Tumour necrosis factor-alpha and interleukin-2 differentially affect hippocampal serotonergic neurotransmission, behavioural activity, body temperature and hypothalamic-pituitary-adrenocortical axis activity in the rat

Central metabolite changes and activation of microglia after peripheral interleukin-2 challenge

Interleukin (IL)-2 regulates the immune response through the proliferation of activated T-cells and also exerts effects on the central nervous system (CNS). Alongside having marked neurobehavioral effects, IL-2 has been suggested to impact on various psychiatric disorders. The immune-CNS communication of IL-2 remains unclear, although, it is suggested that microglia are the source and target of IL-2. Here, we analyzed changes in brain metabolites following a peripheral IL-2 challenge and examined the contribution of microglia in mediating these effects. Rats were assessed by magnetic resonance spectroscopy (MRS) in a 9.4 T scanner for baseline metabolite levels in the prefrontal cortex (PFC) and the hippocampus. After 7 days animals were scanned again following a single injection of IL-2 (2.5 μg/kg) and then tested on the elevated plus-maze for the correlation of IL-2-induced brain metabolites and measures of anxiety. In another experiment CD25(+) microglia cells were determined. A separate group of rats was injected either with IL-2 or vehicle, and afterward the PFC and hippocampus were dissected and fluorescence activated cell sorting (FACS) analysis was performed. The MRS scans in the intra-individual study design showed a significant increase in myo-inositol in the analyzed regions. A significant correlation of anxiety-like measures and myo-inositol, a marker for microglia activity, was found in the hippocampus. The FACS analysis showed a significant increase in CD25(+) microglia in the hippocampus compared to controls. The results support the role of microglia as a mediator in the immune-CNS communication and the effects of peripheral IL-2.

Prenatal lipopolysaccharide treatment enhances MK-801-induced psychotomimetic effects in rats

The aim of this study was to evaluate the effect of prenatal lipopolysaccharide (LPS) treatment, which is an animal developmental model of schizophrenia, on MK-801-induced psychotomimetic behavioral changes and brain aminergic system activity in adult offspring. Repeated LPS (1 mg/kg) injection in rats, that had started from 7th day of pregnancy and was continued every second day till delivery, resulted in a long-lasting disruption of prepulse inhibition (PPI) and elevation of locomotor activity in their offspring. The prenatally LPS-treated rats showed hypersensitivity to MK-801 (0.1 and 0.4 mg/kg) as evidenced by the enhancement of acoustic startle amplitude, reduced PPI, and enhanced locomotor activity. These behavioral changes were accompanied by a decrease in the dopamine and its metabolite, DOPAC concentration in the frontal cortex, enhanced dopaminergic system activity in the striatum and no changes in noradrenaline (NA) level. Furthermore, the significant augmentation of 5-HT and 5-HIAA content in the frontal cortex of females only was detected. No changes in the cortical NA tissue level were found. Summing up, the present study demonstrated that the activation of the immune system in prenatal period led to persistent behavioral hypersensitivity to psychotomimetic action of a non-competitive NMDA receptor antagonist, and attention/information processing deficits. The foregoing data indicate that prenatal administration of LPS model some of the clinical aspects of schizophrenia and these behavioral effects are connected with neurochemical changes.

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.

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.

Effects of cytokines and infections on brain neurochemistry

Administration of cytokines to animals can elicit many effects on the brain, particularly neuroendocrine and behavioral effects. Cytokine administration also alters neurotransmission, which may underlie these effects. The most well studied effect is the activation of the hypothalamo-pituitary-adrenocortical (HPA) axis, especially that by interleukin-1 (IL-1). Peripheral and central administration of IL-1 also induces norepinephrine (NE) release in the brain, most markedly in the hypothalamus. Small changes in brain dopamine (DA) are occasionally observed, but these effects are not regionally selective. IL-1 also increases brain concentrations of tryptophan, and the metabolism of serotonin (5-HT) throughout the brain in a regionally nonselective manner. Increases of tryptophan and 5-HT, but not NE, are also elicited by IL-6, which also activates the HPA axis, although it is much less potent in these respects than IL-1. IL-2 has modest effects on DA, NE and 5-HT. Like IL-6, tumor necrosis factor-α (TNFα) activates the HPA axis, but affects NE and tryptophan only at high doses. The interferons (IFN's) induce fever and HPA axis activation in man, but such effects are weak or absent in rodents. The reported effects of IFN's on brain catecholamines and serotonin have been very varied. However, interferon-γ, and to a lesser extent, interferon-α, have profound effects on the catabolism of tryptophan, effectively reducing its concentration in plasma, and may thus limit brain 5-HT synthesis.Administration of endotoxin (LPS) elicits responses similar to those of IL-1. Bacterial and viral infections induce HPA activation, and also increase brain NE and 5-HT metabolism and brain tryptophan. Typically, there is also behavioral depression. These effects are strikingly similar to those of IL-1, suggesting that IL-1 secretion, which accompanies many infections, may mediate these responses. Studies with IL-1 antagonists, support this possibility, although in most cases the antagonism is incomplete, suggesting the existence of multiple mechanisms. Because LPS is known to stimulate the secretion of IL-1, IL-6 and TNFα, it seems likely that these cytokines mediate at least some of the responses, but studies with antagonists indicate that there are multiple mechanisms. The neurochemical responses to cytokines are likely to underlie the endocrine and behavioral responses. The NE response to IL-1 appears to be instrumental in the HPA activation, but other mechanisms exist. Neither the noradrenergic nor the serotonergic systems appear to be involved in the major behavioral responses. The significance of the serotonin response is unknown.

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.

Cytokines, stressors, and clinical depression: augmented adaptation responses underlie depression pathogenesis

By influencing the central nervous system, cytokines, which regulate immune function innately and adaptively, may play a key role in mediating depression-like neuro-behavioral changes. However, the similarity between cytokine and stressor-effects in animal models raises a question about the degree to which behavioral and neurochemical outcomes of cytokine challenge represent depressive disorder per se. The present review attempts to illustrate the degree of overlap between cytokines and stressors with respect to their effects on neurochemistry and behavior in animal models. The review also shows how short-term effects of cytokine exposure in typical animals may be discerned from characteristics that might otherwise be described as depression-like. By comparing outcomes of immune challenge in typical rodent strains (e.g., Sprague-Dawley [SD], Wistar) and an accepted animal model of depression (e.g., Fawn Hooded [FH] rodent strain), differences between short-term effects of cytokines and depression-like characteristics in rodents are demonstrated. Additionally, because it is known that preexisting vulnerability to depression may affect outcomes of immune challenge, we further compare immunological, biochemical and behavioral effects of cytokines between SD and FH rodent strains. Interestingly, the acute neurochemical and behavioral effects of the cytokine interleukin 1alpha (IL-1alpha) reveal stressor-like responses during behavioral habituation in both strains, though this appears to a stronger degree in FH animals. Further, the subacute response to IL-1alpha vastly differed between strains, indicating differences in adaptive mechanisms. Thus, stressor-like effects of immune challenge, particularly in FH animals, provide validation for recent "cross-sensitization" models of depression pathogenesis that incorporate immune factors.

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.

Cytokines and major depression

In the research field of psychoneuroimmunology, accumulating evidence has indicated the existence of reciprocal communication pathways between nervous, endocrine and immune systems. In this respect, there has been increasing interest in the putative involvement of the immune system in psychiatric disorders. In the present review, the role of proinflammatory cytokines, such as interleukin (IL)-1, tumour necrosis factor (TNF)-alpha and interferon (IFN)-gamma, in the aetiology and pathophysiology of major depression, is discussed. The 'cytokine hypothesis of depression' implies that proinflammatory cytokines, acting as neuromodulators, represent the key factor in the (central) mediation of the behavioural, neuroendocrine and neurochemical features of depressive disorders. This view is supported by various findings. Several medical illnesses, which are characterised by chronic inflammatory responses, e.g. rheumatoid arthritis, have been reported to be accompanied by depression. In addition, administration of proinflammatory cytokines, e.g. in cancer or hepatitis C therapies, has been found to induce depressive symptomatology. Administration of proinflammatory cytokines in animals induces 'sickness behaviour', which is a pattern of behavioural alterations that is very similar to the behavioural symptoms of depression in humans. The central action of cytokines may also account for the hypothalamic-pituitary-adrenal (HPA) axis hyperactivity that is frequently observed in depressive disorders, as proinflammatory cytokines may cause HPA axis hyperactivity by disturbing the negative feedback inhibition of circulating corticosteroids (CSs) on the HPA axis. Concerning the deficiency in serotonergic (5-HT) neurotransmission that is concomitant with major depression, cytokines may reduce 5-HT levels by lowering the availability of its precursor tryptophan (TRP) through activation of the TRP-metabolising enzyme indoleamine-2,3-dioxygenase (IDO). Although the central effects of proinflammatory cytokines appear to be able to account for most of the symptoms occurring in depression, it remains to be established whether cytokines play a causal role in depressive illness or represent epiphenomena without major significance.

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.

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.

Sensitization to the neuroendocrine, central monoamine and behavioural effects of murine tumor necrosis factor-alpha: peripheral and central mechanisms

Systemic administration of murine tumour necrosis factor-alpha (mTNF-alpha; 0.1-2.0 microg, i.p.) dose-dependently increased plasma corticosterone and augmented monoamine utilization within the paraventricular nucleus of the hypothalamus (PVN), locus coeruleus, medial prefrontal cortex (PFC), central and medial amygdala. A time-dependent sensitization was induced in mice, wherein reexposure to mTNF-alpha 28 days (but not 1 day) following the initial cytokine treatment provoked marked signs of illness (diminished activity, ptosis, piloerection) and increased plasma corticosterone levels. Serotonin (5-HT) activity was augmented upon mTNF-alpha reexposure at the 1- or 28-day intervals in the PFC and medial amygdala, respectively. Intracerebroventricular (i.c.v.; 1-500 ng) mTNF-alpha did not promote illness, but modestly increased plasma corticosterone levels. Neither the illness nor the corticosterone changes were subject to a sensitization upon i.c.v. cytokine reexposure. Acute i.c.v. mTNF-alpha increased norepinephrine (NE), 5-HT and dopamine (DA) activity within the PVN and median eminence/arcuate nucleus complex (ME/ARC), and NE utilization within the central amygdala. Subsequent i.c.v. mTNF-alpha further enhanced the hypothalamic monoamine variations. Finally, systemic (i.p.) mTNF-alpha pretreatment did not proactively influence sickness or corticosterone responses upon later i.c.v. cytokine challenge, but augmented locus coeruleus NE activity and 5-HT and DA utilization within the ME/ARC. It is suggested that the sensitization with respect to sickness and corticosterone activity in response to mTNF-alpha reflect the involvement of peripheral mechanisms. Moreover, it appears that mTNF-alpha promotes central neurochemical plasticity through independent central and peripheral mechanisms.

In vitro cytokine secretion in individuals with schizophrenia: results, confounding factors, and implications for further research

The present paper reviews the results of all publications on in vitro cytokine secretion in patients with schizophrenia, as published by March 2001. The authors supply easy to read tables with respect to the individual cytokines and soluble cytokine receptors investigated, the in vitro methodology used, characterization of the patient samples, and the results on cytokine secretion as stated in these studies. Inconsistent results, e.g., regarding in vitro secretion of IL-2 with 11/18 studies finding decreased secretion, 5/18 finding no change, and 2/18 finding increases, cannot systematically be correlated with any methodological procedures nor any diagnostic subtypes, per se. However, factors such as medication and cigarette smoking are likely to play a role. The authors suggest that more hypothesis-driven research, together with more carefully designed studies, as well as better communication between basic or animal researchers and clinicians might help to answer the question of whether there are meaningful peripheral changes in the immune system related to schizophrenia.

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.

Variability of interactions between neuroendocrine and immunological functions in physiological aging and dementia of the Alzheimer's type

A link between neuroendocrine and immunological changes has been suggested in the pathophysiology of dementia of the Alzheimer's type (DAT). Healthy young and old subjects and patients with DAT were recruited to evaluate the chrononeuroendocrine organization of cortisol, GH, and melatonin (MLT) secretions. The study was carried out together with the evaluation of natural killer (NK) cell function: cytotoxic activity (NKCC) and TNF-alpha and IFN-gamma release after exposure to IL-2 (100 U/mL). Moreover, a cerebral morphometric analysis of hippocampus and temporal lobe (MRI) was performed. The activation of hypothalamo-pituitary-adrenal (HPA) axis and the decrease of GH, and MLT nocturnal peaks were associated with normal NKCC and TNF-alpha/IFN-gamma in healthy elderly subjects, whereas in DAT patients the same neuroendocrine changes occurred together with abnormal NKCC (spontaneous and IL-2/IFN-beta-modulated) and with alterations of TNF-alpha/INF-gamma generation from NK. Moreover significant correlations among the increase of NKCC and TNF-alpha and the decrease of cognitive function were found in the DAT group. These correlations were associated with the impairment of nocturnal GH and MLT levels and with the relatively higher serum cortisol concentrations. Moreover, the impairment of cortisol suppression after dexamethasone (1 mg orally at 23:00) was significantly correlated with the increase of spontaneous release of TNF-alpha and with IL-2-modulated NKCC. Finally the imunoneuroendocrine alterations found in DAT were associated with the reduction of cerebral volume in hippocampus and temporal lobes. Taken together these data indicate that the immunoneuroendocrine balance is maintained in physiological aging, whereas NK immune dysregulation in DAT could contribute to altering the neuroendocrine functions and to extend the progression of neurodegeneration and dementia.

Dissociation between synaptic depression and block of long-term depression induced by raising the temperature in rat hippocampal slices

The influence of raising the bath temperature (39 degrees C) on synaptic transmission and neuronal plasticity was studied in the CA1 region of the rat hippocampus using an extracellular recording technique. Increasing the bath temperature from 32 to 39 degrees C resulted in a depression of field excitatory postsynaptic potential (fEPSP). Application of the selective A(1) receptor agonist, 2-chloro-adenosine (2-CADO, 1 microM) reduced the fEPSP and subsequently occluded the raised temperature-induced synaptic depression. On the other hand, the selective adenosine A(1) receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX) blocked depression of fEPSP produced by raising the temperature. These results suggest that raising temperature-induced synaptic depression is due to an alteration of extracellular adenosine concentration. Long-term depression (LTD) could be reliably induced by the standard low-frequency stimulation (LFS, 1 Hz for 15 min) protocol at 32 degrees C but not at 39 degrees C. The raised temperature-induced block of LTD was mimicked by 2-CADO. Unexpectedly, despite the presence of DPCPX, LFS still could not elicit LTD. NMDA receptor-mediated synaptic component (fEPSP(NMDA)) was decreased when increasing the temperature to 39 degrees C and DPCPX failed to reverse such a depression. The increase in the NMDA response in 0.1 mM Mg(++) compared with 1 mM Mg(++) was significantly greater at 32 degrees C than at 39 degrees C. These results suggest that, by increasing the sensitivity of Mg(++) block, an increase in temperature modulates NMDA responses and thereby inhibits the induction of LTD.

Central monoamine and plasma corticosterone changes induced by a bacterial endotoxin: sensitization and cross-sensitization effects

Low doses of lipopolysaccharide, tumour necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), or exposure to a stressor (restraint) increased plasma corticosterone levels. In animals pretreated with lipopolysaccharide, a marked sensitization of the corticosterone response was evident upon subsequent exposure to lipopolysaccharide, TNF-alpha, or restraint, 1 day later. As well, the sickness-inducing effects of lipopolysaccharide, TNF-alpha and IL-1 beta were markedly increased in mice pretreated with lipopolysaccharide. The sensitization effects were marked when the second treatment was administered 1 day after lipopolysaccharide administration, but not when a 28-day interval elapsed. In a second experiment, TNF-alpha influenced monoamine functioning in the paraventricular nucleus of the hypothalamus and within extrahypothalamic regions, including the central amygdala, locus coeruleus, prefrontal cortex. Moreover, serotonin activity within the central amygdala, as well as dopamine activity within the prefrontal cortex, were subject to a sensitization effect in animals pretreated with lipopolysaccharide 1 day earlier. Macrophage depletion by a suspension of clodronate liposomes attenuated the plasma corticosterone changes induced by TNF-alpha, but did not affect the sensitization. In contrast, the acute effects of TNF-alpha on central neurotransmitters were unaffected by the liposome suspension, but this treatment prevented the sensitization. These data may be relevant to clinical situations in which individuals exposed to bacterial infections may be rendered more susceptible to the behavioural and neurochemical effects of subsequently encountered stressors and immunological challenges.

Neurobehavioral alterations in mice with a targeted deletion of the tumor necrosis factor-alpha gene: implications for emotional behavior

Tumor necrosis factor-alpha (TNF-alpha) is emerging as an important modulator of the function of the central nervous system. In the present study, we investigated a role of endogenous TNF-alpha in cognitive and emotional function using mice with targeted deletions of the TNF-alpha gene. TNF-alpha-(-/-) mice showed normal diurnal rhythms of spontaneous locomotor activity and cognitive functions. Emotional behavior in the mutant mice, however, was significantly altered, which manifested in the performance in the open-field, elevated plus maze, and forced swimming tests. The altered performance in the elevated plus maze test was significantly alleviated by treatment with diazepam. Postmortem brain analysis of TNF-alpha-(-/-) mice revealed a significant increase in serotonin metabolism in the brain. These findings suggest a role for endogenous TNF-alpha in emotional behavior, which may possibly be related to alterations of serotonine metabolism.

The brain mineralocorticoid receptor: greedy for ligand, mysterious in function

Glucocorticoids exert their regulatory effects on the hypothalamic-pituitary-adrenocortical axis via two types of corticosteroid receptors: the glucocorticoid receptor and the mineralocorticoid receptor. Whereas the glucocorticoid receptor has a broad distribution in the brain, highest levels of mineralocorticoid receptor are found in the hippocampus. Based on the differential occupancy profile by endogenous glucocorticoids, glucocorticoid receptors are thought to mediate negative feedback signals of elevated glucocorticoid levels, whereas mineralocorticoid receptors control the inhibitory tone of the hippocampus on hypothalamic-pituitary-adrenocortical axis activity. Dysfunction of mineralocorticoid receptors and glucocorticoid receptors are thought to be implicated in stress-related psychiatric diseases such as major depression. Because of its intriguing features, we focus in this review on the mineralocorticoid receptor and provide data which reveal novel aspects of the pharmacology and physiology of mineralocorticoid receptors. Newly obtained results are presented, which help to solve the paradox of why dexamethasone binds with high affinity to mineralocorticoid receptors in vitro, yet binds poorly in vivo. Until recently, mineralocorticoid receptor protein and mRNA levels could only be routinely studied with in vitro cytosol binding assays, in vitro and in vivo receptor autoradiography, Northern blot analysis, and in situ hybridization. These methods are unfortunately hampered by several flaws, such as the necessity of adrenalectomy, no or poor neuroanatomical resolution, the fact that mRNA does not provide the same information as protein, or combinations of these factors. We present immunohistochemical data on mineralocorticoid receptors in the brain obtained by using commercially available antibodies, which alleviate many of these shortcomings. Furthermore, an in vivo microdialysis method is presented which allows the assessment of free corticosterone levels in the brain, which is critical for the study of the pharmacological basis of mineralocorticoid receptor (and glucocorticoid receptor) function. Finally, a novel aspect of the regulation of mineralocorticoid receptors is described which provides evidence that this receptor system is dynamically regulated. In conjunction with previously reported effects of antidepressants, these results have initiated a new concept on the cause of the hypothalamic-pituitary-adrenocortical axis disturbances often seen in stress-related psychiatric disorders such as major depression.

Abnormal cytokine and adrenocortical hormone regulation in myotonic dystrophy

Metabolic-endocrine dysfunctions, including hyperinsulinemia, hypertriglyceridemia, increased fat mass, and dysregulation of the hypothalamic-pituitary-adrenal axis, are common in myotonic dystrophy (MD). We hypothesized that increased production of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) may be important underlying mechanisms. We studied the diurnal rhythmicity of cytokines and cortisol, ACTH, and dehydroepiandrosterone in 18 men with adult onset MD and 18 controls. Morning levels of androstenedione, 17-hydroxyprogesterone, testosterone, and insulin were also determined. Genetic analyses were performed, including calculation of allele sizes. Median circulating 24-h levels of IL-6 (P < 0.001), TNF-alpha (P = 0.05), ACTH (P < 0.05), and cortisol (P < 0.05) were all significantly increased in MD, whereas dehydroepiandrosterone levels were decreased (P < 0.001). The diurnal rhythms of these cytokines/ hormones were disturbed in patients. Morning testosterone levels were decreased and insulin levels increased (P < 0.01 for both). Patients with high body fat mass had significantly increased insulin levels and decreased morning levels of cortisol, ACTH, and testosterone. IL-6 and TNF-alpha levels are increased and adrenocortical hormone regulation is disturbed in MD. Adiposity may contribute to these disturbances, which may be of importance for decreased adrenal androgen hormone production and metabolic, muscular, and neuropsychiatric dysfunction in MD.

Plasma levels of tumor necrosis factor-alpha in patients with panic disorder: effect of alprazolam therapy

Plasma concentrations of tumor necrosis factor-alpha (TNF-alpha) were measured in 10 outpatients with panic disorder, twice (at a 48-h interval) before and twice on days 30-32 of treatment with alprazolam (2-2.5 mg/day), and twice in 10 age- and sex-matched healthy controls. TNF-alpha concentrations did not differ in patients and control subjects, either before therapy or at days 30-32 of therapy. In five patients before therapy, and in three of them after therapy, TNF-alpha values were higher than the maximal concentrations of the cytokine in controls.

Delivery of herpes simplex virus amplicon-based vectors to the dentate gyrus does not alter hippocampal synaptic transmission in vivo

Herpes simplex virus type-1 (HSV) amplicon vectors containing neuroprotective genes can alter cell physiology and enhance survival following various insults. However, to date, little is known about effects of viral infection itself (independent of the gene delivered) on neuronal physiology. Electrically-evoked synaptic responses are routinely recorded to measure functional alterations in the nervous system and were used here to assess the potential capability of HSV vectors to disrupt physiology of the hippocampus (a forebrain structure involved in learning that is highly susceptible to necrotic insult, making it a frequent target in gene therapy research). Population excitatory post-synaptic potentials (EPSPs) were recorded in the dentate gyrus (DG) and in area CA3 in vivo 72 h after infusion of an HSV vector expressing a reporter gene (lacZ) or vehicle into the DG. Evoked perforant path (PP-DG) or mossy fiber (MF-CA3) EPSPs slope values measured across input/output (I/O) curves were not altered by infection. Paired-pulse facilitation at either recording site was also unaffected. X-gal-positive granule cells surrounded the recording electrode (PP-DG recording) and stimulating electrode tracts (MF-CA3 recording) in animals that received vector, suggesting that we had measured function, at least in part, in infected neurons. Because of the negative electrophysiological result, we sought to deliver a gene with an HSV amplicon which would affect the measured endpoints, as a positive control. Delivery of calbindin D28kpotentiated PP-DG synaptic strength, indicating that our recording system could detect alterations due to vector expression. Thus, the data indicate that HSV vectors are benign, in regard to effects on synaptic function, and support the use of these vectors as a safe method to deliver selected genes to the central nervous system.

Sensitization to the effects of tumor necrosis factor-alpha: neuroendocrine, central monoamine, and behavioral variations

Consistent with the proposition that cytokines act as immunotransmitters between the immune system and the brain, systemic administration of the proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha; 1.0-4.0 microg) induced mild illness in CD-1 mice, increased plasma corticosterone concentrations, and altered central norepinephrine, dopamine, and serotonin turnover. The actions of TNF-alpha were subject to a time-dependent sensitization effect. After reexposure to a subeffective dose of the cytokine (1.0 microgram) 14-28 d after initial treatment, marked illness was evident (reduced consumption of a palatable substance and diminished activity and social exploration), coupled with an elevation of plasma corticosterone levels. In contrast, cytokine reexposure 1-7 d after initial treatment did not elicit illness, and at the 1 d interval the corticosterone response to the cytokine was reduced. The increase of norepinephrine release within the paraventricular nucleus of the hypothalamus, as reflected by elevated accumulation of 3-methoxy-4-hydroxyphenylglycol, was augmented at the longer reexposure intervals. In contrast, within the central amygdala and the prefrontal cortex TNF-alpha reexposure at the 1 d interval was associated with a pronounced sensitization-like effect, which was not apparent at longer intervals. Evidently, systemic TNF-alpha proactively influences the response to subsequent treatment; however, the nature of the effects (i.e., the behavioral, neuroendocrine, and central transmitter alterations) vary over time after initial cytokine treatment. It is suggested that the sensitization may have important repercussions with respect to cognitive effects of TNF-alpha and may also be relevant to analyses of the neuroprotective or neurodestructive actions of cytokines.