GABA receptor agonists: pharmacological spectrum and therapeutic actions

Therapeutic efficacy of the BKCa channel opener chlorzoxazone in a mouse model of Fragile X syndrome

Fragile X syndrome (FXS) is an X-linked neurodevelopmental disorder characterized by several behavioral abnormalities, including hyperactivity, anxiety, sensory hyper-responsiveness, and autistic-like symptoms such as social deficits. Despite considerable efforts, effective pharmacological treatments are still lacking, prompting the need for exploring the therapeutic value of existing drugs beyond their original approved use. One such repurposed drug is chlorzoxazone which is classified as a large-conductance calcium-dependent potassium (BKCa) channel opener. Reduced BKCa channel functionality has been reported in FXS patients, suggesting that molecules activating these channels could serve as promising treatments for this syndrome. Here, we sought to characterize the therapeutic potential of chlorzoxazone using the Fmr1-KO mouse model of FXS which recapitulates the main phenotypes of FXS, including BKCa channel alterations. Chlorzoxazone, administered either acutely or chronically, rescued hyperactivity and acoustic hyper-responsiveness as well as impaired social interactions exhibited by Fmr1-KO mice. Chlorzoxazone was more efficacious in alleviating these phenotypes than gaboxadol and metformin, two repurposed treatments for FXS that do not target BKCa channels. Systemic administration of chlorzoxazone modulated the neuronal activity-dependent gene c-fos in selected brain areas of Fmr1-KO mice, corrected aberrant hippocampal dendritic spines, and was able to rescue impaired BKCa currents recorded from hippocampal and cortical neurons of these mutants. Collectively, these findings provide further preclinical support for BKCa channels as a valuable therapeutic target for treating FXS and encourage the repurposing of chlorzoxazone for clinical applications in FXS and other related neurodevelopmental diseases.

Imidazopyridine-based kinase inhibitors as potential anticancer agents: A review

Considering the fundamental role of protein kinases in the mechanism of protein phosphorylation in critical cellular processes, their dysregulation, especially in cancers, has underscored their therapeutic relevance. Imidazopyridines represent versatile scaffolds found in abundant bioactive compounds. Given their structural features, imidazopyridines have possessed pivotal potency to interact with different protein kinases, inspiring researchers to carry out numerous structural variations. In this comprehensive review, we encompass an extensive survey of the design and biological evaluations of imidazopyridine-based small molecules as potential agents targeting diverse kinases for anticancer applications. We describe the structural elements critical to inhibitory potency, elucidating their key structure-activity relationships (SAR) and mode of actions, where available. We classify these compounds into two groups: Serine/threonine and Tyrosine inhibitors. By highlighting the promising role of imidazopyridines in kinase inhibition, we aim to facilitate the design and development of more effective, targeted compounds for cancer treatment.

Non-medical use of baclofen: A case series and review of the literature

BACKGROUND

Baclofen is widely used for spastic disorders and, most recently, for addictive disorders. The first signals of baclofen abuse occurred in the last decade. This study aims to assess the motives, diversion sources, and routes of administration associated with the non-medical use of baclofen and examine health problems related to the non-medical use of baclofen.

METHODS

Spontaneous reports of baclofen abuse reported to the addictovigilance centre of East France were analysed. A literature search was conducted using PubMed®, Web of Sciences®, and Google Scholar® databases. Both investigations were performed in February 2021 without a time limit.

RESULTS

Forty-six cases were analysed (33 from the literature review and 13 from the addictovigilance base). Baclofen's non-medical use mainly affected male subjects with addictive history, but issues of primary abuse in subjects without any substance abuse history were also observed. Euphoria search was the most common reason for misuse. The route of administration included oral, snorting, and sublingual use. Some cases involving illegal sources were also observed. Most patients misusing baclofen presented severe complications, mainly represented by neurological and respiratory disturbances. Physical and psychological dependence on baclofen was observed in three persons.

CONCLUSIONS

Although baclofen abuse remains relatively infrequent or (most likely) underestimated, this study helped confirm baclofen's intrinsic abuse potential and make visible the baclofen-abuse-related health visible harms. Careful consideration and benefit-risk analysis should be employed when prescribing baclofen, and emergency departments should be aware of baclofen dangers in abuse situations.

MCFF-MTDDI: multi-channel feature fusion for multi-typed drug-drug interaction prediction

Adverse drug-drug interactions (DDIs) have become an increasingly serious problem in the medical and health system. Recently, the effective application of deep learning and biomedical knowledge graphs (KGs) have improved the DDI prediction performance of computational models. However, the problems of feature redundancy and KG noise also arise, bringing new challenges for researchers. To overcome these challenges, we proposed a Multi-Channel Feature Fusion model for multi-typed DDI prediction (MCFF-MTDDI). Specifically, we first extracted drug chemical structure features, drug pairs' extra label features, and KG features of drugs. Then, these different features were effectively fused by a multi-channel feature fusion module. Finally, multi-typed DDIs were predicted through the fully connected neural network. To our knowledge, we are the first to integrate the extra label information into KG-based multi-typed DDI prediction; besides, we innovatively proposed a novel KG feature learning method and a State Encoder to obtain target drug pairs' KG-based features which contained more abundant and more key drug-related KG information with less noise; furthermore, a Gated Recurrent Unit-based multi-channel feature fusion module was proposed in an innovative way to yield more comprehensive feature information about drug pairs, effectively alleviating the problem of feature redundancy. We experimented with four datasets in the multi-class and the multi-label prediction tasks to comprehensively evaluate the performance of MCFF-MTDDI for predicting interactions of known-known drugs, known-new drugs and new-new drugs. In addition, we further conducted ablation studies and case studies. All the results fully demonstrated the effectiveness of MCFF-MTDDI.

Behavioral dysregulation exacerbated by intrathecal baclofen in an adolescent with severe traumatic brain injury

This is a case of a 19-year-old male with a history of remote severe traumatic brain injury (TBI) with an intrathecal baclofen (ITB) pump for dystonia management. Given concern for lack of efficacy despite increasing doses of ITB, his catheter was evaluated and found to be epidural rather than intrathecal. The baclofen dose was down-titrated and he underwent catheter revision. Post-op, his baclofen dose was up-titrated and complicated by significant behavioral changes including aggressive physical and verbal behaviors resulting in hospitalization. Work-up was negative for infection, new neurologic pathology, and epileptic activity. Psychiatric medications were adjusted but the behaviors persisted. Due to concern that the increased baclofen dose was causing his mood instability, his pump was down-titrated. As the dosage decreased, the frequency of outbursts also decreased. Throughout these dose adjustments, his dystonia remained stable and overall functional status improved. This is one of the first cases demonstrating that ITB may exacerbate mood instability in patients with TBI.

Molecular Targets of Cannabinoids Associated with Depression

Novel therapeutic strategies are needed to address depression, a major neurological disorder affecting hundreds of millions of people worldwide. Cannabinoids and their synthetic derivatives have demonstrated numerous neurological activities and may potentially be developed into new treatments for depression. This review highlights cannabinoid (CB) receptors, monoamine oxidase (MAO), N-methyl-D-aspartate (NMDA) receptor, gamma-aminobutyric acid (GABA) receptor, and cholecystokinin (CCK) receptor as key molecular targets of cannabinoids that are associated with depression. The anti-depressant activity of cannabinoids and their binding modes with cannabinoid receptors are discussed, providing insights into rational design and discovery of new cannabinoids or cannabimimetic agents with improved druggable properties.

Reactive Depression: Lost in Translation!

The old classification of depression as reactive and endogenous, which are still observed in clinical practice, both cannot be accommodated under the current rubric of major depression. This is because psychiatric nosology under the Diagnostic and Statistical Manual of Mental Disorders (DSM) and its latest fifth edition (DSM-V) is still descriptive and not etiologic. The aim of this review was to revisit reactive and endogenous categories of depression from the perspective of today's understanding of etiological pathways. From an epigenetic perspective, the old dichotomy of reactive versus endogenous is interrelated through the impact of the environment (e.g., stress). This includes familial or prenatal depression, where the environmental impact is before birth, or childhood depression, where the early life stress is the precipitating factor to genetic susceptibility. In conclusion, searching for both environmental impact (e.g., stressors) and genetic predispositions in depression, even at a clinical level, could help clinicians with better therapeutic decisions.

The potential efficacy of GABAB antagonists in depression

Evidence for the potential utility of GABA_B antagonists has been assembled from a variety of sources, including clinical experience with the GABA_B agonist baclofen, murine genetic GABA_B knock-outs, rodent studies of GABA_B receptor expression and function following treatment with antidepressant therapies, animal models of depression, and some functional and post mortem data from human subjects. Definitive testing of GABA_B antagonists in depression, however, still awaits the development of potent, selective and brain-penetrant compounds for human testing.

Baclofen Abuse due to Its Hypomanic Effect in Patients with Alcohol Dependence and Comorbid Major Depressive Disorder

Baclofen is a gamma-aminobutyric acid type B receptor agonist used as an anti-craving agent for treatment of alcohol dependence. It has gained popularity in the recent times because it is well tolerated even in patients with hepatic impairments. Herein we are summarizing the latest literature about baclofen induced hypomania and are reporting a case of baclofen abuse because of its mood elevating property in a patient of alcohol dependence with comorbid major depressive disorder. Literature review and case study of a 36-year-old male with alcohol dependence with comorbid major depressive disorder was prescribed with tablet baclofen as an anti-craving agent along with antidepressant medicines. The patients who did not improve with conventional antidepressant therapy started feeling better in terms of his mood symptoms on taking tablet baclofen. Owing to the mood elevating property he started abusing baclofen. Despite its safety profile in hepatic impairment, one must be very cautious in prescribing baclofen because of its mood altering property which may account for its abuse potentiality.

The comparative effects of aminoglycoside antibiotics and muscle relaxants on electrical field stimulation response in rat bladder smooth muscle

It has been reported that several aminoglycoside antibiotics have a potential of prolonging the action of non-depolarizing muscle relaxants by drug interactions acting pre-synaptically to inhibit acetylcholine release, but antibiotics itself also have a strong effect on relaxing the smooth muscle. In this study, four antibiotics of aminoglycosides such as gentamicin, streptomycin, kanamycin and neomycin were compared with skeletal muscle relaxants baclofen, tubocurarine, pancuronium and succinylcholine, and a smooth muscle relaxant, papaverine. The muscle strips isolated from the rat bladder were stimulated with pulse trains of 40 V in amplitude and 10 s in duration, with pulse duration of 1 ms at the frequency of 1-8 Hz, at 1, 2, 4, 6, 8 Hz respectively. To test the effect of four antibiotics on bladder smooth muscle relaxation, each of them was treated cumulatively from 1 μM to 0.1 mM with an interval of 5 min. Among the four antibiotics, gentamicin and neomycin inhibited the EFS response. The skeletal muscle relaxants (baclofen, tubocurarine, pancuronium and succinylcholine) and inhibitory neurotransmitters (GABA and glycine) did not show any significant effect. However, papaverine, had a significant effect in the relaxation of the smooth muscle. It was suggested that the aminoglycoside antibiotics have inhibitory effect on the bladder smooth muscle.

Pharmacologic Treatment with GABA(B) Receptor Agonist of Methamphetamine-Induced Cognitive Impairment in Mice

Methamphetamine (METH) is a highly addictive drug, and addiction to METH has increased to epidemic proportions worldwide. Chronic use of METH causes psychiatric symptoms, such as hallucinations and delusions, and long-term cognitive deficits, which are indistinguishable from paranoid schizophrenia. The GABA receptor system is known to play a significant role in modulating the dopaminergic neuronal system, which is related to behavioral changes induced by drug abuse. However, few studies have investigated the effects of GABA receptor agonists on cognitive deficits induced by METH. In the present review, we show that baclofen, a GABA receptor agonist, is effective in treating METH-induced impairment of object recognition memory and prepulse inhibition (PPI) of the startle reflex, a measure of sensorimotor gating in mice. Acute and repeated treatment with METH induced a significant impairment of PPI. Furthermore, repeated but not acute treatment of METH resulted in a long-lasting deficit of object recognition memory. Baclofen, a GABA(B) receptor agonist, dose-dependently ameliorated the METH-induced PPI deficits and object recognition memory impairment in mice. On the other hand, THIP, a GABA(A) receptor agonist, had no effect on METH-induced cognitive deficits. These results suggest that GABA(B) receptors may constitute a putative new target in treating cognitive deficits in chronic METH users.

Eszopiclone facilitation of the antidepressant efficacy of fluoxetine using a social defeat stress model

This study analyzed the interaction of the sleep aid eszopiclone (ESZ) and antidepressant fluoxetine (FLX) on social defeat stress (SDS) in the mouse. Beta adrenoreceptors, brain-derived neurotrophic factor (BDNF) and cAMP response element binding protein (CREB) expression in the hippocampus and frontal cortex were also analyzed. Subjects were adult male 'intruder' C57/B6 mice that were exposed to a retired 'resident' male breeder ICR mouse in this animal's home cage for a 5 min period for each of 10 consecutive days, and the resident established physical dominance. The following day, all animals were assigned to one of four drug treatment groups, and treatment was given for up to 18 days: vehicle, ESZ only (3mg/kg), FLX (10mg/kg) only, or ESZ+FLX. A social interaction test was given on days 1, 5, 10, and 15 of drug treatment to assess SDS. Results showed that the ESZ+FLX group spent less time in avoidance zones during the interaction test at days 1 and 5, and more time in the interaction zone at day 5 compared to defeated mice given vehicle. All drug treatment groups spent more time in the interaction zone compared to defeated mice given vehicle on day 1 as well as day 10. SDS completely dissipated by the fourth interaction test according to both behavioral measures. Neurochemically, SDS did not produce changes in any marker analyzed. This study shows the combination of ESZ and FLX alleviated SDS, but a neurochemical correlate remains elusive.

Oral baclofen reduces visceral pain-related pseudo-affective responses to colorectal distension in rats: relation between plasma exposure and efficacy

OBJECTIVE

We previously showed that activation of GABA(B) receptors by intravenous baclofen reduces pseudo-affective responses to colorectal distension in rats. Here we evaluate the potential clinical significance of these observations.

MATERIAL AND METHODS

Clinically relevant colorectal distension protocols were used to assess the effects of oral baclofen on visceromotor and autonomic cardiovascular responses in conscious rats. Plasma levels of baclofen were monitored to provide clinical relevance to the doses used. Conscious female Sprague-Dawley rats were subjected to repeated noxious colorectal distension (12 × 80 mmHg), ascending-phasic colorectal distension (10-80 mmHg, 10 mmHg increments) or ramp colorectal distension (10 min ramp at 8 mmHg/min). Visceromotor and cardiovascular responses (mean arterial blood pressure and heart rate) were monitored. Pain-related response thresholds were assessed using ascending-phasic and ramp colorectal distension.

RESULTS

Baclofen (1-10 μmol/kg, p.o.) reduced the visceromotor response to colorectal distension, reaching a 40% maximal inhibition (p < 0.05). The highest dose (10 μmol/kg, p.o.) also inhibited pain-related cardiovascular responses in telemetrized rats (50-55% reduction in colorectal distension-evoked hypertensive and tachycardic responses; p < 0.05). Similar thresholds for pain-related visceromotor responses were determined during ramp or ascending-phasic colorectal distension (34.1 ± 1.9 and 31.7 ± 3.2 mmHg, respectively). Baclofen (10 μmol/kg, p.o.) increased thresholds to 71.1 ± 3.7 and 77.5 ± 1.8 mmHg during ramp and ascending-phasic colorectal distension, respectively (p < 0.001). Plasma levels of baclofen were 3.3 ± 0.2 μmol/l at 90 min post-dosing, corresponding to the end of the colorectal distension procedure.

CONCLUSIONS

Oral baclofen, at plasma levels similar to those reported safe and within a therapeutic range in humans, produced significant visceral anti-nociceptive effects in rats.

Development of micro-electrode array based tests for neurotoxicity: assessment of interlaboratory reproducibility with neuroactive chemicals

Neuronal assemblies within the nervous system produce electrical activity that can be recorded in terms of action potential patterns. Such patterns provide a sensitive endpoint to detect effects of a variety of chemical and physical perturbations. They are a function of synaptic changes and do not necessarily involve structural alterations. In vitro neuronal networks (NNs) grown on micro-electrode arrays (MEAs) respond to neuroactive substances as well as the in vivo brain. As such, they constitute a valuable tool for investigating changes in the electrophysiological activity of the neurons in response to chemical exposures. However, the reproducibility of NN responses to chemical exposure has not been systematically documented. To this purpose six independent laboratories (in Europe and in USA) evaluated the response to the same pharmacological compounds (Fluoxetine, Muscimol, and Verapamil) in primary neuronal cultures. Common standardization principles and acceptance criteria for the quality of the cultures have been established to compare the obtained results. These studies involved more than 100 experiments before the final conclusions have been drawn that MEA technology has a potential for standard in vitro neurotoxicity/neuropharmacology evaluation. The obtained results show good intra- and inter-laboratory reproducibility of the responses. The consistent inhibitory effects of the compounds were observed in all the laboratories with the 50% Inhibiting Concentrations (IC(50)s) ranging from: (mean ± SEM, in μM) 1.53 ± 0.17 to 5.4 ± 0.7 (n = 35) for Fluoxetine, 0.16 ± 0.03 to 0.38 ± 0.16 μM (n = 35) for Muscimol, and 2.68 ± 0.32 to 5.23 ± 1.7 (n = 32) for Verapamil. The outcome of this study indicates that the MEA approach is a robust tool leading to reproducible results. The future direction will be to extend the set of testing compounds and to propose the MEA approach as a standard screen for identification and prioritization of chemicals with neurotoxicity potential.

Consciousness recovery induced by intrathecal baclofen administration after subarachnoid hemorrhage -two case reports-

Two patients with subarachnoid hemorrhage recovered consciousness after intrathecal baclofen administration using an implanted intrathecal baclofen pump delivering 50 microg per day using a simple infusion mode. Intrathecal baclofen resulted in significant reduction of spasticity 3 months after the implantation. Case 1 was reduced to a completely bedridden state with spasticity and could slightly move her fingers following commands. However, the patient could eat food and wash her face with minimal assistance at 3 months after the implantation, and could stand up in the parallel bars with assistance and speak several words at 8 months. Case 2 was in a completely bedridden state at 10 months after onset and could neither drink water nor follow instructions. However, the patient became oriented and could eat by herself within 3 to 4 weeks of implantation. She could walk with a cane and use the stairs with minimal assistance at 2 and 3 months after implantation. The patient could speak fluently within 6 months of implantation. Flatulence and dysuria happened during the screening test, but these symptoms were not repeated after implantation of a pump-catheter-system and continuous intrathecal baclofen infusion. Continuous intrathecal baclofen infusion caused both improvement in muscle tone and spasms and consciousness recovery from the vegetative state. This therapy is a strong candidate treatment for patients with spasticity and consciousness disturbance.

Complete suppression of craving in alcohol-dependent individuals: is it possible?

A recent preliminary, single-case report suggested that baclofen not only reduces but may completely suppress craving in alcohol-dependent individuals. The current article summarizes the neurobiological basis of drug craving, and the pharmacological targets that have been shown to be involved in modulating such craving. The potential usefulness of agents that suppress craving is discussed. However, beyond individual case reports, no evidence is available to indicate that targeting a single neurobiological pathway will be sufficient to completely suppress craving in unselected individuals. Nevertheless, subgroups that carry specific characteristics associated with single receptor dysfunction might benefit from a targeted treatment. Further research is needed to enable clinicians to detect these subgroups.

Involvement of pallidotegmental neurons in methamphetamine- and MK-801-induced impairment of prepulse inhibition of the acoustic startle reflex in mice: reversal by GABAB receptor agonist baclofen

We have previously demonstrated that pallidotegmental GABAergic neurons play a crucial role in prepulse inhibition (PPI) of the startle reflex in mice through the activation of GABA(B) receptors in pedunculopontine tegmental neurons. In this study, we investigated whether PPI disruption induced by methamphetamine (METH) or MK-801 is associated with the dysfunction of pallidotegmental neurons. Furthermore, we examined the effects of baclofen, a GABA(B) receptor agonist, on METH- and MK-801-induced PPI impairment. Acute treatment with METH (3 mg/kg, subcutaneouly (s.c.)) and MK-801 (>0.3 mg/kg, s.c.) significantly disrupted PPI, accompanied by the suppression of c-Fos expression in lateral globus pallidus induced by PPI. Furthermore, acute treatment with METH and MK-801 stimulated c-Fos expression in the caudal pontine reticular nucleus (PnC) in mice subjected to the PPT test, although PPI alone had no effect on c-Fos expression. Repeated treatment with 1 mg/kg METH for 7 days, which did not affect PPI acutely, showed similar effects on PPI and c-Fos expression to acute treatment with METH (3 mg/kg). Baclofen dose-dependently ameliorated PPI impairment induced by acute treatment with METH (3 mg/kg) and MK-801 (1 mg/kg), and decreased METH- and MK-801-stimulated c-Fos expression in PnC to the basal level. These results suggest that dysfunction of pallidotegmental neurons is involved in PPI disruption caused by METH and MK-801 in mice. GABA(B) receptor may constitute a putative target in treating neuropsychiatric disorders with sensorimotor gating deficits, such as schizophrenia and METH psychosis.

GABAB receptor agonist baclofen improves methamphetamine-induced cognitive deficit in mice

In this study, we investigated the effects of GABA(A) and GABA(B) receptor agonists on the methamphetamine-induced impairment of recognition memory in mice. Repeated treatment with methamphetamine at a dose of 1 mg/kg for 7 days induced an impairment of recognition memory. Baclofen, a GABA(B) receptor agonist, ameliorated the repeated methamphetamine-induced cognitive impairment, although gaboxadol, a GABA(A) receptor agonist, had no significant effect. GABA(B) receptors may constitute a putative new target in treating cognitive deficits in patients suffering from schizophrenia, as well as methamphetamine psychosis.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

Psychopharmacology of topiramate: from epilepsy to bipolar disorder

Topiramate (TPM) is one of the novel antiepileptic drugs and exhibits a wide range of mechanisms of action. Efficacy of TPM has been demonstrated in partial-onset seizures and primary generalized seizures in adults and children, as both monotherapy and adjunctive therapy. More recently, TPM has been proposed as an add-on treatment for patients with lithium-resistant bipolar disorder, especially those displaying rapid-cycling and mixed states. This paper reviews the multiple mechanisms of action and the tolerability profile of TPM in the light of its therapeutic potential in affective disorders. Studies of TPM in bipolar disorder are evaluated, and the efficacy and tolerability issues as a mood stabilizing agent are discussed.

Clinical studies implementing glutamate neurotransmission in mood disorders

Emerging evidence suggests that the amino acid neurotransmitter systems are associated with the pathophysiology and treatment of mood disorders. Recent advances in the areas of molecular neurobiology, pharmacology, and magnetic resonance spectroscopy (MRS) now provide better tools to probe the function of the amino acid neurotransmitter systems and are affording us the opportunity to better investigate the relationship of these systems to mood disorders. Here we review the available literature in the field and suggest a possible pathophysiological model that may account for the many of the findings.

Transport of pharmacologically active proline derivatives by the human proton-coupled amino acid transporter hPAT1

Several proline derivatives such as L-azetidine-2-carboxylic acid, cis-4-hydroxy-L-proline, and 3,4-dehydro-DL-proline prevent procollagen from folding into a stable triple-helical conformation, thereby reducing excessive deposition of collagen in fibrotic processes and the growth of tumors. This study was performed to investigate whether the recently discovered human proton-coupled amino acid transporter 1 (hPAT1) is capable of transporting such pharmacologically relevant proline derivatives and also GABA analogs. Uptake of L-[3H]proline and [3H]glycine in Caco-2 cells was Na+-independent but strongly H+-dependent. The L-proline uptake was saturable and mediated by a single transport system (hPAT1) with an affinity constant of 2.0 +/- 0.2 mM. The uptake of L-[3H]proline was inhibited by D-proline, trans-4-hydroxy-L-proline, cis-4-hydroxy-L-proline, cis-4-hydroxy-D-proline, 3,4-dehydro-DL-proline, L-azetidine-2-carboxylic acid, 3-amino-1-propanesulfonic acid, D- and L-pipecolic acid, l-thiaproline, and many others. Apical uptake and transepithelial flux of L-[3H]proline across Caco-2 cell monolayers were strongly inhibited by proline derivatives in proportions corresponding to their respective affinity constants at hPAT1. The basolateral to apical flux of L-[3H]proline was only 8% of that in the opposite direction. Apical uptake of unlabeled L-proline, cis-4-hydroxy-L-proline, and L-azetidine-2-carboxylic acid was stimulated by an inside directed H+ gradient 2- to 3-fold. Total apical to basolateral flux of proline derivatives was moderately correlated with their inhibitory potency for L-[3H]proline uptake and flux inhibition. We conclude that 1) the substrate specificity of hPAT1 is very much broader than so far reported and 2) the system accepts therapeutically relevant proline and GABA derivatives. hPAT1 is a promising candidate for new ways of oral drug delivery.

Baclofen reverses the reduction in prepulse inhibition of the acoustic startle response induced by dizocilpine, but not by apomorphine

RATIONALE

Since baclofen, the prototypical GABA(B) receptor agonist, is known to reduce the activity of dopaminergic mesolimbic neurons, a putative antipsychotic property of this compound has been suggested, but the evidence for this is still controversial.

OBJECTIVES

The aim of the present study was to elucidate the effects of baclofen on the prepulse inhibition (PPI) of the acoustic startle response (ASR), a behavioral paradigm considered to be one of the most powerful tools for the evaluation of sensorimotor gating and for the screening of antipsychotics.

METHODS

We tested the effects of baclofen (1.25, 2.5, 5 and 10 mg/kg IP) in rats, per se and in co-treatment with some of the substances known to induce a robust reduction of PPI, such as apomorphine (0.25 mg/kg SC) and dizocilpine (0.1 mg/kg SC). Finally, in order to ascertain whether the effects of baclofen could be ascribed to its activity on GABA(B) receptors, we analyzed whether its action could be prevented by pretreatment with SCH 50911, a selective GABA(B) receptor antagonist (20 mg/kg IP). All the experiments were carried out using standard procedures for the assessment of PPI of the ASR.

RESULTS

Baclofen per se produced no significant change in PPI parameters. Moreover, while no effect on apomorphine-mediated alterations in PPI parameters was observed, baclofen proved able to reverse dizocilpine-induced PPI disruption, and this effect was significantly prevented by SCH 50911. On the other hand, this last compound exhibited no effects per se at the same dose.

CONCLUSIONS

These results indicate that GABA(B) receptors are implicated in the neurobiological circuitry accounting for glutamatergic action in sensorimotor gating, and therefore can be proposed as putative new targets in the pharmacological therapy of psychotic disorders. Further studies should be addressed to evaluate more closely the clinical efficacy of baclofen in this respect.

GABA and schizophrenia: a review of basic science and clinical studies

BACKGROUND

A converging body of evidence implicates the gamma-aminobutyric acid (GABA) neurotransmitter system in the pathogenesis of schizophrenia.

METHODS

The authors review neuroscience literature and clinical studies investigating the role of the GABA system in the pathophysiology of schizophrenia. First, a background on the GABA system is provided, including GABA pharmacology and neuroanatomy of GABAergic neurons. Results from basic science schizophrenia animal models and human studies are reviewed. The role of GABA in cognitive dysfunction in schizophrenia is then presented, followed by a discussion of GABAergic compounds used in monotherapy or adjunctively in clinical schizophrenia studies.

RESULTS

In basic studies, reductions in GABAergic neuronal density and abnormalities in receptors and reuptake sites have been identified in several cortical and subcortical GABA systems. A model has been developed suggesting GABA's role (including GABA-dopamine interactions) in schizophrenia. In several clinical studies, the use of adjunctive GABA agonists was associated with greater improvement in core schizophrenia symptoms.

CONCLUSIONS

Alterations in the GABA neurotransmitter system are found in clinical and basic neuroscience schizophrenia studies as well as animal models and may be involved in the pathophysiology of schizophrenia. The interaction of GABA with other well-characterized neurotransmitter abnormalities remains to be understood. Future studies should elucidate the potential therapeutic role for GABA ligands in schizophrenia treatment.

Jamais vu episodes in relationship to baclofen treatment: a case report

A 37-year-old man presented with new onset jamais vu episodes. Jamais vu is a mental state characterized by a sense of unfamiliarity in a familiar situation. The patient's episodes of jamais vu were unrelated to any known factor other than his use of baclofen. The episodes, which occurred as each baclofen dose wore off, resolved after the baclofen dose that triggered it was discontinued. The patient has had no recurrence of jamais vu states after discontinuation of his baclofen. This is the first known case report of jamais vu episodes caused by baclofen. Although jamais vu episodes can occur in healthy persons, they are known to occur more frequently in persons with epilepsy, fatigue, psychologic states, or intoxications. This case suggests that medications should be considered as a possible cause of jamais vu episodes.

GABA(B) receptor agonists for the treatment of drug addiction: a review of recent findings

A growing preclinical and clinical literature suggests that GABA(B) receptor agonists promote abstinence and reduce the use of cocaine, heroin, alcohol and nicotine. The purpose of this paper is to critically review these data. GABA(B) receptor agonists, such as baclofen, appear to reduce the reinforcing effects of abused drugs in animal models under multiple experimental procedures. This occurs at doses that have little effect on responding for other positive reinforcers such as food or water. We review evidence that these potential therapeutic effects may be mediated by modulation of mesolimbic dopamine neurons. This review also examines the preliminary clinical data from studies of the efficacy of baclofen for treatment of cocaine, alcohol, and nicotine dependence. We suggest that these preliminary data provide a rationale for conducting more systematic studies of the effects of GABA(B) receptor agonists as treatment for drug abuse. This line of research may also improve our understanding of the neurochemical mechanisms underlying the drug dependence process.

Progressive Myoclonic Epilepsies

The treatment of progressive myoclonus epilepsy (PME) remains a major therapeutic challenge in neurology. Generalized convulsive seizures are often well controlled through classic antiepileptic drugs (AEDs) like valproate and clonazepam, whereas myoclonus, the main symptom that is affecting patients most in their daily life, is usually refractory to standard AEDs. Alternative therapy concepts have been and still are investigated. Among the new drugs, zonisamide and piracetam have shown the most promising results as add-on treatments. Other therapeutic approaches, like the use of antioxidants, 5-hydroxytryptophan (5-HTP), and baclofen should also be taken into consideration for the treatment of intractable cases of PME. Nonpharmacologic treatment options such as diet and physical therapy should always be considered, because they may save costs and side effects. In some instances, the occasional use of alcohol has shown beneficial effects.

Effects of vigabatrin intake on brain GABA activity as monitored by spectrally edited magnetic resonance spectroscopy and positron emission tomography

A deficit in gamma-aminobutyric acid (GABA) levels in the brain or the cerebrospinal fluid (CSF) is found in many epilepsy patients. Frequency and severity of seizures may be reduced by treatment with GABA increasing medicaments as e.g. vigabatrin, an irreversible inhibitor of GABA-transaminase. For a better understanding of the associated effects, healthy volunteers were examined with magnetic resonance spectroscopy (MRS) and positron emission tomography (PET) before and after intake of different doses of vigabatrin. For the MRS examinations, a dedicated localized spectral editing method was developed to determine GABA levels. The 11C-flumazenil (FMZ)-PET protocol allowed determination of GABA-A receptor binding. The results show a clear and dose-dependent increase in the brain GABA levels after the medication period as compared to the baseline values. The GABA-A receptor binding, on the other hand, did not change significantly.

The effect of GABAergic system activity on hyperthermia-induced seizures in rats

Developing rats were given GABA antagonists and agonist before electrical seizure discharges were induced by heating the brain of a rat with infra-red rays. The thresholds for the GABA antagonist groups were significantly lower than that for the control, and the threshold for the GABA agonist group was significantly higher than that for the control. These results support the hypothesis that reduced GABAergic system activity underlies febrile seizures.

A review of the preclinical pharmacology of tiagabine: a potent and selective anticonvulsant GABA uptake inhibitor

We review the neurochemical and behavioral profile of the selective gamma-aminobutyric acid (GABA) uptake inhibitor, (R)-N-(4,4-di-(3-methylthien-2-yl)but-3-enyl) nipecotic acid hydrochloride [tiagabine (TGB), previously termed NNC 05-0328, NO 05-0328, and NO-328], which is currently in phase III clinical trials for epilepsy. TGB is a potent, and specific GABA uptake inhibitor. TGB lacks significant affinity for other neurotransmitter receptor binding sites and/or uptake sites. In electrophysiological experiments in hippocampal slices in culture, TGB prolonged the inhibitory postsynaptic potentials (IPSP) and inhibitory postsynaptic currents (IPSC) in the CA1 and CA3 produced by the addition of exogenous GABA. In vivo microdialysis shows that TGB also increases extracellular GABA overflow in a dose-dependent manner. Together these biochemical data suggest that the in vitro and in vivo mechanism of action of TGB is to inhibit GABA uptake specifically, resulting in an increase in GABAergic mediated inhibition in the brain. TGB is a potent anticonvulsant agent against methyl-6,7-dimethyoxy-4-ethyl-B-carboline-3-carboxylate (DMCM)-induced clonic convulsions (mice), subcutaneous pentylenetetrazol (PTZ)-induced tonic convulsions (mice and rats), sound-induced convulsions in DBA/2 mice and genetically epilepsy-prone rats (GEPR), and electrically induced convulsions in kindled rats. TGB is partially efficacious, against subcutaneous PTZ-induced clonic convulsions, and photically induced myoclonus in Papio papio. TGB is weakly efficacious in the intravenous PTZ seizure threshold test and the maximal electroshock seizure (MES) test and produces only partial protection against bicuculline (BIC)-induced convulsions in rats. The overall biochemical and anticonvulsant profile of TGB suggests potential utility in the treatment of chronic seizure disorders such as generalized clonic-tonic epilepsy (GTCS), photomyoclonic seizures, myoclonic petit mal epilepsy, and complex partial epilepsy.

The GABAB antagonist, CGP 35348, antagonizes the effects of baclofen, gamma-butyrolactone and HA 966 on rat striatal dopamine synthesis

The effects of the new GABAB antagonist, CGP 35348 (3-aminopropane-diethoxymethylphosphinic acid), on rat striatal dopamine synthesis and the increases thereof, caused by (-)-baclofen, gamma-butyrolactone (GBL), and HA 966 (3-amino-1-hydroxypyrrolid-2-one), were investigated. CGP 35348 did not alter dopamine synthesis on its own up to the highest dose tested (500 mg/kg i.p.). However, it antagonized the increase elicited by 50 mg/kg s.c. (-)-baclofen at doses above 100 mg/kg i.p.; at 500 mg/kg i.p. this antagonism disappeared within about 6 h of interval between the administration of the compound and (-)-baclofen. CGP 35348 also clearly and significantly attenuated the effects of graded doses of GBL and HA 966 at 500 mg/kg i.p., but was unable to alter the responses elicited by 0.3 mg/kg i.p. haloperidol or 10 mg/kg i.p. tetrabenazine. This indicates that the compound did not generally attenuate increases of dopamine synthesis. It is likely that its GABAB antagonistic properties are responsible for the attenuation of the effect of (-)-baclofen, and our results suggest that this compound is useful for the characterization of the role of GABAB receptors in vivo, e.g. in behaviour. On the other hand, they also suggest the possibility that GBL and HA 966 elicit their effects on dopamine synthesis by means of an interaction with GABAB receptors; a weak in vitro interaction with the latter in radioligand binding experiments has been found for GBL, but not for HA 966.

Direct evidence for central action of PCPGABA to stimulate gastric acid secretion by intracisternal injection

PCPGABA, injected into the cisterna magna, significantly stimulated gastric acid secretion in the perfused rat stomach preparation. This secretagogue action was dose-dependent (0.5-2 micrograms/rat). The peak response occurred within 60 min and lasted up to 100 min. The secretagogue action by PCPGABA was completely reduced by truncal vagotomy. Intracisternal injection of 5-aminovaleric acid, a GABAB-receptor antagonist, did not alter basal gastric acid output, and it also failed to antagonize the acid secretory response to intracisternal PCPGABA. These results demonstrate that intracisternal PCPGABA caused hypersecretion of acid through vagal dependent mechanisms partially independent of GABAB-receptors.

Specificity of vigabatrin for the GABAergic system in human epilepsy

The therapeutic action of vigabatrin (gamma vinyl GABA, GVG) has been reported to be mediated by GABAergic neurotransmission. In the present study, we evaluated different neurotransmitter systems in the cerebrospinal fluid (CSF) of patients with complex partial epilepsy, before and during GVG treatment. The markers of the GABAergic system (free GABA, total GABA, homocarnosine) showed a two- to threefold elevation. There was also an increase in glycine during the 6 months of GVG treatment. In contrast, we did not find any constant CSF changes in either excitatory amino acids or in markers of the cholinergic (acetylcholinesterase), dopaminergic (homovanillic acid), serotonergic (5-hydroxyindoleacetic acid), or peptidergic (somatostatin, prolactin, beta-endorphin) systems. This finding (except an elevation in glycine) was in agreement with previous studies which suggest a specific action of GVG on the GABAergic system. The role of glycine in antiepileptic efficacy of GVG needs further evaluation.

A simple model of the interactions of dopamine, acetylcholine and GABA in movement disorders seen in psychiatry

Many of the movement disorders seen in psychiatric patients are determined, in large part, by the dynamic balance of dopamine and acetylcholine in the basal ganglia. Gamma aminobutyric acid has effects on various neurotransmitters in the brain including dopamine and acetylcholine and is therefore relevant to a discussion of movement disorders. Laboratory and clinical data on the effects of gamma aminobutyric acid on dopamine and acetylcholine are discussed, and a simple model of the interactions of these transmitters, with respect to movement disorders seen in psychiatry, is discussed. Some clinical evidence that is consistent with this model is discussed. Guidelines for the use of gamma aminobutyric acid--like drugs for movement disorders seen in clinical practice are also discussed.

Effect of gamma-vinyl GABA treatment on cholinergic and aminergic neurotransmission and on cyclic nucleotides in human complex partial epilepsy--a CSF study

1. Gamma-vinyl GABA (GVG) is a new anticonvulsant drug that enhances levels of GABA in the brain by irreversibly inhibiting GABA transaminase. 2. To further evaluate the effects and mechanism of action of GVG in the human brain, we measured acetylcholinesterase (AChE) activity and levels of homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA), cyclic nucleotides (cAMP, cGMP), total GABA (TGABA), and GVG in CSF of 78 patients with complex partial epilepsy. The CSF samples were taken at baseline and after 3 months of GVG administration (3 g GVG per day). Thereafter, the responders (= 50% decrease in number of seizures) were divided (double-blind) into two groups that received either 1.5 g or 3 g of GVG per day for the next 3 months. The third CSF sample was taken after this double-blind period. 3. TGABA levels were increased during the GVG treatment (p less than 0.001). In the whole group of patients AChE, HVA, 5-HIAA, and cAMP did not differ from baseline values, cGMP levels were slightly elevated after 3 months of GVG administration (p = 0.019), but were no longer elevated after 6 months. Responders had slightly lower AChE activity than nonresponders (p = 0.041). After 6 months of drug treatment the cGMP levels of patients receiving 1.5 g of GVG did not differ from those receiving 3 g. 4. In conclusion, GVG administration elevates levels of TGABA in the CSF without any clear of constant change to cholinergic and aminergic transmission or effect on cyclic nucleotides. Our study further emphasizes the specific mechanism of action of GVG via GABAergic transmission.

One to three day dose intervals during subchronic treatment of epileptic gerbils with gamma-vinyl GABA: anticonvulsant efficacy and alterations in regional brain GABA levels

gamma-Vinyl GABA (GVG), an irreversible inhibitor of GABA degradation, was administered to seizure-susceptible gerbils at different dosage regimens. After acute i.p. administration, GVG dose dependently protected the animals against air blast-induced seizures with an ED50 of 50 mg/kg. After oral administration, GVG exerted similar anticonvulsant potency. However, during subchronic daily oral dosing of 100 mg/kg GVG, tolerance developed to the anticonvulsant effect of the treatment. No tolerance was observed with daily oral dosing of 50 mg/kg or every other day and every third day dosing of 100 mg/kg GVG. The disadvantage of the two latter dosage regimens was that no sufficient seizure control was obtained on the days between two administrations. Determination of GABA levels in 11 brain regions of gerbils after subchronic treatment with the different dosage regimens of GVG indicated that tolerance during treatment with daily administration of 100 mg/kg GVG could be the consequence of feedback reduction of GABA synthesis. The data suggest that the choice of suitable dosage regimens for chronic treatment is more critical with GVG than with other antiepileptic drugs, because compensatory mechanisms within the GABA system may develop when GVG-induced GABA accumulation is too marked.

Delayed transneuronal death of substantia nigra neurons prevented by gamma-aminobutyric acid agonist

In an investigation of the mechanism by which brain lesions result in delayed degeneration of neurons remote from the site of injury, neurons within the caudate nucleus of rats were destroyed by local injection of the excitotoxin ibotenic acid. Treatment resulted in the rapid degeneration of the striatonigral pathway including projections containing the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and delayed transneuronal death of neurons in the substantia nigra pars reticulata. The distribution of nigral cell loss corresponded to the loss of GABAergic terminals. Neuronal death was prevented by long-term intraventricular infusion of the GABA agonist muscimol. Delayed transneuronal degeneration may be produced by neuronal disinhibition consequent to loss of inhibitory inputs. Replacement of inhibitory transmitters by suitable drugs may prevent some forms of delayed neuronal death.

Quantitative evaluation of the potencies of GABA-receptor agonists and antagonists using the rat hippocampal slice preparation

CA1 population spikes recorded in the rat hippocampal slice were used to assess quantitatively the potencies of GABA-receptor agonists and antagonists on mammalian CNS neurones. Apart from GABA itself, GABA A-receptor agonists inhibited the CA1 population spikes with potencies that correlated closely (r = 0.96) with their ability to displace [3H]-GABA from GABAA-binding sites. The low potency of GABA in this preparation was attributed to the action of uptake processes as the GABA uptake inhibitor, cis-4-hydroxynipecotic acid (2 X 10(-4) M), produced an approximate 6 fold increase in the potency of GABA whilst having no effect on the potency of 4,5,6,7-tetrahydroisoxazolo [5,4-c] pyridin-3-ol (THIP), a GABAA-receptor agonist which is not a substrate for the GABA uptake system. The inhibitory effects of the selective GABAA-receptor agonists isoguvacine and muscimol were antagonized by bicuculline methochloride, which shifted the dose-response curves to the right in a parallel manner. The Schild plots for bicuculline methochloride against isoguvacine and muscimol had slopes of 1 and gave pA2 values of 6.24 and 6.10, respectively. Picrotoxin also antagonized the inhibitory effects of isoguvacine and produced parallel shifts to the right of the dose-response curve. However, the Schild plot for picrotoxin had a slope significantly less than unity (0.82) and gave a pA2 value of 6.89. The novel GABAA-receptor antagonist, pitrazepin, antagonized the inhibitory effects of isoguvacine in an apparently competitive manner. The Schild plot had a slope of 1 and gave a pA2 of 6.69. 6 The inhibitory effects of baclofen, GABA and kojic amine were not antagonized by GABAAreceptor antagonists and were presumed to be mediated by actions at GABA5-receptors. 7 The inhibitory effects of THIP and isoguvacine were antagonized with the same potency by bicuculline methobromide. These results do not support the suggestion that THIP acts preferentially at a 'synaptic' bicuculline-sensitive, GABA receptor. 8 It is concluded that the CAI population spike in the rat hippocampal slice is a useful test system for the quantitative analysis of both GABAA- and GABAB-receptor agonists and antagonists.

Development of tolerance to the anticonvulsant effect of GABAmimetic drugs in genetically epilepsy-prone gerbils

Three drugs which increase GABA-mediated inhibitory neurotransmission in the brain, namely the GABA degradation inhibitors aminooxyacetic acid (AOAA) and gamma-acetylenic GABA (GAG), and the GABA receptor agonist THIP (gaboxadol), were administered to epilepsy-prone gerbils via subcutaneously implanted osmotic minipumps for 2 weeks. The antiepileptic drugs valproic acid (VPA) and diazepam were also included in the experiments. After one day of constant rate application, all GABAmimetics markedly suppressed seizure activity induced in the gerbils by air blast stimulation, but anticonvulsant efficacy of the drugs was lost after 8 and 14 days of treatment. With VPA, only moderate anticonvulsant effects were found because only sub-therapeutic drug levels (about 40 micrograms/ml plasma) were reached via minipump administration. The experiments with diazepam could only be evaluated in part because of instability of the drug in aqueous solution. Determination of brain GABA metabolism in the gerbils indicated that reduction of GABA synthesis may be responsible, at least in part, for development of tolerance to the anticonvulsant effects of AOAA and GAG.

Experimental basis for the antidepressant action of the GABA receptor agonist progabide

gamma-Aminobutyric acid (GABA) receptor agonists (e.g. progabide) are effective in behavioral tests predictive of antidepressant drug action. Also, these compounds, by changing the firing rate of the corresponding neurons, accelerate norepinephrine turnover (without changes in postsynaptic receptor density) and decrease 5-hydroxytryptamine (5-HT) liberation (with up-regulation of 5-HT2 receptors). At variance, tricyclic antidepressants block monoamine reuptake and cause down-regulation of beta-adrenergic and 5-HT2 receptors. Progabide exerts an antidepressant action which is indistinguishable from that of imipramine. The different modes of action of GABA receptor agonists and tricyclics, as well as alterations of GABA-related parameters by tricyclics, challenge the classical monoaminergic hypothesis of depression and suggest that GABA-mediated mechanisms play a role in this disorder.