10.1586/14737175.2013.811191 747ISSN 1473-7175© 2013 Informa UK Ltdwww.expert-reviews.com

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Schizophrenia is a severe and chronic psychi-atric disorder, affecting approximately 1% of the global population. Accumulating data from epidemiological studies show that maternal viral infection during pregnancy plays a key role in the etiology of schizophrenia. Initial studies highlighted an increased occurrence of schizophrenia among individuals who were fetuses in the second trimester of gestation during influenza epidemics, although further studies have failed to replicate this associa-tion [1]. Birth cohort studies provided further support for the hypothesis that maternal infection (viral, protozoal or bacterial) could increase the risk for schizophrenia in adult offspring [1]. This strongly implies that prenatal immune activation plays a role in the etiology of schizophrenia.

The viral mimic polyriboinosinic-polyribocyt-idylic acid (poly [I:C]), a synthetic analogue of dsRNA, has been widely used to induce immune activation during pregnancy in animal models of schizophrenia [1–4]. Poly (I:C) is also known

to act as a Toll-like receptor-3 agonist. Prenatal poly (I:C) exposure during pregnancy leads to cytokine imbalances in the fetal environ-ment [2]. Both prenatal maternal infection and postnatal exposure to psychological trauma are environmental risk factors for the development of psychiatric disorders, such as schizophrenia [5]. The ‘two-hit’ hypothesis of schizophrenia, where genetic factors and subsequent psycho-logical stress result in schizophrenia, is widely recognized as the most likely disease pathway [6].

Prenatal immune activation, recognized as the first hit, and environmental stress at the peripubertal stage, recognized as the second hit, contribute to the development of schizophrenia. However, it is very difficult to detect the syner-gistic effects of prenatal immune activation and peripubertal stress in adult behavioral func-tions [7]. Giovanoli et al. reported that maternal immune activation during pregnancy and sub-sequent peripubertal stress induced synergistic pathological effects on behavioral abnormalities relevant to schizophrenia in adults [8]. This lends

Noriko Yoshimi1,2, Takashi Futamura2 and Kenji Hashimoto*1

1Division of Clinical Neuroscience, Chiba University Center for Forensic Mental Health, Chiba, Japan2Qs Research Institute, Otsuka Pharmaceutical Co., Ltd., Tokushima, Japan*Author for correspondence: Tel.: +81 43 226 2517 Fax: +81 43 226 2561 hashimoto@faculty.chiba-u.jp

Evaluation of: Giovanoli S, Engler H, Engler A et al. Stress in puberty unmasks latent neuropathological consequences of prenatal immune activation in mice. Science 339(6123), 1095–1099 (2013).

Epidemiological studies show that maternal viral infection during pregnancy plays a key role in the etiology of neurodevelopmental disorders, such as schizophrenia and autism. Prenatal maternal immune activation and peripubertal psychological stress are key environmental risk factors for neurodevelopmental disorders. Viral mimic polyriboinosinic-polyribocytidylic acid is known to act as a Toll-like receptor-3 agonist. Polyriboinosinic-polyribocytidylic acid has been typically used to establish this rodent model of prenatal immune activation. Recently, Giovanoli et al. reported on a new neurodevelopmental model of schizophrenia based on combined prenatal immune activation and peripubertal stress. In this report, we place these findings into context and discuss their significance.

Prenatal immune activation and subsequent peripubertal stress as a new model of schizophreniaExpert Rev. Neurother. 13(7), 747–750 (2013)

Expert Review of Neurotherapeutics

© 2013 Informa UK Ltd

10.1586/14737175.2013.811191

1473-7175

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THEMED ARTICLE ❙ Schizophrenia

Keywords: auditory sensory gating • dopamine • microglial activation • peripubertal stress • prenatal immune activation • schizophrenia • two-hit model

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support to the theory that mental illnesses with delayed onset involve multiple environmental hits.

Methods & resultsThis new model of schizophrenia is based on the two-hit hypothesis of psychiatric disorders [8]. The first- and second-hit stressors are maternal immune activation by poly (I:C) and peripubertal stress, respectively. Stress protocols included the following five distinct stressors: electric foot shock, restraint stress, swimming stress, water deprivation or repeated changing of home cages, applied on alternate days, between postnatal days 30 and 40. To avoid the possibility of ceiling effects, a low dose of poly (I:C) (1.0 mg/kg intravenously) was administered compared with the typical dose (5.0 mg/kg intravenously) [9]. Prenatal immune activation and peripubertal stress resulted in synergistic effects in the development of sensory gating deficiency, as assessed by prepulse inhibition of the acoustic startle response. Furthermore, behavioral hyperactivity in response to the psychostimulant amphetamine and the NMDA receptor antagonist dizocilpine was shown. Neither immune activation nor stress alone affected sensory gating or behavior in response to these psychotomimetic drugs. Abnormalities in these behavioral patterns became evident only after combined exposure to both environmental factors. Interestingly, application of stress during adolescence at postnatal days 50–60 did not elicit a synergistic interaction with prenatal immune activation. This indicates the presence of a discrete temporal window for stress induction in order to promote synergistic effects.

In the elevated plus maze test, peripubertal stress exposure increased anxiety-like behavior at both peripubertal and adult ages, independent of prenatal immune activation, suggesting that the development of anxiety-like behavior was independent of prenatal immune activation. Furthermore, observed behavioral abnormali-ties also appear to be independent of changes in the hypothalamus–pituitary–adrenal axis, since plasma levels of corticosterone, the main effector hormone of the hypothalamus–pituitary–adrenal axis, were unchanged. It was previously reported that increased sensitivity to methamphetamine was present in adult, but not juve-nile, offspring after prenatal poly (I:C) exposure [10], indicating a hyperdopaminergic state in these adult offspring. Interestingly, increased levels of dopamine in the hippocampus occurred only after combined exposure to a prenatal immune challenge and peri-pubertal stress, implying that the synergistic effects between the two adverse events precipitate hippocampal dopamine imbalance in adults [8]. Further studies on this phenomenon are needed.

Prenatal immune activation and peripubertal stress had lit-tle impact on neuroimmunological changes in adult animals. However, at the peripubertal stage, levels of CD68 and CD11b, markers of activated microglia, were elevated in the hippocam-pus and prefrontal cortex after combined immune activation and stress. These changes were also accompanied by increased levels of proinflammatory cytokines IL-1β and TNF-α [8]. In addition, mRNA levels of CD200, CD200R and CD47, the effectors of neuron–microglia inhibitory signaling, were decreased synergisti-cally by combined prenatal immune activation and exposure to acute restraint stress [8].

Discussion & significanceThis article showed that prenatal immune activation and peri-pubertal stress induced synergistic effects on the development of sensory-gating prepulse inhibition deficits, as well as promoting behavioral hyperactivity in response to psychotomimetic drugs. Furthermore, prenatal immune activation and peripubertal stress exerted synergistic effects on the expression of activated microglial markers in the hippocampus and prefrontal cortex, along with proinflammatory cytokines in the hippocampus at peripuber-tal stages. A meta-analysis of clinical studies showed that some cytokines such as IL-1β, IL-6 and TGF-β may be state markers for acute schizophrenia, while others such as IL-12, IFN-γ, TNF-α and the soluble IL-2 receptor may be trait markers [11]. Moreover, a PET study using (R)-[11C]PK11195 showed that activated micro-glia were detected in the brains of patients with schizophrenia [12]. Taken together, it appears that inflammatory processes induced by microglial activation may contribute to the pathophysiology of schizophrenia, raising the possibility that anti-inflammatory drugs such as minocycline could potentially be used as therapeutic agents [13].

Since most viruses do not cross the placenta, these findings beg the question: how does maternal prenatal poly (I:C) exposure induce schizophrenia-like behavior in offspring? It would appear that in response to maternal poly (I:C) exposure, the placenta undergoes dramatic upregulation of proinflammatory cytokines and activation of decidual immune cells [14]. In this manner, abnormal levels of cytokines induced by maternal poly (I:C) injection are transferred to the fetus through the placenta, and this may cause the observed fetal brain structure and behavio-ral abnormalities. Typically, maternal injection of poly (I:C) increased expression of IL-6, TNF-α and IL-10 within 4 h, with levels receding 24 h after injection [8]. Unfortunately, cytokine expression in maternal placenta after poly (I:C) injection was not examined in this study. These measurements should form part of any future studies.

Expert commentary & five-year viewThis article suggests that prenatal immune activation and peri-pubertal stress act in synergy to induce schizophrenic-like behav-ior in offspring at adult stages. These behavioral changes are accompanied by immunological alterations in the hippocampus and prefrontal cortex at peripubertal ages. However, the precise mechanisms connecting behavioral alterations and immuno-logical changes induced by this two-hit model are still unclear. Recent studies report alterations in the brain’s glutamatergic system (e.g., GluN1 subunit of the NMDA receptor and metabo-tropic glutamate receptor mGluR2) after prenatal poly (I:C) exposure [15,16]. Given the importance of the glutamatergic sys-tem in the pathophysiology of schizophrenia [17], further detailed studies examining the connection between this system and the synergistic effect of prenatal immune activation and peripubertal stress are needed.

DISC1 is a known risk factor for major psychiatric dis-eases, such as schizophrenia and major depression. Prenatal immune activation by poly (I:C) exposure produced behavioral

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Key issues

• Prenatal maternal infection and postnatal exposure to psychological trauma are two environmental risk factors for the onset of neurodevelopmental disorders, such as schizophrenia.

• Prenatal immune activation is mimicked by administration of polyriboinosinic-polyribocytidylic acid in pregnancy.

• Prenatal immune activation and subsequent peripubertal stress induce synergistic effects on adult behavioral abnormalities relevant to schizophrenia.

• Prenatal immune activation and peripubertal stress mediate neuroimmunological changes at the peripubertal, but not adult, stages.

• The application of stress in adolescence does not produce an interactive effect with prenatal immune activation.

• Prenatal infection can ‘prime’ a developing animal’s sensitivity to subsequent postnatal environmental challenges.

abnormalities, such as elevated anxiety, depression-like behav-iors, altered patterns of sociability and attenuated reactivity to stress, that were not present in untreated mutant human DISC1 mice [18]. Recently Niwa et al. reported that the environmental stressor, ‘isolation stress during adolescence’, could elicit molecu-lar, neurochemical and behavioral deficits, but only when com-bined with a dominant-negative DISC1 [19]. These two reports highlight gene–environment interactions in the pathophysiology of behavioral abnormalities relevant to psychiatric disorders.

By contrast, Sun et al. reported that a predictable, chronic-mild stress during adolescence attenuated depressive and anxiety-like behavior caused by chronic unpredictable stress later on in adult-hood [20]. This implies that modulated stress in adolescence con-fers protection against depression and anxiety induced by stress in adulthood. It also raises the possibility that predictable and unpredictable stressors may induce different effects.

In conclusion, stress experienced during peripubertal and ado-lescent ages may confer risk and protective factors, respectively, on brain development. This in turn reshapes neural circuits that regulate future responses to stress and adversity. Further detailed studies are required to examine how psychological stress during developmental stages can affect the onset of psychiatric diseases in adults.

Financial & competing interests disclosureThis study was supported by a Grant-in-Aid for Scientific Research on Innovative Areas of the Ministry of Education, Culture, Sports, Science and Technology, Japan (K Hashimoto). N Yoshimi and T Futamura are employers of Otsuka Pharmaceutical Co., Ltd. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript.

No writing assistance was utilized in the production of this manuscript.

ReferencesPapers of special note have been highlighted as:• of interest•• of considerable interest

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