Cytokine-induced release of ceramide-enriched exosomes as a mediator of cell death signaling in an oligodendroglioma cell line.

Th1 pro-inflammatory cytokines, i.e., TNF-α and IFN-γ, in combination are known to induce cell death in several cell types, including oligodendrocytes, but the mechanism of their synergistic cytotoxicity is unclear. Although ceramide (Cer) has been implicated in cytokine- and stress-induced cell death, its intracellular levels alone cannot explain cytokine synergy. We considered the possibility that Cer released as part of extracellular vesicles may contribute to cytokine-induced synergistic cell death. Using a human oligodendroglioma (HOG) cell line as a model, here we show that exosomes derived from TNF-α-treated "donor" cells, while being mildly toxic to fresh cultures (similar to individual cytokines), induce enhanced cell death when added to IFN-γ-primed target cultures in a fashion resembling the effect of cytokine combination. Further, the sphingolipid profiles of secreted exosomes, as determined by HPLC-MS/MS, revealed that the treatment with the cytokines time-dependently induced the formation and exosomal release, in particular of C16-, C24-, and C24:1-Cer species; C16-, C24-, and C24:1-dihydroCer species; and C16-, C24-, and C24:1-SM species. Finally, exogenous C6-Cer or C16-Cer mimicked and enhanced the cytotoxic effects of the cytokines upon HOG cells, thereby supporting the cell death-signaling role of extracellular Cer.

Optogenetic stimulation of astrocytes in the posterior hypothalamus increases sleep at night in C57BL/6J mice.

A distributed network of neurons regulates wake, non-rapid eye movement (NREM) sleep, and REM sleep. However, there are also glia in the brain, and there is growing evidence that neurons and astroglia communicate intimately to regulate behaviour. To identify the effect of optogenetic stimulation of astrocytes on sleep, the promoter for the astrocyte-specific cytoskeletal protein, glial fibrillary acidic protein (GFAP) was used to direct the expression of channelrhodopsin-2 (ChR2) and the linked reporter gene, enhanced yellow fluorescent protein (EYFP), in astrocytes. rAAV-GFAP-ChR2 (H134R)-EYFP or rAAV-GFAP-EYFP was microinjected (750 nL) into the posterior hypothalamus (bilateral) of mice. Three weeks later baseline sleep was recorded (0 Hz) and 24 h later optogenetic stimulation applied during the first 6 h of the lights-off period. Mice with ChR2 were given 5, 10 or 30 Hz stimulation for 6 h (10-ms pulses; 1 mW; 1 min on 4 min off). At least 36 h elapsed between the stimulation periods (5, 10, 30 Hz) and although 0 Hz was always first, the order of the other three stimulation rates was randomised. In mice with ChR2 (n = 7), 10 Hz, but not 5 or 30 Hz stimulation increased both NREM and REM sleep during the 6-h period of stimulation. Delta power did not increase. In control mice (no ChR2; n = 5), 10 Hz stimulation had no effect. This study demonstrates that direct stimulation of astrocytes powerfully induces sleep during the active phase of the sleep-wake cycle and underlines the inclusion of astrocytes in network models of sleep-wake regulation.

The role of tumor progression locus 2 protein kinase in glial inflammatory response.

Tumor progression locus 2 (Tpl2)/cancer Osaka thyroid kinase is a newer member of MAP3K family that is now known for its essential role in tumor necrosis factor-aplha (TNFα) expression in macrophages, but its pro-inflammatory signaling, if any, in glia is unknown. When cultures of murine microglia and astrocytes were exposed to lipopolysaccharide, there was a rapid activation (i.e., phosphorylation) of Tpl2 in parallel to the activation of down-stream effector MAPKs, that is, extracellular signal regulated kinase (ERK), p38 MAPK and C-Jun N-terminal kinase (JNK). Pre-incubation of the cultures with a Tpl2 inhibitor selectively suppressed the activation of the primary down-stream target, that is, ERK relative to p38 MAPK and JNK. That Tpl2 activation was functionally involved in glial inflammatory response was indicated by a reduced release of the cytokines, i.e. TNFα and the expression of inducible nitric oxide synthase in the presence of the kinase inhibitor. Furthermore, over-expression of a wild-type Tpl2 construct in C-6 glia resulted in an enhanced transcriptional activation of inducible nitric oxide synthase, while transfection with a dominant negative form of Tpl-2 had the opposite effect. The findings assign an important pro-inflammatory signaling function for Tpl2 pathway in glial cells.

Engineering an in situ crosslinkable hydrogel for enhanced remyelination.

Remyelination has to occur to fully regenerate injured spinal cords or brain tissues. A growing body of evidence has suggested that exogenous cell transplantation is one promising strategy to promote remyelination. However, direct injection of neural stem cells or oligodendrocyte progenitor cells (OPCs) to the lesion site may not be an optimal therapeutic strategy due to poor viability and functionality of transplanted cells resulted from the local hostile tissue environment. The overall objective of this study was to engineer an injectable biocompatible hydrogel system as a supportive niche to provide a regeneration permissive microenvironment for transplanted OPCs to survive, functionally differentiate, and remyelinate central nervous system (CNS) lesions. A highly biocompatible hydrogel, based on thiol-functionalized hyaluronic acid and thiol-functionalized gelatin, which can be crosslinked by poly-(ethylene glycol) diacrylate (PEGDA), was used. These hydrogels were optimized first regarding cell adhesive properties and mechanical properties to best support the growth properties of OPCs in culture. Transplanted OPCs with the hydrogels optimized in vitro exhibited enhanced survival and oligodendrogenic differentiation and were able to remyelinate demyelinated axons inside ethidium bromide (EB) demyelination lesion in adult spinal cord. This study provides a new possible therapeutic approach to treat CNS injuries in which cell therapies may be essential.

cAMP-induced decrease in cell-surface laminin receptor and cellular prion protein attenuates amyloid-ß uptake and amyloid-ß-induced neuronal cell death.

Previous studies have shown that amyloid-ß oligomers (AßO) bind with high affinity to cellular prion protein (PrPC ). The AßO-PrPC complex binds to cell-surface co-receptors, including the laminin receptor (67LR). Our current studies revealed that in Neuroscreen-1 cells, 67LR is the major co-receptor involved in the cellular uptake of AßO and AßΟ-induced cell death. Both pharmacological (dibutyryl-cAMP, forskolin and rolipram) and physiological (pituitary adenylate cyclase-activating polypeptide) cAMP-elevating agents decreased cell-surface PrPC and 67LR, thereby attenuating the uptake of AßO and the resultant neuronal cell death. These cAMP protective effects are dependent on protein kinase A, but not dependent on the exchange protein directly activated by cAMP. Conceivably, cAMP protects neuronal cells from AßO-induced cytotoxicity by decreasing cell-surface-associated PrPC and 67LR.

Targets for AD treatment: conflicting messages from γ-secretase inhibitors.

Current evidence suggests that Alzheimer's disease (AD) is a multi-factorial disease that starts with accumulation of multiple proteins. We have previously proposed that inhibition of γ-secretase may impair membrane recycling causing neurodegeneration starting at synapses (Sambamurti K., Suram A., Venugopal C., Prakasam A., Zhou Y., Lahiri D. K. and Greig N. H. A partial failure of membrane protein turnover may cause Alzheimer's disease: a new hypothesis. Curr. Alzheimer Res., 3, 2006, 81). We also proposed familal AD mutations increase Aß42 by inhibiting γ-secretase. Herein, we discuss the failure of Eli Lilly's γ-secretase inhibitor, semagacestat, in clinical trials in the light of our hypothesis, which extends the problem beyond toxicity of Aß aggregates. We elaborate that γ-secretase inhibitors lead to accumulation of amyloid precursor protein C-terminal fragments that can later be processed by γ-secretase to yields bursts of Aß to facilitate aggregation. Although we do not exclude a role for toxic Aß aggregates, inhibition of γ-secretase can affect numerous substrates other than amyloid precursor protein to affect multiple pathways and the combined accumulation of multiple peptides in the membrane may impair its function and turnover. Taken together, protein processing and turnover pathways play an important role in maintaining cellular homeostasis and unless we clearly see consistent disease-related increase in their levels or activity, we need to focus on preserving their function rather than inhibiting them for treatment of AD and similar diseases.

Linking cardiometabolic disorders to sporadic Alzheimer's disease: a perspective on potential mechanisms and mediators.

There is increasing evidence that the incidence of Alzheimer's disease (AD) is significantly influenced by cardiovascular risk factors in association with a cluster of metabolic diseases including diabetes and atherosclerosis. The shared risk is also reflected in the dietary and lifestyle links to both metabolic disorders and AD-type cognitive dysfunction. Recent studies with genetic and diet-induced animal models have begun to illuminate convergent mechanisms and mediators between these two categories of disease conditions with distinct tissue-specific pathologies. Although it is clear that peripheral inflammation and insulin resistance are central to the pathogenesis of the disorders of metabolic syndrome, it seems that the same mechanisms are also in play across the blood-brain barrier that lead to AD-like molecular and cognitive changes. This review highlights these convergent mechanisms and discusses the role of cerebrovascular dysfunction as a conduit to brain emergence of these pathogenic processes that might also represent future therapeutic targets in AD in common with metabolic disorders.

Mild pericyte deficiency is associated with aberrant brain microvascular flow in aged PDGFRß+/- mice.

The receptor tyrosine kinase PDGFRß is essential for pericyte migration to the endothelium. In mice lacking one allele of PDGFRß (PDGFRß+/-), previous reports have described an age-dependent loss of pericytes in the brain, leading to cerebrovascular dysfunction and subsequent neurodegeneration reminiscent of that seen in Alzheimer's disease and vascular dementia. We examined 12-20-month-old PDGFRß+/- mice to better understand how pericyte loss affects brain microvascular structure and perfusion in vivo. We observed a mild reduction of cortical pericyte number in PDGFRß+/- mice (27% fewer cell bodies) compared to controls, but no decrease in pericyte coverage of the endothelium. This mild degree of pericyte loss caused no discernable change in cortical microvascular density, length, basal diameter or reactivity to hypercapnia. Yet, it was associated with an increase in basal blood cell velocity, primarily in pre-capillary arterioles. Taken together, our results suggest that mild pericyte loss can lead to aberrant cerebral blood flow despite a lack of apparent effect on microvascular structure and reactivity.

Organizational hierarchy and structural diversity of microvascular pericytes in adult mouse cortex.

Smooth muscle cells and pericytes, together called mural cells, coordinate many distinct vascular functions. Canonically, smooth muscle cells are ring-shaped and cover arterioles with circumferential processes, whereas pericytes extend thin processes that run longitudinally along capillaries. In between these canonical mural cell types are cells with features of both smooth muscle cells and pericytes. Recent studies suggest that these transitional cells are critical for controlling blood flow to the capillary bed during health and disease, but there remains confusion on how to identify them and where they are located in the brain microvasculature. To address this issue, we measured the morphology, vascular territory, and α-smooth muscle actin content of structurally diverse mural cells in adult mouse cortex. We first imaged intact 3D vascular networks to establish the locations of major gradations in mural cell appearance as arterioles branched into capillaries. We then imaged individual mural cells occupying the regions within these gradations. This revealed two transitional cells that were often similar in appearance, but with sharply contrasting levels of α-smooth muscle actin. Our findings highlight the diversity of mural cell morphologies in brain microvasculature, and provide guidance for identification and categorization of mural cell types.

High cholesterol-induced neuroinflammation and amyloid precursor protein processing correlate with loss of working memory in mice.

Recent findings suggest that hypercholesterolemia may contribute to the onset of Alzheimer's disease-like dementia but the underlying mechanisms remain unknown. In this study, we evaluated the cognitive performance in rodent models of hypercholesterolemia in relation to neuroinflammatory changes and amyloid precursor protein (APP) processing, the two key parameters of Alzheimer's disease pathogenesis. Groups of normal C57BL/6 and low density lipoprotein receptor (LDLR)-deficient mice were fed a high fat/cholesterol diet for an 8-week period and tested for memory in a radial arm maze. It was found that the C57BL/6 mice receiving a high fat diet were deficient in handling an increasing working memory load compared with counterparts receiving a control diet while the hypercholesterolemic LDLR-/- mice showed impaired working memory regardless of diet. Immunohistochemical analysis revealed the presence of activated microglia and astrocytes in the hippocampi from high fat-fed C57BL/6 mice and LDLR-/- mice. Consistent with a neuroinflammatory response, the hyperlipidemic mice showed increased expression of cytokines/mediators including tumor necrosis factor-alpha, interleukin-1beta and -6, nitric oxide synthase 2, and cycloxygenase 2. There was also an induced expression of the key APP processing enzyme i.e. beta-site APP cleaving enzyme 1 in both high fat/cholesterol-fed C57BL/6 and LDLR-/- mice accompanied by an increased generation of C-terminal fragments of APP. Although ELISA for beta-amyloid failed to record significant changes in the non-transgenic mice, a threefold increase in beta-amyloid 40 accumulation was apparent in a strain of transgenic mice expressing wild-type human APP on high fat/cholesterol diet. The findings link hypercholesterolemia with cognitive dysfunction potentially mediated by increased neuroinflammation and APP processing in a non-transgenic mouse model.

Pericyte Structural Remodeling in Cerebrovascular Health and Homeostasis.

The biology of brain microvascular pericytes is an active area of research and discovery, as their interaction with the endothelium is critical for multiple aspects of cerebrovascular function. There is growing evidence that pericyte loss or dysfunction is involved in the pathogenesis of Alzheimer's disease, vascular dementia, ischemic stroke and brain injury. However, strategies to mitigate or compensate for this loss remain limited. In this review, we highlight a novel finding that pericytes in the adult brain are structurally dynamic in vivo, and actively compensate for loss of endothelial coverage by extending their far-reaching processes to maintain contact with regions of exposed endothelium. Structural remodeling of pericytes may present an opportunity to foster pericyte-endothelial communication in the adult brain and should be explored as a potential means to counteract pericyte loss in dementia and cerebrovascular disease. We discuss the pathophysiological consequences of pericyte loss on capillary function, and the biochemical pathways that may control pericyte remodeling. We also offer guidance for observing pericytes in vivo, such that pericyte structural remodeling can be more broadly studied in mouse models of cerebrovascular disease.

Dynamic Remodeling of Pericytes In Vivo Maintains Capillary Coverage in the Adult Mouse Brain.

Direct contact and communication between pericytes and endothelial cells is critical for maintenance of cerebrovascular stability and blood-brain barrier function. Capillary pericytes have thin processes that reach hundreds of micrometers along the capillary bed. The processes of adjacent pericytes come in close proximity but do not overlap, yielding a cellular chain with discrete territories occupied by individual pericytes. Little is known about whether this pericyte chain is structurally dynamic in the adult brain. Using in vivo two-photon imaging in adult mouse cortex, we show that while pericyte somata were immobile, the tips of their processes underwent extensions and/or retractions over days. The selective ablation of single pericytes provoked exuberant extension of processes from neighboring pericytes to contact uncovered regions of the endothelium. Uncovered capillary regions had normal barrier function but were dilated until pericyte contact was regained. Pericyte structural plasticity may be critical for cerebrovascular health and warrants detailed investigation.

A role for p38 mitogen-activated protein kinase in the regulation of the serotonin transporter: evidence for distinct cellular mechanisms involved in transporter surface expression.

The serotonin transporter (SERT) is regulated by various signaling mechanisms that may operate to maintain appropriate levels of synaptic serotonin (5-HT). We demonstrate that one of the mitogen-activated protein kinases (MAPKs), p38 MAPK, regulates SERT. Treatment of rat midbrain synaptosomes with p38 MAPK-specific inhibitors, PD169316 [4-(4-fluorophenyl)-2-(4-nitrophenyl)-5-(4-pyridyl)-1H-imidazole] or SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole], reduced 5-HT uptake. An additive SERT inhibition by PD169316 and beta-phorbol 12-myristate 13-acetate (beta-PMA) indicated the involvement of a protein kinase C (PKC)-independent MAPK pathway. Kinetic studies indicated a significant decrease in the transport capacity (V(max)) after PD169316 treatment of synaptosomes. Biotinylation studies showed reduced SERT proteins in the plasma membrane of synaptosomes after p38 MAPK inhibition and PKC activation. Phosphorylation studies using synaptosomes revealed decreased SERT phosphorylation by PD169316 but increased phosphorylation by beta-PMA. d-Amphetamine enhanced SERT basal phosphorylation and PD169316 blocked this effect. SERT interaction with protein phosphatase 2A catalytic subunit and syntaxin 1A decreased after PD169316 or beta-PMA treatment of synaptosomes. In synaptosomes, PKC activation but not p38 MAPK inhibition resulted in SERT redistribution from cholesterolrich lipid raft fractions to nonlipid raft fractions. The presence of phospho-p38 MAPK in synaptosomes and human embryonic kidney 293 (HEK-293) cells suggested the presence of constitutively active p38 MAPK in these preparations. Cotransfection of HEK-293 cells with SERT and a constitutively active form of MAP kinase kinase 3b(E) [MKK3b(E)] increased 5-HT transport, and RNA interference targeted to p38 MAPK inhibited 5-HT uptake, confirming the involvement of active p38 MAPK in SERT expression. Although PD169316 inhibited SERT insertion to the plasma membrane, beta-PMA increased SERT internalization in HEK-293 cells. Together, these results indicate a distinct role of p38 MAPK in SERT regulation.

Hypoxia/reoxygenation differentially modulates NF-kappaB activation and iNOS expression in astrocytes and microglia.

Hypoxia/ischemic brain injury accompanies an inflammatory response involving an activation of glial cells. This study, using an in vitro model, investigated the signaling mechanisms mediating hypoxic responses of the two glial cell types (astrocytes and microglia) in relation to the expression of inducible nitric oxide synthase (iNOS). In cultures of rat brain microglia and astrocytes, hypoxia (8 h) followed by reoxygenation (24 h) (H/O) had little (microglia) or no (astrocytes) effect on the expression of iNOS. However, H/O elicited opposite effects on lipopolysaccharide (LPS) induction of iNOS in the two cell types: it potentiated LPS induction of iNOS in microglia but inhibited this response in astrocytes. Similar differential effects of hypoxia were observed on the production of tumor necrosis factor-alpha (TNFalpha). In contrast, there was an upregulation of hemoxygenase- 1 (HO-1), a counter-regulatory pathway, with astrocytes showing a bigger induction than microglia. While hypoxic activation of mitogen-activated protein kinases (MAPKs) was similar in the two glial types, the activation pattern of NFkappaB was clearly different: hypoxia stimulated the activation of NFkappaB pathway and NFkappaB-dependent transcription in microglia but not in astrocytes. Lastly, the two cell types displayed differential vulnerabilities to hypoxia-induced cell death, the astrocytes being relatively more resistant than microglia.

C-Jun N-terminal kinase (JNK) regulation of iNOS expression in glial cells: predominant role of JNK1 isoform.

The mitogen-activated protein kinases (MAPKs) play a central role in mediating the activation and transcriptional responses of diverse cells, including glia. c-Jun N-terminal kinase (JNK), a member of the MAPK family, is activated by a variety of stress and proinflammatory signals and in turn phosphorylates its downstream substrates including nuclear factors, leading to transcriptional activation of target genes. There are at least three subtypes of JNK (i.e., JNKs 1-3) that may play isoform-specific roles. This study examined the role of JNK isoforms in the induction of inducible nitric oxide synthase (iNOS) in astrocytes in response to lipopolysachharide (LPS) and interferon (IFN)-gamma. While an inhibitor of the JNK pathway (SP600125) inhibited iNOS expression, ectopic expression of a constitutively active form of MEKK1 (MAPK/ERK kinase kinase- 1), an upstream activator of JNK, led to an induction of co-transfected iNOS promoter activity and, in the presence of LPS, to an enhanced expression of iNOS. RNA knockdown studies with JNK subtype-specific short-interfering RNA (siRNA), indicated that JNK1- but not JNK2- nor JNK3-specific siRNA, interfered with LPS/IFNgamma induction of iNOS. It is concluded that, of the three JNK forms, JNK1 is the major mediator of iNOS induction and perhaps, inflammatory signaling in general, in glial cells.

MAP kinase regulation of IP10/CXCL10 chemokine gene expression in microglial cells.

Interferongamma inducible protein-10 (IP10 or CXCL10), a Th-1 affiliated chemokine, is expressed by activated glial cells and may contribute to the trafficking of immune cells in the inflamed central nervous system. This study examines the regulation of the expression of this chemokine in cultured microglial cells focusing on the roles of mitogen-activated protein (MAP) kinase cascades. Exposure of a mouse microglial cell line, BV-2, to lipopolysaccharide (LPS) and IFNgamma led to an induction of IP10 mRNA and protein as determined by RT-PCR and ELISA, respectively. This induction was suppressed by pharmacological inhibitors of p38 MAPK (i.e., SB203580) and c-Jun N-terminal kinase (JNK, SP600125), suggesting the involvement of the two kinases in IP10 expression. LPS also induced the activity of an IP10 promoter reporter (luciferase) construct transfected into BV-2 cells in a MAP kinase- and NFkappaB-dependent manner. The use of deletion constructs revealed that the kinase-targeted sequences were within the region between -533 bp and -332 bp upstream of the transcriptional start site. Co-transfection of IP10 luciferase with the active forms of the upstream kinases in the MAP kinase cascades, i.e., MAPK kinase-3 (MKK3), MKK6 (the immediately upstream activators of p38 kinase) and a MAP3K, i.e., TGFbeta-activated kinase-1 (TAK1), produced a marked stimulation of the promoter activity. The results of this study indicate that the MAP kinase cascades prominently regulate IP10 gene expression in microglial cells.

Vasculoprotection as a Convergent, Multi-Targeted Mechanism of Anti-AD Therapeutics and Interventions.

Using a variety of animal models of Alzheimer's disease (AD), there have been a number of recent studies reporting varying degrees of success with anti-AD therapeutics. The efficacies are often discussed in terms of the modulatory effects of the compounds tested on identified or assumed targets among the known (or proposed) pathogenic and neuroprotective mechanisms, largely within the context of the dominant amyloid cascade hypothesis. However, it is clear that several of the relatively more efficacious treatments tend to be multifunctional and target multiple pathological processes associated with AD including most commonly, oxidative and metabolic stress and neuroinflammation. Increasing evidence suggests that vascular and neurodegenerative pathologies often co-exist and that neurovascular dysfunction plays a critical role in the development or progression of AD. In this review, we will discuss the significance of vasculoprotection or neurovascular unit integrity as a common, multi-targeted mechanism underlying the reported efficacy of a majority of anti-AD therapeutics--amyloid-targeted or otherwise--while providing a strong support for future neurovascular-based treatment strategies and interventions.

Cholesterol and Alzheimer's disease: clinical and experimental models suggest interactions of different genetic, dietary and environmental risk factors.

Alzheimer's disease (AD) is a progressive senile dementia characterized by deposition of a 4 kDa peptide of 39-42 residues known as amyloid beta-peptide (Abeta) in the form of senile plaques and the microtubule associated protein tau as paired helical filaments. Genetic studies have identified mutations in the Abeta precursor protein (APP) as the key triggers for the pathogenesis of AD. Other genes such as presenilins 1 and 2 (PS1/2) and apolipoprotein E (APOE) also play a critical role in increased Abeta deposition. Several biochemical and molecular studies using transfected cultured cells and transgenic animals point to mechanisms by which Abeta is generated and aggregated to trigger the neurodegeneration that may cause AD. Three important enzymes collectively known as 'secretases' participate in APP processing leading to the generation of either Abeta or non-amyloid proteins. However, the mechanisms of neurotoxicity of Abeta and the role of APP function in AD remain important unanswered questions. Although early studies recognized the loss of cholesterol and other lipids in the brain, these findings have been poorly connected with AD pathogenesis, despite the identification of the epsilon4 allele of APOE as a major risk factor in AD. The recent finding that cholesterol can modulate the yield of potentially toxic Abeta has boosted research on its role in AD. Consequently, several cholesterol-reducing drugs are currently being evaluated for the treatment of AD. The present review summarizes our current understanding of the relationship of AD pathogenesis with cholesterol, lipids and other genetic and environmental risk factors.

Expression of the CB1 cannabinoid receptor in macrophage-like cells from brain tissue: immunochemical characterization by fusion protein antibodies.

Antibodies designed to recognize a 74 amino acid sequence of the N- or C-terminal domain of the rat CB1 cannabinoid receptor detected a 58 kDa protein in immunoblots of brain and various cells known to express the CB1 cannabinoid receptor. A human B-lymphoblastoid cell line and macrophage-like cells from neonatal rat brain were also positive for CB1 receptor-like immunoreactivity. Immunocytochemical analysis performed with isolated Fab fragments showed surface staining in NG108-15 cells and brain macrophage like cells which also express MHC class II antigens. The data suggest a plausible role for CB1 receptors in the immune function of brain.