Activation of Medial Prefrontal Cortex by Phencyclidine is Mediated via a Hippocampo-prefrontal Pathway

Sequential deactivation across the thalamus-hippocampus-mPFC pathway during loss of consciousness

How consciousness is lost in states such as sleep or anesthesia remains a mystery. To gain insight into this phenomenon, we conducted concurrent recordings of electrophysiology signals in the anterior cingulate cortex and whole-brain functional magnetic resonance imaging (fMRI) in rats exposed to graded propofol, undergoing the transition from consciousness to unconsciousness. Our results reveal that upon the loss of consciousness (LOC), as indicated by the loss of righting reflex, there is a sharp increase in low-frequency power of the electrophysiological signal. Additionally, simultaneously measured fMRI signals exhibit a cascade of deactivation across a pathway including the hippocampus, thalamus, and medial prefrontal cortex (mPFC) surrounding the moment of LOC, followed by a broader increase in brain activity across the cortex during sustained unconsciousness. Furthermore, sliding window analysis demonstrates a temporary increase in synchrony of fMRI signals across the hippocampus-thalamus-mPFC pathway preceding LOC. These data suggest that LOC might be triggered by sequential activities in the hippocampus, thalamus and mPFC, while wide-spread activity increases in other cortical regions commonly observed during anesthesia-induced unconsciousness might be a consequence, rather than a cause of LOC. Taken together, our study identifies a cascade of neural events unfolding as the brain transitions into unconsciousness, offering critical insight into the systems-level neural mechanisms underpinning LOC.

Ketamine promptly normalizes excess norepinephrine and enhances dopamine neuronal activity in Wistar Kyoto rats

Ketamine acts primarily by blocking the N-methyl-D-aspartate (NMDA) receptor at the phencyclidine site. The rapid antidepressant properties of ketamine were demonstrated in the clinic and several behavioral models of depression in rodents. We hypothesized that the normalization of abnormal activity of monoamine neurons in Wistar Kyoto (WKY) rats contributes to the rapid antidepressant effects of ketamine. A single administration of ketamine (10 mg/kg, i. p) or saline was administered to anesthetized WKY rats before in vivo electrophysiological recordings of dorsal raphe nucleus (DRN) serotonin (5-HT), locus coeruleus (LC) norepinephrine (NE) and ventral tegmental area (VTA) dopamine (DA) neuronal activity. Pyramidal neurons from the medial prefrontal cortex (mPFC) were also recorded before and after a ketamine injection. In the VTA, ketamine elicited a significant increase in the population activity of DA neurons. This enhancement was consistent with findings in other depression-like models in which such a decreased population activity was observed. In the LC, ketamine normalized increased NE neuron burst activity found in WKY rats. In the DRN, ketamine did not significantly reverse 5-HT neuronal activity in WKY rats, which is dampened compared to Wistar rats. Ketamine did not significantly alter the neuronal activity of mPFC pyramidal neurons. These findings demonstrate that ketamine normalized NE neuronal activity and enhanced DA neuronal activity in WKY rats, which may contribute to its rapid antidepressant effect.

A Comprehensive Review of the Current Status of the Cellular Neurobiology of Psychedelics

Psychedelic substances have gained significant attention in recent years for their potential therapeutic effects on various psychiatric disorders. This review delves into the intricate cellular neurobiology of psychedelics, emphasizing their potential therapeutic applications in addressing the global burden of mental illness. It focuses on contemporary research into the pharmacological and molecular mechanisms underlying these substances, particularly the role of 5-HT2A receptor signaling and the promotion of plasticity through the TrkB-BDNF pathway. The review also discusses how psychedelics affect various receptors and pathways and explores their potential as anti-inflammatory agents. Overall, this research represents a significant development in biomedical sciences with the potential to transform mental health treatments.

Circuit-Based Approaches to Understanding Corticostriatothalamic Dysfunction Across the Psychosis Continuum

Dopamine is known to play a role in the pathogenesis of psychotic symptoms, but the mechanisms driving dopaminergic dysfunction in psychosis remain unclear. Considerable attention has focused on the role of corticostriatothalamic (CST) circuits, given that they regulate and are modulated by the activity of dopaminergic cells in the midbrain. Preclinical studies have proposed multiple models of CST dysfunction in psychosis, each prioritizing different brain regions and pathophysiological mechanisms. A particular challenge is that CST circuits have undergone considerable evolutionary modification across mammals, complicating comparisons across species. Here, we consider preclinical models of CST dysfunction in psychosis and evaluate the degree to which they are supported by evidence from human resting-state functional magnetic resonance imaging studies conducted across the psychosis continuum, ranging from subclinical schizotypy to established schizophrenia. In partial support of some preclinical models, human studies indicate that dorsal CST and hippocampal-striatal functional dysconnectivity are apparent across the psychosis spectrum and may represent a vulnerability marker for psychosis. In contrast, midbrain dysfunction may emerge when symptoms warrant clinical assistance and may thus be a trigger for illness onset. The major difference between clinical and preclinical findings is the strong involvement of the dorsal CST in the former, consistent with an increasing prominence of this circuitry in the primate brain. We close by underscoring the need for high-resolution characterization of phenotypic heterogeneity in psychosis to develop a refined understanding of how the dysfunction of specific circuit elements gives rise to distinct symptom profiles.

Disrupted gut microbiota aggravates working memory dysfunction induced by high-altitude exposure in mice

BACKGROUND

The widely accepted microbiome-gut-brain axis (MGBA) hypothesis may be essential for explaining the impact of high-altitude exposure on the human body, especially brain function. However, studies on this topic are limited, and the underlying mechanism remains unclear. Therefore, this study aimed to determine whether high-altitude-induced working memory dysfunction could be exacerbated with gut microbiota disruption.

METHODS AND RESULTS

C57BL/6 mice were randomly divided into three groups: control, high-altitude exposed (HAE), and high-altitude exposed with antibiotic treatment (HAE-A). The HAE and HAE-A groups were exposed to a low-pressure oxygen chamber (60-65 kPa) simulating the altitude of 3,500-4,000 m for 14 days, The air pressure level for the control group was maintained at 94.5 kPa. Antibiotic water (mixed with 0.2 g/L of ciprofloxacin and 1 g/L of metronidazole) was provided to the HAE-A group. Based on the results of the novel object test and P300 in the oddball behavioral paradigm training test, working memory dysfunction was aggravated by antibiotic treatment. We determined the antioxidant capacity in the prefrontal cortex and found a significant negative influence (p < 0.05) of disturbed gut microbiota on the total antioxidant capacity (T-AOC) and malondialdehyde (MDA) content, as well as the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px). The same trend was also observed in the apoptosis-related functional protein content and mRNA expression levels in the prefrontal cortex, especially the levels of bcl-2, Bax, and caspase-3. The high-altitude environment and antibiotic treatment substantially affected the richness and diversity of the colonic microbiota and reorganized the composition and structure of the microbial community. S24-7, Lachnospiraceae, and Lactobacillaceae were the three microbial taxa with the most pronounced differences under the stimulation by external factors in this study. In addition, correlation analysis between colonic microbiota and cognitive function in mice demonstrated that Helicobacteraceae may be closely related to behavioral results.

CONCLUSION

Disrupted gut microbiota could aggravate working memory dysfunction induced by high-altitude exposure in mice, indicating the existence of a link between high-altitude exposure and MGBA.

Diagnosis and Clinical Features in Autoimmune-Mediated Movement Disorders

Movement disorders are common manifestations in autoimmune-mediated encephalitis. This group of diseases is suspected to be triggered by infection or neoplasm. Certain phenotypes correlate with specific autoantibody-related neurological disorders, such as orofacial-lingual dyskinesia with N-methyl-D-aspartate receptor encephalitis and faciobrachial dystonic seizures with leucine-rich glioma-inactivated protein 1 encephalitis. Early diagnosis and treatment, especially for autoantibodies targeting neuronal surface antigens, can improve prognosis. In contrast, the presence of autoantibodies against intracellular neuronal agents warrants screening for underlying malignancy. However, early clinical diagnosis is challenging because these diseases can be misdiagnosed. In this article, we review the distinctive clinical phenotypes, magnetic resonance imaging findings, and current treatment options for autoimmune-mediated encephalitis.

P2X7 Receptor-Dependent Layer-Specific Changes in Neuron-Microglia Reactivity in the Prefrontal Cortex of a Phencyclidine Induced Mouse Model of Schizophrenia

Background: It has been consistently reported that the deficiency of the adenosine triphosphate (ATP) sensitive purinergic receptor P2X7 (P2X7R) ameliorates symptoms in animal models of brain diseases.

Objective: This study aimed to investigate the role of P2X7R in rodent models of acute and subchronic schizophrenia based on phencyclidine (PCP) delivery in animals lacking or overexpressing P2X7R, and to identify the underlying mechanisms involved.

Methods: The psychotomimetic effects of acute i.p. PCP administration in C57Bl/6J wild-type, P2X7R knockout (P2rx7^−/−) and overexpressing (P2X7-EGFP) young adult mice were quantified. The medial prefrontal cortex (mPFC) of P2rx7^−/− and heterozygous P2X7-EGFP acutely treated animals was characterized through immunohistochemical staining. The prefrontal cortices of young adult P2rx7^−/− and P2rx7^tg/+ mice were examined with tritiated dopamine release experiments and the functional properties of the mPFC pyramidal neurons in layer V from P2rx7^−/− mice were assessed by patch-clamp recordings. P2rx7^−/− animals were subjected to a 7 days subchronic systemic PCP treatment. The animals working memory performance and PFC cytokine levels were assessed.

Results: Our data strengthen the hypothesis that P2X7R modulates schizophrenia-like positive and cognitive symptoms in NMDA receptor antagonist models in a receptor expression level-dependent manner. P2X7R expression leads to higher medial PFC susceptibility to PCP-induced circuit hyperactivity. The mPFC of P2X7R knockout animals displayed distinct alterations in the neuronal activation pattern, microglial organization, specifically around hyperactive neurons, and were associated with lower intrinsic excitability of mPFC neurons.

Conclusions: P2X7R expression exacerbated PCP-related effects in C57Bl/6J mice. Our findings suggest a pleiotropic role of P2X7R in the mPFC, consistent with the observed behavioral phenotype, regulating basal dopamine concentration, layer-specific neuronal activation, intrinsic excitability of neurons in the mPFC, and the interaction of microglia with hyperactive neurons. Direct measurements of P2X7R activity concerning microglial ramifications and dynamics could help to further elucidate the molecular mechanisms involved.

Autoimmunity and NMDA receptor in brain disorders: Where do we stand?

Over the past decades, the identification of autoimmune encephalitis in which patients express autoantibodies directed against neurotransmitter receptors has generated great hope to shed new light on the molecular mechanisms underpinning neurological and psychiatric conditions. Among these autoimmune encephalitides, the discovery of autoantibodies directed against the glutamatergic NMDA receptor (NMDAR-Ab), in the anti-NMDAR encephalitis, has provided some key information on how complex neuropsychiatric symptoms can be caused by a deficit in NMDAR signalling. Yet, NMDAR-Abs have also been detected in several neurological and psychiatric conditions, as well as in healthy individuals. In addition, these various NMDAR-Abs appear to have different molecular properties and pathogenicities onto receptors and synaptic functions. Here, we discuss the current view on the variety of NMDAR-Abs and, in particular, how these autoantibodies can lead to receptor dysfunction in neuronal networks. Since our mechanistic understanding on patients' NMDAR-Abs is still in its infancy, several complementary processes can be proposed and further in-depth molecular and cellular investigations will surely reveal key insights. Autoantibodies represent a great opportunity to gain knowledge on the etiology of neuropsychiatric disorders and pave the way for innovative therapeutic strategies. ONE SENTENCE SUMMARY: Current view on patients' autoantibody against NMDAR.

Ketamine: The final frontier or another depressing end?

Two decades ago, the observation of a rapid and sustained antidepressant response after ketamine administration provided an exciting new avenue in the search for more effective therapeutics for the treatment of clinical depression. Research elucidating the mechanism(s) underlying ketamine's antidepressant properties has led to the development of several hypotheses, including that of disinhibition of excitatory glutamate neurons via blockade of N-methyl-d-aspartate (NMDA) receptors. Although the prominent understanding has been that ketamine's mode of action is mediated solely via the NMDA receptor, this view has been challenged by reports implicating other glutamate receptors such as AMPA, and other neurotransmitter systems such as serotonin and opioids in the antidepressant response. The recent approval of esketamine (Spravato™) for the treatment of depression has sparked a resurgence of interest for a deeper understanding of the mechanism(s) underlying ketamine's actions and safe therapeutic use. This review aims to present our current knowledge on both NMDA and non-NMDA mechanisms implicated in ketamine's response, and addresses the controversy surrounding the antidepressant role and potency of its stereoisomers and metabolites. There is much that remains to be known about our understanding of ketamine's antidepressant properties; and although the arrival of esketamine has been received with great enthusiasm, it is now more important than ever that its mechanisms of action be fully delineated, and both the short- and long-term neurobiological/functional consequences of its treatment be thoroughly characterized.

Motivation, risk-taking and sensation seeking behavior in propofol anesthesia exposed peripubertal rats

Adolescent neurodevelopment confer vulnerability to the actions of treatments that produce adaptations in neurocircuitry underlying motivation, impulsivity and reward. Considering wide usage of a sedative-hypnotic agent propofol in clinical practice, we examined whether propofol is a challenging treatment for peripubertal brain. Motivation/hedonic behavior (sucrose preference test), approach/avoidance behavior (elevated plus maze test) and response to dissociative drug phencyclidine (PCP) were studied in peripubertal rats (the rodent model of periadolescence) after propofol anesthesia exposure (PAE). Neurodegeneration (Fluoro-Jade staining) and the expression of proteins (Western blot) involved in excitatory synaptic transmission and activity-dependent synaptic stabilization in the medial prefrontal cortex (mPFC) and striatum (components of motivation/reward circuitry; process both appetitive and aversive events) were examined as well. In peripubertal rats PAE produced 1) transient brain-region specific changes in the expression of N-methyl-d-aspartate (NMDA) receptor subunits NR2A and NR2B, PSD-95 and N-cadherin, without neurotoxicity, 2) hyperlocomotor response to PCP, 3) no changes in preference for palatable 1% sucrose solution and a decrease in food eaten, 4) preference for 20% sucrose solution without changes in food eaten, 5) stretch-attended postures and open arms entries in the elevated plus maze test. Overall, these novel findings show that PAE leaves transient synaptic trace recognized as early form of synaptic plasticity related to passive drug exposure in the brain systems implicated in motivation/reward, increases drug-responsiveness, favors risk-taking and preference of novel/intense stimuli repairing otherwise present motivational deficiency. These findings accentuate multifaceted response to propofol in peripuberty and the importance of environmental stability for the most favorable neurobehavioral recovery.

Neuro-behavioral effects after systemic administration of MK-801 and disinhibition of the anterior thalamic nucleus in rats: Potential relevance in schizophrenia

Non-competitive N-methyl-d-aspartate receptor (NMDA-R) antagonists have been suggested to evoke psychotomimetic-like behaviors by selectively targeting GABAergic elements in cortical and thalamic circuits. In previous studies, we had reported the involvement of the reticular and anterior thalamic nuclei (ATN) in the MK-801-evoked hyperactivity and other motor alterations. Consistent with the possibility that these responses were mediated by thalamic disinhibition, we examined the participation of cortical and hippocampal areas innervated by ATN in the responses elicited by the systemic administration of MK-801 (0.2 mg/kg) and compared them to the effects produced by the microinjection of a subconvulsive dose of bicuculline (GABA_A receptor antagonist) in the ATN. We used the expression of Fos related antigen 2 (Fra-2) as a neuronal activity marker in the ATN and its projection areas such as hippocampus (HPC), retrosplenial cortex (RS), entorhinal cortex (EC) and medial prefrontal cortex (mPFC). Dorsal (caudate-putamen, CPu) and ventral striatum (nucleus accumbens, core and shell, NAc,co and NAc,sh) were also studied. Behavioral and brain activation results suggest a partial overlap after the effect of MK-801 administration and ATN disinhibition. MK-801 and ATN disinhibition increases locomotor activity and disorganized movements, while ATN disinhibition also reduces rearing behavior. A significant increase in Fra-2 immunoreactivity (Fra-2-IR) in the ATN, mPFC (prelimbic area, PrL) and NAc,sh was observed after MK-801, while a different pattern of Fra-2-IR was detected following ATN disinhibition (e.g., increase in DG and NAc,sh, and decrease in PrL cortex). Overall, our data may contribute to the understanding of dysfunctional neural circuits involved in schizophrenia.

Neural and neuronal discoordination in schizophrenia: From ensembles through networks to symptoms

Despite the substantial knowledge accumulated by past research, the exact mechanisms of the pathogenesis of schizophrenia and causal treatments still remain unclear. Deficits of cognition and information processing in schizophrenia are today often viewed as the primary and core symptoms of this devastating disorder. These deficits likely result from disruptions in the coordination of neuronal and neural activity. The aim of this review is to bring together convergent evidence of discoordinated brain circuits in schizophrenia at multiple levels of resolution, ranging from principal cells and interneurons, neuronal ensembles and local circuits, to large-scale brain networks. We show how these aberrations could underlie deficits in cognitive control and other higher order cognitive-behavioural functions. Converging evidence from both animal models and patients with schizophrenia is presented in an effort to gain insight into common features of deficits in the brain information processing in this disorder, marked by disruption of several neurotransmitter and signalling systems and severe behavioural outcomes.

NMDAR Hypofunction Animal Models of Schizophrenia

The N-methyl-d-aspartate receptor (NMDAR) hypofunction hypothesis has been proposed to help understand the etiology and pathophysiology of schizophrenia. This hypothesis was based on early observations that NMDAR antagonists could induce a full range of symptoms of schizophrenia in normal human subjects. Accumulating evidence in humans and animal studies points to NMDAR hypofunctionality as a convergence point for various symptoms of schizophrenia. Here we review animal models of NMDAR hypofunction generated by pharmacological and genetic approaches, and how they relate to the pathophysiology of schizophrenia. In addition, we discuss the limitations of animal models of NMDAR hypofunction and their potential utility for therapeutic applications.

Age-dependent effects of (+)-MK801 treatment on glutamate release and metabolism in the rat medial prefrontal cortex

NMDAR antagonist treatments in adolescent/young adult rodents are associated with augmented glutamate (Glu) release and perturbed Glu/glutamine (Gln) metabolism in the medial prefrontal cortex (mPFC) resembling those found in first-episode schizophrenia. Few studies, however, investigated NMDAR antagonist-induced changes in the adult mPFC and whether there is an age-dependence to this end. In this study, the effects of acute/repeated (+)-MK801 treatment on Glu release/metabolism were measured in the mPFC of male adolescent (postnatal day 30) and adult (14 weeks) rats. Acute (+)-MK801 treatment at 0.5 mg/kg body weight induced an approximately 4-fold increase of extracellular Glu concentration in the adolescent rats, and repeated treatment for 6 consecutive days significantly increased the levels of Glu + Gln (Glx) and glial metabolites 7 days after the last dose. Histologically (+)-MK801 treatments induced reactive astrocytosis and elevated oxidative stress in the mPFC of adolescent rats, without causing evident neuronal degeneration in the region. All (+)-MK801-induced changes observed in the mPFC of adolescent rats were not present or evident in the adult rats, suggesting that the treatments might have caused less disinhibition in the adult mPFC than in the adolescent mPFC. In conclusion, the effects of (+)-MK801 treatments on the Glu release/metabolism in the mPFC were found to be age-dependent; and the adult mPFC is likely equipped with more robust neurobiological mechanisms to preserve excitatory-inhibitory balance in response to NMDAR hypofunction.

Can N-Methyl-D-Aspartate Receptor Hypofunction in Schizophrenia Be Localized to an Individual Cell Type?

Hypofunction of N-methyl-D-aspartate glutamate receptors (NMDARs), whether caused by endogenous factors like auto-antibodies or mutations, or by pharmacological or genetic manipulations, produces a wide variety of deficits which overlap with-but do not precisely match-the symptom spectrum of schizophrenia. In order to understand how NMDAR hypofunction leads to different components of the syndrome, it is necessary to take into account which neuronal subtypes are particularly affected by it in terms of detrimental functional alterations. We provide a comprehensive overview detailing findings in rodent models with cell type-specific knockout of NMDARs. Regarding inhibitory cortical cells, an emerging model suggests that NMDAR hypofunction in parvalbumin (PV) positive interneurons is a potential risk factor for this disease. PV interneurons display a selective vulnerability resulting from a combination of genetic, cellular, and environmental factors that produce pathological multi-level positive feedback loops. Central to this are two antioxidant mechanisms-NMDAR activity and perineuronal nets-which are themselves impaired by oxidative stress, amplifying disinhibition. However, NMDAR hypofunction in excitatory pyramidal cells also produces a range of schizophrenia-related deficits, in particular maladaptive learning and memory recall. Furthermore, NMDAR blockade in the thalamus disturbs thalamocortical communication, and NMDAR ablation in dopaminergic neurons may provoke over-generalization in associative learning, which could relate to the positive symptom domain. Therefore, NMDAR hypofunction can produce schizophrenia-related effects through an action on various different circuits and cell types.

Distinct and Dynamic ON and OFF Neural Ensembles in the Prefrontal Cortex Code Social Exploration

The medial prefrontal cortex (mPFC) is important for social behavior, but the mechanisms by which mPFC neurons code real-time social exploration remain largely unknown. Here we utilized miniScopes to record calcium activities from hundreds of excitatory neurons in the mPFC while mice freely explored restrained social targets in the absence or presence of the psychedelic drug phencyclidine (PCP). We identified distinct and dynamic ON and OFF neural ensembles that displayed opposing activities to code real-time behavioral information. We further illustrated that ON and OFF ensembles tuned to social exploration carried information of salience and novelty for social targets. Finally, we showed that dysfunctions in these ensembles were associated with abnormal social exploration elicited by PCP. Our findings underscore the importance of mPFC ON and OFF neural ensembles for proper exploratory behavior, including social exploration, and pave the way for future studies elucidating neural circuit dysfunctions in psychiatric disorders.

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.

Phencyclidine Discoordinates Hippocampal Network Activity But Not Place Fields

We used the psychotomimetic phencyclidine (PCP) to investigate the relationships among cognitive behavior, coordinated neural network function, and information processing within the hippocampus place cell system. We report in rats that PCP (5 mg/kg, i.p.) impairs a well learned, hippocampus-dependent place avoidance behavior in rats that requires cognitive control even when PCP is injected directly into dorsal hippocampus. PCP increases 60-100 Hz medium-freguency gamma oscillations in hippocampus CA1 and these increases correlate with the cognitive impairment caused by systemic PCP administration. PCP discoordinates theta-modulated medium-frequency and slow gamma oscillations in CA1 LFPs such that medium-frequency gamma oscillations become more theta-organized than slow gamma oscillations. CA1 place cell firing fields are preserved under PCP, but the drug discoordinates the subsecond temporal organization of discharge among place cells. This discoordination causes place cell ensemble representations of a familiar space to cease resembling pre-PCP representations despite preserved place fields. These findings point to the cognitive impairments caused by PCP arising from neural discoordination. PCP disrupts the timing of discharge with respect to the subsecond timescales of theta and gamma oscillations in the LFP. Because these oscillations arise from local inhibitory synaptic activity, these findings point to excitation-inhibition discoordination as the root of PCP-induced cognitive impairment.SIGNIFICANCE STATEMENT Hippocampal neural discharge is temporally coordinated on timescales of theta and gamma oscillations in the LFP and the discharge of a subset of pyramidal neurons called "place cells" is spatially organized such that discharge is restricted to locations called a cell's "place field." Because this temporal coordination and spatial discharge organization is thought to represent spatial knowledge, we used the psychotomimetic phencyclidine (PCP) to disrupt cognitive behavior and assess the importance of neural coordination and place fields for spatial cognition. PCP impaired the judicious use of spatial information and discoordinated hippocampal discharge without disrupting firing fields. These findings dissociate place fields from spatial cognitive behavior and suggest that hippocampus discharge coordination is crucial to spatial cognition.

Role of mGlu5 Receptors and Inhibitory Neurotransmission in M1 Dependent Muscarinic LTD in the Prefrontal Cortex: Implications in Schizophrenia

Selective potentiation of the mGlu₅ subtype of metabotropic glutamate (mGlu) receptor using positive allosteric modulators (PAMs) has robust cognition-enhancing effects in rodent models that are relevant for schizophrenia. Until recently, these effects were thought to be due to potentiation of mGlu₅-induced modulation of NMDA receptor (NMDAR) currents and NMDAR-dependent synaptic plasticity. However, "biased" mGlu₅ PAMs that do not potentiate mGlu₅ effects on NMDAR currents show efficacy that is similar to that of prototypical mGlu₅ PAMs, suggesting that NMDAR-independent mechanisms must be involved in these actions. We now report that synaptic activation of mGlu₅ is required for a form of long-term depression (mLTD) in mouse prefrontal cortex (PFC) that is induced by activation of M₁ muscarinic acetylcholine (mAChR) receptors, which was previously thought to be independent of mGlu₅ activation. Interestingly, a biased mGlu₅ PAM, VU0409551, that does not potentiate mGlu₅ modulation of NMDAR currents, potentiated induction of mLTD. Furthermore, coactivation of mGlu₅ and M₁ receptors increased GABA_A-dependent inhibitory tone in the PFC pyramidal neurons, which likely contributes to the observed mLTD. Finally, systemic administration of the biased mGlu₅ PAM reversed deficits in mLTD and associated cognitive deficits in a model of cortical disruption caused by repeated phencyclidine exposure that is relevant for schizophrenia and was previously shown to be responsive to selective M₁ muscarinic receptor PAMs. These studies provide exciting new insights into a novel mechanism by which mGlu₅ PAMs can reverse deficits in PFC function and cognition that is independent of modulation of NMDAR currents.

Buspirone Counteracts MK-801-Induced Schizophrenia-Like Phenotypes through Dopamine D3 Receptor Blockade

Background: Several efforts have been made to develop effective antipsychotic drugs. Currently, available antipsychotics are effective on positive symptoms, less on negative symptoms, but not on cognitive impairment, a clinically relevant dimension of schizophrenia. Drug repurposing offers great advantages over the long-lasting, risky and expensive, de novo drug discovery strategy. To our knowledge, the possible antipsychotic properties of buspirone, an azapirone anxiolytic drug marketed in 1986 as serotonin 5-HT_1A receptor (5-HT_1AR) partial agonist, have not been extensively investigated despite its intriguing pharmacodynamic profile, which includes dopamine D₃ (D₃R) and D₄ receptor (D₄R) antagonist activity. Multiple lines of evidence point to D₃R as a valid therapeutic target for the treatment of several neuropsychiatric disorders including schizophrenia. In the present study, we tested the hypothesis that buspirone, behaving as dopamine D₃R antagonist, may have antipsychotic-like activity.

Materials and Methods: Effects of acute administration of buspirone was assessed on a wide-range of schizophrenia-relevant abnormalities induced by a single administration of the non-competitive NMDAR antagonist MK-801, in both wild-type mice (WT) and D₃R-null mutant mice (D₃R^-/-).

Results: Buspirone (3 mg⋅kg^-1, i.p.) was devoid of cataleptogenic activity in itself, but resulted effective in counteracting disruption of prepulse inhibition (PPI), hyperlocomotion and deficit of temporal order recognition memory (TOR) induced by MK-801 (0.1 mg⋅kg^-1, i.p.) in WT mice. Conversely, in D₃R^-/- mice, buspirone was ineffective in preventing MK-801-induced TOR deficit and it was only partially effective in blocking MK-801-stimulated hyperlocomotion.

Conclusion: Taken together, these results indicate, for the first time, that buspirone, might be a potential therapeutic medication for the treatment of schizophrenia. In particular, buspirone, through its D₃R antagonist activity, may be a useful tool for improving the treatment of cognitive deficits in schizophrenia that still represents an unmet need of this disease.

Altered functional connectivity in patients with post-stroke memory impairment: A resting fMRI study

Post-stroke memory dysfunction (PMD) is one of the most common forms of cognitive impairment among stroke survivors. However, only a limited number of studies have directly investigated the neural mechanisms associated with memory decline. The aim of the present study was to identify dynamic changes in the functional organization of the default mode network (DMN) and the dorsal attention network of patients with PMD. A total of 27 patients with PMD who experienced a stroke in the right hemisphere were enrolled in the current study, along with 27 healthy control subjects matched by age, sex, and educational level. A behavioral examination and functional magnetic resonance imaging scan were performed. The data were analyzed using an independent component analysis method. The results revealed a significantly increased functional connectivity between the DMN and prefrontal cortex (left middle/inferior frontal and left precentral gyri), temporal regions (left superior temporal gyrus), and bilateral and posterior cingulate gyri/precuneus (P<0.001). There was also a significantly decreased functional connectivity between the DMN and right middle temporal gyrus, left uvula, and right inferior parietal lobule, and between the dorsal attention network and prefrontal cortex (left precentral/inferior and right inferior/middle frontal gyri), right inferior parietal gyrus, and right insula (P<0.001). These results suggest that the stroke affected both the lesioned and contralesional hemispheres. The prefrontal cortex, temporal regions, insula, and posterior cingulate gyrus/precuneus serve a crucial role in memory processing.

Molecular and Cellular Mechanisms of Rapid-Acting Antidepressants Ketamine and Scopolamine

Major depressive disorder (MDD) is a prevalent neuropsychiatric disease that causes profound social and economic burdens. The impact of MDD is compounded by the limited therapeutic efficacy and delay of weeks to months of currently available medications. These issues highlight the need for more efficacious and faster-acting treatments to alleviate the burdens of MDD. Recent breakthroughs demonstrate that certain drugs, including ketamine and scopolamine, produce rapid and long-lasting antidepressant effects in MDD patients. Moreover, preclinical work has shown that the antidepressant actions of ketamine and scopolamine in rodent models are caused by an increase of extracellular glutamate, elevated BDNF, activation of the mammalian target of rapamycin complex 1 (mTORC1) cascade, and increased number and function of spine synapses in the prefrontal cortex (PFC). Here we review studies showing that both ketamine and scopolamine elicit rapid antidepressant effects through converging molecular and cellular mechanisms in the PFC. In addition, we discuss evidence that selective antagonists of NMDA and muscarinic acetylcholine (mACh) receptor subtypes (i.e., NR2B and M1-AChR) in the PFC produce comparable antidepressant responses. Furthermore, we discuss evidence that ketamine and scopolamine antagonize inhibitory interneurons in the PFC leading to disinhibition of pyramidal neurons and increased extracellular glutamate that promotes the rapid antidepressant responses to these agents. Collectively, these studies indicate that specific NMDA and mACh receptor subtypes on GABAergic interneurons are promising targets for novel rapid-acting antidepressant therapies.

Spatial and temporal boundaries of NMDA receptor hypofunction leading to schizophrenia

The N-methyl-d-aspartate receptor hypofunction is one of the most prevalent models of schizophrenia. For example, healthy subjects treated with uncompetitive N-methyl-d-aspartate receptor antagonists elicit positive, negative, and cognitive-like symptoms of schizophrenia. Patients with anti-N-methyl-d-aspartate receptor encephalitis, which is likely caused by autoantibody-mediated down-regulation of cell surface N-methyl-d-aspartate receptors, often experience psychiatric symptoms similar to schizophrenia initially. However, where and when N-methyl-d-aspartate receptor hypofunction occurs in the brain of schizophrenic patients is poorly understood. Here we review the findings from N-methyl-d-aspartate receptor antagonist and autoantibody models, postmortem studies on N-methyl-d-aspartate receptor subunits, as well as the global and cell-type-specific knockout mouse models of subunit GluN1. We compare various conditional GluN1 knockout mouse strains, focusing on the onset of N-methyl-d-aspartate receptor deletion and on the cortical cell-types. Based on these results, we hypothesize that N-methyl-d-aspartate receptor hypofunction initially occurs in cortical GABAergic neurons during early postnatal development. The resulting GABA neuron maturation deficit may cause reduction of intrinsic excitability and GABA release, leading to disinhibition of pyramidal neurons. The cortical disinhibition in turn could elicit glutamate spillover and subsequent homeostatic down regulation of N-methyl-d-aspartate receptor function in pyramidal neurons in prodromal stage. These two temporally-distinct N-methyl-d-aspartate receptor hypofunctions may be complimentary, as neither alone may not be able to fully explain the entire schizophrenia pathophysiology. Potential underlying mechanisms for N-methyl-d-aspartate receptor hypofunction in cortical GABA neurons are also discussed, based on studies of naturally-occurring N-methyl-d-aspartate receptor antagonists, neuregulin/ErbB4 signaling pathway, and theoretical analysis of excitatory/inhibitory balance.

Effects of Hallucinogens on Neuronal Activity

Hallucinogens evoke sensory, perceptual, affective, and cognitive effects that may be useful to understand the neurobiological basis of mood and psychotic disorders. The present chapter reviews preclinical research carried out in recent years in order to better understand the action of psychotomimetic agents such as the noncompetitive NMDA receptor (NMDA-R) antagonists and serotonergic hallucinogens. Our studies have focused on the mechanisms through which these agents alter cortical activity. Noncompetitive NMDA-R antagonists, such as phencyclidine (PCP) and MK-801 (dizocilpine), as well as the serotonergic hallucinogens DOI and 5-MeO-DMT, produce similar effects on cellular and population activity in prefrontal cortex (PFC); these effects include alterations of pyramidal neuron discharge (with an overall increase in firing), as well as a marked attenuation of the low frequency oscillations (0.2-4 Hz) to which neuronal discharge is coupled in anesthetized rodents. PCP increases c-fos expression in excitatory neurons from various cortical and subcortical areas, particularly the thalamus. This effect of PCP involves the preferential blockade of NMDA-R on GABAergic neurons of the reticular nucleus of the thalamus, which provides feedforward inhibition to the rest of thalamic nuclei. It is still unknown whether serotonergic hallucinogens also affect thalamocortical networks. However, when examined, similar alterations in other cortical areas, such as the primary visual cortex (V1), have been observed, suggesting that these agents affect cortical activity in sensory and associative areas. Interestingly, the disruption of PFC activity induced by PCP, DOI and 5-MeO-DMT is reversed by classical and atypical antipsychotic drugs. This effect suggests a possible link between the mechanisms underlying the disruption of perception by multiple classes of hallucinogenic agents and the therapeutic efficacy of antipsychotic agents.

Prefrontal Cortex-Mediated Impairments in a Genetic Model of NMDA Receptor Hypofunction Are Reversed by the Novel M1 PAM VU6004256

Abnormalities in the signaling of the N-methyl-d-aspartate subtype of the glutamate receptor (NMDAR) within cortical and limbic brain regions are thought to underlie many of the complex cognitive and affective symptoms observed in individuals with schizophrenia. The M₁ muscarinic acetylcholine receptor (mAChR) subtype is a closely coupled signaling partner of the NMDAR. Accumulating evidence suggests that development of selective positive allosteric modulators (PAMs) of the M₁ receptor represent an important treatment strategy for the potential normalization of disruptions in NMDAR signaling in patients with schizophrenia. In the present studies, we evaluated the effects of the novel and highly potent M₁ PAM, VU6004256, in ameliorating selective prefrontal cortical (PFC)-mediated physiologic and cognitive abnormalities in a genetic mouse model of global reduction in the NR1 subunit of the NMDAR (NR1 knockdown [KD]). Using slice-based extracellular field potential recordings, deficits in muscarinic agonist-induced long-term depression (LTD) in layer V of the PFC in the NR1 KD mice were normalized with bath application of VU6004256. Systemic administration of VU6004256 also reduced excessive pyramidal neuron firing in layer V PFC neurons in awake, freely moving NR1 KD mice. Moreover, selective potentiation of M₁ by VU6004256 reversed the performance impairments of NR1 KD mice observed in two preclinical models of PFC-mediated learning, specifically the novel object recognition and cue-mediated fear conditioning tasks. VU6004256 also produced a robust, dose-dependent reduction in the hyperlocomotor activity of NR1 KD mice. Taken together, the current findings provide further support for M₁ PAMs as a novel therapeutic approach for the PFC-mediated impairments in schizophrenia.

Postnatal Administration of Dizocilpine Inhibits Neuronal Excitability in PFC and Induces Social Deficits Detected by MiceProfiler

Schizophrenia is a devastating mental disease with social deficit as its core component of negative symptoms, which could be induced in rodents by dizocilpine (MK-801), a noncompetitive NMDA receptor antagonist. NMDA receptors are highly expressed during the postnatal period. However, less attention has been paid to the effects of postnatal MK-801 administration on social interaction. In this study, we evaluated the effects of postnatal administration of MK-801 on social interaction and explored the possible mechanisms. Postnatal day-7 mice were intraperitoneally injected with MK-801 twice daily for 5 days, and their social interaction repertoire was monitored by a computerized video in the 10th week. The contact event, relative position event, stop-state, and dynamic event were analyzed with MiceProfiler automatic idTracker system. The results showed that MK-801 reduced the number of the contact events, relative position events, and stop-states, while increased the number and duration of dynamic events. These changes implied that MK-801-injected mice had indifference and lower motivation in social interaction and could be a useful model for studies on the social deficit of schizophrenia. The prefrontal cortex is the key region for social interaction behaviors. Slice patch clamp was performed to analyze the cellular excitability of prefrontal cortical neurons after postnatal treatment with MK-801 in mice. The results demonstrated that MK-801 injection reduced the frequency and amplitude of action potentials, but increased the frequency of miniature inhibitory postsynaptic currents. These data illustrated that the excitability of neurons in the prefrontal cortex was inhibited. Finally, immunoblotting data demonstrated that MK-801 significantly decreased the levels of sirtuin 1 (SIRT1) and phosphorylated protein kinase B (p-PKB) in the prefrontal cortex (both P < 0.05). Taken together, our results indicated that administration of MK-801 to postnatal mice induces social interaction deficits possibly due to inhibiting the neuronal excitability and decreasing the levels of SIRT1 and p-PKB in the prefrontal cortex.

Activation of a ventral hippocampus-medial prefrontal cortex pathway is both necessary and sufficient for an antidepressant response to ketamine

A single sub-anesthetic dose of ketamine exerts rapid and sustained antidepressant effects. Here, we examined the role of the ventral hippocampus (vHipp)-medial prefrontal cortex (mPFC) pathway in ketamine's antidepressant response. Inactivation of the vHipp with lidocaine prevented the sustained, but not acute, antidepressant-like effect of ketamine as measured by the forced swim test (FST). Moreover, optogenetic as well as pharmacogenetic specific activation of the vHipp-mPFC pathway using DREADDs (designer receptors exclusively activated by designer drugs) mimicked the antidepressant-like response to ketamine; importantly, this was pathway specific, in that activation of a vHipp to nucleus accumbens circuit did not do this. Furthermore, optogenetic inactivation of the vHipp/mPFC pathway at the time of FST completely reversed ketamine's antidepressant response. In addition, we found that a transient increase in TrkB receptor phosphorylation in the vHipp contributes to ketamine's sustained antidepressant response. These data demonstrate that activity in the vHipp-mPFC pathway is both necessary and sufficient for the antidepressant-like effect of ketamine.

Acute phencyclidine administration induces c-Fos-immunoreactivity in interneurons in cortical and subcortical regions

Dysfunction of N-Methyl-d-aspartate receptors (NMDARs) is believed to underlie some of the symptoms in schizophrenia, and non-competitive NMDAR antagonists (including phencyclidine (PCP)) are widely used as pharmacological schizophrenia models. Furthermore, mounting evidence suggests that impaired γ-aminobutyric acid (GABA) neurotransmission contributes to the cognitive deficits in schizophrenia. Thus alterations in GABAergic interneurons have been observed in schizophrenia patients and animal models. Acute systemic administration of PCP increases levels of c-Fos in several cortical and subcortical areas, but whether such induction occurs in specific populations of GABAergic interneuron subtypes still remains to be established. We performed an immunohistochemical analysis of the PCP-induced c-Fos-immunoreactivity (IR) in parvalbumin (PV) and calbindin (CB) interneuron subtypes in the cortex and thalamus of rats. A single dose of PCP (10mg/kg, s.c.) significantly increased total number of c-Fos-IR in: (1) the prelimbic, infralimbic, anterior cingulate, ventrolateral orbital, motor, somatosensory and retrosplenial cortices as well as the nucleus accumbens (NAc), field CA1 of the hippocampus (CA1) field of hippocampus and mediodorsal thalamus (MD); (2) PV-IR cells in the ventrolateral orbitofrontal and retrosplenial cortices and CA1 field of hippocampus; and (3) CB-IR cells in the motor cortex. Overall, our data indicate that PCP activates a wide range of cortical and subcortical brain regions and that a substantial part of this activation is present in GABAergic interneurons in certain regions. This suggests that the psychotomimetic effect of PCP may be mediated via GABAergic interneurons.

Preclinical Magnetic Resonance Imaging and Spectroscopy Studies of Memory, Aging, and Cognitive Decline

Neuroimaging provides for non-invasive evaluation of brain structure and activity and has been employed to suggest possible mechanisms for cognitive aging in humans. However, these imaging procedures have limits in terms of defining cellular and molecular mechanisms. In contrast, investigations of cognitive aging in animal models have mostly utilized techniques that have offered insight on synaptic, cellular, genetic, and epigenetic mechanisms affecting memory. Studies employing magnetic resonance imaging and spectroscopy (MRI and MRS, respectively) in animal models have emerged as an integrative set of techniques bridging localized cellular/molecular phenomenon and broader in vivo neural network alterations. MRI methods are remarkably suited to longitudinal tracking of cognitive function over extended periods permitting examination of the trajectory of structural or activity related changes. Combined with molecular and electrophysiological tools to selectively drive activity within specific brain regions, recent studies have begun to unlock the meaning of fMRI signals in terms of the role of neural plasticity and types of neural activity that generate the signals. The techniques provide a unique opportunity to causally determine how memory-relevant synaptic activity is processed and how memories may be distributed or reconsolidated over time. The present review summarizes research employing animal MRI and MRS in the study of brain function, structure, and biochemistry, with a particular focus on age-related cognitive decline.

Potentiation of M1 Muscarinic Receptor Reverses Plasticity Deficits and Negative and Cognitive Symptoms in a Schizophrenia Mouse Model

Schizophrenia patients exhibit deficits in signaling of the M1 subtype of muscarinic acetylcholine receptor (mAChR) in the prefrontal cortex (PFC) and also display impaired cortical long-term depression (LTD). We report that selective activation of the M1 mAChR subtype induces LTD in PFC and that this response is completely lost after repeated administration of phencyclidine (PCP), a mouse model of schizophrenia. Furthermore, discovery of a novel, systemically active M1 positive allosteric modulator (PAM), VU0453595, allowed us to evaluate the impact of selective potentiation of M1 on induction of LTD and behavioral deficits in PCP-treated mice. Interestingly, VU0453595 fully restored impaired LTD as well as deficits in cognitive function and social interaction in these mice. These results provide critical new insights into synaptic changes that may contribute to behavioral deficits in this mouse model and support a role for selective M1 PAMs as a novel approach for the treatment of schizophrenia.

Contribution of N-methyl-D-aspartate receptors to attention and episodic spatial memory during senescence

A decrease in N-methyl-D-aspartate receptor (NMDAR) function is associated with age-related cognitive impairments. However, NMDAR antagonists are prescribed for cognitive decline associated with age-related neurodegenerative disease, raising questions as to the role of NMDAR activity in cognitive function during aging. The current studies examined effects of NMDAR blockade on cognitive task that are sensitive to aging. Young and middle-age rats were trained on the five-choice serial reaction time task (5-CSRTT) and challenged with MK-801 (0.025, 0.05, and 0.1mg/kg or vehicle). Attention deficits were apparent in middle-age and performance of young and middle-age rats was enhanced for low doses of MK-801 (0.025 and 0.05). The beneficial effects on attention were reversed by the highest dose of MK-801. Older animals exhibited a delay-dependent impairment of episodic spatial memory examined on a delayed-matching to place water maze task. Similarly, a low dose of MK-801 (0.05mg/kg) impaired performance with increasing delay and aged animals were more susceptible to disruption by NMDAR blockade. Despite MK-801 impairment of episodic spatial memory, MK-801 had minimal effects on spatial reference memory. Our results confirm that NMDARs contribute to rapidly acquired and flexible spatial memory and support the idea that a decline in NMDAR function contributes to the age-related impairments in cognition.

Serotonin Modulation of Prefronto-Hippocampal Rhythms in Health and Disease

There is mounting evidence that most cognitive functions depend upon the coordinated activity of neuronal networks often located far from each other in the brain. Ensembles of neurons synchronize their activity, generating oscillations at different frequencies that may encode behavior by allowing an efficient communication between brain areas. The serotonin system, by virtue of the widespread arborisation of serotonergic neurons, is in an excellent position to exert strong modulatory actions on brain rhythms. These include specific oscillatory activities in the prefrontal cortex and the hippocampus, two brain areas essential for many higher-order cognitive functions. Psychiatric patients show abnormal oscillatory activities in these areas, notably patients with schizophrenia who display psychotic symptoms as well as affective and cognitive impairments. Synchronization of neural activity between the prefrontal cortex and the hippocampus seems to be important for cognition and, in fact, reduced prefronto-hippocampal synchrony has been observed in a genetic mouse model of schizophrenia. Here, we review recent advances in the field of neuromodulation of brain rhythms by serotonin, focusing on the actions of serotonin in the prefrontal cortex and the hippocampus. Considering that the serotonergic system plays a crucial role in cognition and mood and is a target of many psychiatric treatments, it is surprising that this field of research is still in its infancy. In that regard, we point to future investigations that are much needed in this field.

The hippocampo-prefrontal pathway: a possible therapeutic target for negative and cognitive symptoms of schizophrenia

The hippocampo-prefrontal (H-PFC) pathway has been linked to cognitive and emotional disturbances in several psychiatric disorders including schizophrenia. Preclinical evidence from the NMDA receptor antagonism rodent model of schizophrenia shows severe pathology selective to the H-PFC pathway. It is speculated that there is an increased excitatory drive from the hippocampus to the prefrontal cortex due to dysfunctions in the H-PFC plasticity, which may serve as the basis for the behavioral consequences observed in this rodent model. Thus, the H-PFC pathway is currently emerging as a promising therapeutic target for the negative and cognitive symptom clusters of schizophrenia. Here, we have reviewed the physiological, pharmacological and functional characteristics of the H-PFC pathway and we propose that allosteric activation of glutamatergic and cholinergic neurotransmission can serve as a plausible therapeutic approach.

Cannabidiol attenuates sensorimotor gating disruption and molecular changes induced by chronic antagonism of NMDA receptors in mice

BACKGROUND

Preclinical and clinical data suggest that cannabidiol (CBD), a major non-psychotomimetic compound from Cannabis sativa, induces antipsychotic-like effects. However, the antipsychotic properties of repeated CBD treatment have been poorly investigated. Behavioral changes induced by repeated treatment with glutamate N-methyl-D-aspartate receptor (NMDAR) antagonists have been proposed as an animal model of schizophrenia-like signs. In the present study, we evaluated if repeated treatment with CBD would attenuate the behavioral and molecular modifications induced by chronic administration of one of these antagonists, MK-801.

METHODS

Male C57BL/6J mice received daily i.p. injections of MK-801 (0.1, 0.5, or 1mg/kg) for 14, 21, or 28 days. Twenty-four hours after the last injection, animals were submitted to the prepulse inhibition (PPI) test. After that, we investigated if repeated treatment with CBD (15, 30, and 60mg/kg) would attenuate the PPI impairment induced by chronic treatment with MK-801 (1mg/kg; 28 days). CBD treatment began on the 6th day after the start of MK-801 administration and continued until the end of the treatment. Immediately after the PPI, the mice brains were removed and processed to evaluate the molecular changes. We measured changes in FosB/ΔFosB and parvalbumin (PV) expression, a marker of neuronal activity and a calcium-binding protein expressed in a subclass of GABAergic interneurons, respectively. Changes in mRNA expression of the NMDAR GluN1 subunit gene (GRN1) were also evaluated. CBD effects were compared to those induced by the atypical antipsychotic clozapine.

RESULTS

MK-801 administration at the dose of 1mg/kg for 28 days impaired PPI responses. Chronic treatment with CBD (30 and 60mg/kg) attenuated PPI impairment. MK-801 treatment increased FosB/ΔFosB expression and decreased PV expression in the medial prefrontal cortex. A decreased mRNA level of GRN1 in the hippocampus was also observed. All the molecular changes were attenuated by CBD. CBD by itself did not induce any effect. Moreover, CBD effects were similar to those induced by repeated clozapine treatment.

CONCLUSIONS

These results indicate that repeated treatment with CBD, similar to clozapine, reverses the psychotomimetic-like effects and attenuates molecular changes observed after chronic administration of an NMDAR antagonist. These data support the view that CBD may have antipsychotic properties.

Reelin has a preventive effect on phencyclidine-induced cognitive and sensory-motor gating deficits

Reelin has recently attracted attention because of its connection to several neuropsychiatric diseases. We previously reported the finding that prior transplantation of GABAergic neuron precursor cells into the medial prefrontal cortex (mPFC) of mice significantly prevented the induction of cognitive and sensory-motor gating deficits induced by phencyclidine (PCP). The majority of the precursor cells transplanted into the mPFC of the recipient mice differentiated into members of a somatostatin/Reelin-expressing class of GABAergic interneurons. These findings raised the possibility that Reelin secreted by the transplanted cells plays an important role in preventing the deficits induced by PCP. In this study, we investigated whether Reelin itself has a preventive effect on PCP-induced behavioral phenotypes by injecting conditioned medium containing Reelin into the lateral ventricle of the brains of 6- to 7-week-old male mice before administrating PCP. Behavioral analyses showed that the prior Reelin injection had a preventive effect against induction of the cognitive and sensory-motor gating deficits associated with PCP. Moreover, one of the types of Reelin receptor was found to be expressed by neurons in the mPFC. The results of this study point to the Reelin signaling pathway as a candidate target for the pharmacologic treatment of neuropsychiatric diseases.

Analysis of central mechanism of cognitive training on cognitive impairment after stroke: Resting-state functional magnetic resonance imaging study

OBJECTIVE

To investigate the central mechanism of cognitive training in patients with stroke, using resting state (RS) functional magnetic resonance imaging (fMRI).

METHODS

Patients with stroke and executive function and memory deficit were randomized to receive computer-assisted cognitive training (treatment group; total 60 h training over 10 weeks) or no training (control group). All participants received neuropsychological assessment and RS fMRI at baseline and 10 weeks.

RESULTS

Patients in the treatment group (n = 16) showed increased functional connectivity (FC) of the hippocampus with the frontal lobe (right inferior, right middle, left middle, left inferior and left superior frontal gyrus) and left parietal lobe at 10 weeks compared with baseline. Patients in the control group (n = 18) showed decreased FC of the left hippocampus-right occipital gyrus, and right hippocampus-right posterior lobe of cerebellum and left superior temporal gyrus. Significant correlations were found between improved neuropsychological scores and increased FC of the hippocampus with the frontal lobe and left parietal lobe in the treatment group only.

CONCLUSIONS

Increased RS FC of the hippocampus with the frontal and parietal lobes may be an important mechanism of cognitive recovery after stroke.

Disruption of thalamocortical activity in schizophrenia models: relevance to antipsychotic drug action

Non-competitive NMDA receptor antagonists are widely used as pharmacological models of schizophrenia due to their ability to evoke the symptoms of the illness. Likewise, serotonergic hallucinogens, acting on 5-HT(2A) receptors, induce perceptual and behavioural alterations possibly related to psychotic symptoms. The neurobiological basis of these alterations is not fully elucidated. Data obtained in recent years revealed that the NMDA receptor antagonist phencyclidine (PCP) and the serotonergic hallucinogen 1-(2,5-dimethoxy-4-iodophenyl-2-aminopropane; DOI) produce a series of common actions in rodent prefrontal cortex (PFC) that may underlie psychotomimetic effects. Hence, both agents markedly disrupt PFC function by altering pyramidal neuron discharge (with an overall increase) and reducing the power of low frequency cortical oscillations (LFCO; < 4 Hz). In parallel, PCP increased c-fos expression in excitatory neurons of various cortical areas, the thalamus and other subcortical structures, such as the amygdala. Electrophysiological studies revealed that PCP altered similarly the function of the centromedial and mediodorsal nuclei of the thalamus, reciprocally connected with PFC, suggesting that its psychotomimetic properties are mediated by an alteration of thalamocortical activity (the effect of DOI was not examined in the thalamus). Interestingly, the observed effects were prevented or reversed by the antipsychotic drugs clozapine and haloperidol, supporting that the disruption of PFC activity is intimately related to the psychotomimetic activity of these agents. Overall, the present experimental model can be successfully used to elucidate the neurobiological basis of schizophrenia symptoms and to examine the potential antipsychotic activity of new drugs in development.

The role of the hippocampo-prefrontal cortex system in phencyclidine-induced psychosis: a model for schizophrenia

Phencyclidine (PCP) is a psychotomimetic drug that induces schizophrenia-like symptoms in healthy individuals and exacerbates pre-existing symptoms in patients with schizophrenia. PCP also induces behavioral and cognitive abnormalities in non-human animals, and PCP-treated animals are considered a reliable pharmacological model of schizophrenia. However, the exact neural mechanisms by which PCP modulates behavior are not known. During the last decade several studies have indicated that disturbed activity of the prefrontal cortex (PFC) may be closely related to PCP-induced psychosis. Systemic administration of PCP produces long-lasting activation of medial PFC (mPFC) neurons in rats, almost in parallel with augmentation of locomotor activity and behavioral stereotypies. Later studies have showed that such PCP-induced behavioral abnormalities are ameliorated by prior administration of drugs that normalize or inhibit excess excitability of PFC neurons. Similar activation of mPFC neurons is not induced by systemic injection of a typical psychostimulant such as methamphetamine, even though behavioral hyperactivity is induced to almost the same level. This suggests that the neural circuits mediating PCP-induced psychosis are different to those mediating methamphetamine-induced psychosis. Locally applied PCP does not induce excitation of mPFC neurons, indicating that PCP-induced tonic excitation of mPFC neurons is mediated by inputs from regions outside the mPFC. This hypothesis is strongly supported by experimental results showing that local perfusion of PCP in the ventral hippocampus, which has dense fiber projections to the mPFC, induces tonic activation of mPFC neurons with accompanying augmentation of behavioral abnormalities. In this review we summarize current knowledge on the neural mechanisms underlying PCP-induced psychosis and highlight a possible involvement of the PFC and the hippocampus in PCP-induced psychosis.

Functional structure of the intermediate and ventral hippocampo-prefrontal pathway in the prefrontal convergent system

The hippocampo-prefrontal pathway is a unique projection that connects distant ends of the cerebral cortex. The direct hippocampo-prefrontal projection arises from the ventral to intermediate third of the hippocampus, but not from the dorsal third. It forms a funnel-shaped structure that collects information from the large hippocampal area and projects it to the prefrontal cortex. The anatomical regional differentiation of the projection has not been described. The hippocampal region is differentiated into structural and behavioural roles. For example, it has been shown that the ventral, but not the dorsal, hippocampus reciprocally connects with the amygdala and influences emotional behaviours. These data imply that hippocampal variation along the dorso-ventral axis is contained within the hippocampo-prefrontal pathway. Here, we present electrophysiological studies that demonstrate regional differences in short- but not long-term plasticity in the intermediate/posterior-dorsal and ventral routes of the hippocampo-prefrontal pathway. Furthermore, behavioural studies revealed that each route appears to play a different role in working memory. These results suggest that hippocampal regional information is processed through different routes, with the integration of individual regulatory functions in the prefrontal convergent system.

The effect of non-competitive NMDA receptor antagonist MK-801 on neuronal activity in rodent prefrontal cortex: an animal model for cognitive symptoms of schizophrenia

Schizophrenia affects about 1% of the world population and is a major socio-economical problem in ours societies. Cognitive symptoms are particularly resistant to current treatments and are believed to be closely related to an altered function of prefrontal cortex (PFC). Particularly, abnormalities in the plasticity processes in the PFC are a candidate mechanism underlying cognitive symptoms, and the recent evidences in patients are in line with this hypothesis. Animal pharmacological models of cognitive symptoms, notably with non-competitive NMDA receptor antagonists such as MK-801, are commonly used to investigate the underlying cellular and molecular mechanisms of schizophrenia. However, it is still unknown whether in these animal models, impairments in plasticity of PFC neurons are present. In this article, we briefly summarize the current knowledge on the effect of non-competitive NMDA receptor antagonist MK-801 on medial PFC (mPFC) neuronal activity and then introduce a form of plasticity found after acute exposure to MK-801, which was accompanied by cognitive deficits. These observations suggest a potential correlation between cognitive deficits and the aberrant plasticity in the mPFC in the animal model of schizophrenia.

Neonatal administration of phencyclidine decreases the number of putative inhibitory interneurons and increases neural excitability to auditory paired clicks in the hippocampal CA3 region of freely moving adult mice

Animals exposed to phencyclidine (PCP) during the neonatal period have fewer GABAergic interneurons in the corticolimbic area, including the hippocampus, and exhibit abnormal behaviors after attaining maturation that correspond with schizophrenic symptoms. Since a lack of inhibitory interneurons in the hippocampus has also been reported in postmortem studies of patients with schizophrenia, the deficit may induce abnormal activity of hippocampal neurons that underlies pathological states in schizophrenia. However, it remains unclear how PCP treatment during the neonatal period affects the discharge activity of hippocampal neurons in adulthood. In the current study, single unit responses of hippocampal CA3 neurons to paired auditory clicks were recorded in freely moving mice repeatedly injected with PCP or saline during the neonatal period. The recorded neurons were classified into two subpopulations, narrow-spike neurons and broad-spike neurons, based on the spike width. The spontaneous discharge rate was higher in the narrow-spike neurons than in the broad-spike neurons, indicating that the narrow-spike neurons correspond with hippocampal inhibitory neurons. The proportion of narrow-spike neurons was significantly smaller in neonatally PCP-treated mice than in saline-treated mice. The broad-spike neurons that exhibited a response magnitude to the second click as large as that to the first click (E/E-type response) showed longer response duration to the paired clicks in PCP-treated mice than in the saline-treated mice. Further, the number of neurons with E/E-type response was higher in the PCP-treated mice than in the saline-treated mice. Finally, the attenuation of an auditory-evoked potential component, N40, to the second click (sensory gating) was blunted in the PCP-treated mice when compared with that in the saline-treated mice. These results suggest that the neonatal administration of PCP induced a deficit of inhibitory interneurons and altered discharge activity of neurons in the hippocampal CA3 region to the paired clicks, thereby inducing the deficit in sensory gating.

Importance of inter-hemispheric prefrontal connection in the effects of non-competitive NMDA receptor antagonists

Previous studies have shown that systemic, but not unilateral intra-prefrontal cortex administration of non-competitive NMDA antagonists, increased prefrontal activity, the cortical efflux of serotonin, and induced stereotypies. In this work we used in-vivo microdialysis and immunohistochemistry to test the hypothesis as to whether MK-801 and ketamine need to act on both prefrontal cortices to reproduce these neurochemical and behavioural changes. Dialysis probes were implanted in the medial prefrontal cortex, and extracellular serotonin as well as behavioural stereotypies was measured after systemic administration of MK-801 and ketamine (1 mg/kg and 25 mg/kg, respectively), and unilateral and bilateral perfusion of both drugs (300 μm and 3 mm, respectively). Additionally, the prefrontal (glutamatergic) level of activity was measured using c-Fos immunohistochemistry. Systemic and bilateral (but not unilateral) prefrontal administration of MK-801 and ketamine increased serotonin efflux whereas only systemic administration of both drugs produced hyperlocomotion and stereotypies. The unilateral perfusion of 1 μm tetrodotoxin in the medial prefrontal cortex reduced increases of serotonin in both hemispheres, the expression of c-Fos in the contralateral side, and stereotypy scores after systemic NMDA antagonists. Our results support the hypothesis that a bilateral impairment of cortical inhibition in the medial prefrontal cortex is needed for non-competitive NMDA antagonists to induce the state of pyramidal cell hyperactivity and concurrent efflux of serotonin. Furthermore, hyperlocomotion and stereotypies produced by MK-801 and ketamine do not appear to result from changes in the activity of prefrontal cortex although this structure exerts some control over these behaviours.

PDE4 inhibition enhances hippocampal synaptic plasticity in vivo and rescues MK801-induced impairment of long-term potentiation and object recognition memory in an animal model of psychosis

Inhibition of phosphodiesterase type 4 (PDE4) by rolipram (4-(3-(cyclopentyloxy)-4-methoxyphenyl)-pyrrolidin-2-one) has been the focus of many behavioral and molecular studies in the recent years. Rolipram exhibits memory-enhancing effects in rodents. In vitro studies have shown that long-term potentiation (LTP), which may comprise a cellular substrate for learning, is also enhanced by rolipram. However, effects have not been assessed in vivo. Rolipram has antipsychotic properties. Psychosis affects cognition and in animal models of psychosis LTP is impaired. In this study, we investigated if PDE4 inhibition improves LTP in healthy animals in vivo and if PDE4 inhibition rescues impaired LTP and prevents object recognition memory deficits in an animal model of psychosis. Recordings were made from the hippocampus of adult, freely behaving Wistar rats. Thirty minutes after treatment with rolipram or vehicle, a tetanus was applied to the medial perforant path to elicit short-term potentiation (STP) in the dentate gyrus. At this time-point, radioimmunoassay revealed that rolipram significantly elevated cyclic adenosine monophosphate levels in the dorsal hippocampus, in line with reports by others that rolipram mediates decreased PDE4 activity. In healthy animals, both intracerebroventricular and subcutaneous treatment with rolipram facilitated STP into LTP, suggesting that PDE4 inhibition may have a permissive role in plasticity mechanisms that are relevant for learning and memory. One week after a single systemic treatment with the irreversible N-methyl-D-aspartate antagonist, MK801, LTP and object recognition memory were significantly impaired, but could be rescued by PDE4 inhibition. These data suggest that the relief of cognitive disturbances in psychosis models by rolipram may be mediated in part by a rescue of hippocampal LTP.

Glutamatergic synaptic dysregulation in schizophrenia: therapeutic implications

Schizophrenia affects approximately 1% of the population and continues to be associated with poor outcome because of the limited efficacy of and noncompliance with existing antipsychotic medications. An alternative hypothesis invoking the excitatory neurotransmitter, glutamate, arose out of clinical observations that NMDA receptor antagonists, the dissociative anesthetics like ketamine, can replicate in normal individuals the full range of symptoms of schizophrenia including psychosis, negative symptoms, and cognitive impairments. Low dose ketamine can also re-create a number of physiologic abnormalities characteristic of schizophrenia. Postmortem studies have revealed abnormalities in endogenous modulators of NMDA receptors in schizophrenia as well as components of a postsynaptic density where NMDA receptors are localized. Gene association studies have revealed several genes that affect NMDA receptor function whose allelic variants are associated with increased risk for schizophrenia including genes encoding D-amino acid oxidase, its modulator G72, dysbindin, and neuregulin. The parvalbumin-positive, fast-firing GABAergic interneurons that provide recurrent inhibition to cortical-limbic pyramidal neurons seem to be most sensitive to NMDA receptor hypofunction. As a consequence, disinhibition of glutamatergic efferents disrupts cortical processing, causing cognitive impairments and negative symptoms, and drives subcortical dopamine release, resulting in psychosis. Drugs designed to correct the cortical-limbic dysregulated glutamatergic neurotransmission show promise for reducing negative and cognitive symptoms of schizophrenia as well as its positive symptoms.

Expression of parvalbumin and glutamic acid decarboxylase-67 after acute administration of MK-801. Implications for the NMDA hypofunction model of schizophrenia

RATIONALE

A reduction of GABAergic markers in postmortem tissue is consistently found in schizophrenia. This is generally mediated by a decreased expression of the calcium-binding protein, parvalbumin (PV), and the 67-kDa isoform of the GABA synthesizing enzyme glutamic acid decarboxylase (GAD(67)). Similar reductions of PV or GAD(67) are observed after repeated exposure to N-methyl-D-aspartate (NMDA) receptor antagonists but less attention has been paid to what occurs after their acute administration.

OBJECTIVES

Here, we have used in situ hybridization to examine the expression of PV and GAD(67) mRNAs at 4 h and 24 h after an acute administration of MK-801 (1 mg/kg).

RESULTS

Four hours after MK-801, the expression of PV mRNA decreased only in dentate gyrus of the hippocampus. Twenty four hours after this treatment, a reduction of the levels of PV mRNA was found in the medial prefrontal, orbitofrontal and entorhinal cortices, hippocampus and the basolateral nucleus of the amygdala. In contrast, no changes in the expression of GAD(67) were observed in any of the brain regions examined. Interestingly, the reduction in PV mRNA expression is observed in discrete corticolimbic subregions that have been implicated in schizophrenia, which is coincident with changes observed in postmortem tissue of schizophrenia brain.

CONCLUSIONS

These findings indicate that acute administration of a NMDA antagonist delineate a pattern of changes in GABAergic markers different from those observed in postmortem tissue in schizophrenia inasmuch as only deficits in parvalbumin (but not GAD(67)) were seen.

Activation of thalamocortical networks by the N-methyl-D-aspartate receptor antagonist phencyclidine: reversal by clozapine

BACKGROUND

Noncompetitive N-methyl-D-aspartate receptor antagonists are widely used as pharmacological models of schizophrenia. Their neurobiological actions are still poorly understood, although the prefrontal cortex (PFC) appears as a key target area.

METHODS

We examined the effect of phencyclidine (PCP) on neuronal activity of the mediodorsal (MD) and centromedial (CM) thalamic nuclei, reciprocally connected with the PFC, using extracellular recordings (n = 50 neurons from 35 Wistar rats) and c-fos expression.

RESULTS

Phencyclidine (.25 mg/kg intravenous [IV]) markedly disorganized the activity of MD/CM neurons, increasing (424%) and decreasing (41%) the activity of 57% and 20% of the recorded neurons, respectively (23% remained unaffected). Phencyclidine reduced delta oscillations (.15-4 Hz) as assessed by recording local field potentials. The subsequent clozapine administration (1 mg/kg IV) reversed PCP effects on neuronal discharge and delta oscillations. Double in situ hybridization experiments revealed that PCP (10 mg/kg intraperitoneal [IP]) markedly increased c-fos expression in glutamatergic neurons of several cortical areas (prefrontal, somatosensory, retrosplenial, entorhinal) and in thalamic nuclei, including MD/CM. Phencyclidine also increased c-fos expression in the amygdala; yet, it had a small effect in the hippocampus. Phencyclidine did not increase c-fos expression in gamma-aminobutyric acidergic cells except in hippocampus, amygdala, somatosensory, and retrosplenial cortices. Clozapine (5 mg/kg IP) had no effect by itself but significantly prevented PCP-induced c-fos expression.

CONCLUSIONS

Phencyclidine likely exerts its psychotomimetic action by increasing excitatory neurotransmission in thalamo-cortico-thalamic networks involving, among others, PFC, retrosplenial, and somatosensory cortices. The antipsychotic action of clozapine includes, among other actions, an attenuation of the neuronal hyperactivity in thalamocortical networks.