxyandrostane-17-carbonitrile] that was also the most effective blocker of T currents. Compared with previously studied 5alpha-reduced steroids, these 5beta-reduced steroids are more efficacious blockers of neuronal T-type Ca(2+) channels and are potentially useful as new experimental reagents for understanding the role of neuronal T-type Ca(2+) channels in peripheral pain pathways.