Effects of multiple exposures to d-cycloserine on extinction of conditioned fear in rats

A fear conditioned cue orchestrates a suite of behaviors in rats

Pavlovian fear conditioning has been extensively used to study the behavioral and neural basis of defensive systems. In a typical procedure, a cue is paired with foot shock, and subsequent cue presentation elicits freezing, a behavior theoretically linked to predator detection. Studies have since shown a fear conditioned cue can elicit locomotion, a behavior that - in addition to jumping, and rearing - is theoretically linked to imminent or occurring predation. A criticism of studies observing fear conditioned cue-elicited locomotion is that responding is non-associative. We gave rats Pavlovian fear discrimination over a baseline of reward seeking. TTL-triggered cameras captured 5 behavior frames/s around cue presentation. Experiment 1 examined the emergence of danger-specific behaviors over fear acquisition. Experiment 2 examined the expression of danger-specific behaviors in fear extinction. In total, we scored 112,000 frames for nine discrete behavior categories. Temporal ethograms show that during acquisition, a fear conditioned cue suppresses reward seeking and elicits freezing, but also elicits locomotion, jumping, and rearing - all of which are maximal when foot shock is imminent. During extinction, a fear conditioned cue most prominently suppresses reward seeking, and elicits locomotion that is timed to shock delivery. The independent expression of these behaviors in both experiments reveals a fear conditioned cue to orchestrate a temporally organized suite of behaviors.

Enhanced fear limits behavioral flexibility in Shank2-deficient mice

Background

A core symptom of autism spectrum disorder (ASD) is repetitive and restrictive patterns of behavior. Cognitive inflexibility has been proposed as a potential basis for these symptoms of ASD. More generally, behavioral inflexibility has been proposed to underlie repetitive and restrictive behavior in ASD. Here, we investigated whether and how behavioral flexibility is compromised in a widely used animal model of ASD.

Methods

We compared the behavioral performance of Shank2-knockout mice and wild-type littermates in reversal learning employing a probabilistic classical trace conditioning paradigm. A conditioned stimulus (odor) was paired with an unconditioned appetitive (water, 6 µl) or aversive (air puff) stimulus in a probabilistic manner. We also compared air puff-induced eye closure responses of Shank2-knockout and wild-type mice.

Results

Male, but not female, Shank2-knockout mice showed impaired reversal learning when the expected outcomes consisted of a water reward and a strong air puff. Moreover, male, but not female, Shank2-knockout mice showed stronger anticipatory eye closure responses to the air puff compared to wild-type littermates, raising the possibility that the impairment might reflect enhanced fear. In support of this contention, male Shank2-knockout mice showed intact reversal learning when the strong air puff was replaced with a mild air puff and when the expected outcomes consisted of only rewards.

Limitations

We examined behavioral flexibility in one behavioral task (reversal learning in a probabilistic classical trace conditioning paradigm) using one ASD mouse model (Shank2-knockout mice). Thus, future work is needed to clarify the extent to which our findings (that enhanced fear limits behavioral flexibility in ASD) can explain the behavioral inflexibility associated with ASD. Also, we examined only the relationship between fear and behavioral flexibility, leaving open the question of whether abnormalities in processes other than fear contribute to behavioral inflexibility in ASD. Finally, the neurobiological mechanisms linking Shank2-knockout and enhanced fear remain to be elucidated.

Conclusions

Our results indicate that enhanced fear suppresses reversal learning in the presence of an intact capability to learn cue-outcome contingency changes in Shank2-knockout mice. Our findings suggest that behavioral flexibility might be seriously limited by abnormal emotional responses in ASD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13229-022-00518-1.

Berberine Facilitates Extinction and Prevents the Return of Fear

Exposure to a catastrophic event or intense stimulation can trigger fear memories, and the threatening memories persist even over a lifetime. Exposure therapy is based on extinction learning and is widely used to treat fear-related disorders, but its effect on remote fear memory is modest. Berberine, an isoquinoline alkaloid derived from Coptis chinensis or Berberis spp., has been recently reported to exert a diversity of pharmacological effects on the central nervous system, such as facilitating extinction of drug memory. Here, we explored the effect of berberine on extinction of fear memory using a classical contextual fear conditioning (CFC) paradigm, which is Pavlovian conditioning, can rapidly create fear memories related to contexts. Twenty-four hours or 30 days after CFC training, mice were subjected to context extinction (10 days) to extinguish their behaviors and treated with 12.5 or 25 mg/kg berberine intragastrically 1 or 6 h after each extinction session, followed by reinstatement and spontaneous recovery tests. The results showed that intragastric administration of 25 mg/kg berberine 1 h after extinction significantly promoted the extinction of recent and remote fear memories and prevented reinstatement and spontaneous recovery of extinguished fear in mice. These findings indicate that berberine combined with extinction training could serve as a promising novel avenue for the treatment of fear-related disorders.

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.

D-cycloserine augmentation of cognitive behavioral therapy for delusions: A randomized clinical trial

OBJECTIVE

D-cycloserine (DCS) promotes consolidation of extinction learning. This study extends earlier work by examining whether DCS can enhance cognitive behavioral therapy (CBT) for delusions.

METHODS

Adults reporting moderate or greater delusions were randomly assigned to receive 50 mg of DCS or placebo prior to 10 weekly CBT sessions. The primary outcome was change in severity of delusions measured with the Psychotic Symptom Rating Scale delusion subscale (PSYRATS-D). Secondary outcomes included persistence of response at 3 and 6 month follow-up and the effects of DCS on memory consolidation and cognitive flexibility. Fifty-eight participants were randomized and 44 completed the trial.

RESULTS

The DCS and placebo groups did not differ in change from baseline to end of CBT on PSYRATS-D, nor did DCS improve memory consolidation or cognitive flexibility compared to placebo. However, at the 3 month follow-up visit (week 24), 47% of participants who completed treatment with DCS reported a 20% or greater decrease on PSYRATS-D compared to 15% in the placebo group (p = .04). Change in distress across CBT sessions interacted with treatment group to predict change from baseline to week 24 in PSYRATS-D total score (p = .03) such that response at week 24 was greatest in DCS-treated participants who experienced a decrease in distress during CBT sessions.

CONCLUSIONS

DCS augmentation of CBT did not improve delusions compared to placebo during treatment; however, DCS was associated with a higher response rate at 3-month follow-up. DCS may produce a delayed therapeutic effect, associated with successful CBT sessions, but this finding requires replication.

Randomized controlled trial of d-cycloserine in cocaine dependence: Effects on contingency management and cue-induced cocaine craving in a naturalistic setting

Cocaine dependence constitutes a significant public health concern. This randomized, double-blind, placebo-controlled trial tested a novel approach to reducing cocaine use among cocaine-dependent individuals with d-cycloserine, a drug known to enhance learning and some learning-based therapies. Urine samples and cocaine craving were assessed across three phases: induction (Weeks 1-2), treatment (Weeks 3-5; urinalysis-based contingency management plus exposure therapy), and posttreatment (Weeks 6-7). During the treatment phase, either 50 mg of d-cycloserine or placebo was administered after delivery of urinalysis feedback with potential monetary reward and before exposure therapy sessions in naturalistic contexts individually associated with cocaine use. d-cycloserine significantly improved learning on an operant laboratory task. Contingency management significantly reduced cocaine use and craving. d-cycloserine did not significantly affect cocaine use or craving in the treatment phase. Craving significantly increased for the d-cycloserine group during the post treatment phase. Therefore, although the study showed that d-cycloserine was capable of improving learning, enhancement of learning-based therapy was not observed. Moreover, no differences in behavioral measures of cocaine demand (cocaine purchasing task) or monetary or sexual delay discounting were observed across phases or between groups in any phase. These results are somewhat consistent with previous findings suggesting that d-cycloserine administration increases cocaine craving, although they differ from other findings showing that d-cycloserine administration reduces alcohol or nicotine cravings. Methodological variables (e.g., guided vs. unguided exposure therapy sessions, length of extinction exposure) likely play a role in dissimilar findings observed across studies. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

A randomized, placebo-controlled laboratory study of the effects of D-cycloserine on sexual memory consolidation in women

RATIONALE AND OBJECTIVE

The aim of this study was to investigate the possible facilitating effect of the partial NMDA receptor agonist D-cycloserine (DCS) on memory consolidation of conditioned sexual responses and to examine the capability of DCS to reduce context-specificity of learning.

METHODS

In a randomized placebo-controlled double-blind trial, 50 healthy females were exposed to a differential conditioning procedure. Two pictures of a male abdomen were used as conditional stimuli (CSs), of which one (the CS+) was followed by the unconditional stimulus (US), a genital vibrotactile stimulus. After the conditioning session on day 1, participants received either 125 mg of DCS or a placebo. The effects of DCS on affect, sexual arousal and US expectancy in response to the CS+ and CS- were examined 24 h after the conditioning procedure.

RESULTS

A main effect of DCS was found on affect at the first test trials (p = 0.04, η_p² = 0.09), and a similar non-significant but trend level effect was found for sexual arousal (p = 0.06, η_p² = 0.07), which appeared to persist over a longer time (p = 0.07, η_p² = 0.08). Unexpectedly, ratings of positive affect and sexual arousal in response to both the CS+ and the CS- were higher in the DCS condition compared to the control condition, possibly indicating that DCS administration reduced stimulus specificity. Since the results did not show clear evidence for context learning, we were not able to test effects on context-specificity of learning.

CONCLUSION

Although largely inconclusive, the results provide tentative support for a facilitating effect of DCS on affect and sexual arousal in response to stimuli that were presented in a sexual conditioning procedure, however, no conclusions can be drawn about effects of DCS on sexual reward learning, since the design and results do not lend themselves to unambiguous interpretation.

Glutamate Systems in DSM-5 Anxiety Disorders: Their Role and a Review of Glutamate and GABA Psychopharmacology

Serotonin reuptake inhibitors and benzodiazepines are evidence-based pharmacological treatments for Anxiety Disorders targeting serotonin and GABAergic systems, respectively. Although clearly effective, these medications fail to improve anxiety symptoms in a significant proportion of patients. New insights into the glutamate system have directed attention toward drugs that modulate glutamate as potential alternative treatments for anxiety disorders. Here we summarize the current understanding of the potential role of glutamate neurotransmission in anxiety disorders and highlight specific glutamate receptors that are potential targets for novel anxiety disorder treatments. We also review clinical trials of medications targeting the glutamate system in DSM-5 anxiety disorders. Understanding the role of the glutamate system in the pathophysiology of anxiety disorder may aid in developing novel pharmacological agents that are effective in treating anxiety disorders.

N-methyl-d-aspartate-type glutamate receptor modulators and related medications for the enhancement of auditory system plasticity in schizophrenia

Deficits in N-methyl-d-aspartate-type (NMDAR) function contribute to cognitive deficits in schizophrenia, particularly dysfunction in neuroplasticity, defined as reduced learning during training on exercises that place implicit, increasing demands on early sensory (auditory and visual) information processing. Auditory mismatch negativity (MMN) can be both a target engagement biomarker for the NMDAR and a proxy measure of neurophysiological plasticity. This review covers the evidence for using NMDAR modulator and related compounds for enhancement of cognition, with a particular focus on early auditory processing/plasticity. Compounds covered include glycine site agonists, glycine and system A-type transporter inhibitors, d-amino acid oxidase inhibitors, memantine and nicotinic alpha-7 acetylcholine receptor agonists. As opposed to daily treatment studies focusing on schizophrenia in general, intermittent, non-daily treatment combining NMDAR modulators with neuroplasticity-based paradigms, using MMN as target-engagement biomarkers show promise as treatments to both remediate plasticity deficits and overall functional deficits.

Learn to forget: Does post-exposure administration of d-cycloserine enhance fear extinction in agoraphobia?

The use of d-cycloserine (DCS) to augment exposure based therapy for anxiety disorders has shown mixed, although overall positive effects. Aim of the present study was to examine post-exposure administration of DCS in patients with agoraphobia with or without panic disorder. 73 patients with agoraphobia (with or without panic disorder) were treated with 12 sessions of cognitive behavioral therapy (CBT) including 3 exposures. Following successful exposure patients were given double blind either placebo or 50 mg of DCS. Primary outcome criterion was change in the Panic and Agoraphobia Scale (PAS) between CBT session t1, t4 (+∼2 months), t10 (+∼3 months) und t11 (+∼4 months). During the course of CBT the patients' symptomatology decreased significantly as measured by primary and secondary outcome criteria, however, without an additional benefit for DCS treated patients. Exploratory sub-group analyses for severely ill patients and patients with high anxiety and strong habituation during exposure showed that DCS administration was associated with increased improvement during the 1-month follow-up period (t10 - t11) with medium to large effect sizes (range in effect size η²_p from .06 to .25). Our study results are consistent with recent research on DCS, indicating a beneficial augmentative effect for sub-groups of anxiety patients. The lack of an overall DCS effect for the whole patient sample might be explained by a dual mechanism in fear conditioning and extinction with different cognitive processes being involved during exposure depending on the degree of anxiety experienced by the patient.

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.

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.

No Effects of D-Cycloserine Enhancement in Exposure With Response Prevention Therapy in Panic Disorder With Agoraphobia: A Double-Blind, Randomized Controlled Trial

PURPOSE/BACKGROUND

D-cycloserine (DCS) is a partial N-methyl-D-aspartate receptor agonist that potentially augments response to exposure therapy in anxiety disorders by enhancing extinction learning. This randomized, double-blinded, placebo-controlled augmentation trial examined (1) the effectiveness of adding 125 mg of DCS to exposure therapy (before or directly after the first 6 treatment sessions) in patients with panic disorder with agoraphobia and (2) the effectiveness of DCS augmentation preceding exposure relative to DCS augmentation directly postexposure.

METHODS/PROCEDURES

Fifty-seven patients were allocated to 1 of 3 medication conditions (placebo and pre-exposure and postexposure DCS) as an addition to 6 exposure sessions within a 12-session exposure and response prevention protocol. The primary outcome measure was the mean score on the "alone" subscale of the Mobility Inventory (MI).

FINDINGS/RESULTS

No differences were found in treatment outcome between DCS and placebo, administered either pre-exposure or postexposure therapy, although at 3-month follow-up, the DCS postexposure group compared with DCS pre-exposure, exhibited greater symptom reduction on the MI-alone subscale. Ancillary analyses in specific subgroups (responders vs nonresponders, early vs late responders, severely vs mildly affected patients) did not reveal any between-group DCS versus placebo differences. Finally, the study did not find an effect of DCS relative to placebo to be specific for successful exposure sessions.

IMPLICATIONS/CONCLUSIONS

This study does not find an effect of augmentation with DCS in patients with severe panic disorder and agoraphobia administered either pretreatment or directly posttreatment sessions. Moreover, no preferential effects are revealed in specific subgroups nor in successful exposure sessions. Yet, a small effect of DCS administration postexposure therapy cannot be ruled out, given the relatively small sample size of this study.

Effects of Augmenting N-Methyl-D-Aspartate Receptor Signaling on Working Memory and Experience-Dependent Plasticity in Schizophrenia: An Exploratory Study Using Acute d-cycloserine

Cognitive deficits in schizophrenia have been hypothesized to reflect N-methyl-D-aspartate receptor (NMDAR) dysfunction. However, the mechanisms through which the NMDAR contributes to individual cognitive functions differ. To explore how NMDAR signaling relates to specific cognitive deficits in schizophrenia, we tested the effects of enhancing NMDAR signaling on working memory and experience-dependent plasticity using d-cycloserine (DCS). Plasticity was assessed using an EEG paradigm that utilizes high-frequency visual stimulation (HFvS) to induce neural potentiation, and 2 learning tasks, the information integration (IIT) and weather prediction (WPT) tasks. Working memory was assessed using an N-back task. Forty-five schizophrenia patients were randomized to receive a single 100 mg DCS dose (SZ-DCS; n = 24) or placebo (SZ-PLC; n = 21) in a double-blind, between-groups design. Testing occurred on a single day after placebo or DCS administration; baseline values were not obtained. DCS did not affect plasticity, as indicated by similar neural potentiation, and similar IIT and WPT learning between groups. However, among patients who successfully engaged in the working memory task (ie, performed above chance), SZ-DCS (n = 17) showed superior 2-back performance compared to SZ-PLC (n = 16). Interestingly, SZ-DCS also showed larger pre-HFvS neural responses during the LTP task. Notably, this pattern of DCS effects is the opposite of those found in our prior study of healthy adults. Results are consistent with target engagement of the NMDAR by DCS, but suggest that NMDAR signaling was not translated into synaptic plasticity changes in schizophrenia. Results highlight the importance of considering how distinct NMDAR-associated processes contribute to individual cognitive deficits in schizophrenia.

Association of schizophrenia onset age and white matter integrity with treatment effect of D-cycloserine: a randomized placebo-controlled double-blind crossover study

BACKGROUND

It has been reported that drugs which promote the N-Methyl-D-aspartate-type glutamate receptor function by stimulating the glycine modulatory site in the receptor improve negative symptoms and cognitive dysfunction in schizophrenia patients being treated with antipsychotic drugs.

METHODS

We performed a placebo-controlled double-blind crossover study involving 41 schizophrenia patients in which D-cycloserine 50 mg/day was added-on, and the influence of the onset age and association with white matter integrity on MR diffusion tensor imaging were investigated for the first time. The patients were evaluated using the Positive and Negative Syndrome Scale (PANSS), Scale for the Assessment of Negative Symptoms (SANS), Brief Assessment of Cognition in Schizophrenia (BACS), and other scales.

RESULTS

D-cycloserine did not improve positive or negative symptoms or cognitive dysfunction in schizophrenia. The investigation in consideration of the onset age suggests that D-cycloserine may aggravate negative symptoms of early-onset schizophrenia. The better treatment effect of D-cycloserine on BACS was observed when the white matter integrity of the sagittal stratum/ cingulum/fornix stria terminalis/genu of corpus callosum/external capsule was higher, and the better treatment effect on PANSS general psychopathology (PANSS-G) was observed when the white matter integrity of the splenium of corpus callosum was higher. In contrast, the better treatment effect of D-cycloserine on PANSS-G and SANS-IV were observed when the white matter integrity of the posterior thalamic radiation (left) was lower.

CONCLUSION

It was suggested that response to D-cycloserine is influenced by the onset age and white matter integrity.

TRIAL REGISTRATION

UMIN Clinical Trials Registry (number UMIN000000468 ). Registered 18 August 2006.

The Efficacy of Augment of D-Cycloserine and Cognitive-behavioral Therapy on Adolescent with one Type of Anxiety Disorders: A Double-blind Randomized Controlled Trial

Background:

This study was designed to investigating the effect of combining D-cycloserine (DCS) and cognitive-behavioral therapy (CBT) on adolescent with at least one type of anxiety disorders.

Materials and Methods:

The present study was conducted as a double-blind randomized controlled trial on 36 adolescent with anxiety disorders. Patients were assessed in two groups. In addition to 4 sessions of weekly CBT in both groups; case group, received a 50-mg DCS capsules, control group, received Placebo daily for a month. Patients received DCS capsules or placebo 1 h before sessions of CBT. Age, sex, kind of anxiety disorders “screen for child anxiety related disorders (SCARED)” and “cognitive abilities test (CATS)” scores were evaluated and compared between groups.

Results:

The mean age of the studied patients (29 females (80.6%) and 7 males (19.4%)) was 14.1 ± 1.8 years. The most frequent anxiety disorder among the study population was generalized social disorder (GAD) (77.7%). Age, sex and the frequency of anxiety disorders were not statistically significant between the study groups (P > 0.05). The mean score of “SCARED” and “CATS” at before starting the treatment, after treatment and three month after the treatment were not statistically significant between groups (P > 0.05). Also, decrease in values of “SCARED” and “CATS” during the evaluation time periods was not statistically significant between groups (P > 0.05).

Conclusions:

Findings of this study showed that there has been no difference in symptoms improvement in adolescent with anxiety disorder who received treatment protocol including 4 sessions of CBT, weekly, together with 50 mgs of DCS compared to the patients of the control group.

D-cycloserine in Schizophrenia: New Strategies for Improving Clinical Outcomes by Enhancing Plasticity

Background

Dysregulation of N-methyl D-aspartate (NMDA) receptor signaling is strongly implicated in schizophrenia. Based on the ketamine model of NMDA receptor hypoactivity, therapeutic approaches designed to maintain a sustained increase in agonist activity at the glycine site of the NMDA receptor have produced promising, although inconsistent, efficacy for negative symptoms.

Methods

A review of the published literature on D-cycloserine (DCS) pharmacology in animal models and in clinical studies was performed. Findings relevant to DCS effects on memory and plasticity and their potential clinical application to schizophrenia were summarized.

Results

Studies in animals and clinical trials in patients with anxiety disorders have demonstrated that single or intermittent dosing with DCS enhances memory consolidation. Preliminary trials in patients with schizophrenia suggest that intermittent dosing with DCS may produce persistent improvement of negative symptoms and enhance learning when combined with cognitive behavioral therapy for delusions or with cognitive remediation. The pharmacology of DCS is complex, since it acts as a “super agonist” at NMDA receptors containing GluN2C subunits and, under certain conditions, it may act as an antagonist at NMDA receptors containing GluN2B subunits.

Conclusions

There are preliminary findings that support a role for D-cycloserine in schizophrenia as a strategy to enhance neuroplasticity and memory. However, additional studies with DCS are needed to confirm these findings. In addition, clinical trials with positive and negative allosteric modulators with greater specificity for NMDA receptor subtypes are needed to identify the optimal strategy for enhancing neuroplasticity in schizophrenia.

d-Cycloserine enhances durability of social skills training in autism spectrum disorder

BACKGROUND

d-Cycloserine (DCS) enhances extinction learning across species, but it has proven challenging to identify consistent benefit of DCS when added to therapeutic interventions. We conducted a placebo-controlled trial of DCS to potentiate social skills training in autism spectrum disorder (ASD) but found substantial improvement in both the DCS and placebo groups at the conclusion of active treatment. Here, we assess the impact of DCS 11 weeks following active treatment to evaluate the impact of DCS on treatment response durability.

METHODS

Study participants included 60 outpatient youth with ASD, ages 5-11 years, all with IQ above 70, and significantly impaired social functioning who completed a 10-week active treatment phase during which they received weekly single doses of 50 mg of DCS or placebo administered 30 min prior to group social skills training. Following the 10-week active treatment phase, blinded follow-up assessments occurred at week 11 and week 22. The primary outcome measure for our durability of treatment evaluation was the parent-rated social responsiveness scale (SRS) total raw score at week 22.

RESULTS

Analysis of the SRS total raw score demonstrated significant decrease for the DCS group compared to the placebo group (p = 0.042) indicating greater maintenance of treatment effect in the DCS group. DCS was well tolerated, with irritability being the most frequently reported adverse effect in both groups.

CONCLUSIONS

The findings of this study suggest that DCS may help youth with ASD to maintain skills gained during sort-term social skills training. Larger-scale studies with longer follow-up will be necessary to further understand the long-term impact of DCS paired with structured social skills training.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT01086475.

d-cycloserine addition to exposure sessions in the treatment of patients with obsessive-compulsive disorder

BACKGROUND

Preliminary studies have shown that the addition of the partial NMDA-agonist d-cycloserine (DCS) might be promising in enhancing the results of exposure therapy in obsessive-compulsive disorder (OCD). We examined the effect of DCS addition to exposure therapy in a somewhat larger sample of OCD patients with special attention to subgroups, because of the heterogeneity of OCD.

METHODS

A randomized, double-blind, placebo controlled trial was conducted in 39 patients with OCD. Patients received 6 guided exposure sessions, once a week. One hour before each session 125mg DCS or placebo was administered.

RESULTS

Scores on the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) declined more in the DCS group than in the placebo group, but the difference did not reach statistical significance (P=0.076, partial η²=0.13). Response percentages also did not differ between the DCS and the placebo group (37% and 15% respectively). In the 'cleaning/contamination' subgroup a significant effect was found in favour of DCS (P=0.033, partial η²=0.297).

CONCLUSIONS

The results of this study did not support the application of DCS to exposure therapy in OCD. Some specific aspects need further investigation: efficacy of DCS in a larger 'cleaning/contamination' (sub-)group, DCS addition only after successful sessions, interaction with antidepressants.

Acute high-intensity noise induces rapid Arc protein expression but fails to rapidly change GAD expression in amygdala and hippocampus of rats: Effects of treatment with D-cycloserine

Tinnitus is a devastating auditory disorder impacting a growing number of people each year. The aims of the current experiment were to assess neuronal mechanisms involved in the initial plasticity after traumatic noise exposure that could contribute to the emergence of tinnitus and to test a potential pharmacological treatment to alter this early neural plasticity. Specifically, this study addressed rapid effects of acute noise trauma on amygdalo-hippocampal circuitry, characterizing biomarkers of both excitation and inhibition in these limbic regions, and compared them to expression of these same markers in primary auditory cortex shortly after acute noise trauma. To assess excitatory plasticity, activity-regulated cytoskeleton-associated (Arc) protein expression was evaluated in male rats 45 min after bilateral exposure to acute high-intensity noise (16 kHz, 115 dB SPL, for 1 h), sufficient to cause acute cochlear trauma, a common cause of tinnitus in humans and previously shown sufficient to induce tinnitus in rat models of this auditory neuropathology. Western blot analyses confirmed that up-regulation of amygdalo-hippocampal Arc expression occurred rapidly post-noise trauma, corroborating several lines of evidence from our own and other laboratories indicating that limbic brain structures, i.e. outside of the classical auditory pathways, exhibit plasticity early in the initiation of tinnitus. Western blot analyses revealed no noise-induced changes in amygdalo-hippocampal expression of glutamate decarboxylase (GAD), the biosynthetic enzyme required for GABAergic inhibition. No changes in either Arc or GAD protein expression were observed in primary auditory cortex in this immediate post-noise exposure period, confirming other reports that auditory cortical plasticity may not occur until later in the development of tinnitus. As a further control, our experiments compared Arc protein expression between groups exposed to the quiet background of a sound-proof chamber to those exposed not only to the traumatic noise described above, but also to an intermediate, non-traumatic noise level (70 dB SPL) for the same duration in each of these three brain regions. We found that non-traumatic noise did not up-regulate Arc protein expression in these brain regions. To see if changes in Arc expression due to acute traumatic noise exposure were stress-related, we compared circulating serum corticosterone in controls and rats exposed to traumatic noise at the time when changes in Arc were observed, and found no significant differences in this stress hormone in our experimental conditions. Finally, the ability of D-cycloserine (DCS; an NMDA-receptor NR1 partial agonist) to reduce or prevent the noise trauma-related plastic changes in the biomarker, Arc, was tested. D-cycloserine prevented traumatic noise-induced up-regulation of Arc protein expression in amygdala but not in hippocampus, suggesting that DCS alone is not fully effective in eliminating regionally-specific early plastic changes after traumatic noise exposure.

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).

Acute inhibition of mGluR5 disrupts behavioral flexibility

Conditioned cues can sometimes elicit maladaptive responses as seen in the post-traumatic stress disorder (PTSD). Lack of effective fear extinction, which involves a process of inhibitory learning, is hypothesized to associate with PTSD. In this study, we tested the effect of acute pharmacological inhibition of mGluR5 activity on the extinction of fear memory and spatial memory. Intraperitoneal injection of the mGluR5 (metabotropic glutamate receptor 5) antagonist MPEP [2-Methyl-6-(phenylethynyl) pyridine hydrochloride] allowed the retrieval but prevented the extinction of contextual fear memory in mice. Without altering locomotor activity, MPEP inhibited the acquisition but not the consolidation of contextual fear memory. Further, administration of MPEP blocked the extinction of spatial memory in the Morris water maze paradigm. Our data suggest a necessary role of mGluR5 in regulating certain aspects of behavioral flexibility.

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.

D-Cycloserine in Neuropsychiatric Diseases: A Systematic Review

D-Cycloserine, known from tuberculosis therapy, has been widely introduced to neuropsychiatric studies, since its central active mechanism as a partial NMDA-agonist has been found. In this review, we evaluate its therapeutic potential in neuropsychological disorders and discuss its pitfalls in terms of dosing and application frequency as well as its safety in low-dose therapy. Therefore, we identified 91 clinical trials by performing a Medline search. We demonstrate in part preliminary but increasing evidence that D-cycloserine may be effective in various psychiatric diseases, including schizophrenia, anxiety disorders, addiction, eating disorders, major depression, and autism as well as in neurological diseases, including dementia, Alzheimer's disease, and spinocerebellar degeneration. D-Cycloserine in low-dose therapy is safe, but there is still a need for new drugs with higher specificity to the different N-methyl-D-aspartate-receptor subunits.

The impact of NMDA receptor hypofunction on GABAergic neurons in the pathophysiology of schizophrenia

While the dopamine hypothesis has dominated schizophrenia research for several decades, more recent studies have highlighted the role of fast synaptic transmitters and their receptors in schizophrenia etiology. Here we review evidence that schizophrenia is associated with a reduction in N-methyl-d-aspartate receptor (NMDAR) function. By highlighting postmortem, neuroimaging and electrophysiological studies, we provide evidence for preferential disruption of GABAergic circuits in the context of NMDAR hypo-activity states. The functional relationship between NMDARs and GABAergic neurons is realized at the molecular, cellular, microcircuit and systems levels. A synthesis of findings across these levels explains how NMDA-mediated inhibitory dysfunction may lead to aberrant interactions among brain regions, accounting for key clinical features of schizophrenia. This synthesis of schizophrenia unifies observations from diverse fields and may help chart pathways for developing novel diagnostics and therapeutics.

D-Cycloserine acts via increasing the GluN1 protein expressions in the frontal cortex and decreases the avoidance and risk assessment behaviors in a rat traumatic stress model

D-cycloserine (DCS), an FDA approved anti-tuberculosis drug has extensively been studied for its cognitive enhancer effects in psychiatric disorders. DCS may enhance the effects of fear extinction trainings in animals during exposure therapy and hence we investigated the effects of DCS on distinct behavioral parameters in a predator odor stress model and tested the optimal duration for repeated daily administrations of the agent. Cat fur odor blocks were used to produce stress and avoidance and risk assessment behavioral parameters were used where DCS or saline were used as treatments in adjunct to extinction trainings. We observed that DCS facilitated extinction training by providing further extinction of avoidance responses, risk assessment behaviors and increased the contact with the cue in a setting where DCS was administered before extinction trainings for 3 days without producing a significant tolerance. In amygdala and hippocampus, GluN1 protein expressions decreased 72h after the fear conditioning in the traumatic stress group suggesting a possible down-regulation of NMDARs. We observed that extinction learning increased GluN1 proteins both in the amygdaloid complex and the dorsal hippocampus of the rats receiving extinction training or extinction training with DCS. Our findings also indicate that DCS with extinction training increased GluN1 protein levels in the frontal cortex. We may suggest that action of DCS relies on enhancement of the consolidation of fear extinction in the frontal cortex.

Augmentation of Evidence-Based Psychotherapy for PTSD With Cognitive Enhancers

Exposure-based therapy has proven to be useful to treat various anxiety disorders as well as post-traumatic stress disorder (PTSD). Despite its efficacy, a fair proportion of patients remain symptomatic after treatment. Different lines of research have put considerable efforts to investigate ways to enhance the efficacy of exposure-based therapy, which could ultimately lead to better clinical outcomes for patients. Given that this type of therapy relies on extinction learning principles, neuroscience research has tested different adjuncts that could be used as cognitive enhancers through their impact on extinction learning and its consolidation. The current review will summarize some of the latest compounds that have received attention and show some promise to be used in clinical settings to improve the efficacy of exposure-based therapy.

Drug development in schizophrenia: are glutamatergic targets still worth aiming at?

PURPOSE OF REVIEW

The ketamine model has dominated drug discovery in schizophrenia over the past decade, supported by genetic and postmortem evidence implicating glutamatergic transmission. This review assesses recent successes and disappointments of glutamatergic agents and identifies promising new directions.

RECENT FINDINGS

Strategies focused on enhancing activity of the N-methyl D-aspartate (NMDA) receptor via direct agonists at the glycine site or by inhibition of glycine reuptake have produced modest and often inconsistent evidence of efficacy, as have approaches to reduce excessive glutamate release by lamotrigine or by mGluR2/3 agonists. Strategies targeting α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors have also met with only limited success. Newer approaches include selective allosteric modulation of NMDA receptor subunits and of mGluR5 receptors. In addition, intracellular pathways downstream of NMDA receptors may also provide new treatment targets, as exemplified by phosphodiesterase (PDE) inhibitors.

SUMMARY

Targeting glutamatergic transmission remains one of the most promising strategies in schizophrenia, particularly early in the course of illness, but therapeutic approaches may require greater specificity for receptor subtype type, illness phase, and individual biology in order to enhance efficacy and overcome problems with reproducibility of clinical results.

A novel treatment for tinnitus and tinnitus-related cognitive difficulties using computer-based cognitive training and D-cycloserine

IMPORTANCE

Tinnitus affects more than 40 million people in the Unites States, and cognitive difficulties are among the most commonly associated symptoms.

OBJECTIVE

To test the feasibility and preliminarily the effectiveness of using a putative neuroplasticity-enhancing drug, D-cycloserine, to facilitate a computer-assisted CT program for improving tinnitus bother and related cognitive difficulties.

DESIGN, SETTING, AND PARTICIPANTS

Double-blind, randomized clinical trial at an outpatient academic medical center of 34 participants aged 35 to 65 years with subjective, unilateral or bilateral, nonpulsatile tinnitus of at least 6 months' duration.

INTERVENTIONS

Five weeks of twice-weekly computer-based CT with either 250 mg D-cycloserine or placebo orally prior to computer CT sessions.

MAIN OUTCOMES AND MEASURES

Difference in the change in Tinnitus Functional Index (TFI) score between the 2 groups.

RESULTS

After excluding 1 participant lost to follow-up, 1 who withdrew, 1 who did not complete 90% of sessions, and 1 outlier, 30 participants were included in the analysis. The D-cycloserine plus CT group showed a significant improvement in median TFI score (-5.8 [95% CI, -9.4 to -1.1]) and self-reported cognitive deficits (-4.5 [95% CI, -11.5 to -1.0]), but the placebo group did not (-1.0 [95% CI, -11.7 to 4.9] and -2.0 [95% CI, -5.1 to 2.0], respectively). After controlling for age and duration of tinnitus, there was no significant difference in TFI score change between the 2 groups (P = .41). After confounders were controlled for, the D-cycloserine group demonstrated a significantly greater improvement in self-reported cognitive deficits as compared with the placebo group (P = .03). No serious adverse events were reported.

CONCLUSIONS AND RELEVANCE

Use of a computer-based CT program with a putative neuroplasticity-sensitizing drug, D-cycloserine, was feasible and well tolerated. With the limited sample size, the adjuvant use of D-cycloserine was no more effective than placebo at improving tinnitus bother. The finding that D-cycloserine use was more effective than placebo at improving self-reported cognitive difficulties could be important given the high rate of concern for cognitive deficits in patients with tinnitus. D-cycloserine and other putative neuroplasticity-facilitating agents could be investigated in the future as a strategy to enhance neuroplasticity-based tinnitus treatments.

TRIAL REGISTRATION

clinicaltrials.gov Identifier: NCT01550796.

Pharmacology of cognitive enhancers for exposure-based therapy of fear, anxiety and trauma-related disorders

Pathological fear and anxiety are highly debilitating and, despite considerable advances in psychotherapy and pharmacotherapy they remain insufficiently treated in many patients with PTSD, phobias, panic and other anxiety disorders. Increasing preclinical and clinical evidence indicates that pharmacological treatments including cognitive enhancers, when given as adjuncts to psychotherapeutic approaches [cognitive behavioral therapy including extinction-based exposure therapy] enhance treatment efficacy, while using anxiolytics such as benzodiazepines as adjuncts can undermine long-term treatment success. The purpose of this review is to outline the literature showing how pharmacological interventions targeting neurotransmitter systems including serotonin, dopamine, noradrenaline, histamine, glutamate, GABA, cannabinoids, neuropeptides (oxytocin, neuropeptides Y and S, opioids) and other targets (neurotrophins BDNF and FGF2, glucocorticoids, L-type-calcium channels, epigenetic modifications) as well as their downstream signaling pathways, can augment fear extinction and strengthen extinction memory persistently in preclinical models. Particularly promising approaches are discussed in regard to their effects on specific aspects of fear extinction namely, acquisition, consolidation and retrieval, including long-term protection from return of fear (relapse) phenomena like spontaneous recovery, reinstatement and renewal of fear. We also highlight the promising translational value of the preclinial research and the clinical potential of targeting certain neurochemical systems with, for example d-cycloserine, yohimbine, cortisol, and L-DOPA. The current body of research reveals important new insights into the neurobiology and neurochemistry of fear extinction and holds significant promise for pharmacologically-augmented psychotherapy as an improved approach to treat trauma and anxiety-related disorders in a more efficient and persistent way promoting enhanced symptom remission and recovery.

Mechanisms to medicines: elucidating neural and molecular substrates of fear extinction to identify novel treatments for anxiety disorders

The burden of anxiety disorders is growing, but the efficacy of available anxiolytic treatments remains inadequate. Cognitive behavioural therapy for anxiety disorders focuses on identifying and modifying maladaptive patterns of thinking and behaving, and has a testable analogue in rodents in the form of fear extinction. A large preclinical literature has amassed in recent years describing the neural and molecular basis of fear extinction in rodents. In this review, we discuss how this work is being harnessed to foster translational research on anxiety disorders and facilitate the search for new anxiolytic treatments. We begin by summarizing the anatomical and functional connectivity of a medial prefrontal cortex (mPFC)-amygdala circuit that subserves fear extinction, including new insights from optogenetics. We then cover some of the approaches that have been taken to model impaired fear extinction and associated impairments with mPFC-amygdala dysfunction. The principal goal of the review is to evaluate evidence that various neurotransmitter and neuromodulator systems mediate fear extinction by modulating the mPFC-amygdala circuitry. To that end, we describe studies that have tested how fear extinction is impaired or facilitated by pharmacological manipulations of dopamine, noradrenaline, 5-HT, GABA, glutamate, neuropeptides, endocannabinoids and various other systems, which either directly target the mPFC-amygdala circuit, or produce behavioural effects that are coincident with functional changes in the circuit. We conclude that there are good grounds to be optimistic that the progress in defining the molecular substrates of mPFC-amygdala circuit function can be effectively leveraged to identify plausible candidates for extinction-promoting therapies for anxiety disorders.

Does d-Cycloserine Augmentation of CBT Improve Therapeutic Homework Compliance for Pediatric Obsessive-Compulsive Disorder?

Clinical studies in adults and children with obsessive-compulsive disorder (OCD) have shown that d-cycloserine (DCS) can improve treatment response by enhancing fear extinction learning during exposure-based psychotherapy. Some have hypothesized that improved treatment response is a function of increased compliance and engagement in therapeutic homework tasks, a core component of behavioral treatment. The present study examined the relationship between DCS augmented cognitive-behavioral therapy (CBT) and homework compliance in a double-blind, placebo controlled trial with 30 youth with OCD. All children received 10 CBT sessions, the last seven of which included exposure and response prevention paired with DCS or placebo dosed 1 h before the session started. Results suggested that DCS augmented CBT did not predict improved homework compliance over the course of treatment, relative to the placebo augmented CBT group. However, when groups were collapsed, homework compliance was directly associated with treatment outcome. These findings suggest that while DCS may not increase homework compliance over time, more generally, homework compliance is an integral part of pediatric OCD treatment outcome.

Can fear extinction be enhanced? A review of pharmacological and behavioral findings

There is considerable interest, from both a basic and clinical standpoint, in gaining a greater understanding of how pharmaceutical or behavioral manipulations alter fear extinction in animals. Not only does fear extinction in rodents model exposure therapy in humans, where the latter is a cornerstone of behavioral intervention for anxiety disorders such as post-traumatic stress disorder and specific phobias, but also understanding more about extinction provides basic information into learning and memory processes and their underlying circuitry. In this paper, we briefly review three principal approaches that have been used to modulate extinction processes in animals and humans: a purely pharmacological approach, the more widespread approach of combining pharmacology with behavior, and a purely behavioral approach. The pharmacological studies comprise modulation by: brain derived neurotrophic factor (BDNF), d-cycloserine, serotonergic and noradrenergic drugs, neuropeptides, endocannabinoids, glucocorticoids, histone deacetylase (HDAC) inhibitors, and others. These studies strongly suggest that extinction can be modulated by drugs, behavioral interventions, or their combination, although not always in a lasting manner. We suggest that pharmacotherapeutic manipulations provide considerable promise for promoting effective and lasting fear reduction in individuals with anxiety disorders. This article is part of a Special Issue entitled 'Memory enhancement'.

A pilot randomized controlled trial of D-cycloserine and distributed practice as adjuvants to constraint-induced movement therapy after stroke

Background. Phase III trials of rehabilitation of paresis after stroke have proven the effectiveness of intensive and extended task practice, but they have also shown that many patients do not qualify, because of severity of impairment, and that many of those who are treated are left with clinically significant deficits. Objective. To test the value of 2 potential adjuvants to normal learning processes engaged in constraint-induced movement therapy (CIMT): greater distribution of treatment over time and the coadministration of d-cycloserine, a competitive agonist at the glycine site of the N-methyl-D-aspartate glutamate receptor. Methods. A prospective randomized single-blind parallel-group trial of more versus less condensed therapy (2 vs 10 weeks) and d-cycloserine (50 mg) each treatment day versus placebo (in a 2 × 2 design), as potential adjuvants to 60 hours of CIMT. Results. Twenty-four participants entered the study, and 22 completed it and were assessed at the completion of treatment and 3 months later. Neither greater distribution of treatment nor treatment with d-cycloserine significantly augmented retention of gains achieved with CIMT. Conclusions. Greater distribution of practice and treatment with d-cycloserine do not appear to augment retention of gains achieved with CIMT. However, concentration of CIMT over 2 weeks ("massed practice") appears to confer no advantage either.

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.

Acute stress impairs the retrieval of extinction memory in humans

Extinction training is a form of inhibitory learning that allows an organism to associate a previously aversive cue with a new, safe outcome. Extinction does not erase a fear association, but instead creates a competing association that may or may not be retrieved when a cue is subsequently encountered. Characterizing the conditions under which extinction learning is expressed is important to enhancing the treatment of anxiety disorders that rely on extinction-based exposure therapy as a primary treatment technique. The ventromedial prefrontal cortex, which plays a critical role in the expression of extinction memory, has been shown to be functionally impaired after stress exposure. Further, recent work in rodents has demonstrated that exposure to stress leads to deficits in extinction retrieval, although this has yet to be tested in humans. To explore how stress might influence extinction retrieval in humans, participants underwent a differential aversive learning paradigm, in which one image was probabilistically paired with an aversive shock while the other image denoted safety. Extinction training directly followed, at which point reinforcement was omitted. A day later, participants returned to the lab and either completed an acute stress manipulation (i.e., cold pressor), or a control task, before undergoing an extinction retrieval test. Skin conductance responses and salivary cortisol concentrations were measured throughout each session as indices of fear arousal and neuroendocrine stress response, respectively. The efficacy of our stress induction was established by observing significant increases in cortisol for the stress condition only. We examined extinction retrieval by comparing conditioned responses during the last trial of extinction (day 1) with that of the first trial of re-extinction (day 2). Groups did not differ on initial fear acquisition or extinction, however, a day later participants in the stress group (n=27) demonstrated significantly lower extinction retrieval (i.e., greater fear recovery) than those in the control group (n=25). Our results suggest that acute stress impairs the retrieval of extinction learning and offers insight into why treatment strategies used in the clinic may be challenging to recruit in daily life where stress is pervasive.

Neural mechanisms of pain and alcohol dependence

An association between chronic pain conditions and alcohol dependence has been revealed in numerous studies with episodes of alcohol abuse antedating chronic pain in some people and alcohol dependence emerging after the onset of chronic pain in others. Alcohol dependence and chronic pain share common neural circuits giving rise to the possibility that chronic pain states could significantly affect alcohol use patterns and that alcohol dependence could influence pain sensitivity. The reward and emotional pathways that regulate drug/alcohol addiction also mediate chronic pain. For example, pain-evoked activation of brain learning and brain reward circuitry may modulate cortical processing of pain and central sensitization mediated by mesocorticolimbic circuitry. Imbalance and reorganization of amygdala-mPFC interactions may not only be important for persistent pain, but also for disorders characterized by the abnormal persistence of emotional-affective states such as drug and alcohol addiction. Further studies are necessary to understand how these neural circuits are regulated in comorbid conditions of alcoholism and chronic pain. In addition, long term alcohol use could induce pain symptoms and may exacerbate chronic pain arising from other sources. While prior studies have established a role of neuroendocrine stress axis mediators in alcohol abuse and neurotoxic effects, these studies have not explored the distinction between the individual impact of alcohol and stress hormones. Future studies should explore the mechanisms mediating the contribution of alcohol and stress axis hormones on pain, an important question in our understanding of the neurobiology of alcohol abuse and chronic pain.

Future perspectives on the treatment of cognitive deficits and negative symptoms in schizophrenia

Drug discovery based on classic models for cognitive impairment and negative symptoms of schizophrenia have met with only modest success. Because cognitive impairment and negative symptoms may result from disruptions in neurodevelopment, more complex developmental models that integrate environmental and genetic risk factors are needed. In addition, it has become clear that biochemical pathways involved in schizophrenia form complex, interconnected networks. Points at which risk factors converge, such as brain-derived neurotrophic factor (BDNF) and protein kinase B (AKT), and from which processes involved in neuroplasticity diverge, are of particular interest for pharmacologic interventions. This paper reviews elements of neurodevelopmental models for cognitive deficits and negative symptoms of schizophrenia with the aim of identifying potential targets for interventions.

A randomized, placebo-controlled laboratory study of the effects of D-cycloserine on craving in cocaine-dependent individuals

RATIONALE

D-Cycloserine (DCS), a partial glutamate N-methyl-D-aspartate (NMDA) receptor agonist, enhances extinction of conditioned fear responding; preliminary data suggest that it may facilitate extinction of drug cue reactivity.

OBJECTIVE

This study investigates DCS effects on cocaine cue craving and drug use in cocaine-dependent subjects.

METHODS

Thirty-two subjects were randomly assigned to receive (1) DCS only, (2) DCS before sessions 1 and 3, placebo (PBO) before session 2, or (3) PBO only 15-min before each of 3 1-h cocaine cue exposure sessions conducted 1 day apart. Craving ratings were obtained before, during, and after sessions. Drug use and cue-induced craving were assessed 1 week after the last cue session.

RESULTS

Repeated presentation of cocaine cues resulted in decreased craving both within and between sessions. DCS did not facilitate extinction learning and may have enhanced craving. The group that received three doses of DCS had significantly higher craving than the PBO group at the baseline ratings taken before sessions 2 and 3, as well as significantly higher cue-induced craving at follow-up. The group that received two doses of DCS did not differ from the PBO group. There were no group differences in postextinction cocaine use.

CONCLUSIONS

The reduction of cocaine cue reactivity in the PBO group suggests that the study procedures were sufficient to produce extinction. Under these conditions, DCS did not facilitate extinction and may have enhanced craving. Further studies of glutamatergic agents and extinction in cocaine dependence should include consideration of procedural variables that could have a major impact on study outcomes.

Augmentation of exposure therapy with post-session administration of D-cycloserine

BACKGROUND

Pre-session administration of d-cycloserine (DCS) has been found to augment exposure therapy outcomes in a variety of anxiety disorders. To be able to enhance learning only for successful exposure sessions, it would be beneficial to have the option of administering DCS after rather than before the session, a strategy encouraged by pre-clinical work. We believe the present study is the first published report on the efficacy of post-session administration of DCS in humans.

METHOD

Adults (N = 29) with a DSM-IV diagnosis of acrophobia were randomized to receive two sessions of virtual reality exposure therapy (VRE) in combination with placebo or 50 mg of DCS. Instead of administering the pill prior to each of the sessions, as has been done in extant work, we administered the pill immediately following each session. Measures of acrophobia severity were collected at baseline, at each treatment session, 1-week post-treatment, and at 1-month follow-up.

RESULTS

Mixed-effects repeated-measures ANOVAs and GLMMs revealed significant improvement in all outcome measures over time, but no between-group differences were observed. At post-treatment, 63.5% of patients in the placebo condition vs. 60.0% of those in the DCS condition were in remission. At 1-month follow up, 63.4% of those in the placebo condition vs. 66.6% of those in the DCS condition were in remission.

CONCLUSIONS

These findings do not support the application of post-session DCS administration for augmenting the efficacy of exposure-based treatments. Possible reasons for these findings are discussed.

TRIAL REGISTRY

The Trial is registered at ClinicalTrials.gov (NCT01102803).

Low-frequency stimulation of the ventral hippocampus facilitates extinction of contextual fear

Difficulties to treat fear-associated disorders, including posttraumatic stress disorder, are thought to result from dysfunction in fear extinction learning and/or memory. Animal studies on extinction modulation are therefore promising for the development of new treatments. Recent rat studies, including ones using low-frequency stimulation (LFS), have demonstrated that the ventral hippocampus (VH) modulates extinction memory. The present study explores whether the VH also modulates extinction learning. For this, rats were implanted with stimulating electrodes in the VH and experienced contextual fear conditioning, followed 6 or 24 h later by VH LFS and three sessions of extinction training. We found that, whatever the delay used (6 or 24 h), animals that received VH LFS displayed persistent low levels of freezing from the second extinction session, whereas control rats showed low levels of freezing only during the third session. In animals submitted to a stress condition (provoked by a single inescapable foot-shock followed by three sessions of situational reminders) prior to fear conditioning, VH LFS also reduced freezing levels, which, in contrast, remained high in control rats during the course of extinction training. These data suggest that LFS, targeting the VH, may be useful in reducing fear responses during extinction learning.

Facilitation of extinction of operant behaviour in C57Bl/6 mice by chlordiazepoxide and D-cycloserine

RATIONALE AND OBJECTIVE

Effects on the extinction of GABAergic drug, chlordiazepoxide (CDP), and glutamatergic drug, D: -cycloserine (DCS), in C57BL/6 mice were compared.

MATERIALS AND METHODS

Following a palatability test (Experiment 1), Experiments 2-6 involved food-reinforced lever press training followed by extinction sessions at 1- or 4-day intervals. The effects of drugs were examined. Experiment 7 involved a two-lever task.

RESULTS

CDP did not affect food palatability (Experiment 1), but facilitated extinction when administered prior to extinction sessions via intracerebral (Experiment 2) or peripheral administration at 1-day (Experiments 3-7) or 4-day intervals (Experiment 6). Reducing the amount of training prior to extinction reduced the delay in the effect of CDP typically seen, and CDP had a larger effect in early sessions on mice that had received less training (Experiment 3). There was some evidence that CDP could be blocked by flumazenil (Experiment 4), and CDP withdrawal reversed extinction facilitation (Experiments 5 and 7). With 4-day intervals, DCS administered immediately following extinction sessions, or pre-session CDP, facilitated extinction with 48-trial sessions (experiment 6B). With six-trial sessions, the co-administration of post-session DCS enhanced facilitation produced by pre-session CDP (experiment 6A). Finally, CDP facilitated extinction in a dose-related fashion following training on a two-lever food-reinforced task (Experiment 7).

CONCLUSIONS

The findings are consistent with the hypotheses that two neurotransmitter systems have different roles in operant extinction and that glutamatergic systems are involved in extinction learning and GABAergic systems involved in the expression of that learning. This parallels findings with extinction following Pavlovian conditioning, which has been more extensively investigated.

D-cycloserine: an evolving role in learning and neuroplasticity in schizophrenia

As a partial agonist at the glycine site of the NMDA receptor, D-cycloserine (DCS) has been viewed as lacking potency to fully test the NMDA receptor hypofunction theory of schizophrenia. However, findings of full agonist activity at a subset of NMDA receptors that may have particular relevance to schizophrenia, plus a growing body of evidence demonstrating enhancement of learning and neuroplasticity in animal models, suggest novel therapeutic strategies with DCS in schizophrenia. Preliminary studies with once-weekly administration have supported this potential new role for DCS in schizophrenia by demonstrating benefit for negative symptoms, memory consolidation, and facilitation of cognitive behavioral therapy for delusions.

Cannabinoid facilitation of fear extinction memory recall in humans

A first-line approach to treat anxiety disorders is exposure-based therapy, which relies on extinction processes such as repeatedly exposing the patient to stimuli (conditioned stimuli; CS) associated with the traumatic, fear-related memory. However, a significant number of patients fail to maintain their gains, partly attributed to the fact that this inhibitory learning and its maintenance is temporary and conditioned fear responses can return. Animal studies have shown that activation of the cannabinoid system during extinction learning enhances fear extinction and its retention. Specifically, CB1 receptor agonists, such as Δ9-tetrahydrocannibinol (THC), can facilitate extinction recall by preventing recovery of extinguished fear in rats. However, this phenomenon has not been investigated in humans. We conducted a study using a randomized, double-blind, placebo-controlled, between-subjects design, coupling a standard Pavlovian fear extinction paradigm and simultaneous skin conductance response (SCR) recording with an acute pharmacological challenge with oral dronabinol (synthetic THC) or placebo (PBO) 2 h prior to extinction learning in 29 healthy adult volunteers (THC = 14; PBO = 15) and tested extinction retention 24 h after extinction learning. Compared to subjects that received PBO, subjects that received THC showed low SCR to a previously extinguished CS when extinction memory recall was tested 24 h after extinction learning, suggesting that THC prevented the recovery of fear. These results provide the first evidence that pharmacological enhancement of extinction learning is feasible in humans using cannabinoid system modulators, which may thus warrant further development and clinical testing. This article is part of a Special Issue entitled 'Cognitive Enhancers'.