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

Adrenalectomy exacerbates stress-induced impairment in fear discrimination: A causal role for kynurenic acid?

Impaired activity of the hypothalamic-pituitary axis and reduced blood levels of glucocorticoids (GCs) are signature features of stress-related maladies. Recent evidence suggests a possible role of the tryptophan metabolite kynurenic acid (KYNA) in this context. Here we investigated possible causal relationships in adult male rats, using stress-induced fear discrimination as a translationally relevant behavioral outcome measure. One week following adrenalectomy (ADX) or sham surgery, animals were for 2 h either physically restrained or exposed to a predator odor, which caused a much milder stress response. Extracellular KYNA levels were determined before, during and after stress by in vivo microdialysis in the prefrontal cortex. Separate cohorts underwent a fear discrimination procedure starting immediately after stress termination. Different auditory conditioned stimuli (CS) were either paired with a foot shock (CS+) or non-reinforced (CS-). One week later, fear was assessed by re-exposing the animals to each CS. Separate groups of rats were treated with the KYNA synthesis inhibitor BFF-816 prior to stress initiation to test a causal role of KYNA in fear discrimination. Restraint stress raised extracellular KYNA levels by ∼85 % in ADX rats for several hours, and these animals were unable to discriminate between CS+ and CS-. Both effects were prevented by BFF-816 and were not observed after exposure to predator odor or in sham-operated rats. These findings suggest that a causal connection exists between adrenal function, stress-induced KYNA increases, and behavioral deficits. Pharmacological inhibition of KYNA synthesis may therefore be an attractive, novel option for the treatment of stress-related disorders.

Probing cognitive flexibility in Shank2-deficient mice: Effects of D-cycloserine and NMDAR signaling hub dynamics

Neurodevelopmental disorders such as autism spectrum disorder (ASD) have a heterogeneous etiology but are largely associated with genetic factors. Robust evidence from recent human genetic studies has linked mutations in the Shank2 gene to idiopathic ASD. Modeling these Shank2 mutations in animal models recapitulates behavioral changes, e.g. impaired social interaction and repetitive behavior of ASD patients. Shank2-deficient mice exhibit NMDA receptor (NMDAR) hypofunction and associated behavioral deficits. Of note, NMDARs are strongly implicated in cognitive flexibility. Their hypofunction, e.g. observed in schizophrenia, or their pharmacological inhibition leads to impaired cognitive flexibility. However, the association between Shank2 mutations and cognitive flexibility is poorly understood. Using Shank2-deficient mice, we explored the role of Shank2 in cognitive flexibility measured by the attentional set shifting task (ASST) and whether ASST performance in Shank2-deficient mice can be modulated by treatment with the partial NMDAR agonist D-cycloserine (DCS). Furthermore, we investigated the effects of Shank2 deficiency, ASST training, and DCS treatment on the expression level of NMDAR signaling hub components in the orbitofrontal cortex (OFC), including NMDAR subunits (GluN2A, GluN2B, GluN2C), phosphoglycerate dehydrogenase and serine racemase. Surprisingly, Shank2 deficiency did not affect ASST performance or alter the expression of the investigated NMDAR signaling hub components. Importantly, however, DCS significantly improved ASST performance, demonstrating that positive NMDAR modulation facilitates cognitive flexibility. Furthermore, DCS increased the expression of GluN2A in the OFC, but not that of other NMDAR signaling hub components. Our findings highlight the potential of DCS as a pharmacological intervention to improve cognitive flexibility impairments downstream of NMDAR modulation and substantiate the key role of NMDAR in cognitive flexibility.

Cognitive Flexibility in Mice: Effects of Puberty and Role of NMDA Receptor Subunits

Cognitive flexibility refers to the ability to adapt flexibly to changing circumstances. In laboratory mice, we investigated whether cognitive flexibility is higher in pubertal mice than in adult mice, and whether this difference is related to the expression of distinct NMDA receptor subunits. Using the attentional set shifting task as a measure of cognitive flexibility, we found that cognitive flexibility was increased during puberty. This difference was more pronounced in female pubertal mice. Further, the GluN2A subunit of the NMDA receptor was more expressed during puberty than after puberty. Pharmacological blockade of GluN2A reduced the cognitive flexibility of pubertal mice to adult levels. In adult mice, the expression of GluN2A, GluN2B, and GluN2C in the orbitofrontal cortex correlated positively with performance in the attentional set shifting task, whereas in pubertal mice this was only the case for GluN2C. In conclusion, the present study confirms the observation in humans that cognitive flexibility is higher during puberty than in adulthood. Future studies should investigate whether NMDA receptor subunit-specific agonists are able to rescue deficient cognitive flexibility, and whether they have the potential to be used in human diseases with deficits in cognitive flexibility.

Metabotropic glutamate receptor function and regulation of sleep-wake cycles

Metabotropic glutamate (mGlu) receptors are the most abundant family of G-protein coupled receptors and are widely expressed throughout the central nervous system (CNS). Alterations in glutamate homeostasis, including dysregulations in mGlu receptor function, have been indicated as key contributors to multiple CNS disorders. Fluctuations in mGlu receptor expression and function also occur across diurnal sleep-wake cycles. Sleep disturbances including insomnia are frequently comorbid with neuropsychiatric, neurodevelopmental, and neurodegenerative conditions. These often precede behavioral symptoms and/or correlate with symptom severity and relapse. Chronic sleep disturbances may also be a consequence of primary symptom progression and can exacerbate neurodegeneration in disorders including Alzheimer's disease (AD). Thus, there is a bidirectional relationship between sleep disturbances and CNS disorders; disrupted sleep may serve as both a cause and a consequence of the disorder. Importantly, comorbid sleep disturbances are rarely a direct target of primary pharmacological treatments for neuropsychiatric disorders even though improving sleep can positively impact other symptom clusters. This chapter details known roles of mGlu receptor subtypes in both sleep-wake regulation and CNS disorders focusing on schizophrenia, major depressive disorder, post-traumatic stress disorder, AD, and substance use disorder (cocaine and opioid). In this chapter, preclinical electrophysiological, genetic, and pharmacological studies are described, and, when possible, human genetic, imaging, and post-mortem studies are also discussed. In addition to reviewing the important relationships between sleep, mGlu receptors, and CNS disorders, this chapter highlights the development of selective mGlu receptor ligands that hold promise for improving both primary symptoms and sleep disturbances.

Sex-dependent effects of microglial reduction on impaired fear extinction induced by single prolonged stress

Single prolonged stress (SPS) is a preclinical rodent model for studying post-traumatic stress disorder (PTSD)-like behaviors. Previously we found that increased expression of the microglial marker Iba-1 in the ventral hippocampus after SPS exposure was associated with impaired fear extinction, suggesting that microglial activity contributed to the SPS-induced behavioral changes. To test this, we examined whether reducing microglia with the colony-stimulating factor 1 receptor blocker, PLX3397, in the diet would prevent the SPS-induced extinction impairment. Male rats exposed to SPS showed enhanced fear acquisition and impaired fear extinction memory. Adding PLX3397 to the diet prevented these behavioral changes. In contrast, PLX3397 did not prevent SPS from impairing fear extinction memory in the female rats. Despite the sex-dependent behavioral effects, we found a reduced number and area fraction of Iba-1+ microglia in both male and female rats suggesting that PLX3397 had similar effects on microglia in both sexes. Altogether, these results suggest that microglia contribute to the behavioral changes induced by SPS in male but not female rats.

D-cycloserine rescues scopolamine-induced deficits in cognitive flexibility in rats measured by the attentional set-shifting task

Cognitive flexibility facilitates adaptions to a changing environment in humans and animals and can be assessed with the attentional set shifting task (ASST). In various learning paradigms for laboratory rodents, the partial NMDA receptor agonist D-cycloserine has been found to have pro-cognitive effects. However, D-cycloserine has not yet been investigated for its effects on cognitive flexibility. The aim of the present study was to determine whether D-cycloserine is able to improve cognitive flexibility measured by the ASST in rats. Rats were first pre-treated with the muscarinic antagonist scopolamine (0.5 mg/kg) before the D-cycloserine administrations (20 mg/kg) to induce deficits in ASST performance. Our findings showed impaired ASST performance after scopolamine administration with significant effects on reversal phases and extra-dimensional shift. D-cycloserine treatment selectively improved the performance in the extra-dimensional shift and the last reversal phase, where scopolamine effects were most pronounced. These findings suggest that D-cycloserine can rescue deficits in cognitive flexibility.

Differential effects of d- and l-enantiomers of govadine on distinct forms of cognitive flexibility and a comparison with dopaminergic drugs

RATIONALE

There is an urgent need for novel drugs for treating cognitive deficits that are defining features of schizophrenia. The individual d- and l-enantiomers of the tetrahydroprotoberberine (THPB) d,l-govadine have been proposed for the treatment of cognitive deficiencies and positive symptoms of schizophrenia, respectively.

OBJECTIVES

We examined the effects of d-, l-, or d,l-govadine on two distinct forms of cognitive flexibility perturbed in schizophrenia and compared them to those induced by a selective D1 receptor agonist and D2 receptor antagonist.

METHODS

Male rats received d-, l-, or d,l-govadine (0.3, 0.5, and 1.0 mg/kg), D1 agonist SKF81297(0.1, 0.3, and 1.0 mg/kg), or D2 antagonist haloperidol (0.1-0.2 mg/kg). Experiment 1 used a strategy set-shifting task (between-subjects). In experiment 2, well-trained rats were tested on a probabilistic reversal task (within-subjects).

RESULTS

d-Govadine improved set-shifting across all doses, whereas higher doses of l-govadine impaired set-shifting. SKF81297 reduced perseverative errors at the lowest dose. Low/high doses of haloperidol increased/decreased set-shifting errors, the latter "improvement" attributable to impaired retrieval of a previous acquired rule. Probabilistic reversal performance was less affected by these drugs, but d-govadine reduced errors during the first reversal, whereas l-govadine impaired initial discrimination learning. d,l-Govadine had no reliable cognitive effects but caused psychomotor slowing like l-govadine and haloperidol.

CONCLUSIONS

These findings further highlight differences between two enantiomers of d,l-govadine that may reflect differential modulation of D1 and D2 receptors. These preclinical findings give further impetus to formal clinical evaluation of d-govadine as a treatment for cognitive deficiencies related to schizophrenia.

Optimizing behavior therapy for youth with Tourette's disorder

Tourette's Disorder (TD) is characterized by tics that cause distress and impairment. While treatment guidelines recommend behavior therapy as a first-line intervention, patients with TD may exhibit limited therapeutic response. Given the need to improve treatment outcomes, this study examined the efficacy of augmenting behavior therapy with D-cycloserine (DCS) to reduce tic severity in a placebo-controlled quick-win/fast-fail trial. Twenty youth with TD completed a baseline assessment to characterize tic severity, premonitory urges, medical history, and psychiatric comorbidity. Youth were randomly assigned to receive a single session of habit reversal training (HRT) augmented by either 50 mg of DCS or placebo. Two bothersome tics on the Hopkins Motor/Vocal Tic Scale (HM/VTS) were targeted for treatment during HRT. One week after the HRT session, youth completed a posttreatment assessment to evaluate change in the severity of bothersome tics. All assessments were completed by independent evaluators masked to treatment group. There was a Treatment Group by Time Interaction in favor of DCS-augmented HRT (p < 0.01), controlling for baseline tic severity, tic medication, and attention deficit hyperactivity disorder. Follow-up comparisons revealed small group differences at the treatment visit (d = 0.27), with the DCS group exhibiting slightly greater severity for targeted tics. There was a large group difference at posttreatment, in which the DCS group exhibited lower severity for targeted tics (d = 1.30, p < 0.001) relative to the placebo group. Findings demonstrate the preliminary enhancement of tic severity reductions by augmenting HRT with DCS compared with placebo augmentation.

Cognitive Flexibility Training Improves Extinction Retention Memory and Enhances Cortical Dopamine With and Without Traumatic Stress Exposure

Stress exposure can cause lasting changes in cognition, but certain individual traits, such as cognitive flexibility, have been shown to reduce the degree, duration, or severity of cognitive changes following stress. Both stress and cognitive flexibility training affect decision making by modulating monoamine signaling. Here, we test the role cognitive flexibility training, and high vs. low cognitive flexibility at the individual level, in attenuating stress-induced changes in memory and monoamine levels using the single prolonged stress (SPS) rodent model of traumatic stress in male Sprague-Dawley rats. Exposure to SPS can heighten fear responses to conditioned cues (i.e., freezing) after a fear association has been extinguished, referred to as a deficit in extinction retention. This deficit is thought to reflect an impairment in context processing that is characteristic of posttraumatic stress disorder (PTSD). During a cognitive flexibility training we assessed individual variability in cognitive skills and conditioned rats to discriminately use cues in their environment. We found that cognitive flexibility training, alone or followed by SPS exposure, accelerated extinction learning and decreased fear responses over time during extinction retention testing, compared with rats not given cognitive flexibility training. These findings suggest that cognitive flexibility training may improve context processing in individuals with and without traumatic stress exposure. Individual performance during the reversal phase of the cognitive flexibility training predicted subsequent context processing; individuals with high reversal performance exhibited a faster decrease in freezing responses during extinction retention testing. Thus, high reversal performance predicted enhanced retention of extinction learning over time and suggests that cognitive flexibility training may be a strategy to promote context processing. In a brain region vital for maintaining cognitive flexibility and fear suppression, the prelimbic cortex (PLC), cognitive flexibility training also lastingly enhanced dopamine (DA) and norepinephrine (NE) levels, in animals with and without traumatic stress exposure. In contrast, cognitive flexibility training prior to traumatic stress exposure decreased levels of DA and its metabolites in the striatum, a region mediating reflexive decision making. Overall, our results suggest that cognitive flexibility training can provide lasting benefits by enhancing extinction retention, a hallmark cognitive effect of trauma, and prelimbic DA, which can maintain flexibility across changing contexts.

Recent advances in the neurobiology of posttraumatic stress disorder: A review of possible mechanisms underlying an effective pharmacotherapy

Recent progress in the field of neurobiology supported by clinical evidence gradually reveals the mystery of human brain functioning. So far, many psychiatric disorders have been described in great detail, although there are still plenty of cases that are misunderstood. These include posttraumatic stress disorder (PTSD), which is a unique disease that combines a wide range of neurobiological changes, which involve disturbances of the hypothalamic-pituitary-adrenal gland axis, hyperactivation of the amygdala complex, and attenuation of some hippocampal and cortical functions. Such multiplicity results in differential symptomatology, including elevated anxiety, nightmares, fear retrieval episodes that may trigger delusions and hallucinations, sleep disturbances, and many others that strongly interfere with the quality of the patient's life. Because of widespread neurological changes and the disease manifestation, the pharmacotherapy of PTSD remains unclear and requires a multidimensional approach and involvement of polypharmacotherapy. Hopefully, more and more neuroscientists and clinicians will study PTSD, which will provide us with new information that would possibly accelerate establishment of well-tolerated and effective pharmacotherapy. In this review, we have focused on neurobiological changes regarding PTSD, addressing the most disturbed brain structures and neurotransmissions, as well as discussing in detail the recently taken and novel therapeutic paths.