M2 Cortex Circuitry and Sensory-Induced Behavioral Alterations in Huntington's Disease: Role of Superior Colliculus.

Early and progressive cortico-striatal circuit alterations have been widely characterized in Huntington's disease (HD) patients. Cortical premotor area, M2 cortex in rodents, is the most affected cortical input to the striatum from early stages in patients and is associated to the motor learning deficits present in HD mice. Yet, M2 cortex sends additional long-range axon collaterals to diverse output brain regions beyond basal ganglia. Here, we aimed to elucidate the contribution of M2 cortex projections to HD pathophysiology in mice. Using fMRI, M2 cortex showed most prominent functional connectivity alterations with the superior colliculus (SC) in symptomatic R6/1 HD male mice. Structural alterations were also detected by tractography, although diffusion weighted imaging measurements suggested preserved SC structure and similar electrophysiological responses were obtained in the SC on optogenetic stimulation of M2 cortical axons. Male and female HD mice showed behavioral alterations linked to SC function, including decreased defensive behavioral responses toward unexpected stimuli, such as a moving robo-beetle, and decreased locomotion on an unexpected flash of light. Additionally, GCamp6f fluorescence recordings with fiber photometry showed that M2 cortex activity was engaged by the presence of a randomly moving robo-bettle, an effect absent in HD male mice. Moreover, acute chemogenetic M2 cortex inhibition in WT mice shift behavioral responses toward an HD phenotype. Collectively, our findings highlight the involvement of M2 cortex activity in visual stimuli-induced behavioral responses, which are deeply altered in the R6/1 HD mouse model.SIGNIFICANCE STATEMENT Understanding brain circuit alterations in brain disorders is critical for developing circuit-based therapeutic interventions. The cortico-striatal circuit is the most prominently disturbed in Huntington's disease (HD); and particularly, M2 cortex has a prominent role. However, the same M2 cortical neurons send additional projections to several brain regions beyond striatum. We characterized new structural and functional circuitry alterations of M2 cortex in HD mouse models and found that M2 cortex projection to the superior colliculus (SC) was deeply impaired. Moreover, we describe differential responses to unexpected sensory stimulus in HD mouse models, which relies on SC function. Our data highlight the involvement of M2 cortex in SC-dependent sensory processing and its alterations in HD pathophysiology.

A consensus protocol for functional connectivity analysis in the rat brain.

Task-free functional connectivity in animal models provides an experimental framework to examine connectivity phenomena under controlled conditions and allows for comparisons with data modalities collected under invasive or terminal procedures. Currently, animal acquisitions are performed with varying protocols and analyses that hamper result comparison and integration. Here we introduce StandardRat, a consensus rat functional magnetic resonance imaging acquisition protocol tested across 20 centers. To develop this protocol with optimized acquisition and processing parameters, we initially aggregated 65 functional imaging datasets acquired from rats across 46 centers. We developed a reproducible pipeline for analyzing rat data acquired with diverse protocols and determined experimental and processing parameters associated with the robust detection of functional connectivity across centers. We show that the standardized protocol enhances biologically plausible functional connectivity patterns relative to previous acquisitions. The protocol and processing pipeline described here is openly shared with the neuroimaging community to promote interoperability and cooperation toward tackling the most important challenges in neuroscience.

Spatio-temporal metabolic rewiring in the brain of TgF344-AD rat model of Alzheimer's disease.

Brain damage associated with Alzheimer's disease (AD) occurs even decades before the symptomatic onset, raising the need to investigate its progression from prodromal stages. In this context, animal models that progressively display AD pathological hallmarks (e.g. TgF344-AD) become crucial. Translational technologies, such as magnetic resonance spectroscopy (MRS), enable the longitudinal metabolic characterization of this disease. However, an integrative approach is required to unravel the complex metabolic changes underlying AD progression, from early to advanced stages. TgF344-AD and wild-type (WT) rats were studied in vivo on a 7 Tesla MRI scanner, for longitudinal quantitative assessment of brain metabolic profile changes using MRS. Disease progression was investigated at 4 time points, from 9 to 18 months of age, and in 4 regions: cortex, hippocampus, striatum, and thalamus. Compared to WT, TgF344-AD rats replicated common findings in AD patients, including decreased N-acetylaspartate in the cortex, hippocampus and thalamus, and decreased glutamate in the thalamus and striatum. Different longitudinal evolution of metabolic concentration was observed between TgF344-AD and WT groups. Namely, age-dependent trajectories differed between groups for creatine in the cortex and thalamus and for taurine in cortex, with significant decreases in Tg344-AD animals; whereas myo-inositol in the thalamus and striatum showed greater increase along time in the WT group. Additional analysis revealed divergent intra- and inter-regional metabolic coupling in each group. Thus, in cortex, strong couplings of N-acetylaspartate and creatine with myo-inositol in WT, but with taurine in TgF344-AD rats were observed; whereas in the hippocampus, myo-inositol, taurine and choline compounds levels were highly correlated in WT but not in TgF344-AD animals. Furthermore, specific cortex-hippocampus-striatum metabolic crosstalks were found for taurine levels in the WT group but for myo-inositol levels in the TgF344-AD rats. With a systems biology perspective of metabolic changes in AD pathology, our results shed light into the complex spatio-temporal metabolic rewiring in this disease, reported here for the first time. Age- and tissue-dependent imbalances between myo-inositol, taurine and other metabolites, such as creatine, unveil their role in disease progression, while pointing to the inadequacy of the latter as an internal reference for quantification.

Structural connectivity and subcellular changes after antidepressant doses of ketamine and Ro 25-6981 in the rat: an MRI and immuno-labeling study.

Ketamine has rapid and robust antidepressant effects. However, unwanted psychotomimetic effects limit its widespread use. Hence, several studies examined whether GluN2B-subunit selective NMDA antagonists would exhibit a better therapeutic profile. Although preclinical work has revealed some of the mechanisms of action of ketamine at cellular and molecular levels, the impact on brain circuitry is poorly understood. Several neuroimaging studies have examined the functional changes in the brain induced by acute administration of ketamine and Ro 25-6981 (a GluN2B-subunit selective antagonist), but the changes in the microstructure of gray and white matter have received less attention. Here, the effects of ketamine and Ro 25-6981 on gray and white matter integrity in male Sprague-Dawley rats were determined using diffusion-weighted magnetic resonance imaging (DWI). In addition, DWI-based structural brain networks were estimated and connectivity metrics were computed at the regional level. Immunohistochemical analyses were also performed to determine whether changes in myelin basic protein (MBP) and neurofilament heavy-chain protein (NF200) may underlie connectivity changes. In general, ketamine and Ro 25-6981 showed some opposite structural alterations, but both compounds coincided only in increasing the fractional anisotropy in infralimbic prefrontal cortex and dorsal raphe nucleus. These changes were associated with increments of NF200 in deep layers of the infralimbic cortex (together with increased MBP) and the dorsal raphe nucleus. Our results suggest that the synthesis of NF200 and MBP may contribute to the formation of new dendritic spines and myelination, respectively. We also suggest that the increase of fractional anisotropy of the infralimbic and dorsal raphe nucleus areas could represent a biomarker of a rapid antidepressant response.

Brain connectivity during Alzheimer's disease progression and its cognitive impact in a transgenic rat model.

The research of Alzheimer's disease (AD) in its early stages and its progression till symptomatic onset is essential to understand the pathology and investigate new treatments. Animal models provide a helpful approach to this research, since they allow for controlled follow-up during the disease evolution. In this work, transgenic TgF344-AD rats were longitudinally evaluated starting at 6 months of age. Every 3 months, cognitive abilities were assessed by a memory-related task and magnetic resonance imaging (MRI) was acquired. Structural and functional brain networks were estimated and characterized by graph metrics to identify differences between the groups in connectivity, its evolution with age, and its influence on cognition. Structural networks of transgenic animals were altered since the earliest stage. Likewise, aging significantly affected network metrics in TgF344-AD, but not in the control group. In addition, while the structural brain network influenced cognitive outcome in transgenic animals, functional network impacted how control subjects performed. TgF344-AD brain network alterations were present from very early stages, difficult to identify in clinical research. Likewise, the characterization of aging in these animals, involving structural network reorganization and its effects on cognition, opens a window to evaluate new treatments for the disease.

Resting State Networks in the TgF344-AD Rat Model of Alzheimer's Disease Are Altered From Early Stages.

A better and non-invasive characterization of the preclinical phases of Alzheimer's disease (AD) is important to advance its diagnosis and obtain more effective benefits from potential treatments. The TgF344-AD rat model has been well characterized and shows molecular, behavioral and brain connectivity alterations that resemble the silent period of the pathology. Our aim was to longitudinally investigate functional brain connectivity in established resting-state networks (RSNs) obtained by independent component analysis (ICA) in a cohort of TgF344-AD and control rats every 3 months, from 5 to 18 months of age, to cover different stages of the disease. Before each acquisition, working memory performance was evaluated by the delayed non match-to-sample (DNMS) task. Differences in the temporal evolution were observed between groups in the amplitude and shape of the somatosensorial and sensorimotor networks but not in the whole default mode network (DMN). Subsequent high dimensional ICA analysis showed early alterations in the anterior DMN subnetwork activity of TgF344-AD rats compared to controls. Performance of DNMS task was positively correlated with somatosensorial network at 5 months of age in the wild-type (WT) animals but not in the Tg-F344 rats. At different time points, DMN showed negative correlation with cognitive performance in the control group while in the transgenic group the correlation was positive. In addition, behavioral differences observed at 5 months of age correlated with alterations in the posterior DMN subnetwork. We have demonstrated that functional connectivity using ICA represents a useful biomarker also in animal models of AD such as the TgF344AD rats, as it allows the identification of alterations associated with the progression of the disease, detecting differences in specific networks even at very early stages.

Role of Serotonin and Noradrenaline in the Rapid Antidepressant Action of Ketamine.

Depression is a chronic and debilitating illness that interferes severely with many human behaviors, and is the leading cause of disability in the world. There is data suggesting that deficits in serotonin neurotransmission can contribute to the development of depression. Indeed, >90% of prescribed antidepressant drugs act by increasing serotonergic transmission at the synapse. However, this increase is offset by a negative feedback operating at the level of the cell body of the serotonin neurons in the raphe nuclei. In the present work, we demonstrate: first, the intracortical infusion of ketamine induced an antidepressant-like effect in the forced swim test, comparable to that produced by systemic ketamine; second, systemic and intracortical ketamine increased serotonin and noradrenaline efflux in the prefrontal cortex, but not in the dorsal raphe nucleus; third, systemic and intracortical administration of ketamine increased the efflux of glutamate in the prefrontal cortex and dorsal raphe nucleus; fourth, systemic ketamine did not alter the functionality of 5-HT1A receptors in the dorsal raphe nucleus. Taken together, these findings suggest that the antidepressant-like effects of ketamine are caused by the stimulation of the prefrontal projection to the dorsal raphe nucleus and locus coeruleus caused by an elevated glutamate in the medial prefrontal cortex, which would stimulate release of serotonin and noradrenaline in the same area. The impact of both monoamines in the antidepressant response to ketamine seems to have different time frames.

Early brain connectivity alterations and cognitive impairment in a rat model of Alzheimer's disease.

BACKGROUND: Animal models of Alzheimer's disease (AD) are essential to understanding the disease progression and to development of early biomarkers. Because AD has been described as a disconnection syndrome, magnetic resonance imaging (MRI)-based connectomics provides a highly translational approach to characterizing the disruption in connectivity associated with the disease. In this study, a transgenic rat model of AD (TgF344-AD) was analyzed to describe both cognitive performance and brain connectivity at an early stage (5 months of age) before a significant concentration of ß-amyloid plaques is present. METHODS: Cognitive abilities were assessed by a delayed nonmatch-to-sample (DNMS) task preceded by a training phase where the animals learned the task. The number of training sessions required to achieve a learning criterion was recorded and evaluated. After DNMS, MRI acquisition was performed, including diffusion-weighted MRI and resting-state functional MRI, which were processed to obtain the structural and functional connectomes, respectively. Global and regional graph metrics were computed to evaluate network organization in both transgenic and control rats. RESULTS: The results pointed to a delay in learning the working memory-related task in the AD rats, which also completed a lower number of trials in the DNMS task. Regarding connectivity properties, less efficient organization of the structural brain networks of the transgenic rats with respect to controls was observed. Specific regional differences in connectivity were identified in both structural and functional networks. In addition, a strong correlation was observed between cognitive performance and brain networks, including whole-brain structural connectivity as well as functional and structural network metrics of regions related to memory and reward processes. CONCLUSIONS: In this study, connectivity and neurocognitive impairments were identified in TgF344-AD rats at a very early stage of the disease when most of the pathological hallmarks have not yet been detected. Structural and functional network metrics of regions related to reward, memory, and sensory performance were strongly correlated with the cognitive outcome. The use of animal models is essential for the early identification of these alterations and can contribute to the development of early biomarkers of the disease based on MRI connectomics.

Effects of Orientation and Anisometry of Magnetic Resonance Imaging Acquisitions on Diffusion Tensor Imaging and Structural Connectomes.

Diffusion-weighted imaging (DWI) quantifies water molecule diffusion within tissues and is becoming an increasingly used technique. However, it is very challenging as correct quantification depends on many different factors, ranging from acquisition parameters to a long pipeline of image processing. In this work, we investigated the influence of voxel geometry on diffusion analysis, comparing different acquisition orientations as well as isometric and anisometric voxels. Diffusion-weighted images of one rat brain were acquired with four different voxel geometries (one isometric and three anisometric in different directions) and three different encoding orientations (coronal, axial and sagittal). Diffusion tensor scalar measurements, tractography and the brain structural connectome were analyzed for each of the 12 acquisitions. The acquisition direction with respect to the main magnetic field orientation affected the diffusion results. When the acquisition slice-encoding direction was not aligned with the main magnetic field, there were more artifacts and a lower signal-to-noise ratio that led to less anisotropic tensors (lower fractional anisotropic values), producing poorer quality results. The use of anisometric voxels generated statistically significant differences in the values of diffusion metrics in specific regions. It also elicited differences in tract reconstruction and in different graph metric values describing the brain networks. Our results highlight the importance of taking into account the geometric aspects of acquisitions, especially when comparing diffusion data acquired using different geometries.

Activation of AMPA Receptors Mediates the Antidepressant Action of Deep Brain Stimulation of the Infralimbic Prefrontal Cortex.

Although deep brain stimulation (DBS) has been used with success in treatment-resistant depression, little is known about its mechanism of action. We examined the antidepressant-like activity of short (1 h) DBS applied to the infralimbic prefrontal cortex in the forced swim test (FST) and the novelty-suppressed feeding test (NSFT). We also used in vivo microdialysis to evaluate the release of glutamate, γ-aminobutyric acid, serotonin, dopamine, and noradrenaline in the prefrontal cortex and c-Fos immunohistochemistry to determine the brain regions activated by DBS. One hour of DBS of the infralimbic prefrontal cortex has antidepressant-like effects in FST and NSFT, and increases prefrontal efflux of glutamate, which would activate AMPA receptors (AMPARs). This effect is specific of the infralimbic area since it is not observed after DBS of the prelimbic subregion. The activation of prefrontal AMPARs would result in a stimulation of prefrontal output to the brainstem, thus increasing serotonin, dopamine, and noradrenaline in the prefrontal cortex. Further, the activation of prefrontal AMPARs is necessary and sufficient condition for the antidepressant response of 1 h DBS.

DWI and complex brain network analysis predicts vascular cognitive impairment in spontaneous hypertensive rats undergoing executive function tests.

The identification of biomarkers of vascular cognitive impairment is urgent for its early diagnosis. The aim of this study was to detect and monitor changes in brain structure and connectivity, and to correlate them with the decline in executive function. We examined the feasibility of early diagnostic magnetic resonance imaging (MRI) to predict cognitive impairment before onset in an animal model of chronic hypertension: Spontaneously Hypertensive Rats. Cognitive performance was tested in an operant conditioning paradigm that evaluated learning, memory, and behavioral flexibility skills. Behavioral tests were coupled with longitudinal diffusion weighted imaging acquired with 126 diffusion gradient directions and 0.3 mm(3) isometric resolution at 10, 14, 18, 22, 26, and 40 weeks after birth. Diffusion weighted imaging was analyzed in two different ways, by regional characterization of diffusion tensor imaging (DTI) indices, and by assessing changes in structural brain network organization based on Q-Ball tractography. Already at the first evaluated times, DTI scalar maps revealed significant differences in many regions, suggesting loss of integrity in white and gray matter of spontaneously hypertensive rats when compared to normotensive control rats. In addition, graph theory analysis of the structural brain network demonstrated a significant decrease of hierarchical modularity, global and local efficacy, with predictive value as shown by regional three-fold cross validation study. Moreover, these decreases were significantly correlated with the behavioral performance deficits observed at subsequent time points, suggesting that the diffusion weighted imaging and connectivity studies can unravel neuroimaging alterations even overt signs of cognitive impairment become apparent.

Is the acute NMDA receptor hypofunction a valid model of schizophrenia?

Several genetic, neurodevelopmental, and pharmacological animal models of schizophrenia have been established. This short review examines the validity of one of the most used pharmacological model of the illness, ie, the acute administration of N-methyl-D-aspartate (NMDA) receptor antagonists in rodents. In some cases, data on chronic or prenatal NMDA receptor antagonist exposure have been introduced for comparison. The face validity of acute NMDA receptor blockade is granted inasmuch as hyperlocomotion and stereotypies induced by phencyclidine, ketamine, and MK-801 are regarded as a surrogate for the positive symptoms of schizophrenia. In addition, the loss of parvalbumin-containing cells (which is one of the most compelling finding in postmortem schizophrenia brain) following NMDA receptor blockade adds construct validity to this model. However, the lack of changes in glutamic acid decarboxylase (GAD(67)) is at variance with human studies. It is possible that changes in GAD(67) are more reflective of the neurodevelopmental condition of schizophrenia. Finally, the model also has predictive validity, in that its behavioral and transmitter activation in rodents are responsive to antipsychotic treatment. Overall, although not devoid of drawbacks, the acute administration of NMDA receptor antagonists can be considered as a good model of schizophrenia bearing a satisfactory degree of validity.

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.

Unraveling monoamine receptors involved in the action of typical and atypical antipsychotics on glutamatergic and serotonergic transmission in prefrontal cortex.

The systemic administration of noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists has been considered as a pharmacological model of schizophrenia. In the present work, we used in vivo microdialysis to examine: first, the effects of MK-801, on the efflux of glutamate and serotonin (5-HT) in the medial prefrontal cortex (mPFC) of the rat; second, whether the MK-801-induced changes in the cortical efflux of both transmitters could be blocked by atypical (clozapine and olanzapine) and classical (haloperidol and chlorpromazine) antipsychotic drugs given intra-mPFC; and third, the role of local blockade of dopamine D(2)/D(3)/D(4), serotonin 5-HT(2A) and alpha(1)-adrenergic receptors as well as agonism at dopamine D(1)/D(5) and 5-HT(1A) receptors in the mPFC on the increased efflux of glutamate and 5-HT elicited by MK-801. The four antipsychotic drugs blocked the MK-801-induced increase in glutamate, whereas only clozapine and olanzapine were able to block the increased efflux of 5-HT. Furthermore, M100907 (5-HT(2A) antagonist), BAY x 3702 (5-HT(1A) agonist) and prazosin (alpha(1)-adrenergic antagonist) blocked the MK-801-induced increase of 5-HT and glutamate in the mPFC. In contrast, raclopride (D(2)/D(3) antagonist) and L-745,870 (D(4) antagonist) were able to prevent the increased efflux of glutamate (but not that of 5-HT) elicited by MK-801. SKF-38393 (dopamine D(1)/D(5) agonist) also prevented the MK-801-induced increase of glutamate in the mPFC, but the same effect on cortical 5-HT was reached only at the highest concentration tested. We suggest that the blockade of an exacerbated 5-HT release in the mPFC induced by NMDA antagonists can be a characteristic of atypical antipsychotic drugs. Moreover, we propose that D(2)/D(3)/D(4) receptor antagonists would act predominantly on a subpopulation of GABAergic interneurons of the mPFC, thus enhancing cortical inhibition, which would prevent an excessive glutamatergic transmission. Dopamine D(1)/D(5) agonists would further stimulate GABA release from other subpopulation of interneurons controlling cortical output to the dorsal raphe nucleus. Atypical antipsychotic drugs might further act upon 5-HT(2A), 5-HT(1A) and alpha(1)-adrenoceptors present in pyramidal cells (including those projecting to the dorsal raphe nucleus), which would directly inhibit an excessive excitability of these cells.

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

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

Clozapine and haloperidol differently suppress the MK-801-increased glutamatergic and serotonergic transmission in the medial prefrontal cortex of the rat.

The administration of noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine and ketamine has been shown to increase the extracellular concentration of glutamate and serotonin (5-HT) in the medial prefrontal cortex (mPFC). In the present work, we used in vivo microdialysis to examine the effects of the more potent noncompetitive NMDA receptor antagonist, MK-801, on the efflux of glutamate and 5-HT in the mPFC, and whether the MK-801-induced changes in the cortical efflux of both transmitters could be blocked by clozapine and haloperidol given systemically or intra-mPFC. The systemic, but not the local administration of MK-801, induced an increased efflux of 5-HT and glutamate, which suggests that the NMDA receptors responsible for these effects are located outside the mPFC, possibly in GABAergic neurons that tonically inhibit glutamatergic inputs to the mPFC. The MK-801-induced increases of extracellular glutamate and 5-HT were dependent on nerve impulse and the activation of mPFC AMPA/kainate receptors as they were blocked by tetrodotoxin and NBQX, respectively. Clozapine and haloperidol blocked the MK-801-induced increase in glutamate, whereas only clozapine was able to block the increased efflux of 5-HT. The local effects of clozapine and haloperidol paralleled those observed after systemic administration, which emphasizes the relevance of the mPFC as a site of action of these antipsychotic drugs in offsetting the neurochemical effects of MK-801. The ability of clozapine to block excessive cortical 5-HT efflux elicited by MK-801 might be related to the superior efficacy of this drug in treating negative/cognitive symptoms of schizophrenia.

Clozapine and olanzapine, but not haloperidol, suppress serotonin efflux in the medial prefrontal cortex elicited by phencyclidine and ketamine.

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP) and ketamine can evoke psychotic symptoms in normal individuals and schizophrenic patients. Here, we have examined the effects of PCP (5 mg/kg) and ketamine (25 mg/kg) on the efflux of serotonin (5-HT) in the medial prefrontal cortex (mPFC) and their possible blockade by the antipsychotics, clozapine, olanzapine and haloperidol, as well as ritanserin (5-HT2A/2C receptor antagonist) and prazosin (alpha1-adrenoceptor antagonist). The systemic administration, but not the local perfusion, of the two NMDA receptor antagonists markedly increased the efflux of 5-HT in the mPFC. The atypical antipsychotics clozapine (1 mg/kg) and olanzapine (1 mg/kg), and prazosin (0.3 mg/kg), but not the classical antipsychotic haloperidol (1 mg/kg), reversed the PCP- and ketamine-induced increase in 5-HT efflux. Ritanserin (5 mg/kg) was able to reverse only the effect of PCP. These findings indicate that an increased serotonergic transmission in the mPFC is a functional consequence of NMDA receptor hypofunction and this effect is blocked by atypical antipsychotic drugs.