Interleukin-2 suppresses established long-term potentiation and inhibits its induction in the rat hippocampus

Preventive exercise attenuates IL-2-driven mood disorders in multiple sclerosis

BACKGROUND

Elevated levels of specific proinflammatory molecules in the cerebrospinal fluid (CSF) have been associated with disability progression, enhanced neurodegeneration and higher incidence of mood disorders in people with multiple sclerosis (MS). Studies in animal models of MS suggest that preventive exercise may play an immunomodulatory activity, with beneficial effects on both motor deficits and behavioral alterations. Here we explored the impact of lifestyle physical activity on clinical presentation and associated central inflammation in a large group of newly diagnosed patients with MS. Furthermore, we addressed the causal link between exercise-mediated immunomodulation and mood symptoms in the animal setting.

METHODS

A cross-sectional study was conducted on 235 relapsing-remitting MS patients at the time of the diagnosis. Patients were divided into 3 groups ("sedentary", "lifestyle physical activity" and "exercise") according to the level of physical activity in the six months preceding the evaluation. Patients underwent clinical, neuropsychological and psychiatric evaluation, magnetic resonance imaging and lumbar puncture for diagnostic purposes. The CSF levels of proinflammatory and anti-inflammatory cytokines were analyzed and compared with a group of 80 individuals with non-inflammatory and non-degenerative diseases. Behavioral and electrophysiological studies were carried out in control mice receiving intracerebral injection of IL-2 or vehicle. Behavior was also assessed in mice with experimental autoimmune encephalomyelitis (EAE), animal model of MS, reared in standard (sedentary group) or running wheel-equipped (exercise group) cages.

RESULTS

In exercising MS patients, depression and anxiety were reduced compared to sedentary patients. The CSF levels of the interleukin-2 and 6 (IL-2, IL-6) were increased in MS patients compared with control individuals. In MS subjects exercise was associated with normalized CSF levels of IL-2. In EAE mice exercise started before disease onset reduced both behavioral alterations and striatal IL-2 expression. Notably, a causal role of IL-2 in mood disorders was shown. IL-2 administration in control healthy mice induced anxious- and depressive-like behaviors and impaired type-1 cannabinoid (CB1) receptor-mediated neurotransmission at GABAergic synapses, mimicking EAE-induced synaptic dysfunction.

CONCLUSIONS

Our results indicate an immunomodulatory effect of exercise in MS patients, associated with reduced CSF expression of IL-2, which might result in reduced mood disorders. These data suggest that exercise in the early stages may act as a disease-modifying therapy in MS although further longitudinal studies are needed to clarify this issue.

Neural Mechanisms of Cancer Cachexia

Simple Summary

Cancer cachexia is a devastating wasting syndrome that occurs in many illnesses, with signs and symptoms including anorexia, weight loss, cognitive impairment and fatigue. The brain is capable of exerting overarching homeostatic control of whole-body metabolism and is increasingly being recognized as an important mediator of cancer cachexia. Given the increased recognition and discovery of neural mechanisms of cancer cachexia, we sought to provide an in-depth review and update of mechanisms by which the brain initiates and propagates cancer cachexia programs. Furthermore, recent work has identified new molecular mediators of cachexia that exert their effects through their direct interaction with the brain. Therefore, this review will summarize neural mechanisms of cachexia and discuss recently identified neural mediators of cancer cachexia.

Abstract

Nearly half of cancer patients suffer from cachexia, a metabolic syndrome characterized by progressive atrophy of fat and lean body mass. This state of excess catabolism decreases quality of life, ability to tolerate treatment and eventual survival, yet no effective therapies exist. Although the central nervous system (CNS) orchestrates several manifestations of cachexia, the precise mechanisms of neural dysfunction during cachexia are still being unveiled. Herein, we summarize the cellular and molecular mechanisms of CNS dysfunction during cancer cachexia with a focus on inflammatory, autonomic and neuroendocrine processes and end with a discussion of recently identified CNS mediators of cachexia, including GDF15, LCN2 and INSL3.

Intravitreal Interleukin-2 modifies retinal excitatory circuits and retinocollicular innervation

Interleukin-2 is a classical immune cytokine whose neural functions have received little attention. Its levels have been found to be increased in some neuropathologies, such as Alzheimer's disease, multiple sclerosis and uveitis. Mechanistically, it has been demonstrated the role of IL-2 in regulating glutamate and acetylcholine transmission, thus being relevant for CNS physiology. In fact, our previous work showed that an acute intravitreal IL-2 injection during retinotectal development promoted contralateral eye axonal plasticity in the superior colliculus, but the involved mechanisms were not explored. So, our present study aimed to investigate the effect of increased intravitreal IL-2 levels on the retinal glutamatergic and cholinergic signalling required for retinotectal normal development. We showed through HRP neuronal tracing that intravitreal IL-2 also induces ipsilateral eye axonal sprouting. Protein level and/or immunolocalization analysis in the retina confirmed IL-2 pathway activation by increased expression of phospho-STAT-3, coupled to transient (24h) reduced levels of Egr1, PSD-95 and nicotinic acetylcholine receptor β2 subunit, suggesting reduced neural activity and synaptic sites. Also, AChE activity and GluN2B and GluA2 contents were reduced within 96h after IL-2 treatment. Therefore, IL-2-induced retinotectal plasticity might be driven by changes in cholinergic and glutamatergic pathways of the retina.

Enduring neuroimmunological consequences of developmental experiences: From vulnerability to resilience

The immune system is crucial for normal neuronal development and function (neuroimmune system). Both immune and neuronal systems undergo significant postnatal development and are sensitive to developmental programming by environmental experiences. Negative experiences from infection to psychological stress at a range of different time points (in utero to adolescence) can permanently alter the function of the neuroimmune system: given its prominent role in normal brain development and function this dysregulation may increase vulnerability to psychiatric illness. In contrast, positive experiences such as exercise and environmental enrichment are protective and can promote resilience, even restoring the detrimental effects of negative experiences on the neuroimmune system. This suggests the neuroimmune system is a viable therapeutic target for treatment and prevention of psychiatric illnesses, especially those related to stress. In this review we will summarise the main cells, molecules and functions of the immune system in general and with specific reference to central nervous system development and function. We will then discuss the effects of negative and positive environmental experiences, especially during development, in programming the long-term functioning of the neuroimmune system. Finally, we will review the sparse but growing literature on sex differences in neuroimmune development and response to environmental experiences.

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The immune system is essential for development and function of the central nervous system (neuroimmune system)

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Environmental experiences can permanently alter neuroimmune function and associated brain development

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Altered neuroimmune function following negative developmental experiences may play a role in psychiatric illnesses

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Positive experiences can promote resilience and rescue the effects of negative experiences on the neuroimmune system

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The neuroimmune system is therefore a viable therapeutic target for preventing and treating psychiatric illnesses

Neuroimmunological effects of early life experiences

Exposure to adverse experiences during development increases the risk of psychiatric illness later in life. Growing evidence suggests a role for the neuroimmune system in this relationship. There is now substantial evidence that the immune system is critical for normal brain development and behaviour, and responds to environmental perturbations experienced early in life. Severe or chronic stress results in dysregulated neuroimmune function, concomitant with abnormal brain morphology and function. Positive experiences including environmental enrichment and exercise exert the opposite effect, promoting normal brain and immune function even in the face of early life stress. The neuroimmune system may therefore provide a viable target for prevention and treatment of psychiatric illness. This review will briefly summarise the neuroimmune system in brain development and function, and review the effects of stress and positive environmental experiences during development on neuroimmune function. There are also significant sex differences in how the neuroimmune system responds to environmental experiences early in life, which we will briefly review.

The interoceptive hippocampus: Mouse brain endocrine receptor expression highlights a dentate gyrus (DG)-cornu ammonis (CA) challenge-sufficiency axis

The primeval function of the mammalian hippocampus (HPC) remains uncertain. Implicated in learning and memory, spatial navigation, and neuropsychological disorders, evolutionary theory suggests that the HPC evolved from a primeval chemosensory epithelium. Deficits in sensing of internal body status ('interoception') in patients with HPC lesions argue that internal sensing may be conserved in higher vertebrates. We studied the expression patterns in mouse brain of 250 endocrine receptors that respond to blood-borne ligands. Key findings are (i) the proportions and levels of endocrine receptor expression in the HPC are significantly higher than in all other comparable brain regions. (ii) Surprisingly, the distribution of endocrine receptor expression within mouse HPC was found to be highly structured: receptors signaling 'challenge' are segregated in dentate gyrus (DG), whereas those signaling 'sufficiency' are principally found in cornu ammonis (CA) regions. Selective expression of endocrine receptors in the HPC argues that interoception remains a core feature of hippocampal function. Further, we report that ligands of DG receptors predominantly inhibit both synaptic potentiation and neurogenesis, whereas CA receptor ligands conversely promote both synaptic potentiation and neurogenesis. These findings suggest that the hippocampus acts as an integrator of body status, extending its role in context-dependent memory encoding from 'where' and 'when' to 'how I feel'. Implications for anxiety and depression are discussed.

Interleukin-10 and transforming growth factor-β1 facilitate long-term potentiation in CA1 region of hippocampus

It has been shown that pro-inflammatory cytokines preferentially attenuate long-term potentiation (LTP), at the same time the effect of anti-inflammatory cytokines on synaptic plasticity has not been fully studied yet. Here we studied the effect of two anti-inflammatory cytokines - interleukin-10 (IL-10) and transforming growth factor-β1 (TGF-β1) on long-term potentiation. It was found that exogenously added IL-10 as well as TGF-β1 were able to effectively facilitate LTP evoked with ether high frequency or theta burst stimulation protocols in CA1 area of hippocampus. Effectiveness of IL-10 and TGF-β1 on LTP varied depending on the concentration of used cytokine and type of tetanic stimulation protocol used for LTP induction. Overall the positive effect of studied cytokines on LTP was associated with their ability to increase basal synaptic strength at Schaffer collateral - CA1 synapse. At the same time IL-10 and TGF-β1 did not have any effect on short-term plasticity. Our results provide new evidence upon the modulatory effects that anti-inflammatory cytokines exert on synaptic plasticity further highlighting their potency as modulators of neuronal function.

Pretreatment Cancer-Related Cognitive Impairment-Mechanisms and Outlook

Cognitive changes are common in patients with active cancer and during its remission. This has largely been blamed on therapy-related toxicities and diagnosis-related stress, with little attention paid to the biological impact of cancer itself. A plethora of clinical studies demonstrates that cancer patients experience cognitive impairment during and after treatment. However, recent studies show that a significant portion of patients with non-central nervous system (CNS) tumors experience cognitive decline prior to treatment, suggesting a role for tumor-derived factors in modulating cognition and behavior. Cancer-related cognitive impairment (CRCI) negatively impacts a patient’s quality of life, reduces occupational and social functioning, and increases morbidity and mortality. Furthermore, patients with cancer cachexia frequently experience a stark neurocognitive decline, suggesting peripheral tumors exert an enduring toll on the brain during this chronic paraneoplastic syndrome. However, the scarcity of research on cognitive impairment in non-CNS cancers makes it difficult to isolate psychosocial, genetic, behavioral, and pathophysiological factors in CRCI. Furthermore, clinical models of CRCI are frequently confounded by complicated drug regimens that inherently affect neurocognitive processes. The severity of CRCI varies considerably amongst patients and highlights its multifactorial nature. Untangling the biological aspects of CRCI from genetic, psychosocial, and behavioral factors is non-trivial, yet vital in understanding the pathogenesis of CRCI and discovering means for therapeutic intervention. Recent evidence demonstrating the ability of peripheral tumors to alter CNS pathways in murine models is compelling, and it allows researchers to isolate the underlying biological mechanisms from the confounding psychosocial stressors found in the clinic. This review summarizes the state of the science of CRCI independent of treatment and focuses on biological mechanisms in which peripheral cancers modulate the CNS.

Synaptic plasticity modulation by circulating peptides and metaplasticity: Involvement in Alzheimer's disease

Synaptic plasticity is a cellular process involved in learning and memory whose alteration in its two main forms (Long Term Depression (LTD) and Long Term Potentiation (LTP)), is observed in most brain pathologies, including neurodegenerative disorders such as Alzheimer's disease (AD). In humans, AD is associated at the cellular level with neuropathological lesions composed of extracellular deposits of β-amyloid (Aβ) protein aggregates and intracellular neurofibrillary tangles, cellular loss, neuroinflammation and a general brain homeostasis dysregulation. Thus, a dramatic synaptic environment perturbation is observed in AD patients, involving changes in brain neuropeptides, cytokines, growth factors or chemokines concentration and diffusion. Studies performed in animal models demonstrate that these circulating peptides strongly affect synaptic functions and in particular synaptic plasticity. Besides this neuromodulatory action of circulating peptides, other synaptic plasticity regulation mechanisms such as metaplasticity are altered in AD animal models. Here, we will review new insights into the study of synaptic plasticity regulatory/modulatory mechanisms which could influence the process of synaptic plasticity in the context of AD with a particular attention to the role of metaplasticity and peptide dependent neuromodulation.

Astrocytes and synaptic plasticity in health and disease

Activity-dependent synaptic plasticity phenomena such as long-term potentiation and long-term depression are candidate mechanisms for storing information in the brain. Regulation of synaptic plasticity is critical for healthy cognition and learning and this is provided in part by metaplasticity, which can act to maintain synaptic transmission within a dynamic range and potentially prevent excitotoxicity. Metaplasticity mechanisms also allow neurons to integrate plasticity-associated signals over time. Interestingly, astrocytes appear to be critical for certain forms of synaptic plasticity and metaplasticity mechanisms. Synaptic dysfunction is increasingly viewed as an early feature of AD that is correlated with the severity of cognitive decline, and the development of these pathologies is correlated with a rise in reactive astrocytes. This review focuses on the contributions of astrocytes to synaptic plasticity and metaplasticity in normal tissue, and addresses whether astroglial pathology may lead to aberrant engagement of these mechanisms in neurological diseases such as Alzheimer's disease.

Circulating progenitor cells are positively associated with cognitive function among overweight/obese children

Recent evidence has indicated that overweight/obese children may experience cognitive and immune dysfunction, but the underlying mechanisms responsible for the association between overweight/obesity, immune dysfunction, and cognition have yet to be established. The present study aimed to identify a novel link between obesity-induced immune system dysregulation and cognition in preadolescent children. A total of 27 male children (age: 8-10years) were recruited and separated by body mass index (BMI) into healthy weight (HW: 5th-84.9th percentile, n=16) and overweight/obese (OW: ⩾85th percentile, n=11) groups. Adiposity was assessed using dual energy X-ray absorptiometry (DXA), and aspects of executive function were assessed using the Woodcock-Johnson III Tests of Cognitive Abilities. Monocyte populations (CD14(+)CD16(-), CD14(+)CD16(+)) with and without expression of chemokine receptor type 2 (CCR2), and circulating progenitor cells (CPCs: CD34(+)CD45(dim)), in peripheral blood were quantified by flow cytometry. CPCs were isolated by flow sorting and cultured for 24h for collection of conditioned media (CM) that was applied to SH-SY5Y neuroblastomas to examine the paracrine effects of CPCs on neurogenesis. OW had significantly higher quantities of both populations of monocytes (CD14(+)CD16(-): 57% increase; CD14(+)CD16(+): 95% increase, both p<0.01), monocytes expressing CCR2 (CD14(+)CD16(-)CCR2(+): 66% increase; CD14(+)CD16(+)CCR2(+): 168% increase, both p<0.01), and CPCs (47% increase, p<0.05) than HW. CPCs were positively correlated with abdominal adiposity in OW, and negatively correlated in HW with a significant difference between correlations (p<0.05). CPC content was positively correlated with executive processes in OW, and negatively correlated in HW with a significant difference in the strength of the correlations between groups (p<0.05 for correlation between OW and HW). Finally, CPC-CM from OW trended to increase neuroblast viability in vitro relative to HW (1.79 fold, p=0.07). These novel findings indicate that increased content of CPCs among OW children may play a role in preventing decrements in cognitive function via paracrine mechanisms.

Neuroinflammatory changes negatively impact on LTP: A focus on IL-1β

In recent years it has become clear that neuroinflammatory changes develop in the brain with age and that similar, though more profound changes, occur in neurodegenerative conditions and in animal models of neurodegeneration. These changes are linked with deterioration in plasticity and the evidence suggests that a key causative factor is microglial activation and the associated increase in production and release of inflammatory cytokines. Several groups have reported that interleukin (IL)-1β negatively impacts on hippocampal-dependent learning and has an inhibitory effect on LTP although this is concentration-dependent. Similarly other inflammatory cytokines, which are also produced by microglia similarly decrease LTP. The evidence supporting these findings will be reviewed here and will be discussed in the context of considering mechanisms by which the negative impact of neuroinflammation can be ameliorated. This article is part of a Special Issue entitled SI: Brain and Memory.

Loss of Neuronal Phenotype and Neurodegeneration: Effects of T Lymphocytes and Brain Interleukin-2

Loss of neuronal phenotype and reversal of neuronal atrophy have been demonstrated in different models of central nervous system (CNS) injury. These processes may be generalizable to different types of brain neurons and circuitry. The idea that some injured neurons may lose their phenotype and/or atrophy with the potential to rejuvenate is a remarkable and potentially promising form of neuronal plasticity that is not well understood. In this paper, we present some of our laboratory's basic neuroimmunology research showing that peripheral T cells entering the CNS, and brain-derived interleukin-2 (IL-2), play significant roles in these intriguing processes. Our findings suggest, for example, that T cell immunosenesence could be involved in related processes of brain aging and contribute to neurodegenerative disease. Neuroimmunological approaches may provide new insights into yet undiscovered factors and brain mechanisms that regulate changes in neuronal integrity associated with aging and disease. Such findings could have important implications for discovering more effective strategies for treating patients with neurotrauma and neurodegenerative diseases (e.g., Alzheimer's disease).

Selective Neuronal and Brain Regional Expession of IL-2 in IL2P 8-GFP Transgenic Mice: Relation to Sensorimotor Gating

Brain-derived interleukin-2 (IL-2) has been implicated in diseases processes that arise during CNS development (e.g., autism) to neurodegenerative alterations involving neuroinflammation (e.g., Alzheimer’s disease). Progress has been limited, however, because the vast majority of current knowledge of IL-2’s actions on brain function and behavior is based on the use exogenously administered IL-2 to make inferences about the function of the endogenous cytokine. Thus, to identify the cell-type(s) and regional circuitry that express brain-derived IL-2, we used B6.Cg-Tg/ IL2-EGFP17Evr (IL2p8-GFP) transgenic mice, which express green fluorescent protein (GFP) in peripheral immune cells known to produce IL-2. We found that the IL2-GFP transgene was localized almost exclusively to NeuN-positive cells, indicating that the IL-2 is produced primarily by neurons. The IL2-GFP transgene was expressed in discrete nuclei throughout the rostral-caudal extent of the brain and brainstem, with the highest levels found in the cingulate, dorsal endopiriform nucleus, lateral septum, nucleus of the solitary tract, magnocellular/gigantocellular reticular formation, red nucleus, entorhinal cortex, mammilary bodies, cerebellar fastigial nucleus, and posterior interposed nucleus. Having identified IL-2 gene expression in brain regions associated with the regulation of sensorimotor gating (e.g., lateral septum, dorsal endopiriform nucleus, entorhinal cortex, striatum), we compared prepulse inhibition (PPI) of the acoustic startle response in congenic mice bred in our lab that have selective loss of the IL-2 gene in the brain versus the peripheral immune system, to test the hypothesis that brain-derived IL-2 plays a role in modulating PPI. We found that congenic mice devoid of IL-2 gene expression in both the brain and the peripheral immune system, exhibited a modest alteration of PPI. These finding suggest that IL2p8-GFP transgenic mice may be a useful tool to elucidate further the role of brain-derived IL-2 in normal CNS function and disease.

Anatomy of Graft-induced Dyskinesias: Circuit Remodeling in the Parkinsonian Striatum

The goal of researchers and clinicians interested in re-instituting cell based therapies for PD is to develop an effective and safe surgical approach to replace dopamine (DA) in individuals suffering from Parkinson's disease (PD). Worldwide clinical trials involving transplantation of embryonic DA neurons into individuals with PD have been discontinued because of the often devastating post-surgical side-effect known as graft-induced dyskinesia (GID). There have been many review articles published in recent years on this subject. There has been a tendency to promote single factors in the cause of GID. In this review, we contrast the pros and cons of multiple factors that have been suggested from clinical and/or preclinical observations, as well as novel factors not yet studied that may be involved with GID. It is our intention to provide a platform that might be instrumental in examining how individual factors that correlate with GID and/or striatal pathology might interact to give rise to dysfunctional circuit remodeling and aberrant motor output.

Glial cells in (patho)physiology

Neuroglial cells define brain homeostasis and mount defense against pathological insults. Astroglia regulate neurogenesis and development of brain circuits. In the adult brain, astrocytes enter into intimate dynamic relationship with neurons, especially at synaptic sites where they functionally form the tripartite synapse. At these sites, astrocytes regulate ion and neurotransmitter homeostasis, metabolically support neurons and monitor synaptic activity; one of the readouts of the latter manifests in astrocytic intracellular Ca(2+) signals. This form of astrocytic excitability can lead to release of chemical transmitters via Ca(2+) -dependent exocytosis. Once in the extracellular space, gliotransmitters can modulate synaptic plasticity and cause changes in behavior. Besides these physiological tasks, astrocytes are fundamental for progression and outcome of neurological diseases. In Alzheimer's disease, for example, astrocytes may contribute to the etiology of this disorder. Highly lethal glial-derived tumors use signaling trickery to coerce normal brain cells to assist tumor invasiveness. This review not only sheds new light on the brain operation in health and disease, but also points to many unknowns.

Interleukin-2 and the septohippocampal system: intrinsic actions and autoimmune processes relevant to neuropsychiatric disorders

The effects of IL-2 on brain development, function, and disease are the result of IL-2's actions in the peripheral immune system and its intrinsic actions in the central nervous system (CNS). Determining whether, and under what circumstances (e.g., development, acute injury), these different actions of IL-2 are operative in the brain is essential to make significant advances in understanding the multifaceted affects of IL-2 on CNS function and disease, including psychiatric disorders. For several decades, there has been a great deal of speculation about the role of autoimmunity in brain disease. More recently, we have learned a great deal about the role of cytokines on neurobiological processes, and there have been many studies that have found peripheral immune alterations in patients with neurological and neuropsychiatric diseases. Despite a plethora of published literature, almost all of this data in humans is correlative and much of the basic research has understandably relied on simpler models (e.g., in vitro models). Good animal models such as our IL-2 knockout mouse model could provide valuable new insight into understanding how the complex biology of a cytokine such as IL-2 can have simultaneous, dynamic effects on multiple systems (e.g., regulating homeostasis in the brain and immune system, autoimmunity that can affect both systems). Animal models can also provide much needed new data elucidating neuroimmunological and autoimmune processes involved in brain development and disease. Such information may ultimately provide critical new insight into the role of brain cytokines and autoimmunity in prominent neurological and neuropsychiatric diseases (e.g., Alzheimer's disease, autism, multiple sclerosis, schizophrenia).

Interleukin-2 and the brain: dissecting central versus peripheral contributions using unique mouse models

Although many studies have documented peripheral immune alterations in patients with psychiatric and neurological disorders, almost all these data in humans are correlative. The actions of IL-2 on neurodevelopment, function, and disease are the result of both IL-2's actions in the peripheral immune system and intrinsic actions in the CNS. Determining if, and under what conditions (e.g., development, acute injury) these different actions of IL-2 are operative in the brain is essential to make advances in understanding the multifaceted affects of IL-2 on CNS function and disease. Mouse models have provided ways to obtain new insights into how the complex biology of a cytokine such as IL-2 can have simultaneous, dynamic effects on multiple systems (e.g., regulating homeostasis in the brain and immune system, autoimmunity that can affect both systems). Here we describe some of the relevant literature and our research using different mouse models. This includes models such as congenic IL-2 knockout mice bred on immunodeficient backgrounds coupled with immune reconstitution strategies used to dissect neuroimmunological processes involved in the development of septohippocampal pathology, and test the hypothesis that dysregulation of the brain's endogenous neuroimmunological milieu may occur with the loss of brain IL-2 gene expression and be involved in initiating CNS autoimmunity. Use of animal models like these in the field of psychoneuroimmunology may lead to critical advances into our understanding of the role of brain cytokines and autoimmunity in neurodegenerative diseases (e.g., Alzheimer's disease), neurodevelopmental disorder (e.g., autism, schizophrenia), and autoimmune diseases including multiple sclerosis.

Intravitreous interleukin-2 treatment and inflammation modulates glial cells activation and uncrossed retinotectal development

Interleukin-2 (IL-2) plays regulatory functions both in immune and nervous system. However, in the visual system, little is known about the cellular types which respond to IL-2 and its effects. Herein, we investigated the influence of IL-2 in the development of central visual pathways. Lister Hooded rats were submitted to multiple (at postnatal days [PND]7/10/13) or single (at PND10) intravitreous injections of phosphate-buffered saline (PBS) (vehicle), zymosan, or IL-2. IL-2 receptor α subunit was detected in the whole postnatal retina. Chronic treatment with either PBS or IL-2 increases retinal glial fibrillary acidic protein (GFAP) expression, induces intravitreous inflammation revealed by the presence of macrophages, and results in a slight rearrangement of retinotectal axons. Acute zymosan treatment disrupts retinotectal axons distribution, confirming the influence of inflammation on retinotectal pathway reordering. Furthermore, acute IL-2 treatment increases GFAP expression in the retina without inflammation and produces a robust sprouting of the intact uncrossed retinotectal pathway. No difference was observed in glial cells activity in superior colliculus. Taken together, these data suggest that inflammation and interleukin-2 modulate retinal ganglion cells development and the distribution of their axons within central targets.

Novel roles for immune molecules in neural development: implications for neurodevelopmental disorders

Although the brain has classically been considered “immune-privileged”, current research suggests an extensive communication between the immune and nervous systems in both health and disease. Recent studies demonstrate that immune molecules are present at the right place and time to modulate the development and function of the healthy and diseased central nervous system (CNS). Indeed, immune molecules play integral roles in the CNS throughout neural development, including affecting neurogenesis, neuronal migration, axon guidance, synapse formation, activity-dependent refinement of circuits, and synaptic plasticity. Moreover, the roles of individual immune molecules in the nervous system may change over development. This review focuses on the effects of immune molecules on neuronal connections in the mammalian central nervous system – specifically the roles for MHCI and its receptors, complement, and cytokines on the function, refinement, and plasticity of geniculate, cortical and hippocampal synapses, and their relationship to neurodevelopmental disorders. These functions for immune molecules during neural development suggest that they could also mediate pathological responses to chronic elevations of cytokines in neurodevelopmental disorders, including autism spectrum disorders (ASD) and schizophrenia.

Central and peripheral cytokines mediate immune-brain connectivity

The immune system is a homeostatic system that contributes to maintain the constancy of the molecular and cellular components of the organism. Immune cells can detect the intrusion of foreign antigens or alteration of self-components and send information to the central nervous system (CNS) about this kind of perturbations, acting as a receptor sensorial organ. The brain can respond to such signals by emitting neuro/endocrine signals capable of affecting immune reactivity. Thus, the immune system, as other physiologic systems, is under brain control. Under disease conditions, when priorities for survival change, the immune system can, within defined limits, reset brain-integrated neuro-endocrine mechanisms in order to favour immune processes at the expenses of other physiologic systems. In addition, some cytokines initially conceived as immune products, such as IL-1 and IL-6, are also produced in the "healthy" brain by glial cells and even by some neurons. These and other cytokines have the capacity to affect synaptic plasticity acting as mediators of interactions between astrocytes and pre- and post-synaptic neurons that constitute what is actually defined as a tripartite synapse. Since the production of cytokines in the brain is affected by peripheral immune and central neural signals, it is conceivable that tripartite synapses can, in turn, serve as a relay system in immune-CNS communication.

Immune proteins in brain development and synaptic plasticity

Many proteins first identified in the immune system are also expressed in the developing and adult nervous system. Unexpectedly, recent studies reveal that a number of these proteins, in addition to their immunological roles, are essential for the establishment, function, and modification of synaptic connections. These include proinflammatory cytokines (e.g., TNFalpha, IL-6), proteins of the innate immune system (e.g., complement C1q and C3, pentraxins, Dscam), members of the major histocompatibility complex class I (MHCI) family, and MHCI-binding immunoreceptors and their components (e.g., PIRB, Ly49, DAP12, CD3zeta). Understanding how these proteins function in neurons will clarify the molecular basis of fundamental events in brain development and plasticity and may add a new dimension to our understanding of neural-immune interactions in health and disease.

Opiate modulation of IL-1alpha, IL-2, and TNF-alpha transport across the blood-brain barrier

Interleukin-1alpha (IL-1alpha), interleukin-2 (IL-2), and tumor necrosis factor alpha (TNF-alpha) are proinflammatory cytokines with potent neuromodulatory effects and are implicated in the etiology and pathogenesis of various psychological and neurological disorders. The findings that chronic morphine treatment alters both blood-brain barrier (BBB) function and cytokine production raises the possibility that morphine can also modulate cytokine transport across the BBB. Here, we found that acute morphine treatment (12 mg/kg i.p.) did not alter blood-to-brain transport of IL-1alpha, IL-2 or TNF-alpha. Whereas chronic morphine treatment (48 h after implantation of 75 mg morphine pellets) and withdrawal from morphine (10-15 min after an i.p. injection of 1mg/kg of naltroxone 48 h after implantation of 75 mg morphine pellets) did not alter blood-to-brain transport of IL-1alpha or TNF-alpha, both the chronic morphine treatment and withdrawal from morphine groups had increased blood-to-brain transport of IL-2. Typically, the permeability of the BBB to IL-2 is dominated by brain-to-blood efflux, with only limited blood-to-brain transport. Here, we found that chronic morphine and withdrawal from morphine did not alter brain-to-blood efflux, but induced a novel saturable blood-to-brain transport system. Whereas IL-1alpha, IL-2, and TNF-alpha are all proinflammatory cytokines, morphine exposure has individualized effects on their blood-to-brain transport.

Association study of interleukin 2 (IL2) and IL4 with schizophrenia in a Japanese population

Interleukin 2 (IL-2) and IL-4 are pleiotropic cytokines regulating Th1/Th2 balance and have a regulatory activity in brain function. Thus these cytokines have been implicated in the pathophysiology of schizophrenia. The latest studies provided controversial results regarding the genetic associations of these cytokines. The functional polymorphisms, IL2-330T/G and IL4-590C/T, were associated with schizophrenia in a German population, although contradictory findings were also reported in a Korean population. To ascertain whether IL2 and IL4 contribute to vulnerability to schizophrenia, we conducted a moderate-scale case-control (536 patients and 510 controls) association study for seven polymorphisms in Japanese subjects. There were no significant associations of these genes with schizophrenia using either single marker or haplotype analyses. The present study suggests that IL2 and IL4 do not contribute to vulnerability to schizophrenia in the Japanese population.

Neuroinflammation and synaptic plasticity: theoretical basis for a novel, immune-centred, therapeutic approach to neurological disorders

The fascinating capacity that the central nervous system (CNS) has for encoding and retaining memories is thought to be based on activity-dependent forms of synaptic plasticity. The CNS and the immune systems are known to be engaged in an intense bidirectional crosstalk, and glial cells are now viewed as a crucial third element of the synapse. In this opinion article, we review the principal mechanisms by which the immune system, and in particular immune diffusible mediators, influences synaptic transmission and the induction of brain plastic phenomena. Thereafter, we consider the potential implications of inflammation-related overexpression of diffusible mediators in the disruption of synaptic plastic processes and neuronal networks functioning during human neurological diseases. Finally, we propose that a more accurate characterization of the mechanisms underlying the immune-mediated control of synaptic plasticity could represent, in the future, the basis for the development of a novel immune-centred therapeutic approach to neurological disorders.

Spinal injection of IL-2 or IL-15 alters mechanical and thermal withdrawal thresholds in rats

IL-2 and IL-15 were tested for effects on responses to mechanical or thermal stimuli when spinally administered to male Sprague-Dawley rats with surgically implanted intrathecal catheters. Restricted doses of both IL-2 and IL-15 produced increased responsiveness to mechanical stimulation of the hindpaws. This effect lasted up to 48 h. IL-2 had biphasic effects on thermal responses whereas IL-15 produced thermal hypalgesia alone. These effects dissipated within 24h. These results suggest that IL-2 and IL-15 may participate in the generation of hyperalgesia in some pain conditions.

Neurobehavioral and immunological consequences of prenatal immune activation in rats. Influence of antipsychotics

Increasing evidence suggests that pre- or perinatal events that influence the immune system contribute to the development of behavioral or neuropsychiatric disorders. For instance, exposure of pregnant rats to the bacterial endotoxin lipopolysaccharide (LPS) disrupts sensorimotor information processing, as assessed by the prepulse inhibition test (PPI), and also the immune function in adult offspring, which might be of particular relevance as regards schizophrenia. However, the consequences of maternal LPS exposure during pregnancy on synaptic functioning in adult offspring and, more importantly, the therapeutic opportunity to re-establish PPI and immune function have still to be demonstrated. In this work, we analyzed the consequences of prenatal LPS exposure on dopaminergic neurotransmission and presynaptic markers in adult brain areas related to PPI circuitry. In addition, we tested whether oral treatment with the typical antipsychotic drug haloperidol (HAL) could reinstate PPI performances and cytokine serum levels in six-month-old male rats with prenatal LPS exposure. Both sensory information processing deficits and immune anomalies induced by prenatal exposure to LPS were accompanied by changes in dopaminergic neurotransmission and synaptophysin expression. It is important to note that PPI disruption and serum increases in cytokines induced by prenatal LPS exposure were both reversed by HAL. Taken together, these results demonstrate the critical influence of prenatal immune events on the functioning of adult nervous and immune systems, in association with the putative role of the immune system in the development of behavior relevant to schizophrenia.

Interleukin-2 inhibits NMDA receptor-mediated currents directly and may differentially affect subtypes

Using whole-cell patch-clamp recordings, this study investigated the effects of interleukin-2 (IL-2) on N-methyl-d-aspartate (NMDA) receptor-mediated currents (I(NMDA)) in rat cultured hippocampal neurons and human embryonic kidney (HEK) 293 cells expressing recombinant NMDA receptors. We found that IL-2 (0.01-1ng/ml) immediately and significantly decreased peak I(NMDA) in cultured neurons. Interestingly, the peak I(NMDA) induced in HEK 293 cells was also inhibited by IL-2. We also found that IL-2 differentially decreased the peak amplitudes of NR2A- and NR2B-containing NMDA receptor-mediated currents (I(NR2A) and I(NR2B)) by 54+/-5% and 30+/-4%, respectively. These results provide new evidence that IL-2 induces rapid inhibition of peak currents of NMDA receptor-mediated responses with possible NR1/NR2A and NR1/NR2B subtype-differentiation, and suggest that the inhibition is mediated by direct interaction between IL-2 and NMDA receptors.

Leptin facilitates learning and memory performance and enhances hippocampal CA1 long-term potentiation and CaMK II phosphorylation in rats

Leptin, an adipocytokine encoded by an obesity gene and expressed in adipose tissue, affects feeding behavior, thermogenesis, and neuroendocrine status via leptin receptors distributed in the brain, especially in the hypothalamus. Leptin may also modulate the synaptic plasticity and behavioral performance related to learning and memory since: leptin receptors are found in the hippocampus, and both leptin and its receptor share structural and functional similarities with the interleukin-6 family of cytokines that modulate long-term potentiation (LTP) in the hippocampus. We therefore examined the effect of leptin on (1) behavioral performance in emotional and spatial learning tasks, (2) LTP at Schaffer collateral-CA1 synapses, (3) presynaptic and postsynaptic activities in hippocampal CA1 neurons, (4) the intracellular Ca(2+) concentration ([Ca(2+)](i)) in CA1 neurons, and (5) the activity of Ca(2+)/calmodulin protein kinase II (CaMK II) in the hippocampal CA1 tissue that exhibits LTP. Intravenous injection of 5 and/or 50mug/kg, but not of 500mug/kg leptin, facilitated behavioral performance in passive avoidance and Morris water-maze tasks. Bath application of 10(-12)M leptin in slice experiments enhanced LTP and increased the presynaptic transmitter release, whereas 10(-10)M leptin suppressed LTP and reduced the postsynaptic receptor sensitivity to N-methyl-d-aspartic acid. The increase in the [Ca(2+)](i) induced by 10(-10)M leptin was two times greater than that induced by 10(-12)M leptin. In addition, the facilitation (10(-12)M) and suppression (10(-10)M) of LTP by leptin was closely associated with an increase and decrease in Ca(2+)-independent activity of CaMK II. Our results show that leptin not only affects hypothalamic functions (such as feeding, thermogenesis, and neuroendocrine status), but also modulates higher nervous functions, such as the behavioral performance related to learning and memory and hippocampal synaptic plasticity.

Changes in hippocampal IL-15, related cytokines, and neurogenesis in IL-2 deficient mice

Previous studies have demonstrated that interleukin-2 knockout (KO) mice exhibit alterations in hippocampal cytoarchitecture. Several lines of evidence suggest that these variations may result from immune dysregulation and/or autoimmunity. Thus, this study sought to compare adult IL-2 KO mice and wild-type littermates (8-12 weeks of age), the age where differences in hippocampal cytoarchitecture have previously been observed, for differences in measures of neuroimmunological status in the hippocampus. Furthermore, because IL-15 shares the same receptor subunits for signal transduction as IL-2 (IL-2/15Rbeta and gammac) that are enriched in the hippocampus and may induce inflammatory processes in IL-2 KO mice, we sought to test the hypothesis that IL-15 is elevated in the hippocampus of IL-2 KO mice. Compared to wild-type mice, IL-2 KO mice exhibited increased hippocampal protein concentrations of IL-15 as well as IL-12, IP-10, and MCP-1. These cytokine changes, however, did not correlate with levels in the peripheral circulation, and there were no T cells or an increase in MHCII-positive microglia in the hippocampus of IL-2 KO mice. Since elevated levels of certain inflammatory cytokines may impair hippocampal neurogenesis, we also tested the hypothesis that changes in neuroimmunological status would be associated with reductions in neurogenesis of neurons in the dentate gyrus of IL-2 KO mice. Contrary to this hypothesis, compared to wild-type mice, male IL-2 KO mice exhibited increased neurogenesis in both the infrapyramidal and suprapyramidal limbs of the granule cell layer of the dentate gyrus, differences that were not observed between females. These findings indicate that IL-2 gene deletion alters the neuroimmunological status of the mouse hippocampus through a dysregulation of cytokines produced by CNS cells, and in males, these changes are associated with increased hippocampal neurogenesis.

IL-2 deficiency results in altered septal and hippocampal cytoarchitecture: relation to development and neurotrophins

We have found previously that brain IL-2 receptors are enriched in the hippocampal formation, and that loss of this cytokine results in cytoarchitectural alterations in the hippocampus and septum and related behavioral changes in IL-2 knockout (IL-2 KO) mice. These alterations included decreased cholinergic somata in the medial septum/vertical limb of the diagonal band of Broca (MS/vDB) and decreased distance across the infrapyramidal (IP) granule cell layer (GCL) of the dentate gyrus (DG). To extend our previous findings, several experiments were conducted comparing IL-2 KO mice and wild-type littermates to determine (1) whether the GABAergic projection neurons of IL-2 KO mice in this region were also affected; (2) if the reduction in septal cholinergic projection neurons found in adult IL-2 KO mice is present at weaning (and prior to the development of peripheral autoimmune disease); and (3) if loss of IL-2 may result in changes in the neurotrophins, brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF), involved in maintenance of hippocampal neurons. No differences in GABAergic neurons in the MS/vDB were found in adult mice, and the reduction in cholinergic neurons seen in adult IL-2 KO mice was not found in animals at postnatal day 21. The number of neurons in the IP-GCL was also significantly reduced. Compared to wild-type mice, IL-2 KO mice had significantly reduced concentration of BDNF protein and increased concentrations of NGF. These data suggest that the septohippocampal neuronal loss in IL-2 KO mice is selective for the cholinergic neurons and appears to be due to a failure in neuronal maintenance/survival that may be, in part, associated with changes in neurotrophins.

Adenovirus-mediated interleukin-2 gene therapy of nociception

The effect of adenovirus-mediated interleukin-2 (IL-2) gene on rat basal nociceptive response and chronic neuropathic pain was explored. The paw withdrawal latency induced by radiant heat was used to evaluate the antinociceptive effect of adenovirus type 5 (Ad5) and Ad5-IL-2. The results showed that intrathecal delivery of Ad5-IL-2 exhibited obvious antinociceptive effects on basal nociceptive response and chronic neuropathic pain, which were maintained for 3 and 4 weeks, respectively. This suggested that the antinociceptive effect of Ad5-IL-2 on chronic neuropathic pain was greater than its effect on basal nociceptive response. Human IL-2 mRNA was detected by in situ hybridization in the spinal pia mater and parenchyma of the lumbar, sacral, thoracic and cervical regions, and gray matter had higher level of IL-2 expression than white matter. These data demonstrated that the IL-2 gene was transfected into spinal cord regions relevant to pain modulation. The expressed IL-2 protein profile in spinal cord detected by enzyme-linked immunosorbent assay coincided almost exactly with its antinociceptive effect. This supported the hypothesis that the therapeutic effect of IL-2 gene was related to IL-2 protein expression. The study indicates that intrathecal delivery of adenovirus-mediated IL-2 gene has a relatively long antinociceptive effect.

Cytokines and cognition--the case for a head-to-toe inflammatory paradigm

The brain is not only immunologically active of its own accord, but also has complex peripheral immune interactions. Given the central role of cytokines in neuroimmmunoendocrine processes, it is hypothesized that these molecules influence cognition via diverse mechanisms. Peripheral cytokines penetrate the blood-brain barrier directly via active transport mechanisms or indirectly via vagal nerve stimulation. Peripheral administration of certain cytokines as biological response modifiers produces adverse cognitive effects in animals and humans. There is abundant evidence that inflammatory mechanisms within the central nervous system (CNS) contribute to cognitive impairment via cytokine-mediated interactions between neurons and glial cells. Cytokines mediate cellular mechanisms subserving cognition (e.g., cholinergic and dopaminergic pathways) and can modulate neuronal and glial cell function to facilitate neuronal regeneration or neurodegeneration. As such, there is a growing appreciation of the role of cytokine-mediated inflammatory processes in neurodegenerative diseases such as Alzheimer's disease and vascular dementia. Consistent with their involvement as mediators of bidirectional communication between the CNS and the peripheral immune system, cytokines play a key role in the hypothalamic-pituitary-adrenal axis activation seen in stress and depression. In addition, complex cognitive systems such as those that underlie religious beliefs, can modulate the effects of stress on the immune system. Indirect means by which peripheral or central cytokine dysregulation could affect cognition include impaired sleep regulation, micronutrient deficiency induced by appetite suppression, and an array of endocrine interactions. Given the multiple levels at which cytokines are capable of influencing cognition it is plausible that peripheral cytokine dysregulation with advancing age interacts with cognitive aging.

Alterations in septohippocampal cholinergic neurons resulting from interleukin-2 gene knockout

Interleukin-2 (IL-2) has potent effects on acetylcholine (ACh) release from septohippocampal cholinergic neurons and trophic effects on fetal septal and hippocampal neuronal cultures. Previous work from our lab showed that the absence of endogenous IL-2 leads to impaired hippocampal neurodevelopment and related behaviors. We sought to extend this work by testing the hypotheses that the loss of IL-2 would result in reductions in cholinergic septohippocampal neuron cell number and the density of cholinergic axons found in the hippocampus of IL-2 knockout mice. Stereological cell counting and imaging techniques were used to compare C57BL/6-IL-2(-/-) knockout and C57BL/6-IL-2(+/+) wild-type mice for differences in choline acetyltransferase (ChAT)-positive somata in the medial septum and vertical limb of the diagonal band of Broca (MS/vDB) and acetylcholine esterase (AChE)-labeled cholinergic axons in hippocampal projection fields. IL-2 knockout mice had significantly lower numbers (26%) of MS/vDB ChAT-positive cell bodies than wild-type mice; however, there were no differences in striatal ChAT-positive neurons. Although AChE-positive axon density in CA1, CA3b, the internal, and external blades of the dentate gyrus did not differ between the knockout and wild-type mice, the distance across the granular cell layer of the external blade of the dentate gyrus was reduced significantly in IL-2 knockout mice. Further research is needed to determine whether these outcomes in IL-2 knockout mice may be due to the absence of central and/or peripheral IL-2 during brain development or neurodegeneration secondary to autoimmunity.

Interleukin-2 gene therapy of chronic neuropathic pain

Previous research has revealed an antinociceptive (analgesic) effect of interleukin-2 (IL-2) in central and peripheral nervous systems. Unfortunately IL-2 is very short-lived in vivo, so it is impractical to apply IL-2 for analgesia in clinic. This study was performed to evaluate the effect of intrathecal delivery of human IL-2 gene on rat chronic neuropathic pain induced by chronic constriction injury of the sciatic nerve. Human IL-2 cDNA was cloned into pcDNA3 containing a cytomegalovirus promoter. The paw-withdrawal latency induced by radiant heat was used to measure the pain threshold. The results showed that recombinant human IL-2 had a dose-dependent antinociceptive effect, but that this only lasted for 10-25 min. The pcDNA3-IL-2 or pcDNA3-IL-2/lipofectamine complex in contrast also showed dose-dependent antinociceptive effects, but these reached a peak at day 2-3 and were maintained for up to 6 days. Liposome-mediated pcDNA3-IL-2 produced a more powerful antinociceptive effect than pcDNA3-IL-2 alone. The paw-withdrawal latencies were not affected by control treatments such as vehicle, lipofectamine, pcDNA3, or pcDNA3-lipofectamine. In the experimental groups, human IL-2 mRNA was detected by reverse transcription-polymerase chain reaction in the lumbar spinal pia mater, dorsal root ganglion, sciatic nerve, and spinal dorsal horn, but not in gastrocnemius muscle. The expressed IL-2 profile detected by western blot coincided with its mRNA profile except it was present in the spinal dorsal horn at a higher level. Furthermore, human IL-2 assayed by enzyme-linked immunosorbent assay in cerebrospinal fluid could still be detected at day 6, but lower than day 3. The antinociceptive effect of pcDNA3-IL-2 could be blocked by naloxone, showing some relationship of the antinociceptive effect produced by IL-2 gene to the opioid receptors. It is hoped that the new delivery approach of a single intrathecal injection of the IL-2 gene described here may be of some practical use as a part of a gene therapy for treating neuropathic pain.

IL-2/15 receptor-beta gene deletion alters neurobehavioral performance

The common IL-2/15 receptor-beta (IL-2/15Rbeta) is an essential signaling subunit that is shared exclusively by IL-2 and IL-15, and is enriched in the hippocampal formation and related limbic regions. We have previously shown that mice lacking IL-2 exhibit alterations in hippocampal-dependent learning, sensorimotor gating and accompanying reductions in hippocampal infrapyramidal mossy neuronal fiber length. Although the effects of exogenous IL-2 on various aspects of forebrain neuronal function are well documented, it is unclear whether IL-15 has neuromodulatory actions. Here we sought to test the hypothesis that the combined loss of the ability of IL-2 and IL-15 to signal through IL-2/15Rbeta in the brain would influence neurobehavioral performance, in particular spatial learning and memory performance. To test this hypothesis, we compared several different domains of behavior in mice that had one or both IL-2/15Rbeta gene alleles deleted. Compared with C57BL/6-IL-2/15Rbeta+/+ wild-type and C57BL/6-IL-2/15Rbeta+/- heterozygote littermates, C57BL/6-IL-2/15Rbeta-/- knockout mice exhibited a deficit in prepulse inhibition of the acoustic startle reflex (PPI). The IL-2/15Rbeta knockout mice also showed significant reductions in acoustic startle reactivity, and modest differences in behavior in the elevated plus-maze test indicative of reduced levels of fearfulness in response to novelty. The IL-2/15Rbeta knockout mice did not differ in locomotor activity in either the plus-maze or the Morris water-maze, and contrary to our working hypothesis, they did not differ in spatial learning or memory performance in the water-maze. Further studies are required to determine if these behavioral alterations may be attributable to factors such as the loss of the ability of IL-15 and/or IL-2 to modulate limbic neurons, autoimmunity or genetic factors associated with IL-2/15Rbeta gene deletion.

Lipopolysaccharide inhibits induction of long-term potentiation and depression in the rat hippocampal CA1 area

We examined the effects of lipopolysaccharide, a bacterial endotoxin, on synaptic plasticity in the rat hippocampal CA1 area in vitro. Lipopolysaccharide suppressed the induction of long-term potentiation elicited by tetanic stimulation and long-term depression, elicited by low-frequency stimulation of Schaffer collateral-commissural fibres at 10 and 50 microg/ml, respectively. Lipid A (1 microg/ml), the biologically active component of lipopolysaccharide, mimicked the effects of 10 microg/ml lipopolysaccharide on long-term potentiation and depression. Nifedipine, an L-type voltage-sensitive Ca(2+) channel antagonist, did not influence the induction of long-term potentiation and depression, whereas a high concentration of extracellular calcium enabled long-term potentiation induction in the presence of 10 microg/ml lipopolysaccharide. The NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (APV, 50 microM), nifedipine (10 microM) or lipopolysaccharide (10 or 50 microg/ml) partially reduced the magnitude of tetraethylammonium-induced long-term potentiation. Nifedipine combined with lipopolysaccharide completely blocked tetraethylammonium-induced long-term potentiation. Whole-cell voltage clamp recordings showed that lipopolysaccharide suppressed NMDA receptor-mediated excitatory postsynaptic currents (EPSCs). Our results indicate that lipopolysaccharide acutely modifies synaptic plasticity by blocking Ca(2+) entry through NMDA receptors, suggesting a possible mechanism for the amnesic action of bacterial infection.

Interleukin-2 potentiates novelty- and GBR 12909-induced exploratory activity

Interleukin (IL)-2 is a cytokine that influences exploratory behavior and central dopamine activity in rodents, and induces schizophrenic-like behavior and cognitive deficits in humans. We presently report that a single i.p. injection of murine IL-2 (0.05-0.80 microg/mouse) induced significant increases in novelty-induced locomotion and exploration in BALB/c mice. These measures were not significantly altered in mice that were pre-exposed to the test cage prior to cytokine injection. The IL-2-induced behavioral changes were not further augmented by repeated intermittent injections (five daily i.p. injections; 0.4 microg/mouse), however. Nonetheless, during the treatment period, activity scores of IL-2-treated mice significantly exceeded those of mice receiving saline; hence, repeated injections of IL-2 induced a persistent behavioral activation. IL-2 treatment also increased sensitivity to the behavior-stimulating effects of GBR 12909, a highly selective dopamine uptake inhibitor. This effect was a very long-lasting one since the dopamine agonist was administered 6 weeks after cessation of IL-2 treatment. The latter finding indicates that IL-2 interacts with the mesolimbic dopamine system, changing its sensitivity to seemingly different substances. Based on these data, and those of Zalcman and colleagues (S. Zalcman, I. Savina, R.A. Wise, Interleukin-6 increases sensitivity to the locomotor-stimulating effects of amphetamine in rats, Brain Res. 847 (1999) 276-283), it is suggested that cytokines can influence the development of behavioral abnormalities that are characteristic of aberrant mesolimbic dopamine activity via sensitization-like processes.

Cloning the full-length IL-2/15 receptor-beta cDNA sequence from mouse brain: evidence of enrichment in hippocampal formation neurons

Numerous studies have implicated interleukin-2 (IL-2) in various brain processes, and more recently, several studies have also attributed neurobiological actions to interleukin-15 (IL-15). On lymphocytes, receptors for IL-2 and IL-15 share a common subunit, the IL-2/15 receptor-beta (IL-2/15Rbeta) that is essential for intracellular signaling. Although a short segment of IL-2/15Rbeta has been cloned (0.35 kb) from normal brain cells, attempts to isolate the full-length cDNA have been unsuccessful, suggesting the possibility that the genes expressed by brain cells and lymphocytes may differ. Using conventional and anchored PCR cloning strategies, we isolated the full-length cDNA of IL-2/15Rbeta (2038 bp) from well-perfused, normal mouse forebrain. The coding sequence and the adjacent 5' and 3' UTR sequences from brain and lymphocyte were found to be fully homologous. Although evidence of expression of IL-2/15Rbeta can be found in many brain regions using PCR, clear evidence of gene expression by in situ hybridization was detectable only in the hippocampal formation, habenula and piriform cortex. This same pattern of mRNA expression in situ was also observed for the common gamma subunit shared by IL-2 and IL-15. In the hippocampus, IL-2/15Rbeta expression was localized to neurons by high resolution in situ hybridization and evidence of IL-2 receptor protein expression was also detected by radioligand receptor binding using hippocampal homogenates. Comparison of undifferentiated and differentiated, immortalized H19-7 hippocampal neurons showed that IL-2/15Rbeta was constitutively expressed across disparate stages of hippocampal neuronal differentiation. These data indicate that IL-2/15Rbeta may serve to modulate neuronal processes in the hippocampus and associated limbic brain regions.

Lipopolysaccharide induces memory-processing deficits in day-old chicks

Recent evidence has demonstrated that immune activation can result in cognitive deficits due to the actions of the proinflammatory cytokines. These series of studies examined the effects of peripheral administration of lipopolysaccharide (LPS) on the memory processes of day-old chicks trained on a single-trial passive-avoidance task. LPS impaired performance in a dose- and time-dependent manner. Maximal impairment was produced by a dose of 2.5-mg/kg LPS administered 60 min prior to training. Retention tests revealed that deficits in memory processing appeared between 10 and 20 min posttraining. These results demonstrate an inhibitory effect of LPS on memory processing at the transition point from short-term memory to intermediate-term memory.

The inhibitory effects of interleukin-6 on synaptic plasticity in the rat hippocampus are associated with an inhibition of mitogen-activated protein kinase ERK

Several cytokines have short-term effects on synaptic transmission and plasticity that are thought to be mediated by the activation of intracellular protein kinases. We have studied the effects of interleukin-6 (IL-6) on the expression of paired pulse facilitation (PPF), posttetanic potentiation (PTP), and long-term potentiation (LTP) in the CA1 region of the hippocampus as well as on the activation of the signal transducer and activator of transcription-3 (STAT3), the mitogen-activated protein kinase ERK (MAPK/ERK), and the stress-activated protein kinase/c-Jun NH(2)-terminal kinase (SAPK/JNK). IL-6 induced a marked and dose-dependent decrease in the expression of PTP and LTP that could be counteracted by the simultaneous treatment with the tyrosine kinase inhibitor lavendustin A (LavA) but did not significantly affect PPF. The IL-6-induced inhibition of PTP and LTP was accompanied by a simulation of STAT3 tyrosine phosphorylation and an inhibition of MAPK/ERK dual phosphorylation, in the absence of changes in the state of activation of SAPK/JNK. Both effects of IL-6 on STAT3 and MAPK/ERK activation were effectively counteracted by LavA treatment. The results indicate the tyrosine kinases and MAPK/ERK are involved in hippocampal synaptic plasticity and may represent preferential intracellular targets for the actions of IL-6 in the adult nervous system.

Inhibition of nociceptive withdrawal reflex by microinjection of interleukin 2 into rat locus coeruleus

This study was to examine the effects of microinjection of human recombinant interleukin 2 (IL-2) into locus coeruleus (LC) on spinal nociception. Following application of IL-2 (0.1 microl, 10 pM) into LC, the percentage of inhibition of nociceptive C responses of reflex at 3, 9, 15, 21 and 27 min after injection were 88.2 +/-9.4%, 84.0 +/- 11.8%, 89.7 +/- 10.5%, 57.1 +/- 8.7% and 26.3 +/- 12.2%, respectively. Also, the expression of Fos protein in superficial dorsal horn was reduced by 73.01 +/- 13.58% of control (P<0.0001). Naloxone (10 microg, i.p.) completely blocked the IL-2-induced inhibition of C responses. The results clearly show that IL-2 receptors present in LC mediate descending inhibition of the spinal nociception, which may couple with the activation of opioid receptors on LC neurons.

Cytokine and growth factor involvement in long-term potentiation

Hippocampal long-term potentiation (LTP) is one of the best-studied models of learning and memory at the molecular level. While it has long been known that tetanic stimulation causes changes at the synapse within seconds to minutes, recent research has begun to focus on factors that may affect synaptic plasticity on a longer time scale. One group of factors with many of the characteristics predicted for both short- and long-term actions at the synapse is the cytokines and growth factors. In vitro, these proteins can alter neuronal morphology, gene expression, and proliferation, and many cytokines and their receptors are present in the adult CNS. Because brainderived neurotrophic factor (BDNF) is the best-studied synaptic modulator of this class, we begin by discussing the experimental evidence linking BDNF to LTP. Ten cytokines and growth factors that have been examined in the context of hippocampal LTP are then considered. We discuss the effects of LTP on the expression of the cytokines and explore the regulation of synaptic plasticity by exogenous application or antagonist perturbation of these proteins. The available evidence strongly supports a role for these factors in synaptic modulation and should prompt further exploration of their functions at the synapse.

Impaired learning and memory and altered hippocampal neurodevelopment resulting from interleukin-2 gene deletion

Interleukin-2 (IL-2), the protypical T cell growth factor and immunoregulatory cytokine produced by lymphocytes, has been implicated as a brain neurotrophic factor and neuromodulator. The consequences of the absence of endogenous IL-2 on brain development and function were unknown. Brain IL-2 receptors are enriched in the hippocampal formation, an area critical for the acquisition and consolidation of spatial learning and memory. Thus, we tested the hypothesis that mice lacking IL-2 would exhibit alterations in hippocampal-dependent learning and neurodevelopment. Compared with C57BL/6-IL-2+/+ wild-type mice, we observed that C57BL/6-IL-2-/- gene knockout mice had markedly impaired spatial learning and memory in the Morris water maze. No significant deficits in parameters of learning and memory performance were found in severe combined immunodeficient (SCID) mice (C57BL/6scid), however, suggesting that the impaired spatial learning and memory exhibited by IL-2 knockout mice is not attributable to generalized immunodeficiency resulting from the absence of endogenous IL-2. Examination of other domains of behavioral performance showed that the IL-2 knockout and wildtype mice did not differ in measures of fearfulness or locomotor activity in an elevated plus maze, or in reflexive startle responses to auditory stimuli--although prepulse inhibition of acoustic startle (PPI) was increased significantly in IL-2 knockout mice. The spatial learning and memory impairment in IL-2 knockout mice was accompanied by reductions in hippocampal infrapyramidal mossy neuronal fiber length, a factor shown previously to correlate positively with spatial learning ability. These findings indicate that, in addition to being a pivotal cytokine in immune regulation, IL-2 may play a role in the development and regulation of brain neurons involved in spatial learning and memory.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Interleukin-2 and -6 induce behavioral-activating effects in mice

Interleukin (IL)-1, IL-2 and IL-6 influence central monoamine activity in a cytokine-specific manner. We demonstrated that whereas IL-2 increased hypothalamic and hippocampal norepinephrine (NE) utilization, and DA turnover in the prefrontal cortex, IL-6 induced profound elevations of serotonin (5-HT) and mesocortical dopamine (DA) activity in the hippocampus and prefrontal cortex [S. Zalcman, J.M. Green-Johnson, L. Murray, D.M. Nance, D.G. Dyck, H. Anisman, A. H. Greenberg, Cytokine-specific central monoamine alterations following IL-1, -2 and -6 administration, Brain Res. 643 (1994) 40-49]. IL-1, in contrast, induced a wide range of central monoamine alterations. We presently report that these cytokines also differentially influence behavior. Profound reductions in non-ambulatory and ambulatory exploration were induced in BALB/c mice following IL-1 administration. In contrast, IL-2-treated mice displayed significant increases in the time spent engaged in non-ambulatory exploration, digging, rearing (particularly the number of free rears), and in the investigation of a novel stimulus (i.e., increased number and duration of stimulus contacts). IL-6-treated mice, moreover, exhibited significant increases in the time spent engaged in ambulatory exploration, digging and rearing (particularly the number of free rears, which tended to be of short duration). Modest increases in locomotion and grooming were also observed in IL-6-treated animals. Plasma corticosterone levels did not vary significantly as a function of IL-6 treatment. Hence, cytokine-specific behavioral-activating effects were induced following administration of IL-2 and IL-6. We suggest that these effects have adaptive significance and relevance to sickness behavior; however, pathological outcomes (e.g., schizophrenia, anxious-like states, anxious depression, motor abnormalities) could develop should these cytokines be overproduced or dysregulated.

Effect of interleukin-1beta on the ascending and descending reflex in rat small intestine

Acute inflammation of the intestine is associated with motility changes. We investigated the acute effect of inflammatory mediators such as interleukin-1beta, interleukin-2 and tumor necrosis factor-alpha (TNF-alpha) on electrically stimulated ascending and descending reflex responses of the rat small intestine. Exogenous application of interleukin-1beta caused a concentration-dependent inhibition of the oral contraction (0.1 ng/ml: -22.9+/-3.8%, 10 ng/ml: -57.0+/-7.4%, P < 0.05, n=10) but had no effect on anal relaxation. The interleukin-1 receptor antagonist alone had no significant effect on the reflex response, but prevented the inhibitory effect of interleukin-1beta (10 ng/ml: -3.9+/-11.4%, n=8). Interleukin-2 and TNF-alpha had no significant effect on the oral contractile and the anal inhibitory response (n.s., n=10). Using reverse transcriptase polymerase chain reaction (RT-PCR) the presence of mRNA of the interleukin-1 receptor was demonstrated in the rat small intestine. Preincubation of the preparation with indomethacin (10(-6) M), the histamine H1 receptor antagonist, pyrilamine (10(-8) M), and the histamine H3 receptor antagonist, clobenpropit (10(-8) M), decreased the oral contraction by 60.1+/-7.7%, 42.8+/-6.9% and 44.4+/-14.2% as well as the anal relaxation. These data suggest that acute administration of interleukin-1beta inhibits the ascending and descending contractile reflex pathway and this effect seems not to be mediated by prostaglandins or histamine receptors.