Viral infection of developing GABAergic neurons in a model of hippocampal disinhibition

Schizophrenia and Influenza at the Centenary of the 1918-1919 Spanish Influenza Pandemic: Mechanisms of Psychosis Risk

Associations between influenza infection and psychosis have been reported since the eighteenth century, with acute "psychoses of influenza" documented during multiple pandemics. In the late 20th century, reports of a season-of-birth effect in schizophrenia were supported by large-scale ecological and sero-epidemiological studies suggesting that maternal influenza infection increases the risk of psychosis in offspring. We examine the evidence for the association between influenza infection and schizophrenia risk, before reviewing possible mechanisms via which this risk may be conferred. Maternal immune activation models implicate placental dysfunction, disruption of cytokine networks, and subsequent microglial activation as potentially important pathogenic processes. More recent neuroimmunological advances focusing on neuronal autoimmunity following infection provide the basis for a model of infection-induced psychosis, potentially implicating autoimmunity to schizophrenia-relevant protein targets including the N-methyl-D-aspartate receptor. Finally, we outline areas for future research and relevant experimental approaches and consider whether the current evidence provides a basis for the rational development of strategies to prevent schizophrenia.

Immune aging, dysmetabolism, and inflammation in neurological diseases

As we age, the immune system undergoes a process of senescence accompanied by the increased production of proinflammatory cytokines, a chronic subclinical condition named as “inflammaging”. Emerging evidence from human and experimental models suggest that immune senescence also affects the central nervous system and promotes neuronal dysfunction, especially within susceptible neuronal populations. In this review we discuss the potential role of immune aging, inflammation and metabolic derangement in neurological diseases. The discovery of novel therapeutic strategies targeting age-linked inflammation may promote healthy brain aging and the treatment of neurodegenerative as well as neuropsychiatric disorders.

The Long and the Short of it: Gene and Environment Interactions During Early Cortical Development and Consequences for Long-Term Neurological Disease

Cortical development is a complex amalgamation of proliferation, migration, differentiation, and circuit formation. These processes follow defined timescales and are controlled by a combination of intrinsic and extrinsic factors. It is currently unclear how robust and flexible these processes are and whether the developing brain has the capacity to recover from disruptions. What is clear is that there are a number of cognitive disorders or conditions that are elicited as a result of disrupted cortical development, although it may take a long time for the full pathophysiology of the conditions to be realized clinically. The critical window for the manifestation of a neurodevelopmental disorder is prolonged, and there is the potential for a complex interplay between genes and environment. While there have been extended investigations into the genetic basis of a number of neurological and mental disorders, limited definitive associations have been discovered. Many environmental factors, including inflammation and stress, have been linked to neurodevelopmental disorders, and it may be that a better understanding of the interplay between genes and environment will speed progress in this field. In particular, the development of the brain needs to be considered in the context of the whole materno-fetal unit as the degree of the metabolic, endocrine, or inflammatory responses, for example, will greatly influence the environment in which the brain develops. This review will emphasize the importance of extending neurodevelopmental studies to the contribution of the placenta, vasculature, cerebrospinal fluid, and to maternal and fetal immune response. These combined investigations are more likely to reveal genetic and environmental factors that influence the different stages of neuronal development and potentially lead to the better understanding of the etiology of neurological and mental disorders such as autism, epilepsy, cerebral palsy, and schizophrenia.

Developmental pathology, dopamine, stress and schizophrenia

Psychological stress is a contributing factor for a wide variety of neuropsychiatric diseases including substance use disorders, anxiety, depression and schizophrenia. However, it has not been conclusively determined how stress augments the symptoms of these diseases. Here we review evidence that the ventral hippocampus may be a site of convergence whereby a number of seemingly discrete risk factors, including stress, may interact to precipitate psychosis in schizophrenia. Specifically, aberrant hippocampal activity has been demonstrated to underlie both the elevated dopamine neuron activity and associated behavioral hyperactivity to dopamine agonists in a verified animal model of schizophrenia. In addition, stress, psychostimulant drug use, prenatal infection and select genetic polymorphisms all appear to augment ventral hippocampal function that may therefore exaggerate or precipitate psychotic symptoms. Such information is critical for our understanding into the pathology of psychiatric disease with the ultimate aim being the development of more effective therapeutics.

Neonatal intrahippocampal immune challenge alters dopamine modulation of prefrontal cortical interneurons in adult rats

BACKGROUND

Among the diverse animal models proposed for schizophrenia, the neonatal ventral hippocampal lesion (NVHL) is one of the most widely used. However, its construct validity can be questioned because there is no evidence of a lesion present in schizophrenia. Other approaches that have tried to capture environmental influences on development include diverse models of maternal infection.

METHODS

As the early postnatal days in rodents are equivalent to the third trimester of human pregnancy in terms of brain development, we decided to test whether a neonatal immune challenge with an injection of the bacterial endotoxin lipopolysaccharide (LPS) into the ventral hippocampus caused deficits in interneuron function similar to those reported for the NVHL.

RESULTS

Neonatal LPS injection caused a persistent elevation in cytokines in several brain regions, deficits in prepulse inhibition of the acoustic startle response, and a loss of the periadolescent maturation in the response of prefrontal cortical fast-spiking interneurons to dopamine.

CONCLUSIONS

The same phenotypes elicited by a NVHL can be obtained with an intrahippocampal immune challenge, suggesting that perinatal environmental factors can affect adult prefrontal interneuron maturation during adolescence.

Potential microbial origins of schizophrenia and their treatments

Schizophrenia is a severe brain disease that affects approximately 1% of the world's population. Extensive study into the indication of and causes of this disease has been ongoing for decades. Historical review of research into associated abnormalities and markers common in schizophrenic patients has demonstrated a correlation with potential microbial origins in the development of the disease. While infectious etiologies could be responsible for some cases of schizophrenia, no consistent use of anti-infective agents has been developed for its prevention or treatment. Elucidation of the mechanisms for infectious roots of schizophrenia may open new avenues for effective treatment.

The neurodevelopmental hypothesis of schizophrenia, revisited

While multiple theories have been put forth regarding the origin of schizophrenia, by far the vast majority of evidence points to the neurodevelopmental model in which developmental insults as early as late first or early second trimester lead to the activation of pathologic neural circuits during adolescence or young adulthood leading to the emergence of positive or negative symptoms. In this report, we examine the evidence from brain pathology (enlargement of the cerebroventricular system, changes in gray and white matters, and abnormal laminar organization), genetics (changes in the normal expression of proteins that are involved in early migration of neurons and glia, cell proliferation, axonal outgrowth, synaptogenesis, and apoptosis), environmental factors (increased frequency of obstetric complications and increased rates of schizophrenic births due to prenatal viral or bacterial infections), and gene-environmental interactions (a disproportionate number of schizophrenia candidate genes are regulated by hypoxia, microdeletions and microduplications, the overrepresentation of pathogen-related genes among schizophrenia candidate genes) in support of the neurodevelopmental model. We relate the neurodevelopmental model to a number of findings about schizophrenia. Finally, we also examine alternate explanations of the origin of schizophrenia including the neurodegenerative model.

Gene-environment interaction and covariation in schizophrenia: the role of obstetric complications

While genetic factors account for a significant proportion of liability to schizophrenia, a body of evidence attests to a significant environmental contribution. Understanding the mechanisms through which genetic and environmental factors coalesce in influencing schizophrenia is critical for elucidating the pathways underlying psychotic illness and for developing primary prevention strategies. Although obstetric complications (OCs) remain among the most well-documented environmental indicators of risk for schizophrenia, the pathogenic role they play in the etiology of schizophrenia continues to remain poorly understood. A question of major importance is do these factors result from a genetic diathesis to schizophrenia (as in gene-environment covariation), act additively or interactively with predisposing genes for the disorder in influencing disease risk, or independently cause disease onset? In this review, we evaluate 3 classes of OCs commonly related to schizophrenia including hypoxia-associated OCs, maternal infection during pregnancy, and maternal stress during pregnancy. In addition, we discuss several mechanisms by which OCs impact on genetically susceptible brain regions, increasing constitutional vulnerability to neuromaturational events and stressors later in life (ie, adolescence), which may in turn contribute to triggering psychosis.

Congenital viral infections of the brain: lessons learned from lymphocytic choriomeningitis virus in the neonatal rat

The fetal brain is highly vulnerable to teratogens, including many infectious agents. As a consequence of prenatal infection, many children suffer severe and permanent brain injury and dysfunction. Because most animal models of congenital brain infection do not strongly mirror human disease, the models are highly limited in their abilities to shed light on the pathogenesis of these diseases. The animal model for congenital lymphocytic choriomeningitis virus (LCMV) infection, however, does not suffer from this limitation. LCMV is a well-known human pathogen. When the infection occurs during pregnancy, the virus can infect the fetus, and the developing brain is particularly vulnerable. Children with congenital LCMV infection often have substantial neurological deficits. The neonatal rat inoculated with LCMV is a superb model system of human congenital LCMV infection. Virtually all of the neuropathologic changes observed in humans congenitally infected with LCMV, including microencephaly, encephalomalacia, chorioretinitis, porencephalic cysts, neuronal migration disturbances, periventricular infection, and cerebellar hypoplasia, are reproduced in the rat model. Within the developing rat brain, LCMV selectively targets mitotically active neuronal precursors. Thus, the targets of infection and sites of pathology depend on host age at the time of infection. The rat model has further shown that the pathogenic changes induced by LCMV infection are both virus-mediated and immune-mediated. Furthermore, different brain regions simultaneously infected with LCMV can undergo widely different pathologic changes, reflecting different brain region-virus-immune system interactions. Because the neonatal rat inoculated with LCMV so faithfully reproduces the diverse neuropathology observed in the human counterpart, the rat model system is a highly valuable tool for the study of congenital LCMV infection and of all prenatal brain infections In addition, because LCMV induces delayed-onset neuronal loss after the virus has been cleared, the neonatal rat infected with LCMV may be an excellent model system to study neurodegenerative or psychiatric diseases whose etiologies are hypothesized to be virus-induced, such as autism, schizophrenia, and temporal lobe epilepsy.

Biopsychosocial model in Depression revisited

There are two fundamental etiological perspectives about mental disorders; biomedical and psychosocial. The biopsychosocial model has claimed to integrate these two perspectives in a scientific way, signalling their interconnection and interdependence. To that end, it used a systemic conceptual framework, taking advantage of the possibilities which it offers to establish general principles for diverse systems, independently of their physical, biological or sociological nature. In recent years, drawing on the theory of systems, theories have been developing of the dynamic non-linear systems, applicable to networks of a large quantity of densely interconnected elements (also called complex systems), like the mind or the brain. We believe that this revised systemic conceptual framework can bring integrative ideas to apply to Depression, such as the "binding dysfunction" concept we use in this article. According to this, vulnerability or predisposition to Depression would be associated with the imbalance between activating and inhibiting interactions (between some cognitions and emotions at a mental level, and between certain neuronal groups at a cerebral level). Precipitating factors would imply the increase of the activation level over this pattern of cognitions and emotions, or over those neuronal systems. When stress goes beyond the vulnerability threshold an excessive positive feedback between cognitions and emotions would appear (and between groups of neurons) with insufficient inhibitory control to mitigate it, which would imply a mental/cerebral dissociation in dominions of different level of activation. As a consequence, the generation and dissolution of patterns of cerebral and mental activation will no longer have the dynamism and flexibility that permits an optimal interaction with the environment ("binding dysfunction"). Therefore, our hypothesis is that the person with Depression will suffer at a cerebral level a functional dissociation in neural dominions (some rigidly hyperactive and others rigidly hypoactive) in determined locations, which would be a different combination from those found in other mental disorders. At a mental level, this would correlate with a functional dissociation in several cognitive-emotive dominions; some corresponds to over activated patterns of "depressive" cognitions and emotions that for that reason invade the consciousness frequently, intrusively and repetitively; meanwhile there are other alternative hypoactive emotions and cognitions that do not manage to become powerful enough to avoid the consequent distortion in the communication with the environment.

Immunological aetiology of major psychiatric disorders: evidence and therapeutic implications

Historically, immunological research in psychiatry was based on empirical findings and early epidemiological studies indicating a possible relationship between psychiatric symptoms and acute infectious diseases. However, aetiopathological explanations for psychiatric disorders are no longer closely related to acute infection. Nevertheless, immune hypotheses have been discussed in schizophrenia, affective disorders and infantile autism in the last decades. Although the variability between the results of the epidemiological studies conducted to date is strikingly high, there is still some evidence that the immune system might play a role in the aetiopathogenesis of these three psychiatric diseases, at least in subgroups of patients. In anxiety disorders immunological research is still very much in its infancy, and the few and inconsistent data of immune changes in these patients are believed to reflect the influence of short- or long-term stress exposure. Nevertheless, there are also some hints raising the possibility that autoimmune mechanisms could interrupt neurotransmission, which would be of significance in certain patients with anxiety and panic disorders. Drug and alcohol (ethanol) dependence are not believed to be primarily influenced by an immunological aetiology. On the other hand, immune reactions due to different drugs of abuse and alcohol may directly or indirectly influence the course of concomitant somatic diseases. In different organic brain disorders the underlying somatic disease is defined as a primary immune or autoimmune disorder, for instance HIV infection or systemic lupus erythematosus (SLE). For other neurodegenerative disorders, such as Alzheimer's disease, immunoaetiopathological mechanisms are supported by experimental and clinical studies. Treatment strategies based on immune mechanisms have been investigated in patients with schizophrenia and affective disorders. Furthermore, some antipsychotics and most antidepressants are known to have direct or indirect effects on the immune system. Different immunotherapies have been used in autism, including transfer factor, pentoxifylline, intravenous immunoglobulins and corticosteroids. Immunosuppressive and/or immunomodulating agents are well established methods for treating the neuropsychiatric sequelae of immune or autoimmune disorders, for example AIDS and SLE. Therapeutic approaches in Alzheimer's disease also apply immunological methods such as strategies of active/passive immunisation and NSAIDs. Considering the comprehensive interactive network between mind and body, future research should focus on approaches linking targets of the different involved systems.

Maternal immune activation leads to behavioral and pharmacological changes in the adult offspring

Maternal exposure to viral infection has been associated with an increased risk of schizophrenia in the offspring, and it has been suggested that the maternal immune response may interfere with normal fetal brain development. Although studies in rodents have shown that perinatal viral infections can lead to neuropathological and behavioral abnormalities considered relevant to schizophrenia, it is not clear whether these consequences are due to the infection itself or to the maternal immune response to infection. We show that an induction of maternal immune stimulation without exposure to a virus by injecting pregnant dams with the synthetic cytokine releaser polyriboinosinic-polyribocytidilic acid (poly I:C) leads to abnormal behavioral and pharmacological responses in the adult offspring. As in schizophrenia, these offspring displayed excessive behavioral switching, manifested in the loss of latent inhibition and in rapid reversal learning. Consistent with the clinical pharmacology of schizophrenia, both deficits were alleviated by antipsychotic treatment. In addition, these offspring displayed increased sensitivity to the locomotor-stimulating effects of MK-801, pointing to developmental alterations of the dopaminergic and/or glutamatergic systems. Prenatal poly I:C administration did not produce learning deficits in classical fear conditioning, active avoidance, discrimination learning and water maze. These results show that the maternal immune response is sufficient to cause behavioral and pharmacological alterations relevant to schizophrenia in the adult offspring.

Immune activation during pregnancy in rats leads to a postpubertal emergence of disrupted latent inhibition, dopaminergic hyperfunction, and altered limbic morphology in the offspring: a novel neurodevelopmental model of schizophrenia

Prenatal exposure to infection is associated with increased liability to schizophrenia, and it is believed that such an association is mediated by the maternal immune response, in particular, the proinflammatory cytokines released by the maternal immune system, which may disrupt fetal brain development. Impaired capacity to ignore irrelevant stimuli is one of the central deficits in schizophrenia, and is manifested, among others, in loss of latent inhibition (LI), a phenomenon whereby repeated inconsequential pre-exposure to a stimulus impairs its subsequent capacity to signal significant consequences. We tested the effects of prenatal immune activation induced by peripheral administration of the synthetic cytokine releaser polyriboinosinic-polyribocytidilic acid (poly I : C) to pregnant dams, on LI in juvenile and adult offspring. Consistent with the characteristic maturational delay of schizophrenia, prenatal immune activation did not affect LI in the juvenile offspring, but led to LI disruption in adulthood. Both haloperidol (0.1 mg/kg) and clozapine (5 mg/kg) reinstated LI in the adult offspring. In addition, prenatal immune activation led to a postpubertal emergence of increased sensitivity to the locomotor-stimulating effects of amphetamine and increased in vitro striatal dopamine release, as well as to morphological alterations in the hippocampus and the entorhinal cortex in the adult offspring, consistent with the well-documented mesolimbic dopaminergic and temporolimbic pathology in schizophrenia. These results suggest that prenatal poly I : C administration may provide a neurodevelopmental model of schizophrenia that reproduces a putative inducing factor; mimics the temporal course as well as some central abnormalities of the disorder; and predicts responsiveness to antipsychotic drugs. Neuropsychopharmacology (2003) 28, 1778-1789. advance online publication, 16 July 2003; doi:10.1038/sj.npp.1300248

Post-pubertal emergence of disrupted latent inhibition following prenatal immune activation

RATIONALE

There is evidence pointing to an association between prenatal exposure to infection and increased liability to schizophrenia, and it has been suggested that the maternal immune response, in particular, the release of pro-inflammatory cytokines, may interfere with normal fetal brain development. Impaired capacity to ignore irrelevant stimuli is considered one of the central deficits in schizophrenia, and is manifested, among others, in disrupted latent inhibition (LI).

OBJECTIVES

To test the effects of prenatal immune activation on LI in juvenile and adult offspring.

METHODS

Pregnant rats were injected with the synthetic cytokine releaser polyriboinosinic-polyribocytidilic acid (poly I:C, 4 mg/kg) on gestational day 15. LI was assessed in 35-day and 3-month-old offspring using a thirst motivated conditioned emotional response procedure.

RESULTS

Consistent with the characteristic maturational delay of schizophrenia, prenatal immune activation did not affect LI in the juvenile offspring but led to a post-pubertal emergence of LI disruption. In addition, pronounced alterations in hippocampal morphology resembling those found in schizophrenia, were evident in the adult offspring.

CONCLUSIONS

These results support the hypothesis that immune activation during pregnancy may lead to long-term abnormalities mimicking those observed in schizophrenia.

Neonatal viral infection decreases neuronal progenitors and impairs adult neurogenesis in the hippocampus

While adult neurogenesis has been demonstrated in the hippocampal dentate gyrus of several mammalian species, including humans, the impact of viral infections has not been well studied. To examine this question we used a model in which neonatal rats are infected with lymphocytic choriomeningitis virus (LCMV) leading to a gradual loss of dentate granule cells (DGCs), which becomes fully evident in adulthood. Stereological cell counts performed 8 months after infection revealed that the loss of mature DGCs was accompanied by an 84.2% reduction in proliferation of DGCs as measured by BrdU uptake. Moreover, there was a severe loss of Mash1-labeled neuronal progenitor cells (87 and 83% decrease in the granule cell layer and hilus, respectively). Thus, neurogenesis is impaired in this model of chronic DGC loss, perhaps due to a virus-induced impoverishment of DGC neuronal progenitors. The LCMV model could be exploited to examine pathophysiological mechanisms of neurodegeneration and to test pharmacological strategies aimed at increasing neurogenesis or rescuing multipotent progenitors.

Critical role for glial cells in the propagation and spread of lymphocytic choriomeningitis virus in the developing rat brain

Inoculation of the neonatal rat with lymphocytic choriomeningitis virus (LCMV) results in the selective infection of several neuronal populations and in focal pathological changes. However, the pathway by which LCMV reaches the susceptible neurons has not been described, and the nature and time course of the pathological changes induced by the infection are largely unknown. This study examined the sequential migration of LCMV in the developing rat brain and compared the pathological changes among infected brain regions. The results demonstrate that astrocytes and Bergmann glia cells are the first cells of the brain parenchyma infected with LCMV and that the virus spreads across the brain principally via contiguous glial cells. The virus then spreads from glial cells into neurons. However, not all neurons are susceptible to infection. LCMV infects neurons in only four specific brain regions: the cerebellum, olfactory bulb, dentate gyrus, and periventricular region. The virus is then cleared from glial cells but persists in neurons. LCMV induces markedly different pathological changes in each of the four infected regions. The cerebellum undergoes an acute and permanent destruction, while the olfactory bulb is acutely hypoplastic but recovers fully with age. Neurons of the dentate gyrus are unaffected in the acute phase but undergo a delayed-onset mortality. In contrast, the periventricular region has neither acute nor late-onset cell loss. Thus, LCMV infects four specific brain regions in the developing brain by spreading from glial cells to neurons and then induces substantially different pathological changes with diverse time courses in each of the four infected regions.

Maternal infection: window on neuroimmune interactions in fetal brain development and mental illness

Direct viral infection of the developing brain can have disastrous consequences for the fetus. More subtle and perhaps more insidious are viral infections of the pregnant mother, which can have long-lasting effects such as an increased risk of schizophrenia in the offspring. A recent mouse model has shown that respiratory infection in the pregnant mother leads to marked behavioral and pharmacological abnormalities in the offspring, some of which are relevant for schizophrenia and autism. This effect on fetal brain development might be caused by the maternal antiviral immune response, possibly mediated by cytokines.

Psychopathology and the binding problem

Normal cerebral activity is based on the rapid and flexible alternation of an enormous quantity of spatial-temporal patterns of neuronal firing. There is still much to learn about the mechanism that keeps neurons on functionally closed assemblies firing in synchronization. The explanation of this mechanism is the so called 'binding problem', one of the cornerstones of the generation of perceptions, cognitions, emotions and behaviours by the brain, and therefore of great interest to psychiatry. We explore the possibility that a 'binding dysfunction' could be a relevant aspect of the pathophysiology of mental disorders, and apply it to schizophrenia, in an attempt to integrate some neurobiological findings in this disorder, including a few preliminary data obtained with transcranial magnetic stimulation (TMS).

Schizophrenia and viral infection during neurodevelopment: a focus on mechanisms

The task of defining schizophrenia pathogenesis has fascinated and frustrated researchers for nearly a century. In recent years, unprecedented advances from diverse fields of study have given credence to both viral and developmental theories. This review considers possible mechanisms by which viral and developmental processes may interact to engender schizophrenia. Many of the current controversies in schizophrenia pathogenesis are reviewed in light of the viral hypothesis, including: epidemiological findings and the role of a genetic diathesis, phenotype heterogeneity, abnormalities in excitatory and inhibitory neurotransmitter systems, anomalous cerebral latereralization, and static vs progressive disease. The importance of animal models in elucidating the impact of viral infections on developing neurons is illustrated by recent studies in which neonatal rats are infected with lymphocytic choriomeningitis virus in order to examine alterations in hippocampal circuitry. Finally, consideration is given to a new hypothesis that some cases of schizophrenia could be instigated by a viral infection that disrupts developing inhibitory circuits, consequently unleashing glutamatergic neurotransmission leading to selective excitotoxicity, and a degenerative disease course.