Microglia play a role in mediating the effects of cytokines on the structure and function of the rat pineal gland

Microglia in Circumventricular Organs: The Pineal Gland Example

The circumventricular organs (CVOs) are unique areas within the central nervous system. They serve as a portal for the rest of the body and, as such, lack a blood-brain barrier. Microglia are the primary resident immune cells of the brain parenchyma. Within the CVOs, microglial cells find themselves continuously challenged and stimulated by local and systemic stimuli, even under steady-state conditions. Therefore, CVO microglia in their typical state often resemble the activated microglial forms found elsewhere in the brain as they are responding to pathological conditions or other stressors. In this review, I focus on the dynamics of CVO microglia, using the pineal gland as a specific CVO example. Data related to microglia heterogeneity in both homeostatic and unhealthy environments are presented and discussed, including those recently generated by using advanced single-cell and single-nucleus technology. Finally, perspectives in the CVO microglia field are also included.Summary StatementMicroglia in circumventricular organs (CVOs) continuously adapt to react differentially to the diverse challenges they face. Herein, I discuss microglia heterogeneity in CVOs, including pineal gland. Further studies are needed to better understand microglia dynamics in these unique brain areas. .

Daily and LPS-induced variation of endocrine mediators in cururu toads (Rhinella icterica)

Increased plasma glucocorticoids (corticosterone - CORT, in amphibians) and melatonin (MEL) are associated with the daily activity phase and with environmental darkness, respectively. Besides, CORT and MEL also play pivotal immunomodulatory roles in several vertebrates. Herein we described the daily profile of plasma MEL and CORT for Rhinella icterica toads in captivity. Thereafter, we investigated the effects of lipopolysaccharide (LPS)-induced systemic inflammation on the production of CORT and MEL in the R. icterica. Captive toads showed CORT and MEL diurnal variation typical of nocturnal species, with increased values for CORT at ZT12 (18 h) and MEL peak at ZT18 (24 h). LPS-induced hormonal changes included increased plasma CORT and decreased ocular and plasma MEL when compared to those from toads treated with saline 2 h post-injection. Our results demonstrated the presence of a diurnal CORT and MEL variation in toads. We also showed the crosstalk between CORT and MEL during the toad's systemic inflammation in response to an immune challenge with LPS. Additionally, our results demonstrated that anuran eyes' MEL production might be regulated during the inflammatory processes.

Neuromodulation of the Pineal Gland via Electrical Stimulation of Its Sympathetic Innervation Pathway

Stimulation of the pineal gland via its sympathetic innervation pathway results in the production of N-acetylserotonin and melatonin. Melatonin has many therapeutic roles and is heavily implicated in the regulation of the sleep-wake cycle. In addition, N-acetylserotonin has recently been reported to promote neurogenesis in the brain. Upregulation of these indoleamines is possible via neuromodulation of the pineal gland. This is achieved by electrical stimulation of structures or fibres in the pineal gland sympathetic innervation pathway. Many studies have performed such pineal neuromodulation using both invasive and non-invasive methods. However, the effects of various experimental variables and stimulation paradigms has not yet been reviewed and evaluated. This review summarises these studies and presents the optimal experimental protocols and stimulation parameters necessary for maximal upregulation of melatonin metabolic output.

Immune-pineal axis - acute inflammatory responses coordinate melatonin synthesis by pinealocytes and phagocytes

Melatonin is well known for its circadian production by the pineal gland, and there is a growing body of data showing that it is also produced by many other cells and organs, including immune cells. The chronobiotic role of pineal melatonin, as well as its protective effects in vitro and in vivo, have been extensively explored. However, the interaction between the chronobiotic and defence functions of endogenous melatonin has been little investigated. This review details the current knowledge regarding the coordinated shift in melatonin synthesis from the pineal gland (circadian and monitoring roles) to the regulation of acute immune responses via immune cell production and autocrine effects, producing systemic interactions termed the immune-pineal axis. An acute inflammatory response drives the transcription factor, NFκB, to switch melatonin synthesis from pinealocytes to macrophages/microglia and, upon acute inflammatory resolution, back to pinealocytes. The potential pathophysiological relevance of immune-pineal axis dysregulation is highlighted, with both research and clinical implications, across several medical conditions, including host/parasite interaction, neurodegenerative diseases and cancer. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Dual Effect of Catecholamines and Corticosterone Crosstalk on Pineal Gland Melatonin Synthesis

BACKGROUND/AIM

The nocturnal production of melatonin by the pineal gland is triggered by sympathetic activation of adrenoceptors and may be modulated by immunological signals. The effect of glucocorticoids on nocturnal melatonin synthesis is controversial; both stimulatory and inhibitory effects have been reported. During pathophysiological processes, an increased sympathetic tonus could result in different patterns of adrenoceptor activation in the pineal gland. Therefore, in this investigation, we evaluated whether the pattern of adrenergic stimulation of the pineal gland drives the direction of the glucocorticoid effect on melatonin production.

METHODS

The corticosterone effect on the pineal hormonal production induced by β-adrenoceptor or β+α1-adrenoceptor activation was evaluated in cultured glands. We also investigated whether the in vivo lipopolysaccharide (LPS)-induced inhibition of melatonin is dependent on the interaction of glucocorticoids and the α1-adrenoceptor in adrenalectomized animals and on the in vivo blockade of glucocorticoid receptors (GRs) or the α1-adrenoceptor.

RESULTS

Corticosterone potentiated β-adrenoceptor-induced pineal melatonin synthesis, whilst corticosterone-dependent inhibition was observed when melatonin production was induced by β+α1-adrenoceptors agonists. The inhibitory effect of corticosterone is mediated by GR, as it was abolished in the presence of a GR antagonist. Moreover, LPS-induced reduction in melatonin nocturnal plasma content was reversed by adrenalectomy and by antagonizing GR or α1-adrenoceptors.

CONCLUSIONS

The dual effect of corticosterone on pineal melatonin synthesis is determined by the activation pattern of adrenoceptors (β or β+α1) in the gland during GR activation, suggesting that increased activation of the sympathetic system and the hypothalamic-pituitary-adrenal axis are necessary for the control of melatonin production during defense responses.

Cellular Basis of Pineal Gland Development: Emerging Role of Microglia as Phenotype Regulator

The adult pineal gland is composed of pinealocytes, astrocytes, microglia, and other interstitial cells that have been described in detail. However, factors that contribute to pineal development have not been fully elucidated, nor have pineal cell lineages been well characterized. We applied systematic double, triple and quadruple labeling of cell-specific markers on prenatal, postnatal and mature rat pineal gland tissue combined with confocal microscopy to provide a comprehensive view of the cellular dynamics and cell lineages that contribute to pineal gland development. The pineal gland begins as an evagination of neuroepithelium in the roof of the third ventricle. The pineal primordium initially consists of radially aligned Pax6+ precursor cells that express vimentin and divide at the ventricular lumen. After the tubular neuroepithelium fuses, the distribution of Pax6+ cells transitions to include rosette-like structures and later, dispersed cells. In the developing gland all dividing cells express Pax6, indicating that Pax6+ precursor cells generate pinealocytes and some interstitial cells. The density of Pax6+ cells decreases across pineal development as a result of cellular differentiation and microglial phagocytosis, but Pax6+ cells remain in the adult gland as a distinct population. Microglial colonization begins after pineal recess formation. Microglial phagocytosis of Pax6+ cells is not common at early stages but increases as microglia colonize the gland. In the postnatal gland microglia affiliate with Tuj1+ nerve fibers, IB4+ blood vessels, and Pax6+ cells. We demonstrate that microglia engulf Pax6+ cells, nerve fibers, and blood vessel-related elements, but not pinealocytes. We conclude that microglia play a role in pineal gland formation and homeostasis by regulating the precursor cell population, remodeling blood vessels and pruning sympathetic nerve fibers.

Interleukin-1 β Modulates Melatonin Secretion in Ovine Pineal Gland: Ex Vivo Study

The study was designed to determine the effect of proinflammatory cytokine, interleukin- (IL-) 1β, on melatonin release and expression enzymes essential for this hormone synthesis: arylalkylamine-N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIOMT) in ovine pineal gland, taking into account the immune status of animals before sacrificing. Ewes were injected by lipopolysaccharide (LPS; 400 ng/kg) or saline, two hours after sunset during short day period (December). Animals were euthanized three hours after the injection. Next, the pineal glands were collected and divided into four explants. The explants were incubated with (1) medium 199 (control explants), (2) norepinephrine (NE; 10 µM), (3) IL-1β (75 pg/mL), or (4) NE + IL-1β. It was found that IL-1β abolished (P < 0.05) NE-induced increase in melatonin release. Treatment with IL-1β also reduced (P < 0.05) expression of AA-NAT enzyme compared to NE-treated explants. There was no effect of NE or IL-1β treatment on gene expression of HIOMT; however, the pineal fragments isolated from LPS-treated animals were characterized by elevated (P < 0.05) expression of HIOMT mRNA and protein compared to the explants from saline-treated ewes. Our study proves that IL-1β suppresses melatonin secretion and its action seems to be targeted on the reduction of pineal AA-NAT protein expression.

Modulation of pineal melatonin synthesis by glutamate involves paracrine interactions between pinealocytes and astrocytes through NF-κB activation

The glutamatergic modulation of melatonin synthesis is well known, along with the importance of astrocytes in mediating glutamatergic signaling in the central nervous system. Pinealocytes and astrocytes are the main cell types in the pineal gland. The objective of this work was to investigate the interactions between astrocytes and pinealocytes as a part of the glutamate inhibitory effect on melatonin synthesis. Rat pinealocytes isolated or in coculture with astrocytes were incubated with glutamate in the presence of norepinephrine, and the melatonin content, was quantified. The expression of glutamate receptors, the intracellular calcium content and the NF-κB activation were analyzed in astrocytes and pinealocytes. TNF-α's possible mediation of the effect of glutamate was also investigated. The results showed that glutamate's inhibitory effect on melatonin synthesis involves interactions between astrocytes and pinealocytes, possibly through the release of TNF-α. Moreover, the activation of the astrocytic NF-κB seems to be a necessary step. In astrocytes and pinealocytes, AMPA, NMDA, and group I metabotropic glutamate receptors were observed, as well as the intracellular calcium elevation. In conclusion, there is evidence that the modulation of melatonin synthesis by glutamate involves paracrine interactions between pinealocytes and astrocytes through the activation of the astrocytic NF-κB transcription factor and possibly by subsequent TNF-α release.

Glia-pinealocyte network: the paracrine modulation of melatonin synthesis by tumor necrosis factor (TNF)

The pineal gland, a circumventricular organ, plays an integrative role in defense responses. The injury-induced suppression of the pineal gland hormone, melatonin, which is triggered by darkness, allows the mounting of innate immune responses. We have previously shown that cultured pineal glands, which express toll-like receptor 4 (TLR4) and tumor necrosis factor receptor 1 (TNFR1), produce TNF when challenged with lipopolysaccharide (LPS). Here our aim was to evaluate which cells present in the pineal gland, astrocytes, microglia or pinealocytes produced TNF, in order to understand the interaction between pineal activity, melatonin production and immune function. Cultured pineal glands or pinealocytes were stimulated with LPS. TNF content was measured using an enzyme-linked immunosorbent assay. TLR4 and TNFR1 expression were analyzed by confocal microscopy. Microglial morphology was analyzed by immunohistochemistry. In the present study, we show that although the main cell types of the pineal gland (pinealocytes, astrocytes and microglia) express TLR4, the production of TNF induced by LPS is mediated by microglia. This effect is due to activation of the nuclear factor kappa B (NF-kB) pathway. In addition, we observed that LPS activates microglia and modulates the expression of TNFR1 in pinealocytes. As TNF has been shown to amplify and prolong inflammatory responses, its production by pineal microglia suggests a glia-pinealocyte network that regulates melatonin output. The current study demonstrates the molecular and cellular basis for understanding how melatonin synthesis is regulated during an innate immune response, thus our results reinforce the role of the pineal gland as sensor of immune status.

Global daily dynamics of the pineal transcriptome

Transcriptome profiling of the pineal gland has revealed night/day differences in the expression of a major fraction of the genes active in this tissue, with two-thirds of these being nocturnal increases. A set of over 600 transcripts exhibit two-fold to >100-fold daily differences in abundance. These changes appear to be primarily attributable to adrenergic-cyclic-AMP-dependent mechanisms, which are controlled via a neural pathway that includes the suprachiasmatic nucleus, the master circadian oscillator. In addition to melatonin synthesis, night/day differences in gene expression impact genes associated with several specialized functions, including the immune/inflammation response, photo-transduction, and thyroid hormone/retinoic acid biology. The following nonspecialized cellular features are also affected: adhesion, cell cycle/cell death, cytoskeleton, DNA modification, endothelium, growth, RNA modification, small molecule biology, transcription factors, vesicle biology, signaling involving Ca(2+), cyclic nucleotides, phospholipids, mitogen-activated protein kinases, the Wnt signaling pathway, and protein phosphorylation.

The Perivascular Phagocyte of the Mouse Pineal Gland: an Antigen‐Presenting Cell

The perivascular space of the rat pineal gland is known to contain phagocytic cells that are immunoreactive for leukocyte antigens, and thus they appear to belong to the macrophage/microglial cell line. These cells also contain MHC class II proteins. We investigated this cell type in the pineal gland of mice. Actively phagocytosing cells with a prominent lysosomal system were found in the pericapillary spaces of the mouse pineal gland following intravenous injection of horseradish peroxidase. The cells also exhibited strong acid phosphatase activity. Perivascular cells were immunopositive for MHC class II protein and for CD68, a marker of monocytes/phagocytes. This study verifies that perivascular phagocytes with antigen-presenting properties are present in the mouse pineal gland.

Melatonin as pharmacologic support in burn patients: a proposed solution to thermal injury-related lymphocytopenia and oxidative damage

OBJECTIVE

To review the data that support the clinical use of melatonin in the treatment of burn patients, with special emphasis on the stimulation of the oxidative defense system, the immune system, circadian rhythm of sleep/wakefulness, and the reduction in the toxicity of therapeutic agents used in the treatment of burn victims.

DATA SOURCE

A MEDLINE/PubMed search from 1975 to July 2006 was conducted.

STUDY SELECTION

The screening of the literature was examined using the key words: burn patients, lymphocytopenia, skin oxidative stress, antioxidant, melatonin, and free radicals.

DATA EXTRACTION AND SYNTHESIS

Thermal injury often causes damage to multiple organs remote from the original burn wound and may lead to multiple organ failure. Animal models and burn patients exhibit elevated free radical generation that may be causative in the local wound response and in the development of burn shock and distant organ injury. The suppression of nonspecific resistance and the disturbance in the adaptive immune system makes burn patients vulnerable to infections. Moreover, there is loss of sleep and the toxicity produced by drugs habitually used in the clinic for burn patients. Melatonin is a powerful antioxidant and is a potent protective agent against damage after experimental thermal injury. Some actions of melatonin as a potential supportive pharmacologic agent in burn patients include its: role as a scavenger of both oxygen and nitrogen-based reactants, stimulation of the activities of a variety of antioxidative enzymes, reduction in proinflammatory cytokines, inhibition of adhesion molecules, chronobiotic effects, and reduction in the toxicity of the drugs used in protocols to treat thermal injury patients.

CONCLUSIONS

These combined actions of melatonin, along with its low toxicity and its ability to penetrate all morphophysiologic membranes, could make it a ubiquitously acting and highly beneficial molecule in burn patients.

Pharmacological utility of melatonin in the treatment of septic shock: experimental and clinical evidence

Sepsis is a major cause of mortality in critically ill patients and develops as a result of the host response to infection. In recent years, important advances have been made in understanding the pathophysiology and treatment of sepsis. Mitochondria play a central role in the intracellular events associated with inflammation and septic shock. One of the current hypotheses for the molecular mechanisms of sepsis is that the enhanced nitric oxide (NO) production by mitochondrial nitric oxide synthase (mtNOS) leads to excessive peroxynitrite (ONOO-) production and protein nitration, impairing mitochondrial function. Despite the advances in understanding of its pathophysiology, therapy for septic shock remains largely symptomatic and supportive. Melatonin has well documented protective effects against the symptoms of severe sepsis/shock in both animals and in humans; its use for this condition significantly improves survival. Melatonin administration counteracts mtNOS induction and respiratory chain failure, restores cellular and mitochondrial redox status, and reduces proinflammatory cytokines. Melatonin clearly prevents multiple organ failure, circulatory failure, and mitochondrial damage in experimental sepsis, and reduces lipid peroxidation, indices of inflammation and mortality in septic human newborns. Considering these effects of melatonin and its virtual absence of toxicity, the use of melatonin (along with conventional therapy) to preserve mitochondrial bioenergetics as well as to limit inflammatory responses and oxidative damage should be seriously considered as a treatment option in both septic newborn and adult patients. This review summarizes the data that provides a rationale for using melatonin in septic shock patients.

Effect of TNF-alpha on the melatonin synthetic pathway in the rat pineal gland: basis for a 'feedback' of the immune response on circadian timing

A retino-hypothalamic-sympathetic pathway drives the nocturnal surge of pineal melatonin production that determines the synchronization of pineal function with the environmental light/dark cycle. In many studies, melatonin has been implicated in the modulation of the inflammatory response. However, scant information on the feedback action of molecules present in the blood on the pineal gland during the time course of an inflammatory response is available. Here we analyzed the effect of tumor necrosis factor-alpha (TNF-alpha) and corticosterone on the transcription of the Aa-nat, hiomt and 14-3-3 protein genes in denervated pineal glands of rats stimulated for 5 hr with norepinephrine, using real-time reverse transcription-polymerase chain reaction. The transcription of Aa-nat, a gene encoding the key enzyme in melatonin biosynthesis, together with the synthesis of the melatonin precursor N-acetylserotonin, was inhibited by TNF-alpha. This inhibition was transient, and a preincubation of TNF-alpha for more than 24 hr had no detectable effect. In fact, a protein(s) transcribed, later on, as shown by cycloheximide, was responsible for the reversal of the inhibition of Aa-nat transcription. In addition, corticosterone induced a potentiation of norepinephrine-induced Aa-nat transcription even after 48 hr of incubation. These data support the hypothesis that the nocturnal surge in melatonin is impaired at the beginning of an inflammatory response and restored either during the shutdown of an acute response or in a chronic inflammatory pathology. Here, we introduce a new molecular pathway involved in the feedback of an inflammatory response on pineal activity, and provide a molecular basis for understanding the expression of circadian timing in injured organisms.

Characterization of lymphocyte subsets over a 24-hour period in Pineal-Associated Lymphoid Tissue (PALT) in the chicken

BACKGROUND

Homeostatic trafficking of lymphocytes in the brain has important relevance to the understanding of CNS disease processes. The pineal gland of the chicken contains large accumulations of lymphocytes that suggest an important role related to homeostatic circadian neuro-immune interactions. The purpose of this initial study was to characterize the lymphocyte subsets in the pineal gland and quantitate the distribution and frequency of lymphocyte phenotypes at two time points over the 24-hour light:dark cycle.

RESULTS

PALT comprised approximately 10% of the total pineal area. Image analysis of immunocytochemically stained sections showed that the majority of lymphocytes were CD3+ (80%) with the remaining 20% comprising B-cells and monocytes (Bu-1+), which tended to distribute along the periphery of the PALT. T-cell subsets in PALT included CD4+ (75-80%), CD8+ (20-25%), TCRalphabeta/Vbeta1+ (60%), and TCRgammadelta+ (15%). All of the T-cell phenotypes were commonly found within the interfollicular septa and follicles of the pineal gland. However, the ratios of CD8+/CD4+ and TCRgammadelta+/TCRalphabeta/Vbeta1+ within the pineal tissue were each 1:1, in contrast to the PALT where the ratios of CD8+/CD4+ and TCRgammadelta+/TCRalphabeta/Vbeta1+ each approximated 1:4. Bu-1+ cells were only rarely seen in the pineal interstitial spaces, but ramified Bu-1+ microglia/macrophages were common in the pineal follicles. Effects of the 24-h light:dark cycle on these lymphocyte-pineal interactions were suggested by an increase in the area of PALT, a decline in the density of TCRalphabeta/Vbeta1+ cells, and a decline in the area density of Bu-1+ microglia at the light:dark interphase (1900 h) compared to the dark:light interphase (0700 h).

CONCLUSION

The degree of lymphocyte infiltration in the pineal suggests novel mechanisms of neuro-immune interactions in this part of the brain. Our results further suggest that these interactions have a temporal component related to the 24-hour light:dark cycle and that CD8+ and TCRgammadelta+ T-cells are preferentially recruited to the pineal follicles. Pineal microglia/macrophages were common and represent an important candidate for mediating these lymphocyte-pineal interactions via secretion of cytokines and chemokines.

Reactive changes of interstitial glia and pinealocytes in the rat pineal gland challenged with cell wall components from gram-positive and -negative bacteria

Lipopolysaccharide (LPS), the major proinflammatory component of gram-negative bacteria, is well known to induce sepsis and microglial activation in the CNS. On the contrary, the effect of products from gram-positive bacteria especially in areas devoid of blood-brain barrier remains to be explored. In the present study, a panel of antibodies, namely, OX-6, OX-42 and ED-1 was used to study the response of microglia/macrophages in the pineal gland of rats given an intravenous LPS or lipoteichoic acid (LTA). These antibodies recognize MHC class II antigens, complement type 3 receptors and unknown lysosomal proteins in macrophages, respectively. In rats given LPS (50 microg/kg) injection and killed 48 h later, the cell density and immunoexpression of OX-6, OX-42 and ED-1 in pineal microglia/macrophages were markedly increased. In rats receiving a high dose (20 mg/kg) of LTA, OX-42 and OX-6, immunoreactivities in pineal microglia/macrophages were also enhanced, but that of ED-1 was not. In addition, both bacterial toxins induced an increase in astrocytic profiles labelled by glial fibrillary acid protein. An interesting feature following LPS or LTA treatment was the lowering effect on serum melatonin, enhanced serotonin immunolabelling and cellular vacuolation as studied by electron microscopy in pinealocytes. The LPS- or LTA-induced vacuoles appeared to originate from the granular endoplasmic reticulum as well as the Golgi saccules. The present results suggest that LPS and LTA could induce immune responses of microglia/macrophages and astroglial activation in the pineal gland. Furthermore, the metabolic and secretory activity of pinealocytes was modified by products from both gram-positive and -negative bacteria.

Regional heterogeneity in immunoreactive macrophages/microglia in the rat pineal gland

Using specific macrophage antibodies (OX-42, OX-6, ED-1 and ED-2), this study examined the distribution of macrophages/microglia in the pineal gland of adult rats. Except for ED-2, all antibodies labeled distinct subpopulations of macrophages/microglia in the gland; ED-2 labeling was hardly detectable. The quantitative study showed that the pineal macrophages/microglia (PMM) expressing complement type 3 receptors (OX-42) were more numerous than those expressing the major histocompatibility complex class II antigen (OX-6) or unknown cytoplasmic/lysosomal antigens (ED-1). The PMM were ubiquitous, especially the OX-42 labeled cells which were distributed from the dorsal to the ventral aspect of the gland. The macrophages/microglia labeled with OX-6 or ED-1 were localized mainly in the intermediate portion of the pineal gland. Immunolabeled cells were sparsely distributed in the distal portion of the pineal gland. A notable feature was that the OX-6 labeled macrophages/microglia showed a proximal-distal gradient in cell density. Another interesting feature was the occurrence of prominent cell aggregations around the larger blood vessels. These cells were mostly round and exhibited different immunoreactivity. Confocal microscopic study with triple immunolabeling further revealed that individual PMM cell possessed two or more different antigens (ED-1+/OX-6+, OX-42+/OX-6+ or OX-42+/ED-1+). Remarkably, a large population co-expressed ED-1+/OX-6+/OX-42+. The present results show that the expression of immunoreactive molecules in PMM varies in topographical distribution of the cells. It is suggested that this may be linked to their immunoregulatory functions in the gland.

Effect of endotoxin on expression of TNF receptors and transport of TNF-alpha at the blood-brain barrier of the rat

The transport mechanism mediating brain uptake of tumor necrosis factor (TNF)-alpha has been studied. When (125)I-labeled rat TNF-alpha was used in internal carotid artery perfusions in rats, the cytokine showed transcytosis through the blood-brain barrier in intact form (permeability-surface area product 0.34 +/- 0.13 microl. min(-1). g(-1)). Uptake was inhibited by low nanomolar concentrations of unlabeled rat TNF-alpha. Human TNF-alpha, which does not interact with the p80 TNF receptor in rodents, showed no brain uptake. mRNA expression of both p60 and p80 receptors could be demonstrated in native brain microvessel preparations. These transcripts increased to 149% (p60) and 127% (p80) of control 4 h after a systemic immune stimulation (2 mg/kg bacterial endotoxin ip). Lipopolysaccharide treatment did not alter the rate of brain uptake of TNF-alpha measured between 4 and 24 h later. In conclusion, a receptor-mediated mechanism is responsible for the transcytosis of TNF-alpha. Saturable transport, requiring the p80 receptor, occurs at concentrations encountered under pathophysiological conditions and therefore constitutes a relevant mechanism of communication between the immune system and the brain.