Strain-dependent behavioral alterations induced by peripheral interleukin-1 challenge in neonatal mice

Convergent evidence shows a positive association of interleukin-1 gene complex locus with susceptibility to schizophrenia in the Caucasian population

Recent genetic studies have revealed that the Interleukin-1 (IL1) gene complex (IL1 alpha, IL1 beta and IL1 receptor antagonist) is associated with schizophrenia, but contradictory findings have also been reported. We investigated the association of the IL1 gene complex locus and schizophrenia using meta-analytic techniques, covering all published data up to January 2010, to restrict to the most commonly reported 4 single nucleotide polymorphisms (SNP). We also explored potential sources of heterogeneity and to investigate whether ancestry and study design moderated any association. The combined allele-wise odds ratio (OR) for schizophrenia of the rs16944 (IL1B gene; T-511C) polymorphism was 0.86 (95% CI: 0.77to 0.96).When applying stratified analysis to this polymorphism, the pooled allele-wise OR was 0.88 (95% CI, 0.79 to 0.97) in 10 population-based studies and 0.85 (95% CI: 0.73 to 0.99) in Caucasian samples. In a stratified analysis of the rs1143634 (IL1B gene; T3953C) polymorphism, the pooled genotype-wise results in a dominant model were also statistically significant both in a population-based study subgroup with summary OR of 0.64 (95% CI: 0.41 to 0.99) and a Caucasian population subgroup with summary OR of 0.62 (95% CI: 0.40 to 0.97). Neither combined nor stratified analyses found any association of the rs1800587 (IL1A gene; T-889C) or rs1794068 (IL1RA Gene; IL1RN_86 bp; T/C) with schizophrenia susceptibility. Our study suggests the IL1B gene or the IL1 gene complex may play a moderate role in the etiology of schizophrenia in the Caucasian population.

Cytokine hypothesis of schizophrenia pathogenesis: evidence from human studies and animal models

The pathogenesis of schizophrenia has yet to be fully characterized. Gene-environment interactions have been found to play a crucial role in the vulnerability to this disease. Among various environmental factors, inflammatory immune processes have been most clearly implicated in the etiology and pathology of schizophrenia. Cytokines, regulators of immune/inflammatory reactions and brain development, emerge as part of a common pathway of genetic and environmental components of schizophrenia. Maternal infection, obstetric complications, neonatal hypoxia and brain injury all recruit cytokines to mediate inflammatory processes. Abnormal expression levels of specific cytokines such as epidermal growth factor, interleukins (IL) and neuregulin-1 are found both in the brain and peripheral blood of patients with schizophrenia. Accordingly, cytokines have been proposed to transmit peripheral immune/inflammatory signals to immature brain tissue through the developing blood-brain barrier, perturbing structural and phenotypic development of the brain. This cytokine hypothesis of schizophrenia is also supported by modeling experiments in animals. Animals treated with specific cytokines of epidermal growth factor, IL-1, IL-6, and neuregulin-1 as embryos or neonates exhibit schizophrenia-like behavioral abnormalities after puberty, some of which are ameliorated by treatment with antipsychotics. In this review, we discuss the neurobiological mechanisms underlying schizophrenia and novel antipsychotic candidates based on the cytokine hypothesis.

Models of neurodevelopmental abnormalities in schizophrenia

The neurodevelopmental hypothesis of schizophrenia asserts that the underlying pathology of schizophrenia has its roots in brain development and that these brain abnormalities do not manifest themselves until adolescence or early adulthood. Animal models based on developmental manipulations have provided insight into the vulnerability of the developing fetus and the importance of the early environment for normal maturation. These models have provided a wide range of validated approaches to answer questions regarding environmental influences on both neural and behavioral development. In an effort to better understand the developmental hypothesis of schizophrenia, animal models have been developed, which seek to model the etiology and/or the pathophysiology of schizophrenia or specific behaviors associated with the disease. Developmental models specific to schizophrenia have focused on epidemiological risk factors (e.g., prenatal viral insult, birth complications) or more heuristic models aimed at understanding the developmental neuropathology of the disease (e.g., ventral hippocampal lesions). The combined approach of behavioral and neuroanatomical evaluation of these models strengthens their utility in improving our understanding of the pathophysiology of schizophrenia and developing new treatment strategies.

Animal models of gene-environment interactions in schizophrenia

The pathogenesis of schizophrenia and related mental illnesses likely involves multiple interactions between susceptibility genes of small effects and environmental factors. Gene-environment interactions occur across different stages of neurodevelopment to produce heterogeneous clinical and pathological manifestations of the disease. The main obstacle for mechanistic studies of gene-environment interplay has been the paucity of appropriate experimental systems for elucidating the molecular pathways that mediate gene-environment interactions relevant to schizophrenia. Recent advances in psychiatric genetics and a plethora of experimental data from animal studies allow us to suggest a new approach to gene-environment interactions in schizophrenia. We propose that animal models based on identified genetic mutations and measurable environment factors will help advance studies of the molecular mechanisms of gene-environment interplay.

Characterization of parabrachial subnuclei in mice with regard to salt tastants: possible independence of taste relay from visceral processing

We examined whether salt taste and/or abdominal illness were dealt within different subnuclei in the parabrachial nucleus (PBN) in mice, using retrograde tracing methods and c-Fos-like immunoreactivity (FLI) detection procedures. Some PBN subnuclei have distinct functions and receive various sensory inputs from the nucleus of the solitary tract (NTS) and other areas and relay them to the higher order nuclei such as the thalamus. The afferent-dependent pattern of FLI has been investigated in the PBN. However, it is unclear in which PBN subnuclei the tastants induce c-Fos, or whether PBN subnuclei process taste inputs separately from other inputs, or integrate them. After the tracer injections into the thalamic taste relay, the retrograde labeled cells revealed the taste relay cells in the PBN at the boundary with the superior cerebellar peduncle of both the inner part of the external lateral subnucleus and the medial subnucleus and in the waist area. On the other hand, NaCl intake induced intense FLI in the dorsal lateral subnucleus, whereas LiCl intake yielded intense FLI in both the dorsal lateral subnucleus and the outer part of the external lateral subnucleus. Thus, the present findings that subnuclei relaying taste information to the thalamus do not yield FLI in response to salt taste and abdominal illness indicate that they lack FLI yielding pathways or that they are independent from the subnuclei processing salt taste and visceral information via c-Fos in mice.

Common behavioral influences of the ErbB1 ligands transforming growth factor alpha and epiregulin administered to mouse neonates

Ligands for epidermal growth factor (EGF) receptor (ErbB1), such as EGF, transforming growth factor alpha (TGFalpha), and epiregulin, are enriched in body fluids and blood and regulate development of various peripheral organs. It remains however how such circulating polypeptide growth factors influence brain development and function. Here, we performed peripheral injections of TGFalpha and epiregulin to mouse neonates and evaluated immediate physical and neurochemical development and later behavioral consequences. Subcutaneous administration of TGFalpha and epiregulin increased phosphorylation of brain ErbB1, suggesting their effects on brain development. Repeated their injections similarly enhanced physical development of eyelid opening and tooth eruption during early postnatal stage and resulted in abnormal behavioral traits in the adult stage. Acoustic startle responses of mice treated with these growth factors as neonates were enhanced and prepulse inhibition was decreased without an apparent correlation between prepulse inhibition level and startle intensity. Locomotor activity and fear-learning performance with tone and context cues were not altered, however. These results suggest that circulating ErbB1 ligands in the periphery of neonates have some common influences on later behavioral traits. Abnormal ErbB1 ligand production at neonatal and potentially prenatal stages might therefore associate with neurodevelopmental disorders such as schizophrenia.

The cellular and behavioral consequences of interleukin-1 alpha penetration through the blood-brain barrier of neonatal rats: a critical period for efficacy

Proinflammatory cytokines circulating in the periphery of early postnatal animals exert marked influences on their subsequent cognitive and behavioral traits and are therefore implicated in developmental psychiatric diseases such as schizophrenia. Here we examined the relationship between the permeability of the blood-brain barrier to interleukin-1 alpha (IL-1 alpha) in neonatal and juvenile rats and their later behavioral performance. Following s.c. injection of IL-1 alpha into rat neonates, IL-1 alpha immunoreactivity was first detected in the choroid plexus, brain microvessels, and olfactory cortex, and later diffused to many brain regions such as neocortex and hippocampus. In agreement, IL-1 alpha administration to the periphery resulted in a marked increase in brain IL-1 alpha content of neonates. Repeatedly injecting IL-1 alpha to neonates triggered astrocyte proliferation and microglial activation, followed by behavioral abnormalities in startle response and putative prepulse inhibition at the adult stage. Analysis of covariance with a covariate of startle amplitude suggested that IL-1 alpha administration may influence prepulse inhibition. However, adult rats treated with IL-1 alpha as neonates exhibited normal learning ability as measured by contextual fear conditioning, two-way passive shock avoidance, and a radial maze task and had no apparent sign of structural abnormality in the brain. In comparison, when IL-1 alpha was administered to juveniles, the blood-brain barrier permeation was limited. The increases in brain IL-1 alpha content and immunoreactivity were less pronounced following IL-1 alpha administration and behavioral abnormalities were not manifested at the adult stage. During early development, therefore, circulating IL-1 alpha efficiently crosses the blood-brain barrier to induce inflammatory reactions in the brain and influences later behavioral traits.

Lack of association between the interleukin-1 gene complex and schizophrenia in a Japanese population

Interleukin-1 (IL1) is an inflammatory cytokine and exerts neurodegenerative effects in the brain. Several studies have indicated that IL1 is likely to be involved in the pathogenesis of schizophrenia. Recent genetic studies have revealed that the IL1 gene complex (IL,1 alpha, IL1, beta and IL1 receptor antagonist) was associated with schizophrenia, although contradictory findings have also been reported. To assess whether the IL1 gene complex was implicated in vulnerability to schizophrenia, the authors conducted a case-control association study (416 patients with schizophrenia and 440 control subjects) for nine polymorphisms in Japanese subjects. The authors found no association between the IL1 gene complex polymorphisms and schizophrenia using either single-marker or haplotype analyses. The results of the present study suggest that the IL1 gene complex does not play a major role in conferring susceptibility to schizophrenia in the Japanese population.

Recent progress in animal modeling of immune inflammatory processes in schizophrenia: implication of specific cytokines

Epidemiologic studies demonstrate significant environmental impact of maternal viral infection and obstetric complications on the risk of schizophrenia and indicate their detrimental influences on brain development in this disorder. Based on these findings, animal models for schizophrenia have been established using double stranded RNA, bacterial lipopolysaccharides, hippocampal lesion, or prenatal/perinatal ischemia. Key molecules regulating such immune/inflammatory reactions are cytokines, which are also involved in brain development, regulating dopaminergic and GABAergic differentiation, and synaptic maturation. Specific members of the cytokine family, such as interleukin-1, epidermal growth factor, and neuregulin-1, are induced after infection and brain injury; therefore, certain cytokines are postulated to have a central role in the neurodevelopmental defects of schizophrenia. Recently, to test this hypothesis, a variety of cytokines were administered to rodent pups. Cytokines administered in the periphery penetrated the immature blood-brain barrier and perturbed phenotypic neural development. Among the many cytokines examined, epidermal growth factor (or potentially other ErbB1 ligands) and interleukin-1 specifically induced the most severe and persistent behavioral and cognitive abnormalities, most of which were ameliorated by antipsychotics. These animal experiments illustrate that, during early development, these cytokine activities in the periphery perturbs normal brain development and impairs later psychobehavioral and/or cognitive traits. The neurodevelopmental and behavioral consequences of prenatal/perinatal cytokine activity are compared with those of other schizophrenia models and cytokine interactions with genes are also discussed in this review.