Opposite effect of phencyclidine on activity-regulated cytoskeleton-associated protein (Arc) in juvenile and adult limbic rat brain regions

NeuropsychopharmARCology: Shaping Neuroplasticity through Arc/ Arg3.1 Modulation

Activity-regulated cytoskeleton-associated protein (aka activity-regulated gene Arg3.1) belongs to the effector gene family of the immediate early genes. This family encodes effector proteins, which act directly on cellular homeostasis and function. Arc/Arg3.1 is localized at dendritic processes, allowing the protein local synthesis on demand, and it is considered a reliable index of activity- dependent synaptic changes. Evidence also exists showing the critical role of Arc/Arg3.1 in memory processes. The high sensitivity to changes in neuronal activity, its specific localization as well as its involvement in long-term synaptic plasticity indeed make this effector gene a potential, critical target of the action of psychotropic drugs. In this review, we focus on antipsychotic and antidepressant drugs as well as on psychostimulants, which belong to the category of drugs of abuse but can also be used as drugs for specific disorders of the central nervous system (i.e., Attention Deficit Hyperactivity Disorder). It is demonstrated that psychotropic drugs with different mechanisms of action converge on Arc/Arg3.1, providing a means whereby Arc/Arg3.1 synaptic modulation may contribute to their therapeutic activity. The potential translational implications for different neuropsychiatric conditions are also discussed, recognizing that the treatment of these disorders is indeed complex and involves the simultaneous regulation of several dysfunctional mechanisms.

Arc Regulates Transcription of Genes for Plasticity, Excitability and Alzheimer’s Disease

The immediate early gene Arc is a master regulator of synaptic function and a critical determinant of memory consolidation. Here, we show that Arc interacts with dynamic chromatin and closely associates with histone markers for active enhancers and transcription in cultured rat hippocampal neurons. Both these histone modifications, H3K27Ac and H3K9Ac, have recently been shown to be upregulated in late-onset Alzheimer’s disease (AD). When Arc induction by pharmacological network activation was prevented using a short hairpin RNA, the expression profile was altered for over 1900 genes, which included genes associated with synaptic function, neuronal plasticity, intrinsic excitability, and signalling pathways. Interestingly, about 100 Arc-dependent genes are associated with the pathophysiology of AD. When endogenous Arc expression was induced in HEK293T cells, the transcription of many neuronal genes was increased, suggesting that Arc can control expression in the absence of activated signalling pathways. Taken together, these data establish Arc as a master regulator of neuronal activity-dependent gene expression and suggest that it plays a significant role in the pathophysiology of AD.

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.

Effect of pharmacological manipulations on Arc function

Activity-regulated cytoskeleton-associated protein (Arc) is a brain-enriched immediate early gene that regulates important mechanisms implicated in learning and memory. Arc levels are controlled through a balance of induction and degradation in an activity-dependent manner. Arc further undergoes multiple post-translational modifications that regulate its stability, localization and function. Recent studies demonstrate that these features of Arc can be pharmacologically manipulated. In this review, we discuss some of these compounds, with an emphasis on drugs of abuse and psychotropic drugs. We also discuss inflammatory states that regulate Arc.

Early-life blockade of NMDA receptors induces epigenetic abnormalities in the adult medial prefrontal cortex: possible involvement in memory impairment in trace fear conditioning

RATIONALE

Several findings indicate that early-life dysfunction of N-methyl-D-aspartate (NMDA) receptors might cause schizophrenia-like abnormalities in adulthood that might be induced by impairments in epigenetic regulation.

OBJECTIVES

In the present study, we investigated whether postnatal blockade of NMDA receptors (within the first 3 weeks of life) by the competitive antagonist CGP 37849 (CGP) might affect some epigenetic markers in the adult medial prefrontal cortex (mPFC).

METHODS

Histone H3 phosphorylation at serine 10 (H3S10ph), histone H3 acetylation at lysine 9 or 14 (H3K9ac or H3K14ac, respectively), or expression of histone deacetylase (HDAC) 2, HDAC5, myocyte enhancer factor (MEF) 2D and activity-regulated cytoskeleton-associated protein (Arc) were analysed. Moreover, we also evaluated whether the deacetylase inhibitor sodium butyrate (SB; 1.2 mg/kg, ip) could prevent behavioural and neurochemical changes in the mPFC induced by CGP during memory retrieval in the trace fear conditioning paradigm.

RESULTS

The results showed that CGP administration increased the number of H3S10ph nuclei but did not affect H3K9ac and H3K14ac or HDAC2 protein levels. However, CGP administration altered the HDAC5 mRNA and protein levels and increased the mRNA and protein levels of MEF2D. CGP also increased Arc mRNA, which was correlated with an increase in the amount of Arc DNA bound to MEF2D. SB given 2 h after training prevented impairment of the freezing response and disruption of epigenetic markers (H3S10ph, HDAC5, MEF2D) and Arc expression during memory retrieval induced by CGP administration.

CONCLUSIONS

The early-life blockade of NMDA receptors impairs some epigenetic regulatory processes in the mPFC that are involved in fear memory formation.

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.

Repeated Blockade of NMDA Receptors During Adolescence Impairs Reversal Learning and Disrupts GABAergic Interneurons in Rat Medial Prefrontal Cortex

Adolescence is of particular significance to schizophrenia, since psychosis onset typically occurs in this critical period. Based on the N-methyl-D-aspartate (NMDA) receptor hypofunction hypothesis of schizophrenia, in this study, we investigated whether and how repeated NMDA receptor blockade during adolescence would affect GABAergic interneurons in rat medial prefrontal cortex (mPFC) and mPFC-mediated cognitive functions. Specifically, adolescent rats were subjected to intraperitoneal administration of MK-801 (0.1, 0.2, 0.4 mg/kg), a non-competitive NMDA receptor antagonist, for 14 days and then tested for reference memory and reversal learning in the water maze. The density of parvabumin (PV)-, calbindin (CB)- and calretinin (CR)-positive neurons in mPFC was analyzed at either 24 h or 7 days after drug cessation. We found that MK-801 treatment delayed reversal learning in the water maze without affecting initial acquisition. Strikingly, MK-801 treatment also significantly reduced the density of PV⁺ and CB⁺ neurons, and this effect persisted for 7 days after drug cessation at the dose of 0.2 mg/kg. We further demonstrated that the reduction in PV⁺ and CB⁺ neuron densities was ascribed to a downregulation of the expression levels of PV and CB, but not to neuronal death. These results parallel the behavioral and neuropathological changes of schizophrenia and provide evidence that adolescent NMDA receptors antagonism offers a useful tool for unraveling the etiology of the disease.

Perinatal phencyclidine administration decreases the density of cortical interneurons and increases the expression of neuregulin-1

RATIONALE

Perinatal phencyclidine (PCP) administration in rat blocks the N-methyl D-aspartate receptor (NMDAR) and causes symptoms reminiscent of schizophrenia in human. A growing body of evidence suggests that alterations in γ-aminobutyric acid (GABA) interneuron neurotransmission may be associated with schizophrenia. Neuregulin-1 (NRG-1) is a trophic factor important for neurodevelopment, synaptic plasticity, and wiring of GABA circuits.

OBJECTIVES

The aim of this study was to determine the long-term effects of perinatal PCP administration on the projection and local circuit neurons and NRG-1 expression in the cortex and hippocampus.

METHODS

Rats were treated on postnatal day 2 (P2), P6, P9, and P12 with either PCP (10 mg/kg) or saline. Morphological studies and determination of NRG-1 expression were performed at P70.

RESULTS

We demonstrate reduced densities of principal neurons in the CA3 and dentate gyrus (DG) subregions of the hippocampus and a reduction of major interneuronal populations in all cortical and hippocampal regions studied in PCP-treated rats compared with controls. For the first time, we show the reduced density of reelin- and somatostatin-positive cells in the cortex and hippocampus of animals perinatally treated with PCP. Furthermore, an increase in the numbers of perisomatic inhibitory terminals around the principal cells was observed in the motor cortex and DG. We also show that perinatal PCP administration leads to an increased NRG-1 expression in the cortex and hippocampus.

CONCLUSION

Taken together, our findings demonstrate that perinatal PCP administration increases NRG-1 expression and reduces the number of projecting and local circuit neurons, revealing complex consequences of NMDAR blockade.

Behavioral and neurochemical consequences of cortical oxidative stress on parvalbumin-interneuron maturation in rodent models of schizophrenia

Oxidative stress, in response to the activation of the superoxide-producing enzyme Nox2, has been implicated in the schizophrenia-like behavioral dysfunction that develops in animals that were subject to either neonatal NMDA receptor-antagonist treatment or social isolation. In both of these animal models of schizophrenia, an environmental insult occurring during the period of active maturation of the fast-spiking parvalbumin-positive (PV+) interneuronal circuit leads to a diminished expression of parvalbumin in GABA-inhibitory neurons when animals reach adulthood. The loss of PV+ interneurons in animal models had been tentatively attributed to the death of these neurons. However, present results show that for the perinatal NMDA-R antagonist model these interneurons are still alive when animals are 5-6 weeks of age even though they have lost their phenotype and no longer express parvalbumin. Alterations in parvalbumin expression and sensory-evoked gamma-oscillatory activity, regulated by PV+ interneurons, are consistently observed in schizophrenia. We propose that cortical networks consisting of faulty PV+ interneurons interacting with pyramidal neurons may be responsible for the aberrant oscillatory activity observed in schizophrenia. Thus, oxidative stress during the maturation window for PV+ interneurons by alteration of normal brain development, leads to the emergence of schizophrenia-like behavioral dysfunctions when subjects reach early adulthood.

Tonic NMDA receptor-mediated current in prefrontal cortical pyramidal cells and fast-spiking interneurons

Tonically activated neuronal currents mediated by N-methyl-d-aspartate receptors (NMDARs) have been hypothesized to contribute to normal neuronal function as well as to neuronal pathology resulting from excessive activation of glutamate receptors (e.g., excitotoxicity). Whereas cortical excitatory cells are very vulnerable to excitotoxic insult, the data regarding resistance of inhibitory cells (or interneurons) are inconsistent. Types of neurons with more pronounced tonic NMDAR current potentially associated with the activation of extrasynaptic NMDARs could be expected to be more vulnerable to excessive activation by glutamate. In this study, we compared tonic activation of NMDARs in excitatory pyramidal cells and inhibitory fast-spiking interneurons in prefrontal cortical slices. We assessed tonic NMDAR current by measuring holding current shift as well as noise reduction following NMDAR blockade after removal of spontaneous glutamate release. In addition, we compared NMDAR miniature excitatory postsynaptic currents (EPSCs) in both cell types. We have demonstrated for the first time that tonic NMDAR currents are present in inhibitory fast-spiking interneurons. We found that the magnitude of tonic NMDAR current is similar in pyramidal cells and fast-spiking interneurons, and that quantal release of glutamate does not significantly impact tonic NMDAR current.

Asenapine effects on cognitive and monoamine dysfunction elicited by subchronic phencyclidine administration

PURPOSE

Repeated, intermittent administration of the psychotropic NMDA antagonist phencyclidine (PCP) to laboratory animals causes impairment in cognitive and executive functions, modeling important sequelae of schizophrenia; these effects are thought to be due to a dysregulation of neurotransmission within the prefrontal cortex. Atypical antipsychotic drugs have been reported to have measurable, if incomplete, effects on cognitive dysfunction in this model, and these effects may be due to their ability to normalize a subset of the physiological deficits occurring within the prefrontal cortex. Asenapine is an atypical antipsychotic approved in the US for the treatment of schizophrenia and for the treatment, as monotherapy or adjunctive therapy to lithium or valproate, of acute manic or mixed episodes associated bipolar I disorder. To understand its cognitive and neurochemical actions more fully, we explored the effects of short- and long-term dosing with asenapine on measures of cognitive and motor function in normal monkeys and in those previously exposed for 2 weeks to PCP; we further studied the impact of treatment with asenapine on dopamine and serotonin turnover in discrete brain regions from the same cohort.

METHODS

Monkeys were trained to perform reversal learning and object retrieval procedures before twice daily administration of PCP (0.3 mg/kg intra-muscular) or saline for 14 days. Tests confirmed cognitive deficits in PCP-exposed animals before beginning twice daily administration of saline (control) or asenapine (50, 100, or 150 μg/kg, intra-muscular). Dopamine and serotonin turnover were assessed in 15 specific brain regions by high-pressure liquid chromatography measures of the ratio of parent amine to its major metabolite.

RESULTS

On average, PCP-treated monkeys made twice as many errors in the reversal task as did control monkeys. Asenapine facilitated reversal learning performance in PCP-exposed monkeys, with improvements at trend level after 1 week of administration and reaching significance after 2-4 weeks of dosing. In week 4, the improvement with asenapine 150 μg/kg (p = 0.01) rendered the performance of PCP-exposed monkeys indistinguishable from that of normal monkeys without compromising fine motor function. Asenapine administration (150 μg/kg twice daily) produced an increase in dopamine and serotonin turnover in most brain regions of control monkeys and asenapine (50-150 μg/kg) increased dopamine and serotonin turnover in several brain regions of subchronic PCP-treated monkeys. No significant changes in the steady-state levels of dopamine or serotonin were observed in any brain region except for the central amygdala, in which a significant depletion of dopamine was observed in PCP-treated control monkeys; asenapine treatment reversed this dopamine depletion. A significant decrease in serotonin utilization was observed in the orbitofrontal cortex and nucleus accumbens in PCP monkeys, which may underlie poor reversal learning. In the same brain regions, dopamine utilization was not affected. Asenapine ameliorated this serotonin deficit in a dose-related manner that matched its efficacy for reversing the cognitive deficit.

CONCLUSIONS

In this model of cognitive dysfunction, asenapine produced substantial gains in executive functions that were maintained with long-term administration. The cognition-enhancing effects of asenapine and the neurochemical changes in serotonin and dopamine turnover seen in this study are hypothesized to be primarily related to its potent serotonergic and noradrenergic receptor binding properties, and support the potential for asenapine to reduce cognitive dysfunction in patients with schizophrenia and bipolar disorder.

Persisting cognitive deficits induced by low-dose, subchronic treatment with MK-801 in adolescent rats

Cognitive impairments have been proposed as a core feature of schizophrenia. Studies have shown that chronic or subchronic treatment with N-methyl-d-aspartate (NMDA) antagonists could induce cognitive deficits that resemble the symptoms of schizophrenia, yet few studies have investigated the effects of repeated NMDA blockade during adolescence on cognition. In the current study, adolescent, male rats were treated with an intraperitoneal injection of MK-801 (0.05, 0.1, and 0.2mg/kg) once daily for 14days. They were then tested 24h and 14days after drug cessation, respectively, in a series of behavioural tasks, including the object recognition task, the object-in-context recognition task and the working memory task of the Morris water maze (MWM). Results showed that object-in-context recognition and spatial working memory in the MWM were significantly impaired by repeated MK-801 treatment when animals were tested 24h after drug cessation, but object recognition was left intact. In particular, such deficits were observed 14days after drug cessation in the 0.2mg/kg group. The cognition-impairing effect of MK-801 could not be attributed to malnutrition or alterations in motor functions. Taken together, this study may provide support for establishing an animal model of cognitive deficits of schizophrenia based on low-dose, repeated treatment of MK-801 during adolescence.

Failure of NMDA receptor hypofunction to induce a pathological reduction in PV-positive GABAergic cell markers

Reduction in cortical presynaptic markers, notably parvalbumin (PV), for the chandelier subtype of inhibitory γ-amino-butyric acid (GABA) interneurons is a highly replicated post-mortem finding in schizophrenia. Evidence from genetic and pharmacological studies implicates hypofunction of N-methyl-d-aspartate receptor (NMDAR)-mediated glutamatergic signaling as a critical component of the pathophysiology of schizophrenia. Serine racemase (SR) produces the endogenous NMDAR co-agonist d-serine, and disruption of the SR gene results in reduced NMDAR signaling. SR null mutant (-/-) mice were used to study the link between NMDAR hypofunction and decreased PV expression, assessed by immunoreactive (IR) cell density in the medial prefrontal cortex and hippocampus and protein levels in brain homogenates from the frontal cortex and hippocampus. Contrary to expectations, SR -/- mice showed modest elevations in PV-IR cell density and no difference in PV expression in brain homogenate. To control for these surprising results, we investigated PV expression in mice and rats following subchronic phencyclidine or ketamine treatments in adulthood. PV expression was not affected by drug these treatment in either species, failing to reproduce previously published findings. Our findings challenge the hypothesis that pathological deficits in PV expression are simply a consequence of NMDAR hypofunction.