GABA(B) receptor modulators potentiate baclofen-induced depression of dopamine neuron activity in the rat ventral tegmental area

Neurobiology of alcohol seeking behavior

Alcohol addiction is a chronic relapsing brain disease characterized by an impaired ability to stop or control alcohol use despite adverse consequences. A main challenge of addiction treatment is to prevent relapse, which occurs in more than >50% of newly abstinent patients with alcohol disorder within 3 months. In people suffering from alcohol addiction, stressful events, drug-associated cues and contexts, or re-exposure to a small amount of alcohol trigger a chain of behaviors that frequently culminates in relapse. In this review, we first present the preclinical models that were developed for the study of alcohol seeking behavior, namely the reinstatement model of alcohol relapse and compulsive alcohol seeking under a chained schedule of reinforcement. We then provide an overview of the neurobiological findings obtained using these animal models, focusing on the role of opioids systems, corticotropin-release hormone and neurokinins, followed by dopaminergic, glutamatergic, and GABAergic neurotransmissions in alcohol seeking behavior.

Effects of pharmacological inhibition of the centrally-projecting Edinger-Westphal nucleus on ethanol-induced conditioned place preference and body temperature

Alcohol use disorder is a chronic disease characterized in part by repeated relapsing events. Exposure to environmental stimuli or cues that have previously been associated with the effects of alcohol can promote relapse through the triggering of craving for alcohol. Therefore, identifying and characterizing neuronal populations that may regulate these associations is of the upmost importance. Previous studies have implicated the centrally-projecting Edinger Westphal nucleus (EWcp) in this process, as the EWcp is both sensitive to, and can regulate alcohol intake. To date however, it is unclear if the EWcp is involved in the formation or expression of these alcohol-cue associations. As such, the present studies examined the involvement of the EWcp in male DBA/2J mice in the acquisition and expression of place preference for an alcohol-paired cue using the conditioned place preference (CPP) procedure. Pharmacological inhibition of the EWcp via the GABAA and GABAB receptor agonists muscimol and baclofen did not affect either the acquisition or the expression of CPP. Follow up studies did find however, that pharmacological inhibition of the EWcp increased body temperature and prevented alcohol-induced increases in c-Fos expression in the EWcp. When considered in light of previous studies, the present results indicate that the EWcp may be involved in the regulation of alcohol self-administration, and not conditioned alcohol-seeking. Additionally, the present studies provide further evidence for the involvement of the EWcp in thermoregulation and help elucidate the molecular mechanisms by which alcohol increases c-Fos in the EWcp.

Effects of fendiline on cocaine-seeking behavior in the rat

RATIONALE

L-type Ca(2+) channels (LTCC) and GABAB receptors are both possible targets in the development of new pharmacological compounds for cocaine addiction. Drugs that target either receptor attenuate a wide range of cocaine-seeking behaviors in the rat. However, there is no current human-approved pharmacotherapeutic intervention for psychostimulant addiction.

OBJECTIVES

This study examined the effects of a human-approved LTCC blocker, fendiline, on cocaine-taking and cocaine-seeking behavior in rats. The effects of combining fendiline with the GABAB receptor agonist baclofen on cocaine self-administration were also tested.

METHODS

Male Wistar rats were trained to self-administer cocaine, and the effects of fendiline pretreatment (vehicle, 1.78, 3.16, 5.62 mg/kg, intraperitoneal (IP)) were tested on progressive ratio responding and cue- and drug-induced reinstatement. The effects of baclofen (vehicle, 0.56, 1.78, 3.16, 5.62 mg/kg, IP) combined with fendiline (5.62 mg/kg, IP) were tested on progressive ratio responding. Control experiments measured locomotor activity and lever pressing for food in rats that received both baclofen and fendiline prior to the test session.

RESULTS

Acute injections of fendiline prior to cue- or drug-induced reinstatement significantly attenuated lever-pressing behavior (p < 0.05). Fendiline and baclofen, but not fendiline alone, not only significantly attenuated breakpoints, but also impaired general motor behavior and naturalistic reinforcement (p < 0.05).

CONCLUSION

These data suggest that the LTCC blocker fendiline may represent a novel pharmacotherapeutic intervention to prevent reinstatement to cocaine seeking. Also, co-administration of fendiline and baclofen not only can attenuate the motivation to take cocaine, but also impairs general motor behavior and naturalistic reinforcement.

GABA(B) modulation of dopamine release in the nucleus accumbens core

Modulation of the concentration of dopamine (DA) released from dopaminergic terminals in the nucleus accumbens (NAc) influences behaviours such as the motivation to obtain drugs of abuse. γ-Aminobutyric acid type B (GABAB ) receptors are expressed throughout the mesolimbic circuit, including in the NAc, and baclofen, an agonist of GABAB receptors, can decrease drug-seeking behaviours. However, the mechanism by which GABAB receptors modulate terminal DA release has not been well studied. We explored how baclofen modulates the concentration of DA released from terminals in the NAc core using fast-scan cyclic voltammetry in brain slices from adult male C57BL/6J mice. We found that baclofen concentration-dependently decreased single pulse-evoked DA release. This effect was blocked by the GABAB antagonist, CGP 52432, but not by a nicotinic acetylcholine receptor antagonist. Suppression of DA release by a saturating concentration of baclofen was sustained for up to 1 h. The effect of baclofen was reduced with electrical stimulations mimicking burst firing of DA neurons. Similar to the D2 receptor agonist, quinpirole, baclofen reduced the probability of DA release, supporting a mechanistic overlap with D2 receptors. Baclofen-mediated suppression of DA release persisted after a locomotor-sensitizing cocaine treatment, indicating that GABAB receptors on DA terminals were not altered by cocaine exposure. These data suggest that baclofen-mediated suppression of terminal DA release is due to GABAB activation on DA terminals to reduce the probability of DA release. This effect does not readily desensitize, and persists regardless of chronic cocaine treatment.

Effects of co-administration of the GABAB receptor agonist baclofen and a positive allosteric modulator of the GABAB receptor, CGP7930, on the development and expression of amphetamine-induced locomotor sensitization in rats

BACKGROUND

Several of the behavioral effects of amphetamine (AMPH) are mediated by an increase in dopamine neurotransmission in the nucleus accumbens. However, evidence shows that γ-aminobutyric acid B (GABAB) receptors are involved in the behavioral effects of psychostimulants, including AMPH. Here, we examined the effects of co-administration of the GABAB receptor agonist baclofen and a positive allosteric modulator of the GABAB receptor, CGP7930, on AMPH-induced locomotor sensitization.

METHODS

In a series of experiments, we examined whether baclofen (2.0, 3.0 and 4.0 mg/kg), CGP7930 (5.0, 10.0 and 20.0 mg/kg), or co-administration of CGP7930 (5.0, 10.0 and 20.0 mg/kg) with a lower dose of baclofen (2.0 mg/kg) could prevent the development and expression of locomotor sensitization produced by AMPH (1.0 mg/kg).

RESULTS

The results showed that baclofen treatment prevented both the development and expression of AMPH-induced locomotor sensitization in a dose-dependent manner. Furthermore, the positive allosteric modulator of the GABAB receptor, CGP7930, increased the effects of a lower dose of baclofen on AMPH-induced locomotor sensitization under both conditions.

CONCLUSION

These data provide further evidence that GABAB receptor ligands may modulate psychostimulant-induced behaviors.

Effect of fendiline on the maintenance and expression of methamphetamine-induced conditioned place preference in Sprague-Dawley rats

RATIONALE

Fendiline is a GABAB receptor-positive allosteric modulator and L-type Ca²⁺ channel blocker that is safe for human use. Based on these pharmacological properties, fendiline may be useful to disrupt associative memories that can drive relapse to drug use in drug-addicted individuals

OBJECTIVE

The current study evaluated the potential of fendiline to inhibit the maintenance and expression of learned associations between methamphetamine (meth) and an environmental context using conditioned place preference (CPP) in rats, to model for the associative learning that occurs during drug abuse by humans

METHODS

Following meth conditioning (1 mg/kg), fendiline (5 mg/kg) was administered at various post-conditioning times to ascertain if there was a temporal window during which fendiline would be effective.

RESULTS

Two once-daily injections of fendiline did not influence the maintenance of CPP regardless of the post-conditioning treatment time while 10 once-daily fendiline treatments inhibited CPP maintenance (p < 0.05). Fendiline administered immediately prior to the CPP test inhibited expression of meth-induced CPP in rats with a fendiline treatment history of 10 once-daily injections (p < 0.05) or those that received two injections that corresponded to the last 2 days of the 10-day treatment (p < 0.05). Fendiline did not produce preference or aversion on its own, nor did it alter motivated motor behavior.

CONCLUSION

Maintenance and expression of meth CPP is mitigated by repeated fendiline treatments when administered during the days that precede CPP testing. Reduction in the significance of meth-associated cues can reduce relapse; therefore, fendiline may be of value for addiction therapy in abstinent, meth-addicted humans.

Cocaine-induced adaptations in metabotropic inhibitory signaling in the mesocorticolimbic system

The addictive properties of psychostimulants such as cocaine are rooted in their ability to activate the mesocorticolimbic dopamine (DA) system. This system consists primarily of dopaminergic projections arising from the ventral tegmental area (VTA) and projecting to the limbic and cortical brain regions, such as the nucleus accumbens (NAc) and prefrontal cortex (PFC). While the basic anatomy and functional relevance of the mesocorticolimbic DA system is relatively well-established, a key challenge remaining in addiction research is to understand where and how molecular adaptations and corresponding changes in function of this system facilitate a pathological desire to seek and take drugs. Several lines of evidence indicate that inhibitory signaling, particularly signaling mediated by the Gi/o class of heterotrimeric GTP-binding proteins (G proteins), plays a key role in the acute and persistent effects of drugs of abuse. Moreover, recent evidence argues that these signaling pathways are targets of drug-induced adaptations. In this review we discuss inhibitory signaling pathways involving DA and the inhibitory neurotransmitter GABA in two brain regions - the VTA and PFC - that are central to the effects of acute and repeated cocaine exposure and represent sites of adaptations linked to addiction-related behaviors including sensitization, craving, and relapse.

GABAB receptor activation attenuates the stimulant but not mesolimbic dopamine response to ethanol in FAST mice

Neural processes influenced by γ-aminobutyric acid B (GABA(B)) receptors appear to contribute to acute ethanol sensitivity, including the difference between lines of mice bred for extreme sensitivity (FAST) or insensitivity (SLOW) to the locomotor stimulant effect of ethanol. One goal of the current study was to determine whether selection of the FAST and SLOW lines resulted in changes in GABA(B) receptor function, since the lines differ in sensitivity to the GABA(B) receptor agonist baclofen and baclofen attenuates the stimulant response to ethanol in FAST mice. A second goal was to determine whether the baclofen-induced reduction in ethanol stimulation in FAST mice is associated with an attenuation of the mesolimbic dopamine response to ethanol. In Experiment 1, the FAST and SLOW lines were found to not differ in GABA(B) receptor function (measured by baclofen-stimulated [(35)S]GTPγS binding) in whole brain or in several regional preparations, except in the striatum in one of the two replicate sets of selected lines. In Experiment 2, baclofen-induced attenuation of the locomotor stimulant response to ethanol in FAST mice was not accompanied by a reduction in dopamine levels in the nucleus accumbens, as measured by microdialysis. These data suggest that, overall, GABA(B) receptor function does not play an integral role in the genetic difference in ethanol sensitivity between the FAST and SLOW lines. Further, although GABA(B) receptors do modulate the locomotor stimulant response to ethanol in FAST mice, this effect does not appear to be due to a reduction in tonic dopamine signaling in the nucleus accumbens.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

Allosteric modulators of GABA(B) receptors: mechanism of action and therapeutic perspective

γ-aminobutyric acid (GABA) plays important roles in the central nervous system, acting as a neurotransmitter on both ionotropic ligand-gated Cl^--channels, and metabotropic G-protein coupled receptors (GPCRs). These two types of receptors called GABA_A (and C) and GABA_B are the targets of major therapeutic drugs such as the anxiolytic benzodiazepines, and antispastic drug baclofen (lioresal®), respectively. Although the multiplicity of GABA_A receptors offer a number of possibilities to discover new and more selective drugs, the molecular characterization of the GABA_B receptor revealed a unique, though complex, heterodimeric GPCR. High throughput screening strategies carried out in pharmaceutical industries, helped identifying new compounds positively modulating the activity of the GABA_B receptor. These molecules, almost devoid of apparent activity when applied alone, greatly enhance both the potency and efficacy of GABA_B agonists. As such, in contrast to baclofen that constantly activates the receptor everywhere in the brain, these positive allosteric modulators induce a large increase in GABA_B-mediated responses only WHERE and WHEN physiologically needed. Such compounds are then well adapted to help GABA to activate its GABA_B receptors, like benzodiazepines favor GABA_A receptor activation. In this review, the way of action of these molecules will be presented in light of our actual knowledge of the activation mechanism of the GABA_B receptor. We will then show that, as expected, these molecules have more pronounced in vivo responses and less side effects than pure agonists, offering new potential therapeutic applications for this new class of GABA_B ligands.

Neurogenetics of dopaminergic receptor supersensitivity in activation of brain reward circuitry and relapse: proposing "deprivation-amplification relapse therapy" (DART)

BACKGROUND AND HYPOTHESIS

It is well known that after prolonged abstinence, individuals who use their drug of choice experience a powerful euphoria that often precipitates relapse. While a biological explanation for this conundrum has remained elusive, we hypothesize that this clinically observed "supersensitivity" might be tied to genetic dopaminergic polymorphisms. Another therapeutic conundrum relates to the paradoxical finding that the dopaminergic agonist bromocriptine induces stronger activation of brain reward circuitry in individuals who carry the DRD2 A1 allele compared with DRD2 A2 allele carriers. Because carriers of the A1 allele relative to the A2 allele of the DRD2 gene have significantly lower D2 receptor density, a reduced sensitivity to dopamine agonist activity would be expected in the former. Thus, it is perplexing that with low D2 density there is an increase in reward sensitivity with the dopamine D2 agonist bromocriptine. Moreover, under chronic or long-term therapy with D2 agonists, such as bromocriptine, it has been shown in vitro that there is a proliferation of D2 receptors. One explanation for this relates to the demonstration that the A1 allele of the DRD2 gene is associated with increased striatal activity of L-amino acid decarboxylase, the final step in the biosynthesis of dopamine. This appears to be a protective mechanism against low receptor density and would favor the utilization of an amino acid neurotransmitter precursor like L-tyrosine for preferential synthesis of dopamine. This seems to lead to receptor proliferation to normal levels and results in significantly better treatment compliance only in A1 carriers.

PROPOSAL AND CONCLUSION

We propose that low D2 receptor density and polymorphisms of the D2 gene are associated with risk for relapse of substance abuse, including alcohol dependence, heroin craving, cocaine dependence, methamphetamine abuse, nicotine sensitization, and glucose craving. With this in mind, we suggest a putative physiological mechanism that may help to explain the enhanced sensitivity following intense acute dopaminergic D2 receptor activation: "denervation supersensitivity." Rats with unilateral depletions of neostriatal dopamine display increased sensitivity to dopamine agonists estimated to be 30 to 100 x in the 6-hydroxydopamine (6-OHDA) rotational model. Given that mild striatal dopamine D2 receptor proliferation occurs (20%-40%), it is difficult to explain the extent of behavioral supersensitivity by a simple increase in receptor density. Thus, the administration of dopamine D2 agonists would target D2 sensitization and attenuate relapse, especially in D2 receptor A1 allele carriers. This hypothesized mechanism is supported by clinical trials utilizing amino acid neurotransmitter precursors, enkephalinase, and catechol-O-methyltransferase (COMT) enzyme inhibition, which have resulted in attenuated relapse rates in reward deficiency syndrome (RDS) probands. If future translational research reveals that dopamine agonist therapy reduces relapse in RDS, it would support the proposed concept, which we term "deprivation-amplification relapse therapy" (DART). This term couples the mechanism for relapse, which is "deprivation-amplification," especially in DRD2 A1 allele carriers with natural D2 agonist therapy utilizing amino acid precursors and COMT and enkepalinase inhibition therapy.

Fendiline-evoked [Ca2+]i rises and non-Ca2+-triggered cell death in human oral cancer cells

The effect of fendiline on cytosolic free Ca(2+) concentrations ([Ca(2+)](i)) and proliferation has not been explored in human oral cancer cells. This study examined whether fendiline altered Ca(2+) levels and caused cell death in OC2 human oral cancer cells. [Ca(2+)](i) and cell viability were measured using the fluorescent dyes fura-2 and WST-1, respectively. Fendiline at concentrations above 10 microM increased [Ca(2+)](i) in a concentration-dependent manner. The Ca(2+) signal was reduced partly by removing extracellular Ca(2+). The fendiline-induced Ca(2+) influx was sensitive to blockade of L-type Ca(2+) channel blockers. In Ca(2+)-free medium, after pretreatment with 50 microM fendiline, 1 microM thapsigargin (an endoplasmic reticulum Ca(2+) pump inhibitor)-induced [Ca(2+)](i) rises were inhibited; and conversely, thapsigargin pretreatment nearly abolished fendiline-induced [Ca(2+)](i) rises. Inhibition of phospholipase C with 2 microM U73122 did not change fendiline-induced [Ca(2+)](i) rises. At concentrations between 5 and 25 microM, fendiline killed cells in a concentration-dependent manner. The cytotoxic effect of 15 microM fendiline was not reversed by prechelating cytosolic Ca(2+) with BAPTA/AM. Collectively, in OC2 cells, fendiline induced [Ca(2+)](i) rises by causing Ca(2+) release from the endoplasmic reticulum and Ca(2+) influx from L-type Ca(2+) channels. Furthermore, fendiline-caused cytotoxicity was not via a preceding [Ca(2+)](i) rise.

Inactivation of the maternal fragile X gene results in sensitization of GABAB receptor function in the offspring

Fragile X syndrome is an X-linked disorder caused by the inactivation of the FMR1 gene, with symptoms ranging from impaired cognitive functions to seizures, anxiety, sensory abnormalities, and hyperactivity. Although fragile X syndrome is considered a typical Mendelian disorder, we have recently reported that the environment, specifically the fmr1(+/-) or fmr1(-/-) [H or knockout (KO)] maternal environment, elicits on its own a partial fragile X-like phenotype and can contribute to the overall phenotype of fmr1(-/0) (KO) male offspring. Genetically fmr1(+/0) (WT) males born to H females (H(maternal) > WT(offspring)), similar to KO male offspring born to H and KO mothers (H > KO and KO > KO), exhibit locomotor hyperactivity. These mice also showed reduced D(2) autoreceptor function, indicating a possible diminished feedback inhibition of dopamine (DA) release in the nigrostriatal and mesolimbic systems. The GABAergic system also regulates DA release, in part via presynaptic GABA(B) receptors (Rs) located on midbrain dopaminergic neurons. Here, we show that the locomotor inhibitory effect of the GABA(B)R agonist baclofen [4-amino-3-(4-chlorophenyl)-butanoic acid] is enhanced in all progeny of mutant mothers (H > WT, H > KO, and KO > KO) compared with WT > WT mice, irrespective of their own genotype. However, increased sensitivity to baclofen was selective and limited to the locomotor response because the muscle-relaxant and sedative effects of the drug were not altered by the maternal environment. These data show that GABA(B)R sensitization, traditionally induced pharmacologically, can also be elicited by the fmr1-deficient maternal environment.

Characterization of (R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one as a positive allosteric modulator of GABAB receptors

BACKGROUND AND PURPOSE

As baclofen is active in patients with anxiety disorders, GABAB receptors have been implicated in the modulation of anxiety. To avoid the side effects of baclofen, allosteric enhancers of GABAB receptors have been studied to provide an alternative therapeutic avenue for modulation of GABAB receptors. The aim of this study was to characterize derivatives of (R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one (rac-BHFF) as enhancers of GABAB receptors.

EXPERIMENTAL APPROACH

Enhancing properties of rac-BHFF were assessed in the Chinese hamster ovary (CHO)-Galpha16-hGABA(B1a,2a) cells by Fluorometric Imaging Plate Reader and GTPgamma[35S]-binding assays, and in rat hippocampal slices by population spike (PS) recordings. In vivo activities of rac-BHFF were assessed using the loss of righting reflex (LRR) and stress-induced hyperthermia (SIH) models.

KEY RESULTS

In GTPgamma[35S]-binding assays, 0.3 microM rac-BHFF or its pure enantiomer (+)-BHFF shifted the GABA concentration-response curve to the left, an effect that resulted in a large increase in both GABA potency (by 15.3- and 87.3-fold) and efficacy (149% and 181%), respectively. In hippocampal slices, rac-BHFF enhanced baclofen-induced inhibition of PS of CA1 pyramidal cells. In an in vivo mechanism-based model in mice, rac-BHFF increased dose-dependently the LRR induced by baclofen with a minimum effective dose of 3 mg kg(-1) p.o. rac-BHFF (100 mg kg(-1) p.o.) tested alone had no effect on LRR nor on spontaneous locomotor activity, but exhibited anxiolytic-like activity in the SIH model in mice.

CONCLUSIONS AND IMPLICATIONS

rac-BHFF derivatives may serve as valuable pharmacological tools to elucidate the pathophysiological roles played by GABAB receptors in the central and peripheral nervous systems.

CGP7930: a positive allosteric modulator of the GABAB receptor

CGP7930 (3-(3',5'-Di-tert-butyl-4'-hydroxy)phenyl-2,2-dimethylpropanol) is a positive allosteric modulator of the metabotropic GABAB receptor. CGP7930 has been found to modulate the GABAB receptor in the open, or high affinity, state increasing agonist affinity for the receptor and signal transduction efficacy following agonist stimulation. The GABAB heteromeric subunit B2, involved in signal transduction but not ligand binding, seems to be the site of action of CGP7930 and similar allosteric modulators. When administered alone in naïve animals, CGP7930 acts as an anxiolytic in rodents without other overt behavioral effects and has also been demonstrated to reduce self-administration of nicotine, cocaine, or alcohol in rodents, suggesting that "fine tuning" of the GABAB receptor by positive allosteric modulators may be able to regulate abuse of these drugs. Baclofen, the GABAB agonist, is currently finding use in treating addiction and various other disorders, but this can result in off-target effects and tolerance. CGP7930 when co-administered with baclofen enhances its potency, which could in theory minimize deleterious effects. Further study of CGP7930 is required, but this compound, and others like it, holds potential in a clinical setting.

Effects of GABAB receptor agents on cocaine priming, discrete contextual cue and food induced relapses

In the present study we investigated the effects of the GABA(B) receptor antagonist (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH 50911), the agonists baclofen and 3-aminopropyl(methyl)phosphinic acid (SKF 97541), and the allosteric positive modulator 3,5-bis(1,1-dimethylethyl)-4-hydroxy-beta,beta-dimethylbenzenepropanol (CGP 7930) on cocaine seeking behavior. The effects of the above drugs on the reinstatement of responding induced by natural reinforcer (food) were also studied. Male Wistar rats were trained to self-administer either cocaine (0.5 mg/kg/infusion) or food (sweet milk) and responding on the reinforcer-paired lever was extinguished. Reinstatement of responding was induced by a noncontingent presentation of the self-administered reinforcer (10 mg/kg cocaine, i.p.), a discrete contextual cue, or a contingent presentation of food. SCH 50911 (3-10 mg/kg) dose-dependently attenuated responding on the previously cocaine-paired lever during both reinstatement conditions, with slightly greater efficacy at reducing conditioned cue reinstatement. At the same time, it failed to alter reinstatement of food-seeking behavior. Baclofen (1.25-5 mg/kg) and SKF 97541 (0.03-0.3 mg/kg) attenuated cocaine- or food-seeking behavior; the effect of the drug appeared more effective for cocaine-seeking than food-seeking. CGP 7930 (10-30 mg/kg) reduced cocaine seeking without affecting food-induced reinstatement on reward seeking. Our results indicate that tonic activation of GABA(B) receptors is required for cocaine seeking behavior in rats. Moreover, the GABA(B) receptor antagonist SCH 50911 was effective in reducing relapse to cocaine at doses that failed to alter reinstatement of food-seeking behavior (present study), basal locomotor activity, cocaine and food self-administration (Filip et al., submitted for publication), suggesting its selective effects on motivated drug-seeking behavior. The potent inhibitory responses on cocaine seeking behavior were also seen following the GABA(B) receptor agonists or the allosteric positive modulator, however, doses of baclofen and SKF 97541 that inhibited cocaine-seeking were only threefold lower of those that inhibited food-seeking. In addition, the direct GABA(B) receptor agonists and the allosteric positive modulator cause decreases in cocaine or food self-administration (Filip et al., submitted for publication), indicating their nonspecific effects on relapse to drug-seeking and drug-taking behavior. In conclusion, the GABA(B) receptor antagonist SCH 50911 seems to be viable treatment for reducing cocaine craving and preventing relapse, while the GABA(B) receptor allosteric positive modulator CGP 7930 may hold the highest promise for attenuating cue-evoked relapses to cocaine as well as the direct rewarding properties of cocaine.

Reducing effect of the positive allosteric modulator of the GABA(B) receptor, GS39,783, on alcohol self-administration in alcohol-preferring rats

RATIONALE AND OBJECTIVES

The positive allosteric modulator of the GABA(B) receptor, GS39,783, has recently been found to suppress acquisition and maintenance of alcohol drinking behavior in selectively bred Sardinian alcohol-preferring (sP) rats exposed to the standard, homecage two-bottle "alcohol vs water" choice regimen. The present study was designed to extend the characterization of the "anti-alcohol" effects of GS39,783 to oral self-administration of alcohol under an operant procedure.

MATERIALS AND METHODS

Two separate groups of male sP rats were trained to lever-press (on an FR4 schedule) to orally self-administer alcohol (15%, v/v) or sucrose (0.3%, w/v) in daily 30-min sessions. Once lever-pressing behavior reached stable levels, the effect of GS39,783 (0, 25, 50, and 100 mg/kg, i.g.) on responding for alcohol and sucrose was determined.

RESULTS

Pretreatment with GS39,783 resulted in a significant, dose-dependent reduction in responding for alcohol; at the dose of 100 mg/kg GS39,783, the number of lever responses for alcohol was reduced by approximately 50% in comparison to vehicle-treated rats. The effect of GS39,783 on alcohol self-administration was specific, as responding for sucrose was completely unaffected by pretreatment with GS39,783.

CONCLUSIONS

These data demonstrate the capability of GS39,783 to attenuate the reinforcing properties of alcohol in alcohol-preferring rats. These data constitute a further piece of experimental evidence in support of the hypothesized role for the GABA(B) receptor in the control of alcohol drinking and reinforcement.

Differential involvement of ventral tegmental GABA(A) and GABA(B) receptors in the regulation of the nucleus accumbens dopamine response to stress

Evidence indicates that dopamine (DA) transmission in nucleus accumbens (NAcc) is modulated by glutamate (GLUT) projections from medial prefrontal cortex (PFC) to NAcc and the ventral tegmental area (VTA). Local NMDA receptor blockade in NAcc has previously been shown to enhance the DA stress response in this region as well as in the VTA. This raises the possibility that the NAcc DA stress response is regulated by GLUT acting at NMDA receptors located on NAcc GABA output neurons that project to the VTA where GABA is known to regulate DA cell activity. Thus, in the present study, we used voltammetry to examine the effects of intra-VTA administration of GABA(A) and GABA(B) agonists and antagonists on restraint stress-induced increases in NAcc DA. The results show that local VTA GABA(B) receptor activation with baclofen (0.01, 0.1 and 1.0 nmol) dose-dependently inhibited the NAcc DA stress response whereas GABA(B) receptor blockade with phaclofen had the opposite effect, resulting in a dose-dependent potentiation of the stress response. A similar potentiation of the NAcc DA stress response was observed following VTA GABA(A) receptor blockade with bicuculline, but only at the highest dose (1.0 nmol). Interestingly, intra-VTA injection of the GABA(A) receptor agonist, muscimol, at the lowest dose (0.01 nmol) but not at the higher doses (0.1 or 1.0 nmol) also potentiated the NAcc DA stress response, suggesting an action mediated primarily at GABA(A) receptors located on non-DA neurons. These results indicate that the NAcc DA stress response is regulated by GABA afferents to VTA DA cells and that this action is differentially mediated by GABA(A) and GABA(B) receptors. The data suggest that the relevant GABA(B) receptors are located on DA neurons whereas the GABA(A) receptors are located on GABA interneurons and perhaps also on DA cells. The present findings are also consistent with the idea that the corticofugal GLUT input to NAcc indirectly regulates stress-induced DA release in this region through the GABA feedback pathway to VTA.

GABA(B) receptor-positive modulation decreases selective molecular and behavioral effects of cocaine

Exposure to cocaine induces selective behavioral and molecular adaptations. In rodents, acute cocaine induces increased locomotor activity, whereas prolonged drug exposure results in behavioral locomotor sensitization, which is thought to be a consequence of drug-induced neuroadaptive changes. Recent attention has been given to compounds activating GABA(B) receptors as potential antiaddictive therapies. In particular, the principle of allosteric positive GABA(B) receptor modulators is very promising in this respect, as positive modulators lack the sedative and muscle relaxant properties of full GABA(B) receptor agonists such as baclofen. Here, we investigated the effects of systemic application of the GABA(B) receptor-positive modulator GS39783 (N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4, 6-diamine) in animals treated with acute and chronic cocaine administration. Both GS39783 and baclofen dose dependently attenuated acute cocaine-induced hyperlocomotion. Furthermore, both compounds also efficiently blocked cocaine-induced Fos induction in the striatal complex. In chronic studies, GS39783 induced a modest attenuation of cocaine-induced locomotor sensitization. Chronic cocaine induces the accumulation of the transcription factor deltaFosB and upregulates cAMP-response-element-binding protein (CREB) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32). GS39783 blocked the induction/activation of DARPP-32 and CREB in the nucleus accumbens and dorsal striatum and partially inhibited deltaFosB accumulation in the dorsal striatum. In summary, our data provide evidence that GS39783 attenuates the acute behavioral effects of cocaine exposure in rodents and in addition prevents the induction of selective long-term adaptive changes in dopaminergic signaling pathways. Further investigation of GABA(B) receptor-positive modulation as a novel therapeutic strategy for the treatment of cocaine dependence and possibly other drugs of abuse is therefore warranted.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

The GABAB receptor allosteric modulator CGP7930, like baclofen, reduces operant self-administration of ethanol in alcohol-preferring rats

GABA systems have been implicated as targets for ethanol at the cellular, molecular and behavioural level. The present study was designed to further examine the potential of the GABA(B) receptor as a target for regulating operant alcohol responding. Given that the prototypic agonist, baclofen, reduces the self-administration of alcohol, we hypothesized that the GABA(B) receptor allosteric modulator, CGP7930, might have similar actions but a reduced side-effect profile. In this context, inbred alcohol-preferring (iP) rats were trained to respond for 10% v/v ethanol in a fixed ratio paradigm; all drug testing was performed under an FR3 schedule. Both baclofen and CGP7930 independently reduced voluntary responding for 10% ethanol in a dose-related manner. Neither drug impacted upon responding for water. A combination of subthreshold doses of baclofen and CGP7930 was also able to reduce operant responding for ethanol, suggesting that CGP7930 is indeed acting to facilitate GABA(B) receptor-mediated signalling in this paradigm. These data demonstrate the potential of positive allosteric modulators of metabotropic GABA(B) receptors to regulate alcohol responding.

Differential Modulation by the GABABReceptor Allosteric Potentiator 2,6-Di-tert-butyl-4-(3-hydroxy-2,2-dimethylpropyl)-phenol (CGP7930) of Synaptic Transmission in the Rat Hippocampal CA1 Area

The recently discovered GABAB receptor-positive allosteric modulators enhanced the potency and efficacy of GABAB receptor agonists in in vitro experiments. These GABAB modulators also attenuated reward and anxiety in behavioral experiments without causing the untoward side effects associated with GABAB receptor activation by agonist administration and hence exhibited potential therapeutic utility. However, the underlying molecular mechanisms enabling the GABAB allosteric modulators to dissociate from the GABAB agonistic side effects remain elusive. To address this question, we have examined the effects of a typical GABAB modulator, 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethylpropyl)-phenol (CGP7930), on GABAB receptor-mediated modulations of both the excitatory and the delayed inhibitory components of hippocampal CA1 synaptic transmission. Using baclofen as an agonist and a multielectrode recording system, we recorded GABAB receptor-mediated modulations of both the field excitatory postsynaptic potentials and the population spikes simultaneously, as well as the paired-pulse inhibition of the population spike. We found that CGP7930 selectively enhanced the baclofen-induced modulation of synaptic inhibition without having any significant effects on the synaptic excitation. Our experiments have therefore revealed a pathway-selective differential modulation of synaptic transmission by CGP7930. This finding provides a synaptic mechanism to support the hypothesis that GABAB potentiators may be a better therapeutic alternative than GABAB agonists for central nervous system disorders.

Clinical potential of GABAB receptor modulators

Metabotropic gamma-aminobutyric acid(B) (GABAB) receptors for the major inhibitory transmitter GABA, together with metabotropic glutamate (mGLuRs) receptors, the extracellular calcium-sensing receptors (CaSRs), some V2R pheromone receptors and T1R taste receptors, belong to the family of 3 G-protein-coupled receptors (GPCRs). GABAB receptors are known to control neuronal excitability and modulate synaptic neurotransmission, playing a very important role in many physiological activities. These receptors are widely expressed and distributed in the nervous system and have been implicated in a variety of neurodegenerative and pathophysiological disorders including epilepsy, spasticity, chronic pain, depression, schizophrenia and drug addiction. To form a functional receptor entity, GABAB receptors must exist as a heterodimer consisting of GABAB1 and GABAB2 receptor subtypes with two 7-transmembrane proteins, and these subunits arise from distinct genes. The GABAB1 subunit binds the endogenous ligand within its extracellular N-terminus, whilst the GABAB2 subunit is not only essential for the correct trafficking of the GABAB1 subunit to the cell surface, but is also responsible for the interaction of the receptor with its cognate G-protein. Allosteric modulation has recently been recognized as an alternative pharmacological approach to gain selectivity in drug action. It is now generally accepted that modulators acting at the allosteric sites provide a novel perspective for the development of subtype-selective agents acting at GPCRs. These agents interact with allosteric binding sites quite separate from the highly conserved agonist binding region. In this review, we present a new class of phenylalkylamines, based on the lead compound fendiline, that are potent positive potentiators of GABAB receptor-mediated function and discuss their putative clinical applications. It is proposed that these new modulators may have therapeutic value in GABAB receptor pharmacology and are capable of selectively modifying GABAB receptor function. The allosteric modulators are offering an attractive and novel means to identify new leads, that are devoid of side effects associated with GABAB receptor agonists, and may, therefore, represent a major advance in the drug discovery process.

The positive allosteric modulator GS39783 enhances GABA(B) receptor-mediated inhibition of cyclic AMP formation in rat striatum in vivo

We studied the effects of the positive allosteric modulator GS39783 on GABA(B) receptors at a biochemical level in vivo. Changes in extracellular levels of cyclic AMP following GABA(B) receptor activation were monitored in the striatum of freely moving rats using microdialysis. Locally applied GABA(B) agonist R(-)-baclofen inhibited cyclic AMP formation stimulated by a water-soluble forskolin analogue in a concentration-dependent manner (EC50 7.3 microM, maximal inhibition 40%). The selective GABA(B) antagonist CGP56999 reversed R(-)-baclofen-induced cyclic AMP inhibition to control levels, but not higher. Orally applied GS39783 lacked effects on its own but, together with a threshold concentration of R(-)-baclofen (1 microM), significantly decreased cyclic AMP formation in a dose-dependent fashion. Effects of GS39783 were revoked with CGP56999, showing dependence on GABA(B) receptor activation and suggesting allosteric modulation as a mechanism of action in vivo. Administered with a maximally active dose of R(-)-baclofen, GS39783 failed to further inhibit cyclic AMP formation. The data obtained with CGP56999 and the lack of effect of GS39783 alone suggest that there is no detectable endogenous activation of GABA(B) receptors controlling cyclic AMP formation in rat striatum. To our knowledge, these results provide the first biochemical demonstration of in vivo activity of a G protein-coupled receptor-positive allosteric modulator.

Reducing effect of the positive allosteric modulators of the GABA(B) receptor, CGP7930 and GS39783, on alcohol intake in alcohol-preferring rats

The gamma-aminobutyric acidB (GABA(B)) receptor full agonists, baclofen and CGP44532, have been found to suppress different aspects of alcohol drinking behavior, including acquisition and maintenance, in selectively bred Sardinian alcohol-preferring (sP) rats. The present study was designed to assess whether this capability extends to the recently synthesized, positive allosteric modulators of the GABA(B) receptor, 2,6-Di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) and N,N'-dicyclopentyl-2-methylsulfanyl-5-nitro-pyrimidine-4,6-diamine (GS39783). In the "acquisition" experiments, CGP7930 (0, 25, 50 and 100 mg/kg; i.g.) and GS39783 (0, 6.25, 12.5 and 25 mg/kg; i.g.) were administered for 5 consecutive days to alcohol-naive sP rats. In the "maintenance" experiments, (0, 50 and 100 mg/kg; i.g.) and GS39783 (0, 50 and 100 mg/kg; i.g.) were administered for 5 consecutive days to alcohol-experienced sP rats. Alcohol intake was evaluated under the standard, homecage 2-bottle "alcohol (10%, v/v) vs water" regimen with unlimited access for 24 h/day. Both CGP7930 and GS39783 dose-dependently suppressed the acquisition of alcohol drinking behavior. In the "maintenance" experiments, CGP7930 and GS39783 reduced daily alcohol intake by 30-40% only at the highest dose when compared to vehicle-treated rats; this effect tended to vanish on continuing treatment. The results of the present study suggest that positive allosteric modulation of the GABA(B) receptor produced an effect on alcohol drinking behavior similar to that produced by GABA(B) receptor full agonists. These data also suggest that positive allosteric modulation of the GABA(B) receptor may constitute a potential strategy for developing new drugs for treating alcohol dependence.