CGP 36,742, an orally active GABAB receptor antagonist, facilitates memory in a social recognition test in rats

GABAB Receptors and Cognitive Processing in Health and Disease

GABA_B receptors are implicated in numerous central nervous system-based behaviours and mechanisms, including cognitive processing in preclinical animal models. Homeostatic changes in the expression and function of these receptors across brain structures have been found to affect cognitive processing. Numerous preclinical studies have focused on the role of GABA_B receptors in learning, memory and cognition per se with some interesting, although sometimes contradictory, findings. The majority of the existing clinical literature focuses on alterations in GABA_B receptor function in conditions and disorders whose main symptomatology includes deficits in cognitive processing. The aim of this chapter is to delineate the role of GABA_B receptors in cognitive processes in health and disease of animal models and human clinical populations. More specifically, this review aims to present literature on the role of GABA_B receptors in animal models with cognitive deficits, especially those of learning and memory. Further, it aims to capture the progress and advances of research studies on the effects of GABA_B receptor compounds in neurodevelopmental and neurodegenerative conditions with cognitive dysfunctions. The neurodevelopmental conditions covered include autism spectrum disorders, fragile X syndrome and Down's syndrome and the neurodegenerative conditions discussed are Alzheimer's disease, epilepsy and autoimmune anti-GABA_B encephalitis. Although some findings are contradictory, results indicate a possible therapeutic role of GABA_B receptor compounds for the treatment of cognitive dysfunction and learning/memory impairments for some of these conditions, especially in neurodegeneration. Moreover, future research efforts should aim to develop selective GABA_B receptor compounds with minimal, if any, side effects.

Influence of hippocampal GABAB receptor inhibition on memory in rats with acute β-amyloid toxicity

The neurotransmitter γ-aminobutyric acid (GABA) is involved in the process of memory. It has been reported that the inhibition of GABA_B receptors has beneficial effects on cognition. The aim of this study was to investigate the role of CGP35348 (a GABA_B receptor antagonist) on dentate gyrus GABA_B receptor inhibition and its effects on learning and memory impairments that had been induced in adult male rats by microinjection of β-amyloid (Aβ). Seventy Wistar male rats were randomly divided into seven groups: control, sham (receiving the Aβ vehicle only), Aβ, Aβ + CGP35348 (1, 10, and 100 μg/μL), and CGP35348 alone (10 μg/μL). Memory impairment was induced by unilateral interventricular microinjection of Aβ (6 μg/6 μL). Rats were cannulated bilaterally in the dentate gyrus, and then, they were treated for 20 consecutive days. Learning and memory were assessed using the novel object recognition and passive avoidance learning tests. The discrimination index and the step-through latency were significantly increased in the Aβ + CGP35348 group in comparison to the Aβ only group (P < 0.05 and P < 0.01, respectively). Data showed that the discrimination index was decreased in the Aβ + CGP35348 group in comparison with the control group (P < 0.05) and sham group (P < 0.01). Moreover, the step-through latency was significantly decreased in the Aβ + CGP35348 group in comparison to the control and sham groups (P < 0.01). Data from this study indicated that intra-hippocampal microinjection of the GABA_B receptor antagonist counteracts the learning, memory, and cognitive impairments induced by Aβ. It can be concluded that the GABA_B receptor antagonist is a possible therapeutic agent against the progression of acute Aβ toxicity-induced memory impairment.

GABAergic inhibitory neurons as therapeutic targets for cognitive impairment in schizophrenia

Schizophrenia is considered primarily as a cognitive disorder. However, functional outcomes in schizophrenia are limited by the lack of effective pharmacological and psychosocial interventions for cognitive impairment. GABA (gamma-aminobutyric acid) interneurons are the main inhibitory neurons in the central nervous system (CNS), and they play a critical role in a variety of pathophysiological processes including modulation of cortical and hippocampal neural circuitry and activity, cognitive function-related neural oscillations (eg, gamma oscillations) and information integration and processing. Dysfunctional GABA interneuron activity can disrupt the excitatory/inhibitory (E/I) balance in the cortex, which could represent a core pathophysiological mechanism underlying cognitive dysfunction in schizophrenia. Recent research suggests that selective modulation of the GABAergic system is a promising intervention for the treatment of schizophrenia-associated cognitive defects. In this review, we summarized evidence from postmortem and animal studies for abnormal GABAergic neurotransmission in schizophrenia, and how altered GABA interneurons could disrupt neuronal oscillations. Next, we systemically reviewed a variety of up-to-date subtype-selective agonists, antagonists, positive and negative allosteric modulators (including dual allosteric modulators) for α5/α3/α2 GABA_A and GABA_B receptors, and summarized their pro-cognitive effects in animal behavioral tests and clinical trials. Finally, we also discuss various representative histone deacetylases (HDAC) inhibitors that target GABA system through epigenetic modulations, GABA prodrug and presynaptic GABA transporter inhibitors. This review provides important information on current potential GABA-associated therapies and future insights for development of more effective treatments.

Role of GABA(B) receptors in learning and memory and neurological disorders

Although it is evident from the literature that altered GABAB receptor function does affect behavior, these results often do not correspond well. These differences could be due to the task protocol, animal strain, ligand concentration, or timing of administration utilized. Because several clinical populations exhibit learning and memory deficits in addition to altered markers of GABA and the GABAB receptor, it is important to determine whether altered GABAB receptor function is capable of contributing to the deficits. The aim of this review is to examine the effect of altered GABAB receptor function on synaptic plasticity as demonstrated by in vitro data, as well as the effects on performance in learning and memory tasks. Finally, data regarding altered GABA and GABAB receptor markers within clinical populations will be reviewed. Together, the data agree that proper functioning of GABAB receptors is crucial for numerous learning and memory tasks and that targeting this system via pharmaceuticals may benefit several clinical populations.

Effects of a Proprietary Standardized Orthosiphon stamineus Ethanolic Leaf Extract on Enhancing Memory in Sprague Dawley Rats Possibly via Blockade of Adenosine A 2A Receptors

The aim of the study was to explore a propriety standardized ethanolic extract from leaves of Orthosiphon stamineus Benth in improving impairments in short-term social memory in vivo, possibly via blockade of adenosine A_2A receptors (A2AR). The ethanolic extract of O. stamineus leaves showed significant in vitro binding activity of A2AR with 74% inhibition at 150 μg/ml and significant A2AR antagonist activity with 98% inhibition at 300 μg/mL. A significant adenosine A₁ receptor (A1R) antagonist activity with 100% inhibition was observed at 300 μg/mL. Its effect on learning and memory was assessed via social recognition task using Sprague Dawley rats whereby the ethanolic extract of O. stamineus showed significant (p < 0.001) change in recognition index (RI) at 300 mg/kg and 600 mg/kg p.o and 120 mg/kg i.p., respectively, compared to the vehicle control. In comparison, the ethanolic extract of Polygonum minus aerial parts showed small change in inflexion; however, it remained insignificant in RI at 200 mg/kg p.o. Our findings suggest that the ethanolic extract of O. stamineus leaves improves memory by reversing age-related deficits in short-term social memory and the possible involvement of adenosine A₁ and adenosine A_2A as a target bioactivity site in the restoration of memory.

Effects of disrupting medial prefrontal cortex GABA transmission on decision-making in a rodent gambling task

RATIONALE

Decision-making is a complex cognitive process that is mediated, in part, by subregions of the medial prefrontal cortex (PFC). Decision-making is impaired in a number of psychiatric conditions including schizophrenia. Notably, people with schizophrenia exhibit reductions in GABA function in the same PFC areas that are implicated in decision-making. For example, expression of the GABA-synthesizing enzyme GAD67 is reduced in the dorsolateral PFC of people with schizophrenia.

OBJECTIVES

The goal of this experiment was to determine whether disrupting cortical GABA transmission impairs decision-making using a rodent gambling task (rGT).

METHODS

Rats were trained on the rGT until they reached stable performance and then were implanted with guide cannulae aimed at the medial PFC. Following recovery, the effects of intra-PFC infusions of the GABAA receptor antagonist bicuculline methiodide (BMI) or the GABA synthesis inhibitor L-allylglycine (LAG) on performance on the rGT were assessed.

RESULTS

Intracortical infusions of BMI (25 ng/μl/side), but not LAG (10 μg/μl/side), altered decision-making. Following BMI infusions, rats made fewer advantageous choices. Follow-up experiments suggested that the change in decision-making was due to a change in the sensitivity to the punishments, rather than a change in the sensitivity to reward magnitudes, associated with each outcome. LAG infusions increased premature responding, a measure of response inhibition, but did not affect decision-making.

CONCLUSIONS

Blocking GABAA receptors, but not inhibiting cortical GABA synthesis, within the medial PFC affects decision-making in the rGT. These data provide proof-of-concept evidence that disruptions in GABA transmission can contribute to the decision-making deficits in schizophrenia.

Age-related changes in rostral basal forebrain cholinergic and GABAergic projection neurons: relationship with spatial impairment

Both cholinergic and GABAergic projections from the rostral basal forebrain contribute to hippocampal function and mnemonic abilities. While dysfunction of cholinergic neurons has been heavily implicated in age-related memory decline, significantly less is known regarding how age-related changes in codistributed GABAergic projection neurons contribute to a decline in hippocampal-dependent spatial learning. In the current study, confocal stereology was used to quantify cholinergic (choline acetyltransferase [ChAT] immunopositive) neurons, GABAergic projection (glutamic decarboxylase 67 [GAD67] immunopositive) neurons, and total (neuronal nuclei [NeuN] immunopositive) neurons in the rostral basal forebrain of young and aged rats that were first characterized on a spatial learning task. ChAT immunopositive neurons were significantly but modestly reduced in aged rats. Although ChAT immunopositive neuron number was strongly correlated with spatial learning abilities among young rats, the reduction of ChAT immunopositive neurons was not associated with impaired spatial learning in aged rats. In contrast, the number of GAD67 immunopositive neurons was robustly and selectively elevated in aged rats that exhibited impaired spatial learning. Interestingly, the total number of rostral basal forebrain neurons was comparable in young and aged rats, regardless of their cognitive status. These data demonstrate differential effects of age on phenotypically distinct rostral basal forebrain projection neurons, and implicate dysregulated cholinergic and GABAergic septohippocampal circuitry in age-related mnemonic decline.

Blockade of GABA(B) receptors completely reverses age-related learning impairment

Impaired cognitive functions are well-described in the aging process. GABA(B) antagonists can facilitate learning and memory in young subjects, but these agents have not been well-characterized in aging. Here we show a complete reversal of olfactory discrimination learning deficits in cognitively-impaired aged Fischer 344 rats using the GABA(B) antagonist CGP55845, such that drug treatment restored performance to that on par with young and cognitively-unimpaired aged subjects. There was no evidence that this improved learning was due to enhanced olfactory detection abilities produced by the drug. These results highlight the potential of targeting GABA(B) receptors to ameliorate age-related cognitive deficits and demonstrate the utility of olfactory discrimination learning as a preclinical model for testing novel therapies to improve cognitive functions in aging.

Influence of positive allosteric modulators on GABA(B) receptor coupling in rat brain: a scintillation proximity assay characterisation of G protein subtypes

Little is known concerning coupling of cerebral GABA(B) receptors to G protein subtypes, and the influence of positive allosteric modulators (PAMs) has not been evaluated. These questions were addressed by an antibody-capture/scintillation proximity assay strategy. GABA concentration-dependently enhanced the magnitude of [(35)S]GTPgammaS binding to Galphao and, less markedly, Galphai(1/3) in cortex, whereas Gq and Gs/olf were unaffected. (R)-baclofen and SKF97581 likewise activated Galphao and Galphai(1/3), expressing their actions more potently than GABA. Similar findings were acquired in hippocampus and cerebellum, and the GABA(B) antagonist, CGP55845A, abolished agonist-induced activation of Galphao and Galphai(1/3) in all structures. The PAMs, GS39783, CGP7930 and CGP13501, inactive alone, enhanced efficacy and potency of agonist-induced [(35)S]GTPgammaS binding to Galphao in all regions, actions abolished by CGP55845A. In contrast, they did not modify efficacies at Galphai(1/3). Similarly, in human embryonic kidney cells expressing GABA(B(1a+2)) or GABA(B(1b+2)) receptors, allosteric modulators did not detectably enhance efficacy of GABA at Galphai(1/3), though they increased its potency. To summarise, GABA(B) receptors coupled both to Galphao and to Galphai, but not Gq and Gs/olf, in rat brain. PAMs more markedly enhanced efficacy of coupling to Go versus Gi(1/3). It will be of interest to confirm these observations employing complementary techniques and to evaluate their potential therapeutic significance.

Effects of GABAB receptor antagonists CGP63360, CGP76290A and CGP76291A on learning and memory processes in rodents

Data in literature that use methods for studying the learning and memory processes suggest that GABA and especially GABAB receptor antagonists may be active against amnesia. The aim of our study was to examine the effects of three new GABAB-antagonists on learning and memory processes. Active and passive avoidance tests with negative reinforcement in rats were used. The rats treated with different GABAB receptor antagonists showed improving effects in both tests (active and passive avoidances)on learning as well as on memory retention. There are some differences in their activities, probably due to its chemical structures. The phosphinic analogue CGP63360A is potent to the point that the benzoic one CGP76290A and the left isomer of the benzoic analogue CGP76291A has no effect. It may be concluded that the obtained results on the GABAB receptor antagonists could contribute to their pharmacological characteristics and might be of interest for potential clinical implication.

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.

SGS742: the first GABA(B) receptor antagonist in clinical trials

The GABA(B) receptor antagonist SGS742 (CGP36742) displays pronounced cognition enhancing effects in mice, young and old rats and in Rhesus monkeys in active and passive avoidance paradigms, in an eight-arm radial maze and a Morris water maze and in a social learning task. SGS742 blocks the late inhibitory postsynaptic potential and the paired-pulse inhibition of population spikes recorded from CA1 pyramidal neurons of the hippocampus of rats in vitro and in vivo. SGS742 significantly enhances the release of glutamate, aspartate, glycine and somatostatin in vivo. Chronic administration of SGS742 causes an up-regulation of GABA(B) receptors in the frontal cortex of rats. Single doses cause a significant enhancement of the mRNA and protein levels of NGF and BDNF in the cortex and hippocampus of rats. The observed antidepressant effects of SGS742 in rats may be explained by these findings. SGS742 was well tolerated in experimental animals as well as in young and elderly human volunteers with an absolute bioavailability in humans of 44%. In a Phase II double-blind, placebo-controlled study in 110 patients with mild cognitive impairment (MCI), oral administration of SGS742 at a dose of 600 mg t.i.d. for 8 weeks significantly improved attention, in particular choice reaction time and visual information processing as well as working memory measured as pattern recognition speed. A second Phase II clinical trial in 280 Alzheimer's disease patients is underway.

Gamma-hydroxybutyric acid: neurobiology and toxicology of a recreational drug

gamma-Hydroxybutyric acid (GHB) is a short-chain fatty acid that occurs naturally in mammalian brain where it is derived metabolically from gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the brain. GHB was synthesised over 40 years ago and its presence in the brain and a number of aspects of its biological, pharmacological and toxicological properties have been elucidated over the last 20-30 years. However, widespread interest in this compound has arisen only in the past 5-10 years, primarily as a result of the emergence of GHB as a major recreational drug and public health problem in the US. There is considerable evidence that GHB may be a neuromodulator in the brain. GHB has multiple neuronal mechanisms including activation of both the gamma-aminobutyric acid type B (GABA(B)) receptor, and a separate GHB-specific receptor. This complex GHB-GABA(B) receptor interaction is probably responsible for the protean pharmacological, electroencephalographic, behavioural and toxicological effects of GHB, as well as the perturbations of learning and memory associated with supra-physiological concentrations of GHB in the brain that result from the exogenous administration of this drug in the clinical context of GHB abuse, addiction and withdrawal. Investigation of the inborn error of metabolism succinic semialdehyde deficiency (SSADH) and the murine model of this disorder (SSADH knockout mice), in which GHB plays a major role, may help dissect out GHB- and GABA(B) receptor-mediated mechanisms. In particular, the mechanisms that are operative in the molecular pathogenesis of GHB addiction and withdrawal as well as the absence seizures observed in the GHB-treated animals.

Molecular structure and physiological functions of GABA(B) receptors

GABA(B) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. The cloning of the first GABA(B) receptor cDNAs in 1997 revived interest in these receptors and their potential as therapeutic targets. With the availability of molecular tools, rapid progress was made in our understanding of the GABA(B) system. This led to the surprising discovery that GABA(B) receptors need to assemble from distinct subunits to function and provided exciting new insights into the structure of G protein-coupled receptors (GPCRs) in general. As a consequence of this discovery, it is now widely accepted that GPCRs can exist as heterodimers. The cloning of GABA(B) receptors allowed some important questions in the field to be answered. It is now clear that molecular studies do not support the existence of pharmacologically distinct GABA(B) receptors, as predicted by work on native receptors. Advances were also made in clarifying the relationship between GABA(B) receptors and the receptors for gamma-hydroxybutyrate, an emerging drug of abuse. There are now the first indications linking GABA(B) receptor polymorphisms to epilepsy. Significantly, the cloning of GABA(B) receptors enabled identification of the first allosteric GABA(B) receptor compounds, which is expected to broaden the spectrum of therapeutic applications. Here we review current concepts on the molecular composition and function of GABA(B) receptors and discuss ongoing drug-discovery efforts.

GABA(B) receptor antagonist CGP-36742 enhances somatostatin release in the rat hippocampus in vivo and in vitro

Here, we show the modulation of somatostatin functions in the hippocampus by the orally active 'cognition enhancer' GABA(B) receptor antagonist, (3-aminopropyl)n-butylphosphinic acid (CGP-36742), both in vivo and in vitro. Using high-pressure liquid chromatography-coupled electrospray mass spectrometry, we measured a two-fold increase in the extracellular level of somatostatin to CGP-36742 application in the hippocampus of anaesthetised rats. The basal release of [125I]somatostatin in the synaptosomal fraction was increased by CGP-36742 in concentrations lower than 1 muM. Simultaneous measurement of [14C]Glu and [3H]gamma-aminobutyric-acid ([3H]GABA) showed that CGP-36742 increased their basal release. However, prior [125I]somatostatin application suppressed the increase in the basal release of [14C]Glu and induced a net decrease in the basal release of [3H]GABA. Somatostatin application had a similar effect. In slices, CGP-36742 increased the postsynaptic effect of somatostatin on CA1 pyramidal cells. These results suggest a pre- and postsynaptic functional 'cross-talk' between coexisting GABA(B) and somatostatin receptors in the rat hippocampus.

Neonatal nitric oxide synthase inhibition: social interaction deficits in adulthood and reversal by antipsychotic drugs

Nitric oxide synthase (NOS) is thought to migrate improperly during development in the brains of schizophrenic patients. Also it is known that nitric oxide (NO) effects synaptogenesis during development of the CNS. Previously we have shown that neonatal treatment with a NOS inhibitor effects an animal's sensitivity to amphetamine and PCP. In the present study, neonatal rats were challenged with a NOS inhibitor (L-nitroarginine, 10mg/kg, s.c.) daily on post-natal days (PD) three, four and five. L-Nitroarginine (L-NoArg) treated male rats at adulthood (PD56 and older) had a deficit in social interaction (SI) when placed in an environment with another foreign male rat and this deficit was reproducible on a weekly basis for at least five weeks. Haloperidol failed to significantly reverse this deficit before pronounced secondary effects on general behavior were seen at high doses. However, the atypical antipsychotics, clozapine and olanzapine, were able to significantly reverse this deficit at doses which did not effect baseline SI values. In a separate cohort of animals the effect of DOI was investigated, this was done to ascertain if there was a differential sensitivity of serotonergic pathways in this model. There was no difference in the behavioral score elicited from control or NoArg-treated rats. It is suggested that the SI deficits seen here may be more sensitive to atypical antipsychotics rather than haloperidol.

AMPA and GABA(B) receptor antagonists and their interaction in rats with a genetic form of absence epilepsy

The effects of combined and single administration of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, 7,8-methylenedioxy-1-(4-aminophenyl)-4-methyl-3-acetyl-4,5-dihydro-2,3-benzodiazepine (LY 300164), and of the GABA(B) receptor antagonist gamma-aminopropyl-n-butyl-phosphinic acid (CGP 36742), on spontaneously occurring spike-wave discharges were investigated in WAG/Rij rats. LY 300164 had minor effects; only the highest dose (16 mg/kg) reduced the number of spike-wave discharges in a short time window. CGP 36742 was more effective as it significantly reduced the number of spike-wave discharges and shortened their duration at the doses of 25 and 100 mg/kg. The ED(50) values for the inhibition of spike-wave discharges by LY 300164 and CGP 36742 in a time window 30-60 min after injection were 15.5 and 16.6 mg/kg, respectively. The ED(50) of CGP 36742 was reduced to 8.0 mg/kg when this antagonist was administered in combination with LY 300164 (6 mg/kg). The interaction between the two antagonists appeared to be additive according to isobolographic analysis. Importantly, CGP 36742 and LY 300164 administered either alone or in combination had no apparent effects on behavior. These results may provide information for a rational approach to polytherapy for the treatment of generalized absence epilepsy.

Epilepsy, hyperalgesia, impaired memory, and loss of pre- and postsynaptic GABA(B) responses in mice lacking GABA(B(1))

GABA(B) (gamma-aminobutyric acid type B) receptors are important for keeping neuronal excitability under control. Cloned GABA(B) receptors do not show the expected pharmacological diversity of native receptors and it is unknown whether they contribute to pre- as well as postsynaptic functions. Here, we demonstrate that Balb/c mice lacking the GABA(B(1)) subunit are viable, exhibit spontaneous seizures, hyperalgesia, hyperlocomotor activity, and memory impairment. Upon GABA(B) agonist application, null mutant mice show neither the typical muscle relaxation, hypothermia, or delta EEG waves. These behavioral findings are paralleled by a loss of all biochemical and electrophysiological GABA(B) responses in null mutant mice. This demonstrates that GABA(B(1)) is an essential component of pre- and postsynaptic GABA(B) receptors and casts doubt on the existence of proposed receptor subtypes.

The GABA-B antagonist CGP 36742 prevent PTZ-kindling-provoked amnesia in rats

Deficit in active and inhibitory avoidance behaviour has been found in pentylenetetrazole (PTZ)-kindled rats. This supports the view that memory deficit is an integral part of epilepsy. In the present study we examined the effect of the GABA B antagonist CGP 36742 on memory deficit induced by PTZ-kindling in shuttle-box- and step-down-trained rats. The retention in CGP 36742-treated animals was significantly improved compared to the kindled controls. The mechanisms of action of CGP 36742 is considered. The favourable effect of the GABA B antagonist in cases of amnesia provoked by PTZ-kindling might be of interest in clinical practice.

Social memory is impaired in neonatally ibotenic acid lesioned rats

Postnatal lesion with ibotenic acid in the ventral hippocampus produces several behavioural effects that resemble the symptoms of schizophrenia. In the present study, we tested neonatally lesioned 1-year-old Sprague-Dawley rats for their social memory. It was found that social memory is worsened in lesioned rats. Subchronic treatment with haloperidol (0.025 mg/kg body weight) partly ameliorated this impairment. It was suggested that social memory might be a useful paradigm to test clinically used and potential antipsychotic drugs for their effects on learning and memory processes.

The GABA(B) receptor antagonist CGP36742 attenuates the baclofen- and scopolamine-induced deficit in Morris water maze task in rats

Effects of CGP36742 (3-aminopropyl-n-butylphosphinic acid), an orally active GABA(B) receptor antagonist, on the baclofen- and scopolamine-induced deficit of place learning in the Morris water maze task were examined in rats. Rats were given four training trials per day with the submerged platform at a fixed location in the maze for 4 days. On day 4, the rats were required to swim in the pool without the platform after the fourth training trial (probe test). Intraperitoneal injection of baclofen (4 mg/kg) or scopolamine (0.3 mg/kg) significantly increased the escape latency to reach the platform and decreased the duration in the quadrant where the platform had been originally located. Increased latency in the training trials and decreased duration in the probe test induced by baclofen or scopolamine were significantly attenuated by oral administration of CGP36742 at doses of 10 and 30 mg/kg. In the rotarod test, CGP36742 at a dose of 100 mg/kg but not at doses of 10 or 30 mg/kg antagonized the baclofen-induced motor incoordination. Thus, there was dissociation between the effective doses of CGP36742 in the learning task and those in the sensory motor test. These results suggest the possible involvement of cholinergic systems as well as GABA(B) receptor systems in the CGP36742 action.

Effects of GABAB receptor antagonists on learning and memory retention in a rat model of absence epilepsy

A variety of animal models of absence epilepsy have been described and among these exists a genetically susceptible strain of rat (genetic absence epilepsy rats of Strasbourg (GAERS)). These rats produce periods of behavioural arrest with simultaneous production of cortical spike and wave discharges (SWD). GABAB receptor antagonists suppress completely the production of these spike and wave discharges. GABAB receptor ligands have also been reported to affect cognitive performance in rodents. The present study examined the cognitive performance of GAERS and the influence of GABAB receptor antagonists on this activity. Rats were injected intraperitoneally once per day with saline or a GABAB receptor antagonist (CGP 36742 (3-amino-propyl-n-butyl-phosphinic acid) 100 mg/kg; CGP 56433 ([3-(1-(S)-[(3-(cyclohexylmethyl)hydroxy phosphinyl]-2-(S) hydroxy propyl] amino]ethyl]benzoic acid) ([3-{1-(S)-[{3-(cyclohexylmethyl)hydroxy phosphinyl]-2-(S) hydroxy propyl] amino]ethyl]benzoic acid) 1 mg/kg or CGP 61334 ([3-({[3-[(diethoxymethyl)hydroxy phosphinyl]propyl] amino}methyl]-benzoic acid (1 mg/kg). A two-way active avoidance test paradigm with negative reinforcement was used. Untreated GAERS performed significantly better than non-epileptic rats (P < 0.05) and this enhancement in cognitive performance was sustained in rats treated with the GABAB receptor antagonists.

The GABAB receptor antagonist CGP 36,742 and the nootropic oxiracetam facilitate the formation of long-term memory

The memory-enhancing effects of a single treatment with the GABAB antagonist CGP 36,742 (10 mg/kg) or the nootropic agent oxiracetam (100 mg/kg) given immediately after a learning experience ('post-trial') remain detectable for at least 4 months thereafter. This indicates that in all probability these substances facilitate the formation of the long-term memory trace.