Prefrontal stimulation of GABAA receptors counteracts the corticolimbic hyperactivity produced by NMDA antagonists in the prefrontal cortex of the rat

Cell type-specific genetic reconstitution of CB1 receptor subsets to assess their role in exploratory behaviour, sociability, and memory

Several studies support the notion that exploratory behaviour depends on the functionality of the cannabinoid type 1 (CB1) receptor in a cell type-specific manner. Mice lacking the CB1 receptor in forebrain GABAergic or dorsal telencephalic glutamatergic neurons have served as essential tools revealing the necessary CB1 receptor functions in these two neuronal populations. However, whether these specific CB1 receptor populations are also sufficient within the endocannabinoid system for wild-type-like exploratory behaviour has remained unknown. To evaluate cell-type-specific sufficiency of CB1 receptor signalling exclusively in dorsal telencephalic glutamatergic neurons (Glu-CB1-RS) or in forebrain GABAergic neurons (GABA-CB1-RS), we utilised a mouse model in which CB1 receptor expression can be reactivated conditionally at endogenous levels from a complete CB1-KO background. The two types of conditional CB1-rescue mice were compared with CB1 receptor-deficient [no reactivation (Stop-CB1)] and wild-type [ubiquitous reactivation of endogenous CB1 receptor (CB1-RS)] controls to investigate the behavioural consequences. We evaluated social and object exploratory behaviour in four different paradigms. Remarkably, the reduced exploration observed in Stop-CB1 animals was rescued in Glu-CB1-RS mice and sometimes even surpassed CB1-RS (wild-type) exploration. In contrast, GABA-CB1-RS animals showed the lowest exploratory drive in all paradigms, with an even stronger phenotype than Stop-CB1 mice. Interestingly, these effects weakened with increasing familiarity with the environment, suggesting a causal role for altered neophobia in the observed phenotypes. Taken together, using our genetic approach, we were able to substantiate the opposing role of the CB1 receptor in dorsal telencephalic glutamatergic versus forebrain GABAergic neurons regarding exploratory behaviour.

Long-term potentiation prevents ketamine-induced aberrant neurophysiological dynamics in the hippocampus-prefrontal cortex pathway in vivo

N-methyl-D-aspartate receptor (NMDAr) antagonists such as ketamine (KET) produce psychotic-like behavior in both humans and animal models. NMDAr hypofunction affects normal oscillatory dynamics and synaptic plasticity in key brain regions related to schizophrenia, particularly in the hippocampus and the prefrontal cortex. It has been shown that prior long-term potentiation (LTP) occluded the increase of synaptic efficacy in the hippocampus-prefrontal cortex pathway induced by MK-801, a non-competitive NMDAr antagonist. However, it is not clear whether LTP could also modulate aberrant oscillations and short-term plasticity disruptions induced by NMDAr antagonists. Thus, we tested whether LTP could mitigate the electrophysiological changes promoted by KET. We recorded HPC-PFC local field potentials and evoked responses in urethane anesthetized rats, before and after KET administration, preceded or not by LTP induction. Our results show that KET promotes an aberrant delta-high-gamma cross-frequency coupling in the PFC and an enhancement in HPC-PFC evoked responses. LTP induction prior to KET attenuates changes in synaptic efficiency and prevents the increase in cortical gamma amplitude comodulation. These findings are consistent with evidence that increased efficiency of glutamatergic receptors attenuates cognitive impairment in animal models of psychosis. Therefore, high-frequency stimulation in HPC may be a useful tool to better understand how to prevent NMDAr hypofunction effects on synaptic plasticity and oscillatory coordination in cortico-limbic circuits.

Infralimbic GluN2A-Containing NMDA Receptors Modulate Reconsolidation of Cocaine Self-Administration Memory

Addiction is characterized by high relapse susceptibility, and relapse can be triggered by drug-associated cues. Cue presentation induces retrieval of the drug-cue memory, which becomes labile and must be reconsolidated into long-term storage. Repeated unpaired cue presentation, however, promotes extinction. Cue-reactivity can be reduced by blocking reconsolidation or facilitating extinction, which are mediated by NMDA receptors (NMDArs). However, the role of NMDArs in either process following self-administration is unclear. Thus, to determine their role in extinction, rats learned to self-administer cocaine before receiving injections of the NMDAr antagonist CPP immediately after four 45-min extinction sessions. During a subsequent 90-min extinction retention test, CPP-treated rats lever pressed less than saline-treated rats indicating that NMDAr blockade facilitated extinction or disrupted drug-cue memory reconsolidation. In addition, infusing CPP into the infralimbic medial prefrontal cortex (IL-mPFC), a structure implicated in extinction, before four 45-min or immediately after four 30min extinction sessions, had similar results during the extinction retention tests. Next, the GluN2A-selective antagonist NVP or GluN2B-selective antagonist Ro25 was infused into IL-mPFC or nucleus accumbens (NAc) shell, another structure implicated in extinction, after four 45-min extinction sessions. Blocking GluN2A-, but not GluN2B-, containing NMDArs, in IL-mPFC or NAc shell reduced lever pressing during the extinction retention tests. Finally, to dissociate reconsolidation from extinction, NVP was infused into IL-mPFC after four 10-min reactivation sessions, which resulted in reduced lever pressing during the retention test. These results indicate that IL-mPFC GluN2A-containing NMDArs modulate reconsolidation, and suggest a novel treatment strategy, as reducing cue reactivity could limit relapse susceptibility.

Chemogenetic approach to model hypofrontality

Clinical evidence suggests that the prefrontal cortex (PFC) is hypofunctional in disorders including schizophrenia, drug addiction, and attention-deficit/hyperactivity disorder (ADHD). In schizophrenia, hypofrontality has been further suggested to cause both the negative and cognitive symptoms, and overactivity of dopamine neurons that project to subcortical areas. The latter may contribute to the development of positive symptoms of the disorder. Nevertheless, what causes hypofrontality and how it alters dopamine transmission in subcortical structures remain unclear due, in part, to the difficulty in modeling hypofrontality using previous techniques (e.g. PFC lesioning, focal cooling, repeated treatment with psychotomimetic drugs). We propose that the use of designer receptors exclusively activated by designer drugs (DREADDs) chemogenetic technique will allow precise interrogations of PFC functions. Combined with electrophysiological recordings, we can investigate the effects of PFC hypofunction on activity of dopamine neurons. Importantly, from a drug target discovery perspective, the use of DREADDs will enable us to examine whether chemogenetically enhancing PFC activity will reverse the behavioral abnormalities associated with PFC hypofunction and dopamine neuron overactivity, and also explore druggable targets for the treatment of schizophrenia and other disorders associated with abnormalities via modulation of the G-protein coupled receptor signaling pathway. In conclusion, the use of the DREADDs technique has several advantages over other previously employed strategies to simulate PFC hypofunction not only in terms of disease modeling but also from the viewpoint of drug target discovery.

Sex Differences Distinguish Intracortical Glutamate Receptor-Mediated Regulation of Extracellular Dopamine Levels in the Prefrontal Cortex of Adult Rats

Executive functions of the prefrontal cortex (PFC) are sensitive to local dopamine (DA) levels. Although sex differences distinguish these functions and their dysfunction in disease, the basis for this is unknown. We asked whether sex differences might result from dimorphisms in the glutamatergic mechanisms that regulate PFC DA levels. Using antagonists selective for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptors, we compared drug effects on in vivo microdialysis DA measurements in the PFC of adult male and female rats. We found that baseline DA levels were similar across sex, AMPA antagonism decreased PFC DA in both sexes, and NMDA antagonism increased DA in males but decreased DA in females. We also found that, at subseizure-producing drug levels, γ-aminobutyric acid (GABA)-A antagonism did not affect DA in either sex but that GABA-B antagonism transiently increased PFC DA in both sexes, albeit more so in females. Finally, when NMDA antagonism was coincident with GABA-B antagonism, PFC DA levels in males responded as if to GABA-B antagonism alone, whereas in females, DA effects mirrored those induced by NMDA antagonism. Taken together, these data suggest commonalities and fundamental differences in the intracortical amino acid transmitter mechanisms that regulate DA homeostasis in the male and female rat PFCs.

Breakdown of the striatal-default mode network loop in schizophrenia

The striatum has been shown to be a core region in schizophrenia with functional and structural deficits. Previous studies have confirmed the schizophrenia-related functional connectivity between the striatal and cortical regions. However, among these, few studies have attempted to determine the directional flow of the influence. In the present study, we used resting-state fMRI to explore the directed connectivity between the striatum and the cortical regions in schizophrenia. Employing a Granger causality analysis, we observed a significant failure of the directed inhibitory influence of the striatum on the default mode network (DMN) in schizophrenia. Furthermore, the reciprocal influence of the DMN on the striatum was also significantly reduced. These findings provide compelling evidence for a breakdown of the striatum-DMN loop in schizophrenia. This abnormal connectivity could be related to clinical variables. In conclusion, our study suggests that abnormally directed influences between the striatum and the DMN might be a biomarker of schizophrenia and also reveals a potential target for treatment.

Hypofunction of prefrontal cortex NMDA receptors does not change stress-induced release of dopamine and noradrenaline in amygdala but disrupts aversive memory

RATIONALE

A dysfunction of prefrontal cortex has been associated with the exacerbated response to stress observed in schizophrenic patients and high-risk individuals to develop psychosis. The hypofunction of NMDA glutamatergic receptors induced by NMDA antagonists produces cortico-limbic hyperactivity, and this is used as an experimental model to resemble behavioural abnormalities observed in schizophrenia.

OBJECTIVES

The aim of the present study was to investigate whether injections of NMDA antagonists into the medial prefrontal cortex of the rat change (1) the increases of dopamine, noradrenaline and corticosterone concentrations produced by acute stress in amygdala, and (2) the acquisition of aversive memory related to a stressful event.

METHODS

Male Wistar rats were implanted with guide cannulae to perform microdialysis and bilateral microinjections (0.5 μl/side) of the NMDA antagonist 3-[(R)-2-carboxypiperazin-4-yl]-propyl-1-phophonic acid (CPP) (25 and 100 ng). Prefrontal injections were performed 60 min before restraint stress in microdialysis experiments, or training (footshock; 0.6 mA, 2 s) in inhibitory avoidance test. Retention latency was evaluated 24 h after training as an index of aversive memory.

RESULTS

Acute stress increased amygdala dialysate concentrations of dopamine (160% of baseline), noradrenaline (145% of baseline) and corticosterone (170% of baseline). Prefrontal injections of CPP did not change the increases of dopamine, noradrenaline or corticosterone produced by stress. In contrast, CPP significantly reduced the retention latency in the inhibitory avoidance test.

CONCLUSIONS

These results suggest that the hypofunction of prefrontal NMDA receptors does not change the sensitivity to acute stress of dopamine and noradrenaline projections to amygdala but impairs the acquisition of aversive memory.

Impaired cognition after stimulation of P2Y1 receptors in the rat medial prefrontal cortex

We hypothesize that cortical ATP and ADP accumulating in the extracellular space, eg during prolonged network activity, contribute to a decline in cognitive performance in particular via stimulation of the G protein-coupled P2Y1 receptor (P2Y1R) subtype. Here, we report first evidence on P2Y1R-mediated control of cognitive functioning in rats using bilateral microinfusions of the selective agonist MRS2365 into medial prefrontal cortex (mPFC). MRS2365 attenuated prepulse inhibition of the acoustic startle reflex while having no impact on startle amplitude. Stimulation of P2Y1Rs deteriorated performance accuracy in the delayed non-matching to position task in a delay dependent manner and increased the rate of magazine entries consistent with both working memory disturbances and impaired impulse control. Further, MRS2365 significantly impaired performance in the reversal learning task. These effects might be related to MRS2365-evoked increase of dopamine observed by microdialysis to be short-lasting in mPFC and long-lasting in the nucleus accumbens. P2Y1Rs were identified on pyramidal cells and parvalbumin-positive interneurons, but not on tyrosine hydroxylase-positive fibers, which argues for an indirect activation of dopaminergic afferents in the cortex by MRS2365. Collectively, these results suggest that activation of P2Y1Rs in the mPFC impairs inhibitory control and behavioral flexibility mediated by increased mesocorticolimbic activity and local disinhibition.

Impact of prenatal ischemia on behavior, cognitive abilities and neuroanatomy in adult rats with white matter damage

Early brain damage, such as white matter damage (WMD), resulting from perinatal hypoxia-ischemia in preterm and low birth weight infants represents a high risk factor for mortality and chronic disabilities, including sensory, motor, behavioral and cognitive disorders. In previous studies, we developed a model of WMD based on prenatal ischemia (PI), induced by unilateral ligation of uterine artery at E17 in pregnant rats. We have shown that PI reproduced some of the main deficits observed in preterm infants, such as white and gray matter damage, myelination deficits, locomotor, sensorimotor, and short-term memory impairments, as well as related musculoskeletal and neuroanatomical histopathologies [1-3]. Here, we determined the deleterious impact of PI on several behavioral and cognitive abilities in adult rats, as well as on the neuroanatomical substratum in various related brain areas. Adult PI rats exhibited spontaneous exploratory and motor hyperactivity, deficits in information encoding, and deficits in short- and long-term object memory tasks, but no impairments in spatial learning or working memory in watermaze tasks. These results were in accordance with white matter injury and damage in the medial and lateral entorhinal cortices, as detected by axonal degeneration, astrogliosis and neuronal density. Although there was astrogliosis and axonal degeneration in the fornix, hippocampus and cingulate cortex, neuronal density in the hippocampus and cingulate cortex was not affected by PI. Levels of spontaneous hyperactivity, deficits in object memory tasks, neuronal density in the medial and lateral entorhinal cortices, and astrogliosis in the fornix correlated with birth weight in PI rats. Thus, this rodent model of WMD based on PI appears to recapitulate the main neurobehavioral and neuroanatomical human deficits often observed in preterm children with a perinatal history of ischemia.

Circuit specific functions of cannabinoid CB1 receptor in the balance of investigatory drive and exploration

Well balanced novelty seeking and exploration are fundamental behaviours for survival and are found to be dysfunctional in several psychiatric disorders. Recent studies suggest that the endocannabinoid (eCB) system is an important control system for investigatory drive. Pharmacological treatment of rodents with cannabinergic drugs results in altered social and object investigation. Interestingly, contradictory results have been obtained, depending on the treatment, drug concentration and experimental conditions. The cannabinoid type 1 (CB1) receptor, a central component of the eCB system, is predominantly found at the synapses of two opposing neuronal populations, i.e. on inhibitory GABAergic and excitatory glutamatergic neurons. In the present study, using different transgenic mouse lines, we aimed at investigating the impact of CB1 receptor inactivation in glutamatergic or GABAergic neurons on investigatory behaviour. We evaluated animate (interaction partner) and inanimate (object) exploratory behaviour in three different paradigms. We show that exploration was increased when CB1 receptor was deleted from cortical and striatal GABAergic neurons. No effect was observed when CB1 receptor was deleted specifically from dopamine receptor D1-expressing striatal GABAergic medium spiny neurons. In contrast, deletion of CB1 receptor from cortical glutamatergic neurons resulted in a decreased exploration. Thus, our results indicate that exploratory behaviour is accurately balanced in both, the social and non-social context, by the eCB system via CB1 receptor activation on cortical glutamatergic and GABAergic neurons. In addition, the results could explain the contradictory findings of previous pharmacological studies and could further suggest a possibility to readjust an imbalance in exploratory behaviour observed in psychiatric disorders.

Na+,K+-ATPase activity is increased in rats subjected to chronic administration of ketamine

De Luca RD, Fraga DB, Ghedim FV, Kolling J, Ferreira AGK, Cunha AA, Wyse ATS, Zugno AI. Na+,K+-ATPase activity is increased in rats subjected to chronic administration of ketamine.

Objective: Schizophrenia is a devastating psychiatric disorder. Symptoms of schizophrenia can be divided into positive, negative and cognitive, and the physiopathology is still been unknown. Na+,K+-ATPase is a protein in its role as a maintainer of fluid balance in all mammals and alterations in this enzyme could cause brain abnormalities. The aim of our study was to investigate the activity of this enzyme in rats submitted to chronic administration of ketamine.

Methods: Adult male Wistar rats were submitted to sub-anaesthetic doses of the 25 mg/kg ketamine by seven consecutive days and the Na+,K+-ATPase activity was analysed in prefrontal and hippocampus of rats.

Results: We observed an increase in Na+,K+-ATPase activity in prefrontal cortex administration of 25 mg/kg ketamine. However, ketamine has no effect in hippocampus.

Conclusion: This evidence indicates that the alteration in Na+,K+-ATPase may be related with glutamatergic system and consequently could be related to the development of schizophrenia.

Effects of pharmacological manipulations of NMDA-receptors on deliberation in the Multiple-T task

Both humans and non-human animals have the ability to navigate and make decisions within complex environments. This ability is largely dependent upon learning and memory processes, many of which are known to depend on NMDA-sensitive receptors. When humans come to difficult decisions they often pause to deliberate over their choices. Similarly, rats pause at difficult choice points. This behavior, known as vicarious trial and error (VTE), is hippocampally dependent and entails neurophysiological representations of expectations of future outcomes in hippocampus and downstream structures. In order to determine the dependence of VTE behaviors on NMDA-sensitive receptors, we tested rats on a Multiple-T choice task with a reward-delivery reversal known to elicit VTE. Rats under the influence of NMDA-receptor antagonists (CPP) showed a significant reduction in VTE, particularly at the reward reversal, implying a role for NMDA-sensitive receptors in the generation of vicarious trial and error behaviors.

GABAergic interneuron origin of schizophrenia pathophysiology

Hypofunction of N-methyl-d-aspartic acid-type glutamate receptors (NMDAR) induced by the systemic administration of NMDAR antagonists is well known to cause schizophrenia-like symptoms in otherwise healthy subjects. However, the brain areas or cell-types responsible for the emergence of these symptoms following NMDAR hypofunction remain largely unknown. One possibility, the so-called "GABAergic origin hypothesis," is that NMDAR hypofunction at GABAergic interneurons, in particular, is sufficient for schizophrenia-like effects. In one attempt to address this issue, transgenic mice were generated in which NMDARs were selectively deleted from cortical and hippocampal GABAergic interneurons, a majority of which were parvalbumin (PV)-positive. This manipulation triggered a constellation of phenotypes--from molecular and physiological to behavioral--resembling characteristics of human schizophrenia. Based on these results, and in conjunction with previous literature, we argue that during development, NMDAR hypofunction at cortical, PV-positive, fast-spiking interneurons produces schizophrenia-like effects. This review summarizes the data demonstrating that in schizophrenia, GABAergic (particularly PV-positive) interneurons are disrupted. PV-positive interneurons, many of which display a fast-spiking firing pattern, are critical not only for tight temporal control of cortical inhibition but also for the generation of synchronous membrane-potential gamma-band oscillations. We therefore suggest that in schizophrenia the specific ability of fast-spiking interneurons to control and synchronize disparate cortical circuits is disrupted and that this disruption may underlie many of the schizophrenia symptoms. We further argue that the high vulnerability of corticolimbic fast-spiking interneurons to genetic predispositions and to early environmental insults--including excitotoxicity and oxidative stress--might help to explain their significant contribution to the development of schizophrenia.