The 5-HT receptor antagonist M100,907 prevents extracellular glutamate rising in response to NMDA receptor blockade in the mPFC

Efficacy of 5-HT2A antagonists on negative symptoms in patients with schizophrenia: A meta-analysis

Negative symptoms have a major impact on the prognosis of schizophrenia, but have proven more difficult to improve or treat with antipsychotic medication. The aim of this meta-analysis is to evaluate the efficacy of 5-HT2A antagonist treatments on negative symptoms in patients with schizophrenia. After a systematic search, all randomized, double-blind and placebo-controlled trials evaluating the efficacy of 5-HT2A antagonists were included. Standardized mean differences were calculated between quantitative data from treatment and placebo groups, and odds ratios were calculated between qualitative data from treatment and placebo groups. Ten studies were included in the analysis. A significantly greater decrease in negative symptoms and global symptomatology was found in the 5-HT2A antagonist group compared with the placebo group, but no difference was found for positive symptoms. At the end of the studies, a lower extra-pyramidal symptoms score was found in the 5-HT2A antagonist group. No significant difference was found for the drop-out rate or for the rate of serious adverse effects, but a higher rate of treatment-emergent adverse effects was found in the 5-HT2A antagonist group. Our meta-analysis shows that 5-HT2A antagonists demonstrate a favorable benefit/risk profile and could be useful in the treatment of negative symptoms in patients with schizophrenia.

Basolateral Amygdala Serotonin 2C Receptor Regulates Emotional Disorder-Related Symptoms Induced by Chronic Methamphetamine Administration

Globally, methamphetamine (MA) is the second most abused drug, with psychotic symptoms being one of the most common adverse effects. Emotional disorders induced by MA abuse have been widely reported both in human and animal models; however, the mechanisms underlying such disorders have not yet been fully elucidated. In this study, a chronic MA administration mouse model was utilized to elucidate the serotonergic pathway involved in MA-induced emotional disorders. After 4 weeks of MA administration, the animals exhibited significantly increased depressive and anxious symptoms. Molecular and morphological evidence showed that chronic MA administration reduced the expression of the 5-hydroxytryptamine (5-HT) rate-limiting enzyme, tryptophan hydroxylase 2, in the dorsal raphe and the concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid in the basolateral amygdala (BLA) nuclei. Alterations in both 5-HT and 5-HT receptor levels occurred simultaneously in BLA; quantitative polymerase chain reaction, western blotting, and fluorescence analysis revealed that the expression of the 5-HT2C receptor (5-HT_2CR) increased. Neuropharmacology and virus-mediated silencing strategies confirmed that targeting 5-HT_2CR reversed the depressive and anxious behaviors induced by chronic MA administration. In the BLA, 5-HT_2CR-positive cells co-localized with GABAergic interneurons. The inactivation of 5-HT_2CR ameliorated impaired GABAergic inhibition and decreased BLA activation. Thus, herein, for the first time, we report that the abnormal regulation of 5-HT_2CR is involved in the manifestation of emotional disorder-like symptoms induced by chronic MA use. Our study suggests that 5-HT_2CR in the BLA is a promising clinical target for the treatment of MA-induced emotional disorders.

Role of the medial prefrontal cortex in the effects of rapid acting antidepressants on decision-making biases in rodents

Major depressive disorder is a significant and costly cause of global disability. Until the discovery of the rapid acting antidepressant (RAAD) effects of ketamine, treatments were limited to drugs that have delayed clinical benefits. The mechanism of action of ketamine is currently unclear but one hypothesis is that it may involve neuropsychological effects mediated through modulation of affective biases (where cognitive processes such as learning and memory and decision-making are modified by emotional state). Previous work has shown that affective biases in a rodent decision-making task are differentially altered by ketamine, compared to conventional, delayed onset antidepressants. This study sought to further investigate these effects by comparing ketamine with other NMDA antagonists using this decision-making task. We also investigated the subtype selective GluN2B antagonist, CP-101,606 and muscarinic antagonist scopolamine which have both been shown to have RAAD effects. Both CP-101,606 and scopolamine induced similar positive biases in decision-making to ketamine, but the same effects were not seen with other NMDA antagonists. Using targeted medial prefrontal cortex (mPFC) infusions, these effects were localised to the mPFC. In contrast, the GABA_A agonist, muscimol, induced general disruptions to behaviour. These data suggest that ketamine and other RAADs mediate a specific effect on affective bias which involves the mPFC. Non-ketamine NMDA antagonists lacked efficacy and we also found that temporary inactivation of the mPFC did not fully recapitulate the effects of ketamine, suggesting a specific mechanism.

Glutamate in schizophrenia: Neurodevelopmental perspectives and drug development

Research into the neurobiological processes that may lead to the onset of schizophrenia places growing emphasis on the glutamatergic system and brain development. Preclinical studies have shown that neurodevelopmental, genetic, and environmental factors contribute to glutamatergic dysfunction and schizophrenia-related phenotypes. Clinical research has suggested that altered brain glutamate levels may be present before the onset of psychosis and relate to outcome in those at clinical high risk. After psychosis onset, glutamate dysfunction may also relate to the degree of antipsychotic response and clinical outcome. These findings support ongoing efforts to develop pharmacological interventions that target the glutamate system and could suggest that glutamatergic compounds may be more effective in specific patient subgroups or illness stages. In this review, we consider the updated glutamate hypothesis of schizophrenia, from a neurodevelopmental perspective, by reviewing recent preclinical and clinical evidence, and discuss the potential implications for novel therapeutics.

Recapitulation of Neuropsychiatric Behavioral Features in Mice Using Acute Low-dose MK-801 Administration

Despite some innate limitations, animal models are a potent investigative tool when used to model specific symptoms of a disorder. For example, MK-801, an N-methyl-D-aspartate receptor antagonist, is used as a pharmacological tool to induce symptoms found in some neuropsychiatric disorders. However, a close examination of literature suggests that the application window of MK-801 doses is relatively narrow between individual behavioral paradigms, necessitating careful characterization of the evoked behavioral aberrations and the doses used to induce them. Moreover, variation in behaviors depending on the animal strain, gender of the subject, and the timing of administration is observed, making it difficult to compare the behavioral characteristics reported in different studies. We aim to characterize the behavioral aberrations induced by different doses of MK-801 in CD-1 mice and create a ready reference for future studies. We used CD-1 mice to recapitulate behavioral impairments resulting from acute administration of MK-801. In 0.1 mg kg⁻¹, we observed diminished spontaneous alteration during the Y-maze test, while 0.12 mg kg⁻¹ resulted in hyperlocomotion and social deficit. Mice treated with 0.2 and 0.3 mg kg⁻¹ of MK-801 demonstrated a decreased self-grooming. Finally, all doses significantly impaired cliff avoidance behaviors suggesting increased impulsivity. These results affirm that MK-801 can effectively model various symptoms of different neuropsychiatric disorders in a dose-dependent manner. The observed sensitivity against spatial-memory impairment and impulsive behaviors at low concentration of MK-801 suggest that MK801 may modulate cognitive function and impulsivity in even lower concentration before it can modulate other behavioral domains.

Neuromodulation of Attention

Attention is critical to high-level cognition and attention deficits are a hallmark of neurologic and neuropsychiatric disorders. Although years of research indicates that distinct neuromodulators influence attentional control, a mechanistic account that traverses levels of analysis (cells, circuits, behavior) is missing. However, such an account is critical to guide the development of next-generation pharmacotherapies aimed at forestalling or remediating the global burden associated with disorders of attention. Here, we summarize current neuroscientific understanding of how attention affects single neurons and networks of neurons. We then review key results that have informed our understanding of how neuromodulation shapes these neuron and network properties and thereby enables the appropriate allocation of attention to relevant external or internal events. Finally, we highlight areas where we believe hypotheses can be formulated and tackled experimentally in the near future, thereby critically increasing our mechanistic understanding of how attention is implemented at the cellular and network levels.

Blockade of Glutamate Receptors within the Prelimbic Cortex Attenuate Concentration of Excitatory Amino Acids in the Morphine Self-administration in Rats

Background

The attitude of research on addiction has been done on the key role of glutamate. As a regard, the prelimbic cortex (PrL) has an important role in addiction, learning, and memory. We tried to investigate the level of glutamate and aspartate concentration after glutamate receptors blockade in this region in the morphine-addicted rats.

Materials and Methods

In this study, we examined the effects of local infusion of the N-methyl-D-aspartate receptor and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonists, 2-amino-5-phosphonovaleric acid (AP5), and 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), into the PrL cortex on the level of excitatory amino acids (EAAs) and glycine. After 11 days of self-administration, the prelimbic area of the brain was taken out, and the EAAs and glycine concentration was measured by high-performance liquid chromatography.

Results

Morphine resulted in the significant increase in the EAAs concentration within this area (P ≤ 0.001). Microinjection of AP5 into this region before using of morphine significantly decreased the morphine-induced glutamate and aspartate concentration (P ≤ 0.001). CNQX had the same effect and significantly reduced the EAAs concentration compared to the morphine group (P ≤ 0.001). In addition, microinjection of AP5 and CNQX simultaneously increased glycine concentration (P ≤ 0.001).

Conclusions

These results show that morphine stimulates the EAAs release in the prelimbic area. It seems that microinjection of AP5 or CNQX in this region is effective in reducing morphine-induced EAA. It is suggested that EAA transmission in the PrL cortex may be a possible target for treatment of morphine addiction.

Prefrontal cortex executive processes affected by stress in health and disease

Prefrontal cortical executive functions comprise a number of cognitive capabilities necessary for goal directed behavior and adaptation to a changing environment. Executive dysfunction that leads to maladaptive behavior and is a symptom of psychiatric pathology can be instigated or exacerbated by stress. In this review we survey research addressing the impact of stress on executive function, with specific focus on working memory, attention, response inhibition, and cognitive flexibility. We then consider the neurochemical pathways underlying these cognitive capabilities and, where known, how stress alters them. Finally, we review work exploring potential pharmacological and non-pharmacological approaches that can ameliorate deficits in executive function. Both preclinical and clinical literature indicates that chronic stress negatively affects executive function. Although some of the circuitry and neurochemical processes underlying executive function have been characterized, a great deal is still unknown regarding how stress affects these processes. Additional work focusing on this question is needed in order to make progress on developing interventions that ameliorate executive dysfunction.

Bidirectional variation in glutamate efflux in the medial prefrontal cortex induced by selective positive and negative allosteric mGluR5 modulators

Dysregulation of prefrontal cortical glutamatergic signalling via NMDA receptor hypofunction has been implicated in cognitive dysfunction and impaired inhibitory control in such neuropsychiatric disorders as schizophrenia, attention‐deficit hyperactivity disorder and drug addiction. Although NMDA receptors functionally interact with metabotropic glutamate receptor 5 (mGluR5), the consequence of this interaction for glutamate release in the prefrontal cortex (PFC) remains unknown. We therefore investigated the effects of positive and negative allosteric mGluR5 modulation on changes in extracellular glutamate efflux in the medial PFC (mPFC) induced by systemic administration of the non‐competitive NMDA receptor antagonist dizocilpine (or MK801) in rats. Extracellular glutamate efflux was measured following systemic administration of the positive allosteric mGluR5 modulator [S‐(4‐Fluoro‐phenyl)‐{3‐[3‐(4‐fluoro‐phenyl)‐[1,2,4]‐oxadiazol‐5‐yl]‐piperidin‐1‐yl}‐methanone] (ADX47273; 100 mg/kg, p.o.) and negative allosteric mGluR5 modulator [2‐chloro‐4‐{[1‐(4‐fluorophenyl)‐2,5‐dimethyl‐1H‐imidazol‐4‐yl]ethynyl}pyridine] (RO4917523; 0.3 mg/kg, p.o.), using a wireless glutamate biosensor in awake, freely moving rats. The effect of MK801 (0.03–0.06 mg/kg, s.c.) on mPFC glutamate efflux was also investigated in addition to the effects of MK801 (0.03 mg/kg, s.c.) following ADX47273 (100 mg/kg, p.o.) pre‐treatment. ADX47273 produced a sustained increase in glutamate efflux and increased the effect of NMDA receptor antagonism on glutamate efflux in the mPFC. In contrast, negative allosteric mGluR5 modulation with RO4917523 decreased glutamate efflux in the mPFC. These findings indicate that positive and negative allosteric mGluR5 modulators produce long lasting and opposing actions on extracellular glutamate efflux in the mPFC. Positive and negative allosteric modulators of mGluR5 may therefore be viable therapeutic agents to correct abnormalities in glutamatergic signalling present in a range of neuropsychiatric disorders.

Differences in trait impulsivity do not bias the response to pharmacological drug challenge in the rat five-choice serial reaction time task

RATIONALE

Maladaptive impulsivity is symptomatic of several neuropsychiatric disorders including schizophrenia, attention-deficit hyperactivity disorder (ADHD), and substance abuse disorders; paradigms designed to assess the underlying neurobiology of this behavior are essential for the discovery of novel therapeutic agents. Various models may be used to assess impulsivity as measured by the five-choice serial reaction time task (5-CSRTT), including variable inter-trial interval (ITI) sessions, the selection of extreme high and low impulsivity phenotypes from a large outbred population of rats, as well as pharmacological challenges.

OBJECTIVES

The aim of this study is to evaluate if pharmacological challenge models for impulsivity are biased by underlying differences in impulsivity phenotype.

METHODS

Extreme high and low impulsivity phenotypes were selected in the 5-CSRTT, and dose-dependent effects of various pharmacological challenges, namely MK-801, yohimbine, and cocaine, were evaluated on task performance, specifically accuracy and premature responses.

RESULTS

All three compounds increased premature responding, while a decrease in attentional performance occurred following MK-801 and yohimbine administration. No differences in drug-induced impulsivity between rats selected for high or low impulsivity or in parameters indicative of attentional performance could be determined.

CONCLUSIONS

Our findings indicate that different pharmacological challenges increase impulsivity on the 5-CSRTT, with modest effects on attention. These effects were not influenced by underlying differences in impulsivity phenotype, which is an important prerequisite to reliably use these challenge models to screen and profile compounds with putative anti-impulsive characteristics.

Neural Stem Cell Transplantation Induces Stroke Recovery by Upregulating Glutamate Transporter GLT-1 in Astrocytes

Ischemic stroke is the leading cause of disability, but effective therapies are currently widely lacking. Recovery from stroke is very much dependent on the possibility to develop treatments able to both halt the neurodegenerative process as well as to foster adaptive tissue plasticity. Here we show that ischemic mice treated with neural precursor cell (NPC) transplantation had on neurophysiological analysis, early after treatment, reduced presynaptic release of glutamate within the ipsilesional corticospinal tract (CST), and an enhanced NMDA-mediated excitatory transmission in the contralesional CST. Concurrently, NPC-treated mice displayed a reduced CST degeneration, increased axonal rewiring, and augmented dendritic arborization, resulting in long-term functional amelioration persisting up to 60 d after ischemia. The enhanced functional and structural plasticity relied on the capacity of transplanted NPCs to localize in the peri-ischemic and ischemic area, to promote the upregulation of the glial glutamate transporter 1 (GLT-1) on astrocytes and to reduce peri-ischemic extracellular glutamate. The upregulation of GLT-1 induced by transplanted NPCs was found to rely on the secretion of VEGF by NPCs. Blocking VEGF during the first week after stroke reduced GLT-1 upregulation as well as long-term behavioral recovery in NPC-treated mice. Our results show that NPC transplantation, by modulating the excitatory-inhibitory balance and stroke microenvironment, is a promising therapy to ameliorate disability, to promote tissue recovery and plasticity processes after stroke.

SIGNIFICANCE STATEMENT

Tissue damage and loss of function occurring after stroke can be constrained by fostering plasticity processes of the brain. Over the past years, stem cell transplantation for repair of the CNS has received increasing interest, although underlying mechanism remain elusive. We here show that neural stem/precursor cell transplantation after ischemic stroke is able to foster axonal rewiring and dendritic plasticity and to induce long-term functional recovery. The observed therapeutic effect of neural precursor cells seems to underlie their capacity to upregulate the glial glutamate transporter on astrocytes through the vascular endothelial growth factor inducing favorable changes in the electrical and molecular stroke microenvironment. Cell-based approaches able to influence plasticity seem particularly suited to favor poststroke recovery.

Effects of Antipsychotic Administration on Brain Glutamate in Schizophrenia: A Systematic Review of Longitudinal 1H-MRS Studies

Schizophrenia is associated with brain glutamate dysfunction, but it is currently unclear whether antipsychotic administration can reduce the extent of glutamatergic abnormality. We conducted a systematic review of proton magnetic resonance spectroscopy (¹H-MRS) studies examining the effects of antipsychotic treatment on brain glutamate levels in schizophrenia. The Medline database was searched to identify relevant articles published until December 2016. Inclusion required that studies examined longitudinal changes in brain glutamate metabolites in patients with schizophrenia before and after initiation of first antipsychotic treatment or a switch in antipsychotic treatment. The searches identified eight eligible articles, with baseline and follow-up measures in a total of 168 patients. The majority of articles reported a numerical reduction in brain glutamate metabolites with antipsychotic treatment, and the estimated overall mean reduction of 6.5% in Glx (the combined signal from glutamate and glutamine) across brain regions. Significant reductions in glutamate metabolites in at least one brain region were reported in four of the eight studies, and none of the studies reported a significant glutamatergic increase after antipsychotic administration. Relationships between the degree of change in glutamate and the degree of improvement in symptoms have been inconsistent but may provide limited evidence that antipsychotic response may be associated with lower glutamate levels before treatment and a greater extent of glutamatergic reduction during treatment. Further longitudinal, prospective studies of glutamate and antipsychotic response are required to confirm these findings.

Potential involvement of serotonergic signaling in ketamine's antidepressant actions: A critical review

A single i.v. infusion of ketamine, classified as an N-methyl-d-aspartate (NMDA) receptor antagonist, may alleviate depressive symptoms within hours of administration in treatment resistant depressed patients, and the antidepressant effect may last for several weeks. These unique therapeutic properties have prompted researchers to explore the mechanisms mediating the antidepressant effects of ketamine, but despite many efforts, no consensus on its antidepressant mechanism of action has been reached. Recent preclinical reports have associated the neurotransmitter serotonin (5-hydroxytryptamine; 5-HT) with the antidepressant-like action of ketamine. Here, we review the current evidence for a serotonergic role in ketamine's antidepressant effects. The pharmacological profile of ketamine may include equipotent activity on several non-NMDA targets, and the current hypotheses for the mechanisms responsible for ketamine's antidepressant activity do not appear to preclude the possibility that non-glutamate neurotransmitters are involved in the antidepressant effects. At multiple levels, the serotonergic and glutamatergic systems interact, and such crosstalk could support the notion that changes in serotonergic neurotransmission may impact ketamine's antidepressant potential. In line with these prospects, ketamine may increase 5-HT levels in the prefrontal cortex of rats, plausibly via hippocampal NMDA receptor inhibition and activation of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors. In addition, a number of preclinical studies suggest that the antidepressant-like effects of ketamine may depend on endogenous activation of 5-HT receptors. Recent imaging and behavioral data predominantly support a role for 5-HT1A or 5-HT1B receptors, but the full range of 5-HT receptors has currently not been systematically investigated in this context. Furthermore, the nature of any 5-HT dependent mechanism in ketamine's antidepressant effect is currently not understood, and therefore, more studies are warranted to confirm this hypothesis and explore the specific pathways that might implicate 5-HT.

Roles of the NMDA Receptor and EAAC1 Transporter in the Modulation of Extracellular Glutamate by Low and High Affinity AMPA Receptors in the Cerebellum in Vivo: Differential Alteration in Chronic Hyperammonemia

The roles of high- and low-affinity AMPA receptors in modulating extracellular glutamate in the cerebellum remain unclear. Altered glutamatergic neurotransmission is involved in neurological alterations in hyperammonemia, which differently affects high- and low-affinity AMPA receptors. The aims were to assess by in vivo microdialysis (a) the effects of high- and low-affinity AMPA receptor activation on extracellular glutamate in the cerebellum; (b) whether chronic hyperammonemia alters extracellular glutamate modulation by high- and/or low-affinity AMPA receptors; and (c) the contribution of NMDA receptors and EAAC1 transporter to AMPA-induced changes in extracellular glutamate. In control rats, high affinity receptor activation does not affect extracellular glutamate but increases glutamate if NMDA receptors are blocked. Low affinity AMPA receptor activation increases transiently extracellular glutamate followed by reduction below basal levels and return to basal values. The reduction is associated with transient increased membrane expression of EAAC1 and is prevented by blocking NMDA receptors. Blocking NMDA receptors with MK-801 induces a transient increase in extracellular glutamate which is associated with reduced membrane expression of EAAC1 followed by increased membrane expression of the glutamate transporter GLT-1. Chronic hyperammonemia does not affect responses to activation of low affinity AMPA receptors. Activation of high affinity AMPA receptors increases extracellular glutamate in hyperammonemic rats by an NMDA receptor-dependent mechanism. In conclusion, these results show that there is a tightly controlled interplay between AMPA and NMDA receptors and an EAAC1 transporter in controlling extracellular glutamate. Hyperammonemia alters high- but not low-affinity AMPA receptors.

Dissociable effects of mGluR5 allosteric modulation on distinct forms of impulsivity in rats: interaction with NMDA receptor antagonism

RATIONALE

Impaired N-methyl-D-aspartate (NMDA) receptor signalling underlies several psychiatric disorders that express high levels of impulsivity. Although synergistic interactions exist between NMDA receptors and metabotropic glutamate receptor 5 (mGluR5), the significance of this interaction for impulsivity is unknown.

OBJECTIVE

This study aims to investigate the effects of negative and positive allosteric mGluR5 modulation (NAM/PAM) on trait impulsivity and impulsivity evoked by NMDA receptor antagonism in rats.

METHODS

Motor and choice impulsivity were assessed using the five-choice serial reaction time task (5-CSRTT) and delayed-discounting task (DDT), respectively. The effects of RO4917523 and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) (NAMs) and ADX47273 (PAM) were investigated in non-impulsive rats and in trait high- and low-impulsive rats. The effects of these compounds on impulsivity induced by NMDA receptor antagonism (MK801) in the 5-CSRTT were also investigated.

RESULTS

RO4917523 (0.1-1 mg/kg) decreased premature responding and increased omissions but had no effect on locomotor activity up to 0.1 mg/kg. MTEP significantly increased omissions, decreased accuracy and slowed responding but had no effect on premature responding. ADX47273 decreased premature responding at doses that had no effect on locomotor activity. MK801 increased premature responding and impaired attentional accuracy; these deficits were dose dependently rescued by ADX47273 pre-treatment. Allosteric modulation of mGluR5 had no significant effect on choice impulsivity, nor did it modulate general task performance.

CONCLUSIONS

These findings demonstrate that mGluR5 allosteric modulation selectively dissociates motor and choice impulsivity. We further show that mGluR5 PAMs may have therapeutic utility in selectively targeting specific aspects of impulsivity and executive dysfunction.

Synthesis and Characterization of 5-Hydroxy-2-(2-phenylethyl)chromone (5-HPEC) and Its Analogues as Non-nitrogenous 5-HT2B Ligands

The involvement of the neurotransmitter serotonin (5-HT) in numerous physiological functions is often attributed to the diversity of receptors with which it interacts. Ligands targeting serotonin receptor 2B (5-HT2B) have received renewed interest for their potential to help understand the role of 5-HT2B in migraines, drug abuse, neurodegenerative diseases, and irritable bowel syndrome. To date, most of the ligands targeting 5-HT2B have been nitrogen-containing compounds. The natural product 5-hydroxy-2-(2-phenylethyl)chromone (5-HPEC, 5) has been shown previously to act as a non-nitrogenous antagonist for the 5-HT2B receptor (pKi = 5.6). This report describes further progress on the study of the structure-activity relationship of both naturally occurring and synthetic compounds bearing the 2-(2-phenylethyl)chromone scaffold at the 5-HT2B receptor. The inhibitory activity of the newly synthesized compounds (at 10 μM) was tested against each of the 5-HT2 receptors. Following this assay, the binding affinity and antagonism of the most promising compounds were then evaluated at 5-HT2B. Among all the analogues, 5-hydroxy-2-(2-phenylpropyl)chromone (5-HPPC, 22h) emerged as a new lead compound, showing a 10-fold improvement in affinity (pKi = 6.6) over 5-HPEC with reasonable antagonist properties at 5-HT2B. Additionally, ligand docking studies have identified a putative binding pocket for 5-HPPC and have helped understand its improved affinity.

Effects of NMDA receptor antagonists and antipsychotics on high frequency oscillations recorded in the nucleus accumbens of freely moving mice

RATIONALE

Abnormal oscillatory activity associated with N-methyl-D-aspartate (NMDA) receptor hypofunction is widely considered to contribute to the symptoms of schizophrenia.

OBJECTIVE

This study aims to characterise the changes produced by NMDA receptor antagonists and antipsychotics on accumbal high-frequency oscillations (HFO; 130-180 Hz) in mice.

METHODS

Local field potentials were recorded from the nucleus accumbens of freely moving mice.

RESULTS

Systemic injection of ketamine and MK801 both dose-dependently increased the power of HFO and produced small increases in HFO frequency. The atypical antipsychotic drug, clozapine, produced a robust dose-dependent reduction in the frequency of MK801-enhanced HFO, whilst haloperidol, a typical antipsychotic drug, had little effect. Stimulation of NMDA receptors (directly or through the glycine site) as well as activation of 5-HT1A receptors, reduced the frequency of MK801-enhanced HFO, but other receptors known to be targets for clozapine, namely 5-HT2A, 5-HT7 and histamine H3 receptors had no effect.

CONCLUSIONS

NMDA receptor antagonists and antipsychotics produce broadly similar fundamental effects on HFO, as reported previously for rats, but we did observe several notable differences. In mice, HFO at baseline were weak or not detectable unlike rats. Post-injection of NMDA receptor antagonists HFO was also weaker but significantly faster. Additionally, we found that atypical antipsychotic drugs may reduce the frequency of HFO by interacting with NMDA and/or 5-HT1A receptors.

Serotoninergic and dopaminergic modulation of cortico-striatal circuit in executive and attention deficits induced by NMDA receptor hypofunction in the 5-choice serial reaction time task

Executive functions are an emerging propriety of neuronal processing in circuits encompassing frontal cortex and other cortical and subcortical brain regions such as basal ganglia and thalamus. Glutamate serves as the major neurotrasmitter in these circuits where glutamate receptors of NMDA type play key role. Serotonin and dopamine afferents are in position to modulate intrinsic glutamate neurotransmission along these circuits and in turn to optimize circuit performance for specific aspects of executive control over behavior. In this review, we focus on the 5-choice serial reaction time task which is able to provide various measures of attention and executive control over performance in rodents and the ability of prefrontocortical and striatal serotonin 5-HT_1A, 5-HT_2A, and 5-HT_2C as well as dopamine D₁- and D₂-like receptors to modulate different aspects of executive and attention disturbances induced by NMDA receptor hypofunction in the prefrontal cortex. These behavioral studies are integrated with findings from microdialysis studies. These studies illustrate the control of attention selectivity by serotonin 5-HT_1A, 5-HT_2A, 5-HT_2C, and dopamine D₁- but not D₂-like receptors and a distinct contribution of these cortical and striatal serotonin and dopamine receptors to the control of different aspects of executive control over performance such as impulsivity and compulsivity. An association between NMDA antagonist-induced increase in glutamate release in the prefrontal cortex and attention is suggested. Collectively, this review highlights the functional interaction of serotonin and dopamine with NMDA dependent glutamate neurotransmission in the cortico-striatal circuitry for specific cognitive demands and may shed some light on how dysregulation of neuronal processing in these circuits may be implicated in specific neuropsychiatric disorders.

Serotonergic hyperactivity as a potential factor in developmental, acquired and drug-induced synesthesia

Though synesthesia research has seen a huge growth in recent decades, and tremendous progress has been made in terms of understanding the mechanism and cause of synesthesia, we are still left mostly in the dark when it comes to the mechanistic commonalities (if any) among developmental, acquired and drug-induced synesthesia. We know that many forms of synesthesia involve aberrant structural or functional brain connectivity. Proposed mechanisms include direct projection and disinhibited feedback mechanisms, in which information from two otherwise structurally or functionally separate brain regions mix. We also know that synesthesia sometimes runs in families. However, it is unclear what causes its onset. Studies of psychedelic drugs, such as psilocybin, LSD and mescaline, reveal that exposure to these drugs can induce synesthesia. One neurotransmitter suspected to be central to the perceptual changes is serotonin. Excessive serotonin in the brain may cause many of the characteristics of psychedelic intoxication. Excessive serotonin levels may also play a role in synesthesia acquired after brain injury. In brain injury sudden cell death floods local brain regions with serotonin and glutamate. This neurotransmitter flooding could perhaps result in unusual feature binding. Finally, developmental synesthesia that occurs in individuals with autism may be a result of alterations in the serotonergic system, leading to a blockage of regular gating mechanisms. I conclude on these grounds that one commonality among at least some cases of acquired, developmental and drug-induced synesthesia may be the presence of excessive levels of serotonin, which increases the excitability and connectedness of sensory brain regions.

Neurocircuitry of drug reward

In recent years, neuroscientists have produced profound conceptual and mechanistic advances on the neurocircuitry of reward and substance use disorders. Here, we will provide a brief review of intracranial drug self-administration and optogenetic self-stimulation studies that identified brain regions and neurotransmitter systems involved in drug- and reward-related behaviors. Also discussed is a theoretical framework that helps to understand the functional properties of the circuitry involved in these behaviors. The circuitry appears to be homeostatically regulated and mediate anticipatory processes that regulate behavioral interaction with the environment in response to salient stimuli. That is, abused drugs or, at least, some may act on basic motivation and mood processes, regulating behavior-environment interaction. Optogenetics and related technologies have begun to uncover detailed circuit mechanisms linking key brain regions in which abused drugs act for rewarding effects. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Dopamine D1-like and D2-like receptors in the dorsal striatum control different aspects of attentional performance in the five-choice serial reaction time task under a condition of increased activity of corticostriatal inputs

We investigated the interaction between the corticostriatal glutamatergic afferents and dopamine D1-like and D2-like receptors in the dorsomedial striatum (dm-STR) in attention and executive response control in the five-choice serial reaction time (5-CSRT) task. The competitive NMDA receptor antagonist 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP) injected in the mPFC impaired accuracy and increased premature and perseverative responding, raising GLU, DA, and GABA release in the dm-STR. The D1-like antagonist SCH23390 injected in the dm-STR reversed the CPP-induced accuracy deficit but did not affect the increase in perseverative responding. In contrast, the D2-like antagonist haloperidol injected in the dm-STR reduced the CPP-induced increase in perseverative responding but not the accuracy deficit. The different roles of dorsal striatal D1-like and D2-like receptor were further supported by the finding that activation of D1-like receptor in the dm-STR by SKF38393 impaired accuracy but not perseverative responding while the D2-like agonist quinpirole injected in the dm-STR increased perseverative responding but did not affect accuracy. These findings suggest that integration of cortical information by D1-like receptors in the dm-STR is a key mechanism of the input selection process of attention while the integration of corticostriatal signals by D2-like receptors preserves the ability to switch from one act/response to the next in a complex motor sequence, thus providing for behavioral flexibility.

Evidence for involvement of nitric oxide and GABA(B) receptors in MK-801- stimulated release of glutamate in rat prefrontal cortex

Systemic administration of NMDA receptor antagonists elevates extracellular glutamate within prefrontal cortex. The cognitive and behavioral effects of NMDA receptor blockade have direct relevance to symptoms of schizophrenia, and recent studies demonstrate an important role for nitric oxide and GABA(B) receptors in mediating the effects of NMDA receptor blockade on these behaviors. We sought to extend those observations by directly measuring the effects of nitric oxide and GABA(B) receptor mechanisms on MK-801-induced glutamate release in the prefrontal cortex. Systemic MK-801 injection (0.3 mg/kg) to male Sprague-Dawley rats significantly increased extracellular glutamate levels in prefrontal cortex, as determined by microdialysis. This effect was blocked by pre-treatment with the nitric oxide synthase inhibitor L-NAME (60 mg/kg). Reverse dialysis of the nitric oxide donor SNAP (0.5-5 mM) directly into prefrontal cortex mimicked the effect of systemic MK-801, dose-dependently elevating cortical extracellular glutamate. The effect of MK-801 was also blocked by systemic treatment with the GABA(B) receptor agonist baclofen (5 mg/kg). In combination, these data suggest increased nitric oxide formation is necessary for NMDA antagonist-induced elevations of extracellular glutamate in the prefrontal cortex. Additionally, the data suggest GABA(B) receptor activation can modulate the NMDA antagonist-induced increase in cortical glutamate release.

Effects of the 5-HT2C receptor agonist Ro60-0175 and the 5-HT2A receptor antagonist M100907 on nicotine self-administration and reinstatement

The reinforcing effects of nicotine are mediated in part by brain dopamine systems. Serotonin, acting via 5-HT(2A) and 5-HT(2C) receptors, modulates dopamine function. In these experiments we examined the effects of the 5-HT(2C) receptor agonist Ro60-0175 and the 5-HT(2A) receptor antagonist (M100907, volinanserin) on nicotine self-administration and reinstatement of nicotine-seeking. Male Long-Evans rats self-administered nicotine (0.03 mg/kg/infusion, IV) on either a FR5 or a progressive ratio schedule of reinforcement. Ro60-0175 reduced responding for nicotine on both schedules. While Ro60-0175 also reduced responding for food reinforcement, response rates under drug treatment were several-fold higher than in animals responding for nicotine. M100907 did not alter responding for nicotine, or food, on either schedule. In tests of reinstatement of nicotine-seeking, rats were first trained to lever press for IV infusions of nicotine; each infusion was also accompanied by a compound cue consisting of a light and tone. This response was then extinguished over multiple sessions. Injecting rats with a nicotine prime (0.15 mg/kg) reinstated responding; reinstatement was also observed when responses were accompanied by the nicotine associated cue. Ro60-0175 attenuated reinstatement of responding induced by nicotine and by the cue. The effects of Ro60-0175 on both forms of reinstatement were blocked by the 5-HT(2C) receptor antagonist SB242084. M100907 also reduced reinstatement induced by either the nicotine prime or by the nicotine associated cue. The results indicate that 5-HT(2C) and 5-HT(2A) receptors may be potential targets for therapies to treat some aspects of nicotine dependence.

The role of serotonin in the regulation of patience and impulsivity

Classic theories suggest that central serotonergic neurons are involved in the behavioral inhibition that is associated with the prediction of negative rewards or punishment. Failed behavioral inhibition can cause impulsive behaviors. However, the behavioral inhibition that results from predicting punishment is not sufficient to explain some forms of impulsive behavior. In this article, we propose that the forebrain serotonergic system is involved in “waiting to avoid punishment” for future punishments and “waiting to obtain reward” for future rewards. Recently, we have found that serotonergic neurons increase their tonic firing rate when rats await food and water rewards and conditioned reinforcer tones. The rate of tonic firing during the delay period was significantly higher when rats were waiting for rewards than for tones, and rats were unable to wait as long for tones as for rewards. These results suggest that increased serotonergic neuronal firing facilitates waiting behavior when there is the prospect of a forthcoming reward and that serotonergic activation contributes to the patience that allows rats to wait longer. We propose a working hypothesis to explain how the serotonergic system regulates patience while waiting for future rewards.

Dorsal-striatal 5-HT₂A and 5-HT₂C receptors control impulsivity and perseverative responding in the 5-choice serial reaction time task

RATIONAL

Prefrontal cortex (PFC) and dorsal striatum are part of the neural circuit critical for executive attention. The relationship between 5-HT and aspects of attention and executive control is complex depending on experimental conditions and the level of activation of different 5-HT receptors within the nuclei of corticostriatal circuitry.

OBJECTIVE

The present study investigated which 5-HT(2A) and 5-HT(2C) receptors in the dorsomedial-striatum (dm-STR) contribute to executive attention deficit induced by blockade of NMDA receptors in the PFC.

MATERIALS AND RESULTS

Executive attention was assessed by the five-choice serial reaction time task (5-CSRTT), which provides indices of attention (accuracy) and those of executive control over performance such as premature (an index of impulsivity) and perseverative responding. The effects of targeted infusion in dm-STR of 100 and 300 ng/μl doses of the selective 5-HT(2A) antagonist M100907 and 1 and 3 μg/μl doses of 5-HT(2C) agonist Ro60-0175 was examined in animals injected with 50 ng/μl dose of a competitive NMDA receptor antagonist 3-(R)-2-carboxypiperazin-4-phosphonic acid (CPP) in the mPFC. Blockade of NMDA receptors impaired accuracy as well as executive control as shown by increased premature and perseverative responding. The CPP-induced premature and perseverative over-responding were dose-dependently prevented by both M100907 and Ro60-0175. Both drugs partially removed the CPP-induced accuracy deficit but only at the highest dose tested.

CONCLUSIONS

It is suggested that in the dorsal striatum, 5-HT by an action on 5-HT(2A) and 5-HT(2C) receptors may integrate the glutamate corticostriatal inputs critical for different aspects of the 5-CSRT task performance.

Impulsive behaviour induced by both NMDA receptor antagonism and GABAA receptor activation in rat ventromedial prefrontal cortex

RATIONALE

Previous work has demonstrated a profound effect of N-methyl-D: -aspartic acid receptor (NMDAR) antagonism in the infralimbic cortex (IL) to selectively elevate impulsive responding in a rodent reaction time paradigm. However, the mechanism underlying this effect is unclear.

OBJECTIVES

This series of experiments investigated the pharmacological basis of this effect in terms of excitatory and inhibitory neurotransmission. We tested several pharmacological mechanisms that might produce the effect of NMDAR antagonism via disruption or dampening of IL output.

METHODS

Drugs known to affect brain GABA or glutamate function were tested in rats pre-trained on a five-choice serial reaction time task (5-CSRTT) following either their systemic administration or direct administration into the IL.

RESULTS

Systemic lamotrigine administration (15 mg/kg), which attenuates excess glutamate release, did not counteract the ability of the intra-IL NMDAR antagonist 3-((R)-2-carboxypiperazin-4-yl)-propyl-L: -phosphonic acid ((R)-CPP) to increase premature responding on the 5-CSRTT. Putative elevation of local extracellular glutamate via intra-IL infusions of the selective glutamate reuptake inhibitor DL: -threo-β-benzyloxyaspartate as well as local α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor antagonism also had no effect on this task. However, intra-IL infusions of the GABA(A) receptor agonist muscimol produced qualitatively but not quantitatively comparable increases in impulsive responding to those elicited by (R)-CPP. Moreover, the GABA(A) receptor antagonist bicuculline blocked the increase in impulsivity produced by (R)-CPP when infused in the IL.

CONCLUSIONS

These findings implicate glutamatergic and GABAergic mechanisms in the IL in the expression of impulsivity and suggest that excessive glutamate release may not underlie increased impulsivity induced by local NMDA receptor antagonism.

Pre-treatment with the mGlu2/3 receptor agonist LY379268 attenuates DOI-induced impulsive responding and regional c-Fos protein expression

RATIONALE

Overactivation of serotonin (5-hydroxytryptamine, 5-HT)(2A) receptors causes impulsivity and attentional deficits. Since 5-HT(2A) receptors are known to entertain antagonistic interactions with metabotropic glutamate (mGlu)2/3 receptors, this interaction may provide an alternative target for a novel class of antipsychotics.

OBJECTIVES/METHODS

The study characterizes interactions between 5-HT(2A) and mGlu2/3 receptors implicated in impulse control. Hooded Lister rats were trained in a 5-choice serial reaction time task (5-CSRTT) and treated with the 5-HT(2A/2C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropan hydrochloride (DOI, 0.1 mg/kg) and the mGlu2/3 receptor agonist LY379268 (1 mg/kg). In addition, associated drug-induced changes in neuronal activity were assessed via c-Fos immunoreactivity (Fos IR), and co-localization of c-Fos and GABAergic markers was detected using double immunofluorescence labeling.

RESULTS

Systemic DOI caused impulsive overresponding that was attenuated in animals pre-treated with LY379268. LY379268 itself had no significant effect on the rats' performance in the 5-CSRTT. DOI enhanced Fos IR within fronto-cortical and limbic brain structures, and this effect was blocked by LY379268 pre-treatment. Double immunofluorescence labeling showed a specific co-localization of DOI-elicited Fos IR with GABAergic (GAD(67)-positive) cells lacking the calcium-binding protein parvalbumin while LY379268 increased Fos IR in GABAergic and non-GABAergic cells.

CONCLUSION

Our results suggest that impulsivity is possibly due to a primary increase in Glu transmission mediated via 5-HT(2A) receptor activation. Thus, mGlu2/3 receptor agonists might have some potential for treating motor impulsivity-related impairments while their cognitive enhancing effects were not confirmed in this study.

Glutamate and glutathione interplay in a motor neuronal model of amyotrophic lateral sclerosis reveals altered energy metabolism

Impairment of mitochondrial function might contribute to oxidative stress associated with neurodegeneration in amyotrophic lateral sclerosis (ALS). Glutamate levels in tissues of ALS patients are sometimes altered. In neurons, mitochondrial metabolism of exogenous glutamine is mainly responsible for the net synthesis of glutamate, which is a neurotransmitter, but it is also necessary for the synthesis of glutathione, the main endogenous antioxidant. We investigated glutathione synthesis and glutamine/glutamate metabolism in a motor neuronal model of familial ALS. In standard culture conditions (with glutamine) or restricting glutamine or cystine, the level of glutathione was always lower in the cell line expressing the mutant (G93A) human Cu, Zn superoxide dismutase (G93ASOD1) than in the line expressing wild-type SOD1. With glutamine the difference in glutathione was associated with a lower glutamate and impairment of the glutamine/glutamate metabolism as evidenced by lower glutaminase and cytosolic malate dehydrogenase activity. d-β-hydroxybutyrate, as an alternative to glutamine as energy substrate in addition to glucose, reversed the decreases of cytosolic malate dehydrogenase activity and glutamate and glutathione. However, in the G93ASOD1 cell line, in all culture conditions the expression of pyruvate dehydrogenase kinase l protein, which down-regulates pyruvate dehydrogenase activity, was induced, together with an increase in lactate release in the medium. These findings suggest that the glutathione decrease associated with mutant SOD1 expression is due to mitochondrial dysfunction caused by the reduction of the flow of glucose-derived pyruvate through the TCA cycle; it implies altered glutamate metabolism and depends on the different mitochondrial energy substrates.

Effects of aripiprazole, olanzapine, and haloperidol in a model of cognitive deficit of schizophrenia in rats: relationship with glutamate release in the medial prefrontal cortex

RATIONALE

Disruption in cognition is characteristic of psychiatric illnesses such as schizophrenia. Studies of drugs that improve cognition might provide a better insight into the mechanisms underlying cognitive deficits.

OBJECTIVES

We compared the effects of the antipsychotic drugs aripiprazole, olanzapine, and haloperidol on performance deficit in a test of divided and sustained visual attention, the five-choice serial reaction time task (5-CSRTT), which provides information on attentional functioning (accuracy of visual discrimination), response control (measured by anticipatory and perseverative responses) and speed.

METHODS

The cognitive deficit was induced by infusion of the competitive NMDA receptor antagonist 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP) in the rat medial prefrontal cortex (mPFC). In vivo microdialysis was used to compare the effects of aripiprazole, olanzapine and haloperidol on CPP-induced glutamate (GLU) and serotonin (5-HT) release in the mPFC of conscious rats.

RESULTS

Oral aripiprazole (1.0 and 3.0 mg/kg) and olanzapine (0.3 and 1.0 mg/kg), but not haloperidol (0.1 mg/kg), abolished the CPP-induced accuracy deficit and GLU release. Haloperidol and aripiprazole, but not olanzapine, reduced perseverative over-responding, while anticipatory responding was best controlled by olanzapine. However, these effects were not associated with changes in GLU release. No association was found between the effects of these antipsychotics on CPP-induced attentional performance deficits in the 5-CSRTT and 5-HT efflux.

CONCLUSIONS

The data confirm that excessive GLU release in the mPFC is associated with attentional deficits. Thus, suppression of GLU release may be a target for the development of novel antipsychotic drugs with greater effect on some aspects of cognitive deficits.

Sertindole restores attentional performance and suppresses glutamate release induced by the NMDA receptor antagonist CPP

RATIONALE

Blockade of N-methyl-d-aspartic acid (NMDA) receptors in the rat medial prefrontal cortex (mPFC) impairs performance in the five-choice serial reaction time task (5-CSRTT) and increases glutamate (GLU) release. Recent research suggests that excessive GLU release may be critical for attention deficits.

OBJECTIVES

We tested this hypothesis by investigating the effects of the atypical antipsychotics sertindole and clozapine on 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid (CPP)-induced performance deficits in the 5-CSRTT and on the CPP-induced GLU release in the mPFC.

METHODS

The 5-CSRTT, a test of divided and sustained visual attention providing indices of attentional functioning (accuracy of visual discrimination), response control (anticipatory and perseverative responses) and intracortical microdialysis in conscious rats were used to investigate the effects of sertindole and clozapine.

RESULTS

Low doses of sertindole (0.02-0.32 mg/kg) prevented CPP-induced accuracy deficits, anticipatory over-responding and the rise in GLU release. In contrast, doses ranging from 0.6 to 2.5 mg/kg had no effect or even enhanced the effect of CPP on anticipatory responding. Similarly, 2.5 mg/kg sertindole was unable to reverse CPP-induced rise in GLU release. Clozapine (2.5 mg/kg) prevented accuracy deficits and the increase in anticipatory responding and abolished the rise in GLU release induced by CPP.

CONCLUSIONS

These findings show that the ameliorating effects of sertindole and clozapine on NMDA receptor dependent attention deficit is associated with suppression in GLU release in the mPFC. This supports the proposal that suppression in GLU release might be a target for the development of novel drugs aimed at counteracting some aspects of cognitive deficits of schizophrenia.

Blockade of 5-HT2A receptors in the medial prefrontal cortex attenuates reinstatement of cue-elicited cocaine-seeking behavior in rats

RATIONALE

The action of serotonin (5-HT) at the 5-HT(2A) receptor subtype is thought to be involved in cocaine-seeking behavior that is motivated by exposure to drug-associated cues and drug priming. 5-HT(2A) receptors are densely clustered in the ventromedial prefrontal cortex (vmPFC), an area that plays a role in mediating cocaine-seeking behavior.

OBJECTIVES

This study examined the hypothesis that M100907, a 5-HT(2A) receptor antagonist, infused directly in the vmPFC attenuates cue- and cocaine-primed reinstatement of cocaine-seeking behavior.

METHODS

Rats trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues underwent extinction training during which operant responses produced no consequences. Once behavior extinguished, rats were tested for reinstatement of responding elicited by either response-contingent presentations of the cocaine-paired light/tone cues or by cocaine-priming injections (10 mg/kg, i.p.) within 1 min after pretreatment with microinfusions of M100907 (0.1, 0.3, 1.0, or 1.5 μg/0.2 μl/side) into the vmPFC.

RESULTS

Intra-vmPFC M100907 decreased cue-elicited reinstatement at the two highest doses (1.0 and 1.5 μg) but produced only a slight decrease in cocaine-primed reinstatement that was not dose dependent. The decrease in cue reinstatement was not likely due to impaired ability to respond since intra-vmPFC M100907 infusions had minimal effect on cocaine self-administration and no effect on cue-elicited sucrose-seeking behavior, or spontaneous or cocaine-induced locomotion. M100907 infusions into the adjacent anterior cingulate cortex had no effect on cue reinstatement.

CONCLUSIONS

The results suggest that the blockade of 5-HT(2A) receptors in the vmPFC selectively attenuates the incentive motivational effects of cocaine-paired cues.

Dissociable roles of prelimbic and infralimbic cortices, ventral hippocampus, and basolateral amygdala in the expression and extinction of conditioned fear

Current models of conditioned fear expression and extinction involve the basolateral amygdala (BLA), ventral medial prefrontal cortex (vmPFC), and the hippocampus (HPC). There is some disagreement with respect to the specific roles of these structures, perhaps due to subregional differences within each area. For example, growing evidence suggests that infralimbic (IL) and prelimbic (PL) subregions of vmPFC have opposite influences on fear expression. Moreover, it is the ventral HPC (vHPC), rather than the dorsal HPC, that projects to vmPFC and BLA. To help determine regional specificity, we used small doses of the GABA(A) agonist muscimol to selectively inactivate IL, PL, BLA, or vHPC in an auditory fear conditioning and extinction paradigm. Infusions were performed prior to extinction training, allowing us to assess the effects on both fear expression and subsequent extinction memory. Inactivation of IL had no effect on fear expression, but impaired the within-session acquisition of extinction as well as extinction memory. In contrast, inactivation of PL impaired fear expression, but had no effect on extinction memory. Inactivation of the BLA or vHPC impaired both fear expression and extinction memory. Post-extinction inactivations had no effect in any structure. We suggest a model in which amygdala-dependent fear expression is modulated by inputs from PL and vHPC, whereas extinction memory requires extinction-induced plasticity in IL, BLA, and/or vHPC.

Attention deficit induced by blockade of N-methyl D-aspartate receptors in the prefrontal cortex is associated with enhanced glutamate release and cAMP response element binding protein phosphorylation: role of metabotropic glutamate receptors 2/3

The hypothesis that attention deficits induced by the hypofunction of N-methyl d-aspartate (NMDA) receptors in the prefrontal cortex (PFC) might be associated with increased glutamate release and changes in the phosphorylation of the cyclic adenosine monophosphate response element-binding protein on serine 133 (p-S(133)CREB) was investigated in this study. Infusion of 50 ng/side 3-(R)-2-carboxypiperazin-4-propyl-1-phosphonic acid ((R)-CPP), a competitive glutamate NMDA receptor antagonist, into the medial prefrontal cortex (mPFC) of rats performing the five-choice serial reaction time (5-CSRT) task, reduced accuracy of visual discrimination (measured by % correct responses) and enhanced impulsivity (measured by the number of premature responses) and compulsivity (measured by the number of perseverative responses). The mGluR2/3 receptor agonist, LY379268, injected s.c. at 0.1 mg/kg, reduced (R)-CPP-induced impairment in attentional functioning (accuracy) and impulsivity but not compulsive perseveration. In parallel studies using microdialysis technique and Western blot analysis we found that (R)-CPP (100 μM) infused in the medial prefrontal cortex increased glutamate efflux whereas injected in the medial prefrontal cortex at a dose causing impairments in attentional performance (50 ng/side) increased p-S(133)CREB in the frontal cortex (FC), decreased it in the caudate-putamen (CPu) and was without effect in the nucleus accumbens (NAC). LY379268 at the dose effective in reducing (R)-CPP-induced behavioral deficit reduced both the (R)-CPP-induced rise in glutamate efflux in the prefrontal cortex and the increase in p-S(133)CREB in the frontal cortex but was without effect on the decrease in p-S(133)CREB in the caudate-putamen. The data provide evidence that enhanced glutamate release and phosphorylation of cAMP response element binding protein (CREB) on serine 133 may be associated to attention deficit and loss of impulse control. Furthermore they suggest that mGluR2/3 agonists have a therapeutic potential for cognitive deficits.

Antagonism at serotonin 5-HT(2A) receptors modulates functional activity of frontohippocampal circuit

RATIONALE

Several second-generation antipsychotics are characterised by a significant antagonistic effect at serotonin 5-HT(2A) receptors (5-HT(2A)R), a feature that has been associated with lower incidence of extra-pyramidal symptoms and a putative amelioration of positive and negative symptoms experienced by schizophrenic patients. However, the neurofunctional substrate of 5-HT(2A) antagonism and its exact contribution to the complex pharmacological profile of these drugs remain to be elucidated.

OBJECTIVES

Here, we used pharmacological magnetic resonance imaging to map the modulatory effects of the selective 5-HT(2A)R antagonist Ml00907 on the spatiotemporal patterns of brain activity elicited by acute phencyclidine (PCP) challenge in the rat. PCP is a non-competitive NMDA receptor antagonist that induces dysregulation of corticolimbic glutamatergic neurotransmission and produces cognitive impairment and psychotic-like symptoms reminiscent of those observed in schizophrenia.

RESULTS

Pre-administration of M100907 produced focal and region-dependent attenuation of PCP-induced response in frontoseptohippocampal areas. As early studies highlighted a permissive role of 5-HT(2A)R on frontal dopamine release, the role of post-synaptic dopamine D(1) receptors on PCP-induced response was examined by using the potent antagonist SCH23390. Interestingly, SCH23390 did not affect PCP's response in any of the regions examined. This finding rules out a significant contribution of dopamine in the functional changes mapped and, indirectly, the inhibitory effect of M100907, in favour of a glutamatergic origin.

CONCLUSIONS

Our data expand recent evidence suggesting a key role of 5-HT(2A)R in modulating glutamate-mediated cognitive performance in the prefrontal cortex and highlight the whole frontoseptohippocampal circuit as a key functional substrate of 5-HT(2A)R antagonism in normal and disease states.

Enhancement of spatial reversal learning by 5-HT2C receptor antagonism is neuroanatomically specific

We have recently demonstrated that systemic administration of 5-HT(2C) and 5-HT(2A) receptor antagonists significantly enhanced and impaired spatial reversal learning, respectively. In this study, the role of 5-HT(2C) and 5-HT(2A) receptor subtypes in the mediation of these opposing effects was further investigated with respect to neuroanatomical specificity. The roles of 5-HT(2C) and 5-HT(2A) receptors were examined within some of the brain regions implicated in cognitive flexibility, namely the orbitofrontal cortex (OFC), medial prefrontal cortex (mPFC), and nucleus accumbens (NAc), by means of targeted infusions of selective 5-HT(2C) and 5-HT(2A) receptor antagonists (SB 242084 and M100907, respectively). Intra-OFC 5-HT(2C) receptor antagonism produced dose-dependent effects similar to those of systemic administration, i.e., improved spatial reversal learning by reducing the number of trials (all doses: 0.1, 0.3, and 1.0 microg) and perseverative errors to criterion (0.3 and 1.0 microg) compared with controls. However, the highest dose (1.0 microg) showed a nonselective effect, as it also affected retention preceding the reversal phase and decreased learning errors. Intracerebral infusions of SB 242084 into the mPFC or NAc produced no significant effects on any behavioral measures. Similarly, no significant differences were observed with intra-OFC, -mPFC, or -NAc infusions of M100907. These data suggest that the improved performance in reversal learning observed after 5-HT(2C) receptor antagonism is mediated within the OFC. These data also bear on the issue of whether 5-HT(2C) receptor antagonism within the OFC might help elucidate the biological substrate of obsessive-compulsive disorder, offering the potential for therapeutic application.

Blockade of serotonin 2A receptors prevents PCP-induced attentional performance deficit and CREB phosphorylation in the dorsal striatum of DBA/2 mice

RATIONAL AND OBJECTIVE

Functional opposition between N-methyl-D-aspartate and 5-HT(2A) receptors may be a neural mechanism supporting cognitive functions. These systems converge on an intracellular signaling pathway that involves protein kinase A-dependent phosphorylation of different proteins including cyclic adenosine monophosphate response element binding (CREB). Thus, we tested whether selective 5-HT(2A) receptor antagonist, M100907, might abolish phencyclidine (PCP)-induced attentional performance deficit by preventing its effects on transduction mechanisms leading to CREB phosphorylation.

METHODS

Using the five-choice serial reaction time task, the ability of subcutaneous injections of 2.5 and 10 microg/kg of M100907 to abolish the effects of an intraperitoneal injection of 1.5 mg/kg PCP on attentional performance as measured by accuracy (percentage of correct responses) and anticipatory and perseverative responding was assessed in DBA/2 mice. The effects of PCP, M100907, and their combination on S(133)-CREB and T(34)-DARPP32 phosphorylation in the dorsal striatum and prefrontal cortex (PFC) of behaviorally naïve mice were examined using Western blotting technique.

RESULTS

PCP reduced accuracy and increased anticipatory and perseverative responses as well as it increased S(133)-CREB phosphorylation in the dorsal striatum but not in the PFC. Ten microg/kg M100907 abolished the PCP-induced attentional performance deficits and the increase in S(133)-CREB but not T(34)-DARPP32 phosphorylation. By itself, M100907 had no effect on attentional performance or phospho-S(133)-CREB and phospho-T(34)-DARPP32. Interestingly, the effect of PCP on phospho-S(133)-CREB but not on phospho-T(34)-DARPP32 was dependent on endogenous 5-HT.

CONCLUSIONS

The data indicate that blockade of 5-HT(2A) receptors may exert beneficial effects on cognitive deficits through a mechanism linked to striatal S(133)-CREB phosphorylation.

Role of different monoamine receptors controlling MK-801-induced release of serotonin and glutamate in the medial prefrontal cortex: relevance for antipsychotic action

Several studies have demonstrated that systemically administered N-methyl-d-aspartate (NMDA) receptor antagonists increase serotonin (5-HT) and glutamate release in the medial prefrontal cortex (mPFC). Previously we showed that the perfusion of clozapine in the mPFC prevented the MK-801-induced increase in extracellular glutamate and 5-HT whereas haloperidol blocked only the effect of MK-801 on glutamate. To study the contribution of different monoaminergic receptors (for which clozapine and haloperidol exhibit distinct affinities) to these effects, here we used in-vivo microdialysis to examine the role of local blockade of dopamine D2, 5-HT2A and alpha1-adrenergic receptors as well as agonism at dopamine D1 and 5-HT1A receptors in the mPFC on the increased efflux of glutamate and 5-HT elicited by MK-801. The results show that M100907 (5-HT2A antagonist), BAY x 3702 (5-HT1A agonist) and prazosin (alpha1-adrenergic antagonist) blocked the MK-801-induced increase of 5-HT and glutamate in the mPFC. However, raclopride, eticlopride (dopamine D2 antagonists) and SKF-38393 (dopamine D1 agonist) were able to prevent the increased efflux of glutamate (but not that of 5-HT) elicited by MK-801. We propose that D2 receptor antagonists and D1 agonists would act predominantly on a subpopulation of GABAergic interneurons of the mPFC, thus leading to an enhanced cortical inhibition that would prevent an excessive glutamatergic transmission. On the other hand, atypical antipsychotic drugs might further act upon 5-HT2A, 5-HT1A and alpha1-adrenoceptors present in pyramidal cells (including those projecting to the dorsal raphe nucleus), which would directly inhibit an excessive excitability of these cells.

Role of glutamate in schizophrenia: integrating excitatory avenues of research

Schizophrenia is a debilitating lifelong disorder affecting up to 1% of the population worldwide, producing significant financial and emotional hardship for patients and their families. As yet, the causes of schizophrenia and the mechanism of action of antipsychotic drugs are unknown, and many patients do not respond well to currently available medications. Attempts to find risk factors for the disorder using epidemiological methods have shown that schizophrenia is highly heritable, and path analyses predict that the disorder is caused by several genes in combination with nongenetic factors. Therefore, intensive research efforts have been made to identify genes creating vulnerability to schizophrenia and also genes predicting response to treatment. Interactions of the glutamatergic system with dopaminergic and serotonergic circuitry are crucial for normal brain function, and their disruption may be a mechanism by which the pathophysiology of schizophrenia is manifest. Genes within the glutamatergic system are therefore strong candidates for investigation, and these include the glutamate receptor genes in addition to genes encoding neuregulin, dysbindin, D-amino acid oxidase and G72/G30. These genetic studies could eventually reveal new targets for antipsychotic drug treatment, which currently focuses on inhibition of the dopaminergic system. However, a recent breakthrough indicates clinical efficacy of a drug stimulating the metabotropic glutamate receptor II, LY2140023, which has improved efficacy for negative and cognitive symptoms of schizophrenia. Studies of larger patient samples are required to consolidate these data. Further investigation of glutamatergic targets is likely to reinvigorate antipsychotic drug development.

Current status of inverse agonism at serotonin2A (5-HT2A) and 5-HT2C receptors

Contemporary receptor theory was developed to account for the existence of constitutive activity, as defined by the presence of receptor signaling in the absence of any ligand. Thus, ligands acting at a constitutively active receptor, can act as agonists, antagonists, and inverse agonists. In vitro studies have also revealed the complexity of ligand/receptor interactions including agonist-directed stimulus trafficking, a finding that has led to multi-active state models of receptor function. Studies with a variety of cell types have established that the serotonin 5-HT(2A) and 5-HT(2C) receptors also demonstrate constitutive activity and inverse agonism. However, until recently, there has been no evidence to suggest that these receptors also demonstrate constitutive activity and hence reveal inverse agonist properties of ligands in vivo. This paper describes our current knowledge of constitutive activity in vitro and then examines the evidence for constitutive activity in vivo. Both the serotonin 5-HT(2A) and 5-HT(2C) receptors are involved in a number of physiological and behavioral functions and are the targets for treatment of schizophrenia, anxiety, weight control, Parkinsonism, and other disorders. The existence of constitutive activity at these receptors in vivo, along with the possibility of inverse agonism, provides new avenues for drug development.

The 5-hydroxytryptamine2A receptor antagonist R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl-4-piperidinemethanol (M100907) attenuates impulsivity after both drug-induced disruption (dizocilpine) and enhancement (antidepressant drugs) of differential-reinforcement-of-low-rate 72-s behavior in the rat

Previous work has suggested that N-methyl-d-aspartate (NMDA) receptor antagonism and 5-hydroxytryptamine (5-HT)(2A) receptor blockade may enhance and attenuate, respectively, certain types of impulsivity mediated by corticothalamostriatal circuits. More specifically, past demonstrations of synergistic "antidepressant-like" effects of a 5-HT(2A) receptor antagonist and fluoxetine on differential-reinforcement-of-low-rate (DRL) 72-s schedule of operant reinforcement may speak to the role of 5-HT(2A) receptor blockade with respect to response inhibition as an important prefrontal cortical executive function relating to motor impulsivity. To examine the dynamic range over which 5-HT(2A) receptor blockade may exert effects on impulsivity, [R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl-4-piperidinemethanol] (M100907) was examined both alone and in combination with the psychotomimetic NMDA receptor antagonist dizocilpine [e.g., (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate; MK-801] and two different antidepressants, the tricyclic antidepressant desmethylimipramine (DMI) and the monoamine oxidase inhibitor tranylcypromine in rats performing under a DRL 72-s schedule. MK-801 increased the response rate, decreased the number of reinforcers obtained, and exerted a leftward shift in the inter-response time (IRT) distribution as expected. A dose of M100907 that exerted minimal effect on DRL behavior by itself attenuated the psychotomimetic effects of MK-801. Extending previous M100907-fluoxetine observations, addition of a minimally active dose of M100907 to low doses of DMI and tranylcypromine enhanced the antidepressant-like effect of the antidepressants. Therefore, it may be that a tonic excitation of 5-HT(2A) receptors modulates impulsivity and function of corticothalamostriatal circuits over an extensive dynamic range.

Opposing roles for 5-HT2A and 5-HT2C receptors in the nucleus accumbens on inhibitory response control in the 5-choice serial reaction time task

Serotonin (5-HT) is thought to play an important role in the regulation of behavioral inhibition. Studies manipulating 5-HT function in the rodent brain indicate that 5-HT receptors regulate distinct forms of impulsive behavior, including impulsive responding in the 5-choice serial reaction time task (5CSRTT). The present study investigates the loci of effects mediated by 5-HT(2A) and 5-HT(2C) receptors in attention and inhibitory response control using microinfusions targeted at the nucleus accumbens (NAc), prelimbic cortex (PL) and infralimbic cortex (IL). Rats were implanted with bilateral guide cannulas and received infusions of the selective 5-HT(2A) receptor antagonist M100907 (0.1 and 0.3 microg) or selective 5-HT(2C) receptor antagonist SB242084 (0.1 and 0.5 microg) immediately prior to testing. The results show that intra-NAc infusions of M100907 significantly decrease impulsive responding on the 5CSRTT and at the highest dose increased omissions as well. By contrast, infusions of SB242084 into the NAc selectively and dose-dependently increased impulsivity. Neither M100907 nor SB242084 significantly altered impulsive responding following either intra-PL or intra-IL administration. However, SB242084 significantly decreased omissions following intra-PL administration (0.5 microg only). These data reveal opposing effects on impulsivity following 5-HT(2A) and 5-HT(2C) blockade in the NAc. Our results suggest that the NAc, but not the PL or IL, is implicated in the mediation of the effects of M100907 and SB242084 on inhibitory response control during baseline 5CSRTT performance.

Dissociable effects of selective 5-HT2A and 5-HT2C receptor antagonists on serial spatial reversal learning in rats

Serotonin (5-hydroxytryptamine, or 5-HT) is strongly implicated in the ability to shift behavior in response to changing stimulus-reward contingencies. However, there is little information on the contribution of different 5-HT receptors in reversal learning. Thus, we investigated the effects of systemic administration of the 5-HT(2A) antagonist M100907 (0, 0.01, 0.03, and 0.1 mg/kg, i.p.) and the 5-HT(2C) antagonist SB 242084 (0, 0.1, 0.3, and 1.0 mg/kg, i.p.) on the performance of an instrumental two-lever spatial discrimination and serial spatial reversal learning task, where both levers were presented and only one was reinforced. The rat was required to respond on the reinforced lever under a fixed ratio 3 schedule of reinforcement. Following attainment of criterion, a series of within-session reversals was presented. Neither M100907 nor SB 242084 altered performance during spatial discrimination and retention of the previously reinforced contingencies. M100907 significantly impaired reversal learning by increasing both trials to criterion (only at the highest dose) and incorrect responses to criterion in Reversal 1, a pattern of behavior manifested as increased perseverative responding on the previously reinforced lever. In contrast, SB 242084 improved reversal learning by decreasing trials and incorrect responses to criterion in Reversal 1, with significantly fewer perseverative responses. These data support the view that 5-HT(2A) and 5-HT(2C) receptors have distinct roles in cognitive flexibility and response inhibition. The improved performance in reversal learning observed following 5-HT(2C) receptor antagonism suggests these receptors may offer the potential for therapeutic advances in a number of neuropsychiatric disorders where cognitive deficits are a feature, including obsessive-compulsive disorder.