Effects of peripheral cytokine injections on multiple unit activity in the anterior hypothalamic area of the mouse

Circulating brain-reactive autoantibodies and behavioral deficits in the MRL model of CNS lupus

Brain-reactive autoantibodies (BRAA) are hypothesized to play a role in the neuropsychiatric manifestations that accompany systemic lupus erythematosus (SLE). The present study tests the proposed relation between circulating BRAA and behavioral deficits in lupus-prone MRL/lpr mice. Two age-matched cohorts born at different times were used to test the relationship in the context of altered disease severity. Significant correlations between autoimmunity and behavior were detected in both cohorts. These results are the first to report correlations between behavior and autoantibodies to integral membrane proteins of brain, supporting the hypothesis that BRAA contribute to the behavioral dysfunction seen in lupus.

Role of IL-1 beta and 5-HT2 receptors in midbrain periaqueductal gray (PAG) in potentiating defensive rage behavior in cat

Feline defensive rage, a form of aggressive behavior that occurs in response to a threat can be elicited by electrical stimulation of the medial hypothalamus or midbrain periaqueductal gray (PAG). Our laboratory has recently begun a systematic examination of the role of cytokines in the regulation of rage and aggressive behavior. It was shown that the cytokine, interleukin-2 (IL-2), differentially modulates defensive rage when microinjected into the medial hypothalamus and PAG by acting through separate neurotransmitter systems. The present study sought to determine whether a similar relationship exists with respect to interleukin 1-beta (IL-1 beta), whose receptor activation in the medial hypothalamus potentiates defensive rage. Thus, the present study identified the effects of administration of IL-1 beta into the PAG upon defensive rage elicited from the medial hypothalamus. Microinjections of IL-1 beta into the dorsal PAG significantly facilitated defensive rage behavior elicited from the medial hypothalamus in a dose and time dependent manner. In addition, the facilitative effects of IL-1 beta were blocked by pre-treatment with anti-IL-1 beta receptor antibody, while IL-1 beta administration into the PAG had no effect upon predatory attack elicited from the lateral hypothalamus. The findings further demonstrated that IL-1 beta's effects were mediated through 5-HT(2) receptors since pretreatment with a 5-HT(2C) receptors antagonist blocked the facilitating effects of IL-1 beta. An extensive pattern of labeling of IL-1 beta and 5-HT(2C) receptors in the dorsal PAG supported these findings. The present study demonstrates that IL-beta in the dorsal PAG, similar to the medial hypothalamus, potentiates defensive rage behavior and is mediated through a 5-HT(2C) receptor mechanism.

Brain behavior and immunity: twenty years of T cells

During the period from 1987 to 2007, our understanding of nervous system-T lymphocyte bi-directional communication advanced exponentially. Progress in exploring these relationships was aided by the constant development of new, cutting-edge technologies, and by a steady growth in interest, and number, of scientists who recognized the need to conduct cross-disciplinary research. In this brief review of 20 years of Brain, Behavior, and Immunity (BBI), we highlight just a small number of the important studies published in the journal that collectively have provided the foundation for our current understanding of brain, behavior, and, specifically, T cells.

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.

Immune modulation of the hypothalamic-pituitary-adrenal (HPA) axis during viral infection

Compelling data has been amassed indicating that soluble factors, or cytokines, emanating from the immune system can have profound effects on the neuroendocrine system, in particular the hypothalamic- pituitary-adrenal (HPA) axis. HPA activation by cytokines (via the release of glucocorticoids), in turn, has been found to play a critical role in restraining and shaping immune responses. Thus, cytokine-HPA interactions represent a fundamental consideration regarding the maintenance of homeostasis and the development of disease during viral infection. Although reviews exist that focus on the bi-directional communication between the immune system and the HPA axis during viral infection (188,235), others have focused on the immunomodulatory effects of glucocorticoids during viral infection (14,225). This review, however, concentrates on the other side of the bi-directional loop of neuroendocrine-immune interactions, namely, the characterization of HPA axis activity during viral infection and the mechanisms employed by cytokines to stimulate glucocorticoid release.

Cytokine modulation of defensive rage behavior in the cat: role of GABAA and interleukin-2 receptors in the medial hypothalamus

Defensive rage behavior is a form of aggressive behavior occurring in nature in response to a threatening stimulus. It is also elicited by stimulation of the medial hypothalamus and midbrain periaqueductal gray (PAG) and mediated through specific neurotransmitter-receptor mechanisms within these regions. Since interleukin (IL)-2 modulates the release of neurotransmitters linked to aggression and rage, we sought to determine whether IL-2 microinjected into the medial hypothalamus would modulate defensive rage. Microinjections of relatively low doses of IL-2 into the medial hypothalamus significantly suppressed defensive rage elicited from the PAG in a dose-dependent manner and in the absence of signs of sickness behavior. Pre-treatment with an antibody directed against IL-2Ralpha or a GABA(A) receptor antagonist blocked IL-2's suppressive effects upon defensive rage. Since the suppression of defensive rage is also mediated by 5-HT(1) receptors in the medial hypothalamus, a 5-HT(1) antagonist was microinjected into this region as a pretreatment for IL-2; however, it did not block IL-2's suppressive effects. Immunocytochemical data provided anatomical support for these findings by revealing extensive labeling of IL-2Ralpha on neurons in the medial hypothalamus. IL-2 microinjected into the medial hypothalamus did not modulate predatory attack elicited from the lateral hypothalamus. In summary, we provide evidence for a novel role for IL-2 in the medial hypothalamus as a potent suppressor of defensive rage behavior. These effects are mediated through an IL-2-GABA(A) receptor mechanism.

Permeability of the mouse blood-brain barrier to murine interleukin-2: predominance of a saturable efflux system

Interleukin (IL)-2, a T helper (TH)1 cell-derived glycoprotein with potent neuromodulatory effects, is implicated in the etiology and pathogenesis of various psychiatric and neurological disorders. Paralleling these findings, chronic IL-2 intravenous immunotherapy may induce similar psychopathological outcomes. The findings that acute or repeated injections of IL-2 induce motor and cognitive abnormalities in rodents are consistent with these clinical findings, and raise the possibility that IL-2 crosses the blood-brain barrier (BBB) to alter brain function. However, little is known about the ability of IL-2 to enter the brain or whether its effects vary with the chronicity of IL-2 treatment. Here, we found that radioactively labeled mouse IL-2 (I-IL-2) given intravenously entered the brain at a low rate (Ki=0.142+/-0.044microl/g-min) by a non-saturable process. Repeated injections of either IL-2 or vehicle altered the kinetics of entry without producing a net effect on IL-2 entry. When I-IL-2 was given by brain perfusion, the entry rate greatly increased over 10-fold to 2.2+/-0.805microl/g-min. This suggests a circulating factor is retarding the entry of IL-2 into the brain. A paradoxic increase in the rate of I-IL-2 entry into brain occurred when an excess of unlabeled IL-2 was included in the brain perfusate, suggesting a saturable CNS-to-blood efflux system. Intracerebroventricular injection of I-IL-2 with and without unlabeled IL-2 confirmed the presence of a saturable efflux system. We conclude that IL-2 entry into the brain is low because of the absence of a blood-to-brain transporter and further retarded by circulating factors and a CNS-to-blood efflux system. This is the first description of a saturable CNS-to-blood efflux system for a cytokine. We postulate that this efflux system may protect the brain from circulating IL-2.

Cancer anorexia: a model for the understanding and treatment of secondary anorexia

Under normal conditions, food intake is controlled in the hypothalamus by: (i) transducing metabolic/sensorial inputs arising from the periphery into neuronal response; (ii) integrating the information originating from different tissues; and (iii) triggering the appropriate feeding responses. Thus, the anorexia associated with a number of chronic diseases, including cancer, may result from an abnormal input of information to the hypothalamus, or in its defective transduction and integration, or in the induction of exaggerated and inappropriate feeding responses. Currently available data suggest that the pathogenesis of secondary anorexia is multifactorial, and involves most of the neuronal signalling pathways modulating energy intake, including hormones (e.g. leptin), neuropeptides (e.g. NPY), cytokines (e.g. IL-1, IL-6, TNF) and neurotransmitters (e.g. serotonin and dopamine). However, it is unlikely that they represent separate and distinct pathogenic mechanisms, rather it appears that close interrelationships may exist among them. In line with this reasoning, consistent experimental and human data suggest that the hypothalamic serotonergic neurotransmission may represent a major target on which different anorexia-related factors converge. Thus, interfering pharmacologically with hypothalamic serotonin synthesis and activity may represent an effective therapeutic strategy in anorectic patients, as suggested by recent preliminary clinical data.

Interleukin-2-induced increases in climbing behavior: inhibition by dopamine D-1 and D-2 receptor antagonists

Interleukin (IL)-2 is a potent modulator of dopamine activity in the mesocorticolimbic and mesostriatal systems. It is also associated with behavioral changes (increased motor activity) and psychopathological outcomes (schizophrenia, Parkinson's Disease, cognitive deficits) that at least partly reflect aberrations in central dopaminergic transmission. Nonetheless, there is no evidence that a functional link exists between IL-2, dopaminergic processes, and related behavioral changes. We thus determined if IL-2 treatment increases the expression of climbing behavior, a behavior that is linked with dopamine D-1 and/or D-2 receptors and one used to test the efficacy of neuroleptics. IL-2 treatment (5-daily i.p. injections; 0.4 microg/BALB/c mouse) induced a marked 2-fold increase in climbing scores; a single injection had no effect. IL-2-induced increases in climbing behavior were completely blocked by a selective dopamine D-1 receptor antagonist (SCH 23390; 0.05 or 0.2 mg/kg; i.p.), or by a relatively high dose of a D-2 antagonist (sulpiride; 80 mg/kg; i.p.). In contrast, MK-801, a noncompetitive NMDA receptor antagonist, had no effect. This is the first demonstration of a functional link between IL-2, dopaminergic receptors, and behavior. These findings could shed light on the mechanisms by which IL-2 increases vulnerability to psychiatric abnormalities associated with aberrations in central dopaminergic processes.

Neurochemical mechanisms for cancer anorexia

Under normal conditions, the homeostasis of energy intake is maintained in the hypothalamus by 1) transducing metabolic and sensorial inputs arising from the periphery into neuronal response, 2) integrating the information originating from different tissues, and 3) triggering the appropriate feeding responses. If cancer anorexia is considered a disruption of the physiologic mechanisms controlling energy intake, it is conceivable that its pathogenesis may lie in an abnormal input of information to the hypothalamus, its defective transduction and integration, or the induction of exaggerated and inappropriate feeding responses. Currently available data suggest that the pathogenesis of cancer anorexia is multifactorial and involves most of the neuronal signaling pathways modulating energy intake. Thus, a number of factors has been proposed as putative mediators of cancer anorexia, including hormones (e.g., leptin), neuropeptides (e.g., neuropeptide Y), cytokines (e.g., interleukin-1, interleukin-6, tumor necrosis factor), and neurotransmitters (e.g., serotonin and dopamine). However, it is unlikely that they represent separate and distinct pathogenic mechanisms; rather, it appears that close interrelationships may exist among them. In line with this reasoning, consistent experimental and human data suggest that hypothalamic monoaminergic neurotransmission and serotonergic activity in particular may represent a major target on which different anorexia-related factors converge. Thus, interfering pharmacologically with hypothalamic serotonin synthesis and activity has been tested as a therapeutic strategy in anorectic cancer patients with encouraging results. However, more clinical options will be available by revealing the complex interactions between the many factors participating in controlling energy intake under normal and pathologic conditions. Further, modulation of hypothalamic activity also might result in reduced catabolic signals to skeletal muscles, thus improving the cachexia associated with cancer.

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.

Anhedonic and anxiogenic effects of cytokine exposure

Systemic interleukin IL-1 beta, TNF alpha, and IL-2 profoundly influenced central monoamine activity, as well as behavioral outputs. The effects of the various cytokines were clearly distinguishable from one another, although synergistic effects were detected between several of these cytokines and between the actions of cytokines and stressors. Acutely applied IL-2 appeared to affect reward processes, but did not affect anxiety. When chronically administered, this cytokine markedly influenced working memory in a spatial learning test. In contrast to IL-2, both IL-1 beta and TNF alpha appeared to provoke an anxiogenic action, and provoked clear signs of illness. While these cytokines induced anorexia, they did not appear to affect reward processes. IL-1 beta and TNF alpha were found to act synergistically, and the TNF alpha provoked a sensitization with respect to the action of subsequent TNF alpha treatment. The findings indicated that cytokine treatments profoundly influence extrahypothalamic neurochemical functioning and may thus impact on behavioral outputs. Analyses of the behavioral and neurochemical changes elicited by cytokines, and particularly TNF alpha, need to consider not only the immediate impact of such treatments, but also the proactive effects that may be engendered.

Variations of nucleus accumbens dopamine and serotonin following systemic interleukin-1, interleukin-2 or interleukin-6 treatment

The effects of systemically administered interleukin-1beta (1.0 microg), interleukin-6 (1.0 microg) and interleukin-2 (1.0 microg) on in vivo variations of monoamines were assessed in the nucleus accumbens. Administration of interleukin-1beta did not affect extracellular accumbal dopamine, provoked a modest rise of homovanillic acid, and prevented the decline of dihydroxyphenylacetic acid ordinarily seen in saline treated rats. Also, interleukin-1 provoked a modest increase of extracellular 5-hydroxyindoleacetic acid from the nucleus accumbens. Following exposure to the stress of a series of air-puffs, a still greater increase of accumbal 5-hydroxyindoleacetic acid was evident. In contrast to interleukin-1, systemic administration of interleukin-6 and interleukin-2 both induced marked reductions of interstitial dopamine levels. The air-puff exposure further enhanced these effects in rats that had received the cytokine treatment. As well, interleukin-6 and interleukin-2 were both found to reduce the homovanillic acid response associated with the stress, and interleukin-2 promoted a decline of homovanillic acid levels. Treatment with interleukin-6, like that of interleukin-1, prevented the decline of dihydroxyphenylacetic acid ordinarily observed over time, while interleukin-2 was without effect in this respect. Finally, interleukin-6 provoked a modest rise of 5-hydroxyindoleacetic acid, which was most apparent following air-puff exposure, while administration of interleukin-2 did not affect accumbal 5-hydroxyindoleacetic acid. It is suggested that the cytokines may influence the release of biogenic amines in the nucleus accumbens, but the profile of changes were cytokine-specific. As well, it appeared that the cytokines, particularly interleukin-1 and interleukin-6, may act synergistically with the stressor in promoting the amine variations. Systemic administration of cytokines clearly influenced monoamine activity at the nucleus accumbens, a region associated with both rewarding and aversive events. Thus, it may be expected that cytokine treatments may affect behavior. Moreover, it seems that the effects of interleukin-1 and interleukin-6 may be influenced by the presence of stressful stimuli. It ought to be underscored that although cytokines share features with the effects of stressors, most notably the variations of hypothalamic-pituitary-adrenal hormones, the pattern of central neurochemical changes elicited by the cytokines could be distinguished from the amine variations ordinarily associated with stressors.

Potential strategies for ameliorating early cancer anorexia

BACKGROUND

Normally the lateral hypothalamic area (LHA) and the ventromedial nucleus (VMN) interact to regulate food intake (FI), the product of meal number (MN) and meal size (MZ), by changes in neurotransmitters, mainly dopamine and serotonin. Change in LHA dopamine influences meal size; while in VMN, decreasing dopamine and increasing serotonin levels influence meal number. Whether this situation exists in early cancer anorexia was tested in a series of studies to examine the role of the hypothalamus in the pathogenesis of early cancer anorexia.

MATERIALS AND METHODS

In experiment 1, male Fischer tumor-bearing (TB) rats and weight-matched controls had FI, MN, and MZ measured continuously via a computerized rat eater meter. At onset of anorexia, feeding patterns were measured. In experiment 2, the VMN was temporarily blocked with 0.32 microgram of colchicine in TB rats, while TB controls had an equal volume of intra-VMN saline, and changes in feeding patterns were measured. In experiment 3, changes in VMN dopamine and serotonin were measured via microdialysis at anorexia and after tumor resection.

RESULTS

In experiment 1, with the onset of anorexia, food intake decreased significantly in TB rats, initially by a decrease in MN and then by a decrease in both MN and MZ. No change occurred in controls, suggesting that VMN versus LHA played a more significant role in mediation of cancer anorexia. In experiment 2, following VMN block, FI increased significantly in anorectic TB rats, achieved by an almost exclusive increase in MN with minimal change in MZ, thus supporting the role of the VMN in anorexia. In experiment 3, at the onset of anorexia, FI decreased significantly in TB rats versus controls. TB rats had a significant increase in VMN serotonin and a significant decrease in VMN dopamine. After tumor resection, food intake improved and high levels of serotonin normalized with no change in dopamine.

CONCLUSION

Serotoninergic and dopaminergic systems are involved in the etiology of cancer anorexia. The changes in food intake are mediated via the VMN by a decrease in meal number.

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.

The immune system can affect learning: chronic immune complex disease in a rat model

Evidence is presented that the immune system can affect central nervous system functioning, leading to changes in learning. Immune complex disease is induced in rats and their behavior tested using a Lashley maze. Significant differences in behavior were found between the animals with high disease activity and those with low disease activity and the non-disease controls. These changes were not due to uremia and are most likely due to the immune response. There is some evidence immune complex deposits in the choroid plexus may play some role, but not the sole or major role in the behavioral changes. This provides a model by which immunologic processes can cause neuropsychiatric manifestations in autoimmune diseases like lupus, as well as showing that immune processes can affect behavioral functioning.

Inhibition of peripheral interleukin-1 beta-induced hyperalgesia by the intracerebroventricular administration of diclofenac and alpha-melanocyte-stimulating hormone

The present study was undertaken to investigate whether or not the endogeneous mechanisms in the brain can modulate the changes in nociception produced by peripherally-administered interleukin-1 beta (IL-1 beta) in rats. We administered diclofenac and alpha-melanocyte-stimulating hormone (alpha-MSH) into the lateral cerebroventricle (LCV) 10 min before the intraperitoneal (i.p.) injection of recombinant human IL-1 beta (rhIL-1 beta, 1 ng/kg-100 ng/kg) and then observed the changes in nociception using a hot-plate test. The i.p. injection of rhIL-1 beta (10 ng/kg and 100 ng/kg) reduced the paw-withdrawal latency without affecting the colonic temperature. The maximal reduction in the paw-withdrawal latency was observed 30 min after the i.p. injection of rhIL-1 beta at 100 ng/kg. The rhIL-1 beta (100 ng/kg)-induced hyperalgesia was inhibited by the LCV injection of both diclofenac (1 ng) and alpha-MSH (100 ng). The LCV injection of either diclofenac (1 ng) or alpha-MSH (100 ng) was found to have no effect on nociception by itself. These findings therefore suggest that the hyperalgesia induced by peripheral IL-1 beta can be modulated by a cyclooxygenase pathway of the arachidonate and alpha-MSH in the brain.

Cracking the riddle of cancer anorexia

During tumor growth, anorexia and reduced food intake are among the major causes leading to malnutrition and eventually cachexia, which negatively affect patients' outcome. Consistent evidence from our laboratories in rats and humans indicates a key role for ventromedial hypothalamic (VMH) serotonergic system in the development of cancer anorexia. Thus, we postulated that during cancer, increased plasma tryptophan levels (the precursor of serotonin) lead to increased cerebrospinal fluid tryptophan concentrations and increased VMH serotonin synthesis, which then mediates the occurrence of anorexia. However, recent data strongly suggest that factors other than tryptophan supplied to the central nervous system might be involved in the pathogenesis of reduced food intake during tumor growth. Particularly, a significant role appears to be played by interleukin-1 (IL-1). We recently showed that IL-1 infusion in normal rats causes changes in food intake and its determinants, meal number and meal size, similar to those characterizing cancer anorexia, thus supporting the involvement of this cytokine in the development of anorexia. Interestingly, IL-1 and the VMH serotonergic system appear to be closely linked: peripherally infused IL-1 increases brain tryptophan and serotonin concentrations, while intracerebrally infused IL-1 increases neuronal firing rate and serotonin release. We therefore hypothesize that during tumor growth, increased production/secretion of IL-1 occurs, which facilitates the tryptophan supply to the brain. IL-1 can then also act on the VHM itself, where IL-1 receptors exist, to increase its neuronal activity and serotonin release. In other words, we believe that centrally acting IL-1 increases hypothalamic neuronal firing rate and serotonin release, while peripherally acting IL-1 is critical in supplying the hypothalamus with the precursor, tryptophan, in order to maintain the high rate of serotonin synthesis. Also, additional factors recently proposed as mediators of anorexia (including neuropeptide Y and nitric oxide) appear to be part of the hypothesized pathogenic mechanism.

Interleukin-2 decreases accumbal dopamine efflux and responding for rewarding lateral hypothalamic stimulation

Systemic administration of interleukin-2 (IL-2) provoked marked alterations of responding for rewarding brain stimulation from the medial forebrain bundle (MFB). In particular, when animals were tested for ICSS immediately following IL-2 treatment only a modest disturbance of responding was evident. However, if animals were subsequently exposed to repeated daily ICSS sessions (24-168 h) in the drug-free state, rightward shifts in the rate intensity functions and significant increases in reward thresholds were apparent. These results were dependent upon the presence of IL-2 during the initial ICSS session. If animals were tested for ICSS 24 h after IL-2 administration, without an intervening test, performance was unaffected. Evaluation of nonreinforced behavior after IL-2 treatment revealed that ICSS remained under stimulus control and the cytokine did not provoke reward-unrelated performance deficits. Dopamine (DA) activity in the nucleus accumbens has been implicated in goal-directed responding to positively reinforcing stimuli and in the present investigation, using in vivo microdialysis, it was observed that IL-2 markedly reduced DA release from this region. It was suggested that the protracted consequences of IL-2 on ICSS likely do not involve motoric, soporific, attentional or cognitive changes, but may be attributable to its specific actions on motivational arousal, possibly engendered by the cytokine-induced diminution of accumbal DA efflux.

Interleukin-2 as a neuroregulatory cytokine

Interleukin-2 (IL-2), the cytokine also known as T-cell growth factor, has multiple immunoregulatory functions and biological properties not only related to T-cells. In the past decade, substantial evidence accumulated to suggest that IL-2 is also a modulator of neural and neuroendocrine functions. First, extremely potent effects of IL-2 on neural cells were discovered, including activities related to cell growth and survival, transmitter and hormone release and the modulation of bioelectric activities. IL-2 may be involved in the regulation of sleep and arousal, memory function, locomotion and the modulation of the neuroendocrine axis. Second, the concept that IL-2 could act as a neuroregulatory cytokine has been supported by reports on the presence in rodent and human brain tissues of IL-2-like bioactivity, IL-2-like immunoreactivity, IL-2-like mRNA, IL-2 binding sites, IL-2 receptor (IL-2R alpha) and beta chain mRNA and IL-2R immunoreactivity. IL-2 and/or IL-2R molecules mainly localize to the frontal cortex, septum, striatum, hippocampal formation, hypothalamus, locus coeruleus, cerebellum, the pituitary and fiber tracts, such as the corpus callosum, where they are likely expressed by both neuronal and glial cells. Although the molecular biology of the brain IL-2/IL-2R system (including its relation to IL-15/IL-15R alpha) is not yet fully established by cloning and complete sequencing of all respective components, similarities (and to some extent differences) to peripheral counterparts are now apparent. The ability of IL-2 to readily penetrate the blood-brain barrier further suggests that this cytokine could regulate interactions between peripheral tissues and the central nervous system. Taken together, these data suggest that IL-2 of either immune and CNS origin can have access to functional IL-2R molecules on neurons and glia under normal conditions. Additionally, dysregulation of the IL-2/IL-2 receptor system could lead or contribute to functional and pathological alterations in the brain as in the immune system. Understanding the neurobiology of the IL-2/IL-2 receptor system should also help to explain neurologic, neuropsychiatric and neuroendocrine side effects occurring during IL-2 treatment of peripheral and brain tumors. Immunopharmacological manipulation either aiming at the activation or suppression of IL-2 signaling should consider functional interference with constitutive and inducible IL-2 receptors on brain cells in order to fulfil the high expectations associated with the use of this cytokine as a promising agent in immunotherapies, especially of brain tumors.