Stimulation of GABAB receptors in the basal forebrain selectively impairs working memory of rats in the double Y-maze

Rare antibody-mediated and seronegative autoimmune encephalitis: An update

Paralleling advances with respect to more common antibody-mediated encephalitides, such as anti-N-methyl-D-aspartate receptor (NMDAR) and anti-leucine-rich glioma-inactivated 1 (LGI1) Ab-mediated encephalitis, the discovery and characterisation of novel antibody-mediated encephalitides accelerated over the past decade, adding further depth etiologically to the spectrum of antibody-mediated encephalitis. Herein, we review the major mechanistic, clinical features and management considerations with respect to anti-γ-aminobutyric acid B (GABA_B)-, anti-α-amino-3-hydroxy-5-methyl-4-isoxazolepropinoic receptor- (AMPAR), anti-GABA_A-, anti-dipeptidyl-peptidase-like protein-6 (DPPX) Ab-mediated encephalitides, delineate rarer subtypes and summarise findings to date regarding seronegative autoimmune encephalitis.

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.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

Protective effects of maternal administration of curcumin and hesperidin in the rat offspring following repeated febrile seizure: Role of inflammation and TLR4

Neuroinflammation has a key role in seizure generation and perpetuation in the neonatal period, and toll-like receptor 4 (TLR4) pathway has a prominent role in neuroinflammatory diseases. Administration of antioxidants and targeting TLR4 in the embryonic period may protect rat offspring against the next incidence of febrile seizure and its harmful effects. Curcumin and hesperidin are natural compounds with anti-inflammatory and antioxidant properties and have an inhibitory action on TLR4 receptors. We evaluated the effect of maternal administration of curcumin and hesperidin on infantile febrile seizure and subsequent memory dysfunction in adulthood. Hyperthermia febrile seizure was induced on postnatal days 9-11 on male rat pups with 24 h intervals, in a Plexiglas box that was heated to ~45 °C by a heat lamp. We used enzyme-linked immunosorbent assay, Western blotting, malondialdehyde (MDA), and glutathione (GSH) assessment for evaluation of inflammatory cytokine levels, TLR4 protein expression, and oxidative responses in the hippocampal tissues. For assessing working memory and long-term potentiation, the double Y-maze test and Schaffer collateral-CA1 in vivo electrophysiological recording were performed, respectively Our results showed that curcumin and hesperidin decreased TNF-α, IL-10, and TLR4 protein expression and reversed memory dysfunction. However, they did not provoke a significant effect on GSH content or amplitude and slope of recorded fEPSPs in the hippocampus. In addition, curcumin, but not hesperidin, decreased interleukin-1β (IL-1β) and MDA levels. These findings imply that curcumin and hesperidin induced significant protective effects on febrile seizures, possibly via their anti-inflammatory and antioxidant properties and downregulation of TLR4.

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

The role of CA3 GABAB receptors on anxiolytic-like behaviors and avoidance memory deficit induced by D-AP5 with respect to Ca2+ ions

Glutamatergic and GABAergic systems play key roles in the hippocampus and affect the pathogenesis of anxiety- and memory-related processes. Some investigations have assessed the role of balancing the function of these two systems in different areas of the central nervous system (CNS) as an approach to manage the related disorders. We investigated the anxiety and avoidance memory states using the test-retest protocol in the elevated plus maze to understand the role of GABA_B receptors (GABA_BRs) in relation to the NMDA receptor blockade by D-AP5 (an NMDA receptor antagonist). Also, we examined the function of Ca²⁺ ions by blocking its entrance to the cell using SKF96365 (a Ca²⁺ channel blocker). The drugs were injected into the CA3 region before the test. Our data showed that D-AP5 induced anxiolytic-like behaviors and impaired the avoidance memory. Injection of baclofen (a GABA_BR agonist), but not phaclofen (a GABA_BR antagonist) induced anxiolytic-like behaviors. Neither baclofen nor phaclofen altered avoidance memory-related behaviors. When baclofen was injected before D-AP5, it potentiated the anxiolytic-like behaviors induced by D-AP5, but counteracted its effect on avoidance memory. Phaclofen pretreatment attenuated D-AP5-induced anxiolytic-like behaviors, but potentiated its effect on avoidance memory. The effect of baclofen application before D-AP5 on anxiety and phaclofen application before D-AP5 on avoidance memory at the heist doses were accompanied by a decrease in locomotion. The application of SKF96365 did not alter anxiety-like behaviors but induced avoidance memory impairment. SKF96365 application before the combination of baclofen and D-AP5 counteracted the effects produced by the combination of baclofen and D-AP5 on anxiety and memory states. Our findings showed that the CA3 GABA_BRs had a critical role in anxiolytic-like behaviors and avoidance memory deficit induced by D-AP5 and confirmed the role of Ca²⁺ ions in the observed results.

GABA Receptors in the Medial Preoptic Area Modulate the Onset of Oestradiol-Induced Maternal Behaviour in Hysterectomised-Ovariectomised, Pregnant Rats

We studied the participation of GABA neurotransmission in the medial preoptic area (mPOA) with respect to the onset of the pup retrieval response and nest building. Pregnant female rats were implanted with bilateral cannulae in the mPOA on day 12 of pregnancy and, on day 16, the females were hysterectomised and ovariectomised and given 200 μg/kg of oestradiol benzoate. Two days later, the females received one of the following intracerebral drug treatments: GABA_B agonist baclofen (200 ng); GABA_B antagonist phaclofen (1 μg); GABA_A antagonist bicuculline (60 ng); or physiological saline. Five minutes after intracerebral infusion, three foster pups were introduced into the females' home cage. The subjects were observed for pup grouping (retrieval) during 15 min, after which the pups were left with the female. During the next 12 h, an observation was made every 1 h to determine whether the pups had been grouped (retrieved) or not. The GABA_B agonist baclofen reduced the proportion of females retrieving pups from 4 to 8 h following pup introduction. By contrast, both the GABA_A antagonist bicuculline and the GABA_B antagonist phaclofen enhanced the proportion of females retrieving pups during the first 3 h of observation. The latency to pup retrieval in subjects treated with the GABA_B agonist baclofen was significantly longer than that in subjects given any of the antagonists. All females built a nest but baclofen reduced nest quality. These data show that activation of GABA_B receptors in the mPOA has an inhibitory effect on basic maternal behaviours, whereas blockade of either the GABA_A or GABA_B receptor facilitates pup retrieval. It is possible that reduced GABAergic tone in the mPOA is a key element in the initiation of maternal behaviours in postparturient rats.

Opposing Roles of Cholinergic and GABAergic Activity in the Insular Cortex and Nucleus Basalis Magnocellularis during Novel Recognition and Familiar Taste Memory Retrieval

Acetylcholine (ACh) is thought to facilitate cortical plasticity during memory formation and its release is regulated by the nucleus basalis magnocellularis (NBM). Questions remain regarding which neuronal circuits and neurotransmitters trigger activation or suppression of cortical cholinergic activity. During novel, but not familiar, taste consumption, there is a significant increase in ACh release in the insular cortex (IC), a highly relevant structure for taste learning. Here, we evaluate how GABA inhibition modulates cholinergic transmission and its involvement during taste novelty processing and familiar taste memory retrieval. Using saccharin as a taste stimulus in a taste preference paradigm, we examined the effects of injecting the GABAA receptor agonist muscimol or the GABAA receptor antagonist bicuculline into the IC or NBM during learning or retrieval of an appetitive taste memory on taste preference in male Sprague Dawley rats. GABAA receptor agonism and antagonism had opposite effects on cortical ACh levels in novel taste presentation versus familiar taste recognition and ACh levels were associated with the propensity to acquire or retrieve a taste memory. These results indicate that the pattern of cortical cholinergic and GABAergic neuroactivity during novel taste exposure is the opposite of that which occurs during familiar taste recognition and these differing neurotransmitter system states may enable different behavioral consequences. Divergences in ACh and GABA levels may produce differential alterations in excitatory and inhibitory neural processes within the cortex during acquisition and retrieval.

SIGNIFICANCE STATEMENT

During learning and recall, several brain structures act together. This work demonstrates interactions between cortical cholinergic and GABAergic systems during taste learning and memory retrieval. We found that the neuroactivity pattern during novel taste exposure is opposite that which occurs during familiar taste recognition. GABAA receptors must be inactive during novel tasting to enable new memory formation, but must be active and inhibiting acetylcholine release in the cortex to allow memory retrieval. These findings indicate that GABA inhibition modulates cholinergic transmission and that cholinergic-GABAergic system interactions are important during the transition from novel to familiar memory.

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.

M3 muscarinic receptor in the ventral medial prefrontal cortex modulating the expression of contextual fear conditioning in rats

RATIONALE

Basal forebrain cholinergic neurons modulate the activation of cortical neurons by several stimuli such as fear and anxiety. However, the role of the muscarinic receptor in the medial prefrontal cortex (MPFC) in the modulation of the conditioned emotional response (CER) evoked in the model contextual conditioned fear remains unclear.

OBJECTIVES

The objective of this study is to test the hypothesis that inhibition of the muscarinic receptor in ventral MPFC modulates CER observed during animal's re-exposure to the aversive context.

METHODS

Rats implanted with cannulae aimed at the prelimbic (PL) or the infralimbic (IL) were submitted to a high-intensity contextual fear conditioning protocol. Before the test session, they received microinjections of the hemicholinium (choline reuptake blocker), atropine (muscarinic antagonist), J104129 fumarate (M1-M3 muscarinic antagonists), pirenzepine (M1 muscarinic antagonist), neostigmine (inhibitor acetylcholinesterase enzyme), or the systemic administration of the FG7142 (inverse benzodiazepine agonist). Additional independent groups received the neostigmine or FG7142 before the ineffective doses of J104129 fumarate in the low-intensity protocol of contextual fear conditioning.

RESULTS

In the high-intensity protocol, the administration of hemicholinium (1 nmol), atropine (0.06-6 nmol), J104129 fumarate (6 nmol), or pirenzepine (6 nmol) attenuated the expression of CER in rats. However, in the low-intensity protocol, only J10129 fumarate (0.06 nmol) reduced the expression of the CER. Finally, neostigmine (0.1-1 nmol) or FG7142 (8 mg/Kg) increased CER expression, an effect inhibited by the low dose of the J10129 fumarate.

CONCLUSIONS

These results indicated that the blockade of M3 muscarinic receptor in the vMPFC attenuates the CER expression.

Mouse hippocampal GABAB1 but not GABAB2 subunit-containing receptor complex levels are paralleling retrieval in the multiple-T-maze

GABAB receptors are heterodimeric G-protein coupled receptors known to be involved in learning and memory. Although a role for GABAB receptors in cognitive processes is evident, there is no information on hippocampal GABAB receptor complexes in a multiple T maze (MTM) task, a robust paradigm for evaluation of spatial learning. Trained or untrained (yoked control) C57BL/6J male mice (n = 10/group) were subjected to the MTM task and sacrificed 6 h following their performance. Hippocampi were taken, membrane proteins extracted and run on blue native PAGE followed by immunoblotting with specific antibodies against GABAB1, GABAB1a, and GABAB2. Immunoprecipitation with subsequent mass spectrometric identification of co-precipitates was carried out to show if GABAB1 and GABAB2 as well as other interacting proteins co-precipitate. An antibody shift assay (ASA) and a proximity ligation assay (PLA) were also used to see if the two GABAB subunits are present in the receptor complex. Single bands were observed on Western blots, each representing GABAB1, GABAB1a, or GABAB2 at an apparent molecular weight of approximately 100 kDa. Subsequently, densitometric analysis revealed that levels of GABAB1 and GABAB1a but not GABAB2- containing receptor complexes were significantly higher in trained than untrained groups. Immunoprecipitation followed by mass spectrometric studies confirmed the presence of GABAB1, GABAB2, calcium calmodulin kinases I and II, GluA1 and GluA2 as constituents of the complex. ASA and PLA also showed the presence of the two subunits of GABAB receptor within the complex. It is shown that increased levels of GABAB1 subunit-containing complexes are paralleling performance in a land maze.

Genome-Wide Analyses of Working-Memory Ability: A Review

Working memory, a theoretical construct from the field of cognitive psychology, is crucial to everyday life. It refers to the ability to temporarily store and manipulate task-relevant information. The identification of genes for working memory might shed light on the molecular mechanisms of this important cognitive ability and-given the genetic overlap between, for example, schizophrenia risk and working-memory ability-might also reveal important candidate genes for psychiatric illness. A number of genome-wide searches for genes that influence working memory have been conducted in recent years. Interestingly, the results of those searches converge on the mediating role of neuronal excitability in working-memory performance, such that the role of each gene highlighted by genome-wide methods plays a part in ion channel formation and/or dopaminergic signaling in the brain, with either direct or indirect influence on dopamine levels in the prefrontal cortex. This result dovetails with animal models of working memory that highlight the role of dynamic network connectivity, as mediated by dopaminergic signaling, in the dorsolateral prefrontal cortex. Future work, which aims to characterize functional variants influencing working-memory ability, might choose to focus on those genes highlighted in the present review and also those networks in which the genes fall. Confirming gene associations and highlighting functional characterization of those associations might have implications for the understanding of normal variation in working-memory ability and also for the development of drugs for mental illness.

CGP 35348, GABA B receptor antagonist, has a potential to improve neuromuscular coordination and spatial learning in albino mouse following neonatal brain damage

To study the effect of CGP 35348 on learning and memory in albino mice following hypoxia ischemia insult, 10 days old albino mice were subjected to right common carotid artery ligation followed by 8% hypoxia for 25 minutes. Following brain damage, mice were fed on normal rodent diet till they were 13 week old. At this time point, mice were divided into two groups. Group 1 received saline and group 2 intrapertoneally CGP 35348 (1 mg/mL solvent/Kg body weight) for 12 days. A battery of tests used to assess long term neurofunction (Morris water maze, Rota rod and open field) along with brain infarct measurement. Overall CGP 35348 has improved the motor function in male and female albino mice but effects were more pronounced in female albino mice. In open field, CGP 35348 treated female albino mice had demonstrated poor exploratory behavior. During Morris water maze test, gender specific effects were observed as CGP 35348 had improved spatial learning and memory and swimming speed in male albino mice but had no effect in female albino mice following hypoxia ischemia encephalopathy (HIE). We concluded that GABAB receptor antagonists CGP 35348 can be used to improve gender based spatial memory.

Postsynaptic GABAB receptor activity regulates excitatory neuronal architecture and spatial memory

Cognitive dysfunction is a common symptom in many neuropsychiatric disorders and directly correlates with poor patient outcomes. The majority of prolonged inhibitory signaling in the brain is mediated via GABAB receptors (GABABRs), but the molecular function of these receptors in cognition is ill defined. To explore the significance of GABABRs in neuronal activity and cognition, we created mice with enhanced postsynaptic GABABR signaling by mutating the serine 783 in receptor R2 subunit (S783A), which decreased GABABR degradation. Enhanced GABABR activity reduced the expression of immediate-early gene-encoded protein Arc/Arg3.1, effectors that are critical for long-lasting memory. Intriguingly, S783A mice exhibited increased numbers of excitatory synapses and surface AMPA receptors, effects that are consistent with decreased Arc/Arg3.1 expression. These deficits in Arc/Arg3.1 and neuronal morphology lead to a deficit in spatial memory consolidation. Collectively our results suggest a novel and unappreciated role for GABABR activity in determining excitatory neuronal architecture and spatial memory via their ability to regulate Arc/Arg3.1.

Effect of GABAB receptor antagonist (CGP35348) on learning and memory in albino mice

The present study was designed to demonstrate the potential effect of CGP 35348 (GABA_B receptor antagonist) on the learning, memory formation, and neuromuscular coordination in albino mouse. Mice were intrapertoneally injected with 1 mg CGP 35348/mL of distilled water/Kg body weight, while the control animals were injected with equal volume of saline solution. A battery of neurological tests was applied following the intrapertoneal injections. Results of rota rod indicated that CGP 35348 had no effect on neuromuscular coordination in both male (P = 0.528) and female (P = 0.125) albino mice. CGP 35348 treated females demonstrated poor exploratory behavior during open filed for several parameters (time mobile (P = 0.04), time immobile (P = 0.04), rotations (P = 0.04), and anticlockwise rotations (P = 0.038)). The results for Morris water maze (MWM) retention phase indicated that CGP 35348 treated male mice took shorter latency to reach the hidden platform (P = 0.04) than control indicating improved memory. This observation was complemented by the swim strategies used by mice during training days in MWM as CGP 35348 treated males used more direct and focal approach to reach the platform as the training proceeded.

Separate and combined effects of the GABA(B) agonist baclofen and Δ9-THC in humans discriminating Δ9-THC

BACKGROUND

Our previous research with the GABA reuptake inhibitor tiagabine suggested the involvement GABA in the interoceptive effects of Δ9-THC. The aim of the present study was to determine the potential involvement of the GABA(B) receptor subtype by assessing the separate and combined effects of the GABA(B)-selective agonist baclofen and Δ9-THC using pharmacologically specific drug-discrimination procedures.

METHODS

Eight cannabis users learned to discriminate 30 mg oral Δ9-THC from placebo and then received baclofen (25 and 50mg), Δ9-THC (5, 15 and 30 mg) and placebo, alone and in combination. Self-report, task performance and physiological measures were also collected.

RESULTS

Δ9-THC functioned as a discriminative stimulus, produced subjective effects typically associated with cannabinoids (e.g., High, Stoned, Like Drug), elevated heart rate and impaired rate and accuracy on a psychomotor performance task. Baclofen alone (50 mg) substituted for the Δ9-THC discriminative stimulus, and both baclofen doses shifted the discriminative-stimulus effects of Δ9-THC leftward/upward. Similar results were observed on other cannabinoid-sensitive outcomes, although baclofen generally did not engender Δ9-THC-like subjective responses when administered alone.

CONCLUSIONS

These results suggest that the GABA(B) receptor subtype is involved in the abuse-related effects of Δ9-THC, and that GABA(B) receptors were responsible, at least in part, for the effects of tiagabine-induced elevated GABA on cannabinoid-related behaviors in our previous study. Future research should test GABAergic compounds selective for other GABA receptor subtypes (i.e., GABA(A)) to determine the contribution of the different GABA receptors in the effects of Δ9-THC, and by extension cannabis, in humans.

GABAB receptors in reward processes

gamma-aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the brain which acts through different receptor subtypes. Metabotropic GABA(B) receptors are widely distributed throughout the brain. Alterations in GABA signaling through pharmacological activation or deactivation of the GABA(B) receptor regulate behavior and brain reward processes. GABA(B) receptor agonists and, most recently, positive modulators have been found to inhibit the reinforcing effects of drugs of abuse, such as cocaine, amphetamine, nicotine, ethanol, and opiates. This converging evidence of the effects of GABA(B) compounds on the reinforcing properties of addictive drugs is based on behavioral studies that used a variety of procedures with relevance to reward processes and drug abuse liability, including intracranial self-stimulation, intravenous self-administration under both fixed- and progressive-ratio schedules of reinforcement, reinstatement, and conditioned place preference. GABA(B) receptor agonists and positive modulators block the reinforcing effects of drugs of abuse in these animal models. However, GABA(B) receptor agonists also have undesirable side-effects. GABA(B) receptor modulators have potential advantages as medications for drug addiction. These compounds have a better side-effect profile than GABA(B) agonists because they are devoid of intrinsic agonistic activity in the absence of GABA. They only exert their modulatory actions in concert with endogenous GABAergic activity. Thus, GABA(B) receptor positive modulators are promising therapeutics for the treatment of various aspects of dependence (e.g., initiation, maintenance, and relapse) on various drugs of abuse, such as cocaine, nicotine, heroin, and alcohol.

Activation of M2 muscarinic receptors leads to sustained suppression of hippocampal transmission in the medial prefrontal cortex

Cholinergic innervation of the prefrontal cortex is critically involved in arousal, learning and memory. Dysfunction of muscarinic acetylcholine receptors and their downstream signalling pathways has been identified in mental retardation. To assess the role played by the muscarinic receptors at the hippocampal-frontal cortex synapses, an important relay in information storage, we used a newly developed frontal slice preparation in which hippocampal afferent fibres are preserved. Transient activation of muscarinic receptors by carbachol results in a long-lasting depression of synaptic efficacy at the hippocampal but not cortical pathways or local circuitry. On the basis of a combination of electrophysiological, pharmacological and anatomical results, this input-specific muscarinic modulation can be partially attributed to the M2 subtype of muscarinic receptors, possibly through a combination of pre- and postsynaptic mechanisms.

Substitution profile of Delta9-tetrahydrocannabinol, triazolam, hydromorphone, and methylphenidate in humans discriminating Delta9-tetrahydrocannabinol

RATIONALE

Preclinical evidence suggests that non-cannabinoid neurotransmitter systems are involved in the behavioral and physiological effects of cannabinoids, but relatively little research has been conducted in humans.

OBJECTIVES

The aims of this study were to assess whether oral Delta(9)-tetrahydrocannabinol (Delta(9)-THC) would function as a discriminative stimulus in humans and to examine the substitution profile of drugs acting at opioid, GABA, and dopamine systems.

METHODS

Healthy subjects who reported moderate cannabis use were enrolled. Subjects learned to identify when they received oral 25 mg Delta(9)-THC or placebo under double-blind conditions. Once subjects acquired the discrimination (i.e., > or =80% drug-appropriate responding for four consecutive sessions), multiple doses of Delta(9)-THC, the GABA(A) positive modulator triazolam, the micro-opioid agonist hydromorphone and the dopamine reuptake inhibitor methylphenidate were tested to determine if they shared discriminative-stimulus effects with the training dose of Delta(9)-THC.

RESULTS

Eight subjects (N = 8) accurately discriminated Delta(9)-THC and completed the study. The training dose of Delta(9)-THC functioned as a discriminative stimulus and produced prototypical subject-rated drug effects. All of the drugs tested produced significant effects on the self-report questionnaires, but only Delta(9)-THC substituted for the training dose.

CONCLUSION

These results suggest that the discriminative-stimulus effects of Delta(9)-THC in humans are not directly mediated through central neurotransmitter systems acted upon by the drugs tested in this study.

GABAB receptor antagonism abolishes the learning impairments in rats with chronic atypical absence seizures

Chronic atypical absence seizures are a component of the Lennox-Gastaut syndrome, a disorder invariably associated with severe cognitive impairment in children. However, the cause of this intellectual delay remains unclear. The AY9944 model of chronic atypical absence seizures in rats reliably reproduces the electrographic, behavioral, pharmacological and cognitive features of clinical atypical absence. Using this model, we tested the hypothesis that the cognitive impairment associated with this disorder involves a gamma-aminobutyric acid B (GABA(B)) receptor-mediated mechanism. Therefore, we examined the effect of a specific, high affinity GABA(B) receptor antagonist, CGP35348, on the atypical absence seizures, the working memory deficits, and the altered long-term potentiation that we have observed in the AY9944 model. CGP35348 blocked atypical absence seizures, restored long-term potentiation to normal level, and reversed the cognitive deficit in the AY9944-treated animals. However, dose-response studies showed that lower doses of CGP35348 that failed to influence atypical absence seizure activity, completely reversed the spatial working memory deficit. These data suggest that GABA(B) receptor-mediated mechanisms are responsible for the cognitive dysfunction in the AY9944 model of chronic atypical absence seizures and further, that their cognitive impairment is independent of the seizure activity. The data raise the possibility that GABA(B) receptor antagonists may have therapeutic potential for the treatment of cognitive impairment in epilepsy syndromes where atypical absence seizures are a component.

Effect of antidepressants on GABA(B) receptor function and subunit expression in rat hippocampus

Laboratory and clinical studies suggest that depression is associated with changes in the hippocampus and that this brain region is a major target for antidepressant drugs. Given the data suggesting that GABA(B) receptor antagonists display antidepressant properties, the present study was undertaken to assess the effect of antidepressant administration on GABA(B) receptors in the rat hippocampus to determine whether changes in this regional receptor system may play a role in the response to these agents. Rats were administered (i.p.) the monoamine oxidase inhibitors tranylcypromine (10mg/kg) or phenelzine (10mg/kg), the tricyclic antidepressant desipramine (15 mg/kg), or fluoxetine (5mg/kg), a selective serotonin re-uptake inhibitor, once daily for seven consecutive days. Two hours following the last drug treatment the hippocampal tissue was prepared for defining the distribution and quantity of GABA(B) receptor subunits using in situ hybridization and for assessing GABA(B) receptor function by quantifying baclofen-stimulated [(35)S]-GTPgammaS binding. All of these antidepressants selectively increased the expression of the GABA(B(1a)) subunit in hippocampus, having no consistent effect on the expression of GABA(B(1b)) or GABA(B(2)). Moreover, except for fluoxetine, these treatments increased GABA(B) receptor function in this brain region. The results indicate that an enhancement in the production of hippocampal GABA(B(1a)) subunits may be a component of the response to antidepressants, supporting a possible role for this receptor in the symptoms of depression and the treatment of this condition.

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.

Effects of ethanol and GABAB drugs on working memory in C57BL/6J and DBA/2J mice

RATIONALE

It has been suggested that GABA(B) receptors may be part of a neural substrate mediating some of the effects of ethanol.

OBJECTIVE

The purpose of this experiment was to investigate, in mice, the effects of ethanol on working memory in a delayed matching-to position (DMTP) task, and additionally to determine if these effects were modulated by GABA(B) receptors.

METHODS

Female C57BL/6J and DBA/2J mice were trained in the DMTP task, and after asymptotic levels of performance accuracy were achieved, injections (IP) of ethanol, baclofen, or phaclofen were administered. Baclofen or phaclofen were then co-administered with ethanol. Each test was repeated twice.

RESULTS

Ethanol caused deficits in working memory at 2.0 g/kg and higher. The highest dose (2.5 g/kg) produced additional non-specific effects, indicative of sedation. Baclofen increased performance accuracy (2.5 mg/kg), while decreasing the total number of trials completed. When combined with ethanol (1.5 g/kg), baclofen increased memory deficits at the highest dose (7.5 mg/kg). Phaclofen increased performance accuracy at 10 and 30 mg/kg but had no effect on the total number of trials completed. When combined with ethanol (2.5 g/kg), phaclofen did not significantly alter ethanol-induced deficits in performance.

CONCLUSIONS

Analyses of performance accuracy, total trials completed and variables indexing bias and motor impairment indicated that GABA(B) drugs modulate working memory in a behaviorally specific manner. Overall, these receptors may be part of a neural substrate that modulates some of the effects of ethanol.

Baclofen interactions with nicotine in rats: effects on memory

Nicotine has been shown in numerous previous studies to significantly improve memory on the radial-arm maze, yet the critical mechanisms underlying this effect are not fully characterized. Nicotine stimulates the release of a number of neurotransmitters important for memory function including (gamma-aminobutyric acid) GABA. The importance of nicotinic-GABA interactions regarding memory is currently unknown. The purpose of the current study was to determine the interactive effects of nicotine and the GABA agonist baclofen on working memory function as measured by choice accuracy in the radial-arm maze. Female Sprague-Dawley rats trained to asymptotic performance levels on a win-shift eight-arm radial maze task were used for assessment of nicotine-baclofen interactions. Low doses of baclofen improved memory performance while higher doses impaired it. Nicotine, as seen before, improved memory performance. Nicotine also significantly reversed the higher dose baclofen-induced deficit. These data show the importance of both nicotinic and GABA systems in working memory function and the interactions between these two transmitter receptor systems. This not only provides information concerning the neural bases of cognitive performance, it also lends insight into new combination treatments for memory impairment.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effects of intrahippocampal injection of GABAergic drugs on memory retention of passive avoidance learning in rats

The effect of post-training intrahippocampal injection of gamma-aminobutyric acid (GABA) receptor agonists and antagonists, immediately after a training session on memory retention of passive avoidance learning in rats, was measured in the presence and absence of physostigmine. Post-training treatments were carried out in all the experiments. The different doses of the GABAA receptor agonist muscimol (2, 4 and 6 microg/rat) decreased memory retention in rats dose-dependently. The higher response was obtained with 6 microg/rat of the drug. When the GABAA receptor antagonist bicuculline (0.5, 1, 2 and 4 microg/rat) was administered, only one dose of the drug (1 microg/rat) increased memory retention; however, the antagonist reduced the effect of muscimol. The GABAB receptor agonist, baclofen (0.25, 0.5, 1 and 2 microg/rat) also reduced memory retention in the animals. Intrahippocampal injection of lower doses of the GABAB receptor antagonist CGP35348 (P-[3-aminopropyl]-p-diethoxymethyl-phosphinic acid) (2.5, 5, 10 microg/rat) did not effect memory retention, although the higher doses of the drug (25 and 50 microg/rat) decreased memory retention. The doses of antagonist (2.5, 5 and 10 microg/rat), which did not elicit any response alone, reduced the effect of baclofen. The inhibitory response of CGP35348 was also decreased by bicuculline. In another series of experiments, physostigmine improved memory retention. The GABA receptor agonists, muscimol and baclofen, as well as the GABA receptor antagonists bicuculline and CGP35348, decreased the effect of physostigmine. Atropine decreased memory retention by itself and potentiated the response of muscimol and baclofen. It is concluded that GABAA and GABAB receptor activation may be involved in the impairment of memory retention.

Differential effects of bicuculline and muscimol microinjections into the nucleus basalis magnocellularis in taste and place aversive memory formation

The role of the nucleus basalis magnocellularis (NBM) in learning and memory has been demonstrated in different learning paradigms such as conditioned taste aversion (CTA) and inhibitory avoidance (IA). This participation has been related to the cholinergic system, but recent studies have reported the potential role of other neurotransmitters such as GABA. The effects of acute intracerebral administration of the GABAergic antagonist bicuculline (0.05 microg) and the GABAergic agonist muscimol (0.05 microg) into the NBM of male Wistar rats were assessed in CTA and IA learning. In both learning tasks, the drug administration was performed before the acquisition. Taste aversion learning was not affected by the infusion of any of the drugs administered. IA acquisition was not affected by the administration of bicuculline or muscimol, requiring similar number of trials to reach the learning criterion. However, when the rats were tested 24 h later, those injected with bicuculline or muscimol showed an impairment of the IA learning. The present results support a role of the GABAergic system in the consolidation process of IA learning.

CGP 44532, a GABAB receptor agonist, is hedonically neutral and reduces cocaine-induced enhancement of reward

Drugs that alter the function of gamma-aminobutyric acid (GABA) neurotransmission seem to reduce cocaine reinforcement, and as such may be useful in pharmacologically treating cocaine addiction. In the present experiment, the anti-cocaine effects of CGP 44532, a phosphinic acid analogue of GABA, and a highly selective GABA(B) receptor agonist were examined in male Sprague-Dawley rats using brain stimulation reward (BSR) paradigm. In this method, the relationship between the rate of bar pressing and the frequency of stimulation pulses was analyzed in two measures: the maximum rate of responding (MAX) and the frequency necessary to sustain half maximal rate of responding known as the locus of rise (LOR). CGP 44532 was found to be hedonically neutral without producing any measurable effects on performance (MAX). It also dose-dependently reduced cocaine-induced BSR enhancement, in the order of 15-31%, as shown by progressive shifts in LOR towards baseline. Thus, in theory, administration of CGP 44532 might reduce cocaine's hedonic effects, while also maintaining patient compliance. Whether this agent would also be effective at curbing craving, a long-term consequence of drug abuse, remains to be determined.

Sleep is differently modulated by basal forebrain GABA(A) and GABA(B) receptors

There is evidence that GABA plays a major role in sleep regulation. GABA(A) receptor agonists and different compounds interacting with the GABA(A) receptor complex, such as barbiturates and benzodiazepines, can interfere with the sleep/wake cycle. On the other hand, there is very little information about the possible role of GABA(B) receptors in sleep modulation. The nucleus basalis of Meynert (NBM), a cholinergic area in the basal forebrain, plays a pivotal role in the modulation of sleep and wakefulness, and both GABA(A) and GABA(B) receptors have been described within the NBM. This study used unilateral infusions in the NBM to determine the effects of 3-hydroxy-5-aminomethylisoxazole hydrobromide (muscimol hydrobromide, a GABA(A) receptor subtype agonist) and beta-(aminomethyl)-4-chlorobenzenepropanoic acid (baclofen, a GABA(B) receptor subtype agonist) on sleep parameters in freely moving rats by means of polygraphic recordings. Muscimol (0.5 nmol) and baclofen (0.7 nmol) induced an increase in slow-wave sleep and an inhibition of wakefulness. Muscimol, but not baclofen, also caused a decrease in desynchronized sleep parameters. The results reported here indicate that 1) the NBM activation of both GABA(A) and GABA(B) receptors influences the sleep/wake cycle, and 2) GABA(A) but not GABA(B) receptors are important for desynchronized sleep modulation, suggesting that the two GABAergic receptors play different roles in sleep modulation.

Ability of baclofen in reducing alcohol intake and withdrawal severity: I--Preclinical evidence

BACKGROUND

The similarities between the pharmacological effects of the gamma-aminobutyric acid receptor agonist, baclofen, and the alcohol-substituting agent, gamma-hydroxybutyric acid, led us to investigate whether baclofen was capable of reducing (a) ethanol withdrawal syndrome in ethanol-dependent rats and (b) voluntary ethanol intake in ethanol-preferring rats.

METHODS

In experiment 1, Wistar rats were rendered physically dependent on ethanol by the repeated administration of intoxicating doses of ethanol for 6 consecutive days. Baclofen was acutely administered intraperitoneally at doses of 10, 20, and 40 mg/kg. In experiment 2, baclofen (0, 2.5, 5, and 10 mg/kg, intraperitoneally) was administered once a day for 14 consecutive days to ethanol-preferring sP rats that had continuous access to ethanol (10%, v/v) and water under the two-bottle free choice regimen.

RESULTS

In experiment 1, baclofen dose-dependently decreased the intensity of ethanol withdrawal signs; furthermore, 20 mg/kg of baclofen protected from audiogenic seizures in ethanol-withdrawn rats. In experiment 2, baclofen selectively and dose-dependently reduced voluntary ethanol intake; a compensatory increase in water intake left total fluid intake virtually unchanged.

CONCLUSIONS

These results are in close agreement with those of a preliminary clinical study and suggest that baclofen may constitute a novel therapeutic agent for alcoholism.

Role of arginine in immunonutrition

Arginine plays an important role in many physiologic and biologic processes beyond its role as a protein-incorporated amino acid. Dietary supplementation of arginine can enhance wound healing, regulate endocrine activity and potentiate immune activity. Under normal unstressed conditions the arginine requirement of adult humans is fulfilled by endogenous sources, however this is compromised during times of stress, especially in critical illness. These finding have led to use of arginine supplementation as part of an immune-enhancing dietary regimen to help combat the immune suppression seen in such patients. Though the results from studies examining the use of this type of immunonutrition in critically ill patients are far from definitive, they are promising that this mode of therapy may be of some advantage. A better understanding of the in vivo biology of arginine and its metabolism is necessary to truly define a benefit from arginine supplementation.

Septo-hippocampal drug interactions in post-trial memory processing

To determine if serotonin and GABA regulate post-trial memory processing of the cholinergic projection from the septum to the hippocampus, mice were trained on footshock avoidance in a T-maze. Immediately after training, drugs were injected into the septum, hippocampus or both. Retention was tested 1 week after training and drug administration. Ketanserin, a serotonin type 2 receptor antagonist at a dose of 0.5 ng, had no measurable effect on retention, but it reduced the dose of bicuculline, in the septum, or arecoline in the hippocampus that was needed to improve retention. DOI, a serotonin type 2 receptor agonist at a dose of 2.5 ng, had the opposite effect of increasing the doses of bicuculline and arecoline needed to improve retention. Bicuculline, a GABA(A) receptor antagonist at a dose of 0.1 pg, did not affect retention when injected alone into the septum, but it reduced the dose of arecoline needed to improve retention in the hippocampus. Muscimol, a GABA(A) receptor agonist at a dose of 5 ng, injected into the septum, increased the dose of arecoline needed to improve retention. The results of this study are compatible with models that propose that serotonin innervation from the median raphe drives GABA interneurons in the medial septum that synapse on cholinergic neurons projecting to the hippocampus.

Cerebrovascular evidence for a GABAergic modulation of the cholinergic vasodilatatory basalocortical system in the rat

The present work is aimed to study the functional relevance of GABAergic-cholinergic interactions on the modulation of cerebral blood flow (CBF) exerted by the basalocortical system. Injections of GABA into the substantia innominata (SI) induce increases in blood flow in several cortical areas and inhibit partly the increases in cortical blood flow induced by cholinergic activation of this structure. Blockade of local GABAergic receptors by picrotoxin induced almost similar effects. These findings suggest that local GABAergic neurones of the SI exert a complex cortical cerebrovascular modulation at a resting and an activated state.

Immunohistochemical localization of GABA(B) receptors in the rat central nervous system

The recent cloning of two gamma-aminobutyric acid(B) (GABA(B)) receptor isoforms (GABA(B)R1a/b), which are probably splice variants of the same gene transcript, allowed us to develop an antiserum that recognized the receptors in fixed tissue and to map their distribution in the rat central nervous system (CNS). We also investigated whether GABA(B)R1 colocalizes with glutamic acid decarboxylase (GAD), a marker of GABAergic cell bodies and terminals. Although GABA(B)R1-like immunoreactivity (GABA(B)R1-LI) was distributed throughout the CNS, several distinct distribution patterns emerged: (1) all monoaminergic brainstem cell groups appeared to contain very high levels of GABA(B)R1, (2) a very high intensity of GABA(B)R1-LI was observed in the majority of the cholinergic regions in the CNS, with exception of motoneurons of the third through sixth cranial nerve nuclei, and (3) a low density of the receptor was observed in most of the nuclei that contain cell bodies of GABAergic projection neurons. The highest GABA(B)R1 labeling was observed in the thalamus, interpeduncular nucleus and medial habenula. Cell bodies were labeled throughout the neuroaxis. We also observed dense neuropil labeling in many regions, suggesting that this receptor is localized in dendrites and/or axon terminals. However, in immunofluorescent double-labeling experiments for GABA(B)R1 and GAD, we never observed GABA(B)R1-LI in GAD-positive axon terminals; this result suggests that the GABA(B)R1 may not function as an autoreceptor. Double labeling was observed in the cell bodies of Purkinje neurons and in some interneurons. In general, the immunohistochemical localization of the GABA(B)R1 correlates well with physiologic and autoradiographic data on the distribution of GABA(B) receptors, but some critical differences were noted. Thus, it is likely that additional GABA(B) receptor subtypes remain to be identified.

Nucleus basalis injections of N-methyl-D-aspartate enhance memory of rats in the double Y-maze

N-methyl-D-aspartate (NMDA) receptors have been implicated in learning and memory. Many findings show that NMDA receptor antagonists impair memory. Few studies, however, have investigated the role of NMDA receptor agonists in mnemonic function. The present study examined the effects of nucleus basalis magnocellularis (nbm) injections of NMDA on memory. Rats were trained in a two-component double Y-maze task consisting of a spatial discrimination and a delayed alternation. Rats (n = 7) were surgically implanted with bilateral cannulae in the nbm prior to maze training. Once trained, animals received bilateral nbm injections (0.5 microl) of saline (0.9%), NMDA (50, 75, and 100 ng/side), and the benzodiazepine receptor partial inverse agonist N-methyl-beta-carboline-3-carboxamide (FG 7142; 200 ng/side), in a counterbalanced order. During testing, delays (0, 30, 60 s) were introduced. Nbm FG 7142 or NMDA (50 ng/side) produced an improvement in the delayed alternation task. Results support the hypothesis that nbm NMDA receptors are involved in cognitive processes mediating memory.

The neurosteroid pregnenolone sulfate infused into the nucleus basalis increases both acetylcholine release in the frontal cortex or amygdala and spatial memory

The effects of an infusion (5 ng) of the neurosteroid pregnenolone sulfate into the nucleus basalis magnocellularis on acetylcholine release in the frontoparietal cortex and basolateral amygdala were evaluated during the 130 min post-injection in male Sprague-Dawley rats using in vivo microdialysis coupled "on line" with high performance liquid chromatography detection. One week later, the same animals were tested for spatial memory after another infusion of pregnenolone sulfate (5 ng) into the nucleus basalis. Results show that pregnenolone sulfate enhanced acetylcholine release by more than 50% of baseline concentrations in the two structures relative to a control injection. The duration of this effect was longer in cortex (130 min) than in amygdala (30 min). Furthermore, pregnenolone sulfate improved memory performance in a task based upon spatial recognition of a familiar environment. A significant positive correlation (r=0.49) was found between the recognition score in the spatial memory test and the levels of acetylcholine release in the frontoparietal cortex but not in the basolateral amygdala. Therefore, our results suggest that the nucleus basalis magnocellularis-cortical pathway could be in part responsible for the promnesic effect of pregnenolone sulfate. This neurosteroid acts as a negative modulator of the GABA(A) receptor complex and positively modulates the N-methyl-D-aspartate receptor, possibly resulting in a global stimulatory effect on central cholinergic neurotransmission.

The pharmacology of post-trial memory processing in septum

The septum is recognized as important in learning and memory, but relatively little is known about the role of specific neurotransmitter receptors in memory processing in the septum. We evaluated the role of the classical neurotransmitters in mice that were prepared for intraseptal microinfusion of drug solution after footshock avoidance training in T-maze. Retention for the footshock training was determined 1 week after training and drug administration. The results indicated that receptor agonists of dopamine, norepinephrine, glutamate and acetylcholine improved retention, while the antagonists impaired retention. Receptor agonists of serotonin, gamma-amino butyric acid (GABA) and opioids impaired retention, while antagonists improved retention.

Baclofen infused in rat hippocampal formation impairs spatial learning

Recent studies show that baclofen, a selective GABA(B) agonist, impairs different kinds of learning. In the present study we investigated the effect of microinfused baclofen into the hippocampus of male Wistar rats, on the performance in the Morris water maze. Rats of 8-10 weeks of age were implanted with cannulae aimed bilaterally at the hippocampal formation. Baclofen (1 microl of 0.2 mM, 2.0 mM, and 20.0 mM) or sterilized saline was microinfused 1 h before each daily session (3 trials/session, 1 session/day) for 4 days. On the fifth day, the animals did not receive drug or saline injections and the retention of the location of the escape platform was tested in a 30 s free swim trial. Results from the free swim trial indicate that the doses of baclofen used during training affected the ability of the rats to swim to the target quadrant. Although no significant difference compared with the saline group was observed, the experimental rats showed a more generalized swim trajectory in the area of the target and both adjacent quadrants. Moreover, 1 microl of 20.0 mM baclofen also impaired the acquisition. We suggest that baclofen has an impairing action on spatial learning, although more studies should be conducted to reach a more precise conclusion.

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.

Mechanism of colchicine impairment on learning and memory, and protective effect of CGP36742 in mice

Fourteen days after hippocampal microinfusion with colchicine (COL), learning and memory ability of mice was significantly impaired, while glutamate (Glu), gamma-aminobutyric acid (GABA), Glu/GABAB and GABAB receptor levels in the cortex and/or the hippocampus were significantly changed. After treatment with a GABAB receptor antagonist, CGP36742, learning and memory impairment caused by COL could be significantly improved, and the above indices in brain regions reversed. These results suggest GABAB antagonists may have therapeutic value in the treatment of Alzheimer's disease.

Modulation of cognitive processes by transsynaptic activation of the basal forebrain

Each of the neurotransmitter-specific afferents to the basal forebrain (BF) carry different types of information which converge to regulate the activity of cholinergic projections to telencephalic areas. Brainstem monoaminergic and cholinergic inputs are critical for context-dependent arousal. GABAergic afferents are gated by a variety of ascending and descending systems, and in addition provide an intrinsic control of BF output excitability. Corticofugal glutamatergic inputs represent reciprocal connections from sites to which BF afferents project, and carry information about the current level of cortical processing intensity and capacity. Peptidergic inputs arise from hypothalamic sources and locally modulate BF output as a function of motivational and homeostatic processes. The significance of these afferent systems can be studied by examining the behavioral consequences of infusion into the BF of drugs that act on the specific receptor systems. Although traditional analyses suggest that the BF has many behavioral functions that can be subdivided regionally, an analysis of studies employing transsynaptic approaches lead to the conceptualization of the BF as having a uniform function, that of maximizing cortical processing efficiency. The BF is conditionally active during specific episodes of acquisition and processing of behaviorally significant, externally-derived information, and drives cortical targets into a state of readiness by reducing interference and amplifying the processing of relevant stimuli and associations, thus allowing for more efficient processing. This paper describes the transsynaptic approach to studying BF function, reviews the neurobiological and behavioral consequences of altering neurotransmitter-specific inputs to the BF, and explores the functional significance of the BF.

Regional age-related alterations in cholinergic and GABAergic receptors in the rat brain

The age-related changes of cholinergic and gamma-aminobutyric acid (GABA)ergic receptors were studied in 3-week- and 6-, 12-, 18- and 24-month-old Fisher 344 male rat brains using receptor autoradiography. [3H]Quinuclidinyl benzilate (QNB), [3H]hemicholinium-3 (HC) and [3H]muscimol were used to label acetylcholine receptors, acetylcholine reuptake sites and GABAA receptors, respectively. In immature rats (3-week-old), [3H]QNB and [3H]muscimol binding showed a significant increase in most brain areas, compared to adult young animals (6-month-old), whereas [3H]HC binding exhibited a significant increase only in the dentate gyrus, substantia nigra and cerebellum. In contrast, [3H]QNB and [3H]HC binding showed no significant changes in all brain areas during aging. On the other hands, [3H]muscimol binding showed a significant reduction in the substantia nigra and cerebellum of adult mature rats (12-month-old). Thereafter, the age-related reduction in [3H]muscimol binding was observed in all brain areas of aged rats (24-month-old). Our results demonstrate that the GABAergic system is susceptible to aging processes in the central nervous system, whereas the cholinergic system is unaltered by aging. Furthermore, our results suggest significant regional changes in both GABAergic and cholinergic systems in the brain even 3 weeks after birth. These findings suggest that the disturbance in GABAergic-cholinergic interactions may play a key role in age-related neurological deficits and cognitive dysfunction.

Baclofen, a selective GABAB receptor agonist, dose-dependently impairs spatial learning in rats

The present investigation assessed the effects of the selective GABAB receptor agonist baclofen (1, 3, and 6 mg/kg) on spatial learning in the Morris water maze, an aversively motivated spatial learning task. Potential anxiolytic and sedative effects of baclofen were also assessed in an open field. Baclofen dose-dependently reduced locomotion in the open field but had little effect on thigmotaxia (anxiety). In the water maze, baclofen dose-dependently impaired spatial learning and reduced swim speed. During the probe trial given after training, only rats treated with the highest dose of baclofen (6 mg/kg) failed to show a bias for the correct quadrant. Following four additional retraining trials, a second drug-reversal probe trial was given and it was found that rats switched from saline to the highest dose of baclofen (6 mg/kg) showed a bias for the correct quadrant, as did rats switched from the two lowest doses of baclofen (1 and 3 mg/kg) to saline. Rats switched from the highest dose of baclofen (6 mg/kg) to saline failed to show a quadrant bias. Performance on a visible platform task was not impaired by baclofen at any dose. Together these results suggest that baclofen resembles GABAA agonists/positive modulators in that it impairs spatial learning, but not performance of a previously acquired escape response; but differs in that it does not reduce thigmotaxia (anxiety). Potential mechanisms by which baclofen impairs mnemonic processes are discussed.

Involvement of cholinergic systems in the deficit of place learning in Morris water maze task induced by baclofen in rats

Effects of oxotremorine on the deficit of place learning in the Morris water maze task induced by baclofen and scopolamine were examined to determine the involvement of brain cholinergic systems in the deficit of learning induced by baclofen. Rats were given 4 training trials per day with the submerged platform at a fixed location in the maze for 4 days. On day 4, rats were required to swim in the pool without the platform after the 4th training trial (probe test). Baclofen as well as scopolamine dose-dependently increased the escape latency in the training trials. In the probe test, baclofen as well as scopolamine dose-dependently reduced the duration in the quadrant where the platform had been originally located. Increased latency in the training trials and reduced duration in the probe test induced by scopolamine were dose-dependently attenuated by oxotremorine. Increased latency and reduced duration in the baclofen-treated rats were improved by oxotremorine as well as 2-hydroxysaclofen. Baclofen but not scopolamine induced motor incoordination in the rotarod test. Oxotremorine failed to improve motor incoordination induced by baclofen. These results suggest that cholinergic systems may be involved in the deficit of place learning induced by baclofen, and that the ameliorative effects of oxotremorine may not be due to improvement of motor incoordination.