Early prefrontal functional blockade in rats results in schizophrenia-related anomalies in behavior and dopamine

MK-801-induced behavioral and dopaminergic responses in the shell part of the nucleus accumbens in adult male rats are disrupted after neonatal blockade of the ventral subiculum

The present study was conducted in the context of animal modeling of schizophrenia. It investigated in adult male rats, after transient neonatal blockade of the ventral subiculum (VSub), the impact of a very specific non-competitive antagonist of NMDA receptors (MK-801) on locomotor activity and dopaminergic (DAergic) responses in the dorsomedial shell part of the nucleus accumbens (Nacc), a striatal subregion described as the common target region for antipsychotics. The functional neonatal inactivation of the VSub was achieved by local microinjection of tetrodotoxin (TTX) at postnatal day 8 (PND8). Control pups were microinjected with the solvent phosphate buffered saline (PBS). Locomotor responses and DAergic variations in the dorsomedial shell part of the Nacc were measured simultaneously using in vivo voltammetry in awake, freely moving male animals after sc administration of MK-801. The following results were obtained: 1) a dose-dependent increase in locomotor activity in PBS and TTX animals, greater in TTX rats/PBS rats; and 2) divergent DAergic responses for PBS and TTX animals. A decrease in DA levels with a return to around basal values was observed in PBS animals. An increase in DA levels was obtained in TTX animals. The present data suggest that neonatal blockade of the VSub results in disruption in NMDA glutamatergic transmission, causing a disturbance in DA release in the dorsomedial shell in adults male rats. In the context of animal modeling of schizophrenia using the same approach it would be interesting to investigate possible changes in postsynaptic NMDA receptors-related proteins in the dorsomedial shell region in the Nacc.

THC and CBD produce divergent effects on perception and panic behaviours via distinct cortical molecular pathways

Clinical and pre-clinical evidence demonstrates divergent psychotropic effects of THC vs. CBD. While THC can induce perceptual distortions and anxiogenic effects, CBD displays antipsychotic and anxiolytic properties. A key brain region responsible for regulation of cognition and affect, the medial prefrontal cortex (PFC), is strongly modulated by cannabinoids, suggesting that these dissociable THC/CBD-dependent effects may involve functional and molecular interplay within the PFC. The primary aim of this study was to investigate potential interactions and molecular substrates involved in PFC-mediated effects of THC and CBD on differential cognitive and affective behavioural processing. Male Sprague Dawley rats received intra-PFC microinfusions of THC, CBD or their combination, and tested in the latent inhibition paradigm, spontaneous oddity discrimination test, elevated T-maze and open field. To identify local, drug-induced molecular modulation in the PFC, PFC samples were collected and processed with Western Blotting. Intra-PFC THC induced strong panic-like responses that were counteracted with CBD. In contrast, CBD did not affect panic-like behaviours but blocked formation of associative fear memories and impaired latent inhibition and oddity discrimination performance. Interestingly, these CBD effects were dependent upon 5-HT_1A receptor transmission but not influenced by THC co-administration. Moreover, THC induced robust phosphorylation of ERK1/2 that was prevented by CBD, while CBD decreased phosphorylation of p70S6K, independently of THC. These results suggest that intra-PFC infusion of THC promotes panic-like behaviour associated with increased ERK1/2 phosphorylation. In contrast, CBD impairs perceptive functions and latent inhibition via activation of 5-HT_1A receptors and reduced phosphorylation of p70S6K.

Postnatal functional inactivation of the ventral subiculum enhances dopaminergic responses in the core part of the nucleus accumbens following ketamine injection in adult rats

For almost two decades schizophrenia has been considered to be a functional disconnection disorder. This functional disconnectivity between several brain regions could have a neurodevelopmental origin. Various approaches suggest the ventral subiculum (SUB) is a particular target region for neurodevelopemental disturbances in schizophrenia. It is also commonly acknowledged that there is a striatal dopaminergic (DA) dysregulation in schizophrenia which may depend on a subiculo-striatal disconnection involving glutamatergic NMDA receptors. The present study was designed to investigate, in adult rats, the effects of the non-competitive NMDA receptor antagonist ketamine on DA responses in the ventral striatum, or, more specifically, the core part of the nucleus accumbens (Nacc), following postnatal functional inactivation of the SUB. Functional inactivation of the left SUB was carried out by local tetrodotoxin (TTX) microinjection at postnatal day 8 (PND8), i.e. at a critical point in the neurodevelopmental period. DA variations were recorded using in vivo voltammetry in freely moving adult rats (11 weeks). Locomotor activity was recorded simultaneously with the extracellular levels of DA in the core part of the Nacc. Data obtained during the present study showed that after administration of ketamine, the two indexes were higher in TTX animals than PBS animals, the suggestion being that animals microinjected with TTX in the left SUB at PND8 present greater reactivity to ketamine than animals microinjected with PBS. These findings could provide new information regarding the involvement of NMDA glutamatergic receptors in the core part of the Nacc in the pathophysiology of schizophrenia.

Tyrosine phosphorylation of the transmembrane protein SIRPα: Sensing synaptic activity and regulating ectodomain cleavage for synapse maturation

Synapse maturation is a neural activity-dependent process during brain development, in which active synapses preferentially undergo maturation to establish efficient neural circuits in the brain. Defects in this process are implicated in various neuropsychiatric disorders. We have previously reported that a postsynaptic transmembrane protein, signal regulatory protein-α (SIRPα), plays an important role in activity-dependently directing synapse maturation. In the presence of synaptic activity, the ectodomain of SIRPα is cleaved and released and then acts as a retrograde signal to induce presynaptic maturation. However, how SIRPα detects synaptic activity to promote its ectodomain cleavage and synapse maturation is unknown. Here, we show that activity-dependent tyrosine phosphorylation of SIRPα is critical for SIRPα cleavage and synapse maturation. We found that during synapse maturation and in response to neural activity, SIRPα is highly phosphorylated on its tyrosine residues in the hippocampus, a structure critical for learning and memory. Tyrosine phosphorylation of SIRPα was necessary for SIRPα cleavage and presynaptic maturation, as indicated by the fact that a phosphorylation-deficient SIRPα variant underwent much less cleavage and could not drive presynaptic maturation. However, SIRPα phosphorylation did not affect its synaptic localization. Finally, we show that inhibitors of the Src and JAK kinase family suppress neural activity-dependent SIRPα phosphorylation and cleavage. Together, our results indicate that SIRPα phosphorylation serves as a mechanism for detecting synaptic activity and linking it to the ectodomain cleavage of SIRPα, which in turn drives synapse maturation in an activity-dependent manner.

Dopaminergic responses in the core part of the nucleus accumbens to subcutaneous MK801 administration are increased following postnatal transient blockade of the prefrontal cortex

Schizophrenia is a complex and devastating neuropsychiatric disease thought to result from impaired connectivity between several integrative regions, stemming from developmental failures. In particular, the left prefrontal cortex of schizophrenia patients seems to be targeted by such early developmental disturbances. Data obtained over the last three decades support the hypothesis of a dopaminergic dysfunction in schizophrenia. Striatal dopaminergic dysregulation in schizophrenia may result from a dysconnection between the prefrontal cortex and the striatum (dorsal and ventral) involving glutamatergic N-methyl-d-aspartate (NMDA) receptors. In the context of animal modeling of the pathophysiology of schizophrenia, the present study was designed to investigate the effects of MK 801 (dizocilpine) on locomotor activity and dopaminergic responses in the left core part of the nucleus accumbens (ventral striatum) in adult rats following neonatal tetrodotoxin inactivation of the left prefrontal cortex (infralimbic/prelimbic region) at postnatal day 8. Dopaminergic variations were recorded in the nucleus accumbens by means of in vivo voltammetry in freely moving adult animals. Following MK 801 administration, and in comparison to control (PBS) animals, animals microinjected with tetrodotoxin display locomotor hyperactivity and increased extracellular dopamine levels in the core part of the nucleus accumbens. These findings suggest neonatal functional inactivation of the prefrontal cortex may lead to a dysregulation of dopamine release in the core part of the nucleus accumbens involving NMDA receptors. The results obtained may provide new insight into the involvement of NMDA receptors in the pathophysiology of schizophrenia and suggest that future studies should look carefully at the core of the nucleus accumbens.

Neonatal Prefrontal Inactivation Results in Reversed Dopaminergic Responses in the Shell Subregion of the Nucleus Accumbens to NMDA Antagonists

Striatal dopaminergic dysregulation in schizophrenia could result from a prefronto-striatal dysconnectivity, of neurodevelopmental origin, involving N-methyl-d-aspartate (NMDA) receptors. The dorsomedian shell part of the nucleus accumbens is a striatal subregion of particular interest inasmuch as it has been described as the common target region for antipsychotics. Moreover, NMDA receptors located on the dopaminergic endings have been reported in the shell. The present study examines in adult rats the effects of early functional inactivation of the left prefrontal cortex on behavioral and dopaminergic responses in the dorsomedian shell part of the nucleus accumbens following administration of two noncompetitive NMDA receptor antagonists, ketamine, and dizocilpine (MK-801). The results showed that postnatal blockade of the prefrontal cortex led to increased locomotor activity as well as increased extracellular dopamine levels in the dorsomedian shell following administration of both noncompetitive NMDA receptor antagonists, and, more markedly, after treatment with the more specific one, MK-801, whereas decreased dopaminergic levels were observed in respective controls. These data suggest a link between NMDA receptor dysfunctioning and dopamine dysregulation at the level of the dorsomedian shell part of the nucleus accumbens. They may help to understand the pathophysiology of schizophrenia in a neurodevelopmental perspective.

Regional brain dysregulation of Ca(2+)-handling systems in ketamine-induced rat model of experimental psychosis

Chronic N-methyl-D-aspartate receptor (NMDAR) antagonist treatment can provide valuable neurochemical and neuroanatomical models of experimental psychosis. One such antagonist, ketamine, with its short half-time and well-documented psychotomimetic action, has cognitive effects resembling various aspects of schizophrenia-like symptoms. In order to obtain insights into possible relationships between Ca(2+) homeostasis and schizophrenia-related symptoms, we investigate the effects of chronic ketamine administration on intracellular Ca(2+) levels in various brain regions and on the expression level of key members of the neuronal Ca(2+)-handling system in rats. We show increased intracellular [Ca(2+)] in all of the examined brain regions following ketamine treatment but an altered cytosolic Ca(2+) level correlated with hyperlocomotor activity was only established for the cortex and striatum. Our findings also suggest that an imbalance in the expression between the calcium "on" and "off" systems contributes to the deregulation of brain Ca(2+) homeostasis in our ketamine-induced model of experimental psychosis. Identification of the genes whose expression is affected by ketamine treatment indicates their involvement as putative etiological factors in schizophrenia.

Functional imaging of cortical feedback projections to the olfactory bulb

Processing of sensory information is substantially shaped by centrifugal, or feedback, projections from higher cortical areas, yet the functional properties of these projections are poorly characterized. Here, we used genetically-encoded calcium sensors (GCaMPs) to functionally image activation of centrifugal projections targeting the olfactory bulb (OB). The OB receives massive centrifugal input from cortical areas but there has been as yet no characterization of their activity in vivo. We focused on projections to the OB from the anterior olfactory nucleus (AON), a major source of cortical feedback to the OB. We expressed GCaMP selectively in AON projection neurons using a mouse line expressing Cre recombinase (Cre) in these neurons and Cre-dependent viral vectors injected into AON, allowing us to image GCaMP fluorescence signals from their axon terminals in the OB. Electrical stimulation of AON evoked large fluorescence signals that could be imaged from the dorsal OB surface in vivo. Surprisingly, odorants also evoked large signals that were transient and coupled to odorant inhalation both in the anesthetized and awake mouse, suggesting that feedback from AON to the OB is rapid and robust across different brain states. The strength of AON feedback signals increased during wakefulness, suggesting a state-dependent modulation of cortical feedback to the OB. Two-photon GCaMP imaging revealed that different odorants activated different subsets of centrifugal AON axons and could elicit both excitation and suppression in different axons, indicating a surprising richness in the representation of odor information by cortical feedback to the OB. Finally, we found that activating neuromodulatory centers such as basal forebrain drove AON inputs to the OB independent of odorant stimulation. Our results point to the AON as a multifunctional cortical area that provides ongoing feedback to the OB and also serves as a descending relay for other neuromodulatory systems.

Consequences at adulthood of transient inactivation of the parahippocampal and prefrontal regions during early development: new insights from a disconnection animal model for schizophrenia

The psychic disintegration characteristic of schizophrenia is thought to result from a defective connectivity, of neurodevelopmental origin, between several integrative brain regions. The parahippocampal region and the prefrontal cortex are described as the main regions affected in schizophrenia. Interestingly, latent inhibition (LI) has been found to be reduced in patients with schizophrenia, and the existence of a dopaminergic dysfunction is also generally well accepted in this disorder. In the present review, we have integrated behavioral and neurochemical data obtained in a LI protocol involving adult rats subjected to neonatal functional inactivation of the entorhinal cortex, the ventral subiculum or the prefrontal cortex. The data discussed suggest a subtle and transient functional blockade during early development of the aforementioned brain regions is sufficient to induce schizophrenia-related behavioral and dopaminergic abnormalities in adulthood. In summary, these results support the view that our conceptual and methodological approach, based on functional disconnections, is valid for modeling some aspects of the pathophysiology of schizophrenia from a neurodevelopmental perspective.

Abnormalities in white matter microstructure associated with chronic ketamine use

Ketamine is an N-methyl-D-aspartate (NMDA) receptor antagonist that has been found to induce schizophrenia-type symptoms in humans and is a potent and fast-acting antidepressant. It is also a relatively widespread drug of abuse, particularly in China and the UK. Acute administration has been well characterized, but the effect of extended periods of ketamine use-on brain structure in humans-remains poorly understood. We measured indices of white matter microstructural integrity and connectivity in the brain of 16 ketamine users and 16 poly-drug-using controls, and we used probabilistic tractography to quantify changes in corticosubcortical connectivity associated with ketamine use. We found a reduction in the axial diffusivity profile of white matter in a right hemisphere network of white matter regions in ketamine users compared with controls. Within the ketamine-user group, we found a significant positive association between the connectivity profile between the caudate nucleus and the lateral prefrontal cortex and dissociative experiences. These findings suggest that chronic ketamine use may be associated with widespread disruption of white matter integrity, and white matter pathways between subcortical and prefrontal cortical areas may in part predict individual differences in dissociative experiences due to ketamine use.

Memory reconsolidation in aversive and appetitive settings

Memory reconsolidation has been observed across species and in a number of behavioral paradigms. The majority of memory reconsolidation studies have been carried out in Pavlovian fear conditioning and other aversive memory settings, with potential implications for the treatment of post-traumatic stress disorder. However, there is a growing literature on memory reconsolidation in appetitive reward-related memory paradigms, including translational models of drug addiction. While there appears to be substantial similarity in the basic phenomenon and underlying mechanisms of memory reconsolidation across unconditioned stimulus valence, there are also notable discrepancies. These arise both when comparing aversive to appetitive paradigms and also across different paradigms within the same valence of memory. We review the demonstration of memory reconsolidation across different aversive and appetitive memory paradigms, the commonalities and differences in underlying mechanisms and the conditions under which each memory undergoes reconsolidation. We focus particularly on whether principles derived from the aversive literature are applicable to appetitive settings, and also whether the expanding literature in appetitive paradigms is informative for fear memory reconsolidation.

Ketamine increases striatal dopamine release and hyperlocomotion in adult rats after postnatal functional blockade of the prefrontal cortex

Schizophrenia is a complex psychiatric disorder that may result from defective connectivity, of neurodevelopmental origin, between several integrative brain regions. Different anomalies consistent with brain development failures have been observed in patients' left prefrontal cortex (PFC). A striatal dopaminergic functional disturbance is also commonly acknowledged in schizophrenia and could be related to a dysfunctioning of dopamine-glutamate interactions. Non-competitive NMDA antagonists, such as ketamine, can induce psychotic symptoms in healthy individuals and worsen these symptoms in patients with schizophrenia. Our study set out to investigate the consequences of neonatal functional blockade of the PFC for dopaminergic and behavioral reactivity to ketamine in adult rats. Following tetrodotoxin (TTX) inactivation of the left PFC at postnatal day 8, dopaminergic responses induced by ketamine (5 mg/kg, 10 mg/kg, 20 mg/kg sc) were monitored using in vivo voltammetry in the left part of the dorsal striatum in freely moving adult rats. Dopaminergic responses and locomotor activity were followed in parallel. Compared to PBS animals, in rats microinjected with TTX, ketamine challenge induced a greater release of dopamine in the dorsal striatum for the highest dose (20 mg/kg sc) and the intermediate dose (10mg/kg sc). A higher increase in locomotor activity in TTX animals was observed only for the highest dose of ketamine (20 mg/kg sc). These data suggest transient inactivation of the PFC during early development results in greater behavioral and striatal dopaminergic reactivity to ketamine in adulthood. Our study provides an anatomo-functional framework that may contribute toward a better understanding of the involvement of NMDA glutamatergic receptors in schizophrenia.