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

Mechanisms of Action of Dorsal Root Ganglion Stimulation

The dorsal root ganglion (DRG) serves as a pivotal site for managing chronic pain through dorsal root ganglion stimulation (DRG-S). In recent years, the DRG-S has emerged as an attractive modality in the armamentarium of neuromodulation therapy due to its accessibility and efficacy in alleviating chronic pain refractory to conventional treatments. Despite its therapeutic advantages, the precise mechanisms underlying DRG-S-induced analgesia remain elusive, attributed in part to the diverse sensory neuron population within the DRG and its modulation of both peripheral and central sensory processing pathways. Emerging evidence suggests that DRG-S may alleviate pain by several mechanisms, including the reduction of nociceptive signals at the T-junction of sensory neurons, modulation of pain gating pathways within the dorsal horn, and regulation of neuronal excitability within the DRG itself. However, elucidating the full extent of DRG-S mechanisms necessitates further exploration, particularly regarding its supraspinal effects and its interactions with cognitive and affective networks. Understanding these mechanisms is crucial for optimizing neurostimulation technologies and improving clinical outcomes of DRG-S for chronic pain management. This review provides a comprehensive overview of the DRG anatomy, mechanisms of action of the DRG-S, and its significance in neuromodulation therapy for chronic pain.

Quantification of stimulus-evoked tactile allodynia in free moving mice by the chainmail sensitivity test

Chronic pain occurs at epidemic levels throughout the population. Hypersensitivity to touch, is a cardinal symptom of chronic pain. Despite dedicated research for over a century, quantifying this hypersensitivity has remained impossible at scale. To address these issues, we developed the Chainmail Sensitivity Test (CST). Our results show that control mice spend significantly more time on the chainmail portion of the device than mice subject to neuropathy. Treatment with gabapentin abolishes this difference. CST-derived data correlate well with von Frey measurements and quantify hypersensitivity due to inflammation. Our study demonstrates the potential of the CST as a standardized tool for assessing mechanical hypersensitivity in mice with minimal operator input.

Involvement of the Opioidergic Mechanism in the Analgesic Potential of a Novel Indazolone Derivative: Efficacy in the Management of Pain, Neuropathy, and Inflammation Using In Vivo and In Silico Approaches

Indazolones possess interesting pharmacological activities. The search for indazole and indazolone-containing nuclei as drugs is an important research area of medicinal chemistry. The current work aims to evaluate a novel indazolone derivative against in vivo and in silico targets of pain, neuropathy, and inflammation. An indazolone derivative (ID) was synthesized and characterized using advanced spectroscopic techniques. Well-established animal models of abdominal constriction, hot plate, tail immersion, carrageenan paw edema, and Brewer's yeast-induced pyrexia were employed for evaluating the potential of the ID at different doses (20-60 mg kg^-1). Nonselective GABA antagonists, opioid antagonist naloxone (NLX) and pentylenetetrazole (PTZ), were employed to assess the potential role of GABAergic and opioidergic processes. The antineuropathic potential of the drug was evaluated using a vincristine-induced neuropathic pain model. In silico studies were performed to assess any possible interactions of the ID with pain target sites like cyclooxygenases (COX-I/II), GABA_A, and opioid receptors. This study revealed that the selected ID (doses of 20-60 mg kg^-1) efficiently hampered chemically and thermally induced nociceptive responses, producing significant anti-inflammatory and antipyretic effects. These effects produced by the ID were dose-dependent (i.e., 20-60 mg kg^-1 and p range of 0.001-0.01) and significant in comparison to standards (p < 0.001). Antagonistic studies with NLX (1.0 mg kg^-1) and PTZ (15.0 mg kg^-1) revealed the involvement of the opioidergic mechanism rather than the GABAergic mechanism. The ID showed promising anti-static allodynia effects as well. In silico studies revealed preferential binding interactions of the ID with cyclooxygenases (COX-I/II), GABA_A, and opioid receptors. According to the results of the current investigation, the ID may serve in the future as a therapeutic agent for the treatment of pyrexia, chemotherapy-induced neuropathic pain, and nociceptive inflammatory pain.

Excitatory and inhibitory neuronal signaling in inflammatory and diabetic neuropathic pain

Pain, although unpleasant, is an essential warning mechanism against injury and damage of the organism. An intricate network of specialised sensors and transmission systems contributes to reception, transmission and central sensitization of pain. Here, we briefly introduce some of the main aspects of pain signal transmission, including nociceptors and nociceptive signals, mechanisms of inflammatory and neuropathic pain, and the situation of diabetes-associated neuropathic pain. The role of glia-astrocytes, microglia, satellite glia cells-and their specific channels, transporters and signaling pathways is described. A focus is on the contribution of inhibitory synaptic signaling to nociception and a possible role of glycine receptors in glucose-mediated analgesia and treatment-induced diabetic neuropathy. Inhibitory receptors such as GABA_A- and glycine receptors are important contributors to nociceptive signaling; their contribution to altered pain sensation in diabetes may be of clinical relevance, and they could be promising therapeutic targets towards the development of novel analgesics.

N-type calcium channel blockers: a new approach towards the treatment of chronic neuropathic pain

Neuropathic pain (NP) remains maltreated for a wide number of patients by the currently available treatments and little research has been done in finding new drugs for treating NP. Ziconotide (PrialtTM) had been developed as the new drug, which belongs to the class of ω-conotoxin MVIIA. It inhibits N-type calcium channels. Ziconotide is under the last phase of the clinical trial, a new non-narcotic drug for the management of NP. Synthetically it has shown the similarities with ω-conotoxin MVIIA, a constituent of poison found in fish hunting snails (Conus magus). Ziconotide acts by selectively blocking neural N-type voltage-sensitized Ca2+ channels (NVSCCs). Certain herbal drugs also have been studied but no clinical result is there and the study is only limited to preclinical data. This review emphasizes the N-type calcium channel inhibitors, and their mechanisms for blocking calcium channels with their remedial prospects for treating chronic NP.

Electroacupuncture Suppresses CCI-Induced Neuropathic Pain through GABAA Receptors

Neuropathic pain remains a chronic and intractable pain. Recent studies have shown a close relationship between gamma-aminobutyric acid A (GABAA) receptor and neuropathic pain. Spinal cord GABAA receptors are key modulators of pain processing. Electroacupuncture (EA) is currently used worldwide to relieve pain. The immunomodulatory effect of EA in animals has been proposed in previous studies. However, it remains unclear how EA contributes to alleviating neuropathic pain. In this study, the chronic constriction injury (CCI) rat model was used to explore the relationship between GABAA receptor and neuropathic pain. We also investigated whether EA treatment could ameliorate pain hypersensitivity by modulating the GABAA receptor. To determine the function of EA in neurological diseases, in this study, the mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were assessed to determine the threshold of pain. In addition, we used Western blot, immunofluorescence, and real-time quantitative PCR to confirm whether EA treatment relieves pain hypersensitivity by regulating GABAA receptors. The morphology of synapse was examined using an electron microscope. In the present study, EA relieved mechanical allodynia and thermal hyperalgesia. EA also inhibited microglial activation in the spinal cord, accompanied by increased levels of GABAARα2, GABAARα3, and GABAARγ2 subunits. However, the analgesic effect of EA was attenuated by treatment with the GABAA receptor antagonist bicuculine. Overall, the present results indicate that microglia and GABAA receptor might participate in EA analgesia. These results contribute to our understanding of the impact of EA on rats after sciatic nerve compression, providing a theoretical basis for the clinical application of EA analgesia.

The Effect of Pre-emptive Oral Melatonin versus Placebo on Post-operative Analgesia in Infants after Thoracotomy for Closed Cardiac Surgeries: A Randomized Controlled Study

BACKGROUND: Thoracotomy pain is one of the severest types of pain that should be managed properly, especially in children. Opioids are the most widely prescribed analgesics for post-operative pain, but they can have a number of undesirable side effects. Melatonin could be employed as an adjuvant analgesic therapy during procedural discomfort as it had no known major side effects. STUDY DESIGN: This was a double-blinded, controlled randomized study. METHODS: Fifty patients divided randomly into two equal groups. One hour before surgery, children in M group (n = 25) were given 0.5 mg/kg orally of melatonin 3 mg tablets) and patients in Group P (n = 25) received a placebo (5 ml of water by syringe 5 ml) orally. Post-operative pethidine consumption over the 1st 24 h (mg) was recorded, the intraoperative fentanyl consumption (HR and ABP) at baseline, 1 min after induction, at skin incision, and every 30 min till the end of the surgery were recorded. Neonatal Infant Pain Score (NIPS) at 4 h, 6 h, 8 h, 12 h, 18 h, and 24 h postoperatively was recorded. Other reported data include demographic data, extubation time, days of ICU stay, and complications. RESULTS: Total post-operative pethidine consumption (mg) over 24 h was significantly lower in M group than P group (3.48 ± 2.23 vs. 7.68 ± 4.52 p = 0.01). Intraoperative fentanyl consumption (ug) was significantly lower in M group than P group (10.28 ± 4.98 vs. 17.08 ± 7.39 p < 0.001). As regards NIPS, it was statistically lower in M group than P group in all times except at 8 h and 24 h with significant difference. CONCLUSION: Oral melatonin is an effective and safe pre-emptive drug as it reduces the total post-operative pethidine consumption over the first 24 h and decreased post-operative pain scores without any unpleasant effects in pediatrics undergoing closed heart surgery.

6-Aryl-quinazolines as novel GABAB receptor positive allosteric modulators

The systemic use of GABA_B orthosteric agonist baclofen might be limited due to its detrimental properties: sedation and motor impairment. In contrast, GABA_B positive allosteric modulators produce less adverse effects. Using BHF-177 as a starting point, we found a new active scaffold: the 6-aryl-quinazoline scaffold. Further elaborating the scaffold, we identified several in vitro and in vivo active compounds.

Distinct synaptic mechanisms underlying the analgesic effects of γ-aminobutyric acid transporter subtypes 1 and 3 inhibitors in the spinal dorsal horn

ABSTRACT

Normalization of the excitatory and inhibitory balance by increasing the levels of endogenous inhibitory neurotransmitters by blocking their reuptake is a promising therapeutic strategy for relieving chronic pain. Pharmacological blockade of spinal γ-aminobutyric acid (GABA) transporter subtypes 1 and 3 (GAT1 and GAT3) has been reported to generate analgesic effects in animal models of neuropathic pain. Here, we explored the synaptic mechanisms underlying their analgesic effects in the spinal dorsal horn. Whole-cell recordings were made from dorsal horn neurons in spinal slices with attached dorsal roots from adult mice, and the effects of GAT inhibitors on miniature and evoked postsynaptic currents were examined. Behaviorally, GAT inhibitors were intrathecally applied to assess their effects on mechanical hypersensitivity in mice developing neuropathic pain after partial sciatic nerve ligation. The GAT1 inhibitor NNC-711 reduced the frequency of miniature excitatory postsynaptic currents (EPSCs) and the amplitude of C-fiber-mediated EPSCs, and the GAT3 inhibitor SNAP-5114 reduced the amplitude of A-fiber-mediated and C-fiber-mediated EPSCs. These effects were antagonized by the GABAB receptor antagonist CGP55845. Consistently, the analgesic effect of intrathecally injected NNC-711 and SNAP-5114 in mice developing mechanical hypersensitivity after partial sciatic nerve ligation was abolished by CGP55845. Thus, GAT1 and GAT3 inhibitors exert distinct GABAB receptor-mediated inhibitory effects on excitatory synaptic transmission in the spinal dorsal horn, which most likely contributes to their analgesic effects.

GABAB Receptors and Pain

A substantial fraction of the human population suffers from chronic pain states, which often cannot be sufficiently treated with existing drugs. This calls for alternative targets and strategies for the development of novel analgesics. There is substantial evidence that the G protein-coupled GABA_B receptor is involved in the processing of pain signals and thus has long been considered a valuable target for the generation of analgesics to treat chronic pain. In this review, the contribution of GABA_B receptors to the generation and modulation of pain signals, their involvement in chronic pain states as well as their target suitability for the development of novel analgesics is discussed.

Mechanistic evaluation of a novel cyclohexenone derivative's functionality against nociception and inflammation: An in-vitro, in-vivo and in-silico approach

The synthesis of a novel cyclohexanone derivative (CHD; Ethyl 6-(4-metohxyphenyl)-2-oxo-4-phenylcyclohexe-3-enecarboxylate) was described and the subsequent aim was to perform an in vitro, in vivo and in silico pharmacological evaluation as a putative anti-nociceptive and anti-inflammatory agent in mice. Initial in vitro studies revealed that CHD inhibited both cyclooxygenase-2 (COX-2) and 5-lipoxygenase (5-LOX) enzymes and it also reduced mRNA expression of COX-2 and the pro-inflammatory cytokines TNF-α and IL-1β. It was then shown that CHD dose dependently inhibited chemically induced tonic nociception in the abdominal constriction assay and also phasic thermal nociception (i.e. anti-nociception) in the hot plate and tail immersion tests in comparison with aspirin and tramadol respectively. The thermal test outcomes indicated a possible moderate centrally mediated anti-nociception which, in the case of the hot plate test, was pentylenetetrazole (PTZ) and naloxone reversible, implicating GABAergic and opioidergic mechanisms. CHD was also effective against both the neurogenic and inflammatory mediator phases induced in the formalin test and it also disclosed anti-inflammatory activity against the phlogistic agents, carrageenan, serotonin, histamine and xylene compared with standard drugs in edema volume tests. In silico studies indicated that CHD possessed preferential affinity for GABAA, opioid and COX-2 target sites and this was supported by molecular dynamic simulations where computation of free energy of binding also favored the formation of stable complexes with these sites. These findings suggest that CHD has prospective anti-nociceptive and anti-inflammatory properties, probably mediated through GABAergic and opioidergic interactions supplemented by COX-2 and 5-LOX enzyme inhibition in addition to reducing pro-inflammatory cytokine expression. CHD may therefore possess potentially beneficial therapeutic effectiveness in the management of inflammation and pain.

GABAB Receptor Chemistry and Pharmacology: Agonists, Antagonists, and Allosteric Modulators

The GABA_B receptors are metabotropic G protein-coupled receptors (GPCRs) that mediate the actions of the primary inhibitory neurotransmitter, γ-aminobutyric acid (GABA). In the CNS, GABA plays an important role in behavior, learning and memory, cognition, and stress. GABA is also located throughout the gastrointestinal (GI) tract and is involved in the autonomic control of the intestine and esophageal reflex. Consequently, dysregulated GABA_B receptor signaling is associated with neurological, mental health, and gastrointestinal disorders; hence, these receptors have been identified as key therapeutic targets and are the focus of multiple drug discovery efforts for indications such as muscle spasticity disorders, schizophrenia, pain, addiction, and gastroesophageal reflex disease (GERD). Numerous agonists, antagonists, and allosteric modulators of the GABA_B receptor have been described; however, Lioresal^® (Baclofen; β-(4-chlorophenyl)-γ-aminobutyric acid) is the only FDA-approved drug that selectively targets GABA_B receptors in clinical use; undesirable side effects, such as sedation, muscle weakness, fatigue, cognitive deficits, seizures, tolerance and potential for abuse, limit their therapeutic use. Here, we review GABA_B receptor chemistry and pharmacology, presenting orthosteric agonists, antagonists, and positive and negative allosteric modulators, and highlight the therapeutic potential of targeting GABA_B receptor modulation for the treatment of various CNS and peripheral disorders.

Molecular mechanisms of metabotropic GABAB receptor function

Metabotropic γ-aminobutyric acid G protein-coupled receptors (GABA_B) represent one of the two main types of inhibitory neurotransmitter receptors in the brain. These receptors act both pre- and postsynaptically by modulating the transmission of neuronal signals and are involved in a range of neurological diseases, from alcohol addiction to epilepsy. A series of recent cryo-EM studies revealed critical details of the activation mechanism of GABA_B Structures are now available for the receptor bound to ligands with different modes of action, including antagonists, agonists, and positive allosteric modulators, and captured in different conformational states from the inactive apo to the fully active state bound to a G protein. These discoveries provide comprehensive insights into the activation of the GABA_B receptor, which not only broaden our understanding of its structure, pharmacology, and physiological effects but also will ultimately facilitate the discovery of new therapeutic drugs and neuromodulators.

A novel gabapentin analogue assuages neuropathic pain response in chronic sciatic nerve constriction model in rats

Gabapentin (GBP) is an established drug that has been used in the management of symptoms of neuropathy but it is associated with unwanted side effects such as sedation and motor incoordination. The goal of the study was to find out a drug with greater efficacy and safety for the treatment of neuropathic pain. Our previously synthesized GABA analogue (Gabapentsal, GPS) was tested (25-100 mg/kg, i.p) in chronic constriction injury (CCI) induced nociceptive model of static allodynia, dynamic allodynia, thermal hyperalgesia, mechanical hyperalgesia and cold allodynia in rats (Sprague Dawley). Open field and rotarod tests were performed to assess the impact of GPS on the motor performance of the animals. GBP (100 mg/kg, i.p) was used as a standard for comparison. GPS dose dependently reduced static (P <0.001) and dynamic allodynia (P <0.001), thermal hyperalgesia (P <0.001), mechanical hyperalgesia (P < 0.001) and cold allodynia (P < 0.001). In comparison to GBP, GPS failed to alter any significantly the motor performance of rats in both the open field and rotarod assays. These results suggest that GPS is effective in alleviating nociception in CCI neuropathic pain model but free from the side effect of motor discoordination seen in the treatment with GBP. In conclusion, GPS may prove to be a prospectively more effective and safer option in the management of neuropathic syndromes.

Intrathecally administered perampanel alleviates neuropathic and inflammatory pain in rats

Chronic pain conditions such as neuropathic pain and persistent inflammatory pain are difficult to manage. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors modulate nociceptive processing at the spinal dorsal horn. Previous studies have shown that intrathecal AMPA receptor antagonists exert antinociception in various pain states. Perampanel is a selective, noncompetitive inhibitor of the AMPA receptor and used clinically as an antiepileptic drug. Little is known about antinociceptive action of perampanel in the spinal cord. Here, we explored whether intrathecal perampanel attenuates neuropathic and inflammatory pain. A chronic constriction injury (CCI) to the sciatic nerve was induced in male Sprague-Dawley rats. We evaluated the effects of intrathecal perampanel (10, 30, or 100 μg) on mechanical and cold hyperalgesia using the electronic von Frey and cold plate tests, respectively. Normal rats were assessed in terms of inflammatory nociception using the formalin test, and motor function employing the rotarod test. In the CCI rats, spinally applied perampanel inhibited mechanical and cold hyperalgesia dose-dependently. In normal rats, perampanel remarkably suppressed the early- and late-phase responses in the formalin test, and it weakly affected motor performance for a short period at the highest dose. These results suggest that perampanel exerts antinociceptive actions on neuropathic and persistent inflammatory pain in the spinal cord. Perampanel may be safe and beneficial remedy for patients with such pain conditions. In addition, AMPA receptor can be a promising target for treatment of chronic pain.

Reviewing the case for compromised spinal inhibition in neuropathic pain

A striking and debilitating property of the nervous system is that damage to this tissue can cause chronic intractable pain, which persists long after resolution of the initial insult. This neuropathic form of pain can arise from trauma to peripheral nerves, the spinal cord, or brain. It can also result from neuropathies associated with disease states such as diabetes, human immunodeficiency virus/AIDS, herpes, multiple sclerosis, cancer, and chemotherapy. Regardless of the origin, treatments for neuropathic pain remain inadequate. This continues to drive research into the underlying mechanisms. While the literature shows that dysfunction in numerous loci throughout the CNS can contribute to chronic pain, the spinal cord and in particular inhibitory signalling in this region have remained major research areas. This review focuses on local spinal inhibition provided by dorsal horn interneurons, and how such inhibition is disrupted during the development and maintenance of neuropathic pain.

Mechanism of dorsal root ganglion stimulation for pain relief in painful diabetic polyneuropathy is not dependent on GABA release in the dorsal horn of the spinal cord

AIMS

It is hypothesized that dorsal root ganglion stimulation (DRGS), sharing some of the mechanisms of traditional spinal cord stimulation (SCS) of the dorsal columns, induces γ-aminobutyric acid (GABA) release from interneurons in the spinal dorsal horn.

METHODS

We used quantitative immunohistochemical analysis in order to investigate the effect of DRGS on intensity of intracellular GABA-staining levels in the L4-L6 spinal dorsal horn of painful diabetic polyneuropathy (PDPN) animals. To establish the maximal pain relieving effect, we tested for mechanical hypersensitivity to von Frey filaments and animals received 30 minutes of DRGS at day 3 after implantation of the electrode. One day later, 4 Sham-DRGS animals and four responders-to-DRGS received again 30 minutes of DRGS and were perfused at the peak of DRGS-induced pain relief.

RESULTS

No significant difference in GABA-immunoreactivity was observed between DRGS and Sham-DRGS in lamina 1-3 of the spinal levels L4-6 neither ipsilaterally nor contralaterally.

CONCLUSIONS

Dorsal root ganglion stimulation does not induce GABA release from the spinal dorsal horn cells, suggesting that the mechanisms underlying DRGS in pain relief are different from those of conventional SCS. The modulation of a GABA-mediated "Gate Control" in the DRG itself, functioning as a prime Gate of nociception, is suggested and discussed.

Use of Gabapentin in Posterior Spinal Fusion is Associated With Decreased Postoperative Pain and Opioid Use in Children and Adolescents

STUDY DESIGN

Retrospective cohort study.

OBJECTIVE

The objective of this study was to examine associations of gabapentin use with inpatient postoperative daily pain scores and opioid use in children undergoing PSF for AIS.

SUMMARY OF BACKGROUND DATA

Gabapentin use in posterior spinal fusion (PSF) postoperative pain management for adolescent idiopathic scoliosis (AIS) is increasingly common in order to decrease opioid use and improve pain control, though there is conflicting data on dosing and effectiveness to support this practice in real world settings.

METHODS

Retrospective cohort study of children aged 10 to 21 years undergoing PSF for AIS between January 2013 and June 2016 at an urban academic tertiary care center. Adjuvant gabapentin exposure was defined as at least 15 mg/kg/d by postoperative day (POD) 1 with an initial loading dose of 10 mg/kg on day of surgery. Primary outcomes were daily postoperative mean pain score and opioid use [morphine milligram equivalents/kg/day(mme/kg/d)]. Secondary outcomes were short and long-term complications.

RESULTS

Among 129 subjects (mean age, 14.6 y, 74% female, mean coronal cobb, 55.2 degrees), 24 (19%) received gabapentin. Unadjusted GABA exposure was associated with significantly lower opioid use on POD1 and 2 (49% and 31%mme/kg/d, respectively) and lower pain scores (14%) on POD2. Adjusting for preexisting back pain, preoperative coronal Cobb angle, and site, GABA use was associated with significantly lower mean pain scores on POD1 through POD3 (-0.68, P=0.01; -0.86, P=0.002; -0.63, P=0.04). Gabapentin use was also associated with decreased opioid use on POD1 and POD2 (-0.39mme/kg/d, P<0.001; -0.27, P=0.02). There was no difference in complications by gabapentin exposure.

CONCLUSIONS

Addition of gabapentin as adjuvant therapy for adolescent PSF, beginning on day of surgery, is associated with improved pain scores and decreased opioid use in the first 48 to 72 hours postoperatively.

LEVEL OF EVIDENCE

This is a retrospective cohort study, classified as Level III under "Therapeutic Studies Investigating the Results of a Treatment."

Cannabinoid CB2 Agonist AM1710 Differentially Suppresses Distinct Pathological Pain States and Attenuates Morphine Tolerance and Withdrawal

AM1710 (3-(1,1-dimethyl-heptyl)-1-hydroxy-9-methoxy-benzo(c) chromen-6-one), a cannabilactone cannabinoid receptor 2 (CB2) agonist, suppresses chemotherapy-induced neuropathic pain in rodents without producing tolerance or unwanted side effects associated with CB1 receptors; however, the signaling profile of AM1710 remains incompletely characterized. It is not known whether AM1710 behaves as a broad-spectrum analgesic and/or suppresses the development of opioid tolerance and physical dependence. In vitro, AM1710 inhibited forskolin-stimulated cAMP production and produced enduring activation of extracellular signal-regulated kinases 1/2 phosphorylation in human embryonic kidney (HEK) cells stably expressing mCB2. Only modest species differences in the signaling profile of AM1710 were observed between HEK cells stably expressing mCB2 and hCB2. In vivo, AM1710 produced a sustained inhibition of paclitaxel-induced allodynia in mice. In paclitaxel-treated mice, a history of AM1710 treatment (5 mg/kg per day × 12 day, i.p.) delayed the development of antinociceptive tolerance to morphine and attenuated morphine-induced physical dependence. AM1710 (10 mg/kg, i.p.) did not precipitate CB1 receptor-mediated withdrawal in mice rendered tolerant to Δ⁹-tetrahydrocannabinol, suggesting that AM1710 is not a functional CB1 antagonist in vivo. Furthermore, AM1710 (1, 3, 10 mg/kg, i.p.) did not suppress established mechanical allodynia induced by complete Freund's adjuvant (CFA) or by partial sciatic nerve ligation (PSNL). Similarly, prophylactic and chronic dosing with AM1710 (10 mg/kg, i.p.) did not produce antiallodynic efficacy in the CFA model. By contrast, gabapentin suppressed allodynia in both CFA and PSNL models. Our results indicate that AM1710 is not a broad-spectrum analgesic agent in mice and suggest the need to identify signaling pathways underlying CB2 therapeutic efficacy to identify appropriate indications for clinical translation.

α2- and β2-Adrenoreceptor-Mediated Efficacy of the Atypical Antidepressant Agomelatine Combined With Gabapentin to Suppress Allodynia in Neuropathic Rats With Ligated Infraorbital or Sciatic Nerve

Previous data showed that neuropathic pain induced by mechanical lesion of peripheral nerves has specific characteristics and responds differently to alleviating drugs at cephalic versus extracephalic level. This is especially true for tricyclic antidepressants currently used for alleviating neuropathic pain in humans which are less effective against cephalic neuropathic pain. Whether this also applies to the antidepressant agomelatine, with its unique pharmacological properties as MT₁/MT₂ melatonin receptor agonist and 5-HT_2B/5-HT_2C serotonin receptor antagonist, has been investigated in two rat models of neuropathic pain. Acute treatments were performed 2 weeks after unilateral chronic constriction (ligation) injury to the sciatic nerve (CCI-SN) or the infraorbital nerve (CCI-ION), when maximal mechanical allodynia had developed in ipsilateral hindpaw or vibrissal pad, respectively, in Sprague–Dawley male rats. Although agomelatine (45 mg/kg i.p.) alone was inactive, co-treatment with gabapentin, at an essentially ineffective dose (50 mg/kg i.p.) on its own, produced marked anti-allodynic effects, especially in CCI-ION rats. In both CCI-SN and CCI-ION models, suppression of mechanical allodynia by ‘agomelatine + gabapentin’ could be partially mimicked by the combination of 5-HT_2C antagonist (SB 242084) + gabapentin, but not by melatonin or 5-HT_2B antagonist (RS 127445, LY 266097), alone or combined with gabapentin. In contrast, pretreatment by idazoxan, propranolol or the β₂ antagonist ICI 118551 markedly inhibited the anti-allodynic effect of ‘agomelatine + gabapentin’ in both CCI-SN and CCI-ION rats, whereas pretreatment by the MT₁/MT₂ receptor antagonist S22153 was inactive. Altogether these data indicate that ‘agomelatine + gabapentin’ is a potent anti-allodynic combination at both cephalic and extra-cephalic levels, whose action implicates α₂- and β₂-adrenoreceptor-mediated noradrenergic neurotransmission.

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.

Diversity of structure and function of GABAB receptors: a complexity of GABAB-mediated signaling

γ-aminobutyric acid type B (GABA_B) receptors are broadly expressed in the nervous system and play an important role in neuronal excitability. GABA_B receptors are G protein-coupled receptors that mediate slow and prolonged inhibitory action, via activation of Gαi/o-type proteins. GABA_B receptors mediate their inhibitory action through activating inwardly rectifying K⁺ channels, inactivating voltage-gated Ca²⁺ channels, and inhibiting adenylate cyclase. Functional GABA_B receptors are obligate heterodimers formed by the co-assembly of R1 and R2 subunits. It is well established that GABA_B receptors interact not only with G proteins and effectors but also with various proteins. This review summarizes the structure, subunit isoforms, and function of GABA_B receptors, and discusses the complexity of GABA_B receptors, including how receptors are localized in specific subcellular compartments, the mechanism regulating cell surface expression and mobility of the receptors, and the diversity of receptor signaling through receptor crosstalk and interacting proteins.

Post-Operative Cognitive Dysfunction: An exploration of the inflammatory hypothesis and novel therapies

Post-Operative Cognitive Dysfunction (POCD) is a highly prevalent condition with significant clinical, social and financial impacts for patients and their communities. The underlying pathophysiology is becoming increasingly understood, with the role of neuroinflammation and oxidative stress secondary to surgery and anaesthesia strongly implicated. This review aims to describe the putative mechanisms by which surgery-induced inflammation produces cognitive sequelae, with a focus on identifying potential novel therapies based upon their ability to modify these pathways.

G-Protein Coupled Receptors Targeted by Analgesic Venom Peptides

Chronic pain is a complex and debilitating condition associated with a large personal and socioeconomic burden. Current pharmacological approaches to treating chronic pain such as opioids, antidepressants and anticonvulsants exhibit limited efficacy in many patients and are associated with dose-limiting side effects that hinder their clinical use. Therefore, improved strategies for the pharmacological treatment of pathological pain are urgently needed. G-protein coupled receptors (GPCRs) are ubiquitously expressed on the surface of cells and act to transduce extracellular signals and regulate physiological processes. In the context of pain, numerous and diverse families of GPCRs expressed in pain pathways regulate most aspects of physiological and pathological pain and are thus implicated as potential targets for therapy of chronic pain. In the search for novel compounds that produce analgesia via GPCR modulation, animal venoms offer an enormous and virtually untapped source of potent and selective peptide molecules. While many venom peptides target voltage-gated and ligand-gated ion channels to inhibit neuronal excitability and blunt synaptic transmission of pain signals, only a small proportion are known to interact with GPCRs. Of these, only a few have shown analgesic potential in vivo. Here we review the current state of knowledge regarding venom peptides that target GPCRs to produce analgesia, and their development as therapeutic compounds.

The Regulatory Effects of Lateral Hypothalamus Area GABAB Receptor on Gastric Ischemia-Reperfusion Injury in Rats

HIGHLIGHTS

The aim of the research was to determine the functional effects and molecular mechanisms of GABA_B receptor on ischemia reperfusion-induced gastric injury in rats.

The lateral hypothalamus area GABA_B receptor attenuated the ischemia reperfusion-induced gastric injury by up-regulating the production of GABA, GABA_BR, and down-regulating P-GABA_BR in the brain.

This work would provide a new therapeutic strategy for acute gastric injury.

Gastric ischemia-reperfusion (GI-R) injury progression is largely associated with excessive activation of the greater splanchnic nerve (GSN). This study aims to investigate the protective effects of GABA_B receptor (GABA_BR) in the lateral hypothalamic area (LHA) on GI-R injury. A model of GI-R injury was established by clamping the celiac artery for 30 min and then reperfusion for 1 h. The coordinate of FN and LHA was identified in Stereotaxic Coordinates and then the L-Glu was microinjected into FN, GABA_B receptor agonist baclofen, or GABA_B receptor antagonist CGP35348 was microinjected into the LHA, finally the GI-R model was prepared. The expression of GABA_BR, P-GABA_BR, NOX2, NOX4, and SOD in the LHA was detected by western blot, PCR, and RT-PCR. The expression of IL-1β, NOX2, and NXO4 in gastric mucosa was detected by western blot. We found that microinjection of L-Glu into the FN or GABA_B receptor agonist (baclofen) into the LHA attenuated GI-R injury. Pretreatment with GABA_B receptor antagonist CGP35348 reversed the protective effects of FN stimulation or baclofen into the LHA. Microinjection of baclofen into the LHA obviously reduced the expression of inflammatory factor IL-1β, NOX2, and NOX4 in the gastric mucosa.

Conclusion: The protective effects of microinjection of GABA_BR agonist into LHA on GI-R injury in rats could be mediated by up-regulating the production of GABA, GABA_BR, and down-regulating P-GABA_BR in the LHA.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

6-Methoxyflavanone attenuates mechanical allodynia and vulvodynia in the streptozotocin-induced diabetic neuropathic pain

BACKGROUND

Diabetic neuropathy is the most prevalent, persistent and debilitating complication of diabetes mellitus often coupled with vulvodynia that may present as an isolated symptom or as a part of constellation of other neuropathic abnormalities.

OBJECTIVE

Flavonoids have selective affinity for GABA receptors and 6-methoxyflavanone (6-MeOF) is a positive allosteric modulator of GABA responses at human recombinant GABA_A receptors. GABAergic and opioidergic system inhibition have been shown to facilitate neuropathic pain.

METHODS

6-MeOF was evaluated for analgesic effect in the hot plate test and streptozotocin-induced diabetic neuropathic pain in female rats using von Frey hairs. The possible involvement of opioidergic and GABAergic mechanisms was investigated using naloxone and pentylenetetrazole (PTZ) antagonists, respectively. The biodistribution of 6-MeOF in plasma and CNS was examined using a validated HPLC/UV analytical method. The binding affinity of 6-MeOF with opioid and GABA receptors was studied using molecular docking simulation approach.

RESULTS

6-MeOF (10 and 30mg/kg) attenuated the acute phasic thermal nociception in the hot plate test while in the case of streptozotocin-induced diabetic neuropathy model, 6-MeOF (10 and 30mg/kg) produced static/dynamic anti-allodynic (increased paw withdrawal threshold and latency) as well as static/dynamic anti-vulvodynic effects (increased flinching response threshold and latency), when compared to the vehicle and standard gabapentin (75mg/kg). In silico studies depicted the preference of 6-MeOF for the delta- and kappa-opioid and GABA_A receptors. Moreover, the pharmacokinetic profile revealed a quick appearance of 6-MeOF in the systemic circulation and brain areas with maximum concentration observed after 30min in the amygdala, brain stem and cerebral cortex.

CONCLUSION

6-MeOF readily crosses the blood brain barrier and may be effective in attenuating the diabetes-induced allodynia as well as vulvodynia, probably through interactions with the GABAergic and opioidergic systems.

Current Status of the New Antiepileptic Drugs in Chronic Pain

Antiepileptic drugs (AEDs) are extensively used worldwide to treat a wide range of disorders other than epilepsy, such as neuropathic pain, migraine, and bipolar disorder. Due to this situation more than 20 new third-generation AEDs have been introduced in the market recently. The future design of new AEDs must also have potential to help in the non-epileptic disorders. The wide acceptance of second generation AEDs for the management of various non-epileptic disorders has caused the emergence of generics in the market. The wide use of approved AEDs outside epilepsy is based on both economic and scientific reasons. Bipolar disorders, migraine prophylaxis, fibromyalgia, and neuropathic pain represent the most attractive indication expansion opportunities for anticonvulsant developers, providing blockbuster revenues. Strong growth in non-epilepsy conditions will see Pfizer's Lyrica become the market leading brand by 2018. In this review, we mainly focus on the current status of new AEDs in the treatment of chronic pain and migraine prophylaxis. AEDs have a strong analgesic potential and this is demonstrated by the wide use of carbamazepine in trigeminal neuralgia and sodium valproate in migraine prophylaxis. At present, data on the new AEDs for non-epileptic conditions are inconclusive. Not all AEDs are effective in the management of neuropathic pain and migraine. Only those AEDs whose mechanisms of action are match with pathophysiology of the disease, have potential to show efficacy in non-epileptic disorder. For this better understanding of the pathophysiology of the disease and mechanisms of action of new AEDs are essential requirement before initiating pre-clinical and clinical trials. Many new AEDs show good results in the animal model and open-label studies but fail to provide strong evidence at randomized, placebo-controlled trials. The final decision regarding the clinical efficacy of the particular AEDs in a specific non-epileptic disorder should be withdrawal from randomized placebo trials rather than open-label studies; otherwise this may lead to off-label uses of drug. The purpose of the present review is to relate the various mechanisms of action of new AEDs to pathophysiological mechanisms and clinical efficacy in neuropathic pain and migraine.

The Anti-Allodynic Gabapentinoids: Myths, Paradoxes, and Acute Effects

The gabapentinoids (pregabalin and gabapentin) are first line treatments for neuropathic pain. They exert their actions by binding to the α2δ accessory subunits of voltage-gated Ca²⁺ channels. Because these subunits interact with critical aspects of the neurotransmitter release process, gabapentinoid binding prevents transmission in nociceptive pathways. Gabapentinoids also reduce plasma membrane expression of voltage-gated Ca²⁺ channels but this may have little direct bearing on their therapeutic actions. In animal models of neuropathic pain, gabapentinoids exert an anti-allodynic action within 30 minutes but most of their in vitro effects are 30-fold slower, taking at least 17 hours to develop. This difference may relate to increased levels of α2δ expression in the injured nervous system. Thus, in situations where α2δ is experimentally upregulated in vitro, gabapentinoids act within minutes to interrupt trafficking of α2δ subunits to the plasma membrane within nerve terminals. When α2δ is not up-regulated, gabapentinoids act slowly to interrupt trafficking of α2δ protein from cell bodies to nerve terminals. This improved understanding of the mechanism of gabapentinoid action is related to their slowly developing actions in neuropathic pain patients, to the concept that different processes underlie the onset and maintenance of neuropathic pain and to the use of gabapentinoids in management of postsurgical pain.

The positive allosteric GABAB receptor modulator rac-BHFF enhances baclofen-mediated analgesia in neuropathic mice

Neuropathic pain is associated with impaired inhibitory control of spinal dorsal horn neurons, which are involved in processing pain signals. The metabotropic GABAB receptor is an important component of the inhibitory system and is highly expressed in primary nociceptors and intrinsic dorsal horn neurons to control their excitability. Activation of GABAB receptors with the orthosteric agonist baclofen effectively reliefs neuropathic pain but is associated with severe side effects that prevent its widespread application. The recently developed positive allosteric GABAB receptor modulators lack most of these side effects and are therefore promising drugs for the treatment of pain. Here we tested the high affinity positive allosteric modulator rac-BHFF for its ability to relief neuropathic pain induced by chronic constriction of the sciatic nerve in mice. rac-BHFF significantly increased the paw withdrawal threshold to mechanical stimulation in healthy mice, indicating an endogenous GABABergic tone regulating the sensitivity to mechanical stimuli. Surprisingly, rac-BHFF displayed no analgesic activity in neuropathic mice although GABAB receptor expression was not affected in the dorsal horn as shown by quantitative receptor autoradiography. However, activation of spinal GABAB receptors by intrathecal injection of baclofen reduced hyperalgesia and its analgesic effect was considerably potentiated by co-application of rac-BHFF. These results indicate that under conditions of neuropathic pain the GABAergic tone is too low to provide a basis for allosteric modulation of GABAB receptors. However, allosteric modulators would be well suited as an add-on to reduce the dose of baclofen required to achieve analgesia.

Epigenetic regulation of G protein coupled receptor signaling and its implications in psychiatric disorders

G protein-coupled receptors (GPCRs) act as a relay center through which extracellular signals, in the form of neurotransmitters or therapeutics, are converted into an intracellular response, which ultimately shapes the overall response at the tissue and behavioral level. Remarkably in similar ways, epigenetic mechanisms also modulate the expression pattern of a large number of genes in response to the dynamic environment inside and outside of the body, and consequently overall response. Emerging evidences from the pharmacogenomics and preclinical studies clearly suggest that these two distinct mechanisms criss-cross each other in several neurological disorders. At one hand such cross-talks between two distinct mechanisms make disease etiology more challenging to understand, while on the other hand if dealt appropriately, such situations might provide an opportunity to find novel druggable target and strategy for the treatment of complex diseases. In this review article, we have summarized and highlighted the main findings that tie epigenetic mechanisms to GPCR mediated signaling in the pathophysiology of central nervous system (CNS) disorders, including depression, addiction and pain.

Nitrous oxide persistently alleviates pain hypersensitivity in neuropathic rats: A dose-dependent effect

BACKGROUND

Despite numerous pharmacological approaches, there are no common analgesic drugs that produce meaningful relief for the majority of patients with neuropathic pain. Although nitrous oxide (N2O) is a weak analgesic that acts via opioid-dependent mechanisms, it is also an antagonist of the N-methyl-D-aspartate receptor (NMDAR). The NMDAR plays a critical role in the development of pain sensitization induced by nerve injury.

OBJECTIVE

Using the chronic constriction injury of the sciatic nerve in male rats as a preclinical model of neuropathic pain, the first aim of the present study was to evaluate the lowest N2O concentration and the shortest time of N2O postinjury exposure that would produce persistent relief of neuropathic pain. The second aim was to compare the effects of N2O with gabapentin, a reference drug used in human neuropathic pain relief.

METHODS

Changes in the nociceptive threshold were evaluated using the paw pressure vocalization test in rats.

RESULTS

Among the various N2O concentrations tested, which ranged from 25% to 50%, only 50% N2O single exposure for 1 h 15 min induced a persistent (minimum of three weeks) and significant (60%) reduction in pain hypersensitivity. A single gabapentin dose (75 mg⁄kg to 300 mg⁄kg, intraperitoneally) induced an acute (1 h to 1 h 30 min) dose-dependent effect, but not a persistent effect such as that observed with N2O.

CONCLUSIONS

These preclinical results suggest that N2O is advantageous for long-lasting neuropathic pain relief after sciatic nerve injury compared with other drugs used in humans such as gabapentinoids or NMDAR antagonists. The present preclinical study provides a rationale for developing comparative clinical studies.

Baclofen reversed thermal place preference in rats with chronic constriction injury

Chronic constriction injury to the sciatic nerve was used as an animal model of neuropathic pain. Instead of frequently used reflex-based tests we used an operant thermal place preference test to evaluate signs of neuropathic pain and the effect of baclofen administration in rats with neuropathy. Chronic constriction injury was induced by four loose ligations of the sciatic nerve. Thermal place preference (45 °C vs. 22 °C and 45 °C vs. 11 °C) was measured after the ligation and after the administration of baclofen in sham and experimental rats. Rats with the chronic constriction injury spent significantly less time on the colder plate compared to sham operated animals at the combination 45 °C vs. 11 °C. After administration of baclofen (10 mg/kg s.c.), the aversion to the colder plate in rats with chronic constriction injury disappeared. At the combination 45 °C vs. 22 °C, no difference in time spent on colder and/or warmer plate was found between sham and experimental animals. These findings show the importance of cold allodynia evaluation in rats with chronic constriction injury and the effectiveness of baclofen in this neuropathic pain model.

Spinal cord injury induced neuropathic pain: Molecular targets and therapeutic approaches

Neuropathic pain, especially that resulting from spinal cord injury, is a tremendous clinical challenge. A myriad of biological changes have been implicated in producing these pain states including cellular interactions, extracellular proteins, ion channel expression, and epigenetic influences. Physiological consequences of these changes are varied and include functional deficits and pain responses. Developing therapies that effectively address the cause of these symptoms require a deeper knowledge of alterations in the molecular pathways. Matrix metalloproteinases and tissue inhibitors of metalloproteinases are two promising therapeutic targets. Matrix metalloproteinases interact with and influence many of the studied pain pathways. Gene expression of ion channels and inflammatory mediators clearly contributes to neuropathic pain. Localized and time dependent targeting of these proteins could alleviate and even prevent neuropathic pain from developing. Current therapeutic options for neuropathic pain are limited primarily to analgesics targeting the opioid pathway. Therapies directed at molecular targets are highly desirable and in early stages of development. These include transplantation of exogenously engineered cell populations and targeted gene manipulation. This review describes specific molecular targets amenable to therapeutic intervention using currently available delivery systems.

Optoactivation of parvalbumin neurons in the spinal dorsal horn evokes GABA release that is regulated by presynaptic GABAB receptors

Among heterogeneous neural cells in the spinal dorsal horn, parvalbumin (PV)-positive neurons are one subtype of GABA (γ-aminobutyric acid)-containing interneurons. Using an optogenetic approach, we expressed blue light-sensitive cation channel channelrhodopsin-2 (ChR2) via a viral vector on PV neurons in the spinal dorsal horn. Combined with in vitro whole-cell recordings, we activated ChR2 expressed on PV neurons by blue light and recorded GABAA receptor-mediated light-evoked inhibitory postsynaptic currents (L-IPSCs). The L-IPSCs were action potential-dependent and abolished by the GABAA receptor antagonist picrotoxin, indicating a synchronic GABA release from presynaptic terminals. Activation of GABAB receptors (the metabotropic receptors of GABA) on presynaptic terminals by a putative agonist, baclofen, depressed the amplitude of L-IPSCs. This depression was largely occluded by pretreatment with the highly selective Cav2.1 (P/Q-type) Ca(2+) channel blocker ω-agatoxin IVA. N-type Ca(2+) channel blocker ω-conotoxin GVIA showed less effects on either L-IPSCs or baclofen depression. We conclude that optoactivation of PV-ChR2 neurons in the spinal dorsal horn induces GABA release from presynaptic terminals, which is modulated by presynaptic GABAB receptors that are coupled to P/Q-type Ca(2+) channels. Importantly, our studies provide a simple and reliable optogenetic approach to study dorsal horn neural circuits.

Analgesic, anxiolytic and anaesthetic effects of melatonin: new potential uses in pediatrics

Exogenous melatonin is used in a number of situations, first and foremost in the treatment of sleep disorders and jet leg. However, the hypnotic, antinociceptive, and anticonvulsant properties of melatonin endow this neurohormone with the profile of a drug that modulates effects of anesthetic agents, supporting its potential use at different stages during anesthetic procedures, in both adults and children. In light of these properties, melatonin has been administered to children undergoing diagnostic procedures requiring sedation or general anesthesia, such as magnetic resonance imaging, auditory brainstem response tests and electroencephalogram. Controversial data support the use of melatonin as anxiolytic and antinociceptive agents in pediatric patients undergoing surgery. The aim of this review was to evaluate available evidence relating to efficacy and safety of melatonin as an analgesic and as a sedative agent in children. Melatonin and its analogs may have a role in antinociceptive therapies and as an alternative to midazolam in premedication of adults and children, although its effectiveness is still controversial and available data are clearly incomplete.

Comprehensive analysis of the GABAergic system gene expression profile in the anterior cingulate cortex of mice with Paclitaxel-induced neuropathic pain

The supraspinal pathophysiology of the painful neuropathy induced by paclitaxel, a chemotherapeutic agent, is not well understood. The γ-aminobutyric acid (GABA) neurotransmitter system has been implicated in the pathogenesis of neuropathic pain. Gene expression of GABAergic system molecules was examined in the anterior cingulate cortex (ACC) of mice brains, by real-time PCR, during paclitaxel-induced neuropathic pain, because this area is involved in pain perception and modulation that might contribute to neuropathic pain. Paclitaxel treatment resulted in thermal hyperalgesia and in increased GABA transporter-1 (GAT-1) mRNA expression, but not that of other GABA transporters or GABA(A) ergic enzymes in the ACC compared to vehicle treatment. Among the 18 GABA(A) receptor subunits analyzed, only β2, β3, δ, and γ2 had increased mRNA levels, and for the receptor subunit, only GABA(B2) had increased mRNA levels in the ACC of paclitaxel-treated mice, whereas the rest of the GABA receptor subunits were not altered. The mRNA expression of GABAA receptor subunits α6, θ, π, ρ1, ρ2, and ρ3 were not detected in the ACC. In conclusion, these data show that during paclitaxel-induced neuropathic pain there is significant increase in GAT-1 expression in the ACC. GAT-1 is the main transporter of GABA from the synapse, and thus its increased expression possibly results in less GABA at the synapse and dysregulation of the GABAergic system. GAT-1 is a potential therapeutic target for managing paclitaxel-induced neuropathic pain.

Neuropathic pain: role of inflammation, immune response, and ion channel activity in central injury mechanisms

Neuropathic pain (NP) is a significant and disabling clinical problem with very few therapeutic treatment options available. A major priority is to identify the molecular mechanisms responsible for NP. Although many seemingly relevant pathways have been identified, more research is needed before effective clinical interventions can be produced. Initial insults to the nervous system, such as spinal cord injury (SCI), are often compounded by secondary mechanisms such as inflammation, the immune response, and the changing expression of receptors and ion channels. The consequences of these secondary effects myriad and compound those elicited by the primary injury. Chronic NP syndromes following SCI can greatly complicate the clinical treatment of the primary injury and result in high comorbidity. In this review, we will describe physiological outcomes associated with SCI along with some of the mechanisms known to contribute to chronic NP development.

Involvement of nitridergic and opioidergic pathways in the antinociception of gabapentin in the orofacial formalin test in mice

BACKGROUND

Pain is one of the most common problems in clinical medicine. There is considerable evidence that pharmacologic approaches are the most widely used therapeutic options to ameliorate persistent or chronic pain. In this study it was evaluated the effect of l-NAME and naltrexone in the antinociception induced by administration of gabapentin in the orofacial formalin test of mice.

METHODS

The algesiometer assay was performed by the administration of 20 μl of 2% formalin solution injected into the upper right lip of each mouse.

RESULTS

The dose of gabapentin that produces the 50% of the maximum possible effect (ED50) was significantly increased by the pretreatment with l-NAME or naltrexone.

CONCLUSIONS

These results suggest that gabapentin produce antinociception partly via the activation nitridergic pathways and opioid system.

GINIP, a Gαi-interacting protein, functions as a key modulator of peripheral GABAB receptor-mediated analgesia

One feature of neuropathic pain is a reduced GABAergic inhibitory function. Nociceptors have been suggested to play a key role in this process. However, the mechanisms behind nociceptor-mediated modulation of GABA signaling remain to be elucidated. Here we describe the identification of GINIP, a Gαi-interacting protein expressed in two distinct subsets of nonpeptidergic nociceptors. GINIP null mice develop a selective and prolonged mechanical hypersensitivity in models of inflammation and neuropathy. GINIP null mice show impaired responsiveness to GABAB, but not to delta or mu opioid receptor agonist-mediated analgesia specifically in the spared nerve injury (SNI) model. Consistently, GINIP-deficient dorsal root ganglia neurons had lower baclofen-evoked inhibition of high-voltage-activated calcium channels and a defective presynaptic inhibition of lamina IIi interneurons. These results further support the role of unmyelinated C fibers in injury-induced modulation of spinal GABAergic inhibition and identify GINIP as a key modulator of peripherally evoked GABAB-receptors signaling.

Nerve regenerative effects of GABA-B ligands in a model of neuropathic pain

Neuropathic pain arises as a direct consequence of a lesion or disease affecting the peripheral somatosensory system. It may be associated with allodynia and increased pain sensitivity. Few studies correlated neuropathic pain with nerve morphology and myelin proteins expression. Our aim was to test if neuropathic pain is related to nerve degeneration, speculating whether the modulation of peripheral GABA-B receptors may promote nerve regeneration and decrease neuropathic pain. We used the partial sciatic ligation- (PSL-) induced neuropathic model. The biochemical, morphological, and behavioural outcomes of sciatic nerve were analysed following GABA-B ligands treatments. Simultaneous 7-days coadministration of baclofen (10 mg/kg) and CGP56433 (3 mg/kg) alters tactile hypersensitivity. Concomitantly, specific changes of peripheral nerve morphology, nerve structure, and myelin proteins (P0 and PMP22) expression were observed. Nerve macrophage recruitment decreased and step coordination was improved. The PSL-induced changes in nociception correlate with altered nerve morphology and myelin protein expression. Peripheral synergic effects, via GABA-B receptor activation, promote nerve regeneration and likely ameliorate neuropathic pain.

The edible brown seaweed Ecklonia cava reduces hypersensitivity in postoperative and neuropathic pain models in rats

The current study was designed to investigate whether edible brown seaweed Ecklonia cava extracts exhibits analgesic effects in plantar incision and spared nerve injury (SNI) rats. To evaluate pain-related behavior, we performed the mechanical withdrawal threshold (MWT) and thermal hypersensitivity tests measured by von Frey filaments and a hot/cold plate analgesia meter. Pain-related behavior was also determined through analysis of ultrasonic vocalization. The results of experiments showed MWT values of the group that was treated with E. cava extracts by 300 mg/kg significantly increased; on the contrary, number of ultrasonic distress vocalization of the treated group was reduced at 6 h and 24 h after plantar incision operation (62.8%, p < 0.05). Moreover, E. cava 300 mg/kg treated group increased the paw withdrawal latency in hot-and cold-plate tests in the plantar incision rats. After 15 days of continuous treatment with E. cava extracts at 300 mg/kg, the treated group showed significantly alleviated SNI-induced hypersensitivity response by MWT compared with the control group. In conclusion, these results suggest that E. cava extracts have potential analgesic effects in the case of postoperative pain and neuropathic pain in rats.

Effects of repeated dosing with mechanistically distinct antinociceptive ligands in a rat model of neuropathic spinal cord injury pain

A lack of efficacy of some analgesic drugs has been previously described in rats with neuropathic spinal cord injury (SCI) pain. It has been suggested that repeated dosing in these animals over time may eventually lead to efficacy. However, it is also possible that efficacy may diminish over time with repeated dosing. This study evaluated the efficacy of various drugs upon repeated dosing over time in a rat model of SCI pain. Four weeks following an acute spinal cord compression at the mid-thoracic level, rats developed decreased hind paw withdrawal threshold, suggestive of below level neuropathic hypersensitivity. Either cannabinoid (CB) receptor agonist CP 55,940, the anticonvulsant carbamazepine or gabapentin, the antidepressant amitriptyline or vehicle was administered over a period of 7 days. Neither carbamazepine nor amitriptyline demonstrated efficacy either after a single or repeated dosing. Beginning with a 50% efficacious dose of gabapentin, the effect of gabapentin in SCI rats neither increased nor decreased over the treatment period. The antinociceptive effect of CP 55,940 was maintained for the entire treatment period, which was mediated by CB1 but not CB2 receptors. The current data suggest that sustained antinociception can be obtained with some drugs in rats with neuropathic SCI pain. Furthermore, the current data do not substantiate the notion that repeated treatment with initially ineffective drugs will eventually lead to efficacy; treatments that are not acutely effective are unlikely to demonstrate clinical efficacy.