Differential development of tolerance to the functional and behavioral effects of repeated baclofen treatment in rats

Systemic administration of racemic baclofen reduces both acquisition and maintenance of alcohol consumption in male and female mice

Baclofen is a GABAB receptor agonist with proposed use as a treatment for alcohol use disorder (AUD). In preclinical studies, racemic baclofen decreases alcohol consumption in both mice and rats; however, there is a significant disparity in the efficacy of the drug across species. We previously demonstrated that baclofen is enantioselective, with the racemic enantiomer successfully reducing binge-like alcohol consumption during Drinking-in-the-Dark (DID) in C57BL/6J (B6) mice, as well as 24-h consumption during two-bottle choice (2BC) preference drinking in replicate 1 High Alcohol Preferring (HAP) mice. Here we extend these findings by investigating the effects of racemic baclofen on the acquisition and maintenance of alcohol consumption, locomotor activity, and saccharin drinking in two different mouse genotypes and drinking paradigms. Adult male and female B6 mice were allowed free access to 20% (v/v) alcohol for 2 h daily in a 14-day DID procedure. Adult male and female replicate 2 HAP (HAP2) mice were allowed 24-h access to 10% (v/v) alcohol versus tap water in a 2BC procedure for 14 days. Systemic injections of baclofen (0.0 or 3.0 mg/kg) were given 3 h into the dark cycle on days 1-5 in alcohol acquisition experiments and days 6-10 in alcohol maintenance experiments. We found that racemic baclofen significantly reduces acquisition of DID and 2BC alcohol drinking in male and female B6 and HAP2 mice, whereas it only significantly reduces the maintenance of DID alcohol intake in B6 mice. Racemic baclofen did not alter home cage locomotor activity but did alter saccharin intake, suggesting it may have nonspecific effects. The current data add to literature suggesting that smaller doses of racemic baclofen may be an effective treatment of AUD. Future work should focus on the longitudinal efficacy of racemic baclofen in high-drinking mouse genotypes to further investigate whether it is effective for those with a genetic predisposition to AUD.

Cannabinoid Receptor Type 1 Regulates Drug Reward Behavior via Glutamate Decarboxylase 67 Transcription

Interaction of cannabinoid receptor type 1 (CB1) and GABAergic neuronal activity is involved in drug abuse-related behavior. However, its role in drug-dependent Pavlovian conditioning is not well understood. In this study, we aimed to evaluate the effects of a CB1 agonist, JWH-210, on the development of conditioned place preference (CPP)-induced by methamphetamine (METH). Pretreatment with a synthetic cannabinoid, JWH-210 (CB1 agonist), increased METH-induced CPP score and METH-induced dopamine release in acute striatal slices. Interestingly, CB1 was expressed in glutamate decarboxylase 67 (GAD67) positive cells, and overexpression of CB1 increased GAD67 expression, while CB1 knockdown reduced GAD67 expression in vivo and in vitro. GAD67 is known as an enzyme involved in the synthesis of GABA. CB1 knockdown in the mice striatum increased METH-induced CPP. When GAD67 decreased in the mice striatum, mRNA level of CB1 did not change, suggesting that CB1 can regulate GAD67 expression. GAD67 knockdown in the mouse striatum augmented apomorphine (dopamine receptor D2 agonist)–induced climbing behavior and METH-induced CPP score. Moreover, in the human brain, mRNA level of GAD67 was found to be decreased in drug users. Therefore, we suggest that CB1 potentiates METH-induced CPP through inhibitory GABAergic regulation of dopaminergic neuronal activity.

Baclofen-Induced Neuro-Respiratory Toxicity in the Rat: Contribution of Tolerance and Characterization of Withdrawal Syndrome

Baclofen, a γ-amino-butyric acid type-B receptor agonist with exponentially increased use at high-dose to facilitate abstinence in chronic alcoholics, is responsible for increasing poisonings. Tolerance and withdrawal syndromes have been reported during prolonged treatment but their contribution to the variability of baclofen-induced neurotoxicity in overdose is unknown. We studied baclofen-induced effects on rat sedation, temperature, and ventilation and modeled baclofen pharmacokinetics and effect/concentration relationships aiming to investigate the consequences of repeated baclofen pretreatment and to characterize withdrawal syndrome. Baclofen-induced dose-dependent sedation (p <0.01), hypothermia (p <.001) and respiratory depression (p <.01) were altered in repeatedly baclofen-pretreated rats (p <.05). Repeatedly baclofen-pretreated rats did not exhibit respiratory depression following baclofen overdose due to limitations on baclofen-induced increase in inspiratory (p <.01) and expiratory times (p <.01). Only slight hypoxemia without respiratory acidosis was observed. Baclofen discontinuation resulted in hyperlocomotion and non-anxiogenic withdrawal symptoms. Regarding pharmacokinetics, repeated baclofen pretreatment increased the peak concentration (p <.05) and absorption constant rate (p <.05) and reduced the distribution volume (p <.0001) and elimination half-life (p <.05). Analysis of the effect/concentration relationships indicated that plasma baclofen concentration decreases more rapidly than all studied neuro-respiratory effects, in tolerant and non-tolerant rats. Taken together, our findings supported the role of brain distribution in baclofen-induced neurotoxicity expression and its probable involvement in tolerance-related attenuation in addition to physiological adaptations of ventilation. In conclusion, repeated pretreatment attenuates baclofen-attributed neurotoxicity in overdose and results in post-discontinuation withdrawal syndrome. Our findings suggest both pharmacodynamic and pharmacokinetic mechanisms whose relative contributions to the variability of baclofen-induced neurotoxicity in overdose remain to be established.

Baclofen-induced encephalopathy in overdose - Modeling of the electroencephalographic effect/concentration relationships and contribution of tolerance in the rat

Baclofen, a γ-amino-butyric acid type-B receptor agonist with exponentially increased use at high-dose to facilitate abstinence in chronic alcoholics, is responsible for increasing poisonings. Baclofen overdose may induce severe encephalopathy and electroencephalographic (EEG) abnormalities. Whether prior prolonged baclofen treatment may influence the severity of baclofen-induced encephalopathy in overdose has not been established. We designed a rat study to characterize baclofen-induced encephalopathy, correlate its severity with plasma concentrations and investigate the contribution of tolerance. Baclofen-induced encephalopathy was assessed using continuous EEG and scored based on a ten-grade scale. Following the administration by gavage of 116 mg/kg baclofen, EEG rapidly and steadily impaired resulting in the successive onset of deepening sleep followed by generalized periodic epileptiform discharges and burst-suppressions. Thereafter, encephalopathy progressively recovered following similar phases in reverse. Periodic triphasic sharp waves, non-convulsive status epilepticus and even isoelectric signals were observed at the most critical stages. Prior repeated baclofen administration resulted in reduced severity (peak: grade 7 versus 9; peak effect length: 382 ± 40 versus 123 ± 14 min, P = 0.008) and duration of encephalopathy (18 versus > 24 h, P = 0.0007), supporting the acquisition of tolerance. The relationship between encephalopathy severity and plasma baclofen concentrations fitted a sigmoidal E_max model with an anticlockwise hysteresis loop suggesting a hypothetical biophase site of action. The baclofen concentration producing a response equivalent to 50% of E_max was significantly reduced (8947 μg/L, ±11.3% versus 12,728 μg/L, ±24.0% [mean, coefficient of variation], P = 0.03) with prior prolonged baclofen administration. In conclusion, baclofen overdose induces early-onset and prolonged marked encephalopathy that is significantly attenuated by prior repeated baclofen treatment. Our findings suggest a possible role for the blood-brain barrier in the development of tolerance; however, its definitive involvement remains to be demonstrated.

Efficacy and side effects of baclofen and the novel GABAB receptor positive allosteric modulator CMPPE in animal models for alcohol and cocaine addiction

RATIONALE

Preclinical studies suggest that the GABA_B receptor is a potential target for treatment of substance use disorders. However, recent clinical trials report adverse effects in patients treated with the GABA_B receptor agonist baclofen and even question efficacy. How can the discrepancy between preclinical and clinical findings be explained?

OBJECTIVE

To test efficacy and adverse effects of baclofen and the novel GABA_B positive allosteric modulator (PAM) CMPPE in rat addiction models, which were developed in accordance with DSM.

METHODS

We used a well-characterized rat model of long-term alcohol consumption with repeated deprivation phases that result in compulsive alcohol drinking in a relapse situation, and a rat model of long-term intravenous cocaine self-administration resulting in key symptoms of addictive behavior. We tested repeated baclofen (0, 1, and 3 mg/kg; i.p.) and CMPPE doses (0, 10, and 30 mg/kg; i.p.) in relapse-like situations, in either alcohol or cocaine addicted-like rats.

RESULTS

Baclofen produced a weak anti-relapse effect at the highest dose in alcohol addicted-like rats, and this effect was mainly due to the treatment-induced sedation. CMPPE had a better profile, with a dose-dependent reduction of relapse-like alcohol drinking and without any signs of sedation. The cue-induced cocaine-seeking response was completely abolished by both compounds.

CONCLUSION

Positive allosteric modulation of the GABA_B receptor provides efficacy, and no observable side effects in relapse behavior whereas baclofen may cause, not only sedation, but also considerable impairment of food intake or metabolism. However, targeting GABA_B receptors may be effective in reducing certain aspects of addictive-like behavior, such as cue-reactivity.

A Review of the Potential Mechanisms of Action of Baclofen in Alcohol Use Disorder

Baclofen, a GABA-B receptor agonist, is a promising treatment for alcohol use disorder (AUD). Its mechanism of action in this condition is unknown. GABA-B receptors interact with many biological systems potentially involved in AUD, including transduction pathways and neurotransmitter systems. Preclinical studies have shown that GABA-B receptors are involved in memory storage and retrieval, reward, motivation, mood and anxiety; neuroimaging studies in humans show that baclofen produces region-specific alterations in cerebral activity; GABA-B receptor activation may have neuroprotective effects; baclofen also has anti-inflammatory properties that may be of interest in the context of addiction. However, none of these biological effects fully explain the mechanism of action of baclofen in AUD. Data from clinical studies have provided a certain number of elements which may be useful for the comprehension of its mechanism of action: baclofen typically induces a state of indifference toward alcohol; the effective dose of baclofen in AUD is extremely variable from one patient to another; higher treatment doses correlate with the severity of the addiction; many of the side effects of baclofen resemble those of alcohol, raising the possibility that baclofen acts as a substitution drug; usually, however, there is no tolerance to the effects of baclofen during long-term AUD treatment. In the present article, the biological effects of baclofen are reviewed in the light of its clinical effects in AUD, assuming that, in many instances, clinical effects can be reliable indicators of underlying biological processes. In conclusion, it is proposed that baclofen may suppress the Pavlovian association between cues and rewards through an action in a critical part of the dopaminergic network (the amygdala), thereby normalizing the functional connectivity in the reward network. It is also proposed that this action of baclofen is made possible by the fact that baclofen and alcohol act on similar brain systems in certain regions of the brain.

KK-92A, a novel GABAB receptor positive allosteric modulator, attenuates nicotine self-administration and cue-induced nicotine seeking in rats

RATIONALE

GABA_B receptors (GABA_BR) play a critical role in GABAergic neurotransmission in the brain and are thought to be one of the most promising targets for the treatment of drug addiction. GABA_BR positive allosteric modulators (PAMs) have shown promise as potential anti-addictive therapies, as they lack the sedative and muscle relaxant properties of full GABA_B receptor agonists such as baclofen.

OBJECTIVES

The present study was aimed at developing novel, selective, and potent GABA_BR PAMs with efficacy on abuse-related effects of nicotine.

RESULTS

We synthetized ~100 analogs of BHF177, a GABA_BR PAM that has been shown to inhibit nicotine taking and seeking, and tested their activity in multiple cell-based functional assays. Among these compounds, KK-92A displayed superior PAM properties at the GABA_BR. Interestingly, our results revealed the existence of pathway-selective differential modulation of GABA_BR signaling by the structurally related GABA_BR allosteric modulators BHF177 and KK-92A. In vivo, similarly to BHF177, KK-92A inhibited intravenous nicotine self-administration under both fixed- and progressive-ratio schedules of reinforcement in rats. In contrast to BHF177, KK-92A had no effect on food self-administration. Furthermore, KK-92A decreased cue-induced nicotine-seeking behavior without affecting food seeking.

CONCLUSIONS

These results indicate that KK-92A is a selective GABA_BR PAM with efficacy in inhibition of the primary reinforcing and incentive motivational effects of nicotine, and attenuation of nicotine seeking, further confirming that GABA_BR PAMs may be useful antismoking medications.

Effects of acute administration of the GABA(B) receptor agonist baclofen on behavioral flexibility in rats

RATIONALE

The ability to adjust response strategies when faced with changes in the environment is critical for normal adaptive behavior. Such behavioral flexibility is compromised by experimental disruption of cortical GABAergic signaling, as well as in conditions such as schizophrenia and normal aging that are characterized by cortical hyperexcitability. The current studies were designed to determine whether stimulation of GABAergic signaling using the GABA(B) receptor agonist baclofen can facilitate behavioral flexibility.

METHODS

Male Fischer 344 rats were trained in a set-shifting task in which they learned to discriminate between two response levers to obtain a food reward. Correct levers were signaled in accordance with two distinct response rules (rule 1: correct lever signaled by a cue light; rule 2: correct lever signaled by its left/right position). The order of rule presentation varied, but they were always presented sequentially, with the trials and errors to reach criterion performance on the second (set shift) rule providing the measure of behavioral flexibility. Experiments determined the effects of the GABA(B) receptor agonist baclofen (intraperitoneal, 0, 1.0, 2.5, and 4.0 mg/kg) administered acutely before the shift to the second rule.

RESULTS

Baclofen enhanced set-shifting performance. Control experiments demonstrated that this enhancement was not simply due to improved discrimination learning, nor was it due to impaired recall of the initial discrimination rule.

CONCLUSIONS

The results demonstrate that baclofen can facilitate behavioral flexibility, suggesting that GABA(B) receptor agonists may have utility for treating behavioral dysfunction in neuropsychiatric disorders.

Chronic baclofen desensitizes GABA(B)-mediated G-protein activation and stimulates phosphorylation of kinases in mesocorticolimbic rat brain

The GABAB receptor is a therapeutic target for CNS and neuropathic disorders; however, few preclinical studies have explored effects of chronic stimulation. This study evaluated acute and chronic baclofen treatments on GABAB-activated G-proteins and signaling protein phosphorylation as indicators of GABAB signaling capacity. Brain sections from rats acutely administered baclofen (5 mg/kg, i.p.) showed no significant differences from controls in GABAB-stimulated GTPγS binding in any brain region, but displayed significantly greater phosphorylation/activation of focal adhesion kinase (pFAK(Tyr397)) in mesocorticolimbic regions (caudate putamen, cortex, hippocampus, thalamus) and elevated phosphorylated/activated glycogen synthase kinase 3-β (pGSK3β(Tyr216)) in the prefrontal cortex, cerebral cortex, caudate putamen, nucleus accumbens, thalamus, septum, and globus pallidus. In rats administered chronic baclofen (5 mg/kg, t.i.d. for five days), GABAB-stimulated GTPγS binding was significantly diminished in the prefrontal cortex, septum, amygdala, and parabrachial nucleus compared to controls. This effect was specific to GABAB receptors: there was no effect of chronic baclofen treatment on adenosine A1-stimulated GTPγS binding in any region. Chronically-treated rats also exhibited increases in pFAK(Tyr397) and pGSK3β(Tyr216) compared to controls, and displayed wide-spread elevations in phosphorylated dopamine- and cAMP-regulated phosphoprotein-32 (pDARPP-32(Thr34)) compared to acutely-treated or control rats. We postulate that those neuroadaptive effects of GABAB stimulation mediated by G-proteins and their sequelae correlate with tolerance to several of baclofen's effects, whereas sustained signaling via kinase cascades points to cross-talk between GABAB receptors and alternative mechanisms that are resistant to desensitization. Both desensitized and sustained signaling pathways should be considered in the development of pharmacotherapies targeting the GABA system.

Encephalopathy and hypotonia due to baclofen toxicity in a patient with end-stage renal disease

BACKGROUND

Baclofen is a centrally acting gamma-aminobutyric acid agonist used for the symptomatic relief of skeletal muscle spasm and spasticity in traumatic spinal cord lesions, multiple sclerosis, cerebral palsy, and stroke. It is also used in the treatment of chronic hiccups and cocaine abuse. Baclofen-induced central nervous system depression is rare at the usual therapeutic doses. However, patients with impaired renal function are at a higher risk of developing baclofen toxicity, even at a lower dose.

CASE REPORT

A 57-year-old woman with end-stage renal disease on hemodialysis was admitted to our emergency department with progressive confusion and a generalized decrease in muscular tone. There was no obvious metabolic or infectious etiology that could have explained her condition. A comprehensive laboratory and imaging workup was negative. A review of her medication showed that she had recently been prescribed baclofen for muscular spasm. She was diagnosed with baclofen toxicity and was treated with emergent hemodialysis, which improved her mental status and her decreased muscle tone. Repeated sessions of hemodialysis administered on her second and third days of admission ultimately produced sustained clinical improvement and a complete return to her baseline mental status. She was subsequently discharged home with instructions to stay off baclofen.

CONCLUSIONS

Baclofen toxicity is an under-diagnosed condition, especially in patients with renal dysfunction. Physicians should consider baclofen toxicity in patients with suboptimal kidney function on baclofen who present with altered mental status. Emergent hemodialysis and intensive care unit monitoring is recommended.

Acute and chronic alcohol administration: effects on performance of zebrafish in a latent learning task

Alcohol abuse is a major medical problem. Zebrafish have been proposed to model alcohol related human disorders. Alcohol impairs learning and memory. Here, we analyze the effects of alcohol on performance of zebrafish in a recently developed latent learning paradigm. We employ a 2×3×2 experimental design (chronic×acute alcohol treatment×path blocked). The latent learning task had two phases: one, 30min long exploration trials (16 days, 1 trial/day) with left or right path of a complex maze blocked, and two, a subsequent probe trial with all paths open leading to a goal box that now contained stimulus fish. During the 16 days each fish received one of two chronic treatments: freshwater or 0.50% (v/v%) alcohol. Subsequently, fish were immersed for 1h in one of the following solutions: 0.00 (freshwater), 0.50% or 1.00% alcohol, the acute challenge. Behavior of fish was recorded during the probe trial that commenced immediately after the acute treatment. Path choices, latency to leave the start box and to enter the goal box, time spent in the goal box, distance traveled, and duration of freezing were quantified. We found that acute exposure to 1.00% alcohol after chronic freshwater disrupted learning performance, so did exposure to freshwater after chronic alcohol treatment (withdrawal). We also found exposure to chronic alcohol to diminish the effect of subsequent acute alcohol suggesting development of tolerance. Our results demonstrate that analysis of learning performance of zebrafish allows detection of alcohol-induced functional changes. The simplicity and scalability of the employed task also imply the utility of the zebrafish in high throughput drug screens.

Targeting GABAB receptors for anti-abuse drug discovery

INTRODUCTION

There is increasing evidence encouraging the development of drugs that positively modulate the γ-aminobutyric acid type B (GABA(B)) receptor for combating addiction. Compounds that target GABA(B) receptors are unique as anti-abuse therapies because of their impact against multiple addictive drugs.

AREAS COVERED

The authors present the basic information concerning the drug actions of GABA and GABA(B) receptor orthosteric agonists and positive allosteric modulators (PAM). Furthermore, they discuss several recent excellent reviews and newer results pertaining to GABA(B) receptor drug effects on responses to and self-administration of: alcohol (ethanol), nicotine, cocaine, (meth)amphetamine, and opioids. Preclinical and clinical data are considered.

EXPERT OPINION

Clinical data exist only for baclofen and mostly for alcohol use disorders. Additional trials are needed, but effects are promising. Whether PAMs, given alone or in combination with a direct GABA(B) receptor agonist, will be clinically effective and have fewer side effects requires investigation. The sedative effects of baclofen, a Food and Drug Administration (FDA)-approved drug, become less severe over time. Based on existing data, baclofen is well-tolerated. However, genetic and physiological differences are likely to contribute to individual responses to different therapeutic agents. The more immediate development of baclofen as a therapeutic for alcohol use disorders may be of significant benefit to some individuals.

A dose for the wiser is enough: the alcohol benefits for associative learning in zebrafish

This study aimed to test seeking behavior caused by alcohol and the drug effects on learning in the zebrafish, Danio rerio. Three treatments were conducted: acute, chronic and withdrawal, using 0.10%, 0.25%, and 1.00% alcohol and control (0.00%) (vol/vol.%). For the drug seeking behavior, we used a place preference paradigm (shuttle box tank) before and after alcohol exposure in acute (single exposure) and chronic (7 days) treatments. We observed a change in the basal preference due to the association with alcohol only for 0.25% and 1.00% doses in both acute and chronic offering, indicating an alcohol-seeking behavior after the drug exposure. For the learning task, two treatments were tested: chronic alcohol exposure (26 days including the learning period) and alcohol withdrawal (15 days of alcohol exposure before the learning period). During the learning period, fish received light stimulus followed by food in a pre-defined area of the tank for 8 consecutive days. The low dose group (0.10%) learned the task by the 3rd day both in chronic and withdrawal treatments. The higher doses (0.25% and 1.00%) caused a learning impairment in the chronic treatment group, while fish from the alcohol withdrawal treatment displayed learning on the final testing day. Therefore, we suggest that high alcohol doses impair learning and cause drug seeking behavior, even after drug exposure cessation, while low doses positively affect learning and do not cause seeking behavior. Given our results we propose that the zebrafish is a promising model for identifying active compounds, antibodies or genes which modulate the alcohol dual effects: learning improvement and reinforcing behavior.