Comparison of the novel subtype-selective GABAA receptor-positive allosteric modulator NS11394 [3'-[5-(1-hydroxy-1-methyl-ethyl)-benzoimidazol-1-yl]-biphenyl-2-carbonitrile] with diazepam, zolpidem, bretazenil, and gaboxadol in rat models of inflammatory and neuropathic pain

GABAA receptors in epilepsy: Elucidating phenotypic divergence through functional analysis of genetic variants

Normal brain function requires a tightly regulated balance between excitatory and inhibitory neurotransmissions. γ-Aminobutyric acid type A (GABAA) receptors represent the major class of inhibitory ion channels in the mammalian brain. Dysregulation of these receptors and/or their associated pathways is strongly implicated in the pathophysiology of epilepsy. To date, hundreds of different GABAA receptor subunit variants have been associated with epilepsy, making them a prominent cause of genetically linked epilepsy. While identifying these genetic variants is crucial for accurate diagnosis and effective genetic counselling, it does not necessarily lead to improved personalised treatment options. This is because the identification of a variant does not reveal how the function of GABAA receptors is affected. Genetic variants in GABAA receptor subunits can cause complex changes to receptor properties resulting in various degrees of gain-of-function, loss-of-function or a combination of both. Understanding how variants affect the function of GABAA receptors therefore represents an important first step in the ongoing development of precision therapies. Furthermore, it is important to ensure that functional data are produced using methodologies that allow genetic variants to be classified using clinical guidelines such as those developed by the American College of Medical Genetics and Genomics. This article will review the current knowledge in the field and provide recommendations for future functional analysis of genetic GABAA receptor variants.

Antinociceptive Effects of α2/α3-Subtype-Selective GABAA Receptor Positive Allosteric Modulators KRM-II-81 and NS16085 in Male Rats: Behavioral Specificity

Recent studies suggest that among the gamma-aminobutyric acid type A (GABAA)receptor subtype heterogeneity, α2/α3 subunits of GABAA receptors mediate pain processing. Therefore, α2/α3 subtype-selective GABAA receptor-positive allosteric modulators (PAMs) may be candidate analgesics. Antinociceptive effects of α2/α3 subtype-selective GABAA receptor PAMs have been reported, but the behavioral effects of these compounds have not been systematically evaluated. This study examined the behavioral effects of two α2/α3 subtype-selective GABAA receptor PAMs, KRM-II-81 and NS16085, in male rats. The antinociceptive effects of KRM-II-81 and NS16085 were examined using rat models of inflammatory (complete Freund's adjuvant) and neuropathic pain (chronic constriction injury). The effect of KRM-II-81 on affective pain was measured using the place escape/avoidance paradigm (PEAP). Rate-response of food-maintained operant responding, horizontal wire test, and the spontaneous alternation T-maze were assessed to study the side-effect profiles of KRM-II-81 and NS16085. The benzodiazepine midazolam was used as a comparator in these studies. KRM-II-81 and NS16085 attenuated mechanical allodynia but not thermal hyperalgesia in both pain states, and their effects were attenuated by the benzodiazepine receptor antagonist flumazenil. KRM-II-81 attenuated affective pain-related behavior in the PEAP test. In the operant responding procedure and horizontal wire test, only midazolam produced significant effects at the dose that produced maximal antinociception. In the T-maze assay, only midazolam significantly decreased the percentage of alternation at an antinociceptive dose. Thus, KRM-II-81 and NS16085 but not midazolam selectively produced antinociceptive effects. Collectively, these data suggest that α2/α3 subtype-selective GABAA PAMs could be a novel class of analgesics and warrant further investigation. SIGNIFICANCE STATEMENT: This study demonstrates that α2/α3 subtype-selective GABAA PAMs KRM-II-81 and NS16085 produce selective antinociceptive effects devoid of sedation, myorelaxation, and cognitive impairment in two rat models of persistent pain. This study supports the development of α2/α3 subtype-selective GABAA PAMs, rather than classical benzodiazepines, as safe and novel analgesics for pain management.

Effect of intramuscular diazepam infusion on herpes zoster-related pain in older patients: a randomized, double-blind, placebo-controlled trial

OBJECTIVES

This study evaluated the effectiveness, psychological effects, and sleep quality using intramuscular diazepam infusion compared with placebo in patients with herpes zoster (HZ)-related pain.

METHODS

The patients were randomized to either the diazepam or control group. The diazepam group received an intramuscular injection of diazepam for 3 consecutive days, while the control group received an intramuscular injection of 0.9% normal saline. The primary outcome was pain relief on posttreatment day 4, as measured using the Visual Analog Scale (VAS). Moreover, anxiety and depression were evaluated using the Generalized Anxiety Disorder-7 (GAD7) and Patient Health Questionnaire-9 (PHQ9), respectively. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI).

RESULTS

In total, 78 patients were enrolled in the trial. The mean differences in VAS scores between the two groups were 0.62 (P = 0.049) on posttreatment day 3 and 0.66 (P = 0.037) on posttreatment day 4. The effective rates of pain management in the diazepam group ranged from 10.26 to 66.67%, which were higher than those in the control group on posttreatment days 3 and 4 (P < 0.05). The mean difference in PSQI scores between the diazepam and control groups was 1.36 (P = 0.034) on posttreatment day 7. No differences were found in the incidence of analgesia-adverse 1reactions between the diazepam and placebo groups.

CONCLUSIONS

The intramuscular injection of diazepam for 3 consecutive days provides effective pain management and improves the quality of life. Our study suggests that diazepam is more effective than the placebo in patients with HZ-related pain.

TRIAL REGISTRATION

The study was prospectively registered at https://www.isrctn.com/trialist(Registration date: 24/01/2018; Trial ID: ISRCTN12682696).

Current status of GABA receptor subtypes in analgesia

Gamma-aminobutyric acid (GABA), a non-protein-producing amino acid synthesized from the excitatory amino acid glutamate via the enzyme glutamic acid decarboxylase, is extensively found in microorganisms, plants and vertebrates, and is abundantly expressed in the spinal cord and brain. It is the major inhibitory neurotransmitter in the mammalian nervous system. GABA plays crucial roles in the regulation of synaptic transmission, the promotion of neuronal development and relaxation, and the prevention of insomnia and depression. As the major inhibitory neurotransmitter, GABA plays pivotal roles in the regulation of pain sensation, which is initiated by the activation of peripheral nociceptors and transmitted to the spinal cord and brain along nerves. GABA exerts these roles by directly acting on three types of receptors: ionotropic GABAA and GABAC receptors and G protein-coupled GABAB receptor. The chloride-permeable ion channel receptors GABAA and GABAC mediate fast neurotransmission, while the metabotropic GABAB receptor mediates slow effect. Different GABA receptors regulate pain sensation via different signaling pathways. Here we highlight recent updates on the involvement of specific GABA receptors and their subtypes in the process of pain sensation. Further understanding of different GABA receptors and signaling pathways in pain sensation will benefit the development of novel analgesics for pain management by targeting specific GABA receptor subtypes and signaling pathways.

Abnormal Expression of Synaptic and Extrasynaptic GABAA Receptor Subunits in the Dystrophin-Deficient mdx Mouse

Duchenne muscular dystrophy (DMD) is a neurodevelopmental disorder primarily caused by the loss of the full-length Dp427 dystrophin in both muscle and brain. The basis of the central comorbidities in DMD is unclear. Brain dystrophin plays a role in the clustering of central gamma-aminobutyric acid A receptors (GABA_ARs), and its loss in the mdx mouse alters the clustering of some synaptic subunits in central inhibitory synapses. However, the diversity of GABAergic alterations in this model is still fragmentary. In this study, the analysis of in vivo PET imaging of a benzodiazepine-binding site radioligand revealed that the global density of central GABA_ARs is unaffected in mdx compared with WT mice. In contrast, semi-quantitative immunoblots and immunofluorescence confocal imaging in tissue sections revealed complex and differential patterns of alterations of the expression levels and/or clustered distribution of a variety of synaptic and extrasynaptic GABA_AR subunits in the hippocampus, cerebellum, cortex, and spinal cord. Hence, dystrophin loss not only affects the stabilization of synaptic GABA_ARs but also influences the subunit composition of GABA_ARs subtypes at both synaptic and extrasynaptic sites. This study provides new molecular outcome measures and new routes to evaluate the impact of treatments aimed at compensating alterations of the nervous system in DMD.

GABAkines - Advances in the discovery, development, and commercialization of positive allosteric modulators of GABAA receptors

Positive allosteric modulators of γ-aminobutyric acid-A (GABA_A) receptors or GABAkines have been widely used medicines for over 70 years for anxiety, epilepsy, sleep, and other disorders. Traditional GABAkines like diazepam have safety and tolerability concerns that include sedation, motor-impairment, respiratory depression, tolerance and dependence. Multiple GABAkines have entered clinical development but the issue of side-effects has not been fully solved. The compounds that are presently being developed and commercialized include several neuroactive steroids (an allopregnanolone formulation (brexanolone), an allopregnanolone prodrug (LYT-300), Sage-324, zuranolone, and ganaxolone), the α2/3-preferring GABAkine, KRM-II-81, and the α2/3/5-preferring GABAkine PF-06372865 (darigabat). The neuroactive steroids are in clinical development for post-partum depression, intractable epilepsy, tremor, status epilepticus, and genetic epilepsy disorders. Darigabat is in development for epilepsy and anxiety. The imidazodiazepine, KRM-II-81 is efficacious in animal models for the treatment of epilepsy and post-traumatic epilepsy, acute and chronic pain, as well as anxiety and depression. The efficacy of KRM-II-81 in models of pharmacoresistant epilepsy, preventing the development of seizure sensitization, and in brain tissue of intractable epileptic patients bodes well for improved therapeutics. Medicinal chemistry efforts are also ongoing to identify novel and improved GABAkines. The data document gaps in our understanding of the molecular pharmacology of GABAkines that drive differential pharmacological profiles, but emphasize advancements in the ability to successfully utilize GABA_A receptor potentiation for therapeutic gain in neurology and psychiatry.

Neuropathic pain modeling: Focus on synaptic and ion channel mechanisms

Animal models of pain consist of modeling a pain-like state and measuring the consequent behavior. The first animal models of neuropathic pain (NP) were developed in rodents with a total lesion of the sciatic nerve. Later, other models targeting central or peripheral branches of nerves were developed to identify novel mechanisms that contribute to persistent pain conditions in NP. Objective assessment of pain in these different animal models represents a significant challenge for pre-clinical research. Multiple behavioral approaches are used to investigate and to validate pain phenotypes including withdrawal reflex to evoked stimuli, vocalizations, spontaneous pain, but also emotional and affective behaviors. Furthermore, animal models were very useful in investigating the mechanisms of NP. This review will focus on a detailed description of rodent models of NP and provide an overview of the assessment of the sensory and emotional components of pain. A detailed inventory will be made to examine spinal mechanisms involved in NP-induced hyperexcitability and underlying the current pharmacological approaches used in clinics with the possibility to present new avenues for future treatment. The success of pre-clinical studies in this area of research depends on the choice of the relevant model and the appropriate test based on the objectives of the study.

Synergistic antihyperalgesic and antinociceptive effects of morphine and methyl 8-ethynyl-6-(pyridin-2-yl)-4H-benzo[f]imidazo[1,5-a][1,4]diazepine-3-carboxylate (MP-III-024): a positive allosteric modulator at α2GABAA and α3GABAA receptors

RATIONALE

Opioid and GABA_A receptors are both located in central nociceptive pathways, and compounds that activate these receptors have pain-relieving properties. To date, the interactive effects of concurrent administration of these compounds in preclinical models of pain-like behaviors have not been assessed.

OBJECTIVE

The purpose of this study was to examine the interactive effects of the μ-opioid agonist morphine and the α2GABA_A and α3GABA_A receptor positive allosteric modulator methyl 8-ethynyl-6-(pyridin-2-yl)-4H-benzo[f]imidazo[1,5-a][1,4]diazepine-3-carboxylate (MP-III-024) in preclinical models of mechanical hyperalgesia and thermal nociception.

METHODS

The antihyperalgesic and antinociceptive effects of morphine and MP-III-024 administered alone were assessed initially, followed by fixed-ratio mixtures of MP-III-024/morphine combinations. Drug interaction data were analyzed using isobolographic and dose-addition analyses. All studies were conducted in male CD-1 mice.

RESULTS

In the assay of mechanical hyperalgesia, each compound produced dose-dependent antihyperalgesic effects, whereas only morphine was effective on thermal nociception. Fixed-ratio mixtures of MP-III-024/morphine were also dose-dependently effective in both procedures. These drug combination studies revealed that morphine and MP-III-024 produced supra-additive (synergistic) effects in both assays, depending on their relative proportions.

CONCLUSIONS

These results demonstrate an interaction between α2GABA_A and α3GABA_A receptor- and μ-opioid receptor-mediated signals and suggest that combination therapy may be useful for the treatment of pain-related disorders.

Behavioral Battery for Testing Candidate Analgesics in Mice. I. Validation with Positive and Negative Controls

This study evaluated a battery of pain-stimulated, pain-depressed, and pain-independent behaviors for preclinical pharmacological assessment of candidate analgesics in mice. Intraperitoneal injection of dilute lactic acid (IP acid) served as an acute visceral noxious stimulus to produce four pain-related behaviors in male and female ICR mice: stimulation of 1) stretching, 2) facial grimace, 3) depression of rearing, and 4) depression of nesting. Additionally, nesting and locomotion in the absence of the noxious stimulus were used to assess pain-independent drug effects. These six behaviors were used to compare effects of two mechanistically distinct but clinically effective positive controls (ketoprofen and oxycodone) and two negative controls that are not clinically approved as analgesics but produce either general motor depression (diazepam) or motor stimulation (amphetamine). We predicted that analgesics would alleviate all IP acid effects at doses that did not alter pain-independent behaviors, whereas negative controls would not. Consistent with this prediction, ketoprofen (0.1-32 mg/kg) produced the expected analgesic profile, whereas oxycodone (0.32-3.2 mg/kg) alleviated all IP acid effects except depression of rearing at doses lower than those that altered pain-independent behaviors. For the negative controls, diazepam (1-10 mg/kg) failed to block IP acid-induced depression of either rearing or nesting and only decreased IP acid-stimulated behaviors at doses that also decreased pain-independent behaviors. Amphetamine (0.32-3.2 mg/kg) alleviated all IP acid effects but only at doses that also stimulated locomotion. These results support utility of this model as a framework to evaluate candidate-analgesic effects in a battery of complementary pain-stimulated, pain-depressed, and pain-independent behavioral endpoints. SIGNIFICANCE STATEMENT: Preclinical assays of pain and analgesia often yield false-positive effects with candidate analgesics. This study used two positive-control analgesics (ketoprofen, oxycodone) and two active negative controls (diazepam, amphetamine) to validate a strategy for distinguishing analgesics from nonanalgesics by profiling drug effects in a battery of complementary pain-stimulated, pain-depressed, and pain-independent behaviors in male and female mice.

The α2/α3GABAA receptor modulator TPA023B alleviates not only the sensory but also the tonic affective component of chronic pain in mice

ABSTRACT

Diminished synaptic inhibition in the spinal dorsal horn is a major contributor to pathological pain syndromes of neuropathic or inflammatory origin. Drugs that enhance the activity of dorsal horn α2/α3GABAARs normalize exaggerated nociceptive responses in rodents with neuropathic nerve lesions or peripheral inflammation but lack most of the typical side effects of less specific GABAergic drugs. It is however still unknown whether such drugs also reduce the clinically more relevant conscious perception of pain. Here, we investigated the effects of the α2/α3GABAAR subtype-selective modulator TPA023B on the tonic aversive component of pain in mice with peripheral inflammation or neuropathy. In neuropathic mice with a chronic constriction injury of the sciatic nerve, TPA023B not only reversed hyperalgesia to tactile and heat stimuli but also was highly effective in the conditioned place preference test. In the formalin test, TPA023B not only reduced licking of the injected paw but also reversed facial pain expression scores in the mouse grimace scale assay. Taken together, our results demonstrate that α2/α3GABAA receptor subtype-selective modulators not only reduce nociceptive withdrawal responses but also alleviate the tonic aversive components of chronic pain.

A Thermal Place Preference Test for Discovery of Neuropathic Pain Drugs

Developing potent non-opioid pain medications is an integral part of the battle to conquer both chronic pain and the current opioid crisis. Although most screening approaches use in vitro surrogate targets, in vivo screening of analgesic candidates is a necessary preclinical step in drug discovery. Here, we report the design of a new automated behavioral testing apparatus based on the principle of a thermal place preference test (TPPT). This new design can detect, quantify, and differentiate behavioral responses to cold stimuli between sham and chronic constriction injury (CCI) rodents with up to 12 animals tested simultaneously. At an optimized temperature pair of 12.5 °C vs 30.0 °C (±0.5 °C), the TPPT design has captured the antinociceptive effects of morphine and pregabalin on CCI rats in individual 10 min tests. Moreover, it can differentiate analgesic effects by morphine or pregabalin from anxiolytic effects by diazepam. The results, along with the relatively low cost to construct the apparatus and moderately high throughput, make our TPPT design applicable for behavioral studies of chronic pain in rodents and for high-throughput in vivo screening of the next generation of pain medications.

Pentameric Ligand-Gated Ion Channels as Pharmacological Targets Against Chronic Pain

Chronic pain is a common detrimental condition that affects around 20% of the world population. The current drugs to treat chronic pain states, especially neuropathic pain, have a limited clinical efficiency and present significant adverse effects that complicates their regular use. Recent studies have proposed new therapeutic strategies focused on the pharmacological modulation of G-protein-coupled receptors, transporters, enzymes, and ion channels expressed on the nociceptive pathways. The present work intends to summarize recent advances on the pharmacological modulation of pentameric ligand-gated ion channels, which plays a key role in pain processing. Experimental data have shown that novel allosteric modulators targeting the excitatory nicotinic acetylcholine receptor, as well as the inhibitory GABA_A and glycine receptors, reverse chronic pain-related behaviors in preclinical assays. Collectively, these evidences strongly suggest the pharmacological modulation of pentameric ligand-gated ion channels is a promising strategy towards the development of novel therapeutics to treat chronic pain states in humans.

Effects of the α2/α3-subtype-selective GABAA receptor positive allosteric modulator KRM-II-81 on pain-depressed behavior in rats: comparison with ketorolac and diazepam

This study examined effects of the α2/α3-subtype-selective GABAA receptor positive allosteric modulator KRM-II-81 in an assay of pain-related behavioral depression. Adult, male Sprague-Dawley rats responded for electrical brain stimulation in a frequency-rate intracranial self-stimulation (ICSS) procedure. Intraperitoneal injection of 1.8% lactic acid served as an acute noxious stimulus to depress ICSS. Effects of KRM-II-81 were evaluated in the absence and presence of the acid noxious stimulus. The NSAID ketorolac and the benzodiazepine diazepam were tested as comparators. Neither ketorolac nor KRM-II-81 altered ICSS in the absence of the acid noxious stimulus; however, diazepam produced facilitation consistent with its abuse liability. Ketorolac blocked acid-induced depression of ICSS, and effects of 1.0 mg/kg ketorolac lasted for at least 5 h. KRM-II-81 (1.0 mg/kg) produced significant antinociception after 30 min that dissipated by 60 min. Diazepam also attenuated acid-depressed ICSS, but only at doses that facilitated ICSS when administered alone. The lack of ketorolac or KRM-II-81 effects on ICSS in the absence of the acid noxious stimulus suggests low abuse liability for both compounds. The effectiveness of ketorolac to block acid-induced ICSS depression agrees with clinical analgesic efficacy of ketorolac. KRM-II-81 produced significant but less consistent and shorter-acting antinociception than ketorolac.

Natural product incarvillateine aggravates epileptic seizures by inhibiting GABAA currents

A natural monoterpene alkaloid incarvillateine isolated from the plant Incarvillea sinensis is known to relieve inflammatory and neuropathic pain. However, the molecular target for the action of incarvillateine remains elusive. Here, we report that incarvillateine exacerbates epileptic seizures by inhibiting subtypes of γ-Aminobutyric acid type A (GABA_A) receptors. Two-electrode voltage clamp recordings of α₁β₃γ₂, α₂β₃γ₂, α₃β₃γ₂ and α₅β₃γ₂ subtypes expressed in Xenopus oocytes revealed that incarvillateine inhibited the GABA_A currents with IC₅₀ of 25.1 μM, 43.1 μM, 105.1 μM and 93.7 μM, respectively. Whole-cell patch clamp recordings of hippocampal slices confirmed that incarvillateine inhibited spontaneous inhibitory postsynaptic currents (IPSCs), and miniature IPSCs and tonic currents. Moreover, inhibition of GABA_A currents and spontaneous IPSCs by incarvillateine persisted even in the presence of blockers of adenosine receptors. In addition, incarvillateine enhanced epileptic discharges induced by Mg²⁺-free artificial cerebrospinal fluid (ACSF) in hippocampal slices. Furthermore, intracerebral ventricular injections of incarvillateine increased the severity of seizures induced by kainic acid in a dose-dependent manner. Taken together, our data demonstrate that incarvillateine aggravates seizures by inhibition of GABA_A currents and GABAergic synaptic transmissions.

Human pharmacology of positive GABA-A subtype-selective receptor modulators for the treatment of anxiety

Anxiety disorders arise from disruptions among the highly interconnected circuits that normally serve to process the streams of potentially threatening stimuli. The resulting imbalance among these circuits can cause a fundamental misinterpretation of neural sensory information as threatening and can lead to the inappropriate emotional and behavioral responses observed in anxiety disorders. There is considerable preclinical evidence that the GABAergic system, in general, and its α2- and/or α5-subunit-containing GABA(A) receptor subtypes, in particular, are involved in the pathophysiology of anxiety disorders. However, the clinical efficacy of GABA-A α2-selective agonists for the treatment of anxiety disorders has not been unequivocally demonstrated. In this review, we present several human pharmacological studies that have been performed with the aim of identifying the pharmacologically active doses/exposure levels of several GABA-A subtype-selective novel compounds with potential anxiolytic effects. The pharmacological selectivity of novel α2-subtype-selective GABA(A) receptor partial agonists has been demonstrated by their distinct effect profiles on the neurophysiological and neuropsychological measurements that reflect the functions of multiple CNS domains compared with those of benzodiazepines, which are nonselective, full GABA(A) agonists. Normalizing the undesired pharmacodynamic side effects against the desired on-target effects on the saccadic peak velocity is a useful approach for presenting the pharmacological features of GABA(A)-ergic modulators. Moreover, combining the anxiogenic symptom provocation paradigm with validated neurophysiological and neuropsychological biomarkers may provide further construct validity for the clinical effects of novel anxiolytic agents. In addition, the observed drug effects on serum prolactin levels support the use of serum prolactin levels as a complementary neuroendocrine biomarker to further validate the pharmacodynamic measurements used during the clinical pharmacological study of novel anxiolytic agents.

A randomised, placebo-controlled clinical trial with the α2/3/5 subunit selective GABAA positive allosteric modulator PF-06372865 in patients with chronic low back pain

The effect of PF-06372865, a subtype-selective positive allosteric modulator of the γ-aminobutyric acid type A (GABAA) receptor, on chronic low back pain was investigated in a randomised, placebo- and active-controlled phase 2 clinical trial. The parallel treatment group trial consisted of a 1-week single-blind placebo run in the phase, followed by 4-week double-blind treatment. Patients were randomised to receive either PF-06372865, naproxen, or placebo twice a day for 4 weeks. The primary end point was the numerical rating score of low back pain intensity after 4 weeks of active treatment. Secondary end points included the Roland Morris Disability Questionnaire and the Hopkins Verbal Learning Test-Revised. The trial had predefined decision rules based on the probability that PF-06372865 was better than placebo. The study was stopped at the interim analysis for futility. At this time, a total of 222 patients were randomised and the mean PF-06372865 4-week response on the low back pain intensity was 0.16 units higher (worse) than placebo (90% confidence interval -0.28 to 0.60). There were small, statistically significant reductions in the delayed recall test score with PF-06372865, as measured by Hopkins Verbal Learning Test-Revised. The effects of naproxen were in line with expectations. PF-06372865 was well tolerated. The most common treatment-related adverse events in the PF-06372865 arm were somnolence (5 mild and 4 moderate), dizziness (2 mild and 3 moderate), and nausea (2 mild). Although the reason for the lack of analgesic effect is not completely clear, it may be a result of not achieving sufficient receptor occupancy to drive efficacy.

Analgesic potential of PF-06372865, an α2/α3/α5 subtype-selective GABAA partial agonist, in humans

BACKGROUND

This study investigated the analgesic effects of two doses (15 and 65 mg) of PF-06372865, a novel α2/α3/α5 gamma-aminobutyric acid A (GABA_A) subunit selective partial positive allosteric modulator (PAM), compared with placebo and pregabalin (300 mg) as a positive control.

METHODS

We performed a randomised placebo-controlled crossover study (NCT02238717) in 20 healthy subjects, using a battery of pain tasks (electrical, pressure, heat, cold and inflammatory pain, including a paradigm of conditioned pain modulation). Pharmacodynamic measurements were performed at baseline and up to 10 h after dose.

RESULTS

A dose of 15 mg PF-06372865 increased pain tolerance thresholds (PTTs) for pressure pain at a ratio of 1.11 (90% confidence interval [CI]: 1.02, 1.22) compared with placebo. A dose of 65 mg PF-06372865 led to an increase in PTT for the cold pressor at a ratio of 1.17 (90% CI: 1.03, 1.32), and pressure pain task: 1.11 (90% CI: 1.01, 1.21). Pregabalin showed an increase in PTT for pressure pain at a ratio of 1.15 (95% CI: 1.06, 1.26) and cold pressor task: 1.31 (90% CI: 1.16, 1.48).

CONCLUSION

We conclude that PF-06372865 has analgesic potential at doses that do not induce significant sedation or other intolerable adverse events limiting its clinical use. In addition, the present study established the potential role for this battery of pain tasks as a tool in the development of analgesics with a novel mechanism of action, for the treatment of various pain states including neuropathic pain and to establish proof-of-concept.

CLINICAL TRIALS REGISTRATION

NCT0223871.

Improved Synthesis of Anxiolytic, Anticonvulsant and Antinociceptive α2/α3-GABA(A)ergic Receptor Subtype Selective Ligands as Promising Agents to Treat Anxiety, Epilepsy, as well as Neuropathic Pain

An improved synthesis of the anxiolytic, anticonvulsant and antinociceptive compounds: Hz-166, and its bioisosteres 1,2,4-oxadiazole (MP-III-080) and 1,3-oxazole (KRM-II-81) were executed in higher yields and with more facile purification methods (crystallization, etc.) in multigram quantities without column chromatography. In the synthesis of KRM-II-81, an alternative procedure was employed using the selective reducing reagent, potassium diisobutyl-t-butoxy aluminum hydride (PDBBA), to prepare the desired C(3)-aldehyde in the absence of [N(5)-C(6)] imine reduction in good yield on 20 gram scale.

Acute inflammation reveals GABAA receptor-mediated nociception in mouse dorsal root ganglion neurons via PGE2 receptor 4 signaling

Gamma‐aminobutyric acid (GABA) depolarizes dorsal root ganglia (DRG) primary afferent neurons through activation of Cl⁻ permeable GABA_A receptors but the physiologic role of GABA_A receptors in the peripheral terminals of DRG neurons remains unclear. In this study, we investigated the role of peripheral GABA_A receptors in nociception using a mouse model of acute inflammation. In vivo, peripheral administration of the selective GABA_A receptor agonist muscimol evoked spontaneous licking behavior, as well as spinal wide dynamic range (WDR) neuron firing, after pre‐conditioning with formalin but had no effect in saline‐treated mice. GABA_A receptor‐mediated pain behavior after acute formalin treatment was abolished by the GABA_A receptor blocker picrotoxin and cyclooxygenase inhibitor indomethacin. In addition, treatment with prostaglandin E2 (PGE₂) was sufficient to reveal muscimol‐induced licking behavior. In vitro, GABA induced sub‐threshold depolarization in DRG neurons through GABA_A receptor activation. Both formalin and PGE₂ potentiated GABA‐induced Ca²⁺ transients and membrane depolarization in capsaicin‐sensitive nociceptive DRG neurons; these effects were blocked by the prostaglandin E2 receptor 4 (EP4) antagonist AH23848 (10 μmol/L). Furthermore, potentiation of GABA responses by PGE₂ was prevented by the selective Na_v1.8 antagonist A887826 (100 nmol/L). Although the function of the Na⁺‐K⁺‐2Cl^‐ co‐transporter NKCC1 was required to maintain the Cl^‐ ion gradient in isolated DRG neurons, NKCC1 was not required for GABA_A receptor‐mediated nociceptive behavior after acute inflammation. Taken together, these results demonstrate that GABA_A receptors may contribute to the excitation of peripheral sensory neurons in inflammation through a combined effect involving PGE₂‐EP4 signaling and Na⁺ channel sensitization.

Anatomical recovery of the GABAergic system after a complete spinal cord injury in lampreys

Lampreys recover locomotion spontaneously several weeks after a complete spinal cord injury. Dysfunction of the GABAergic system following SCI has been reported in mammalian models. So, it is of great interest to understand how the GABAergic system of lampreys adapts to the post-injury situation and how this relates to spontaneous recovery. The spinal cord of lampreys contains 3 populations of GABAergic neurons and most of the GABAergic innervation of the spinal cord comes from these local cells. GABAB receptors are expressed in the spinal cord of lampreys and they play important roles in the control of locomotion. The aims of the present study were to quantify: 1) the changes in the number of GABAergic neurons and innervation of the spinal cord and 2) the changes in the expression of the gabab receptor subunits b1 and b2 in the spinal cord of the sea lamprey after SCI. We performed complete spinal cord transections at the level of the fifth gill of mature larval lampreys and GABA immunohistochemistry or gabab in situ hybridization experiments. Animals were analysed up to 10 weeks post-lesion (wpl), when behavioural analyses showed that they recovered normal appearing locomotion (stage 6 in the Ayer's scale of locomotor recovery). We observed a significant decrease in the number of GABA-ir cells and fibres 1 h after lesion both rostral and caudal to the lesion site. GABA-ir cell numbers and innervation were recovered to control levels 1 to 2 wpl. At 1, 4 and 10 wpl the expression of gabab1 and gabab2 transcripts was significantly decreased in the spinal cord compared to control un-lesioned animals. This is the first study reporting the quantitative long-term changes in the number of GABAergic cells and fibres and in the expression of gabab receptors in the spinal cord of any vertebrate following a traumatic SCI. Our results show that in lampreys there is a full recovery of the GABAergic neurons and a decrease in the expression of gabab receptors when functional recovery is achieved.

Antinociceptive Effects of a Novel α2/α3-Subtype Selective GABAA Receptor Positive Allosteric Modulator

Pain remains a challenging clinical condition and spinal GABA_A receptors are crucial modulators of pain processing. α2/α3-subtype GABA_A receptors mediate the analgesic actions of benzodiazepines. Positive allosteric modulators (PAMs) at α2/α3-subtype GABA_A receptors may have analgesic potential. Here we report a new selective α2/α3-subtype GABA_A receptor PAM in in vitro and in vivo pain assays. KRM-II-81 demonstrated similar efficacy at α1/α2/α3 GABA_A receptors and negligible efficacy at α4/α5/α6 GABA_A receptors, with α2 and α3-subtypes being 17- and 28-fold more potent than α1 subtypes in HEK-293T cells expressing GABA_A receptors with different α subunits. In contrast, KRM-II-18B showed significant efficacy at α1/α2/α3/ α5 subtypes, with similar potency at α1/α2/α3 subtypes. Both PAMs and morphine dose-dependently decreased 0.6% acetic acid- and 0.32% lactic acid-induced writhing. The effects of both PAMs were reversed by the benzodiazepine receptor antagonist flumazenil, confirming their action at the benzodiazepine binding site of GABA_A receptors. Both PAMS and morphine all dose-dependently reversed 0.32% lactic acid (but not 0.6% acetic acid) induced suppression of nesting behavior. Acetaminophen, but not the PAMs, reversed acid-depressed locomotor activity. Combined, these findings suggest that KRM-II-81 is a selective α2/α3 subtype GABA_A PAM with significant antinociceptive effects in chemical stimulation-induced pain in mice.

Pharmacological and antihyperalgesic properties of the novel α2/3 preferring GABAA receptor ligand MP-III-024

γ-Aminobutyric acid type A (GABAA) receptors are located in spinal nociceptive circuits where they modulate the transmission of pain sensory signals from the periphery to higher centers. Benzodiazepine-type drugs bind to GABAA receptors containing α1, α2, α3, and α5 subunits (α1GABAA, α2GABAA, α3GABAA and α5GABAA receptors, respectively) through which they inhibit the transmission of these signals. In the present study we describe the novel benzodiazepine site positive allosteric modulator modulator methyl 8-ethynyl-6-(pyridin-2-yl)-4H-benzo[f]imidazo[1,5-a][1,4]diazepine-3-carboxylate (MP-III-024). MP-III-024 displayed preference for α2GABAA and α3GABAA receptors relative to α1GABAA and α5GABAA receptors as well as an improved metabolic profile relative to subtype-selective positive modulators that are available currently. Administration of MP-III-024 resulted in a dose- and time-dependent reversal of mechanical hyperalgesia. On locomotor activity and schedule-controlled responding, MP-III-024 was ineffective across the doses tested. These data provide further evidence that α2GABAA and α3GABAA receptors play an important role in the antihyperalgesic effects and may not be involved in some of the undesired effects of benzodiazepine-like drugs. Further, these findings suggest that MP-III-024 is a suitable research tool for investigating the role of α2GABAA and α3GABAA receptors in the behavioral properties of benzodiazepine-like drugs in mice.

Transmission pathways and mediators as the basis for clinical pharmacology of pain

INTRODUCTION

Mediators in pain transmission are the targets of a multitude of different analgesic pharmaceuticals. This review explores the most significant mediators of pain transmission as well as the pharmaceuticals that act on them. Areas covered: The review explores many of the key mediators of pain transmission. In doing so, this review uncovers important areas for further research. It also highlights agents with potential for producing novel analgesics, probes important interactions between pain transmission pathways that could contribute to synergistic analgesia, and emphasizes transmission factors that participate in transforming acute injury into chronic pain. Expert commentary: This review examines current pain research, particularly in the context of identifying novel analgesics, highlighting interactions between analgesic transmission pathways, and discussing factors that may contribute to the development of chronic pain after an acute injury.

Design, synthesis and biological evaluation of novel analgesic agents targeting both cyclooxygenase and TRPV1

Multitarget-directed ligands might offer certain advantages over traditional single-target drugs and/or drug combinations. In the present study, a series of novel analgesic agents targeting both cyclooxygenase and TRPV1 were prepared and evaluated in an effort to optimize properties of previously described lead compounds from piperazine, ethanediamine cores. These compounds were evaluated for antagonism of hTRPV1 activation by capsaicin and the ability to inhibit Ovine COX-1 and human recombinant COX-2 in vitro. The favorable potentials of these test compounds were further characterized in preliminary analgesic and side-effects tests in vivo. On the basis of comprehensive evaluations, compound 8d which showed strong TRPV1 antagonistic activity, middle COX-2 inhibition, weak ulcerogenic action and had no hyperthermia side-effect was considered as a safe candidate for the further development of analgesic drugs.

NS383 Selectively Inhibits Acid-Sensing Ion Channels Containing 1a and 3 Subunits to Reverse Inflammatory and Neuropathic Hyperalgesia in Rats

AIMS

Here, we investigate the pharmacology of NS383, a novel small molecule inhibitor of acid-sensing ion channels (ASICs).

METHODS

ASIC inhibition by NS383 was characterized in patch-clamp electrophysiological studies. Analgesic properties were evaluated in four rat behavioral models of pain.

RESULTS

NS383 inhibited H(+)-activated currents recorded from rat homomeric ASIC1a, ASIC3, and heteromeric ASIC1a+3 with IC50 values ranging from 0.61 to 2.2 μM. However, NS383 was completely inactive at homomeric ASIC2a. Heteromeric receptors containing AISC2a, such as ASIC1a+2a and ASIC2a+3, were only partially inhibited, presumably as a result of stoichiometry-dependent binding. NS383 (10-60 mg/kg, i.p.), amiloride (50-200 mg/kg, i.p.), acetaminophen (100-400 mg/kg, i.p.), and morphine (3-10 mg/kg, i.p.) all dose-dependently attenuated nocifensive behaviors in the rat formalin test, reversed pathological inflammatory hyperalgesia in complete Freund's adjuvant-inflamed rats, and reversed mechanical hypersensitivity in the chronic constriction injury model of neuropathic pain. However, in contrast to acetaminophen and morphine, motor function was unaffected by NS383 at doses at least 8-fold greater than those that were effective in pain models, whilst analgesic doses of amiloride were deemed to be toxic.

CONCLUSIONS

NS383 is a potent and uniquely selective inhibitor of rat ASICs containing 1a and/or 3 subunits. It is well tolerated and capable of reversing pathological painlike behaviors, presumably via peripheral actions, but possibly also via actions within central pain circuits.

GABAergic modulation in central sensitization in humans: a randomized placebo-controlled pharmacokinetic-pharmacodynamic study comparing clobazam with clonazepam in healthy volunteers

Positive allosteric modulators of GABAA receptors (GAMs) acting at specific subtypes of GABAA receptors effectively restore compromised spinal pain control in rodents. Studies addressing a similar antihyperalgesic effect in humans are sparse and are hampered by sedative effects of nonselective GAMs available for use in humans. We present results from a randomized controlled double-blind crossover study in 25 healthy volunteers, which addressed potential antihyperalgesic actions of clobazam (CBZ) and clonazepam (CLN) at mildly sedating equianticonvulsive doses. Clobazam was chosen because of its relatively low sedative properties and CLN because of its use in neuropathic pain. Tolterodine (TLT) was used as an active placebo. The primary outcome parameter was a change in the area of cutaneous UVB irradiation-induced secondary hyperalgesia (ASH), which was monitored for 8 hours after drug application. Sedative effects were assessed in parallel to antihyperalgesia. Compared with TLT, recovery from hyperalgesia was significantly faster in the CBZ and CLN groups (P = 0.009). At the time point of maximum effect, the rate of recovery from hyperalgesia was accelerated by CBZ and CLN, relative to placebo by 15.7% (95% confidence interval [CI] 0.8-30.5), P = 0.040, and 28.6% (95% CI 4.5-52.6), P = 0.022, respectively. Active compounds induced stronger sedation than placebo, but these differences disappeared 8 hours after drug application. We demonstrate here that GAMs effectively reduce central sensitization in healthy volunteers. These results provide proof-of-principle evidence supporting efficacy of GAMs as antihyperalgesic agents in humans and should stimulate further research on compounds with improved subtype specificity.

Analgesia and unwanted benzodiazepine effects in point-mutated mice expressing only one benzodiazepine-sensitive GABAA receptor subtype

Agonists at the benzodiazepine-binding site of GABA_A receptors (BDZs) enhance synaptic inhibition through four subtypes (α1, α2, α3 and α5) of GABA_A receptors (GABA_AR). When applied to the spinal cord, they alleviate pathological pain; however, insufficient efficacy after systemic administration and undesired effects preclude their use in routine pain therapy. Previous work suggested that subtype-selective drugs might allow separating desired antihyperalgesia from unwanted effects, but the lack of selective agents has hitherto prevented systematic analyses. Here we use four lines of triple GABA_AR point-mutated mice, which express only one benzodiazepine-sensitive GABA_AR subtype at a time, to show that targeting only α2GABA_ARs achieves strong antihyperalgesia and reduced side effects (that is, no sedation, motor impairment and tolerance development). Additional pharmacokinetic and pharmacodynamic analyses in these mice explain why clinically relevant antihyperalgesia cannot be achieved with nonselective BDZs. These findings should foster the development of innovative subtype-selective BDZs for novel indications such as chronic pain.

Benzodiazepines (BDZs) target GABA_A receptors to alleviate pain but these also cause side effects. Here the authors use mice in which only one GABA_A receptor is BDZ-sensitive at a time to identify α2GABA_A as the receptor that provides maximal analgesic activity but minimal side-effects in response to BDZs.

The legacy of the benzodiazepine receptor: from flumazenil to enhancing cognition in Down syndrome and social interaction in autism

The study of the psychopharmacology of benzodiazepines continues to provide new insights into diverse brain functions related to vigilance, anxiety, mood, epileptiform activity, schizophrenia, cognitive performance, and autism-related social behavior. In this endeavor, the discovery of the benzodiazepine receptor was a key event, as it supplied the primary benzodiazepine drug-target site, provided the molecular link to the allosteric modulation of GABAA receptors and, following the recognition of GABAA receptor subtypes, furnished the platform for future, more selective drug actions. This review has two parts. In a retrospective first part, it acknowledges the contributions to the field made by my collaborators over the years, initially at Hoffmann-La Roche in Basle and later, in academia, at the University and the ETH of Zurich. In the second part, the new frontier of GABA pharmacology, targeting GABAA receptor subtypes, is reviewed with special focus on nonsedative anxiolytics, antidepressants, analgesics, as well as enhancers of cognition in Down syndrome and attenuators of symptoms of autism spectrum disorders. It is encouraging that a clinical trial has been initiated with a partial inverse agonist acting on α5 GABAA receptors in an attempt to alleviate the cognitive deficits in Down syndrome.

Allosteric Modulation of GABAA Receptors by an Anilino Enaminone in an Olfactory Center of the Mouse Brain

In an ongoing effort to identify novel drugs that can be used as neurotherapeutic compounds, we have focused on anilino enaminones as potential anticonvulsant agents. Enaminones are organic compounds containing a conjugated system of an amine, an alkene and a ketone. Here, we review the effects of a small library of anilino enaminones on neuronal activity. Our experimental approach employs an olfactory bulb brain slice preparation using whole-cell patch-clamp recording from mitral cells in the main olfactory bulb. The main olfactory bulb is a key integrative center in the olfactory pathway. Mitral cells are the principal output neurons of the main olfactory bulb, receiving olfactory receptor neuron input at their dendrites within glomeruli, and projecting glutamatergic axons through the lateral olfactory tract to the olfactory cortex. The compounds tested are known to be effective in attenuating pentylenetetrazol (PTZ) induced convulsions in rodent models. One compound in particular, KRS-5Me-4-OCF₃, evokes potent inhibition of mitral cell activity. Experiments aimed at understanding the cellular mechanism underlying the inhibitory effect revealed that KRS-5Me-4-OCF₃ shifts the concentration-response curve for GABA to the left. KRS-5Me-4-OCF₃ enhances GABA affinity and acts as a positive allosteric modulator of GABA_A receptors. Application of a benzodiazepine site antagonist blocks the effect of KRS-5Me-4-OCF₃ indicating that KRS-5Me-4-OCF₃ binds at the classical benzodiazepine site to exert its pharmacological action. This anilino enaminone KRS-5Me-4-OCF₃ emerges as a candidate for clinical use as an anticonvulsant agent in the battle against epileptic seizures.

The orthosteric GABAA receptor ligand Thio-4-PIOL displays distinctly different functional properties at synaptic and extrasynaptic receptors

BACKGROUND AND PURPOSE

Explorations into the heterogeneous population of native GABA type A receptors (GABAA Rs) and the physiological functions governed by the multiple GABAA R subtypes have for decades been hampered by the lack of subtype-selective ligands.

EXPERIMENTAL APPROACH

The functional properties of the orthosteric GABAA receptor ligand 5-(4-piperidyl)-3-isothiazolol (Thio-4-PIOL) have been investigated in vitro, ex vivo and in vivo.

KEY RESULTS

Thio-4-PIOL displayed substantial partial agonist activity at the human extrasynaptic GABAA R subtypes expressed in Xenopus oocytes, eliciting maximal responses of up to ∼30% of that of GABA at α5 β3 γ2S , α4 β3 δ and α6 β3 δ and somewhat lower efficacies at the corresponding α5 β2 γ2S , α4 β2 δ and α6 β2 δ subtypes (maximal responses of 4-12%). In contrast, it was an extremely low efficacious agonist at the α1 β3 γ2S , α1 β2 γ2S , α2 β2 γ2S , α2 β3 γ2S , α3 β2 γ2S and α3 β3 γ2S GABAA Rs (maximal responses of 0-4%). In concordance with its agonism at extrasynaptic GABAA Rs and its de facto antagonism at the synaptic receptors, Thio-4-PIOL elicited robust tonic currents in electrophysiological recordings on slices from rat CA1 hippocampus and ventrobasal thalamus and antagonized phasic currents in hippocampal neurons. Finally, the observed effects of Thio-4-PIOL in rat tests of anxiety, locomotion, nociception and spatial memory were overall in good agreement with its in vitro and ex vivo properties.

CONCLUSION AND IMPLICATIONS

The diverse signalling characteristics of Thio-4-PIOL at GABAA Rs represent one of the few examples of a functionally subtype-selective orthosteric GABAA R ligand reported to date. We propose that Thio-4-PIOL could be a useful pharmacological tool in future studies exploring the physiological roles of native synaptic and extrasynaptic GABAA Rs.

GABA pharmacology: the search for analgesics

Decades of research have been devoted to defining the role of GABAergic transmission in nociceptive processing. Much of this work was performed using rigid, orthosteric GABA analogs created by Povl Krogsgaard-Larsen and his associates. A relationship between GABA and pain is suggested by the anatomical distribution of GABA receptors and the ability of some GABA agonists to alter nociceptive responsiveness. Outlined in this report are data supporting this proposition, with particular emphasis on the anatomical localization and function of GABA-containing neurons and the molecular and pharmacological properties of GABAA and GABAB receptor subtypes. Reference is made to changes in overall GABAergic tone, GABA receptor expression and activity as a function of the duration and intensity of a painful stimulus or exposure to GABAergic agents. Evidence is presented that the plasticity of this receptor system may be responsible for the variability in the antinociceptive effectiveness of compounds that influence GABA transmission. These findings demonstrate that at least some types of persistent pain are associated with a regionally selective decline in GABAergic tone, highlighting the need for agents that enhance GABA activity in the affected regions without compromising GABA function over the long-term. As subtype selective positive allosteric modulators may accomplish these goals, such compounds might represent a new class of analgesic drugs.

Sh-I-048A, an in vitro non-selective super-agonist at the benzodiazepine site of GABAA receptors: the approximated activation of receptor subtypes may explain behavioral effects

Enormous progress in understanding the role of four populations of benzodiazepine-sensitive GABAA receptors was paralleled by the puzzling findings suggesting that substantial separation of behavioral effects may be accomplished by apparently non-selective modulators. We report on SH-I-048A, a newly synthesized chiral positive modulator of GABAA receptors characterized by exceptional subnanomolar affinity, high efficacy and non-selectivity. Its influence on behavior was assessed in Wistar rats and contrasted to that obtained with 2mg/kg diazepam. SH-I-048A reached micromolar concentrations in brain tissue, while the unbound fraction in brain homogenate was around 1.5%. The approximated electrophysiological responses, which estimated free concentrations of SH-I-048A or diazepam are able to elicit, suggested a similarity between the 10mg/kg dose of the novel ligand and 2mg/kg diazepam; however, SH-I-048A was relatively more active at α1- and α5-containing GABAA receptors. Behaviorally, SH-I-048A induced sedative, muscle relaxant and ataxic effects, reversed mechanical hyperalgesia 24h after injury, while it was devoid of clear anxiolytic actions and did not affect water-maze performance. While lack of clear anxiolytic actions may be connected with an enhanced potentiation at α1-containing GABAA receptors, the observed behavior in the rotarod, water maze and peripheral nerve injury tests was possibly affected by its prominent action at receptors containing the α5 subunit. The current results encourage further innovative approaches aimed at linking in vitro and in vivo data in order to help define fine-tuning mechanisms at four sensitive receptor populations that underlie subtle differences in behavioral profiles of benzodiazepine site ligands.

Antihyperalgesia by α2-GABAA receptors occurs via a genuine spinal action and does not involve supraspinal sites

Drugs that enhance GABAergic inhibition alleviate inflammatory and neuropathic pain after spinal application. This antihyperalgesia occurs mainly through GABAA receptors (GABAARs) containing α2 subunits (α2-GABAARs). Previous work indicates that potentiation of these receptors in the spinal cord evokes profound antihyperalgesia also after systemic administration, but possible synergistic or antagonistic actions of supraspinal α2-GABAARs on spinal antihyperalgesia have not yet been addressed. Here we generated two lines of GABAAR-mutated mice, which either lack α2-GABAARs specifically from the spinal cord, or, which express only benzodiazepine-insensitive α2-GABAARs at this site. We analyzed the consequences of these mutations for antihyperalgesia evoked by systemic treatment with the novel non-sedative benzodiazepine site agonist HZ166 in neuropathic and inflammatory pain. Wild-type mice and both types of mutated mice had similar baseline nociceptive sensitivities and developed similar hyperalgesia. However, antihyperalgesia by systemic HZ166 was reduced in both mutated mouse lines by about 60% and was virtually indistinguishable from that of global point-mutated mice, in which all α2-GABAARs were benzodiazepine insensitive. The major (α2-dependent) component of GABAAR-mediated antihyperalgesia was therefore exclusively of spinal origin, whereas supraspinal α2-GABAARs had neither synergistic nor antagonistic effects on antihyperalgesia. Our results thus indicate that drugs that specifically target α2-GABAARs exert their antihyperalgesic effect through enhanced spinal nociceptive control. Such drugs may therefore be well-suited for the systemic treatment of different chronic pain conditions.

"Bedside-to-Bench" Behavioral Outcomes in Animal Models of Pain: Beyond the Evaluation of Reflexes

Despite the myriad promising new targets and candidate analgesics recently identified in preclinical pain studies, little translation to novel pain medications has been generated. The pain phenotype in humans involves complex behavioral alterations, including changes in daily living activities and psychological disturbances. These behavioral changes are not reflected by the outcome measures traditionally used in rodents for preclinical pain testing, which are based on reflexes evoked by sensory stimuli of different types (mechanical, thermal or chemical). These measures do not evaluate the impact of the pain experience on the global behavior or disability of the animals, and therefore only consider a limited aspect of the pain phenotype. The development of relevant new outcomes indicative of pain to increase the validity of animal models of pain has been increasingly pursued over the past few years. The aim has been to translate "bedside-to-bench" outcomes from the human pain phenotype to rodents, in order to complement traditional pain outcomes by providing a closer and more realistic measure of clinical pain in rodents. This review summarizes and discusses the most important nonstandard outcomes for pain assessment in preclinical studies. The advantages and drawbacks of these techniques are considered, and their potential impact on the validation of potential analgesics is evaluated.