Interaction between benzodiazepine and GABA-A receptors in state-dependent learning

Benzodiazepine-induced anterograde amnesia: detrimental side effect to novel study tool

Benzodiazepines (BZDs) are anxiolytic drugs that act on GABAa receptors and are used to treat anxiety disorders. However, these drugs come with the detrimental side effect of anterograde amnesia, or the inability to form new memories. In this review we discuss, behavioral paradigms, sex differences and hormonal influences affecting BZD-induced amnesia, molecular manipulations, including the knockout of GABAa receptor subunits, and regional studies utilizing lesion and microinjection techniques targeted to the hippocampus and amygdala. Additionally, the relationship between BZD use and cognitive decline related to Alzheimer's disease is addressed, as there is a lack of consensus on whether these drugs are involved in inducing or accelerating pathological cognitive deficits. This review aims to inspire new research directions, as there is a gap in knowledge in understanding the cellular and molecular mechanisms behind BZD-induced amnesia. Understanding these mechanisms will allow for the development of alternative treatments and potentially allow BZDs to be used as a novel tool to study Alzheimer's disease.

Pentylenetetrazol and Morphine Interaction in a State-dependent Memory Model: Role of CREB Signaling

Introduction:

State-dependent (STD) memory is a process, in which the learned information can be optimally retrieved only when the subject is in the state similar to the encoding phase. This phenomenon has been widely studied with morphine. Several studies have reported that Pentylenetetrazole (PTZ) impairs memory in experimental animal models. Due to certain mechanistic interactions between morphine and PTZ, it is hypothesized that PTZ may interfere with the morphine-STD. The cyclic adenosine monophosphate Response Element-Binding (CREB) is considered as the main downstream marker for long-term memory. This study was designed to determine the possible interaction between PTZ and morphine STD and the presumable changes in CREB mRNA.

Methods:

In an Inhibitory Avoidance (IA) model, posttraining morphine (2.5, 5, and 7.5 mg/ kg-i.p.) was used. The pre-test morphine was evaluated for morphine-induced STD memory. Moreover, the effect of a pre-test PTZ (60 mg/kg-i.p.) was studied along with morphine STD. Locomotion testing was carried out using open-field. Eventually, using real-time-PCR, the CREB mRNA changes in the hippocampus were evaluated.

Results:

Posttraining MOR (7.5 mg/kg-i.p.) impaired IA memory (P<0.001). The pre-test injection of similar doses of morphine recovered the morphine-induced memory impairment (P<0.001). The pre-test PTZ impaired the IA memory recall (P<0.001); however, the pre-test PTZ along with morphine STD potentiated the morphine-induced STD (P<0.001). Alterations in CREB mRNA were observed in all groups. No difference was seen in the locomotor activity.

Conclusion:

Presumably, the certain interactive effect of PTZ on morphine-induced STD is mediated through gamma-aminobutyric acid and opioid systems via CREB signaling.

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.

Neurobiological correlates of state-dependent context fear

Retrieval of fear memories can be state-dependent, meaning that they are best retrieved if the brain states at encoding and retrieval are similar. Such states can be induced by activating extrasynaptic γ-aminobutyric acid type A receptors (GABA_AR) with the broad α-subunit activator gaboxadol. However, the circuit mechanisms and specific subunits underlying gaboxadol's effects are not well understood. Here we show that gaboxadol induces profound changes of local and network oscillatory activity, indicative of discoordinated hippocampal–cortical activity, that were accompanied by robust and long-lasting state-dependent conditioned fear. Episodic memories typically are hippocampus-dependent for a limited period after learning, but become cortex-dependent with the passage of time. In contrast, state-dependent memories continued to rely on hippocampal GABAergic mechanisms for memory retrieval. Pharmacological approaches with α-subunit-specific agonists targeting the hippocampus implicated the prototypic extrasynaptic subunits (α₄) as the mediator of state-dependent conditioned fear. Together, our findings suggest that continued dependence on hippocampal rather than cortical mechanisms could be an important feature of state-dependent memories that contributes to their conditional retrieval.

Neurobiological mechanisms of state-dependent learning

State-dependent learning (SDL) is a phenomenon relating to information storage and retrieval restricted to discrete states. While extensively studied using psychopharmacological approaches, SDL has not been subjected to rigorous neuroscientific study. Here we present an overview of approaches historically used to induce SDL, and highlight some of the known neurobiological mechanisms, in particular those related to inhibitory neurotransmission and its regulation by microRNAs (miR). We also propose novel cellular and circuit mechanisms as contributing factors. Lastly, we discuss the implications of advancing our knowledge on SDL, both for most fundamental processes of learning and memory as well as for development and maintenance of psychopathology.

Allopregnanolone induces state-dependent fear via the bed nucleus of the stria terminalis

Gonadal steroids and their metabolites have been shown to be important modulators of emotional behavior. Allopregnanolone (ALLO), for example, is a metabolite of progesterone that has been linked to anxiety-related disorders such as posttraumatic stress disorder. In rodents, it has been shown to reduce anxiety in a number of behavioral paradigms including Pavlovian fear conditioning. We have recently found that expression of conditioned contextual (but not auditory) freezing in rats can be suppressed by infusion of ALLO into the bed nucleus of the stria terminalis (BNST). To further explore the nature of this effect, we infused ALLO into the BNST of male rats prior to both conditioning and testing. We found that suppression of contextual fear occurred when the hormone was present during either conditioning or testing but not during both procedures, suggesting that ALLO acts in a state-dependent manner within the BNST. A shift in interoceptive context during testing for animals conditioned under ALLO provided further support for this mechanism of hormonal action on contextual fear. Interestingly, infusions of ALLO into the basolateral amygdala produced a state-independent suppression of both conditioned contextual and auditory freezing. Altogether, these results suggest that ALLO can influence the acquisition and expression of fear memories by both state-dependent and state-independent mechanisms.

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.

Modulation of muscimol state-dependent memory by α2-adrenoceptors of the dorsal hippocampal area

In the present study, the effects of bilateral intra-dorsal hippocampal (intra-CA1) injections of α2-adrenoceptor agonist and antagonist, on muscimol state-dependent memory were examined in mice. A single-trial step-down passive avoidance task was used for the assessment of memory retention in adult male NMRI mice. Administration of muscimol (0.1 μg/mouse, intra-CA1) 15 min before training or testing induced impairment of memory retention. Injection of the same dose of the drug 15 min before testing restored memory retention impaired under pre-training muscimol influence. Pre-test intra-CA1 administration of the α2-adrenoceptor agonist clonidine (0.5 and 1 μg/mouse) impaired memory retention, although the low dose of the drug (0.25 μg/mouse) did not affect memory retention. Pre-test intra-CA1 administration of the α2-adrenoceptor antagonist yohimbine (1 and 2 μg/mouse) improved memory retention, although the low dose of the drug (0.5 μg/mouse) did not affect memory retention. In other series of experiments, pre-test co-administration of certain doses of clonidine (0.125 and 0.25 μg/mouse, intra-CA1), doses which were ineffective when given alone, and muscimol (0.1 μg/mouse, intra-CA1) significantly inhibited muscimol state-dependent memory. Pre-test intra-CA1 administration of certain doses of yohimbine (0.25 and 0.5 μg/mouse), doses which were ineffective when given alone, improved pre-training muscimol (0.1 μg/mouse)-induced retrieval impairment. Moreover, pre-test co-administration of yohimbine (0.25 and 0.5 μg/mouse, intra-CA1) and muscimol (0.025 μg/mouse, intra-CA1), an ineffective dose, significantly restored the retrieval and induced muscimol state-dependent memory. It may be concluded that the α2-adrenoceptors of the dorsal hippocampal area play an important role in muscimol state-dependent memory.

The use of cognitive enhancers in animal models of fear extinction

In anxiety disorders, such as posttraumatic stress disorders and phobias, classical conditioning pairs natural (unconditioned) fear-eliciting stimuli with contextual or discrete cues resulting in enduring fear responses to multiple stimuli. Extinction is an active learning process that results in a reduction of conditioned fear responses after conditioned stimuli are no longer paired with unconditioned stimuli. Fear extinction often produces incomplete effects and this highlights the relative permanence of bonds between conditioned stimuli and conditioned fear responses. The animal research literature is rich in its demonstration of cognitive enhancing agents that alter fear extinction. This review specifically examines the fear extinguishing effects of cognitive enhancers that act on gamma-aminobutyric acid (GABA), glutamatergic, cholinergic, adrenergic, dopaminergic, and cannabinoid signaling pathways. It also examines the effects of compounds that alter epigenetic and neurotrophic mechanisms in fear extinction. Of these cognitive enhancers, glutamatergic N-methyl d-aspartate (NMDA) receptor agonists, such as D-cycloserine, have enhanced fear extinction in a context-, dose- and time-dependent manner. Agents that function as glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonists, alpha2-adrenergic receptor antagonists (such as yohimbine), neurotrophic factors (brain derived neurotrophic factor or BDNF) and histone deacetylase inhibitors (valproate and sodium butyrate) also improve fear extinction in animals. However, some have anxiogenic effects and their contextual and temporal effects need to be more reliably demonstrated. Various cognitive enhancers produce changes in cortico-amygdala synaptic plasticity through multiple mechanisms and these neural changes enhance fear extinction. We need to better define the changes in neural plasticity produced by these agents in order to develop more effective compounds. In the clinical setting, such use of effective cognitive enhancers with cue exposure therapy, using compounds derived from animal model studies, provides great hope for the future treatment of anxiety disorders.

Behavioral and neural analysis of GABA in the acquisition, consolidation, reconsolidation, and extinction of fear memory

The current review systematically documents the role of gamma-amino-butyric acid (GABA) in different aspects of fear memory-acquisition and consolidation, reconsolidation, and extinction, and attempts to resolve apparent contradictions in the data in order to identify the function of GABA(A) receptors in fear memory. First, numerous studies have shown that pre- and post-training administration of drugs that facilitate GABAergic transmission disrupt the initial formation of fear memories, indicating a role for GABA(A) receptors, possibly within the amygdala and hippocampus, in the acquisition and consolidation of fear memories. Similarly, recent evidence indicates that these drugs are also detrimental to the restorage of fear memories after their reactivation. This suggests a role for GABA(A) receptors in the reconsolidation of fear memories, although the precise neural circuits are yet to be identified. Finally, research regarding the role of GABA in extinction has shown that GABAergic transmission is also disruptive to the formation of newly acquired extinction memories. We argue that contradictions to these patterns are the result of variations in (a) the location of drug infusion, (b) the dosage of the drug and/or (c) the time point of drug administration. The question of whether these GABA-induced memory deficits reflect deficits in retrieval is discussed. Overall, the evidence implies that the processes mediating memory stability consequent to initial fear learning, memory reactivation, and extinction training are dependent on a common mechanism of reduced GABAergic neurotransmission.

Components of lemon essential oil attenuate dementia induced by scopolamine

The anti-dementia effects of s-limonene and s-perillyl alcohol were observed using the passive avoidance test (PA) and the open field habituation test (OFH). These lemon essential oils showed strong ability to improve memory impaired by scopolamine; however, s-perillyl alcohol relieved the deficit of associative memory in PA only, and did not improve non-associative memory significantly in OFH. Analysis of neurotransmitter concentration in some brain regions on the test day showed that dopamine concentration of the vehicle/scopolamine group was significantly lower than that of the vehicle/vehicle group, but this phenomenon was reversed when s-limonene or s-perillyl alcohol were administered before the injection of scopolamine. Simultaneously, we found that these two lemon essential oil components could inhibit acetylcholinesterase activity in vitro using the Ellman method.

Anxiolytic properties of green tea polyphenol (−)-epigallocatechin gallate (EGCG)

Naturally occurring polyphenols are potent antioxidants. Some of these compounds are also ligands for the GABA(A) receptor benzodiazepine site. This feature endows them with sedative properties. Here, the anxiolytic activity of the green tea polyphenol (-)-epigallocatechin gallate (EGCG) was investigated after acute administration in mice, using behavioral tests (elevated plus-maze and passive avoidance tests) and by electrophysiology on cultured hippocampal neurons. Patch-clamp experiments revealed that EGCG (1-10 muM) had no effect on GABA currents. However, EGCG reversed GABA(A) receptor negative modulator methyl beta-carboline-3-carboxylate (beta-CCM) inhibition on GABA currents in a concentration dependent manner. This was also observed at the level of synaptic GABA(A) receptors by recording spontaneous inhibitory synaptic transmission. In addition, EGCG consistently inhibited spontaneous excitatory synaptic transmission. Behavioral tests indicated that EGCG exerted both anxiolytic and amnesic effects just like the benzodiazepine drug, chlordiazepoxide. Indeed, EGCG in a dose-dependent manner both increased the time spent in open arms of the plus-maze and decreased the step-down latency in the passive avoidance test. GABA(A) negative modulator beta-CCM antagonized EGCG-induced amnesia. Finally, state-dependent learning was observable after chlordiazepoxide and EGCG administration using a modified passive avoidance procedure. Optimal retention was observed only when animals were trained and tested in the same state (veh-veh or drug-drug) and significant retrieval alteration was observed in different states (veh-drug or drug-veh). Moreover, EGCG and chlordiazepoxide fully generalized in substitution studies, indicating that they induced indistinguishable chemical states for the brain. Therefore, our data support that EGCG can induce anxiolytic activity which could result from an interaction with GABA(A) receptors.

Sigma1 receptor ligands and related neuroactive steroids interfere with the cocaine-induced state of memory

The present series of experiments examined the involvement of the sigma(1) receptor and related neuroactive steroids in the memory state induced by a very low dose of cocaine. Using a modified passive avoidance procedure in mice, we examined whether cocaine induces state-dependent (StD) learning. Animals trained and tested with saline or the same dose of cocaine (0.1 or 0.3 mg/kg) showed correct retention, measured using two independent parameters: the retention latency and a ratio between the retention latency and the last training latency. Animals trained with cocaine (0.1 mg/kg) and tested with saline or cocaine (0.03, 0.3 mg/kg), or trained with saline and tested with cocaine, showed altered retention parameters, demonstrating that StD occurred. Therefore, cocaine administered before training produced a chemical state used as an endogenous cue to insure optimal retention. Since sigma(1) receptor activation is an important event during the acquisition of cocaine reward, we tested several sigma(1) ligands and related neurosteroids. The sigma(1) agonist igmesine or antagonist BD1047 failed to produce StD, but modified the cocaine state. Among neuroactive steroids, pregnanolone and allopregnanolone, positive modulators of the gamma-aminobutyric acid type A (GABA(A)) receptor, produced StD. However, steroids also acting as sigma(1) agonists, dehydroepiandrosterone (3beta-hydroxy-5alpha-androsten-17-one (DHEA)), pregnenolone, or antagonist, progesterone, failed to induce StD but modified the cocaine state. Furthermore, optimal retention was observed with mice trained with (igmesine or DHEA)+cocaine and tested with a higher dose of cocaine, or with mice trained with (BD1047 or progesterone)+cocaine and tested with vehicle. This study demonstrated that: (i) low doses of cocaine induce a chemical state/memory trace sustaining StD; (ii) modulation of the sigma(1) receptor activation, although insufficient to provoke StD, modulates the cocaine state; (iii) neuroactive steroids exert a unique role in state-dependent vs state-independent learning, via GABA(A) or sigma(1) receptor modulation, and are able to affect the cocaine-induced mnesic trace at low brain concentrations.

The influence of midazolam on active avoidance retrieval and acquisition rate in rats

The purpose of the present study was to examine the influence of midazolam on the retrieval and acquisition rate of two-way active avoidance in rats. In the schedule 2 x 100 trials, the effects of midazolam (0.5-5.0 mg/kg), benzodiazepine binding site antagonist flumazenil (2.5-10.0 mg/kg), specific antagonist of GABA(A) receptor, bicuculline (0.5-4.0 mg/kg), and the blocker of GABA(A) receptor containing Cl(-) channels, picrotoxin (1.0-5.0 mg/kg), on the second day retrieval of avoidance performance were investigated, as well as the influence of the used blockers of GABA(A) receptor function on midazolam effects. Furthermore, the effect of midazolam (1.0 mg/kg) on acquisition rate in the 5 x 50 schedule, as well as the effects of third day treatment changing in that paradigm, was examined. Throughout the study, drugs were given intraperitoneally, 30 min before testing. Midazolam at the dose of 1.0 mg/kg facilitated avoidance retrieval, whereas flumazenil and bicuculline did not significantly change behavior. Picrotoxin (5.0 mg/kg) diminished performance. All three kinds of blockers used abolished facilitatory action of midazolam, confirming GABAergic mediation of the effect of benzodiazepine. Midazolam (1.0 mg/kg) increased acquisition rate during five consecutive days relative to saline, but without significant effect on the first day acquisition. In the case of third day changing of treatments, the intersection of regression rate lines was detected. Results from active avoidance paradigm experimentally support the findings from human studies that in certain circumstances, benzodiazepines, potentiating GABAergic neurotransmission, could produce retrieval-enhancing effects in memory tasks.

[Effects of fluvoxamine on both the desired anxiolytic effect and the adverse motor incoordination and amnesia induced by benzodiazepines]

Fluvoxamine, a selective serotonin reuptake inhibitor, is frequently used along with benzodiazepine anxiolytics in clinics. In this study, the effects of fluvoxamine on the anxiolytic effects as well as adverse effects of benzodiazepines were examined in the light/dark box, rota-rod and passive avoidance tests using mice. Diazepam, ethyl loflazepate and its active metabolite, CM7116, were used as benzodiazepine anxiolytics. The anxiolytic effects of diazepam, ethyl loflazepate and CM7116 were potentiated by intraperitoneal treatment with fluvoxamine at 10 mg/kg, whereas only those of ethyl loflazepate were potentiated by fluvoxamine at 45 mg/kg. The motor incoordination and amnesia induced by ethyl loflazepate and CM7116 were not affected by fluvoxamine, although these adverse effects of diazepam were potentiated by fluvoxamine at 45 mg/kg. Fluvoxamine itself showed no effects in any of the tests. These results suggest that low-dose fluvoxamine potentiates the anxiolytic effects of benzodiazepines, while high-dose fluvoxamine augments the adverse effects depending on the benzodiazepine used. Consequently, when fluvoxamine is administered along with benzodiazepines, the doses of both fluvoxamine and benzodiazepines should be carefully chosen to achieve anxiolytic effects without any adverse results.

Memory of an operant response and of depressed mood retained in activation states of 5-HT(1A) receptors: evidence from rodent models

Three series of studies were conducted to specify the role of 5-HT(1A) receptors in memory; using selective ligands that differentially activate 5-HT(1A) receptors, it was determined whether a change in the activation state of these receptors can lead to deficient retrieval, and whether a so-produced deficit can occur in an animal model of depression. First, in vitro studies of [35S]GTPgammaS binding responses identified ligands that differentially activate 5-HT(1A) receptors in rat hippocampus. WAY 100635, 8-OH-DPAT and flesinoxan induced 5-HT(1A) receptor activation that amounted to -2, +50 and +63%, respectively, of that produced by 5-HT. Second, we determined whether changes in the activation state of 5-HT(1A) receptors could impair the retrieval of an operant response in vivo. Rats treated with either a 5-HT(1A) receptor ligand or saline were trained to lever press for milk reward, and were then tested for retrieval with either the same or another treatment. Animals trained with 8-OH-DPAT retrieved the response when tested in the same state, but not when tested in the saline state, and vice versa. Rats trained with 0.16 mg/kg of 8-OH-DPAT also retrieved the response when tested with the other intermediate-efficacy ligand flesinoxan (0.63 mg/kg), but not when tested in a state of lower-magnitude activation (i.e. with 0.16 mg/kg of WAY 100635). Animals trained with 0.16 mg/kg of WAY 100635 retrieved the response when tested in this same state or with saline, but not when tested in a state of intermediate-magnitude activation (i.e. with 0.16 mg/kg of 8-OH-DPAT). Finally, studies using the forced swimming paradigm indicated that the retrieval of learned immobility was similarly dependent upon the activation state of 5-HT(1A) receptors. The findings indicate that changes in activation states of 5-HT(1A) receptors can impair the retrieval of learned responses. It is suggested that depression may in part be acquired in the course of ontogeny and may be available for retrieval in the same but not in other states; various biological rhythms conceivably define such states.

Clonazepam prevents the development of sensitization to methamphetamine

The GABA-benzodiazepine neurotransmission has been implicated in various forms of plasticity such as kindling and learning. The present study examined the effects of clonazepam (CZP), a GABA-benzodiazepine agonist, on the development of behavioral sensitization to methamphetamine (MA). Rats treated with MA (1 mg/kg, S.C.) for 10 days displayed significantly enhanced motor activity when tested with MA (1 mg/kg) after a 7-8-day withdrawal, indicating the development of behavioral sensitization. Pretreatment with CZP (0.5 and 2.0 mg/kg) prior to MA administration prevented the development of the phenomenon. Rats treated with CZP alone showed no difference in the motor activity compared to those treated with saline. These results suggest that stimulation of GABA-benzodiazepine receptors plays a role in the development of behavioral sensitization.

Acute and delayed effects of alprazolam on flight phobics during exposure

In order to test if a benzodiazepine would enhance or hinder the therapeutic effects of exposure, immediate and delayed effects of alprazolam on flight phobics were assessed by questionnaires and ambulatory physiological recording. Physiological measures included heart rate, skin conductance level and fluctuations, finger temperature, respiratory sinus arrhythmia, and various respiratory measures derived from two bands calibrated for each subject. Twenty-eight women with flying phobia flew twice at a 1-week interval. One and a half hours before flight 1, 14 randomly assigned phobics received double-blind 1 mg of alprazolam and 14 received placebo. On flight 1, alprazolam reduced self-reported anxiety (5.0 vs 7.4) and symptoms (5.3 vs 3.6) more than placebo, but induced an increase in heart rate (114 vs 105 bpm) and respiratory rate (22.7 vs 18.3 breaths/min). Before flight 2, the alprazolam group did not expect to be more anxious than the placebo group (6.7 vs 6.5), but in fact indicated more anxiety during flight (8.5 vs 5.6), and a substantial increase in panic attacks from flight 1 to flight 2 (7% vs 71%). Heart rates in the alprazolam group increased further (123 bpm). Results indicate that alprazolam increases physiological activation under acute stress conditions and hinders therapeutic effects of exposure in flying phobia.

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.

Involvement of GABAB receptor systems in experimental depression: baclofen but not bicuculline exacerbates helplessness in rats

There are two gamma-aminobutyric acid (GABA) hypotheses of the antidepressants action: an increase in GABAA neurotransmission or a decrease in GABAB neurotransmission may contribute to action of antidepressants. In this study, involvement of GABAA and GABAB receptor systems was examined in the learned helplessness paradigm in rats. Rats were injected with bicuculline or baclofen for 14 days. On day 14, the rats were subjected to 15 inescapable shocks. On day 15, they underwent the 40-trial escape test. Baclofen exacerbated the escape failures in the rats subjected to the inescapable shocks, although baclofen had no effects in the animals without shock pre-treatment. Bicuculline failed to influence the escape failures in the rats with the 15-shock pre-treatment. These results suggest that the long-term increase in GABAB neurotransmission but not the long-term attenuation of GABAA neurotransmission may be related to helplessness in rats.

Involvement of GABAB receptor systems in action of antidepressants. II: Baclofen attenuates the effect of desipramine whereas muscimol has no effect in learned helplessness paradigm in rats

Involvement of GABAergic systems in action of antidepressants was examined in the learned helplessness paradigm in rats. Rats were treated with desipramine, baclofen or muscimol for 14 days. On day 14, the rats were subjected to 90 inescapable shocks. On day 15, the rats received the 40-trial escape test. The inescapable shocks induced the subsequent increase in escape failures in the escape test. Desipramine dose-dependently improved the increased escape failures induced by the inescapable shocks. Baclofen attenuated the escape failures-improving effect of desipramine, although baclofen had no effects on the increased escape failures when it was injected alone. Muscimol at any dose failed to influence the increased escape failures. Therefore, it is suggested that the long-term decrease in GABAB neurotransmission may be involved in the action of antidepressants. Our present results do not support the hypothesis that activation of GABAA receptors may contribute to the action of antidepressants.

Involvement of GABA(B) receptor systems in action of antidepressants: baclofen but not bicuculline attenuates the effects of antidepressants on the forced swim test in rats

Involvement of GABAergic systems in action of antidepressants was examined in the forced swim test in rats. Rats were forced to swim in a cylinder for 15 min on day 1 and for 5 min on day 2. Desipramine, mianserin and buspirone, administered after the 15-min swim session on day 1 and before the 5-min swim test on day 2, dose-dependently decreased the duration of immobility in the swim test on day 2. Baclofen attenuated the decreased duration of immobility induced by desipramine, mianserin and buspirone in the swim test, although baclofen did not affect the duration of immobility when it was injected alone. Muscimol dose-dependently decreased the duration of immobility in the swim test on day 2. Bicuculline antagonized the decreased duration of immobility induced by muscimol. However, bicuculline failed to antagonize the decreased duration of immobility induced by desipramine, mianserin and buspirone. These results suggest that GABA(B) but not GABA(A) receptor systems may be involved in action of antidepressants.

Ethanol-induced state-dependent learning is mediated by 5-hydroxytryptamine3 receptors but not by N-methyl-D-aspartate receptor complex

Involvement of N-methyl-D-aspartate (NMDA) receptor complex and 5-hydroxytryptamine3 (5-HT3) receptors in state-dependent learning (SDL) induced by ethanol (EtOH) was investigated in the step-through passive avoidance task in rats. Pre-training injections of EtOH or MK-801 reduced step-through latency in the test session conducted 24 h after the training session. Pre-test as well as pre-training injections of EtOH failed to reduce the latency, while pre-training and pre-test injections of MK-801 reduced the latency. These results show that EtOH but not MK-801 produces SDL. SDL induced by EtOH was blocked by ICS205-930 injected before either the training or test session. However, ICS205-930 failed to block SDL induced by diazepam and muscimol. These results suggest that NMDA receptor complex may not be involved in SDL, and that 5-HT3 receptors may contribute to SDL induced by EtOH but not by diazepam and muscimol.

Involvement of benzodiazepine/GABA-A receptor complex in ethanol-induced state-dependent learning in rats

State-dependent learning (SDL) induced by ethanol (EtOH) was investigated on the step-through passive avoidance task in rats. Pretraining injection of EtOH dose-dependently reduced step-through latency in the test session 24 h after the training. Injection of EtOH (1.0 g/kg) before both the training and test sessions, however, failed to reduce the latency. These results show that EtOH produces SDL. The failure of learning performance in SDL (dissociation in SDL) induced by EtOH was blocked by bicuculline, Ro15-4513 and picrotoxin injected before the training session. The success of learning performance in SDL (non-dissociation in SDL) induced by EtOH was also blocked by bicuculline, Ro15-4513 and picrotoxin injected before the test session. The antagonism of Ro15-4513 against EtOH was blocked by flumazenil. In the substitution test, pretest injection of EtOH produced non-dissociation in SDL in the both of pretraining diazepam-and muscimol-treated rats. On the other hand, neither pretest injection of diazepam nor muscimol produced non-dissociation in the pretraining EtOH-treated rats: asymmetrical cross-substitution between EtOH and diazepam and between EtOH and muscimol was observed. These results suggest that the EtOH-induced SDL is partially mediated by the benzodiazepine (BDZ)/GABA-A receptor complex.

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.

Muscimol induces state-dependent learning in Morris water maze task in rats

Effects of muscimol on the place learning in Morris water maze task were investigated in rats. 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 4 training trials (probe test). Compared to the saline-treated rats, the rats treated with muscimol on day 1-4 showed no modifications of place learning in the training trials and the probe test. However, in the rats treated with muscimol on day 1-3 and treated with saline on day 4, there was increased latency to reach the platform and reduced duration in the quadrant where the platform had been located on day 4. The increased latency in the training trials and reduced duration in the probe test on day 4 was blocked by bicuculline, when bicuculline and muscimol were co-administered on day 1-3, and saline was injected on day 4. Moreover, in the rats treated with muscimol on day 1-3, co-administration of bicuculline and muscimol on day 4 blocked place learning: increased latency in the training trials and reduced duration in the probe test was observed. These results suggest that muscimol induces state-dependent learning (SDL) in Morris water maze task, and that muscimol-induced SDL is mediated by GABAA receptors.

The elevated T-maze: a new animal model of anxiety and memory

In an attempt to analyze different types of anxiety, and at the same time assess memory, a new experimental model was developed. The apparatus, named the elevated T-maze, consisted of three arms of equal dimensions (50 x 10 cm) elevated 50 cm from the ground. One arm, enclosed by 40-cm high walls, was perpendicular to two open arms. The first experimental session was conducted 25 min after IP injection of either drug or saline. To assess inhibitory (passive) avoidance, the rat was placed at the end of the enclosed arm and the time taken to withdraw from this arm was recorded three times in succession. Soon afterwards, the rat was placed at the end of one of the open arms and the time taken to withdraw from this arm was measured, thus estimating one-way escape. To assess memory, inhibitory avoidance and escape were measured again 3 days later, without drug. Dose-response curves were determined for the benzodiazepine anxiolytic and amnestic agent diazepam (DZP, 0.5-4 mg/kg), as well as for ipsapirone (IPS, 0.25-2 mg/kg), an azapirone anxiolytic that is devoid of clinically significant amnestic effects. The doses of 1, 2, and 4 mg/kg DZP and of 1 and 2 mg/kg IPS impaired inhibitory avoidance, an effect that may be viewed as anxiolytic. Inhibitory avoidance remained impaired 3 days later in the rats treated with 1-4 mg/kg DZP, indicating anterograde amnesia. This effect was not due to state-dependent learning, because rats injected both at pretraining and pretesting with 2 mg/kg DZP still showed complete amnesia.(ABSTRACT TRUNCATED AT 250 WORDS)