Effect on memory processing by D-cycloserine, an agonist of the NMDA/glycine receptor

Effect of the NMDA receptor partial agonist, d-cycloserine, on emotional processing and autobiographical memory

BACKGROUND

Studies suggest that d-cycloserine (DCS) may have antidepressant potential through its interaction with the glycine site of the N-methyl-D-aspartate receptor; however, clinical evidence of DCS's efficacy as a treatment for depression is limited. Other evidence suggests that DCS affects emotional learning which may also be relevant for the treatment of depression and anxiety. The aim of the present investigation was to assess the effect of DCS on emotional processing in healthy volunteers and to further characterise its effects on emotional and autobiographical memory.

METHODS

Forty healthy volunteers were randomly allocated to a single dose of 250 mg DCS or placebo in a double-blind design. Three hours later, participants performed an Emotional Test Battery [including Facial Expression Recognition Task (FERT), Emotional Categorisation Task (ECAT), Emotional Recall Task (EREC), Facial Dot-Probe Task (FDOT) and Emotional Recognition Memory Task (EMEM)] and an Autobiographical Memory Test (AMT). Also, participants performed the FERT, EREC and AMT tasks again after 24 h in order to assess longer lasting effects of a single dose of DCS.

RESULTS

DCS did not significantly affect the FERT, EMEM and FDOT performance but significantly increased emotional memory and classification for positive words v. negative words. Also, DCS enhanced the retrieval of more specific autobiographical memories, and this effect persisted at 24 h.

CONCLUSIONS

These findings support the suggestion that low-dose DCS increases specific autobiographical memory retrieval and positive emotional memory. Such effects make it an intriguing agent for further investigation in clinical depression, which is characterised by decreased autobiographical memory specificity and increased negative bias in memory recall. It also underscores the potential role of DCS as an adjunct to cognitive behavioural therapy in depression.

An Overview of the Involvement of D-Serine in Cognitive Impairment in Normal Aging and Dementia

Dementia, of which Alzheimer's disease (AD) is the most common form, is characterized by progressive cognitive deterioration, including profound memory loss, which affects functioning in many aspects of life. Although cognitive deterioration is relatively common in aging and aging is a risk factor for AD, the condition is not necessarily a part of the aging process. The N-methyl-D-aspartate glutamate receptor (NMDAR) and its co-agonist D-serine are currently of great interest as potential important contributors to cognitive function in normal aging and dementia. D-Serine is necessary for activation of the NMDAR and in maintenance of long-term potentiation (LTP) and is involved in brain development, neuronal connectivity, synaptic plasticity and regulation of learning and memory. In this paper, we review evidence, from both preclinical and human studies, on the involvement of D-serine (and the enzymes involved in its metabolism) in regulation of cognition. Potential mechanisms of action of D-serine are discussed in the context of normal aging and in dementia, as is the potential for using D-serine as a potential biomarker and/or therapeutic agent in dementia. Although there is some controversy in the literature, it has been proposed that in normal aging there is decreased expression of serine racemase and decreased levels of D-serine and down-regulation of NMDARs, resulting in impaired synaptic plasticity and deficits in learning and memory. In contrast, in AD there appears to be activation of serine racemase, increased levels of D-serine and overstimulation of NMDARs, resulting in cytotoxicity, synaptic deficits, and dementia.

The glycine site of NMDA receptors: A target for cognitive enhancement in psychiatric disorders

Cognitive dysfunction is a principal determinant of functional impairment in major depressive disorder (MDD) and often persists during periods of euthymia. Abnormalities in the glutamate system, particularly in N-methyl-d-aspartate receptors (NMDARs) activity, have been shown to contribute to both mood and cognitive symptoms in MDD. The current narrative review aims to evaluate the potential pro-cognitive effects of targeting the glycine site of NMDARs in the treatment of psychiatric disorders, with a special focus on how these results may apply to MDD. Literature databases were searched from inception to May 2018 for relevant pre-clinical and clinical studies evaluating antidepressant and pro-cognitive effects of NMDAR glycine site modulators in both MDD and non-MDD samples. Six glycine site modulators with pro-cognitive and antidepressant properties were identified: d-serine (co-agonist), d-cycloserine (partial agonist), d-alanine (co-agonist), glycine (agonist), sarcosine (co-agonist) and rapastinel (partial agonist). Preclinical animal studies demonstrated improved neuroplasticity and pro-cognitive effects with these agents. Numerous proof-of-concept clinical trials demonstrated pro-cognitive and antidepressant effects trans-diagnostically (e.g., in healthy participants, MDD, schizophrenia, anxiety disorders, major neurocognitive disorders). The generalizability of these clinical studies was limited by the small sample sizes and the paucity of studies directly evaluating cognitive effects in MDD samples, as most clinical trials were in non-MDD samples. Taken together, preliminary results suggest that the glycine site of NMDARs is a promising target to ameliorate symptoms of depression and cognitive dysfunction. Additional rigorously designed clinical studies are required to determine the cognitive effects of these agents in MDD.

D-Cycloserine Facilitates Reversal in an Animal Model of Post-traumatic Stress Disorder

Many psychiatric disorders are associated with cognitive dysfunction that is ineffectively treated by existing pharmacotherapies and which may contribute to poor real-world functioning. D-cycloserine (DCS) is a partial N-methyl-D-aspartate (NMDA) agonist that has attracted attention because of its cognitive enhancing properties, including in people with post-traumatic stress disorder (PTSD). Here, we examined the effect of DCS on reversal learning - a type of cognitive flexibility - following exposure to single prolonged stress (SPS), a rodent model of PTSD. Male Sprague Dawley rats (n = 64) were trained to press levers in an operant chamber, matched for performance and assigned to SPS or control (unstressed) groups. Following SPS, rats received three additional lever press sessions, followed by a side bias test on day three. One day later they learned a response discrimination rule (press left or right lever, opposite to side bias) and on a subsequent day were trained (and tested) for reversal to the opposite lever. DCS (15 mg/kg) or vehicle was administered 30 minutes prior to the reversal session. No between-group differences were found in acquisition or retrieval of the initial rule, but a significant drug x stress interaction on response discrimination reversal indicated that DCS had a greater beneficial effect on SPS rats' cognitive flexibility than it did on performance in controls. These findings add to a growing literature on the beneficial effects of DCS for treating a wide variety of deficits that develop following exposure to extreme stress and may have implications for the development of novel pharmacotherapies for PTSD.

Vortioxetine Treatment Reverses Subchronic PCP Treatment-Induced Cognitive Impairments: A Potential Role for Serotonin Receptor-Mediated Regulation of GABA Neurotransmission

Major depressive disorder (MDD) is associated with cognitive impairments that may contribute to poor functional outcomes. Clinical data suggests that the multimodal antidepressant vortioxetine attenuates some cognitive impairments in MDD patients, but the mechanistic basis for these improvements is unclear. One theory suggests that vortioxetine improves cognition by suppressing γ-amino butyric acid (GABA)ergic neurotransmission, thereby increasing glutamatergic activation. Vortioxetine’s effects on cognition, GABA and glutamate neurotransmission have been supported in separate experiments, but no empirical work has directly connected vortioxetine’s cognitive effects to those on GABA and glutamate neurotransmission. In this paper, we attempt to bridge this gap by evaluating vortioxetine’s effects in the subchronic PCP (subPCP) model, which induces impaired cognitive function and altered GABA and glutamate neurotransmission. We demonstrate that acute or subchronic vortioxetine treatment attenuated subPCP-induced deficits in attentional set shifting (AST) performance, and that the selective 5-HT₃ receptor antagonist ondansetron or the 5-HT reuptake inhibitor escitalopram could mimic this effect. Furthermore, acute vortioxetine treatment reversed subPCP-induced object recognition (OR) deficits in rats, while subchronic vortioxetine reversed subPCP-induced Object Recognition and object placement impairments in mice. Finally, subPCP treatment reduced GABA_B receptor expression in a manner that was insensitive to vortioxetine treatment, and subchronic vortioxetine treatment alone, but not in combination with subPCP, significantly increased GABA’s affinity for the GABA_A receptor. These data suggest that vortioxetine reverses cognitive impairments in a model associated with altered GABA and glutamate neurotransmission, further supporting the hypothesis that vortioxetine’s GABAergic and glutamatergic effects are relevant for cognitive function.

The Role of Serine Racemase in the Pathophysiology of Brain Disorders

The N-methyl-d-aspartate receptor (NMDAR) is unique in requiring two agonists to bind simultaneously to open its cation channel: the neurotransmitter, glutamate, and the coagonists, glycine, or d-serine. The Snyder laboratory was the first to clone serine racemase (SR), the enzyme that synthesizes d-serine, and to localize it immunocytochemically. Our laboratory has focused on the role of d-serine in brain disorders. Silencing the expression of SR, a risk gene for schizophrenia (SCZ), in mice (SR-/-), results in a phenotype that closely resembles SCZ including: cortical atrophy, reduced dendritic spine density and complexity, downregulation of parvalbumin-positive cortical GABAergic neurons, and cognitive impairments. This pathology can be reversed by treatment of SR-/- mice with d-serine in adulthood. SR-/- mice also exhibit abnormal response toward abusable substances, such as stimulants. They show reduced behavioral sensitization to d-amphetamine, but fail to extinguish it. Place preference to cocaine is altered, and the hedonic response to it is profoundly impaired as assessed by intracranial self-stimulation. d-cycloserine, a partial agonist at the NMDAR glycine modulatory site, shows therapeutic benefit for treating pathologic anxiety in combination with behavioral therapies. Studies in vitro with cortical culture and in vivo with middle cerebral artery occlusion show that silencing SR provides substantial protection against ischemic neuronal death. Finally, the switch of SR expression from neurons to reactive astrocytes after closed head trauma accounts for the reduced in vivo neuroplasticity, electroencephalogram abnormalities, and cognitive impairments.

Identification of AICP as a GluN2C-Selective N-Methyl-d-Aspartate Receptor Superagonist at the GluN1 Glycine Site

N-methyl-d-aspartate (NMDA)-type ionotropic glutamate receptors mediate excitatory neurotransmission in the central nervous system and are critically involved in brain function. NMDA receptors are also implicated in psychiatric and neurological disorders and have received considerable attention as therapeutic targets. In this regard, administration of d-cycloserine (DCS), which is a glycine site NMDA receptor agonist, can enhance extinction of conditioned fear responses. The intriguing behavioral effects of DCS have been linked to its unique pharmacological profile among NMDA receptor subtypes (GluN1/2A-D), in which DCS is a superagonist at GluN2C-containing receptors compared with glycine and a partial agonist at GluN2B-containing receptors. Here, we identify (R)-2-amino-3-(4-(2-ethylphenyl)-1H-indole-2-carboxamido)propanoic acid (AICP) as a glycine site agonist with unique GluN2-dependent differences in agonist efficacy at recombinant NMDA receptor subtypes. AICP is a full agonist at GluN1/2A (100% response compared with glycine), a partial agonist at GluN1/2B and GluN1/2D (10% and 27%, respectively), and a highly efficacious superagonist at GluN1/2C receptors (353%). Furthermore, AICP potencies are enhanced compared with DCS with EC₅₀ values in the low nanomolar range (1.7 nM at GluN1/2C). We show that GluN1/2C superagonism of AICP and DCS is mediated by overlapping but distinct mechanisms and that AICP selectively enhances responses from recombinant GluN1/2C receptors in the presence of physiological glycine concentrations. This functional selectivity of AICP for GluN2C-containing NMDA receptors is more pronounced compared with DCS, suggesting that AICP can be a useful tool compound for uncovering the roles of GluN2C subunits in neuronal circuit function and in the development of new therapeutic strategies.

Enhancement of Psychosocial Treatment With D-Cycloserine: Models, Moderators, and Future Directions

Advances in the understanding of the neurobiology of fear extinction have resulted in the development of d-cycloserine (DCS), a partial glutamatergic N-methyl-D-aspartate agonist, as an augmentation strategy for exposure treatment. We review a decade of research that has focused on the efficacy of DCS for augmenting the mechanisms (e.g., fear extinction) and outcome of exposure treatment across the anxiety disorders. Following a series of small-scale studies offering strong support for this clinical application, more recent larger-scale studies have yielded mixed results, with some showing weak or no effects. We discuss possible explanations for the mixed findings, pointing to both patient and session (i.e., learning experiences) characteristics as possible moderators of efficacy, and offer directions for future research in this area. We also review recent studies that have aimed to extend the work on DCS augmentation of exposure therapy for the anxiety disorders to DCS enhancement of learning-based interventions for addiction, anorexia nervosa, schizophrenia, and depression. Here, we attend to both DCS effects on facilitating therapeutic outcomes and additional therapeutic mechanisms beyond fear extinction (e.g., appetitive extinction, hippocampal-dependent learning).

Examining the efficacy of d-cycloserine to augment therapeutic learning in depression

Despite advances in individual and combined treatments for major depression, issues with non-response and partial-response remain relatively common, motivating the search for new treatment strategies. This study aims to develop one such novel treatment. In this proof-of-concept study, we are investigating whether the treatment enhancing effects of d-cycloserine (DCS) administration can be extended outside the extinction-learning paradigms where they have been primarily examined. Using uniform delivery of cognitive behavioral therapy (CBT) content via computer-administered interventions for depression, we are assessing the value of pre-session administrations of DCS for retention of therapeutic learning. Recall of this information is evaluated in conjunction with performance on standardized tests of memory recall with both emotional and non-emotional stimuli. Specifically, in a randomized, double-blind trial we will compare the benefits of two pre-session administrations of DCS augmentation to those achieved by similar administrations of modafinil or placebo. Because modafinil is associated with a number of discriminable effects in addition to cognitive enhancement (e.g., feelings of vigor, alertness, positive mood); whereas these effects would not be expected with DCS, we will assess drug context effects in relation to memory augmentation effects.

The Therapeutic Role of d-Cycloserine in Schizophrenia

The ketamine model for schizophrenia has led to several therapeutic strategies for enhancing N-methyl d-aspartate (NMDA) receptor activity, including agonists directed at the glycine receptor site and inhibitors of glycine reuptake. Because ketamine may primarily block NMDA receptors on inhibitory interneurons, drugs that reduce glutamate release have also been investigated as a means of countering a deficit in inhibitory input. These approaches have met with some success for the treatment of negative and positive symptoms, but results have not been consistent. An emerging approach with the NMDA partial agonist, d-cycloserine (DCS), aims to enhance plasticity by intermittent treatment. Early trials have demonstrated benefit with intermittent DCS dosing for negative symptoms and memory. When combined with cognitive remediation, intermittent DCS treatment enhanced learning on a practiced auditory discrimination task and when added to cognitive behavioral therapy, DCS improved delusional severity in subjects who received DCS with the first CBT session. These studies require replication, but point toward a promising strategy for the treatment of schizophrenia and other disorders of plasticity.

Combined Neuropeptide S and D-Cycloserine Augmentation Prevents the Return of Fear in Extinction-Impaired Rodents: Advantage of Dual versus Single Drug Approaches

BACKGROUND

Despite its success in treating specific anxiety disorders, the effect of exposure therapy is limited by problems with tolerability, treatment resistance, and fear relapse after initial response. The identification of novel drug targets facilitating fear extinction in clinically relevant animal models may guide improved treatment strategies for these disorders in terms of efficacy, acceleration of fear extinction, and return of fear.

METHODS

The extinction-facilitating potential of neuropeptide S, D-cycloserine, and a benzodiazepine was investigated in extinction-impaired high anxiety HAB rats and 129S1/SvImJ mice using a classical cued fear conditioning paradigm followed by extinction training and several extinction test sessions to study fear relapse.

RESULTS

Administration of D-cycloserine improved fear extinction in extinction-limited, but not in extinction-deficient, rodents compared with controls. Preextinction neuropeptide S caused attenuated fear responses in extinction-deficient 129S1/SvImJ mice at extinction training onset and further reduced freezing during this session. While the positive effects of either D-cycloserine or neuropeptide S were not persistent in 129S1/SvImJ mice after 10 days, the combination of preextinction neuropeptide S with postextinction D-cycloserine rendered the extinction memory persistent and context independent up to 5 weeks after extinction training. This dual pharmacological adjunct to extinction learning also protected against fear reinstatement in 129S1/SvImJ mice.

CONCLUSIONS

By using the potentially nonsedative anxiolytic neuropeptide S and the cognitive enhancer D-cycloserine to facilitate deficient fear extinction, we provide here the first evidence of a purported efficacy of a dual over a single drug approach. This approach may render exposure sessions less aversive and more efficacious for patients, leading to enhanced protection from fear relapse in the long term.

Differential Effects of D-Cycloserine and ACBC at NMDA Receptors in the Rat Entorhinal Cortex Are Related to Efficacy at the Co-Agonist Binding Site

Partial agonists at the NMDA receptor co-agonist binding site may have potential therapeutic efficacy in a number of cognitive and neurological conditions. The entorhinal cortex is a key brain area in spatial memory and cognitive processing. At synapses in the entorhinal cortex, NMDA receptors not only mediate postsynaptic excitation but are expressed in presynaptic terminals where they tonically facilitate glutamate release. In a previous study we showed that the co-agonist binding site of the presynaptic NMDA receptor is endogenously and tonically activated by D-serine released from astrocytes. In this study we determined the effects of two co-agonist site partial agonists on both presynaptic and postsynaptic NMDA receptors in layer II of the entorhinal cortex. The high efficacy partial agonist, D-cycloserine, decreased the decay time of postsynaptic NMDA receptor mediated currents evoked by electrical stimulation, but had no effect on amplitude or other kinetic parameters. In contrast, a lower efficacy partial agonist, 1-aminocyclobutane-1-carboxylic acid, decreased decay time to a greater extent than D-cycloserine, and also reduced the peak amplitude of the evoked NMDA receptor mediated postsynaptic responses. Presynaptic NMDA receptors, (monitored indirectly by effects on the frequency of AMPA receptor mediated spontaneous excitatory currents) were unaffected by D-cycloserine, but were reduced in effectiveness by 1-aminocyclobutane-1-carboxylic acid. We discuss these results in the context of the effect of endogenous regulation of the NMDA receptor co-agonist site on receptor gating and the potential therapeutic implications for cognitive disorders.

D-cycloserine 24 and 48 hours after asphyxial cardiac arrest has no effect on hippocampal CA1 neuropathology

Stimulation of N-methyl-D-aspartate receptors (NMDAR) contributes to regenerative neuroplasticity following the initial excitotoxic insult during cerebral ischemia. Stimulation of NMDAR with the partial NMDAR agonist D-cycloserine (DCS) improves outcome and restores hippocampal synaptic plasticity in models of closed head injury. We thus hypothesized that DCS would improve outcome following restoration of spontaneous circulation (ROSC) from cardiac arrest (CA). DCS (10 mg/kg, IP) was administered to Sprague-Dawley rats (male, 250-330 g; 63-84 days old) 24 and 48 hours after 6 or 8 minutes of asphyxial CA. Heart rate and blood pressure declined similarly in all groups. Animals showed neurological deficits after 6 and 8 minutes CA (P<0.05, Tukey) and these deficits recovered more quickly after 6 minutes than after 8 minutes of CA. CA decreased the number of healthy neurons within CA1 with no difference between 6 and 8 minutes duration of CA (180.8±27.6 (naïve, n=5) versus 46.3±33.8 (all CA groups, n=27) neurons per mm CA1). DCS had no effect on neurological deficits or CA1 hippocampal cell counts (P>0.05, Tukey).

D-Serine in Neuropsychiatric Disorders: New Advances

D-Serine (DSR) is an endogenous amino acid involved in glia-synapse interactions that has unique neurotransmitter characteristics. DSR acts as obligatory coagonist at the glycine site associated with the N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) and has a cardinal modulatory role in major NMDAR-dependent processes including NMDAR-mediated neurotransmission, neurotoxicity, synaptic plasticity, and cell migration. Since either over- or underfunction of NMDARs may be involved in the pathophysiology of neuropsychiatric disorders; the pharmacological manipulation of DSR signaling represents a major drug development target. A first generation of proof-of-concept animal and clinical studies suggest beneficial DSR effects in treatment-refractory schizophrenia, movement, depression, and anxiety disorders and for the improvement of cognitive performance. A related developing pharmacological strategy is the indirect modification of DSR synaptic levels by use of compounds that alter the function of main enzymes responsible for DSR production and degradation. Accumulating data indicate that, during the next decade, we will witness important advances in the understanding of DSR role that will further contribute to elucidating the causes of neuropsychiatric disorders and will be instrumental in the development of innovative treatments.

Pharmacological enhancement of memory or cognition in normal subjects

The possibility of expanding memory or cognitive capabilities above the levels in high functioning individuals is a topic of intense discussion among scientists and in society at large. The majority of animal studies use behavioral endpoint measures; this has produced valuable information but limited predictability for human outcomes. Accordingly, several groups are pursuing a complementary strategy with treatments targeting synaptic events associated with memory encoding or forebrain network operations. Transcription and translation figure prominently in substrate work directed at enhancement. Notably, the question of why new proteins would be needed for a now-forming memory given that learning-driven synthesis presumably occurred throughout the immediate past has been largely ignored. Despite this conceptual problem, and some controversy, recent studies have reinvigorated the idea that selective gene manipulation is a plausible route to enhancement. Efforts to improve memory by facilitating synaptic encoding of information have also progressed, in part due of breakthroughs on mechanisms that stabilize learning-related, long-term potentiation (LTP). These advances point to a reductionistic hypothesis for a diversity of experimental results on enhancement, and identify under-explored possibilities. Cognitive enhancement remains an elusive goal, in part due to the difficulty of defining the target. The popular view of cognition as a collection of definable computations seems to miss the fluid, integrative process experienced by high functioning individuals. The neurobiological approach obviates these psychological issues to directly test the consequences of improving throughput in networks underlying higher order behaviors. The few relevant studies testing drugs that selectively promote excitatory transmission indicate that it is possible to expand cortical networks engaged by complex tasks and that this is accompanied by capabilities not found in normal animals.

Combining d-cycloserine with motor training does not result in improved general motor learning in neurologically intact people or in people with stroke

Neurological rehabilitation involving motor training has resulted in clinically meaningful improvements in function but is unable to eliminate many of the impairments associated with neurological injury. Thus there is a growing need for interventions that facilitate motor learning during rehabilitation therapy, to optimize recovery. d-Cycloserine (DCS), a partial N-methyl-d-aspartate (NMDA) receptor agonist that enhances neurotransmission throughout the central nervous system (Ressler KJ, Rothbaum BO, Tannenbaum L, Anderson P, Graap K, Zimand E, Hodges L, Davis M. Arch Gen Psychiatry 61: 1136-1144, 2004), has been shown to facilitate declarative and emotional learning. We therefore tested whether combining DCS with motor training facilitates motor learning after stroke in a series of two experiments. Forty-one healthy adults participated in experiment I, and twenty adults with stroke participated in experiment II of this two-session, double-blind study. Session one consisted of baseline assessment, subject randomization, and oral administration of DCS or placebo (250 mg). Subjects then participated in training on a balancing task, a simulated feeding task, and a cognitive task. Subjects returned 1-3 days later for posttest assessment. We found that all subjects had improved performance from pretest to posttest on the balancing task, the simulated feeding task, and the cognitive task. Subjects who were given DCS before motor training, however, did not show enhanced learning on the balancing task, the simulated feeding task, or the associative recognition task compared with subjects given placebo. Moreover, training on the balancing task did not generalize to a similar, untrained balance task. Our findings suggest that DCS does not enhance motor learning or motor skill generalization in neurologically intact adults or in adults with stroke.

d-Cycloserine augmentation of cognitive remediation in schizophrenia

d-Cycloserine (DCS) has been shown to enhance memory and, in a previous trial, once-weekly DCS improved negative symptoms in schizophrenia subjects. We hypothesized that DCS combined with a cognitive remediation (CR) program would improve memory of a practiced auditory discrimination task and that gains would generalize to performance on unpracticed cognitive tasks. Stable, medicated adult schizophrenia outpatients participated in the Brain Fitness CR program 3-5 times per week for 8weeks. Subjects were randomly assigned to once-weekly adjunctive treatment with DCS (50mg) or placebo administered before the first session each week. Primary outcomes were performance on an auditory discrimination task, the MATRICS cognitive battery composite score and the Scale for the Assessment of Negative Symptoms (SANS) total score. 36 subjects received study drug and 32 completed the trial (average number of CR sessions=26.1). Performance on the practiced auditory discrimination task significantly improved in the DCS group compared to the placebo group. DCS was also associated with significantly greater negative symptom improvement for subjects symptomatic at baseline (SANS score ≥20). However, improvement on the MATRICS battery was observed only in the placebo group. Considered with previous results, these findings suggest that DCS augments CR and alleviates negative symptoms in schizophrenia patients. However, further work is needed to evaluate whether CR gains achieved with DCS can generalize to other unpracticed cognitive tasks.

Potentiation of GluN2C/D NMDA receptor subtypes in the amygdala facilitates the retention of fear and extinction learning in mice

NMDA receptors are glutamate receptor ion channels that contribute to synaptic plasticity and are important for many forms of learning and memory. In the amygdala, NMDA receptors are critical for the acquisition, retention, and extinction of classically conditioned fear responses. Although the GluN2B subunit has been implicated in both the acquisition and extinction of conditioned fear, GluN2C-knockout mice show reduced conditioned fear responses. Moreover, D-cycloserine (DCS), which facilitates fear extinction, selectively enhances the activity of GluN2C-containing NMDA receptors. To further define the contribution of GluN2C receptors to fear learning, we infused the GluN2C/GluN2D-selective potentiator CIQ bilaterally into the basolateral amygdala (3, 10, or 30 μg/side) following either fear conditioning or fear extinction training. CIQ both increased the expression of conditioned fear 24 h later and enhanced the extinction of the previously conditioned fear response. These results support a critical role for GluN2C receptors in the amygdala in the consolidation of learned fear responses and suggest that increased activity of GluN2C receptors may underlie the therapeutic actions of DCS.

Glutamate drugs and pharmacogenetics of OCD: a pathway-based exploratory approach

INTRODUCTION

Neuropharmacology research in glutamate-modulating drugs supports their development and use in the management of neuropsychiatric disorders, including major depression, Alzheimer's disorder and schizophrenia. Concomitantly, there is a growing use of these agents used in the treatment of obsessive-compulsive disorder (OCD).

AREAS COVERED

This article provides a review of glutamate-modulating drugs used in the treatment of OCD. Specifically, the authors examine riluzole, N-acetylcysteine, d-cycloserine, glycine, ketamine, memantine and acamprosate as treatments. Furthermore, recent genetic epidemiology research findings are presented with a focus on the positional candidate genes SLC1A1 (a glutamate transporter), ADAR3 (an RNA-editing enzyme), RYR3 (a Ca(2+) channel), PBX1 (a homeobox transcription factor) and a GWAS candidate gene, DLGAP1 (a protein interacting with post-synaptic density). These genetic findings are submitted to a curated bioinformatics database to conform a biological network for discerning potential pharmacological targets.

EXPERT OPINION

In the genetically informed network, known genes and identified key connecting components, including DLG4 (a developmental gene), PSD-95 (a synaptic scaffolding protein) and PSEN1 (presenilin, a regulator of secretase), conform a group of potential pharmacological targets. These potential targets can be explored, in the future, to deliver new therapeutic approaches to OCD. There is also the need to develop a better understanding of neuroprotective mechanisms as a foundation for future OCD drug discovery.

Autism spectrum disorder causes, mechanisms, and treatments: focus on neuronal synapses

Autism spectrum disorder (ASD) is a group of developmental disabilities characterized by impairments in social interaction and communication and restricted and repetitive interests/behaviors. Advances in human genomics have identified a large number of genetic variations associated with ASD. These associations are being rapidly verified by a growing number of studies using a variety of approaches, including mouse genetics. These studies have also identified key mechanisms underlying the pathogenesis of ASD, many of which involve synaptic dysfunctions, and have investigated novel, mechanism-based therapeutic strategies. This review will try to integrate these three key aspects of ASD research: human genetics, animal models, and potential treatments. Continued efforts in this direction should ultimately reveal core mechanisms that account for a larger fraction of ASD cases and identify neural mechanisms associated with specific ASD symptoms, providing important clues to efficient ASD treatment.

NMDA Neurotransmission Dysfunction in Behavioral and Psychological Symptoms of Alzheimer's Disease

Dementia has become an all-important disease because the population is aging rapidly and the cost of health care associated with dementia is ever increasing. In addition to cognitive function impairment, associated behavioral and psychological symptoms of dementia (BPSD) worsen patient’s quality of life and increase caregiver’s burden. Alzheimer’s disease is the most common type of dementia and both behavioral disturbance and cognitive impairment of Alzheimer’s disease are thought to be associated with the N-methyl-D-aspartate (NMDA) dysfunction as increasing evidence of dysfunctional glutamatergic neurotransmission had been reported in behavioral changes and cognitive decline in Alzheimer’s disease. We review the literature regarding dementia (especially Alzheimer’s disease), BPSD and relevant findings on glutamatergic and NMDA neurotransmission, including the effects of memantine, a NMDA receptor antagonist, and NMDA-enhancing agents, such as D-serine and D-cycloserine. Literatures suggest that behavioral disturbance and cognitive impairment of Alzheimer’s disease may be associated with excitatory neurotoxic effects which result in impairment of neuronal plasticity and degenerative processes. Memantine shows benefits in improving cognition, function, agitation/aggression and delusion in Alzheimer’s disease. On the other hand, some NMDA modulators which enhance NMDA function through the co-agonist binding site can also improve cognitive function and psychotic symptoms. We propose that modulating NMDA neurotransmission is effective in treating behavioral and psychological symptoms of Alzheimer’s disease. Prospective study using NMDA enhancers in patients with Alzheimer’s disease and associated behavioral disturbance is needed to verify this hypothesis.

Testing the effects of Δ9-THC and D-cycloserine on extinction of conditioned fear in humans

Preclinical evidence implicates several neurotransmitter systems in the extinction of conditioned fear. These results are of great interest, because the reduction of acquired fear associations is critical in therapies for anxiety disorders. We tested whether findings with respect to the N-methyl-D-aspartate (NMDA) and cannabinoid receptor (CB) systems in animals carry over to healthy human subjects. To that end, we administered selected doses of D-cycloserine (partial NMDA receptor agonist, 250 mg), delta-9-tetrahydrocannabinol (THC, CB(1) receptor agonist, 10 mg), or placebo prior to the extinction session of a 3-day conditioning protocol. D-cycloserine did not affect within-session extinction, or the retention of extinction in healthy human participants, in contrast with patient data but in line with previous reports in healthy volunteers. During extinction training, Δ9-THC reduced conditioned skin conductance responses, but not fear-potentiated startle. This effect was not retained at the retention test 2 days later, suggesting it was dependent on acute effects of the drug. Our findings implicate that facilitation of the CB(1) or NMDA system with the substances used in this study does not affect conditioned fear extinction lastingly in healthy humans. The apparent discrepancy between these findings and the results from (pre-)clinical trials is discussed in terms of room for improvement in these systems in healthy volunteers, and the lack of specificity of THC as a CB(1) agonist.

NMDA receptors and fear extinction: implications for cognitive behavioral therapy

Based primarily on studies that employ Pavlovian fear conditioning, extinction of conditioned fear has been found to be mediated by N-methyl-D-aspartate (NMDA) receptors in the amygdala and medial prefrontal cortex. This led to the discovery that an NMDA partial agonist, D-cycloserine, could facilitate fear extinction when given systemically or locally into the amygdala. Because many forms of cognitive behavioral therapy depend on fear extinction, this led to the successful use of D-cycloserine as an adjunct to psychotherapy in patients with so-called simple phobias (fear of heights), social phobia, obsessive-compulsive behavior, and panic disorder. Data in support of these conclusions are reviewed, along with some of the possible limitations of D-cycloserine as an adjunct to psychotherapy.

CREB-activity and nmnat2 transcription are down-regulated prior to neurodegeneration, while NMNAT2 over-expression is neuroprotective, in a mouse model of human tauopathy

Tauopathies, characterized by neurofibrillary tangles (NFTs) of phosphorylated tau proteins, are a group of neurodegenerative diseases, including frontotemporal dementia and both sporadic and familial Alzheimer's disease. Forebrain-specific over-expression of human tau(P301L), a mutation associated with frontotemporal dementia with parkinsonism linked to chromosome 17, in rTg4510 mice results in the formation of NFTs, learning and memory impairment and massive neuronal death. Here, we show that the mRNA and protein levels of NMNAT2 (nicotinamide mononucleotide adenylyltransferase 2), a recently identified survival factor for maintaining neuronal health in peripheral nerves, are reduced in rTg4510 mice prior to the onset of neurodegeneration or cognitive deficits. Two functional cAMP-response elements (CREs) were identified in the nmnat2 promoter region. Both the total amount of phospho-CRE binding protein (CREB) and the pCREB bound to nmnat2 CRE sites in the cortex and the hippocampus of rTg4510 mice are significantly reduced, suggesting that NMNAT2 is a direct target of CREB under physiological conditions and that tau(P301L) overexpression down-regulates CREB-mediated transcription. We found that over-expressing NMNAT2 or its homolog NMNAT1, but not NMNAT3, in rTg4510 hippocampi from 6 weeks of age using recombinant adeno-associated viral vectors significantly reduced neurodegeneration caused by tau(P301L) over-expression at 5 months of age. In summary, our studies strongly support a protective role of NMNAT2 in the mammalian central nervous system. Decreased endogenous NMNAT2 function caused by reduced CREB signaling during pathological insults may be one of underlying mechanisms for neuronal death in tauopathies.

D-cycloserine facilitates extinction of cocaine self-administration in C57 mice

INTRODUCTION

Cocaine is a highly addictive drug of abuse for which there are currently no medications. In rats and mice d-cycloserine (DCS), a partial NMDA agonist, accelerates extinction of cocaine seeking behavior. Since cues delay extinction here, we evaluated the effects d-cycloserine in extinction with and without the presence of cues.

METHODS

Two doses of DCS (15 and 30 mg/kg) were studied in C57 mice. Mice self-administered cocaine (1 mg/kg) for 2 weeks and then underwent a 20-day extinction period where DCS was administered i.p. immediately following each daily session. Extinction was conducted in some mice with the presence of cocaine-paired cues; while others were in the absence of these cues.

RESULTS

DCS treated mice (either dose) showed significantly reduced lever pressing during extinction with cue exposures when compared with vehicle treated mice. Without cues, animals showed much lower levels of lever pressing but the differences between vehicle and DCS were not significant.

CONCLUSION

DCS accelerated extinction with the presence of cues, but there were no differences on extinction without cues as compared with vehicle. These findings are consistent with DCS disrupting the memory process associated with the cues. Since drug cues are significantly involved in relapse, these findings support research to assess the therapeutic potential of DCS in cocaine addiction.

Evidence-based treatments for PTSD, new directions, and special challenges

This paper provides a current review of existing evidence-based treatments for posttraumatic stress disorder (PTSD), with a description of psychopharmacologic options, prolonged exposure therapy, cognitive processing therapy, and eye movement desensitization and reprocessing, especially as they pertain to military populations. It further offers a brief summary of promising treatments with a developing evidence base, encompassing both psychotherapy and pharmacotherapy. Finally, challenges to the treatment of PTSD are summarized and future directions suggested.

The N-methyl-D-aspartate receptor co-agonist D-cycloserine facilitates declarative learning and hippocampal activity in humans

BACKGROUND

The N-methyl-D-aspartate receptor (NMDAR) is critical for learning-related synaptic plasticity in amygdala and hippocampus. As a consequence, there is considerable interest in drugs targeting this receptor to help enhance amygdala- and hippocampus-dependent learning. A promising candidate in this respect is the NMDAR glycine-binding site partial agonist D-cycloserine (DCS). Accumulating clinical evidence indicates the efficacy of DCS in the facilitation of amygdala-dependent fear extinction learning in patients with phobic, social anxiety, panic, and obsessive-compulsive disorder. An important unresolved question though is whether the use of DCS can also facilitate hippocampus-dependent declarative learning in healthy people as opposed to being restricted to the fear memory domain.

METHODS

In the present study, we investigated whether or not DCS can facilitate hippocampus-dependent declarative learning. We have therefore combined functional magnetic resonance imaging with two different declarative learning tasks and cytoarchitectonic probabilistic mapping of the hippocampus and its major subdivisions in 40 healthy volunteers administered either a 250 mg single oral dose of DCS or a placebo.

RESULTS

We found that DCS facilitates declarative learning as well as blood-oxygen level dependent activity levels in the probabilistically defined cornu ammonis region of the hippocampus. The absence of activity changes in visual control areas underscores the specific action of DCS in the hippocampal cornu ammonis region.

CONCLUSIONS

Our findings highlight NMDAR glycine-binding site partial agonism as a promising pharmacological mechanism for facilitating declarative learning in healthy people.

Structural determinants of D-cycloserine efficacy at the NR1/NR2C NMDA receptors

We have studied relative efficacies of NR1 agonists glycine and d-cycloserine (DCS), and found efficacy to be dependent on the NR2 subunit. DCS shows partial agonism at NR1/NR2B but has higher relative efficacy than glycine at NR1/NR2C receptor. Molecular dynamics (MD) simulations of the NR1/NR2B and NR1/NR2C agonist binding domain dimer suggest only subtle differences in the interactions of DCS with NR1 binding site residues relative to glycine. The most pronounced differences were observed in the NR1/NR2C simulation between the orientation of helices F and G of the NR1 subunit. Interestingly, Helix F was previously proposed to influence receptor gating and to adopt an orientation depending on agonist efficacy. MD simulations and site-directed mutagenesis further suggest a role for residues at the agonist binding domain dimer interface in regulating DCS efficacy. To relate the structural rearrangements to receptor gating, we recorded single-channel currents from outside-out patches containing a single active NR1/NR2C receptor. DCS increased the mean open time and open probability of NR1/NR2C receptors compared with glycine. Maximum likelihood fitting of a gating model for NR1/NR2C receptor activation to the single-channel data suggests that DCS specifically accelerates the rate constant governing a fast gating step and reduces the closing rate. These changes appear to reflect a decreased activation energy for a pregating step and increased stability of the open states. We suggest that the higher efficacy of DCS at NR1/NR2C receptors involves structural rearrangements at the dimer interface and an effect on NR1/NR2C receptor pregating conformational changes.

Neuroligin-1 deletion results in impaired spatial memory and increased repetitive behavior

Neuroligins (NLs) are a family of neural cell-adhesion molecules that are involved in excitatory/inhibitory synapse specification. Multiple members of the NL family (including NL1) and their binding partners have been linked to cases of human autism and mental retardation. We have now characterized NL1-deficient mice in autism- and mental retardation-relevant behavioral tasks. NL1 knock-out (KO) mice display deficits in spatial learning and memory that correlate with impaired hippocampal long-term potentiation. In addition, NL1 KO mice exhibit a dramatic increase in repetitive, stereotyped grooming behavior, a potential autism-relevant abnormality. This repetitive grooming abnormality in NL1 KO mice is associated with a reduced NMDA/AMPA ratio at corticostriatal synapses. Interestingly, we further demonstrate that the increased repetitive grooming phenotype can be rescued in adult mice by administration of the NMDA receptor partial coagonist d-cycloserine. Broadly, these data are consistent with a role of synaptic cell-adhesion molecules in general, and NL1 in particular, in autism and implicate reduced excitatory synaptic transmission as a potential mechanism and treatment target for repetitive behavioral abnormalities.

d-cycloserine reverses the detrimental effects of stress on learning in females and enhances retention in males

Exposure to acute, inescapable stress produces a facilitation of subsequent classical eyeblink conditioning in male rats. The same stress exposure produces a profound deficit in classical eyeblink conditioning in females. Activation of N-methyl-d-aspartate receptors (NMDAr) is necessary for the effect of stress on learning in males while the contribution of NMDAr activation to the deficit in learning after stress is unknown. Here, we tested the influence of d-cycloserine (DCS), a positive modulator of the NMDAr, in stressed or unstressed male and female rats. Groups of males and females were exposed to an acute stressful event. One day later, they began training with four sessions of trace eyeblink conditioning. Each day before training, they were injected with DCS (15mg/kg) or saline. Females treated with DCS during training responded similarly to those that were untreated. However, those that were stressed and the next day treated with the drug during training did not express the typical learning deficit, i.e. they learned to time the CR very well. Because the drug was administered well after the stressor, these data indicate that DCS reversed the negative effects of stress on learning in females. In males, the effect of DCS was subtle, resulting in higher asymptotic responding, and enhanced retention in a drug-free retention test. Thus, as shown previously, training in the presence of an NMDA receptor agonist enhances associative learning and memory retention. In addition, it can reverse learning deficits that have already been induced.

Effects of D-cycloserine on extinction and reconditioning of ethanol-seeking behavior in mice

BACKGROUND

D-cycloserine (DCS), a partial N-methyl-D-aspartate receptor agonist, has been shown to enhance the extinction of both cocaine and amphetamine-induced conditioned place preference (CPP). However, there have been no reports of the effects of DCS on the extinction of ethanol-conditioned behaviors in mice. Thus, the current experiments examined the effects of DCS on the extinction and subsequent reconditioning of ethanol-induced CPP in mice.

METHODS

Male DBA/2J mice received either 2 or 4 pairings of ethanol (2 g/kg) with a conditioned stimulus (CS+) floor cue (and an equal number of saline pairings with a CS- floor cue on alternate days) resulting in either a weak or strong ethanol CPP, respectively. Following conditioning of a strong ethanol CPP mice received saline or 30 mg/kg DCS prior to each of the twelve 30-minute choice extinction trials administered at 48-hour intervals. Mice that had received conditioning of a weak ethanol CPP received saline, 30 or 60 mg/kg DCS immediately before each of the six 30-minute choice extinction trials. Following successful ethanol CPP extinction, mice received reconditioning trials similar to the initial conditioning trials. A final experiment examined the effects 12 DCS pre-exposures (15, 30, and 60 mg/kg) on initial conditioning of ethanol CPP.

RESULTS

First, we showed that 2 doses of DCS (30 and 60 mg/kg) did not have aversive properties that could confound the effects on extinction of CPP (Experiment 1). Second, we showed that DCS (30 and 60 mg/kg) had no effect on the rate of extinction of either strong (Experiment 2) or weak (Experiment 3) ethanol-induced CPP. Interestingly, DCS administered during extinction interfered with reconditioning of ethanol-induced CPP--an effect specific to reconditioning, as DCS pre-exposure did not influence initial ethanol CPP conditioning (Experiment 4).

CONCLUSIONS

These experiments show that although DCS showed no effect on extinction behavior, when given during extinction it interfered with subsequent reconditioning of ethanol CPP. The mechanisms of this effect were not, however, due to nonspecific interference with learning because repeated DCS pre-exposures did not impair initial conditioning of ethanol CPP.

The molecular and cellular biology of enhanced cognition

Most molecular and cellular studies of cognitive function have focused on either normal or pathological states, but recent research with transgenic mice has started to address the mechanisms of enhanced cognition. These results point to key synaptic and nuclear signalling events that can be manipulated to facilitate the induction or increase the stability of synaptic plasticity, and therefore enhance the acquisition or retention of information. Here, we review these surprising findings and explore their implications to both mechanisms of learning and memory and to ongoing efforts to develop treatments for cognitive disorders. These findings represent the beginning of a fundamental new approach in the study of enhanced cognition.

Once-weekly D-cycloserine effects on negative symptoms and cognition in schizophrenia: an exploratory study

BACKGROUND

Daily dosing with d-cycloserine has inconsistently improved negative symptoms in schizophrenia patients, whereas intermittent dosing significantly facilitated exposure-based therapy in two studies of patients with phobic anxiety. In animal models, single-dose administration enhances memory consolidation, but tachyphylaxis develops with repeated dosing. The objective of this exploratory study was to assess whether once-weekly dosing with d-cycloserine will produce persistent improvements in negative symptoms and cognition.

METHODS

Fifty stable adult schizophrenia outpatients treated with any antipsychotic except clozapine were enrolled and 38 were randomized, double-blind, in a parallel-group, eight-week add-on trial of d-cycloserine 50 mg or placebo administered once-weekly. Symptom rating scales and a cognitive battery were administered at baseline and week 8 before the dose of study drug. As an exploratory analysis of memory consolidation, the Logical Memory Test, modified to measure recall after 7 days, was administered at baseline and after the first weekly dose of d-cycloserine. The primary outcome measures were change from baseline to week 8 on the SANS total score and on a composite cognitive score.

RESULTS

Thirty-three subjects (87%) completed the trial. d-cycloserine significantly improved SANS total scores compared to placebo at week 8. Cognitive performance did not improve with d-cycloserine at 8 weeks. Delayed thematic recall on the Logical Memory Test was significantly improved with the first dose of d-cycloserine compared to placebo. Performance on immediate thematic recall and item recall on the Logical Memory Test did not differ between treatments.

CONCLUSIONS

Once-weekly dosing with d-cycloserine for 8 weeks produced persistent improvement of negative symptoms compared to placebo, although statistical significance was, in part, the result of worsening of negative symptoms with placebo. Consistent with animal models, a single dose of d-cycloserine facilitated memory consolidation tested after 7 days on a test of thematic recall. These results must be considered preliminary since a number of outcomes were examined without correction for multiple tests. These findings suggest that once-weekly dosing with d-cycloserine for the treatment of negative symptoms merits further study, as do d-cycloserine effects on memory consolidation.

D-cycloserine enhances short-delay, but not long-delay, conditioned taste aversion learning in rats

NMDA receptors have been implicated in conditioned taste aversion (CTA), a form of associative learning with the unique temporal characteristic of associating taste and toxic stimuli across very long delays. d-cycloserine (DCS), an NMDA receptor agonist, has been shown to enhance short-delay CTA learning. Here we examined the interaction of DCS with varying temporal parameters of CTA. DCS (15 mg/kg) administered prior to the pairing of 0.125% saccharin and LiCl (38 mM, 12 ml/kg) enhanced CTA when there was a short delay between the taste-toxin pairing (10 min), but not when there was a long delay (45 min). DCS activity remained at effective levels over the long delay, because DCS administered 60 min prior to a short-delay pairing enhanced CTA. The interaction of DCS with the delay between taste stimulus onset and LiCl injection was investigated by administering DCS and then 5 min access to saccharin 45 min prior to a short-delay pairing of saccharin and LiCl. DCS failed to enhance CTA in rats pre-exposed to saccharin, even with a short delay between the second saccharin exposure and LiCl injection. These results suggest that DCS enhancement of CTA is dependent on mechanisms underlying gustatory processing during long-delay taste-toxin associations.

A NMDA receptor glycine site partial agonist, GLYX-13, simultaneously enhances LTP and reduces LTD at Schaffer collateral-CA1 synapses in hippocampus

N-methyl-D-aspartate glutamate receptors (NMDARs) are a key route for Ca2+ influx into neurons important to both activity-dependent synaptic plasticity and, when uncontrolled, triggering events that cause neuronal degeneration and death. Among regulatory binding sites on the NMDAR complex is a glycine binding site, distinct from the glutamate binding site, which must be co-activated for NMDAR channel opening. We developed a novel glycine site partial agonist, GLYX-13, which is both nootropic and neuroprotective in vivo. Here, we assessed the effects of GLYX-13 on long-term synaptic plasticity and NMDAR transmission at Schaffer collateral-CA1 synapses in hippocampal slices in vitro. GLYX-13 simultaneously enhanced the magnitude of long-term potentiation (LTP) of synaptic transmission, while reducing long-term depression (LTD). GLYX-13 reduced NMDA receptor-mediated synaptic currents in CA1 pyramidal neurons evoked by low frequency Schaffer collateral stimulation, but enhanced NMDAR currents during high frequency bursts of activity, and these actions were occluded by a saturating concentration of the glycine site agonist d-serine. Direct two-photon imaging of Schaffer collateral burst-evoked increases in [Ca2+] in individual dendritic spines revealed that GLYX-13 selectively enhanced burst-induced NMDAR-dependent spine Ca2+ influx. Examining the rate of MK-801 block of synaptic versus extrasynaptic NMDAR-gated channels revealed that GLYX-13 selectively enhanced activation of burst-driven extrasynaptic NMDARs, with an action that was blocked by the NR2B-selective NMDAR antagonist ifenprodil. Our data suggest that GLYX-13 may have unique therapeutic potential as a learning and memory enhancer because of its ability to simultaneously enhance LTP and suppress LTD.

Effects of REM deprivation and an NMDA agonist on the extinction of conditioned fear

Rapid eye movement sleep (REM) has been implicated in a number of learning and memory tasks. Previous research has demonstrated that REM deprivation impairs the development of extinction of conditioned fear responses. However, the neurobiological mechanisms of this effect remain unclear. The present study investigated the effects of systemic administration of d-cycloserine (DCS), an NMDA agonist, on the extinction of a conditioned fear response following 6 h of REM deprivation. In experiment 1, rats were administered DCS between fear training and REM deprivation. In experiment 2, rats were administered DCS prior to extinction training. The results of experiment 1 indicated that both DCS alone and REM deprivation alone impaired extinction learning. Administration of DCS to REM deprived animals partially, but not completely, reversed the deficit in extinction. The results of experiment 2 indicated that regardless of prior REM deprivation history, DCS facilitated extinction learning. The results provide further evidence for a role of REM in the extinction of cued fear learning and indicate that this effect appears to be partially mediated by NMDA-dependent mechanisms.

Strain-dependent effects of cognitive enhancers in the mouse

A series of cognitive enhancers (CEs) have been reported to increase spatial memory in rodents, information on behavioral effects, however, is limited. The aim of the study was therefore to examine the behavioral effects of three CEs in two well-documented inbred mouse strains. C57BL/6J and DBA/2 mice were administered intraperitonial. D-cycloserine (DCS; NMDA receptor agonist), 1-(4-Amino-5-chloro-2-methoxyphenyl)-3-[1-butyl-4-piperidinyl]-1-propanone hydrochloride (RS67333; 5HT4-receptor agonist), and (R)-4-{[2-(1-methyl-2-pyrrolidinyl)ethyl]thio}phenol hydrochloride (SIB-1553A; beta-4-nicotinic receptor agonist) and tested in the open field (OF), elevated plus maze (EPM), neurological observational battery and rota-rod. Cognitive performance was tested in the Morris water maze. All compounds modified behavioral performance in the OF, DCS showed an anxiolytic effect in the EPM, and differences in the observational battery were observed i.e. vestibular drop was decreased by SIB-1553A and RS67333 treatment in C57BL/6J and increased with DCS treatment in DBA/2 mice. In the rota rod SIB-1553A improved motor performance. DCS effects on learning and memory was comparable to controls whereas the other compounds impaired performance in the Morris water maze. In conclusion, behavioral testing of CEs in the mouse revealed significant changes that may have to be taken into account for evaluation of CEs, interpretation of cognitive studies and warrant further neurotoxicological studies. Moreover, strain-dependent differences were observed that in turn may confound results obtained from behavioral and cognitive testing.

Long-term memory of cocaine-associated context: disruption and reinstatement

Long-term memory of cocaine-associated context was established by conditioned place preference learning. After 1 week, exposure to context in the absence of cocaine (memory retrieval) was paired with one of the following treatments: saline, scopolamine (muscarinic acetylcholine receptor antagonist), dizocilpine (MK-801; noncompetitive N-methyl-D-aspartate antagonist) or D-cycloserine (partial N-methyl-D-aspartate agonist). In subsequent conditioned place preference tests, place preference was suppressed in the drug-treated groups but not saline-treated groups. Results suggest that the amnesic agents, scopolamine and MK-801, disrupted reconsolidation of cocaine-associated contextual memory. In contrast, the mnemonic agent D-cycloserine might have facilitated extinction learning during context exposure in the absence of cocaine. Challenge administration of cocaine reinstated place preference in all groups except the MK-801 group, suggesting that suppression of conditioned response may or may not suppress memory evoked by drug-context reexposure.

Neotic preferences in laboratory rodents: Issues, assessment and substrates

Neotic preference refers to the extent to which animals prefer stimuli of differing novelty value. Degree of novelty is determined by within- and between-trials habituation and amount of temporal (novelty) and spatial change (complexity) in stimulation which in turn will determine the amount of curiosity-based approach (neophilia) or fear-based avoidance (neophobia) of novel stimuli. Tests of genuine neotic preferences enable direct assessments of responsiveness to temporal and spatial changes and include measurements of novel versus familiar locations (such as novelty-related location preferences), responsiveness to stimulus complexity (such as object exploration) and learning for exploratory rewards (such as light-contingent bar-pressing). Effects of brain lesions and peripherally administered drugs have implicated several brain areas and neurotransmitters that subserve memory, fear and reward in neotic preferences namely the hippocampus and ACh (memory), the amygdala, GABA and 5-HT (fear), and the mesolimbic DA reward system. However, more attention should be paid to the complexity of interactions between different brain and neurotransmitter systems and improvements in methodology before conclusions should be drawn about the neurobiological basis of neotic preferences.

Sulfated steroids as endogenous neuromodulators

Central nervous system function is critically dependent upon an exquisitely tuned balance between excitatory synaptic transmission, mediated primarily by glutamate, and inhibitory synaptic transmission, mediated primarily by GABA. Modulation of either excitation or inhibition would be expected to result in altered functionality of finely tuned synaptic pathways and global neural systems, leading to altered nervous system function. Administration of positive or negative modulators of ligand-gated ion channels has been used extensively and successfully in CNS therapeutics, particularly for the induction of sedation and treatment of anxiety, seizures, insomnia, and pain. Excessive activation of excitatory glutamate receptors, such as in cerebral ischemia, can result in neuronal damage via excitotoxic mechanisms. The discovery that neuroactive steroids exert rapid, direct effects upon the function of both excitatory and inhibitory neurotransmitter receptors has raised the possibility that endogenous neurosteroids may play a regulatory role in synaptic transmission by modulating the balance between excitatory and inhibitory neurotransmission. The sites to which neuroactive steroids bind may also serve as targets for the discovery of therapeutic neuromodulators.

Cognitive impairment

As populations continue to age, the prevalence of dementia is expected to increase. AD is by far the most common cause of dementia. The clinical course of dementia represents the challenges that this disease presents. There are no truly effective therapies for treating dementia, and the cost effectiveness of ChEIs has been challenged; however, there has been an explosion of information about AD. Evidence-based practice parameters for diagnosis and management of dementia have been developed. There has been an increased interest in the possible prodromal states of dementia, such as MCI. The concept of MCI has risen in prominence in recent years; it is speculated that initiation of therapies early in the course of disease may be needed for them to be effective. Considering the enormous burdens that AD places on individuals and society, disease-modifying treatments for AD are needed desperately. There are promising avenues for the development of potentially disease-modifying therapies for this devastating disease.

GLYX-13: A monoclonal antibody-derived peptide that acts as an N-methyl-d-aspartate receptor modulator

We previously created a monoclonal antibody (MAb), B6B21, that acts as a partial agonist at the glycine site of the N-methyl-d-aspartate (NMDA) receptor [Moskal, J.R., Schaffner, A.E., 1986. Monoclonal antibodies to the dentate gyrus: immunocytochemical characterization and flow cytometric analysis of hippocampal neurons bearing a unique cell-surface antigen. J. Neurosci. 6, 2045-2053.]. The hypervariable region of the light chain of B6B21 was cloned and sequenced. Peptides were then synthesized based on this sequence information and screened using rat hippocampal membrane preparations to measure [(3)H]MK-801 binding in the presence of 7-chlorokynurenic acid, a glycine site-specific competitive inhibitor of NMDA receptor [Moskal, J.R., Yamamoto, H., Colley, P.A., 2001. The use of antibody engineering to create novel drugs that target N-methyl-d-aspartate receptors. Curr. Drug Targets 2, 331-345.]. Peptides that were able to increase [(3)H]MK-801 binding in a dose-dependent manner under these conditions were named Glyxins. Here we report that GLYX-13, a tetrapeptide (TPPT-amide), was found to readily cross the blood-brain barrier and modulate the NMDA receptor in a glycine-like fashion when examined pharmacologically and electrophysiologically. When GLYX-13 was administered to rats at 0.5-1.0mg/kg i.v., a significant enhancement in learning was observed using a hippocampus-dependent trace eye blink conditioning paradigm. These data indicate that the Glyxins are a new class of NMDA receptor modulators that may have therapeutic potential. Based on the broad agonist range in vitro and the potent cognitive-enhancing properties in a valid in vivo model of learning, GLYX-13 is a new drug candidate with potential for the treatment of cognitive disorders.

Subtle effects of ketamine on memory when administered following stimulus presentation

RATIONALE

N-methyl-D-aspartate (NMDA) receptor antagonists (e.g., PCP, ketamine) have been shown to impair learning/memory. Well documented in animal models, only limited research in humans has been reported. Findings to date are similar to results of animal studies; however, antagonists are typically administered before the learning experience. This may be problematic as memory failure could be secondary to inattention induced by the psychotomimetic effects of these drugs and/or alterations in sensory processing which can degrade the quality of the stimulus, thereby affecting the accuracy of recall.

OBJECTIVE

The objective of the study is to compare the effects of ketamine vs placebo on recall for words when administered after stimulus presentation.

METHODS

In this double-blind crossover study, 24 normal controls were given bolus injections of ketamine (0.3 mg/kg) or placebo. Immediately prior to infusion, subjects were administered a verbal memory test. Delayed recall was measured 45 min postinfusion. Mental status changes were assessed using the Brief Psychiatric Rating Scale.

RESULTS

Subjects experienced a significant increase in psychiatric symptoms that peaked at 20 min. Results indicate no differences between the drug and placebo conditions for the memory task. However, reminiscence (i.e., recall of previously unrecalled items with repeated testing) was significantly reduced following ketamine administration compared to placebo.

CONCLUSIONS

Findings suggest that aspects of memory consolidation are affected by drugs that interfere with NMDA receptor function.

Effects of D-cycloserine on negative symptoms in schizophrenia

INTRODUCTION

The negative and cognitive symptoms of schizophrenia are poorly responsive to neuroleptic treatment. Glutamatergic dysfunction may mediate some of these symptoms. Low dose D-cycloserine (DCS) is a partial agonist at the glycine site of the NMDA-associated receptor complex, noncompetitively enhancing NMDA neurotransmission. Prior studies suggest a beneficial effect of DCS on negative symptoms and cognition. This treatment trial was initiated to confirm and extend these findings.

METHODS

Twenty-two male schizophrenic subjects displaying prominent negative symptoms who were stabilized on typical neuroleptics completed the study. A randomized double-blind parallel group design was used to compare the effects of 50 mg p.o. QD of DCS to placebo over 4 weeks. The two subject groups did not differ significantly in age, age of onset of illness or time on current neuroleptic treatment. Symptoms were rated by means of the SANS, BPRS and Abrams and Taylor rating scale. Cognition was assessed with the Sternberg Memory Test and the Continuous Performance Test.

RESULTS

Both medication groups improved over the 4 weeks of treatment. However, there were no significant differences between the DCS and placebo group on any symptom rating. DCS effects on cognition did not differ from placebo.

DISCUSSION

This study did not detect improvement in negative symptoms or cognitive performance with DCS treatment that has been found in some prior studies. This negative finding may be attributed to small sample size, relatively short duration of treatment and the overall modest effect of DCS. Future studies of DCS should be adequately powered to detect a small to medium effect size and should provide for a longer treatment phase than was used in this study in order to avoid a type II error.