HZ166, a novel GABAA receptor subtype-selective benzodiazepine site ligand, is antihyperalgesic in mouse models of inflammatory and neuropathic pain

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.

[Pharmacological aspects of pain research in Germany]

In spite of several approved analgesics, the therapy of pain still constitutes a challenge due to the fact that the drugs do not exert sufficient efficacy or are associated with severe side effects. Therefore, the development of new and improved painkillers is still of great importance. A number of highly qualified scientists in Germany are investigating signal transduction pathways in pain, effectivity of new drugs and the so far incompletely investigated mechanisms of well-known analgesics in preclinical and clinical studies. The highlights of pharmacological pain research in Germany are summarized in this article.

Advances in the pharmacology of lGICs auxiliary subunits

Ligand-gated ion channels (LGICs) are cell surface integral proteins that mediate the fast neurotransmission in the nervous system. LGICs require auxiliary subunits for their trafficking, assembly and pharmacological modulation. Auxiliary subunits do not form functional homomeric receptors, but are reported to assemble with the principal subunits in order to modulate their pharmacological profiles. For example, nACh receptors are built at least by co-assemble of α and β subunits, and the neuronal auxiliary subunits β3 and α5 and muscle type β, δ, γ, and ϵ determine the agonist affinity of these receptors. Serotonergic 5-HT3B, 5-HT3C, 5-HT3D and 5-HT3E are reported to assemble with the 5-HT3A subunit to modulate its pharmacological profile. Functional studies evaluating the role of γ2 and δ auxiliary subunits of GABAA receptors have made important advances in the understanding of the action of benzodiazepines, ethanol and neurosteroids. Glycine receptors are composed principally by α1-3 subunits and the auxiliary subunit β determines their synaptic location and their pharmacological response to propofol and ethanol. NMDA receptors appear to be functional as heterotetrameric channels. So far, the existence of NMDA auxiliary subunits is controversial. On the other hand, Kainate receptors are modulated by NETO 1 and 2. AMPA receptors are modulated by TARPs, Shisa 9, CKAMP44, CNIH2-3 auxiliary proteins reported that controls their trafficking, conductance and gating of channels. P2X receptors are able to associate with auxiliary Pannexin-1 protein to modulate P2X7 receptors. Considering the pharmacological relevance of different LGICs auxiliary subunits in the present work we will highlight the therapeutic potential of these modulator proteins.

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.

Role of principal ionotropic and metabotropic receptors in visceral pain

Visceral pain is the most common form of pain caused by varied diseases and a major reason for patients to seek medical consultation. It also leads to a significant economic burden due to workdays lost and reduced productivity. Further, long-term use of non-specific medications is also associated with side effects affecting the quality of life. Despite years of extensive research and the availability of several therapeutic options, management of patients with chronic visceral pain is often inadequate, resulting in frustration for both patients and physicians. This is, most likely, because the mechanisms associated with chronic visceral pain are different from those of acute pain. Accumulating evidence from years of research implicates several receptors and ion channels in the induction and maintenance of central and peripheral sensitization during chronic pain states. Understanding the specific role of these receptors will facilitate to capitalize on their unique properties to augment the therapeutic efficacy while at the same time minimizing unwanted side effects. The aim of this review is to provide a concise review of the recent literature that reports on the role of principal ionotropic receptors and metabotropic receptors in the modulation visceral pain. We also include an overview of the possibility of these receptors as potential new targets for the treatment of chronic visceral pain conditions.

Soluble epoxide hydrolase inhibition is antinociceptive in a mouse model of diabetic neuropathy

Neuropathic pain is currently an insufficiently treated clinical condition. There remains a critical need for efficacious therapies without severe side effects to treat the uniquely persistent and tonic pain of neuropathy. Inhibitors of the soluble epoxide hydrolase (sEH) enzyme that stabilize endogenous epoxy fatty acids have demonstrated antihyperalgesia in clinical chronic inflammatory pain and modeled neuropathic pain. Recently, the conditioned place preference assay has been used to evaluate the tonic nature of neuropathy in several animal models. The current experiments use the conditioned place preference assay alongside withdrawal thresholds to investigate the antihyperalgesic efficacy of sEH inhibitors in a murine model of diabetic neuropathy. Here, the sEH inhibitor trans-4-[4-(3-trifluoromethoxyphenyl-1-ureido)-cyclohexyloxy]-benzoic acid (t-TUCB) at 10 mg/kg induced a robust place preference in diabetic neuropathic mice representative of pain relief. Importantly, this effect was absent both in control mice and in sEH-knockout mice at the same dose, indicating that t-TUCB is not positively reinforcing or rewarding. When compared to gabapentin, t-TUCB elicited a similar degree of withdrawal threshold improvement without the same degree of spontaneous locomotion decline in neuropathic mice. Overall, these experiments show that inhibiting the sEH enzyme attenuates chronic pain and offers an alternative to current side-effect-limited therapies to meet this clinical need.

PERSPECTIVE

These experiments demonstrate antihyperalgesia in a murine chronic pain model mediated by inhibiting the sEH enzyme. The results of this study indicate that inhibiting the sEH is a promising alternative for blocking chronic pain.

Tests of unconditioned anxiety - pitfalls and disappointments

The plus-maze, the light-dark box and the open-field are the main current tests of unconditioned anxiety for mice and rats. Despite their disappointing achievements, they remain as popular as ever and seem to play an important role in an ever-growing demand for behavioral phenotyping and drug screening. Numerous reviews have repeatedly reported their lack of consistency and reliability but they failed to address the core question of whether these tests do provide unequivocal measures of fear-induced anxiety, that these measurements are not confused with measures of fear-induced avoidance or natural preference responses - i.e. discriminant validity. In the present report, I examined numerous issues that undermine the validity of the current tests, and I highlighted various flaws in the aspects of these tests and the methodologies pursued. This report concludes that the evidence in support of the validity of the plus-maze, the light/dark box and the open-field as anxiety tests is poor and methodologically questionable.

Allopregnanolone suppresses diabetes-induced neuropathic pain and motor deficit through inhibition of GABAA receptor down-regulation in the spinal cord of diabetic rats

OBJECTIVES

Painful diabetic neuropathy is associated with hyperexcitability and hyperactivity of spinal cord neurons. However, its underlying pathophysiological mechanisms have not been fully clarified. Induction of excitatory/inhibitory neurotransmission imbalance at the spinal cord seems to account for the abnormal neuronal activity in diabetes. Protective properties of neurosteroids have been demonstrated in numerous cellular and animal models of neurodegeneration.

MATERIALS AND METHODS

Here, the protective effects of allopregnanolone, a neurosteroid were investigated in an in vivo model of diabetic neuropathy. The tail-flick test was used to assess the nociceptive threshold. Diabetes was induced by injection of 50 mg/kg (IP) streptozotocin. Seven weeks after the induction of diabetes, the dorsal half of the lumbar spinal cord was assayed for the expression of γ2 subunit of GABAA receptor using semiquantitative RT-PCR.

RESULTS

The data shows that allopregnanolone (5 and 20 mg/kg) markedly ameliorated diabetes-induced thermal hyperalgesia and motor deficit. The weights of diabetic rats that received 5 and 20 mg/kg allopregnanolone did not significantly reduce during the time course of study. Furthermore, this neurosteroid could inhibit GABAA receptor down-regulation induced by diabetes in the rat spinal cord.

CONCLUSION

The data revealed that allopregnanolone has preventive effects against hyperglycemic-induced neuropathic pain and motor deficit which are related to the inhibition of GABAA receptor down-regulation.

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.

Neurosteroid allopregnanolone attenuates high glucose-induced apoptosis and prevents experimental diabetic neuropathic pain: in vitro and in vivo studies

Hyperglycemia plays a critical role in the development of diabetic neuropathy. Hyperglycemia induces oxidative stress in neurons, resulting in neuronal cell apoptosis and dysfunction. Anti-apoptotic properties of neurosteroids have been demonstrated in numerous cellular models of neurodegenerative studies. Here, the protective effects of neurosteroid allopregnanolone were investigated in in vitro and in vivo models of diabetic neuropathy. The data show that glucose decreased the viability of PC12 cells in a concentration-dependent manner. Allopregnanolone at concentrations of 2.5, 5 and 10μM markedly prevented high glucose-induced toxicity in naïve and NGF-treated (neuron-like) PC12 cells. Furthermore, treatment of diabetic rats with allopregnanolone (5 and 20mg/kg) significantly ameliorated diabetic-induced thermal hyperalgesia. Moreover, this neurosteroid inhibited caspase 3 and decreased Bax/Bcl2 ratio in high glucose-treated cells and spinal cord of diabetic rats. In conclusion, the data revealed that allopregnanolone has protective effects against hyperglycemic-induced cellular damage and prevention of cell apoptosis is involved in its mechanisms. Our findings suggest that allopregnanolone has protective effect against pro-apoptotic challenges such as diabetes and hyperglycemia and propose therapeutic potential of neurosteroids in attenuation of diabetic side effects such as neuropathy.

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.

Synthesis and pharmacological properties of new GABA uptake inhibitors

BACKGROUND

γ-Aminobutanoic acid (GABA) is the principal inhibitory neurotransmitter in the mammalian central nervous system. The identification and subsequent development of the GABA transport inhibitors which enhance the GABA-ergic transmission has shown the important role that GABA transporters play in the control of numerous functions of the nervous system. Compounds which inhibit GABA uptake are used as antiepileptic drugs (tiagabine - a selective GAT1 inhibitor), they are also being investigated for other indications, including treatment of psychosis, general anxiety, sleep disorders, drug addiction or acute and chronic pain.

METHODS

In this paper, the synthesis of 2-substituted-4-(1,3-dioxoisoindolin-2-ylo)-butanamides and 2-substituted-4-aminobutanoic acids derivatives is described. These compounds were tested in vitro for their ability to inhibit GABA uptake. The inhibitory potency towards murine plasma membrane GABA transporters (mGAT1-4) was performed as [(3)H]GABA uptake assay based on stably transfected HEK cells. Compound 18, which demonstrated the highest affinity for mGAT1-4 (pIC(50) ranged from 4.42 for mGAT1 to 5.07 for mGAT3), was additionally investigated in several behavioral tests in mice.

RESULTS

Compound 18 increased the locomotor activity (14-38%) and had anxiolytic-like properties in the four-plate test (ED(50) = 9.3 mg/kg). It did not show analgesic activity in acute pain model, namely the hot plate test, however, it was antinociceptive in the acetic acid-induced writhing test (ED(50) = 15.3 mg/kg) and in the formalin model of tonic pain. In the latter assay, it diminished nocifensive behavior in both phases and in the first (neurogenic) phase of this test the obtained ED(50) value (5.3 mg/kg) was similar to morphine (3.0 mg/kg).

CONCLUSION

Compound 18 exhibited significant anxiolytic-like properties and was antinociceptive in some models of pain in mice. Moreover, it did not impair animals' motor coordination in the chimney test. Some of the described pharmacological activities of compound 18 can be partly explained based on its affinity for plasma membrane GABA transporters.

Evaluation of anti-hyperalgesic and analgesic effects of two benzodiazepines in human experimental pain: a randomized placebo-controlled study

Background and Aims

Compounds that act on GABA-receptors produce anti-hyperalgesia in animal models, but little is known on their effects in humans. The aim of this study was to explore the potential usefulness of GABA-agonism for the control of pain in humans. Two agonists at the benzodiazepine-binding site of GABA_A-receptors (clobazam and clonazepam) were studied using multiple experimental pain tests. Positive results would support further investigation of GABA agonism for the control of clinical pain.

Methods

In a randomized double-blind crossover design, 16 healthy male volunteers received clobazam 20 mg, clonazepam 1 mg and tolterodine 1 mg (active placebo). The area of static hyperalgesia after intradermal capsaicin injection was the primary endpoint. Secondary endpoints were: area of dynamic hyperalgesia, response to von Frey hair stimulation, pressure pain thresholds, conditioned pain modulation, cutaneous and intramuscular electrical pain thresholds (1, 5 and 20 repeated stimulation), and pain during cuff algometry.

Results

For the primary endpoint, an increase in the area of static hyperalgesia was observed after administration of placebo (p<0.001), but not after clobazam and clonazepam. Results suggestive for an anti-hyperalgesic effect of the benzodiazepines were obtained with all three intramuscular pain models and with cuff algometry. No effect could be detected with the other pain models employed.

Conclusions

Collectively, the results are suggestive for a possible anti-hyperalgesic effect of drugs acting at the GABA_A-receptors in humans, particularly in models of secondary hyperalgesia and deep pain. The findings are not conclusive, but support further clinical research on pain modulation by GABAergic drugs. Because of the partial results, future research should focus on compounds acting selectively on subunits of the GABA complex, which may allow the achievement of higher receptor occupancy than unselective drugs. Our data also provide information on the most suitable experimental models for future investigation of GABAergic compounds.

Trial Registration

ClinicalTrials.gov NCT01011036

GABA(A) receptor modulation: potential to deliver novel pain medicines?

GAB(A) (γ-aminobutyric acid) is abundantly expressed within the brain, and spinal cord pain circuits where it acts as the principal mediator of fast inhibitory neurotransmission. However, drugs that target GABA(A) receptor function such as the classical benzodiazepines have not been optimised to promote analgesia, are limited by side effects and are not routinely used for this purpose in humans. Compounds such as NS11394, L-838,417, HZ166 and TPA023 all bind to the same benzodiazepine site on the GABA(A) receptor to allosterically modulate receptor function and enhance the actions of GABA. By virtue of their ability to activate selected subtypes of GABA(A) receptors (principally those containing α2, α3 and α5 subunits) these compounds have been shown to possess excellent tolerability profiles in animals. Importantly, a number of these molecules also mediate profound analgesia in animal models of inflammatory and neuropathic pain. Other modulators such as neurosteroids bind to distinct sites on GABA(A) receptor α subunits, possess a unique pharmacology and are capable of targeting alternative GABA(A) receptor expressing populations. Moreover, neurosteroids also have pronounced analgesic actions in animal pain models. The continuing call for novel mechanism of action analgesics to target specific pathologies, especially in clinical neuropathic conditions, emphasizes the need to test modulators of GABA(A) receptor function in both human experimental pain models and pain patients.

Sex-specific differences in GABA(A) -benzodiazepine receptor availability: relationship with sensitivity to pain and tobacco smoking craving

Sex differences exist in tobacco smoking behaviors. Nicotine, the primary addictive ingredient in tobacco smoke, indirectly affects γ-amino butyric acid (GABA) function. Previous studies reported sex-by-smoking interactions in brain GABA levels. The goal of the present study was to evaluate if there is a sex-by-smoking interaction at the GABA(A)-benzodiazepine receptors (GABA(A)-BZRs), as well as relationships between GABA(A)-BZR availability and behavioral variables before and after 1 week of smoking cessation. Twenty-six women (8 non-smokers, age 36.0 ± 13.4 years; 19 smokers, age 34.6 ± 8.9 years) and 25 men (8 non-smokers, age 37.9 ± 13.8 years; 17 smokers, 34.1 ± 12.4 years) were imaged using [123I]iomazenil and single-photon emission computed tomography. Smokers were imaged at baseline 7 hours after the last cigarette. A significantly great number of men were able to abstain from smoking for 1 week (P = 0.003). There were no significant differences in nicotine dependence and cigarette craving, mood or pain sensitivity between male and female smokers. There was a significant effect of gender across all brain regions (frontal, parietal, anterior cingulate, temporal and occipital cortices, and cerebellum; P < 0.05), with all women (smokers and non-smokers combined) having a higher GABA(A)-BZR availability than all men. There was a negative correlation between GABA(A)-BZR availability and craving (P ≤ 0.02) and pain sensitivity (P = 0.04) in female smokers but not male smokers. This study provides further evidence of a sex-specific regulation of GABA(A)-BZR availability in humans and demonstrates the potential for GABA(A)-BZRs to mediate tobacco smoking craving and pain symptoms differentially in female and male smokers.

Search for α3β₂/₃γ2 subtype selective ligands that are stable on human liver microsomes

Selective modulation of specific benzodiazepine receptor (BzR) gamma amino butyric acid-A (GABA(A)) receptor ion channels has been identified as an important method for separating out the variety of pharmacological effects elicited by BzR-related drugs. Importantly, it has been demonstrated that both α2β(2/3)γ2 (α2BzR) and α3BzR (and/or α2/α3) BzR subtype selective ligands exhibit anxiolytic effects with little or no sedation. Previously we have identified several such ligands; however, three of our parent ligands exhibited significant metabolic liability in rodents in the form of a labile ester group. Here eight analogs are reported which were designed to circumvent this liability by utilizing a rational replacement of the ester moiety based on medicinal chemistry precedents. In a metabolic stability study using human liver microsomes, four compounds were found to undergo slower metabolic transformation, as compared to their corresponding ester analogs. These compounds were also evaluated in in vitro efficacy assays. Additionally, bioisostere 11 was evaluated in a rodent model of anxiety. It exhibited anxiolytic activity at doses of 10 and 100mg/kg and was devoid of sedative properties.

Antihyperalgesic effect of the GABA(A) ligand clobazam in a neuropathic pain model in mice: a pharmacokinetic-pharmacodynamic study

Facilitation of spinal GABAergic inhibition with benzodiazepines (BZDs) reverses pain sensitization in animals; however, the use of BZDs in man is limited by their sedative effect. The antihyperalgesic effects of GABA(A) agonists are mediated by GABA(A) receptors containing α2 subunits, whereas sedation is linked to α1 subunit-containing receptors. α2 and α3 selective GABA(A) receptor modulators have been tested in animals but are not yet available for use in human beings. Clobazam is a 1,5-BZD, which exhibits less cognitive side effects than other benzodiazepines. Here, we studied its antihyperalgesic effects in a mouse model of neuropathic pain. Clobazam showed a dose-dependent antihyperalgesic effect in the chronic constriction injury (CCI) model of neuropathic pain, peaking at 1 hr after administration and lasting for 4 hr with no relevant sedation at a dose of 3 mg/kg. At higher doses, the antihyperalgesic effect was stronger, but sedation became significant. The blood and brain kinetics of clobazam were linear over the range of doses tested with a short half-life of the parent compound and a ready penetration of the blood-brain barrier. Clobazam blood concentrations decreased rapidly, falling below the limit of detection at 120 min. after drug application. Its main metabolite, N-desmethyl-clobazam, showed more delayed and prolonged pharmacokinetics, partly explaining why antihyperalgesia persisted when clobazam was no longer detectable in the blood. Considering its therapeutic margin and its pharmacokinetic properties, clobazam would be a valuable compound to assess the role of the GABAergic pathway in pain transmission in human beings.

A novel GABA(A) receptor pharmacology: drugs interacting with the α(+) β(-) interface

GABA(A) receptors are ligand-gated chloride channels composed of five subunits that can belong to different subunit classes. The existence of 19 different subunits gives rise to a multiplicity of GABA(A) receptor subtypes with distinct subunit composition; regional, cellular and subcellular distribution; and pharmacology. Most of these receptors are composed of two α, two β and one γ2 subunits. GABA(A) receptors are the site of action of a variety of pharmacologically and clinically important drugs, such as benzodiazepines, barbiturates, neuroactive steroids, anaesthetics and convulsants. Whereas GABA acts at the two extracellular β(+) α(-) interfaces of GABA(A) receptors, the allosteric modulatory benzodiazepines interact with the extracellular α(+) γ2(-) interface. In contrast, barbiturates, neuroactive steroids and anaesthetics seem to interact with solvent accessible pockets in the transmembrane domain. Several benzodiazepine site ligands have been identified that selectively interact with GABA(A) receptor subtypes containing α2βγ2, α3βγ2 or α5βγ2 subunits. This indicates that the different α subunit types present in these receptors convey sufficient structural differences to the benzodiazepine binding site to allow specific interaction with certain benzodiazepine site ligands. Recently, a novel drug binding site was identified at the α(+) β(-) interface. This binding site is homologous to the benzodiazepine binding site at the α(+) γ2(-) interface and is thus also strongly influenced by the type of α subunit present in the receptor. Drugs interacting with this binding site cannot directly activate but only allosterically modulate GABA(A) receptors. The possible importance of such drugs addressing a spectrum of receptor subtypes completely different from that of benzodiazepines is discussed.

GABA(A) receptor α subunits differentially contribute to diazepam tolerance after chronic treatment

Background

Within the GABA_A-receptor field, two important questions are what molecular mechanisms underlie benzodiazepine tolerance, and whether tolerance can be ascribed to certain GABA_A-receptor subtypes.

Methods

We investigated tolerance to acute anxiolytic, hypothermic and sedative effects of diazepam in mice exposed for 28-days to non-selective/selective GABA_A-receptor positive allosteric modulators: diazepam (non-selective), bretazenil (partial non-selective), zolpidem (α₁ selective) and TPA023 (α_2/3 selective). In-vivo binding studies with [³H]flumazenil confirmed compounds occupied CNS GABA_A receptors.

Results

Chronic diazepam treatment resulted in tolerance to diazepam's acute anxiolytic, hypothermic and sedative effects. In mice treated chronically with bretazenil, tolerance to diazepam's anxiolytic and hypothermic, but not sedative, effects was seen. Chronic zolpidem treatment resulted in tolerance to diazepam's hypothermic effect, but partial anxiolytic tolerance and no sedative tolerance. Chronic TPA023 treatment did not result in tolerance to diazepam's hypothermic, anxiolytic or sedative effects.

Conclusions

Our data indicate that: (i) GABA_A-α₂/α₃ subtype selective drugs might not induce tolerance; (ii) in rodents quantitative and temporal variations in tolerance development occur dependent on the endpoint assessed, consistent with clinical experience with benzodiazepines (e.g., differential tolerance to antiepileptic and anxiolytic actions); (iii) tolerance to diazepam's sedative actions needs concomitant activation of GABA_A-α₁/GABA_A-α₅ receptors. Regarding mechanism, in-situ hybridization studies indicated no gross changes in expression levels of GABA_A α₁, α₂ or α₅ subunit mRNA in hippocampus or cortex. Since selective chronic activation of either GABA_A α₂, or α₃ receptors does not engender tolerance development, subtype-selective GABA_A drugs might constitute a promising class of novel drugs.

Chronic pain states: pharmacological strategies to restore diminished inhibitory spinal pain control

Potentially noxious stimuli are sensed by specialized nerve cells named nociceptors, which convey nociceptive signals from peripheral tissues to the central nervous system. The spinal dorsal horn and the trigeminal nucleus serve as first relay stations for incoming nociceptive signals. At these sites, nociceptor terminals contact a local neuronal network consisting of excitatory and inhibitory interneurons as well as of projection neurons. Blockade of neuronal inhibition in this network causes an increased sensitivity to noxious stimuli (hyperalgesia), painful sensations occurring after activation of non-nociceptive fibers (allodynia), and spontaneous pain felt in the absence of any sensory stimulation. It thus mimics the major characteristics of chronic pain states. Diminished inhibitory pain control in the spinal dorsal horn occurs naturally, e.g., through changes in the function of inhibitory neurotransmitter receptors or through altered chloride homeo-stasis in the course of inflammation or nerve damage. This review summarizes our current knowledge about endogenous mechanisms leading to diminished spinal pain control and discusses possible ways that could restore proper inhibition through facilitation of fast inhibitory neurotransmission.

Beyond classical benzodiazepines: novel therapeutic potential of GABAA receptor subtypes

GABA(A) (γ-aminobutyric acid, type A) receptors are a family of ligand-gated ion channels that are essential for the regulation of central nervous system function. Benzodiazepines - which non-selectively target GABA(A) receptors containing the α1, α2, α3 or α5 subunits - have been in clinical use for decades and are still among the most widely prescribed drugs for the treatment of insomnia and anxiety disorders. However, their use is limited by side effects and the risk of drug dependence. In the past decade, the identification of separable key functions of GABA(A) receptor subtypes suggests that receptor subtype-selective compounds could overcome the limitations of classical benzodiazepines; furthermore, they might be valuable for novel indications such as chronic pain, depression, schizophrenia, cognitive enhancement and stroke.