Giamal N. Luheshi

Interleukin 1 in the brain: biology, pathology and therapeutic target.

The cytokine interleukin 1 (IL-1) has diverse actions in the brain. In normal brain the IL-1 system is expressed at low levels and is upregulated rapidly in response to local or peripheral insults. IL-1 mediates host defence responses to local and systemic disease and injury (e.g. fever, slow-wave sleep, appetite suppression and neuroendocrine responses) and to neuroinflammation and cell death in neurodegenerative conditions, such as stroke and head injury. It has also been implicated in chronic degenerative diseases, in particular, multiple sclerosis, Parkinsons and Alzheimers diseases. The mechanisms regulating the expression and action of IL-1 are poorly understood, but involve multiple effects on neuronal, glial and endothelial cell function. Thus, the IL-1 system provides an attractive and intensely competitive target for therapeutic intervention.

The role of cytokines in mediating effects of prenatal infection on the fetus: implications for schizophrenia

Maternal infections with bacterial or viral agents during pregnancy are associated with an increased incidence of schizophrenia in the offspring at adulthood although little is known about the mechanism by which maternal infection might affect fetal neurodevelopment. Exposure of pregnant rodents to the bacterial endotoxin, lipopolysaccharide (LPS), results in behavioral deficits in the adult offspring that are relevant to schizophrenia. It is however unknown whether these effects are due to the direct action of the inflammatory stimulus on the developing fetus, or due to secondary immune mediators (cytokines) activated at maternal/fetal sites. In this study we sought to elucidate the site of action of LPS, following a single intraperitoneal (i.p.) injection, in pregnant rats at gestation day 18. Animals received 5 μCi of iodinated LPS (125I-LPS) and its distribution was assessed in maternal/fetal tissues (1–8 h). In addition, induction of the inflammatory cytokines, TNF-α, IL-1β and IL-6, was measured in maternal/fetal tissues following maternal LPS challenge (0.05 mg/kg, i.p.) (2–8 h). 125I-LPS was detected in maternal tissues and placenta, but not the fetus. This distribution was accompanied by significant increases in TNF-α, IL-1β and IL-6 in maternal plasma and placenta, but not in fetal liver or brain. A significant increase in IL-1β was however detected in fetal plasma, possibly due to transfer from the maternal circulation or placenta. Collectively, these data suggest that effects of maternal LPS exposure on the developing fetal brain are not mediated by the direct action of LPS, but via indirect actions at the level of the maternal circulation or placenta.

Cytokines and their receptors in the central nervous system: physiology, pharmacology, and pathology.

Numerous cytokines and their receptors have been identified in the brain, where they act as mediators of host defence responses and have direct effects on neuronal and glial function. Experimental tools for studying cytokine actions, their source and control of synthesis in the brain, actions and mechanisms of action will be reviewed here. In particular, the cytokines interleukin-1, interleukin-6, and tumour necrosis factor-alpha have been implicated in the central control of responses to systemic disease and injury and activation of fever, neuroendocrine, immune, and behavioural responses. The recent discovery of specific inhibitors of cytokine synthesis, release, or action may offer significant therapeutic benefit.

Effects of prenatal infection on prepulse inhibition in the rat depend on the nature of the infectious agent and the stage of pregnancy

Maternal infection during pregnancy is a risk factor for some psychiatric illnesses of neurodevelopmental origin such as schizophrenia and autism. In experimental animals, behavioral and neuropathological outcomes relevant to schizophrenia have been observed in offspring of infected dams. However, the type of infectious agent used and gestational age at time of administration have varied. The objective of the present study was to compare the effects of prenatal challenge with different immune agents given at different time windows during gestation on behavioral outcomes in offspring. For this, pregnant rats were administered bacterial endotoxin (lipopolysaccharide, LPS), the viral mimic polyinosinic: polycytidylic acid (poly I:C), or turpentine, an inducer of local inflammation, at doses known to produce fever, at three different stages in pregnancy: embryonic day (E)10-11, E15-16 and E18-19. Prepulse inhibition of acoustic startle (PPI) was later measured in male adult offspring. PPI was significantly decreased in offspring after prenatal LPS treatment at E15-16 and E18-19. Intramuscular injection of pregnant dams with turpentine at E15-16 also decreased PPI in adult offspring. Maternal poly I:C administration had no significant effect on PPI in offspring. In contrast to prenatal LPS exposure, acute LPS administration to naive adult males had no effect on PPI. Thus, prenatal exposure both to a systemic immunogen and to local inflammation at brief periods during later pregnancy produced lasting deficits in PPI in rat offspring. These findings support the idea that maternal infection during critical windows of pregnancy could contribute to sensorimotor gating deficits in schizophrenia.

Expression of interleukin-1 receptors and their role in interleukin-1 actions in murine microglial cells

Interleukin (IL)‐1 is an important mediator of acute brain injury and inflammation, and has been implicated in chronic neurodegeneration. The main source of IL‐1 in the CNS is microglial cells, which have also been suggested as targets for its action. However, no data exist demonstrating expression of IL‐1 receptors [IL‐1 type‐I receptor (IL‐1RI), IL‐1 type‐II receptor (IL‐1RII) and IL‐1 receptor accessory protein (IL‐1RAcP)] on microglia. In the present study we investigated whether microglia express IL‐1 receptors and whether they present target or modulatory properties for IL‐1 actions. RT–PCR analysis demonstrated lower expression of IL‐1RI and higher expression of IL‐1RII mRNAs in mouse microglial cultures compared with mixed glial or pure astrocyte cultures. Bacterial lipopolysaccharide (LPS) caused increased expression of IL‐1RI, IL‐1RII and IL‐1RAcP mRNAs, induced the release of IL‐1β, IL‐6 and prostaglandin‐E2 (PGE2), and activated nuclear factor κB (NF‐κB) and the mitogen‐activated protein kinases (MAPKs) p38, and extracellular signal‐regulated protein kinase (ERK1/2), but not c‐Jun N‐terminal kinase (JNK) in microglial cultures. In comparison, IL‐1β induced the release of PGE2, IL‐6 and activated NF‐κB, p38, JNK and ERK1/2 in mixed glial cultures, but failed to induce any of these responses in microglial cell cultures. IL‐1β also failed to affect LPS‐primed microglial cells. Interestingly, a neutralizing antibody to IL‐1RII significantly increased the concentration of IL‐1β in the medium of LPS‐treated microglia and exacerbated the IL‐1β‐induced IL‐6 release in mixed glia, providing the first evidence that microglial IL‐1RII regulates IL‐1β actions by binding excess levels of this cytokine during brain inflammation.

Circulating leptin mediates lipopolysaccharide-induced anorexia and fever in rats

Anorexia and fever are important features of the hosts response to inflammation that can be triggered by the bacterial endotoxin lipopolysaccharide (LPS) and the appetite suppressant leptin. Previous studies have demonstrated that LPS induces leptin synthesis and secretion in the periphery, and that the action of leptin on appetite suppression and fever are dependent on brain interleukin (IL)‐1β. However, the role of leptin as a neuroimmune mediator of LPS‐induced inflammation has not been fully elucidated. To address this issue, we neutralized circulating leptin using a leptin antiserum (LAS) and determined how this neutralization affected LPS‐induced anorexia, fever and hypothalamic IL‐1β. Adult male rats were separated into four treatment groups, namely LPS + normal sheep serum (NSS), LPS + LAS, saline + LAS and saline + NSS. Intraperitoneal injection of LPS (100 μg kg−1) induced a significant reduction in food intake and body weight, which were significantly reversed in the presence of LAS (1 ml kg−1), 8 and 24 h after treatment. In addition, LPS‐induced fever was significantly attenuated by LAS over the duration of the fever response (8 h). Lipopolysaccharide induced an increase of circulating IL‐6, another potential circulating pyrogen, which was not affected by neutralization of leptin at 2 h. Interleukin‐1β mRNA at 1 and 8 h, and IL‐1 receptor antagonist (ra) at 2 h were significantly upregulated in the hypothalamus of LPS‐treated animals. The induction of these cytokines was attenuated in the presence of LAS. These results are the first to demonstrate that leptin is a circulating mediator of LPS‐induced anorexia and fever, probably through a hypothalamic IL‐1β‐dependent mechanism.

Brain sites of action of endogenous interleukin-1 in the febrile response to localized inflammation in the rat

1 Interleukin (IL)‐1 is a potent endogenous pyrogen which causes fever when injected into a number of brain sites. However, the brain sites at which endogenous IL‐1 acts to influence body temperature remain equivocal. The aim of this study was to determine the effect of local administration of the interleukin‐1 receptor antagonist (IL‐1ra) into specific sites in the hypothalamus, and other brain regions known to contain receptors for IL‐1, on the febrile response of rats to peripheral injection of lipopolysaccharide (LPS) into a subcutaneous air pouch (intrapouch, i.p.o.) that does not lead to LPS appearance in the circulation. 2 Injection of LPS (100 μg kg−1, i.p.o.) induced a rise in body temperature which commenced 1·5 h after injection and was maximal at 3 h (38·9 ± 0·2 °C, compared with 37·0 ± 0·1 °C at 0 h, n= 6, P < 0·001). Intracerebroventricular (i.c.v.) IL‐1ra (500 μg in 5 μl) significantly attenuated LPS fever (IL‐1ra, 37·7 ± 0·2 °C; saline, 38·9 ± 0·2 °C; n= 6, P < 0·001). Unilateral microinjection of IL‐1ra (50 μg in 0·5 μl at 0 + 1 h) into the anterior hypothalamus (AH), paraventricular hypothalamic nucleus (PVH), peri‐subfornical organ, subfornical organ (SFO) or hippocampus (dentate gyrus and CA3 region) also significantly reduced the fever induced by LPS. 3 The same dose of IL‐1ra had no effect on fever when administered into the ventromedial hypothalamus (VMH), organum vasculosum lamina terminalis (OVLT), CA1 field of the hippocampus, striatum or cortex. 4 These data indicate that the action of endogenous IL‐1 in the brain during fever is site specific, acting at the AH, PVH, SFO and hippocampus, but not the VMH, OVLT and striatum or cortex.

Cytokines and Fever

Fever is one of the best examples of responses to injury and infection which depends on neuroimmune interactions. Cytokines function as mediators of fever, acting locally within damaged tissues, circulating factors and the brain. The primary endogenous pyrogens. IL-1, IL-6 and TNF-alpha act at each of these sites and interact with nervous and endocrine systems to modify host defence responses. Understanding the role and mechanisms of cytokine actions in fever is relevant to many other aspects of neuroimmunology, and host responses to pathological challenges.

Effects of prenatal immune activation on hippocampal neurogenesis in the rat

Maternal infection during pregnancy has been associated with an increased risk for the development of schizophrenia, a disorder characterized by abnormalities in hippocampal morphology and function. Neurogenesis occurs in the hippocampus throughout development into adulthood and is believed to modulate hippocampal function. This study used a rat model in which bacterial endotoxin, lipopolysaccharide (LPS), is administered to pregnant dams, to test if prenatal immune activation has acute and/or long term effects on various phases of neurogenesis (proliferation, survival, differentiation) in the hippocampal dentate gyrus of offspring. When LPS was administered to dams on gestation days (GD) 15 and 16, there was decreased proliferation of dentate cells at postnatal day (PD) 14 and decreased survival of cells generated at PD14 in offspring. When prenatal exposure to LPS was later in pregnancy (GD 18 and 19), offspring showed decreased survival of cells generated both at the time of LPS exposure and at PD14. There was no change in cell proliferation or survival in adult offspring at PD60, with prenatal LPS exposure. Co-administration of the cyclo-oxygenase inhibitor, ibuprofen (IBU), together with prenatal LPS on GD 15 and 16, was unable to prevent the deficit in neuronal survival at PD14. IBU blocked LPS-induced fever but did not block LPS-induced increases in plasma cytokines and corticosterone in the pregnant dam. This indicates that deficits in neurogenesis caused by prenatal LPS are not mediated by LPS-induced fever or eicosanoid induction, but could be mediated by LPS-induced increases in maternal cytokines or corticosterone.

Central administration of rat IL-6 induces HPA activation and fever but not sickness behavior in rats

Interleukin (IL)-6 has been proposed to mediate several sickness responses, including brain-mediated neuroendocrine, temperature, and behavioral changes. However, the exact mechanisms and sites of action of IL-6 are still poorly understood. In the present study, we describe the effects of central administration of species-homologous recombinant rat IL-6 (rrIL-6) on the induction of hypothalamic-pituitary-adrenal (HPA) activity, fever, social investigatory behavior, and immobility. After intracerebroventricular administration of rrIL-6 (50 or 100 ng/rat), rats demonstrated HPA and febrile responses. In contrast, rrIL-6 alone did not induce changes in social investigatory and locomotor behavior at doses of up to 400 ng/rat. Coadministration of rrIL-6 (100 ng/rat) and rrIL-1β (40 ng/rat), which alone did not affect the behavioral responses, reduced social investigatory behavior and increased the duration of immobility. Compared with rhIL-6, intracerebroventricular administration of rrIL-6 (100 ng/rat) induced higher HPA responses and early-phase febrile responses. This is consistent with a higher potency of rrIL-6, compared with rhIL-6, in the murine B9 bioassay. We conclude that species-homologous rrIL-6 alone can act in the brain to induce HPA and febrile responses, whereas it only reduces social investigatory behavior and locomotor activity in the presence of IL-1β.