Neuronal, endocrine, and anorexic responses to the T-cell superantigen staphylococcal enterotoxin A: dependence on tumor necrosis factor-alpha

The social environment alters neural responses to a lipopolysaccharide challenge

Sick animals display drastic changes in their behavioral patterns, including decreased activity, decreased food and water intake, and decreased interest in social interactions. These behaviors, collectively called "sickness behaviors", can be socially modulated. For example, when provided with mating opportunities, males of several species show reduced sickness behaviors. While the behavior is known to change, how the social environment affects neural molecular responses to sickness is not known. Here, we used a species, the zebra finch, Taeniopygia guttata, where males have been shown to decrease sickness behaviors when presented with novel females. Using this paradigm, we obtained samples from three brain regions (the hypothalamus, the bed nucleus of the stria terminalis, and the nucleus taeniae) from lipopolysaccharide (LPS) or control treated males housed under four different social environments. Manipulation of the social environment rapidly changed the strength and co-expression patterns of the neural molecular responses to the immune challenge in all brain regions tested, therefore suggesting that the social environment plays a significant role in determining the neural responses to an infection. In particular, brains of males paired with a novel female showed muted immune responses to LPS, as well as altered synaptic signaling. Neural metabolic activity in response to the LPS challenge was also affected by the social environment. Our results provide new insights into the effects of the social environment on brain responses to an infection, thereby improving our understanding of how the social environment can affect health.

Mitochondrial Gene Expression Profiles and Metabolic Pathways in the Amygdala Associated with Exaggerated Fear in an Animal Model of PTSD

The metabolic mechanisms underlying the development of exaggerated fear in post-traumatic stress disorder (PTSD) are not well defined. In the present study, alteration in the expression of genes associated with mitochondrial function in the amygdala of an animal model of PTSD was determined. Amygdala tissue samples were excised from 10 non-stressed control rats and 10 stressed rats, 14 days post-stress treatment. Total RNA was isolated, cDNA was synthesized, and gene expression levels were determined using a cDNA microarray. During the development of the exaggerated fear associated with PTSD, 48 genes were found to be significantly upregulated and 37 were significantly downregulated in the amygdala complex based on stringent criteria (p < 0.01). Ingenuity pathway analysis revealed up- or downregulation in the amygdala complex of four signaling networks – one associated with inflammatory and apoptotic pathways, one with immune mediators and metabolism, one with transcriptional factors, and one with chromatin remodeling. Thus, informatics of a neuronal gene array allowed us to determine the expression profile of mitochondrial genes in the amygdala complex of an animal model of PTSD. The result is a further understanding of the metabolic and neuronal signaling mechanisms associated with delayed and exaggerated fear.

The role of orphanin FQ/nociceptin in neuroplasticity: relationship to stress, anxiety and neuroinflammation

The neuropeptide, orphanin FQ/nociceptin (OFQ/N or simply, nociceptin), is expressed in both neuronal and non-neuronal tissue, including the immune system. In the brain, OFQ/N has been investigated in relation to stress, anxiety, learning and memory, and addiction. More recently, it has also been found that OFQ/N influences glial cell functions, including oligodendrocytes, astrocytes, and microglial cells. However, this latter research is relatively small, but potentially important, when observations regarding the relationship of OFQ/N to stress and emotional functions is taken into consideration and integrated with the growing evidence for its involvement in cells that mediate inflammatory events. This review will first provide an overview and understanding of how OFQ/N has been implicated in the HPA axis response to stress, followed by an understanding of its influence on natural and learned anxiety-like behavior. What emerges from an examination of the literature is a neuropeptide that appears to counteract anxiogenic influences, but paradoxically, without attenuating HPA axis responses generated in response to stress. Studies utilized both central administration of OFQ/N, which was shown to activate the HPA axis, as well as antagonism of NOP-R, the OFQ/N receptor. In contrast, antagonist or transgenic OFQ/N or NOP-R knockout studies, showed augmentation of HPA axis responses to stress, suggesting that OFQ/N may be needed to control the magnitude of the HPA axis response to stress. Investigations of behavior in standard exploratory tests of anxiogenic behavior (eg., elevated plus maze) or learned fear responses have suggested that OFQ/N is needed to attenuate fear or anxiety-like behavior. However, some discrepant observations, in particular, those that involve appetitive behaviors, suggest a failure of NOP-R deletion to increase anxiety. However, it is also suggested that OFQ/N may operate in an anxiolytic manner when initial anxiogenic triggers (eg., the neuropeptide CRH) are initiated. Finally, the regulatory functions of OFQ/N in relation to emotion-related behaviors may serve to counteract potential neuroinflammatory events in the brain. This appears to be evident within the glial cell environment of the brain, since OFQ/N has been shown to reduce the production of proinflammatory cellular and cytokine events. Given that both OFQ/N and glial cells are activated in response to stress, it is possible that there is a possible convergence of these two systems that has important repercussions for behavior and neuroplasticity.

Depression and sickness behavior are Janus-faced responses to shared inflammatory pathways

It is of considerable translational importance whether depression is a form or a consequence of sickness behavior. Sickness behavior is a behavioral complex induced by infections and immune trauma and mediated by pro-inflammatory cytokines. It is an adaptive response that enhances recovery by conserving energy to combat acute inflammation. There are considerable phenomenological similarities between sickness behavior and depression, for example, behavioral inhibition, anorexia and weight loss, and melancholic (anhedonia), physio-somatic (fatigue, hyperalgesia, malaise), anxiety and neurocognitive symptoms. In clinical depression, however, a transition occurs to sensitization of immuno-inflammatory pathways, progressive damage by oxidative and nitrosative stress to lipids, proteins, and DNA, and autoimmune responses directed against self-epitopes. The latter mechanisms are the substrate of a neuroprogressive process, whereby multiple depressive episodes cause neural tissue damage and consequent functional and cognitive sequelae. Thus, shared immuno-inflammatory pathways underpin the physiology of sickness behavior and the pathophysiology of clinical depression explaining their partially overlapping phenomenology. Inflammation may provoke a Janus-faced response with a good, acute side, generating protective inflammation through sickness behavior and a bad, chronic side, for example, clinical depression, a lifelong disorder with positive feedback loops between (neuro)inflammation and (neuro)degenerative processes following less well defined triggers.

Brain-immune interactions and the neural basis of disease-avoidant ingestive behaviour

Neuro-immune interactions are widely manifested in animal physiology. Since immunity competes for energy with other physiological functions, it is subject to a circadian trade-off between other energy-demanding processes, such as neural activity, locomotion and thermoregulation. When immunity is challenged, this trade-off is tilted to an adaptive energy protecting and reallocation strategy that is identified as 'sickness behaviour'. We review diverse disease-avoidant behaviours in the context of ingestion, indicating that several adaptive advantages have been acquired by animals (including humans) during phylogenetic evolution and by ontogenetic experiences: (i) preventing waste of energy by reducing appetite and consequently foraging/hunting (illness anorexia), (ii) avoiding unnecessary danger by promoting safe environments (preventing disease encounter by olfactory cues and illness potentiation neophobia), (iii) help fighting against pathogenic threats (hyperthermia/somnolence), and (iv) by associative learning evading specific foods or environments signalling danger (conditioned taste avoidance/aversion) and/or at the same time preparing the body to counteract by anticipatory immune responses (conditioning immunomodulation). The neurobiology behind disease-avoidant ingestive behaviours is reviewed with special emphasis on the body energy balance (intake versus expenditure) and an evolutionary psychology perspective.

Effects of acute and repeated administration of Staphylococcal enterotoxin A on Morris water maze learning, corticosterone and hippocampal IL-1β and TNFα

Staphylococcal enterotoxin A (SEA) is a bacterial superantigen that induces pronounced T cell expansion and cytokine production. In addition, SEA activates the HPA axis and forebrain regions relevant to cognitive functions. Since learning-related cognitive changes have not been assessed in response to SEA, spatial learning in the Morris water maze (MWM) was determined in male C57BL/6J mice subjected to acute or repeated injections of 5μg SEA or Saline. Injections were given 2h prior to 4-5days of hidden platform sessions. Animals were then rested for 1month and given retraining without further injections. In addition, splenic IL-1β, IL-2 and TNFα, plasma corticosterone, and hippocampal IL-1β and TNFα were measured after the regimen of treatment used in the behavioral experiments. The results showed no learning impairment following acute or repeated SEA challenge. Moreover, when retested 1month later, and without further injections, the SEA group showed more rapid relearning of the MWM. This suggested that coincidental superantigenic T cell activation and training served to promote long-term improvement in recovery of learning. Furthermore, repeated SEA challenge continued to drive increases in plasma corticosterone, but with a compensatory reduction in hippocampal IL-1β. However, while hippocampal TNFα was reduced after acute and repeated SEA treatment, this was not statistically significant. In view of the importance of modest glucocorticoid elevations and hippocampal IL-1β in promoting contextual learning, the data point to the hypothesis that SEA promotes long-term plasticity by restraining disruptive increases in hippocampal IL-1β, and possibly TNFα, during learning.

CpG-C immunotherapeutic efficacy is jeopardized by ongoing exposure to stress: potential implications for clinical use

Bi-directional influences between stress hormones and immune responses have been repeatedly documented, however, in the clinical setting they are rarely considered when immunotherapeutic approaches are used or studied in patients. As some immunotherapeutic treatments have shown great potential in animal models but have had limited success in patients, we hypothesize that ongoing psychological and physiological stress responses in patients, which do not characterize the setting of animal studies, contribute to this discrepancy. In the current study we examined the interaction between ongoing water stress and CpG-C immunotherapy to determine whether stress that precedes immunotherapy can modulate the efficacy of CpG-C immunostimulation. C57BL/6 mice were exposed to water stress or served as controls. Two hours following the commencement of the stress protocol animals were injected with CpG-C, non-CpG, or PBS, and sacrificed 1, 4 or 12h thereafter. We found that in CpG-C-treated animals stress eliminated the elevation of plasma IL-12, and synergistically elevated corticosterone levels. Furthermore, stress markedly reduced the total number of myeloid (33D1(+)), plasmacytoid (mPDCA-1(+)) and plasmacytoid-derived (33D1(+)mPDCA-1(+)) dendritic cells in CpG-C-treated animals, as well as the numbers of these cell sub-types expressing CD11b, CD80 and CD69. These changes were more dramatic in the blood than in the spleen. Overall, these findings indicate that under no-stress conditions CpG-C induces a robust immune response, which is significantly diminished when immunostimulation is attempted during ongoing stress. If these findings hold in humans, potential prophylactic treatments should be found to limit the deleterious effects of ongoing stress on the efficacy of immunotherapy.

Differential sensitivity to endotoxin exposure in young and middle-age mice

Aging can have a profound effect on the neurobehavioral response to immune activation; aged subjects are predisposed to greater deficits in performance and cognitive function in conjunction with an exaggerated neuroinflammatory response. While increased reactivity to an immune insult has been well characterized in aged subjects, the alterations that may exist by middle-age have not been thoroughly investigated. The present study compared the reactions of young (4-month) and middle-age (12-month) male BALB/c mice to an acute or repeated lipopolysaccharide (LPS) challenge(s). The data suggest that in some respects middle-aged mice are more sensitive to endotoxin exposure, as they show enhanced weight loss, splenic cytokine levels, and c-fos expression in the brain following acute LPS administration compared to younger mice. However, acute LPS exposure led to comparable decreases in locomotor activity in young and middle-aged mice. Following repeated LPS administration both age groups showed diminished behavioral and neural reactions to the final LPS challenge, indicating tolerance development. However, the immune system of the middle-aged mice was still mildly responsive to the final LPS exposure, as splenic levels of IL-1beta were significantly elevated. Collectively, the data suggest that middle-age subjects are more sensitive to an immune insult.

Role of opioid receptor like-1 receptor in modulation of endocrine, immunological, and behavioral responses to the T-cell superantigen staphylococcal enterotoxin A

Opioid receptor like-1 receptor (ORL(1)) is selective for orphaninFQ/nociceptin (OFQ/N), a peptide linked to stress. Since immunologic stimuli exert stressor-like effects, the neuroendocrine and behavioral effects of the T-cell superantigen staphylococcal enterotoxin A (SEA) were tested in ORL(1)(-/-) and ORL(1)(+/+) wildtype 129S6 mice. Within 2h of SEA challenge both genotypes showed elevated corticosterone, but only wildtypes were elevated after 4h, and had altered hypothalamic CRH mRNA. Although amygdaloid CRH and TNFalpha mRNA was increased by SEA, this did not vary with genotype. Interestingly, gustatory neophobia due to SEA challenge was augmented in ORL(1)(-/-) mice, although object neophobia tested 4days later was abrogated. These results suggest differential requirements for ORL(1) in the mediation of neuroimmune effects exerted at different times after an immune challenge.

How T-cell-dependent and -independent challenges access the brain: vascular and neural responses to bacterial lipopolysaccharide and staphylococcal enterotoxin B

Bacterial lipopolysaccharide (LPS) is widely used to study immune influences on the CNS, and cerebrovascular prostaglandin (PG) synthesis is implicated in mediating LPS influences on some acute phase responses. Other bacterial products, such as staphylococcal enterotoxin B (SEB), impact target tissues differently in that their effects are T-lymphocyte-dependent, yet both LPS and SEB recruit a partially overlapping set of subcortical central autonomic cell groups. We sought to compare neurovascular responses to the two pathogens, and the mechanisms by which they may access the brain. Rats received iv injections of LPS (2 microg/kg), SEB (1mg/kg) or vehicle and were sacrificed 0.5-3h later. Both challenges engaged vascular cells as early 0.5h, as evidenced by induced expression of the vascular early response gene (Verge), and the immediate-early gene, NGFI-B. Cyclooxygenase-2 (COX-2) expression was detected in both endothelial and perivascular cells (PVCs) in response to LPS, but only in PVCs of SEB-challenged animals. The non-selective COX inhibitor, indomethacin (1mg/kg, iv), blocked LPS-induced activation in a subset of central autonomic structures, but failed to alter SEB-driven responses. Liposome mediated ablation of PVCs modulated the CNS response to LPS, did not affect the SEB-induced activational profile. By contrast, disruptions of interoceptive signaling by area postrema lesions or vagotomy (complete or hepatic) markedly attenuated SEB-, but not LPS-, stimulated central activational responses. Despite partial overlap in their neuronal and vascular response profiles, LPS and SEB appear to use distinct mechanisms to access the brain.

Influence of age on behavioral, immune and endocrine responses to the T-cell superantigen staphylococcal enterotoxin A

Aged subjects are more vulnerable to administration of the endotoxin lipopolysaccharide, but research on age-associated sensitivity to other immune stimulants has been limited. The current study examined the effects of administering the superantigen, staphylococcal enterotoxin A (SEA), to young (4-month-old) and aged (20-month-old) male C57BL/6J mice on consumption of a novel liquid, cytokine production, corticosterone levels, and expression of central mRNA levels of cytokines and corticotropin-releasing hormone. SEA produced exaggerated hypophagia in aged mice, as they showed decreased consumption that persisted for 24 h. SEA increased hypothalamic mRNA levels of interleukin-1beta in the aged, but not the young, mice 2 h after administration. No differences in cytokine expression were observed 24 h after SEA. Both age groups showed increased plasma corticosterone levels 2 h after SEA administration. However, 24 h after SEA exposure the aged, but not the young, mice showed an augmented corticosterone response to the consumption test. Collectively, these data show that aging may exacerbate the behavioral and neuroinflammatory response to superantigen exposure. Further, the present study suggests that immune activation may result in delayed alterations in stress-induced corticosterone production in aged subjects.

A critical review of human endotoxin administration as an experimental paradigm of depression

The syndrome called depression may represent the common final pathway at which different aetiopathogenic processes converge. One such aetiopathogenic process is innate immune system activation. Some depressed patients have increased levels of inflammatory cytokines and other immunologic abnormalities. It is not known whether immune system activation contributes to the pathogenesis of depressive symptoms. Supporting this possibility is the observation that in both rodents and humans, exogenous immune stimuli such as endotoxin can produce symptoms that resemble depression. A new approach to depression research would be to use immune stimuli to elicit depressive symptoms in humans. Here we review each of the symptoms elicited in humans by endotoxin administration, and compare this model to two other immune depression paradigms: interferon-alpha treatment and typhoid vaccine administration, to assess to what degree endotoxin administration represents a valid model of immune depression. We also review corresponding behavioral changes in rodents and the potential molecular pathways through which immune system activation produces each symptom.

Allergic rhinitis induces anxiety-like behavior and altered social interaction in rodents

Epidemiological and clinical studies report higher incidences of anxiety and increased emotional reactivity in individuals suffering from respiratory allergies. To evaluate if respiratory allergies are capable of promoting anxiety-like behavior in rodents, we used models of allergic rhinitis and behavioral evaluations followed by assessment of mRNA for cytokines in relevant brain regions. Mice and rats were sensitized to ovoalbumin or pollen, respectively, following standard sensitization and challenge protocols. After challenge, the animals were evaluated in the open field, elevated plus-maze and resident-intruder tests. Cytokines and corticotropin-releasing factor expression were assessed in several brain regions by real-time RT-PCR and plasma corticosterone concentrations by radioimmunoassay. Mice and rats sensitized and exposed to allergen showed increased anxiety-like behavior and reduced social interaction without any overt behavioral signs of sickness. T-helper type 2 (T(H)2) cytokines were induced in both rats and mice in the olfactory bulbs and prefrontal cortex and remained unchanged in the temporal cortex and hypothalamus. The same results were found for CRF mRNA expression. No differences were observed in corticosterone concentrations 1h after the last behavioral test. These results show that sensitization and challenge with allergens induce anxiety across rodent species and that these effects were paralleled by an increased expression of T(H)2 cytokines and CRF in the prefrontal cortex. These studies provide experimental evidence that sensitized rodents experience neuroimmune-mediated anxiety and reduced social interaction associated with allergic rhinitis.

Influenza virus- and cytokine-immunoreactive cells in the murine olfactory and central autonomic nervous systems before and after illness onset

Influenza virus invades the olfactory bulb (OB) and enhances cytokine mRNAs therein at the time of illness onset. Here we show that viral antigen immunoreactivity co-localized with glial markers in the OB but could not be detected in other brain areas. Interleukin 1beta- and tumor necrosis factor alpha-immunoreactivity co-localized with neuronal markers in olfactory and central autonomic systems, and the number of cytokine-immunoreactive neurons increased at the time of illness onset [15 h post-inoculation (PI)] but not before (10 h PI). These results suggest that the OB virus influences the brain cytokines and therefore the onset of illness.

Relationship of varying patterns of cytokine production to the anorexic and neuroendocrine effects of repeated Staphylococcal enterotoxin A exposure

Staphylococcal enterotoxin A (SEA) is a superantigen that stimulates T cells and induces the production of multiple cytokines. Previous studies have shown that SEA augments gustatory neophobia and activates the hypothalamic-pituitary-adrenal (HPA) axis. This study aimed to determine if the cytokine response, behavioral effects, and HPA axis activation persisted after repeated SEA treatment. Male C57BL/6J mice were given 1-4 intraperitoneal injections of 5 microg SEA, after which food intake, corticosterone, or peripheral cytokines were measured. In a series of experiments, it was found that secondary exposure to SEA two or three days after priming increased corticosterone, but attenuated splenic TNFalpha, while augmenting IL-1beta, IL-2, and IFNgamma. The anorexic response was intact after secondary exposure, but absent after a third injection, which was still able to elevate corticosterone. It is unlikely that IL-1 mediated the persistent effects on corticosterone, since this was increased in groups lacking corticosterone elevations. Similarly, TNFalpha was only modestly elevated under repeated SEA conditions that elevated plasma corticosterone. This attenuation appeared to be inversely related to the levels of IL-10, the production of which incrementally rose with each successive injection. In conclusion, repeated exposure to SEA activates the HPA axis and alters behavior. However, there may be dissociation between the behavioral and endocrine effects of SEA with increased SEA exposure. Furthermore, it is possible that while TNFalpha was previously shown to be important in response to acute SEA-induced HPA axis activation, further exposure to SEA elicits other cytokines that may exert neuromodulatory effects through sensitization and/or synergistic mechanisms.

Campylobacter jejuni infection increases anxiety-like behavior in the holeboard: possible anatomical substrates for viscerosensory modulation of exploratory behavior

The presence of certain bacteria in the gastrointestinal tract influences behavior and brain function. For example, challenge with live Campylobacter jejuni (C. jejuni), a common food-born pathogen, reduces exploration of open arms of the plus maze, consistent with anxiety-like behavior, and activates brain regions associated with autonomic function, likely via a vagal pathway. As yet, however, little is known regarding the interface of immune sensory signals with brain substrates that mediate changes in behavioral states. To address this issue, we challenged mice with either C. jejuni or saline, and 7-8h later assessed anxiety-like behavior using the open holeboard, and used immunohistochemical detection of the protein c-Fos as an activation marker in the brain. C. jejuni treatment was associated with increased avoidance of the center regions of the holeboard, compared to saline-treated controls. Exposure to the holeboard induced activation in multiple brain regions previously implicated in anxiety-like behavior, including the lateral septum (LS), paraventricular (PVN) and dorsomedial hypothalamic nuclei (DMH), basolateral and central nuclei of the amygdala (BLA, CEA), bed nucleus of the stria terminalis (BST) and periaquiductal grey (PAG), compared to homecage controls. In C. jejuni-treated animals c-Fos induction also occurred in autonomic regions, as previously reported. The PVN, BLA, parts of the BST, medial prefrontal (mPFC) and anterior cingulate responded to both C. jejuni treatment and the holeboard, suggesting a role for these regions in the enhanced anxiety-like behavior observed. In saline-treated animals, anxiety-like behavior was predicted by activation in the CEA and BLA, whereas in C. jejuni-treated animals, c-Fos expression in the BST predicted the degree of anxiety-like behavior. These findings implicate the PVN, amygdala and BST as interfaces between gastrointestinal pathogenic challenge and brain regions that mediate behavioral responses to stress, and reinforce these nuclei as anatomical substrates by which viscerosensory stimuli can influence behavior.

Inhibitory effects of lipopolysaccharide on hypothalamic nuclei implicated in the control of food intake

The arcuate nucleus (Arc) and the lateral hypothalamic area (LHA), two key hypothalamic nuclei regulating feeding behavior, express c-Fos, a marker of neuronal activation in fasted animals. This is reversed by refeeding. In the present study we tested whether an anorectic dose of lipopolysaccharide (LPS), the cell wall component of Gram-negative bacteria, also inhibits fasting-induced c-Fos expression in these hypothalamic nuclei. This would suggest that they are involved in anorexia during bacterial infections as well. We also studied whether LPS modulates the activity of orexin-A positive (OX+) LHA neurons. Food deprived BALB/c mice were injected with LPS or saline and were sacrificed 4 or 6h later. Four hours after injection, LPS reduced the number of c-Fos positive cells in the Arc and in the LHA, but had no effect on c-Fos in OX+ neurons. Six hours after injection, LPS reduced c-Fos expression in the LHA, both in the OX- and OX+ neurons, but not in the Arc. These results show that LPS modulates neuronal activity in the Arc and LHA similar to feeding-related stimuli, suggesting that the observed effects might contribute to the anorectic effect of LPS. Thus, physiological satiety signals released during refeeding and anorexia during bacterial infection seem to engage similar neuronal substrates.

Methylmercuric chloride induces activation of neuronal stress circuitry and alters exploratory behavior in the mouse

Methylmercury (MeHg) is a well-known neurotoxicant, responsible for neurological and cognitive alterations. However, there is very little information available on the effects of MeHg administration on activation of murine neuronal pathways involved in the stress response, and whether this is altered as a function of repeated exposure to MeHg. Moreover, interactions between MeHg and other psychogenic and inflammatory stressors have yet to be fully determined. Acute i.p. exposure of male C57BL/6J mice to MeHg (2-8 mg/kg) dose-dependently attenuated exploratory behavior in the open field in the presence and absence of a novel object. In addition, increased numbers of c-Fos immunoreactive cells appeared in response to acute i.p. and i.c.v. MeHg within thalamic (anterior paraventricular nucleus of the thalamus (PVA)/posterior paraventricular nucleus of the thalamus (PV)), hypothalamic (paraventricular nucleus of the hypothalamus (PVN)), central amygdaloid nucleus (CeC), septal and hippocampal (dentate gyrus) nuclei, medial bed nucleus (BSTm) and the locus coeruleus (Lc). The increase in c-Fos positive cells in response to acute i.p. and i.c.v. MeHg did not appear to be influenced further by open field exposure. Repeated administration of MeHg led to an attenuation of most parameters of open field behavior altered by acute MeHg. However, increased c-Fos was significant in the CeC, Dg, supracapsular bed nucleus (BSTs), and Lc. Moreover, open field exposure after repeated treatments resulted in significant c-Fos responses in similar areas. Interestingly, 3 days after the final repeated MeHg dose (2 or 4 mg/kg) c-Fos increases to an immunogenic stressor (LPS) were not affected by MeHg pretreatment. These results demonstrate that systemic exposure to acute and repeated MeHg serves to activate the brain's stress circuitry, and furthermore appears to engage normal neuronal habituation processes.

Infection-induced viscerosensory signals from the gut enhance anxiety: implications for psychoneuroimmunology

Infection and inflammation lead to changes in mood and cognition. Although the "classic" sickness behavior syndrome, involving fatigue, social withdrawal, and loss of appetites are most familiar, other emotional responses accompany immune activation, including anxiety. Recent studies have shown that gastrointestinal bacterial infections lead to enhanced anxiety-like behavior in mice. The bacteria-induced signal is most likely carried by vagal sensory neurons, and occurs early on (within 6h) during the infection. These signals induce evidence of activation in brain regions that integrate viscerosensory information with mood, and potentiate activation in brain regions established as key players in fear and anxiety. The findings underline the importance of viscerosensory signals arising from the gastrointestinal tract in modulation of behaviors appropriate for coping with threats, and suggest that these signals may contribute to affective symptoms associated with gastrointestinal disorders.

Orphanin FQ/nociceptin interactions with the immune system in vivo: gene expression changes in lymphoid organs and regulation of the cytokine response to staphylococcal enterotoxin A

Orphanin FQ/nociceptin (OFQ/N) is a neuropeptide implicated in immunomodulation. Here, we show that endogenous and exogenous OFQ/N modulated the cytokine response to the bacterial superantigen staphylococcal enterotoxin A (SEA). Specifically, OFQ/N enhanced TNFalpha and IFNgamma transcripts in the spleen when injected prior to SEA challenge. Moreover, mice lacking the OFQ/N precursor gene showed diminished TNFalpha and IFNgamma mRNA induction in the spleen following SEA challenge. In addition, mRNA levels of ppOFQ/N and the OFQ/N receptor, NOP, were altered in thymus and spleen after SEA challenge. Overall, this suggests that the OFQ/N system can influence immune function and is itself influenced by immune stimuli.

CNS activational responses to staphylococcal enterotoxin B: T-lymphocyte-dependent immune challenge effects on stress-related circuitry

Staphylococcal enterotoxin B (SEB) is a bacterial superantigen that engages the immune system in a T-lymphocyte-dependent manner and induces a cytokine profile distinct from that elicited by the better-studied bacterial pathogen analog, lipopolysaccharide (LPS). Because of reports of SEB recruiting central nervous system (CNS) host defense mechanisms via pathways in common with LPS, we sought to further characterize central systems impacted by this agent. Rats were treated with SEB at doses of 50-5,000 mug/kg, and killed 0.5-6 hours thereafter. SEB injection produced a discrete pattern of Fos induction in brain that peaked at 2-3 hours postinjection and whose strength was dose-related. Induced Fos expression was predominantly subcortical and focused in a set of interconnected central autonomic structures, including aspects of the bed n. of the stria terminalis, central amygdala and lateral parabrachial nuclei; functionally related (and LPS-responsive) cell groups in the n. solitary tract, ventrolateral medulla, and paraventricular hypothalamic n. (PVH) were, by contrast, weakly responsive. SEB also activated cell groups in the limbic forebrain (lateral septal n, medial prefrontal cortex) and hypothalamic GABAergic neurons, which could account for its failure to elicit reliable increases in Fos-ir or corticotropin-releasing factor (CRF) mRNA in the PVH. SEB nevertheless did provoke reliable pituitary-adrenal secretory responses. The identification of subsets of central autonomic and limbic forebrain structures that are sensitive to SEB provides a basis for a systems-level understanding of the physiological and behavioral effects attributed to the superantigen. Core SEB-responsive cell groups exclude a medullary-PVH circuit implicated in pituitary-adrenal responses to LPS.

Hypothalamic integration of immune function and metabolism

The immune and neuroendocrine systems are closely involved in the regulation of metabolism at peripheral and central hypothalamic levels. In both physiological (meals) and pathological (infections, traumas and tumors) conditions immune cells are activated responding with the release of cytokines and other immune mediators (afferent signals). In the hypothalamus (central integration), cytokines influence metabolism by acting on nucleus involved in feeding and homeostasis regulation leading to the acute phase response (efferent signals) aimed to maintain the body integrity. Peripheral administration of cytokines, inoculation of tumor and induction of infection alter, by means of cytokine action, the normal pattern of food intake affecting meal size and meal number suggesting that cytokines acted differentially on specific hypothalamic neurons. The effect of cytokines-related cancer anorexia is also exerted peripherally. Increase plasma concentrations of insulin and free tryptophan and decrease gastric emptying and d-xylose absorption. In addition, in obesity an increase in interleukin (IL)-1 and IL-6 occurs in mesenteric fat tissue, which together with an increase in corticosterone, is associated with hyperglycemia, dyslipidemias and insulin resistance of obesity-related metabolic syndrome. These changes in circulating nutrients and hormones are sensed by hypothalamic neurons that influence food intake and metabolism. In anorectic tumor-bearing rats, we detected upregulation of IL-1beta and IL-1 receptor mRNA levels in the hypothalamus, a negative correlation between IL-1 concentration in cerebro-spinal fluid and food intake and high levels of hypothalamic serotonin, and these differences disappeared after tumor removal. Moreover, there is an interaction between serotonin and IL-1 in the development of cancer anorexia as well as an increase in hypothalamic dopamine and serotonin production. Immunohistochemical studies have shown a decrease in neuropeptide Y (NPY) and dopamine (DA) and an increase in serotonin concentration in tumor-bearing rats, in first- and second-order hypothalamic nuclei, while tumor resection reverted these changes and normalized food intake, suggesting negative regulation of NPY and DA systems by cytokines during anorexia, probably mediated by serotonin that appears to play a pivotal role in the regulation of food intake in cancer. Among the different forms of therapy, nutritional manipulation of diet in tumor-bearing state has been investigated. Supplementation of tumor bearing rats with omega-3 fatty acid vs. control diet delayed the appearance of tumor, reduced tumor-growth rate and volume, negated onset of anorexia, increased body weight, decreased cytokines production and increased expression of NPY and decreased alpha-melanocyte-stimulating hormone (alpha-MSH) in hypothalamic nuclei. These data suggest that omega-3 fatty acid suppressed pro-inflammatory cytokines production and improved food intake by normalizing hypothalamic food intake-related peptides and point to the possibility of a therapeutic use of these fatty acids. The sum of these data support the concept that immune cell-derived cytokines are closely related with the regulation of metabolism and have both central and peripheral actions, inducing anorexia via hypothalamic anorectic factors, including serotonin and dopamine, and inhibiting NPY leading to a reduction in food intake and body weight, emphasizing the interconnection of the immune and neuroendocrine systems in regulating metabolism during infectious process, cachexia and obesity.