Anxiolytic-like effects of the noncompetitive NMDA antagonist MK 801

MK-801 and cognitive functions: Investigating the behavioral effects of a non-competitive NMDA receptor antagonist

RATIONALE

MK-801 (dizocilpine) is a non-competitive NMDA receptor antagonist originally explored for anticonvulsant potential. Despite its original purpose, its amnestic properties led to the development of pivotal models of various cognitive impairments widely employed in research and greatly impacting scientific progress. MK-801 offers several advantages; however, it also presents drawbacks, including inducing dose-dependent hyperlocomotion or ambiguous effects on anxiety, which can impact the interpretation of behavioral research results.

OBJECTIVES

The present review attempts to summarize and discuss the effects of MK-801 on different types of memory and cognitive functions in animal studies.

RESULTS

A plethora of behavioral research suggests that MK-801 can detrimentally impact cognitive functions. The specific effect of this compound is influenced by variables including developmental stage, gender, species, strain, and, crucially, the administered dose. Notably, when considering the undesirable effects of MK-801, doses up to 0.1 mg/kg were found not to induce stereotypy or hyperlocomotion.

CONCLUSION

Dizocilpine continues to be of significant importance in preclinical research, facilitating the exploration of various procognitive therapeutic agents. However, given its potential undesirable effects, it is imperative to meticulously determine the appropriate dosages and conduct supplementary evaluations for any undesirable outcomes, which could complicate the interpretation of the findings.

Exposure to polystyrene nanoplastics induces an anxiolytic-like effect, changes in antipredator defensive response, and DNA damage in Swiss mice

Although the in vivo toxicity of nanoplastics (NPs) has already been reported in different model systems, their effects on mammalian behavior are poorly understood. Thus, we aimed to evaluate whether exposure to polystyrene (PS) NPs (diameter: 23.03 ± 0.266 nm) alters the behavior (locomotor, anxiety-like and antipredator) of male Swiss mice, induces brain antioxidant activity, and erythrocyte DNA damage. For this, the animals were exposed to NPs for 20 days at different doses (6.5 ng/kg and 6500 ng/kg). Initially, we did not observe any effect of pollutants on the locomotor activity of the animals (inferred via open field test and Basso mouse scale for locomotion). However, we noticed an anxiolytic-like behavior (in the open field test) and alterations in the antipredatory defensive response of mice exposed to PS NPs, when confronted with their predator potential (snake, Pantherophis guttatus). Furthermore, such changes were associated with suppressing brain antioxidant activity, inferred by lower DPPH radical scavenging activity, reduced total glutathione content, as well as the translocation and accumulation of NPs in the brain of the animals. In addition, we noted that the treatments induced DNA damage, evaluated via a single-cell gel electrophoresis assay (comet assay) applied to circulating erythrocytes of the animals. However, we did not observe a dose-response effect for all biomarkers evaluated and the estimated accumulation of PS NPs in the brain. The values of the integrated biomarker response index and the results of the principal component analysis (PCA) and the hierarchical clustering analysis confirmed the similarity between the responses of animals exposed to different doses of PS NPs. Therefore, our study sheds light on how PS NPs can impact mammals and reinforce the ecotoxicological risk associated with the dispersion of these pollutants in natural environments and their uptake by mammals.

Dizocilpine (MK-801) impairs learning in the active place avoidance task but has no effect on the performance during task/context alternation

The prevention of engram interference, pattern separation, flexibility, cognitive coordination and spatial navigation are usually studied separately at the behavioral level. Impairment in executive functions is often observed in patients suffering from schizophrenia. We have designed a protocol for assessing these functions all together as behavioral separation. This protocol is based on alternated or sequential training in two tasks testing different hippocampal functions (the Morris water maze and active place avoidance), and alternated or sequential training in two similar environments of the active place avoidance task. In Experiment 1, we tested, in adult rats, whether the performance in two different spatial tasks was affected by their order in sequential learning, or by their day-to-day alternation. In Experiment 2, rats learned to solve the active place avoidance task in two environments either alternately or sequentially. We found that rats are able to acquire both tasks and to discriminate both similar contexts without obvious problems regardless of the order or the alternation. We used two groups of rats, controls and a rat model of psychosis induced by a subchronic intraperitoneal application of 0.08mg/kg of dizocilpine (MK-801), a non-competitive antagonist of NMDA receptors. Dizocilpine had no selective effect on parallel/sequential learning of tasks/contexts. However, it caused hyperlocomotion and a significant deficit in learning in the active place avoidance task regardless of the task alternation. Cognitive coordination tested by this task is probably more sensitive to dizocilpine than spatial orientation because no hyperactivity or learning impairment was observed in the Morris water maze.

Effects of nano and conventional zinc oxide on anxiety-like behavior in male rats

Objectives:

Current drug therapies for psychological disorders, such as anxiety, are not as effective as expected, and it has been shown that zinc supplements, such as zinc oxide (ZnO), can influence anxiety. ZnO nanoparticles (ZnO NPs) are among the most used nanomaterials produced and applied in many products.

Materials and Methods:

This study investigated the effects of ZnO NPs in comparison with conventional ZnO (cZnO) on anxiety-like behaviors. Adult male Wistar rats were divided into groups: Control (receiving saline 0.9%), ZnO NPs (5, 10, 20 mg/kg), and cZnO (5, 10, 20 mg/kg). All drugs were dispersed in saline 0.9%, and 30 minutes after intraperitoneal (i.p.) injection of drugs, elevated plus maze apparatus was used to evaluate anxiety.

Results:

ZnO NPs (5 mg/kg) and cZnO (10 and 20 mg/kg) significantly increased the percentage of time spent in open arm (open arm time % OAT) compared with the control group (P < 0.05). This indicates the anxiolytic effect of such components; in addition, ZnO NPs (20 mg/kg) reduced locomotor activity (P < 0.05). Serum zinc concentration increased by both anxiolytic dose of components (from 1.75 ± 1.07 (mg/l) in control group to 5.31 ± 0.53 (mg/l) in ZnO NPs (5 mg/kg) and 10.38 ± 0.90 (mg/l) in cZnO (10 mg/kg) groups). Also, all doses increased serum pH (from 7.3 ± 0.05 in control group to 8.1 ± 0.05 in ZnO NPs (5 mg/kg) and 8.05 ± 0.01 in cZnO (10 mg/kg) groups and kept them constant after 24 hours.

Conclusion:

Results indicate that the anxiolytic effect of ZnO NPs is much higher than its conventional form, but the introduction of ZnO NPs, as a new drug for treatment of anxiety disorder, needs further investigations.

Association of oxidative stress to the genesis of anxiety: implications for possible therapeutic interventions

Oxidative stress caused by reactive species, including reactive oxygen species, reactive nitrogen species, and unbound, adventitious metal ions (e.g., iron [Fe] and copper [Cu]), is an underlying cause of various neurodegenerative diseases. These reactive species are an inevitable by-product of cellular respiration or other metabolic processes that may cause the oxidation of lipids, nucleic acids, and proteins. Oxidative stress has recently been implicated in depression and anxiety-related disorders. Furthermore, the manifestation of anxiety in numerous psychiatric disorders, such as generalized anxiety disorder, depressive disorder, panic disorder, phobia, obsessive-compulsive disorder, and posttraumatic stress disorder, highlights the importance of studying the underlying biology of these disorders to gain a better understanding of the disease and to identify common biomarkers for these disorders. Most recently, the expression of glutathione reductase 1 and glyoxalase 1, which are genes involved in antioxidative metabolism, were reported to be correlated with anxiety-related phenotypes. This review focuses on direct and indirect evidence of the potential involvement of oxidative stress in the genesis of anxiety and discusses different opinions that exist in this field. Antioxidant therapeutic strategies are also discussed, highlighting the importance of oxidative stress in the etiology, incidence, progression, and prevention of psychiatric disorders.

Crocins, the active constituents of Crocus Sativus L., counteracted ketamine-induced behavioural deficits in rats

RATIONALE

Experimental evidence indicates that the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine impairs cognition and can mimic certain aspects of positive and negative symptoms of schizophrenia in rodents. Crocins are among the active components of the plant Crocus sativus L. and were found to be effective in different models of psychiatric disorders including anxiety and depression.

OBJECTIVES

The present study was designed to investigate the ability of crocins to counteract schizophrenia-like behavioural deficits produced by ketamine in rats.

METHODS

Crocin's ability to counteract hypermotility, stereotypies and ataxia induced by ketamine was evaluated in a motor activity cage. The ability of crocins to reverse ketamine-induced memory deficits was assessed using the novel object recognition task (NORT). The social interaction test was used in order to examine the effects of crocins on ketamine-induced social withdrawal.

RESULTS

Crocins (50 but not 30 mg/kg, i.p.) attenuated ketamine (25 mg/kg, i.p.)-induced hypermotility, stereotypies and ataxia. In a subsequent study, post-training administration of crocins (15 and 30 mg/kg, i.p.) reversed ketamine (3 mg/kg, i.p.)-induced performance deficits in the NORT. Finally, crocins (50 but not 30 mg/kg, i.p.) counteracted the ketamine (8 mg/kg, i.p.)-induced social isolation in the social interaction test.

CONCLUSIONS

Our findings show that crocins attenuated schizophrenia-like behavioural deficits induced by the non-competitive NMDA receptor antagonist ketamine in rats.

Group II metabotropic glutamate receptor type 2 allosteric potentiators prevent sodium lactate-induced panic-like response in panic-vulnerable rats

Rats with chronic inhibition of GABA synthesis by infusion of l-allyglycine, a glutamic acid decarboxylase inhibitor, into their dorsomedial/perifornical hypothalamus are anxious and exhibit panic-like cardio-respiratory responses to treatment with intravenous (i.v.) sodium lactate (NaLac) infusions, in a manner similar to what occurs in patients with panic disorder. We previously showed that either NMDA receptor antagonists or metabotropic glutamate receptor type 2/3 receptor agonists can block such a NaLac response, suggesting that a glutamate mechanism is contributing to this panic-like state. Using this animal model of panic, we tested the efficacy of CBiPES and THIIC, which are selective group II metabotropic glutamate type 2 receptor allosteric potentiators (at 10-30 mg/kg i.p.), in preventing NaLac-induced panic-like behavioral and cardiovascular responses. The positive control was alprazolam (3mg/kg i.p.), a clinically effective anti-panic benzodiazepine. As predicted, panic-prone rats given a NaLac challenge displayed NaLac-induced panic-like cardiovascular (i.e. tachycardia and hypertensive) responses and "anxiety" (i.e. decreased social interaction time) and "flight" (i.e. increased locomotion) -associated behaviors; however, systemic injection of the panic-prone rats with CBiPES, THIIC or alprazolam prior to the NaLac dose blocked all NaLac-induced panic-like behaviors and cardiovascular responses. These data suggested that in a rat animal model, selective group II metabotropic glutamate type 2 receptor allosteric potentiators show an anti-panic efficacy similar to alprazolam.

Effects of olanzapine, sertindole and clozapine on MK-801 induced visual memory deficits in mice

We investigated the effects of the second generation antipsychotics olanzapine, sertindole and clozapine on visual recognition memory using the novel object recognition (NOR) test in naive and MK-801-treated animals. The effects of drug treatment on locomotion and anxiety were also determined using the open field test. Male Balb-c mice were treated with olanzapine (0.2, 0.4 and 0.6 mg/kg; i.p.), sertindole (0.63, 1.3 and 2.5mg/kg; s.c.) or clozapine (0.5 and 1mg/kg; i.p.), and cognitive deficits were induced by MK-801 (0.2mg/kg; i.p.) administration. Olanzapine treatment decreased the ratio index in the NOR test, whereas sertindole and clozapine had no effect in naive mice. MK-801-induced cognitive impairment was reversed by treatment with olanzapine, sertindole or clozapine. While olanzapine, sertindole and clozapine had no effect on the anxiety of naive mice as determined by the open field test, MK-801 significantly increased the total distance traveled, time spent in the center zone and the velocity of the animals. MK-801-induced effects on locomotion and anxiety in the open field test were reversed by olanzapine, sertindole or clozapine treatment. The results of the present study demonstrated that olanzapine, sertindole and clozapine improved cognition in MK-801 treated mice, and indicate that these drugs have a potential to improve cognition in schizophrenia.

Hippocampal NMDA receptors and anxiety: at the interface between cognition and emotion

David De Wied had a fundamental interest in the brain and behaviour, with a particular interest in the interface between cognition and emotion, and how impairments at this interface could underlie human psychopathology. The NMDA subtype of glutamate receptor is an important mediator of synaptic plasticity and plays a central role in the neurobiological mechanisms of emotionality, as well as learning and memory. NMDA receptor antagonists affect various aspects of emotionality including fear, anxiety and depression, as well as impairing certain forms of learning and memory. The hippocampus is a key brain structure, implicated in both cognition and emotion. Lesion studies in animals have suggested that dorsal and ventral sub-regions of the hippocampus are differentially involved in dissociable aspects of hippocampus-dependent behaviour. Cytotoxic lesions of the dorsal hippocampus (septal pole) in rodents impair spatial learning but have no effect on anxiety, whereas ventral hippocampal lesions reduce anxiety but are without effect on spatial memory. This role for the ventral hippocampus in anxiety is distinct from the role of the amygdala in other aspects of emotional processing, such as fear conditioning. Recent studies with genetically modified mice have shown that NR1 NMDA receptor subunit deletion, specifically from the granule cells of the dentate gyrus, not only impairs short-term spatial memory but also reduces anxiety. This suggests that NMDA receptors in ventral hippocampus may be a key locus supporting the anxiolytic effects of NMDA receptor antagonists. These data support Gray's neuropsychological account of hippocampal function.

A systematic review of off-label uses of memantine for psychiatric disorders

Recent data points to glutamatergic dysfunction in mood disorders, anxiety disorders, obsessive-compulsive disorder, and schizophrenia. Memantine, a drug approved by the FDA for the treatment of moderate to severe Alzheimer's disease that acts at the N-methyl-d-aspartate receptor, has been used off-label for various psychiatric disorders. Although promising, the available data for the use of memantine in these disorders is limited. Given this data, the routine use of memantine for depression, schizophrenia, obsessive-compulsive disorder, substance abuse, pervasive developmental disorders, bipolar disorder, and binge eating disorder cannot be recommended at this time.

Elicited hippocampal theta rhythm: a screen for anxiolytic and procognitive drugs through changes in hippocampal function?

Hippocampal damage produces cognitive deficits similar to dementia and changes in emotional and motivated reactions similar to anxiolytic drugs. The gross electrical activity of the hippocampus contains a marked 'theta rhythm'. This is a relatively high voltage sinusoidal waveform, resulting from synchronous phasic firing of cells, variation in which correlates with behavioural state. Like the hippocampus, theta has been linked to both cognitive and emotional functions. Critically, it has recently been shown that restoration of theta-like rhythmicity can restore lost cognitive function. We review the effects of systemic administration of drugs on hippocampal theta elicited by stimulation of the reticular formation. We conclude that reductions in the frequency of reticular-elicited theta provide what is currently the best in-vivo means of detecting antianxiety drugs. We also suggest that increases in the power of reticular-elicited theta could detect drugs useful in the treatment of disorders, such as dementia, that involve memory loss. We argue that these functionally distinct effects should be seen as indirect and that each results from a change in a single form of cognitive-emotional processing that particularly involves the hippocampus.

The selective mGlu5 receptor antagonist MTEP, similar to NMDA receptor antagonists, induces social isolation in rats

It has repeatedly been shown that uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists can mimic certain aspects of positive and negative symptoms of schizophrenia in human volunteers and laboratory animals. The purpose of the present study was to expand these findings and to determine whether the selective metabotropic glutamate receptor subtype 5 (mGluR5) antagonist, MTEP (3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine), could induce similar effects in Wistar rats. First, MTEP (1.0-10.0 mg/kg; intraperitoneally) after acute and subchronic (daily for 5 days) administration as well as the uncompetitive antagonists of the NMDA receptor of either high affinity, phencyclidine (0.5-4.0 mg/kg; subcutaneously (s.c.)) and (+)-MK-801 (0.03-0.25 mg/kg; s.c.), or low-moderate affinity, ketamine (2.0-16.0 mg/kg; s.c.) and memantine (0.15-20.0 mg/kg; s.c.), following daily administration for 3 days were tested in the social interaction test to determine their ability to reproduce the negative and positive symptoms measured by social isolation and stereotyped behavior, respectively. Second, the compounds were tested in the motility test following acute administration to determine their ability to induce locomotor hyperactivity reflecting the positive symptoms. In line with previous findings, all examined NMDA receptor antagonists produced social interaction deficits, locomotor hyperactivity, and stereotypy except memantine. Notably, this study found that MTEP following both acute and subchronic administration dose-dependently induced social isolation, but did not cause either locomotor hyperactivity or stereotypy. These data demonstrate that social behavior deficits in rats can be caused by both the blockade of the NMDA receptor and the inhibition of mGluR5, whereas mGluR5 antagonists may not independently be able to mimic the positive symptoms.

Panic-prone state induced in rats with GABA dysfunction in the dorsomedial hypothalamus is mediated by NMDA receptors

Rats with chronic inhibition of GABA synthesis and consequently enhanced glutamatergic excitation in the dorsomedial hypothalamus (DMH) develop panic-like responses, defined as tachycardia, tachypnea, hypertension, and increased anxiety as measured by a social interaction (SI) test, after intravenous sodium lactate infusions, a phenomenon similar to patients with panic disorder. Therefore, the present studies tested the role of the postsynaptic NMDA and AMPA type glutamatergic receptors in the lactate-induced panic-like responses in these rats. Rats were fit with femoral arterial and venous catheters and Alzet pumps [filled with the GABA synthesis inhibitor L-allylglycine (L-AG; 3.5 nmol/0.5 microl per hour) or its inactive isomer D-AG] into the DMH. After 4-5 d of recovery only those rats with L-AG pumps exhibited panic-like responses to lactate infusions. Using double immunocytochemistry, we found that rats exhibiting panic-like responses (e.g., L-AG plus lactate) had increased c-Fos immunoreactivity in DMH neurons expressing the NMDA receptor 1 (NR1) subunit, but not those expressing the glutamate receptor 2 and 3 subunits of the AMPA receptors. To confirm this pharmacologically, we tested another group of rats implanted with l-AG pumps with intravenous lactate infusions preceded by injections of either NMDA [aminophosphonopentanoic acid (AP-5) or (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate (MK-801)] or non-NMDA [CNQX or 4-(8-methyl-9H-1,3-dioxolo[4,5-h][2,3]benzodazepin-5-yl)-benzenamine dihydrochloride (GYKI52466)] antagonists into the DMH. Injections of NMDA, but not non-NMDA, antagonists into the DMH resulted in dose-dependent blockade of the tachycardia, tachypnea, hypertension, and SI responses after lactate infusions. These results suggest that NMDA, and not non-NMDA, type glutamate receptors regulate lactate-induced panic-like responses in rats with GABA dysfunction in the DMH.

Advances in the treatment of anxiety: targeting glutamate

Our current psychopharmacological treatments for anxiety disorders evince a number of shortcomings, including troublesome side effects and lack of primary effects. Whereas many new drugs have been developed in the past few decades, most are based on outmoded theories of the pathogenesis of these disorders (i.e., monoamine hypotheses), thus frustrating our ability to create more specific and effective interventions. Recently, however, the neurobiological literature has shown a convergence of findings focusing on the glutamatergic system in anxiety disorders, and the growth of pharmacological tools targeting these receptors has led to the development of novel treatments having anxiolytic effects in humans and animals alike. Additionally, as this system is showing promise as a final common pathway in the pathogenesis of anxiety disorders, we may be able to employ glutamate-specific neuroimaging techniques (e.g., N-acetyl-aspartate, GLX) to both guide treatment decisions and present reliable objective biomarkers for treatment efficacy.

Termination of pseudopregnancy in the rat alters the response to progesterone, chlordiazepoxide, and MK-801 in the elevated plus-maze

RATIONALE

Allopregnanolone, a neurosteroid-reduced metabolite of progesterone, is a well-documented positive modulator of the gamma-aminobutyric type A (GABA(A)) receptor. As has been reported for other positive modulators of the GABA(A) receptor, chronic exposure to neurosteroids is hypothesized to decrease GABA(A) receptor function. Drawing from the literature on chronic exposure to benzodiazepines or alcohol, putative changes in N-methyl-D-aspartate (NMDA) receptor function are also expected after chronic neurosteroid exposure.

OBJECTIVES

To assess the sensitivity of the GABA(A) and NMDA receptors after chronic elevation of neurosteroid produced by termination of pseudopregnancy in behavioral tests of anxiety and sensorimotor coordination.

METHODS

Female rats ovariectomized on day 10 of pseudopregnancy were tested in the elevated plus-maze and on the rotor rod after an acute injection of progesterone (4 mg/0.2 ml, s.c.), chlordiazepoxide (5 or 15 mg/kg, i.p.), or MK-801 (0.025, 0.05, or 0.1 mg/kg, i.p.).

RESULTS

Pseudopregnancy termination produced an anxiogenic-like response in the plus-maze; an acute injection of progesterone restored baseline levels of behavior in this test. Pseudopregnancy termination eliminated the anxiolytic-like, sedative, and ataxic effects of chlordiazepoxide. In contrast, pseudopregnancy termination produced an increased sensitivity to the anxiolytic-like and ataxic effects of MK-801.

CONCLUSIONS

The effects of pseudopregnancy termination on the behavioral response to positive modulators of the GABA(A) receptor are consistent with results from studies in which chronic exposure to neurosteroids decreases the response to acute neurosteroid and benzodiazepine administration. However, unlike the enhanced glutamatergic tone resulting from discontinuation of chronic benzodiazepine or alcohol exposure, the termination of pseudopregnancy apparently decreases NMDA receptor function.

Young investigator award recipient

Dr. Zhongcong Xie started his research career in 1989 with graduate studies in behavioral pharmacology at Wayne State University, Detroit, Michigan.He moved to Harvard Medical School, Boston, Massachusetts in 1994 as a postdoctoral research fellow in both human and animal behavioral research. After years of research training, he returned to the clinical arena in 1997 as an Anesthesiology resident at Massachusetts General Hospital and Harvard Medical School. After completing anesthesia residency training, he decided to become a physician-scientist and continue his research in neuroscience. He joined the Genetics and Aging Research Unit at Massachusetts General Hospital and Harvard Medical School supervised by Dr. Rudolph Tanzi in the year 2000 to study Alzheimer's disease. His research on the neuropathogenesis of Alzheimer's disease has focused primarily on the molecular mechanisms underlying beta-amyloid precursor protein processing and beta-amyloid protein production, especially regarding regulation of the gamma-secretase complex.Additionally, his work has addressed the role of presenilin-related apoptosis in Alzheimer's disease.

SSR181507, a putative atypical antipsychotic with dopamine D2 antagonist and 5-HT1A agonist activities: improvement of social interaction deficits induced by phencyclidine in rats

Social behaviour is frequently impaired in schizophrenic patients, and current antipsychotics appear poorly effective in alleviating this deficit. SSR181507 is a selective dopamine D2 receptor antagonist and 5-HT1A receptor agonist [Neuropsychopharmacology 28 (2003) 2064] with an atypical antipsychotic profile and additional antidepressant/anxiolytic activities [Neuropsychopharmacology 28 (2003) 1889]. Here, we sought to assess the efficacy of SSR181507, and of reference antipsychotics and antidepressant/anxiolytics, to counteract phencyclidine (PCP)-induced social interaction deficit in rats. Pairs of unfamiliar rats were placed for 10 min each day into a dimly lit arena, during four consecutive days. On the test day (5th day), each pair was placed into the arena 30 min after i.p. treatment with PCP (or vehicle) and a challenge compound or vehicle (same for both rats, i.p. or s.c.). The time spent in social interaction was scored during 10 min. PCP (1 mg/kg) decreased social interaction time by about 35%. This effect was fully antagonized by pre-treatment with SSR181507 (1 mg/kg). In contrast, neither haloperidol (0.05 and 0.1 mg/kg) nor clozapine (0.3 and 1 mg/kg) antagonized this PCP-induced deficit. The selective 5-HT1A receptor agonist 8-OH-DPAT (0.025 and 0.05 mg/kg s.c.), but not the anxiolytic diazepam (0.75 and 1.5 mg/kg), also improved social interaction impairment in PCP-treated rats: this would indicate that the 5-HT1A receptor agonist properties of SSR181507 are responsible for the reversal of PCP-induced social deficit. These data suggest that, in addition to its atypical antipsychotic profile and antidepressant/anxiolytic activities, SSR181507 has a potential therapeutic activity in another key feature of schizophrenia poorly controlled by current antipsychotics, namely deterioration in social functioning.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Effects of ketamine on different types of anxiety/fear and related memory in rats with lesions of the median raphe nucleus

The aim of the present study was to determine the involvement of the median raphe serotonergic system in the effects of ketamine on anxiety behaviours and related memory. The effects of ketamine pretreatment (3 and 10 mg/kg, i.p.) on three types of fear-motivated behaviours, unconditioned one-way escape, conditioned avoidance and freezing were tested. Experiments were performed with the inhibitory avoidance apparatus in rats with ibotenic acid lesions of the median raphe nucleus. It was found that 10 mg/kg ketamine had an anxiogenic-like effect on one-way escape type of fear and anxiolytic-like effect on conditioned freezing-related fear; these effects were unaffected by median raphe lesions. Both ketamine doses impaired freezing-related fear memory. Ketamine (10 mg/kg) also produced an anxiolytic-like effect on avoidance type of fear and impaired avoidance memory. The median raphe lesions attenuated the anxiolytic action of the drug on the avoidance type of fear and prevented ketamine-induced avoidance memory impairment. These results suggest that the anxiolytic-like effect of ketamine on avoidance-type fear is mediated through the median raphe serotonergic system.

N-methyl-D-aspartate (NMDA)-mediated corticotropin-releasing factor (CRF) release in cultured rat amygdala neurons

Corticotropin-releasing factor (CRF) plays an important role in the activation of centrally mediated responses to stress. The amygdala, a limbic structure involved in the stress response, has a significant number of CRF cell bodies and CRF receptors. Activation of glutamatergic projections to the amygdala has been implicated in the stress response. Few studies have evaluated neurotransmitter-stimulated CRF release in the amygdala. We measured the effects of glutamate (0.1-1000 microM) and N-methyl-D-aspartate (NMDA, 0.1-1000 microM) on CRF release from the amygdala using primary neuronal cultures from embryonic rat brains (E18-19). Experiments were performed after the cultures grew for 17-20 days. CRF was measured using radioimmunoassay. The excitatory amino acid neurotransmitters, glutamate and NMDA, stimulated CRF release in a concentration-dependent manner. The apparent EC50 values for glutamate and NMDA were 17.5 microM and 12 microM, respectively. Consistent with a NMDA receptor-driven event, glutamate-stimulated CRF release was blocked by the NMDA antagonist, 2-amino-5-phosphonovaleric acid (AP-5, 1-100 microM) and antagonized by the addition of 1.2 mM MgCl2 to the incubation medium. These results implicate an inhibition of CRF release in the amygdala as a possible mechanism for the reported anxiolytic effects of NMDA antagonists.

NMDA receptors mediate lasting increases in anxiety-like behavior produced by the stress of predator exposure--implications for anxiety associated with posttraumatic stress disorder

It has been proposed that NMDA-dependent long-term potentiation (LTP) of limbic system circuits controlling defensive behavior underlies stressor-induced lasting increases in anxiety-like behavior (ALB). Findings in cats given the stress-inducing beta-carboline, FG-7142, support this hypothesis. An animal model of lasting affective change following traumatic stress has recently been developed. In this model, lasting increases in anxiety-like behavior (ALB) assessed in the elevated plus maze are produced by a single 5-min exposure of a rat to a cat. Rats become more anxious in the plus maze for up to 3 weeks after the exposure. The present study demonstrates that blockade of NMDA receptors in rats with MK-801, AP7, or CPP, given 30 min prior to exposure to a cat, prevents the increase in ALB assessed 1 week later. MK-801 or AP7, given 30 min after exposure to a cat, do not prevent the increase in ALB seen 1 week later, however. MK-801, but not CPP or AP7, promotes approaches to cats during exposure. This "fearlessness" may reflect some anxiolytic action of MK-801. Approach to cats following injection of MK-801 was eliminated by prior injection of Prazosin. Prazosin did not interfere with the block of increases in ALB following cat exposure, however. These findings are consistent with the view that NMDA receptors are involved in initiation, but not maintenance of neural changes mediating lasting increases in anxiety following severe stress. The significance of these findings for PTSD are discussed.

The putative AMPA receptor antagonist, LY326325, produces anxiolytic-like effects without altering locomotor activity in rats

Anxiolytic-like effects produced by the novel, water-soluble AMPA/kainate receptor antagonist, LY326325 (3RS,4aRS,6RS,8aRS)-6-[2-(1(2)H-tetrazole-5-yl)e thyl]decahydro-isoquinoline-3-carboxylic acid), were examined in the elevated plus-maze and in a conflict-suppressed drinking situation. Administration of low doses (0.5, 1.2, and 5 mg/kg; i.p., -30 min) of LY326325 to Sprague-Dawley rats did not alter the percentage of entries into the open arms of the plus-maze, whereas only one dose of LY326325 (1 mg/kg) produced a slight, but significant, increase of the time spent in the open arms of the plus maze. In the conflict-suppressed drinking test, similar doses of LY326325 (2.5 and 5 mg/kg; i.p., -30 min) caused a dose-dependent and significant increase of punished drinking behavior without having any significant effects on unpunished drinking. The anxiolytic-like effects of LY326325 in the plus-maze and in the anticonflict tests were observed at doses, which, by themselves, had no influence on various measures of locomotor activity, i.e., horizontal activity, forward locomotion, and corner time. Our data suggest that the putative AMPA/glutamate receptor antagonist, LY326325, produces anxiolytic-like effects similar to those of diazepam in the conflict-suppressed drinking test, but displays considerably weaker anxiety-reducing properties compared to diazepam in the elevated plus-maze.

Intravenous anaesthetics: some cellular sites of action

Intravenous anaesthetics have diverse effects on neurones within the central nervous system. Only those that occur at clinical concentrations are likely to be relevant. The dominant effect of many agents is the potentiation of the inhibitory neurotransmitter gamma amino butyric acid (GABA) by various mechanisms while inhibiting the effects of excitatory transmitters seems to be less dominant, except for ketamine.

Anticonflict effects of a competitive NMDA receptor antagonist and a partial agonist at strychnine-insensitive glycine receptors

Using the conflict drinking Vogel test in rats as a model, in the present study we examined the anxiolytic-like activity of DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist and 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist at strychnine-insensitive glycine receptors associated with the NMDA receptor complex, after their intraperitoneal (i.p.) and intrahippocampal (IHP) administration. CGP 37849, administered in doses of 1.25-5 mg/kg i.p., produced an anticonflict effect in a dose-dependent manner, but was inactive when injected in doses of 0.01-0.1 micrograms IHP. At the same time, when administered in higher doses (10 mg/kg i.p. or 0.3 micrograms IHP), that drug induced motor impairment. On the other hand, ACPC exhibited an anxiolytic-like activity after both i.p. (100-200 mg/kg) and IHP (3-30 micrograms) administration. These results, as well as the literature data on the lack of motor-impairing effects of ACPC, indicate that the latter drug seems to be more advantageous than CGP 37849 as a potential therapeutic agent in the treatment of anxiety disorders. Furthermore, they also show that the hippocampus may be one of the neuroanatomical sites of the anxiolytic-like effect of ACPC, but not of CGP 37849.

Multiple within-day conflict testing to define the time course of anxiolytic drug effects

The present article describes a method for multiple within-day conflict testing to conduct drug treatment time course studies more efficiently. Groups of female Sprague-Dawley rats were trained for conflict testing in a standard one-session/day procedure [conditioned suppression of drinking (CSD)]. In this task, thirsty rats (24 h water-restricted) drink from a tube that is electrified only when a tone is on (approximately 20% of the 10-min session time). In Experiment 1 it was found that there was no significant variation in CSD conflict behavior when subjects were tested at 0600, 1200, or 1800 h using the traditional procedure of one test/day. In Experiment 2, subjects were assigned to treatment groups such that there were three 5-min test sessions per day and the test-retest interval was either 2, 4, or 6 h (centered around 1200 h). Test-retest intervals of 6 h (i.e., tests at 0600, 1200, and 1800 h) resulted in comparable levels of punished responding across the repeated within-day tests, whereas test-retest intervals of 2 h and, to a lesser extent 4 h, resulted in unequal within-day conflict behavior characterized by a greater number of shocks accepted and a greater volume of water consumed during the earliest test periods each day. In another group of rats, it was determined that conflict behavior sampled five times/day in 3-min sessions separated by a 3-h test-retest interval (i.e., tests at 0600, 0900, 1200, 1500, and 1800 h) also resulted in stable conflict behavior across the various within-day test periods. In Experiment 3, it was found that acute IP challenges with anticonflict treatments that exhibit either a long duration of action (phenobarbital: 40 mg/kg) or a significant delay to onset in addition to a long duration (MK-801: 0.20 mg/kg) yielded time course data comparable to those obtained using the traditional one test/day procedure. These findings indicate that the use of multiple within-day conflict testing can greatly increase the efficiency of these procedures, particularly when drug treatment timecourse information is desired.

Anticonflict effect of MK-801 in rats: time course and chronic treatment studies

The present study examined the time course and chronic treatment effects of the noncompetitive N-methyl-D aspartate (NMDA) antagonist, MK-801 (dizocilpine), on conflict behavior in the conditioned suppression of drinking (CSD) paradigm, a repeated-measures conflict task. In daily 10-min sessions, water-restricted rats drank from a tube that was occasionally electrified (0.25- or 0.5-mA shocks signaled by a tone). Trained subjects (4 weeks of CSD testing) exhibited stable baselines for both punished responding and unpunished responding. In the first experiment, the effects of MK-801 administered IP were determined in female and male rats following a range of pretreatment intervals (i.e., 0.5-48 h). In female rats, 0.2 mg/kg MK-801 exerted an anticonflict effect at pretreatment intervals of 10-36 h, but not before 10 h or after 36 h. In male rats, qualitatively similar results were obtained; MK-801 (0.4 mg/kg) exerted anticonflict effects following pretreatment intervals of 6-14 h, but not before 6 or after 14 h. In the second experiment, chronic treatment of female rats with 0.04, 0.01, or 0.2 mg/kg MK-801 resulted in a dose-dependent anticonflict effect in CSD paradigm, which remained stable over the course of 5 weeks of chronic treatment. Punished responding returned to pretreatment levels within 2-3 days after discontinuation of chronic treatment with MK-801. These data suggest that MK-801 exerts a delayed anticonflict effect in both female and male rats with a qualitatively similar pattern, and that there is no tolerance to the anticonflict effect of MK-801 with chronic treatment.

Evidence for an anxiogenic action of AMPA receptor antagonists in the plus-maze test

The effects of the non-NMDA receptor antagonists, the new alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-selective receptor antagonist, LY326325, and the AMPA/kainate-selective receptor antagonist, NBQX (6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-(1H,4H)dione), on plus-maze behavior and locomotor activity were examined. LY326325 induced a dose-dependent decrease in the per cent time spent in open arms as well as in the per cent entries into the open arms. NBQX caused a dose-dependent reduction in the per cent time spent in open arms but had no effect on the per cent entries into the open arms. The behavioral actions of the AMPA receptor antagonists were observed at doses which had no influence on the locomotor activity of the animals. Based upon the current findings it is suggested that AMPA receptor antagonists produce a dose-dependent increase of anxiogenic behavior in the plus-maze test situation.

Kappa opioid agonists produce anxiolytic-like behavior on the elevated plus-maze

The selective kappa agonist U-50,488H was evaluated on the elevated plus-maze test of anxiety. U-50,488H was administered intraperitoneally to male Sprague-Dawley rats 20 min before testing, first in an open field apparatus, then followed immediately on the elevated plus-maze. No significant change in spontaneous locomotor activity was measured in the open field apparatus, suggesting that U-50,488H was devoid of sedative effects in the dose range tested (0.1-1000 micrograms/kg, IP). Doses between 10 and 1000 micrograms/kg produced significant increases in elevated plus-maze behavior that were consistent with anxiolytic actions for U-50,488H. These anxiolytic-like effects were antagonized by naloxone (2.0 mg/kg, IP), suggesting an opioid receptor site of action. In addition, we tested the kappa 1-selective U-50,488H-derivative, U-69,593 (100 micrograms/kg, IP), which was also shown to mimic the anxiolytic-like effects produced by U-50,488H. These results suggest that low doses of the selective kappa 1 agonists U-50,488H and U-69,593 are endowed with anxiolytic properties in rodents and that the kappa opioid system may be involved in the behavioral response to anxiety.

Effects of antagonists at the NMDA receptor complex in two models of anxiety

The effects of an antagonist at the strychnine insensitive glycine site (5,7-dichlorokynurenic acid, i.c.v.), and of noncompetitive (MK-801, i.p.) and competitive (CGP 37849, i.p.; CGP 39551, i.p.; AP-7, i.c.v.) NMDA antagonists were compared with diazepam (i.p.) in two animal models of anxiety (the open field exploratory behavior of non-habituated rats, and the Vogel conflict test). All drugs when applied in appropriate doses increased punished drinking in the Vogel test, without producing any significant changes in free drinking and the stimulus threshold at their lowest anticonflict doses. The effective doses were as follows: diazepam 1.5 and 2.5 mg/kg; MK-801 0.005 and 0.01 mg/kg; CGP 39551 5.0 and 20.0 mg/kg; CGP 37849 1.0 and 2.5 mg/kg; 5,7-dichlorokynurenic acid 5.0 microgram (i.c.v.); AP-7 0.5 microgram (i.c.v.). In the open field diazepam (0.05 mg/kg), MK-801 (0.1 mg/kg), CGP 37849 (0.01, 0.1, 1.0 mg/kg), and AP-7 2.5 micrograms (i.c.v.) significantly increased exploratory activity in the central sectors of the open field (anti-neophobic reaction), without changing motor activity of the rat. MK-801 at the highest tested dose of 0.2 mg/kg significantly stimulated animal locomotor activity. CGP 37849 in the largest dose examined (10 mg/kg) significantly depressed the motor behavior of rats. Overall, it appeared that different NMDA antagonists showed an anxiolytic-like profile, similar to that of the benzodiazepine diazepam. Among different NMDA receptor complex antagonists studied, CGP 37849 was characterized by the largest distinction between the doses showing an anxiolytic-like action in the open field test, and changing rat motor behavior.

Effects of ethanol, chlordiazepoxide, and MK-801 on performance in the elevated-plus maze and on locomotor activity

The effects of ethanol, chlordiazepoxide, and MK-801 on performance in the elevated-plus maze and on activity measured in a circular activity monitor were compared in Sprague-Dawley rats to determine whether these effects of ethanol could be explained by its action on either GABAA or NMDA receptors. Both ethanol and chlordiazepoxide produced an increase in the time spent in the open arms of the elevated-plus maze and in the ratio of open arm to total arm entries, indicative of an anxiolytic action of these drugs. MK-801 did not alter either the time spent in the open arms or the ratio of open to total arm entries. Chlordiazepoxide and MK-801 produced an increase in total arm entries that suggested that these compounds were increasing locomotor activity. Ethanol also increased total arm entries, but the effect was not statistically reliable. Following habituation to an activity monitor, neither ethanol nor chlordiazepoxide increased activity in this task, whereas MK-801 produced a robust increase in locomotion. Additionally, neither ethanol nor chlordiazepoxide blocked the MK-801-induced locomotor stimulation. The latter finding suggests that the effects of ethanol on GABAA receptors was not blocking an increased activity level produced by its antagonism of NMDA. Additionally, these results indicate that the anxiolytic and locomotor action of ethanol in rats parallel the effects of a benzodiazepine and not those of an NMDA antagonist. Finally, these results suggest that the consequence of ethanol's antagonism of NMDA receptor function is more restricted than that produced by MK-801.

Anxiolytic-like effects of N-methyl-D-aspartate-associated glycine receptor ligands in the rat potentiated startle test

Competitive and non-competitive NMDA receptor complex antagonists have been shown to be active in various models of anxiolytic activity. This study examined the effects of ligands at the NMDA receptor-associated glycine site and two competitive NMDA receptor antagonists on the fear-potentiated startle response model for anxiolytic activity. The results show that the NMDA receptor antagonists, 2-amino-7-phosphonoheptanoate (30 mg/kg) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonate (3 mg/kg), the glycine receptor antagonist 7-chlorokynurenate (100 mg/kg), and the glycine receptor partial agonists 3-amino-1-hydroxy-2-pyrrolidinone ((+)-HA-966) (30 mg/kg), 1-aminocyclopropane carboxylate (200-500 mg/kg) and D-cycloserine (30-300 mg/kg) blocked the potentiated startle effect. These results extend the findings of earlier studies showing anxiolytic-like effects of NMDA antagonists and glycine receptor ligands by demonstrating their effectiveness in the rat potentiated startle paradigm. These results also demonstrate the anxiolytic potential of D-cycloserine.