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

Lu AF35700 reverses the phencyclidine-induced disruption of thalamo-cortical activity by blocking dopamine D1 and D2 receptors

Antipsychotic drugs of different chemical/pharmacological families show preferential dopamine (DA) D₂ receptor (D₂-R) vs. D₁ receptor (D₁-R) affinity, with the exception of clozapine, the gold standard of schizophrenia treatment, which shows a comparable affinity for both DA receptors. Here, we examined the ability of Lu AF35700 (preferential D₁-R>D₂-R antagonist), to reverse the alterations in thalamo-cortical activity induced by phencyclidine (PCP), used as a pharmacological model of schizophrenia. Lu AF35700 reversed the PCP-induced alteration of neuronal discharge and low frequency oscillation (LFO, 0.15-4 Hz) in thalamo-cortical networks. Likewise, Lu AF35700 prevented the increased c-fos mRNA expression induced by PCP in thalamo-cortical regions of awake rats. We next examined the contribution of D₁-R and D₂-R to the antipsychotic reversal of PCP effects. The D₂-R antagonist haloperidol reversed PCP effects on thalamic discharge rate and LFO. Remarkably, the combination of sub-effective doses of haloperidol and SCH-23390 (DA D₁-R antagonist) fully reversed the PCP-induced fall in thalamo-cortical LFO. However, unlike with haloperidol, SCH-23390 elicited different degrees of potentiation of the effects of low clozapine and Lu AF35700 doses. Overall, the present data support a synergistic interaction between both DA receptors to reverse the PCP-induced alterations of oscillatory activity in thalamo-cortical networks, possibly due to their simultaneous blockade in direct and indirect pathways of basal ganglia. The mild potentiation induced by SCH-23390 in the case of clozapine and Lu AF35700 suggests that, at effective doses, these agents reverse PCP effects through the simultaneous blockade of both DA receptors.

Altered functional connectivity in psychotic disorder not otherwise specified

BACKGROUND

Few studies have investigated functional connectivity (FC) in patients with psychotic disorder not otherwise specified (PNOS). We sought to identify distinct FC differentiating PNOS from schizophrenia (SZ).

METHODS

In total, 49 patients with PNOS, 42 with SZ, and 55 healthy controls (HC) matched for age, sex, and education underwent functional magnetic resonance imaging (fMRI) brain scans and clinical evaluation. Using six functional networks consisting of 40 regions of interest (ROIs), we conducted ROI to ROI and intra- and inter-network FC analyses using resting-state fMRI (rs-fMRI) data. Correlations of altered FC with symptomatology were explored.

RESULTS

We found common brain connectomics in PNOS and SZ including thalamo-cortical (especially superior temporal gyrus) hyperconnectivity, thalamo-cerebellar hypoconnectivity, and reduced within-thalamic connectivity compared to HC. Additionally, features differentiating the two patient groups included hyperconnectivity between the thalamic subregion and anterior cingulate cortex in PNOS compared to SZ and hyperconnectivity of the thalamic subregions with the posterior cingulate cortex and precentral gyrus in SZ compared to PNOS.

CONCLUSIONS

These findings suggest that PNOS and SZ exhibit both common and differentiating changes in neuronal connectivity. Furthermore, they may support the hypothesis that PNOS should be treated as a separate clinical syndrome with distinct neural connectomics.

Dual 5-HT3 and 5-HT6 Receptor Antagonist FPPQ Normalizes Phencyclidine-Induced Disruption of Brain Oscillatory Activity in Rats

Schizophrenia is a severe mental disorder featuring psychotic, depressive, and cognitive alterations. Current antipsychotic drugs preferentially target dopamine D2-R and/or serotonergic 5-HT2A/1A-R. They partly alleviate psychotic symptoms but fail to treat negative symptoms and cognitive deficits. Here we report on the putative antipsychotic activity of (1-[(3-fluorophenyl)sulfonyl]-4-(piperazin-1-yl)-1H-pyrrolo[3,2-c]quinoline dihydrochloride) (FPPQ), a dual serotonin 5-HT3-R/5-HT6-R antagonist endowed with pro-cognitive properties. FPPQ fully reversed phencyclidine-induced decrease of low-frequency oscillations in the medial prefrontal cortex of anaesthetized rats, a fingerprint of antipsychotic activity. This effect was mimicked by the combined administration of the 5-HT3-R and 5-HT6-R antagonists ondansetron and SB-399 885, respectively, but not by either drug alone. In freely moving rats, FPPQ countered phencyclidine-induced hyperlocomotion and augmentation of gamma and high-frequency oscillations in medial prefrontal cortex, dorsal hippocampus, and nucleus accumbens. Overall, this supports that simultaneous blockade of 5-HT3R and 5-HT6-R-like that induced by FPPQ-can be a new target in antipsychotic drug development.

Discrimination of motor and sensorimotor effects of phencyclidine and MK-801: Involvement of GluN2C-containing NMDA receptors in psychosis-like models

Non-competitive NMDA receptor (NMDA-R) antagonists like ketamine, phencyclidine (PCP) and MK-801 are routinely used as pharmacological models of schizophrenia. However, the NMDA-R subtypes, neuronal types (e.g., GABA vs. glutamatergic neurons) and brain regions involved in psychotomimetic actions are not fully understood. PCP activates thalamo-cortical circuits after NMDA-R blockade in reticular thalamic GABAergic neurons. GluN2C subunits are densely expressed in thalamus and cerebellum. Therefore, we examined their involvement in the behavioral and functional effects elicited by PCP and MK-801 using GluN2C knockout (GluN2CKO) and wild-type mice, under the working hypothesis that psychotomimetic effects should be attenuated in mutant mice. PCP and MK-801 induced a disorganized and meandered hyperlocomotion in both genotypes. Interestingly, stereotyped behaviors like circling/rotation, rearings and ataxia signs were dramatically reduced in GluN2CKO mice, indicating a better motor coordination in absence of GluN2C subunits. In contrast, other motor or sensorimotor (pre-pulse inhibition of the startle response) aspects of the behavioral syndrome remained unaltered by GluN2C deletion. PCP and MK-801 evoked a general pattern of c-fos activation in mouse brain (including thalamo-cortical networks) but not in the cerebellum, where they markedly reduced c-fos expression, with significant genotype differences paralleling those in motor coordination. Finally, resting-state fMRI showed an enhanced cortico-thalamic-cerebellar connectivity in GluN2CKO mice, less affected by MK-801 than controls. Hence, the GluN2C subunit allows the dissection of the behavioral alterations induced by PCP and MK-801, showing that some motor effects (in particular, motor incoordination), but not deficits in sensorimotor gating, likely depend on GluN2C-containing NMDA-R blockade in cerebellar circuits.

Interneuron Heterotopia in the Lis1 Mutant Mouse Cortex Underlies a Structural and Functional Schizophrenia-Like Phenotype

LIS1 is one of the principal genes related to Type I lissencephaly, a severe human brain malformation characterized by an abnormal neuronal migration in the cortex during embryonic development. This is clinically associated with epilepsy and cerebral palsy in severe cases, as well as a predisposition to developing mental disorders, in cases with a mild phenotype. Although genetic variations in the LIS1 gene have been associated with the development of schizophrenia, little is known about the underlying neurobiological mechanisms. We have studied how the Lis1 gene might cause deficits associated with the pathophysiology of schizophrenia using the Lis1/sLis1 murine model, which involves the deletion of the first coding exon of the Lis1 gene. Homozygous mice are not viable, but heterozygous animals present abnormal neuronal morphology, cortical dysplasia, and enhanced cortical excitability. We have observed reduced number of cells expressing GABA-synthesizing enzyme glutamic acid decarboxylase 67 (GAD67) in the hippocampus and the anterior cingulate area, as well as fewer parvalbumin-expressing cells in the anterior cingulate cortex in Lis1/sLis1 mutants compared to control mice. The cFOS protein expression (indicative of neuronal activity) in Lis1/sLis1 mice was higher in the medial prefrontal (mPFC), perirhinal (PERI), entorhinal (ENT), ectorhinal (ECT) cortices, and hippocampus compared to control mice. Our results suggest that deleting the first coding exon of the Lis1 gene might cause cortical anomalies associated with the pathophysiology of schizophrenia.

Lysergic acid diethylamide differentially modulates the reticular thalamus, mediodorsal thalamus, and infralimbic prefrontal cortex: An in vivo electrophysiology study in male mice

BACKGROUND

The reticular thalamus gates thalamocortical information flow via finely tuned inhibition of thalamocortical cells in the mediodorsal thalamus. Brain imaging studies in humans show that the psychedelic lysergic acid diethylamide (LSD) modulates activity and connectivity within the cortico-striato-thalamo-cortical (CSTC) circuit, altering consciousness. However, the electrophysiological effects of LSD on the neurons in these brain areas remain elusive.

METHODS

We employed in vivo extracellular single-unit recordings in anesthetized adult male mice to investigate the dose-response effects of cumulative LSD doses (5-160 µg/kg, intraperitoneal) upon reticular thalamus GABAergic neurons, thalamocortical relay neurons of the mediodorsal thalamus, and pyramidal neurons of the infralimbic prefrontal cortex.

RESULTS

LSD decreased spontaneous firing and burst-firing activity in 50% of the recorded reticular thalamus neurons in a dose-response fashion starting at 10 µg/kg. Another population of neurons (50%) increased firing and burst-firing activity starting at 40 µg/kg. This modulation was accompanied by an increase in firing and burst-firing activity of thalamocortical neurons in the mediodorsal thalamus. On the contrary, LSD excited infralimbic prefrontal cortex pyramidal neurons only at the highest dose tested (160 µg/kg). The dopamine D2 receptor (D₂) antagonist haloperidol administered after LSD increased burst-firing activity in the reticular thalamus neurons inhibited by LSD, decreased firing and burst-firing activity in the mediodorsal thalamus, and showed a trend towards further increasing the firing activity of neurons of the infralimbic prefrontal cortex.

CONCLUSION

LSD modulates firing and burst-firing activity of reticular thalamus neurons and disinhibits mediodorsal thalamus relay neurons at least partially in a D₂-mediated fashion. These effects of LSD on thalamocortical gating could explain its consciousness-altering effects in humans.

Involvement of NMDA receptors containing the GluN2C subunit in the psychotomimetic and antidepressant-like effects of ketamine

Acute ketamine administration evokes rapid and sustained antidepressant effects in treatment-resistant patients. However, ketamine also produces transient perceptual disturbances similarly to those evoked by other non-competitive NMDA-R antagonists like phencyclidine (PCP). Although the brain networks involved in both ketamine actions are not fully understood, PCP and ketamine activate thalamo-cortical networks after NMDA-R blockade in GABAergic neurons of the reticular thalamic nucleus (RtN). Given the involvement of thalamo-cortical networks in processing sensory information, these networks may underlie psychotomimetic action. Since the GluN2C subunit is densely expressed in the thalamus, including the RtN, we examined the dependence of psychotomimetic and antidepressant-like actions of ketamine on the presence of GluN2C subunits, using wild-type and GluN2C knockout (GluN2CKO) mice. Likewise, since few studies have investigated ketamine's effects in females, we used mice of both sexes. GluN2C deletion dramatically reduced stereotyped (circling) behavior induced by ketamine in male and female mice, while the antidepressant-like effect was fully preserved in both genotypes and sexes. Despite ketamine appeared to induce similar effects in both sexes, some neurobiological differences were observed between male and female mice regarding c-fos expression in thalamic nuclei and cerebellum, and glutamate surge in prefrontal cortex. In conclusion, the GluN2C subunit may discriminate between antidepressant-like and psychotomimetic actions of ketamine. Further, the abundant presence of GluN2C subunits in the cerebellum and the improved motor coordination of GluN2CKO mice after ketamine treatment suggest the involvement of cerebellar NMDA-Rs in some behavioral actions of ketamine.

Sleep spindle and slow wave abnormalities in schizophrenia and other psychotic disorders: Recent findings and future directions

Sleep spindles and slow waves are the two main oscillatory activities occurring during NREM sleep. Slow waves are ∼1 Hz, high amplitude, negative-positive deflections that are primarily generated and coordinated within the cortex, whereas sleep spindles are 12-16 Hz, waxing and waning oscillations that are initiated within the thalamus and regulated by thalamo-cortical circuits. In healthy subjects, these oscillations are thought to be responsible for the restorative aspects of sleep and have been increasingly shown to be involved in learning, memory and plasticity. Furthermore, deficits in sleep spindles and, to lesser extent, slow waves have been reported in both chronic schizophrenia (SCZ) and early course psychosis patients. In this article, we will first describe sleep spindle and slow wave characteristics, including their putative functional roles in the healthy brain. We will then review electrophysiological, genetic, and cognitive studies demonstrating spindle and slow wave impairments in SCZ and other psychotic disorders, with particularly emphasis on recent findings in early course patients. Finally, we will discuss how future work, including sleep studies in individuals at clinical high risk for psychosis, may help position spindles and slow waves as candidate biomarkers, as well as novel treatment targets, for SCZ and related psychotic disorders.

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.

Remission from antipsychotic treatment in first episode psychosis related to longitudinal changes in brain glutamate

Neuroimaging studies in schizophrenia have linked elevated glutamate metabolite levels to non-remission following antipsychotic treatment, and also indicate that antipsychotics can reduce glutamate metabolite levels. However, the relationship between symptomatic reduction and change in glutamate during initial antipsychotic treatment is unclear. Here we report proton magnetic resonance spectroscopy (1H-MRS) measurements of Glx and glutamate in the anterior cingulate cortex (ACC) and thalamus in patients with first episode psychosis (n = 23) at clinical presentation, and after 6 weeks and 9 months of treatment with antipsychotic medication. At 9 months, patients were classified into Remission (n = 12) and Non-Remission (n = 11) subgroups. Healthy volunteers (n = 15) were scanned at the same three time-points. In the thalamus, Glx varied over time according to remission status (P = 0.020). This reflected an increase in Glx between 6 weeks and 9 months in the Non-Remission subgroup that was not evident in the Remission subgroup (P = 0.031). In addition, the change in Glx in the thalamus over the 9 months of treatment was positively correlated with the change in the severity of Positive and Negative Syndrome Scale (PANSS) positive, total and general symptoms (P<0.05). There were no significant effects of group or time on glutamate metabolites in the ACC, and no differences between either patient subgroup and healthy volunteers. These data suggest that the nature of the response to antipsychotic medication may be related to the pattern of changes in glutamatergic metabolite levels over the course of treatment. Specifically, longitudinal reductions in thalamic Glx levels following antipsychotic treatment are associated with symptomatic improvement.

Acute ketamine dysregulates task-related gamma-band oscillations in thalamo-cortical circuits in schizophrenia

Hypofunction of the N-methyl-d-aspartate receptor (NMDAR) has been implicated as a possible mechanism underlying cognitive deficits and aberrant neuronal dynamics in schizophrenia. To test this hypothesis, we first administered a sub-anaesthetic dose of S-ketamine (0.006 mg/kg/min) or saline in a single-blind crossover design in 14 participants while magnetoencephalographic data were recorded during a visual task. In addition, magnetoencephalographic data were obtained in a sample of unmedicated first-episode psychosis patients (n = 10) and in patients with chronic schizophrenia (n = 16) to allow for comparisons of neuronal dynamics in clinical populations versus NMDAR hypofunctioning. Magnetoencephalographic data were analysed at source-level in the 1–90 Hz frequency range in occipital and thalamic regions of interest. In addition, directed functional connectivity analysis was performed using Granger causality and feedback and feedforward activity was investigated using a directed asymmetry index. Psychopathology was assessed with the Positive and Negative Syndrome Scale. Acute ketamine administration in healthy volunteers led to similar effects on cognition and psychopathology as observed in first-episode and chronic schizophrenia patients. However, the effects of ketamine on high-frequency oscillations and their connectivity profile were not consistent with these observations. Ketamine increased amplitude and frequency of gamma-power (63–80 Hz) in occipital regions and upregulated low frequency (5–28 Hz) activity. Moreover, ketamine disrupted feedforward and feedback signalling at high and low frequencies leading to hypo- and hyper-connectivity in thalamo-cortical networks. In contrast, first-episode and chronic schizophrenia patients showed a different pattern of magnetoencephalographic activity, characterized by decreased task-induced high-gamma band oscillations and predominantly increased feedforward/feedback-mediated Granger causality connectivity. Accordingly, the current data have implications for theories of cognitive dysfunctions and circuit impairments in the disorder, suggesting that acute NMDAR hypofunction does not recreate alterations in neural oscillations during visual processing observed in schizophrenia.

Topographic deficits in sleep spindle density and duration point to frontal thalamo-cortical dysfunctions in first-episode psychosis

Sleep spindles are NREM sleep EEG oscillations, which are initiated within the thalamus and are regulated by thalamo-cortical circuits. Previous work from our and other research groups has shown marked spindle deficits in patients with schizophrenia (SCZ). However, the presence of spindle impairments at illness onset, including which parameters are most affected, their topographic characteristics, and their relationships with clinical symptoms have yet to be characterized. In this study we performed sleep high density (hd)-EEG recordings in twenty-seven first-episode psychosis (FEP) patients and twenty-three healthy controls (HC). Several spindle parameters-amplitude, duration, and density-were calculated and compared across groups. FEP patients showed reduced spindle duration and density, but not in spindle amplitude relative to HC. These spindles reductions were localized in a frontal area and predicted the severity of FEP patients' negative symptoms. Altogether, these findings indicate that spindle deficits are present at the beginning of psychosis, contribute to clinical symptomatology, and point to frontal thalamo-cortical dysfunctions, thus providing a potential treatment target for early interventions in SCZ and related psychotic disorders.

In vivo glutamate clearance defects in a mouse model of Lafora disease

Lafora disease (LD) is a fatal rare neurodegenerative disorder characterized by epilepsy, neurodegeneration and insoluble polyglucosan accumulation in brain and other peripheral tissues. Although in the last two decades we have increased our knowledge on the molecular basis underlying the pathophysiology of LD, only a small part of the research in LD has paid attention to the mechanisms triggering one of the most lethal features of the disease: epilepsy. Recent studies in our laboratory suggested that a dysfunction in the activity of the mouse astrocytic glutamate transporter 1 (GLT-1) could contribute to epilepsy in LD. In this work, we present new in vivo evidence of a GLT-1 dysfunction, contributing to increased levels of extracellular glutamate in the hippocampus of a mouse model of Lafora disease (Epm2b-/-, lacking the E3-ubiquitin ligase malin). According to our results, Epm2b-/- mice showed an increased neuronal activity, as assessed by c-fos expression, in the hippocampus, an area directly correlated to epileptogenesis. This brain area presented lesser ability to remove synaptic glutamate after local GLT-1 blockade with dihydrokainate (DHK), in comparison to Epm2b+/+ animals, suggesting that these animals have a compromised glutamate clearance when a challenging condition was presented. These results correlate with a hippocampal upregulation of the minor isoform of the Glt-1 gene, named Glt-1b, which has been associated with compensatory mechanisms activated in response to neuronal stress. In conclusion, the hippocampus of Epm2b-/- mice presents an in vivo impairment in glutamate uptake which could contribute to epileptogenesis.

In vivo electrophysiological recordings of the effects of antidepressant drugs

Antidepressant drugs are a standard biological treatment for various neuropsychiatric disorders, yet relatively little is known about their electrophysiologic and synaptic effects on mood systems that set moment-to-moment emotional tone. In vivo electrical recording of local field potentials (LFPs) and single neuron spiking has been crucial for elucidating important details of neural processing and control in many other systems, and yet electrical approaches have not been broadly applied to the actions of antidepressants on mood-related circuits. Here we review the literature encompassing electrophysiologic effects of antidepressants in animals, including studies that examine older drugs, and extending to more recently synthesized novel compounds, as well as rapidly acting antidepressants. The existing studies on neuromodulator-based drugs have focused on recording in the brainstem nuclei, with much less known about their effects on prefrontal or sensory cortex. Studies on neuromodulatory drugs have moreover focused on single unit firing patterns with less emphasis on LFPs, whereas the rapidly acting antidepressant literature shows the opposite trend. In a synthesis of this information, we hypothesize that all classes of antidepressants could have common final effects on limbic circuitry. Whereas NMDA receptor blockade may induce a high powered gamma oscillatory state via direct and fast alteration of glutamatergic systems in mood-related circuits, neuromodulatory antidepressants may induce similar effects over slower timescales, corresponding with the timecourse of response in patients, while resetting synaptic excitatory versus inhibitory signaling to a normal level. Thus, gamma signaling may provide a biomarker (or “neural readout”) of the therapeutic effects of all classes of antidepressants.

The dynamics of disordered dialogue: Prefrontal, hippocampal and thalamic miscommunication underlying working memory deficits in schizophrenia

The prefrontal cortex is central to the orchestrated brain network communication that gives rise to working memory and other cognitive functions. Accordingly, working memory deficits in schizophrenia are increasingly thought to derive from prefrontal cortex dysfunction coupled with broader network disconnectivity. How the prefrontal cortex dynamically communicates with its distal network partners to support working memory and how this communication is disrupted in individuals with schizophrenia remain unclear. Here we review recent evidence that prefrontal cortex communication with the hippocampus and thalamus is essential for normal spatial working memory, and that miscommunication between these structures underlies spatial working memory deficits in schizophrenia. We focus on studies using normal rodents and rodent models designed to probe schizophrenia-related pathology to assess the dynamics of neural interaction between these brain regions. We also highlight recent preclinical work parsing roles for long-range prefrontal cortex connections with the hippocampus and thalamus in normal and disordered spatial working memory. Finally, we discuss how emerging rodent endophenotypes of hippocampal- and thalamo-prefrontal cortex dynamics in spatial working memory could translate into richer understanding of the neural bases of cognitive function and dysfunction in humans.

Can we increase the speed and efficacy of antidepressant treatments? Part II. Glutamatergic and RNA interference strategies

In the second part we focus on two treatment strategies that may overcome the main limitations of current antidepressant drugs. First, we review the experimental and clinical evidence supporting the use of glutamatergic drugs as fast-acting antidepressants. Secondly, we review the involvement of microRNAs (miRNAs) in the pathophysiology of major depressive disorder (MDD) and the use of small RNAs (e.g.., small interfering RNAs or siRNAs) to knockdown genes in monoaminergic and non-monoaminergic neurons and induce antidepressant-like responses in experimental animals. The development of glutamatergic agents is a promising venue for antidepressant drug development, given the antidepressant properties of the non-competitive NMDA receptor antagonist ketamine. Its unique properties appear to result from the activation of AMPA receptors by a metabolite [(2S,6S;2R,6R)-hydroxynorketamine (HNK)] and mTOR signaling. These effects increase synaptogenesis in prefrontal cortical pyramidal neurons and enhance serotonergic neurotransmission via descending inputs to the raphe nuclei. This view is supported by the cancellation of ketamine's antidepressant-like effects by inhibition of serotonin synthesis. We also review existing evidence supporting the involvement of miRNAs in MDD and the preclinical use of RNA interference (RNAi) strategies to target genes involved in antidepressant response. Many miRNAs have been associated to MDD, some of which e.g., miR-135 targets genes involved in antidepressant actions. Likewise, SSRI-conjugated siRNA evokes faster and/or more effective antidepressant-like responses. Intranasal application of sertraline-conjugated siRNAs directed to 5-HT_1A receptors and SERT evoked much faster changes of pre- and postsynaptic antidepressant markers than those produced by fluoxetine.

The antipsychotic drug brexpiprazole reverses phencyclidine-induced disruptions of thalamocortical networks

Brexpiprazole (BREX), a recently approved antipsychotic drug in the US and Canada, improves cognitive dysfunction in animal models, by still largely unknown mechanisms. BREX is a partial agonist at 5-HT_1A and D₂ receptors and antagonist at α_1B- and α_2C-adrenergic and 5-HT_2A receptors all with a similar potency. The NMDA receptor antagonist phencyclidine (PCP), used as pharmacological model of schizophrenia, activates thalamocortical networks and decreases low frequency oscillations (LFO; <4 Hz). These effects are reversed by antipsychotics. Here we assessed the ability of BREX to reverse PCP-induced hyperactivity of thalamocortical circuits, and the involvement of 5-HT_1A receptors in its therapeutic action. BREX reversed PCP-induced neuronal activation at a lower dose in centromedial/mediodorsal thalamic nuclei (CM/MD; 0.5mg/kg) than in pyramidal medial prefrontal cortex neurons (mPFC, 2mg/kg), perhaps due to antagonism at α_1B-adrenoceptors, abundantly expressed in the thalamus. Conversely, a cumulative 0.5 mg/kg dose reversed a PCP-induced LFO decrease in mPFC but not in CM/MD. BREX reduced LFO in both areas, yet with a different dose-response, and moderately excited mPFC neurons. The latter effect was reversed by the 5-HT_1A receptor antagonist WAY-100635. Thus, BREX partly antagonizes PCP-induced thalamocortical hyperactivity, differentially in mPFC versus CM/MD. This regional selectivity may be related to the differential expression of α_1B-, α_2C-adrenergic and 5-HT_2A receptors in both regions and/or different neuronal types. Furthermore, the pro-cognitive properties of BREX may be related to the 5-HT_1A receptor-mediated increase in mPFC pyramidal neuron activity. Overall, the present data provide new insight on the brain elements involved in BREX's therapeutic actions.

Neuronal correlates of ketamine and walking induced gamma oscillations in the medial prefrontal cortex and mediodorsal thalamus

Alterations in the function of the medial prefrontal cortex (mPFC) and its major thalamic source of innervation, the mediodorsal (MD) thalamus, have been hypothesized to contribute to the symptoms of schizophrenia. The NMDAR antagonist ketamine, used to model schizophrenia, elicits a brain state resembling early stage schizophrenia characterized by cognitive deficits and increases in cortical low gamma (40-70 Hz) power. Here we sought to determine how ketamine differentially affects spiking and gamma local field potential (LFP) activity in the rat mPFC and MD thalamus. Additionally, we investigated the ability of drugs targeting the dopamine D4 receptor (D4R) to modify the effects of ketamine on gamma activity as a measure of potential cognitive therapeutic efficacy. Rats were trained to walk on a treadmill to reduce confounds related to hyperactivity after ketamine administration (10 mg/kg s.c.) while recordings were obtained from electrodes chronically implanted in the mPFC and MD thalamus. Ketamine increased gamma LFP power in mPFC and MD thalamus in a similar frequency range, yet did not increase thalamocortical synchronization. Ketamine also increased firing rates and spike synchronization to gamma oscillations in the mPFC but decreased both measures in MD thalamus. Conversely, walking alone increased both firing rates and spike-gamma LFP correlations in both mPFC and MD thalamus. The D4R antagonist alone (L-745,870) had no effect on gamma LFP power during treadmill walking, although it reversed increases induced by the D4R agonist (A-412997) in both mPFC and MD thalamus. Neither drug altered ketamine-induced changes in gamma power or firing rates in the mPFC. However, in MD thalamus, the D4R agonist increased ketamine-induced gamma power and prevented ketamine's inhibitory effect on firing rates. Results provide new evidence that ketamine differentially modulates spiking and gamma power in MD thalamus and mPFC, supporting a potential role for both areas in contributing to ketamine-induced schizophrenia-like symptoms.

Reduced sleep spindle activity point to a TRN-MD thalamus-PFC circuit dysfunction in schizophrenia

Sleep disturbances have been reliably reported in patients with schizophrenia, thus suggesting that abnormal sleep may represent a core feature of this disorder. Traditional electroencephalographic studies investigating sleep architecture have found reduced deep non-rapid eye movement (NREM) sleep, or slow wave sleep (SWS), and increased REM density. However, these findings have been inconsistently observed, and have not survived meta-analysis. By contrast, several recent EEG studies exploring brain activity during sleep have established marked deficits in sleep spindles in schizophrenia, including first-episode and early-onset patients, compared to both healthy and psychiatric comparison subjects. Spindles are waxing and waning, 12-16Hz NREM sleep oscillations that are generated within the thalamus by the thalamic reticular nucleus (TRN), and are then synchronized and sustained in the cortex. While the functional role of sleep spindles still needs to be fully established, increasing evidence has shown that sleep spindles are implicated in learning and memory, including sleep dependent memory consolidation, and spindle parameters have been associated to general cognitive ability and IQ. In this article we will review the EEG studies demonstrating sleep spindle deficits in patients with schizophrenia, and show that spindle deficits can predict their reduced cognitive performance. We will then present data indicating that spindle impairments point to a TRN-MD thalamus-prefrontal cortex circuit deficit, and discuss about the possible molecular mechanisms underlying thalamo-cortical sleep spindle abnormalities in schizophrenia.

Glial GLT-1 blockade in infralimbic cortex as a new strategy to evoke rapid antidepressant-like effects in rats

Ketamine and deep brain stimulation produce rapid antidepressant effects in humans and rodents. An increased AMPA receptor (AMPA-R) signaling in medial prefrontal cortex (mPFC) has been suggested to mediate these responses. However, little research has addressed the direct effects of enhancing glutamate tone or AMPA-R stimulation in mPFC subdivisions. The current study investigates the behavioral and neurochemical consequences of glutamate transporter-1 (GLT-1) blockade or s-AMPA microinfusion in the infralimbic (IL) and prelimbic (PrL) cortex. Owing to the connectivity between the mPFC and raphe nuclei, the role of serotonin is also explored. The bilateral microinfusion of the depolarizing agent veratridine into IL -but not PrL- of rats evoked immediate antidepressant-like responses. The same regional selectivity was observed after microinfusion of dihydrokainic acid (DHK), a selective inhibitor of GLT-1, present in astrocytes. The DHK-evoked antidepressant-like responses appear to be mediated by an AMPA-R-driven enhancement of serotonergic activity, as (i) they were prevented by NBQX 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide disodium salt) and mimicked by s-AMPA; (ii) DHK and s-AMPA elevated similarly extracellular glutamate in IL and PrL, although extracellular 5-HT and c-fos expression in the midbrain dorsal raphe increased only when these agents were applied in IL; and (iii) DHK antidepressant-like responses were prevented by 5-HT synthesis inhibition and mimicked by citalopram microinfusion in IL. These results indicate that an acute increase of glutamatergic neurotransmission selectively in IL triggers immediate antidepressant-like responses in rats, likely mediated by the activation of IL-raphe pathways, which then results in a fast increase of serotonergic activity.

Thalamo-cortical communication, glutamatergic neurotransmission and neural oscillations: A unique window into the origins of ScZ?

The thalamus has recently received renewed interest in systems-neuroscience and schizophrenia (ScZ) research because of emerging evidence highlighting its important role in coordinating functional interactions in cortical-subcortical circuits. Moreover, higher cognitive functions, such as working memory and attention, have been related to thalamo-cortical interactions, providing a novel perspective for the understanding of the neural substrate of cognition. The current review will support this perspective by summarizing evidence on the crucial role of neural oscillations in facilitating thalamo-cortical (TC) interactions during normal brain functioning and their potential impairment in ScZ. Specifically, we will focus on the relationship between NMDA-R mediated (glutamatergic) neurotransmission in TC-interactions. To this end, we will first review the functional anatomy and neurotransmitters in thalamic circuits, followed by a review of the oscillatory signatures and cognitive processes supported by TC-circuits. In the second part of the paper, data from preclinical research as well as human studies will be summarized that have implicated TC-interactions as a crucial target for NMDA-receptor hypofunctioning. Finally, we will compare these neural signatures with current evidence from ScZ-research, suggesting a potential overlap between alterations in TC-circuits as the result of NMDA-R deficits and stage-specific alterations in large-scale networks in ScZ.

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.

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.

Extended neuroleptic administration modulates NMDA-R subunit immunoexpression in the rat neocortex and diencephalon

BACKGROUND

This study aimed to evaluate the effect of extended olanzapine, clozapine and haloperidol administration on NMDA-R subunit immunoexpression in the rat neocortex and diencephalon.

METHODS

To explore NR1, NR2A and NR2B subunit protein expression, densytometric analysis of immunohistochemically stained brain slices was performed.

RESULTS

Interestingly, all neuroleptics caused a downregulation of NMDA-R subunit expression in the thalamus but increased the level of NR1 in the hypothalamus. Olanzapine upregulated hypothalamic NR2A expression, while clozapine and haloperidol decreased hypothalamic levels. We observed no significant changes in NR2B immunoreactivity. None of the studied medications had significant influence on NMDA-R subunit expression in the neocortex.

CONCLUSIONS

Neuroleptic-induced reduction in the expression of thalamic NMDA-R subunits may play an important role in the regulation of glutamatergic transmission disorders in cortico-striato-thalamo-cortical loop in schizophrenia. A decrease in NMDA signaling in this region after long-term neuroleptic administration may also cautiously explain the incomplete effectiveness of these drugs in the therapy of schizophrenia-related cognitive disturbances.

Comparing the actions of lanicemine and ketamine in depression: key role of the anterior cingulate

Intravenous infusion of lanicemine (formerly AZD6765), a low trapping non-selective N-methyl-D-aspartate (NMDA) receptor antagonist, induces antidepressant effects with a similar time course to ketamine. We investigated whether a single dose lanicemine infusion would reproduce the previously reported decrease in subgenual anterior cingulate cortex (sgACC) activity evoked by ketamine, a potential mechanism of antidepressant efficacy. Sixty un-medicated adults meeting the criteria for major depressive disorder were randomly assigned to receive constant intravenous infusions of ketamine, lanicemine or saline during a 60min pharmacological magnetic resonance imaging (phMRI) scan. Both ketamine and lanicemine gradually increased the blood oxygen level dependent signal in sgACC and rostral ACC as the primary outcome measure. No decreases in signal were seen in any region. Interviewer-rated psychotic and dissociative symptoms were minimal following administration of lanicemine. There was no significant antidepressant effect of either infusion compared to saline. The previously reported deactivation of sgACC after ketamine probably reflects the rapid and pronounced subjective effects evoked by the bolus-infusion method used in the previous study. Activation of the ACC was observed following two different NMDA compounds in both Manchester and Oxford using different 3T MRI scanners, and this effect predicted improvement in mood 1 and 7 days post-infusion. These findings suggest that the initial site of antidepressant action for NMDA antagonists may be the ACC (NCT01046630. A Phase I, Multi-centre, Double-blind, Placebo-controlled Parallel Group Study to Assess the pharmacoMRI Effects of AZD6765 in Male and Female Subjects Fulfilling the Criteria for Major Depressive Disorder; http://clinicaltrials.gov/show/NCT01046630).

Phencyclidine-induced disruption of oscillatory activity in prefrontal cortex: Effects of antipsychotic drugs and receptor ligands

The non-competitive NMDA receptor (NMDA-R) antagonist phencyclidine (PCP) markedly disrupts thalamocortical activity, increasing excitatory neuron discharge and reducing low frequency oscillations (LFO, <4Hz) that temporarily group neuronal discharge. These actions are mainly driven by PCP interaction with NMDA-R in GABAergic neurons of the thalamic reticular nucleus and likely underlie PCP psychotomimetic activity. Here we report that classical (haloperidol, chlorpromazine, perphenazine) and atypical (clozapine, olanzapine, quetiapine, risperidone, ziprasidone, aripripazole) antipsychotic drugs--but not the antidepressant citalopram--countered PCP-evoked fall of LFO in the medial prefrontal cortex (mPFC) of anesthetized rats. PCP reduces LFO by breaking the physiological balance between excitatory and inhibitory transmission. Next, we examined the role of different neurotransmitter receptors to reverse PCP actions. D2-R and D1-R blockade may account for classical antipsychotic action since raclopride and SCH-23390 partially reversed PCP effects. Atypical antipsychotic reversal may additionally involve 5-HT1A-R activation (but not 5-HT2A-R blockade) since 8-OH-DPAT and BAYx3702 (but not M100907) fully countered PCP effects. Blockade of histamine H1-R (pyrilamine) and α1-adrenoceptors (prazosin) was without effect. However, the enhancement of GABAA-R-mediated neurotransmission (using muscimol, diazepam or valproate) and the reduction of excitatory neurotransmission (using the mGluR2/3 agonist LY379268 and the preferential kainite/AMPA antagonist CNQX--but not the preferential AMPA/kainate antagonist NBQX) partially or totally countered PCP effects. Overall, these results shed new light on the neurobiological mechanisms used by antipsychotic drugs to reverse NMDA-R antagonist actions and suggest that agents restoring the physiological excitatory/inhibitory balance altered by PCP may be new targets in antipsychotic drug development.

PCP-based mice models of schizophrenia: differential behavioral, neurochemical and cellular effects of acute and subchronic treatments

RATIONALE

N-methyl-D-aspartate receptor (NMDA-R) hypofunction has been proposed to account for the pathophysiology of schizophrenia. Thus, NMDA-R blockade has been used to model schizophrenia in experimental animals. Acute and repeated treatments have been successfully tested; however, long-term exposure to NMDA-R antagonists more likely resembles the core symptoms of the illness.

OBJECTIVES

To explore whether schizophrenia-related behaviors are differentially induced by acute and subchronic phencyclidine (PCP) treatment in mice and to examine the neurobiological bases of these differences.

RESULTS

Subchronic PCP induced a sensitization of acute locomotor effects. Spontaneous alternation in a T-maze and novel object recognition performance were impaired after subchronic but not acute PCP, suggesting a deficit in working memory. On the contrary, reversal learning and immobility in the tail suspension test were unaffected. Subchronic PCP significantly reduced basal dopamine but not serotonin output in medial prefrontal cortex (mPFC) and markedly decreased the expression of tyrosine hydroxylase in the ventral tegmental area. Finally, acute and subchronic PCP treatments evoked a different pattern of c-fos expression. At 1 h post-treatment, acute PCP increased c-fos expression in many cortical regions, striatum, thalamus, hippocampus, and dorsal raphe. However, the increased c-fos expression produced by subchronic PCP was restricted to the retrosplenial cortex, thalamus, hippocampus, and supramammillary nucleus. Four days after the last PCP injection, c-fos expression was still increased in the hippocampus of subchronic PCP-treated mice.

CONCLUSIONS

Acute and subchronic PCP administration differently affects neuronal activity in brain regions relevant to schizophrenia, which could account for their different behavioral effects.

Neuromagnetic oscillations predict evoked-response latency delays and core language deficits in autism spectrum disorders

Previous studies have observed evoked response latency as well as gamma band superior temporal gyrus (STG) auditory abnormalities in individuals with autism spectrum disorders (ASD). A limitation of these studies is that associations between these two abnormalities, as well as the full extent of oscillatory phenomena in ASD in terms of frequency and time, have not been examined. Subjects were presented pure tones at 200, 300, 500, and 1,000 Hz while magnetoencephalography assessed activity in STG auditory areas in a sample of 105 children with ASD and 36 typically developing controls (TD). Findings revealed a profile such that auditory STG processes in ASD were characterized by pre-stimulus abnormalities across multiple frequencies, then early high-frequency abnormalities followed by low-frequency abnormalities. Increased pre-stimulus activity was a 'core' abnormality, with pre-stimulus activity predicting post-stimulus neural abnormalities, group membership, and clinical symptoms (CELF-4 Core Language Index). Deficits in synaptic integration in the auditory cortex are associated with oscillatory abnormalities in ASD as well as patient symptoms. Increased pre-stimulus activity in ASD likely demonstrates a fundamental signal-to-noise deficit in individuals with ASD, with elevations in oscillatory activity suggesting an inability to maintain an appropriate 'neural tone' and an inability to rapidly return to a resting state prior to the next stimulus.

Network-specific cortico-thalamic dysconnection in schizophrenia revealed by intrinsic functional connectivity analyses

BACKGROUND

Cortico-thalamic connections are thought to be abnormal in schizophrenia due to their important roles in sensory relay and higher cognitive control, both of which are affected by this devastating illness. This study tested the cortico-thalamic dysconnection hypothesis in schizophrenia and further explored cortico-thalamic network properties using functional connectivity MRI (fcMRI).

METHODS

Forty-eight participants with schizophrenia and 48 healthy controls underwent resting fMRI scans and clinical evaluations. Six a priori cortical regions of interests (ROIs) were used to derive the six networks: dorsal default mode network (dDMN), fronto-parietal network (FPN), cingulo-opercular network (CON), primary sensorimotor network (SM1), primary auditory network (A1) and primary visual network (V1). The cortico-thalamic connectivity for each network was calculated for each participant and then compared between groups.

RESULTS

A repeated measures analysis of variance (ANOVA) showed significant group×network interactions (F(5, 90)=9.5, P<0.001), which were driven by a significant increase in FC within the SM1 (t(94)=4.1, P<0.001) and A1 (t(94)=4.2, P<0.001) networks in schizophrenics, as well as a significant decrease within the CON (t(94)=-2.8, P=0.04). The cortico-thalamic dysconnection did not correlate with symptom severity, representing a state independent abnormality.

CONCLUSION

The network analysis indicates that cortico-thalamic dysconnection in schizophrenia involves multiple networks and shows network specific changes. The findings provide support for dysfunctional thalamus-related networks in schizophrenia and further elaborate their network properties.

Brain rhythms connect impaired inhibition to altered cognition in schizophrenia

In recent years, schizophrenia research has focused on inhibitory interneuron dysfunction at the level of neurobiology and on cognitive impairments at the psychological level. Reviewing both experimental and computational findings, we show how the temporal structure of the activity of neuronal populations, exemplified by brain rhythms, can begin to bridge these levels of complexity. Oscillations in neuronal activity tie the pathophysiology of schizophrenia to alterations in local processing and large-scale coordination, and these alterations in turn can lead to the cognitive and perceptual disturbances observed in schizophrenia.

Ketamine-Induced Modulation of the Thalamo-Cortical Network in Healthy Volunteers As a Model for Schizophrenia

BACKGROUND

Schizophrenia has been associated with disturbances of thalamic functioning. In light of recent evidence suggesting a significant impact of the glutamatergic system on key symptoms of schizophrenia, we assessed whether modulation of the glutamatergic system via blockage of the N-methyl-D-aspartate (NMDA)-receptor might lead to changes of thalamic functional connectivity.

METHODS

Based on the ketamine model of psychosis, we investigated changes in cortico-thalamic functional connectivity by intravenous ketamine challenge during a 55-minute resting-state scan. Thirty healthy volunteers were measured with pharmacological functional magnetic resonance imaging using a double-blind, randomized, placebo-controlled, crossover design.

RESULTS

Functional connectivity analysis revealed significant ketamine-specific changes within the thalamus hub network, more precisely, an increase of cortico-thalamic connectivity of the somatosensory and temporal cortex.

CONCLUSIONS

Our results indicate that changes of thalamic functioning as described for schizophrenia can be partly mimicked by NMDA-receptor blockage. This adds substantial knowledge about the neurobiological mechanisms underlying the profound changes of perception and behavior during the application of NMDA-receptor antagonists.

Sleep in patients with schizophrenia

Numerous electrophysiological and neuroimaging studies have reported neurophysiological and cognitive deficits in schizophrenia patients during wakefulness. However, these findings have been inconsistently replicated across different groups of patients, thus complicating the identification of underlying neuronal defects. Sleep minimizes possible waking-related confounds, including decreased motivation and presence of active symptoms. Additionally, the two main sleep rhythms, slow waves and spindles, reflect the intrinsic activity of corticothalamic circuits and are associated with cognitive activities, including learning and memory, occurring during wakefulness. In this review I will present the most relevant sleep findings in schizophrenia, with particular emphasis on several recent studies that have consistently reported sleep spindle deficits in patients with schizophrenia. I will then elaborate on how these findings may contribute to a better understanding of the neurobiology of schizophrenia as well as to the development of novel pharmacological and non-pharmacological interventions to ameliorate the symptoms and cognitive impairments of schizophrenia patients.

The thalamic reticular nucleus: a functional hub for thalamocortical network dysfunction in schizophrenia and a target for drug discovery

The thalamus (comprising many distinct nuclei) plays a key role in facilitating sensory discrimination and cognitive processes through connections with the cortex. Impaired thalamocortical processing has long been considered to be involved in schizophrenia. In this review we focus on the thalamic reticular nucleus (TRN) providing evidence for it being an important communication hub between the thalamus and cortex and how it may play a key role in the pathophysiology of schizophrenia. We first highlight the functional neuroanatomy, neurotransmitter localisation and physiology of the TRN. We then present evidence of the physiological roles of the TRN in relation to oscillatory activity, cognition and behaviour. Next we discuss the role of the TRN in rodent models of risk factors for schizophrenia (genetic and pharmacological) and provide evidence for TRN deficits in schizophrenia. Finally we discuss new drug targets for schizophrenia in relation to restoring TRN circuitry dysfunction.

Phencyclidine inhibits the activity of thalamic reticular gamma-aminobutyric acidergic neurons in rat brain

BACKGROUND

The neurobiological basis of action of noncompetitive N-methyl-D-aspartate acid receptor (NMDA-R) antagonists is poorly understood. Electrophysiological studies indicate that phencyclidine (PCP) markedly disrupts neuronal activity with an overall excitatory effect and reduces the power of low-frequency oscillations (LFO; <4 Hz) in thalamocortical networks. Because the reticular nucleus of the thalamus (RtN) provides tonic feed-forward inhibition to the rest of the thalamic nuclei, we examined the effect of PCP on RtN activity, under the working hypothesis that NMDA-R blockade in RtN would disinhibit thalamocortical networks.

METHODS

Drug effects (PCP followed by clozapine) on the activity of RtN (single unit and local field potential recordings) and prefrontal cortex (PFC; electrocorticogram) in anesthetized rats were assessed.

RESULTS

PCP (.25-.5 mg/kg, intravenous) reduced the discharge rate of 19 of 21 RtN neurons to 37% of baseline (p < .000001) and the power of LFO in RtN and PFC to ~20% of baseline (p < .001). PCP also reduced the coherence between PFC and RtN in the LFO range. A low clozapine dose (1 mg/kg intravenous) significantly countered the effect of PCP on LFO in PFC but not in RtN and further reduced the discharge rate of RtN neurons. However, clozapine administration partly antagonized the fall in coherence and phase-locking values produced by PCP.

CONCLUSIONS

PCP activates thalamocortical circuits in a bottom-up manner by reducing the activity of RtN neurons, which tonically inhibit thalamic relay neurons. However, clozapine reversal of PCP effects is not driven by restoring RtN activity and may involve a cortical action.

Reduced mediodorsal thalamic volume and prefrontal cortical spindle activity in schizophrenia

BACKGROUND

We recently found marked deficits in sleep spindles, non-rapid eye movement (NREM) sleep oscillations that are generated within the thalamus and then amplified and sustained in the cortex, in patients with schizophrenia compared to both healthy and psychiatric controls. Here, we investigated the thalamic and cortical contributions to these sleep spindle deficits.

METHODS

Anatomical volume of interest analysis (i.e., thalamic volumes) and electroencephalogram (EEG) source modeling (i.e., spindle-related cortical currents) were performed in patients with schizophrenia and healthy comparison subjects.

FINDINGS

Schizophrenia patients had reduced mediodorsal (MD) thalamic volumes, especially on the left side, compared to healthy controls, whereas whole thalami and lateral geniculate nuclei did not differ between groups. Furthermore, left MD volumes were strongly correlated with the number of scalp-recorded spindles in an anterior frontal region, and cortical currents underlying these anterior frontal spindles were localized in the prefrontal cortex, in Brodmann area (BA) 10. Finally, prefrontal currents at the peak of spindle activity were significantly reduced in schizophrenia patients and correlated with their performance in an abstraction/working memory task.

CONCLUSION

Altogether, these findings point to deficits in a specific thalamo-cortical circuitry in schizophrenia, which is associated with some cognitive deficits commonly reported in those patients.

The natural hallucinogen 5-MeO-DMT, component of Ayahuasca, disrupts cortical function in rats: reversal by antipsychotic drugs

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT) is a natural hallucinogen component of Ayahuasca, an Amazonian beverage traditionally used for ritual, religious and healing purposes that is being increasingly used for recreational purposes in US and Europe. 5MeO-DMT is of potential interest for schizophrenia research owing to its hallucinogenic properties. Two other psychotomimetic agents, phencyclidine and 2,5-dimethoxy-4-iodo-phenylisopropylamine (DOI), markedly disrupt neuronal activity and reduce the power of low frequency cortical oscillations (<4 Hz, LFCO) in rodent medial prefrontal cortex (mPFC). Here we examined the effect of 5-MeO-DMT on cortical function and its potential reversal by antipsychotic drugs. Moreover, regional brain activity was assessed by blood-oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI). 5-MeO-DMT disrupted mPFC activity, increasing and decreasing the discharge of 51 and 35% of the recorded pyramidal neurons, and reducing (-31%) the power of LFCO. The latter effect depended on 5-HT1A and 5-HT2A receptor activation and was reversed by haloperidol, clozapine, risperidone, and the mGlu2/3 agonist LY379268. Likewise, 5-MeO-DMT decreased BOLD responses in visual cortex (V1) and mPFC. The disruption of cortical activity induced by 5-MeO-DMT resembles that produced by phencyclidine and DOI. This, together with the reversal by antipsychotic drugs, suggests that the observed cortical alterations are related to the psychotomimetic action of 5-MeO-DMT. Overall, the present model may help to understand the neurobiological basis of hallucinations and to identify new targets in antipsychotic drug development.

Brain state-dependent abnormal LFP activity in the auditory cortex of a schizophrenia mouse model

In schizophrenia, evoked 40-Hz auditory steady-state responses (ASSRs) are impaired, which reflects the sensory deficits in this disorder, and baseline spontaneous oscillatory activity also appears to be abnormal. It has been debated whether the evoked ASSR impairments are due to the possible increase in baseline power. GABAergic interneuron-specific NMDA receptor (NMDAR) hypofunction mutant mice mimic some behavioral and pathophysiological aspects of schizophrenia. To determine the presence and extent of sensory deficits in these mutant mice, we recorded spontaneous local field potential (LFP) activity and its click-train evoked ASSRs from primary auditory cortex of awake, head-restrained mice. Baseline spontaneous LFP power in the pre-stimulus period before application of the first click trains was augmented at a wide range of frequencies. However, when repetitive ASSR stimuli were presented every 20 s, averaged spontaneous LFP power amplitudes during the inter-ASSR stimulus intervals in the mutant mice became indistinguishable from the levels of control mice. Nonetheless, the evoked 40-Hz ASSR power and their phase locking to click trains were robustly impaired in the mutants, although the evoked 20-Hz ASSRs were also somewhat diminished. These results suggested that NMDAR hypofunction in cortical GABAergic neurons confers two brain state-dependent LFP abnormalities in the auditory cortex; (1) a broadband increase in spontaneous LFP power in the absence of external inputs, and (2) a robust deficit in the evoked ASSR power and its phase-locking despite of normal baseline LFP power magnitude during the repetitive auditory stimuli. The “paradoxically” high spontaneous LFP activity of the primary auditory cortex in the absence of external stimuli may possibly contribute to the emergence of schizophrenia-related aberrant auditory perception.

Identification of developmentally regulated PCP-responsive non-coding RNA, prt6, in the rat thalamus

Schizophrenia and similar psychoses induced by NMDA-type glutamate receptor antagonists, such as phencyclidine (PCP) and ketamine, usually develop after adolescence. Moreover, adult-type behavioral disturbance following NMDA receptor antagonist application in rodents is observed after a critical period at around 3 postnatal weeks. These observations suggest that the schizophrenic symptoms caused by and psychotomimetic effects of NMDA antagonists require the maturation of certain brain neuron circuits and molecular networks, which differentially respond to NMDA receptor antagonists across adolescence and the critical period. From this viewpoint, we have identified a novel developmentally regulated phencyclidine-responsive transcript from the rat thalamus, designated as prt6, as a candidate molecule involved in the above schizophrenia-related systems using a DNA microarray technique. The transcript is a non-coding RNA that includes sequences of at least two microRNAs, miR132 and miR212, and is expressed strongly in the brain and testis, with trace or non-detectable levels in the spleen, heart, liver, kidney, lung and skeletal muscle, as revealed by Northern blot analysis. The systemic administration of PCP (7.5 mg/kg, subcutaneously (s.c.)) significantly elevated the expression of prt6 mRNA in the thalamus at postnatal days (PD) 32 and 50, but not at PD 8, 13, 20, or 24 as compared to saline-treated controls. At PD 50, another NMDA receptor antagonist, dizocilpine (0.5 mg/kg, s.c.), and a schizophrenomimetic dopamine agonist, methamphetamine (4.8 mg/kg, s.c.), mimicked a significant increase in the levels of thalamic prt6 mRNAs, while a D2 dopmamine receptor antagonist, haloperidol, partly inhibited the increasing influence of PCP on thalamic prt6 expression without its own effects. These data indicate that prt6 may be involved in the pathophysiology of the onset of drug-induced schizophrenia-like symptoms and schizophrenia through the possible dysregulation of target genes of the long non-coding RNA or microRNAs in the transcript.

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.

A systematic review of the effects of NMDA receptor antagonists on oscillatory activity recorded in vivo

Distinct frequency bands can be differentiated from neuronal ensemble recordings, such as local field potentials or electrocorticogram recordings. Recent years have witnessed a rapid acceleration of research examining how N-methyl-D-aspartate receptor (NMDAR) antagonists influence fundamental frequency bands in cortical and subcortical brain regions. Herein, we systematically review findings from in vivo studies with a focus on delta, theta, gamma and more recently identified high-frequency oscillations. We also discuss some of the current hypotheses that are considered to account for the actions of NMDAR antagonists on these frequency bands. The data emphasize a close relationship between altered oscillatory activity and NMDAR blockade, with both local and large-scale networks accounting for their effects. These findings may have fundamental implications for the psychotomimetic effects produced by NMDAR antagonists.

The hippocampal-prefrontal pathway: the weak link in psychiatric disorders?

While the hippocampal formation and the prefrontal cortex each have a well-established role in cognitive and mnemonic processes, the extent and manner in which these structures interact to achieve these functions has not been fully delineated. Recent research in rodents compellingly supports the idea that the projection of neurons extending from the CA1 region of the hippocampus and from the subiculum to the prefrontal cortex, referred to here as the H-PFC pathway, is critically involved in aspects of cognition related to executive function and to emotional regulation. Concurrently, it is becoming evident that persons suffering from schizophrenia, depression, and post-traumatic stress disorder display structural anomalies and aberrant functional coupling within the hippocampal-prefrontal circuit. Considering that these disorders involve varying degrees of cognitive impairment and emotional dysregulation, dysfunction in the H-PFC pathway might therefore be the common element of their pathophysiology. This overlap might also be intertwined with the pathway's evident susceptibility to stress and with its relationship to the amygdala. In consequence, the H-PFC pathway is a potentially crucial element of the pathophysiology of several psychiatric diseases, and it offers a specific target for therapeutic intervention, which is consistent with the recent emphasis on reframing psychiatric diseases in terms of brain circuits.

Afferent drive of medial prefrontal cortex by hippocampus and amygdala is altered in MAM-treated rats: evidence for interneuron dysfunction

Evidence indicates that the prefrontal cortex and its regulation by afferent inputs are disrupted in schizophrenia. Using a validated rat model of schizophrenia based on prenatal administration of the mitotoxin methyl azoxymethanol acetate (MAM), we examined the convergent projections from the ventral hippocampus (vHipp) and the basolateral amygdala (BLA) in the medial prefrontal cortex (mPFC). In vivo extracellular recordings were done in anesthetized rats to assess how prior stimulation of the BLA or vHipp input to the mPFC affected mPFC responses to subsequent stimulation of these regions. The interstimulus interval (ISI) of the BLA and vHipp pulse stimulation was varied randomly between 0 and 130 ms, and the probability of evoked spike response in the mPFC measured. We found that BLA input increased vHipp-evoked spike probability at ISIs 40-130 ms, but decreased spike probability at ISIs 10-20 ms. This would be consistent with activation of inhibitory interneurons at shorter ISIs by BLA stimulation. In contrast, in MAM-treated rats BLA stimulation increased vHipp-evoked spike probability in mPFC at all ISIs tested. Given that interneurons are driven primarily by N-methyl-D-aspartate (NMDA) channel activation, the effects of the NMDA channel blocker, phencyclidine (PCP), were tested. PCP was found to completely attenuate the inhibitory effect of BLA input on vHipp-evoked responses in mPFC at shorter ISIs, causing the response in control rats treated with PCP to resemble that observed in the MAM rat. In contrast to the effects of BLA stimulation on vHipp-mPFC-evoked responses, there was no inhibitory period when examining the effects of vHipp stimulation on BLA-mPFC-evoked responses in control rats, but in MAM-treated rats there was a significant inhibition at short intervals. Thus, both affective input arising from the BLA and context-dependent input from the vHipp exert a modulatory effect on mPFC neural activity in response to these inputs. Whereas the BLA potentiated vHipp input to the mPFC at long intervals, there was a short-interval inhibitory period that appeared to be mediated by an NMDA-dependent drive of interneurons. This inhibitory modulation was absent in the model of schizophrenia and following PCP, which is consistent with an interneuron disruption in this disorder.

Antipsychotic compounds differentially modulate high-frequency oscillations in the rat nucleus accumbens: a comparison of first- and second-generation drugs

Improved understanding of the actions of antipsychotic compounds is critical for a better treatment of schizophrenia. Abnormal oscillatory activity has been found in schizophrenia and in rat models of the disease. N-Methyl-D-aspartic acid receptor (NMDAR) antagonists, used to model certain features of schizophrenia, increase the frequency and power of high-frequency oscillations (HFO, 130-180 Hz) in the rat nucleus accumbens, a brain region implicated in schizophrenia pathology. Antipsychotics can be classified as first- and second-generation drugs, the latter often reported to have wider benefit in humans and experimental models. This prompted the authors to examine the pre- and post-treatment effects of clozapine, risperidone (second-generation drugs) and sulpiride and haloperidol (first-generation drugs) on ketamine and MK801-enhanced accumbal HFO. Both NMDAR antagonists increased HFO frequency. In contrast, clozapine and risperidone markedly and dose-dependently reduced the frequency of spontaneous and NMDAR-antagonist-enhanced HFO, whilst a moderate effect was found for sulpiride and a much weaker effect for haloperidol. Unexpectedly, we found reductions in HFO frequency were associated with an increase in its power. These findings indicate that modulation of accumbal HFO frequency may be a fundamental effect produced by antipsychotic compounds. Of the drugs investigated, first- and second-generation compounds could be dissociated by their potency on this measure. This effect may partially explain the differences in the clinical profile of these drugs.

The electrophysiological signature of motivational salience in mice and implications for schizophrenia

According to the aberrant-salience hypothesis, attribution of motivational salience is severely disrupted in patients with schizophrenia. To provide a translational approach for investigating underlying mechanisms, neural correlates of salience attribution were examined in normal mice and in a MK-801 model of schizophrenia. Electrophysiological responses to standard and deviant tones were assessed in the medial prefrontal cortex (mPFC) using an auditory oddball paradigm. Motivational salience was induced by aversive conditioning to the deviant tone. Analysis of the auditory evoked potential (AEP) showed selective modulation of the late frontal negativity (LFN) by motivational salience, which persisted throughout a 4-week delay. MK-801, an N-methyl-D-aspartic acid receptor antagonist, abolished this differential response to motivational salience in conditioned mice. In contrast, a pronounced LFN response was observed towards the deviant, ie, perceptually salient tone, in nonconditioned mice. The finding of a selective modulation of a late frontal slow wave suggests increased top-down processing and emotional evaluation of motivationally salient stimuli. In particular, the LFN is discussed as the mouse analog to the human stimulus preceding negativity, which reflects preparatory processes in anticipation of reward or punishment. MK-801 led to a disruption of the normal response in conditioned and nonconditioned mice, including an aberrantly increased LFN in nonconditioned mice. This pattern of 'false-negative' and 'false-positive' responses suggests a degradation of salience attribution, which points to mPFC responses to be relevant for translational research on cognitive alterations in schizophrenia.