The endocannabinoid system is still poorly understood. Recently, the basic elements that constitute it, i.e., membrane receptors, endogenous ligands, and mechanisms for termination of the signaling process, have been partially characterized. There is a considerable lack of information, however, concerning the distribution, concentration, and function of those components in the human body, particularly during pathological events. We have studied the status of some of the components of the endocannabinoid system, fatty acid amide hydrolase and cannabinoid CB1 and CB2 receptors, in postmortem brains from patients with Alzheimer's disease. Using specific polyclonal antibodies, we have performed immunohistochemical analysis in hippocampus and entorhinal cortex sections from brains of Alzheimer's disease patients. Our results show that both fatty acid amide hydrolase and cannabinoid CB2 receptors are abundantly and selectively expressed in neuritic plaque-associated astrocytes and microglia, respectively, whereas the expression of CB1 receptors remains unchanged. In addition, the hydrolase activity seems to be elevated in the plaques and surrounding areas. Thus, some elements of the endocannabinoid system may be postulated as possible modulators of the inflammatory response associated with this neurodegenerative process and as possible targets for new therapeutic approaches.

The cannabinoid system is known to be important in neuronal regulation, but is also capable of modulating immune function. Although the CNS resident microglial cells have been shown to express the CB 2 subtype of cannabinoid receptor during non-immune-mediated pathological conditions, little is known about the expression of the cannabinoid system during immune-mediated CNS pathology. To examine this question, we measured CB 2 receptor mRNA expression in the CNS of mice with experimental autoimmune encephalomyelitis (EAE) and, by real-time PCR, found a 100-fold increase in CB 2 receptor mRNA expression during EAE onset. We next determined whether microglial cells specifically express the CB 2 receptor during EAE, and found that activated microglial cells expressed 10-fold more CB 2 receptor than microglia in the resting state. To determine the signals required for the up-regulation of the CB 2 receptor, we cultured microglial cells with combinations of c-interferon (IFN-c) and granulocyte) macrophage-colony stimulating factor (GM-CSF), which both promote microglial cell activation and are expressed in the CNS during EAE, and found that they synergized, resulting in an eight to 10-fold increase in the CB 2 receptor. We found no difference in the amount of the CB 2 receptor ligand, 2-arachidonylglycerol (2-AG), in the spinal cord during EAE. These data demonstrate that microglial cell activation is accompanied by CB 2 receptor up-regulation, suggesting that this receptor plays an important role in microglial cell function in the CNS during autoimmune-induced inflammation.

The cannabinoid system is immunomodulatory and has been targeted as a treatment for the central nervous system (CNS) autoimmune disease multiple sclerosis. Using an animal model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), we investigated the role of the CB(1) and CB(2) cannabinoid receptors in regulating CNS autoimmunity. We found that CB(1) receptor expression by neurons, but not T cells, was required for cannabinoid-mediated EAE suppression. In contrast, CB(2) receptor expression by encephalitogenic T cells was critical for controlling inflammation associated with EAE. CB(2)-deficient T cells in the CNS during EAE exhibited reduced levels of apoptosis, a higher rate of proliferation and increased production of inflammatory cytokines, resulting in severe clinical disease. Together, our results demonstrate that the cannabinoid system within the CNS plays a critical role in regulating autoimmune inflammation, with the CNS directly suppressing T-cell effector function via the CB(2) receptor.

The plant-derived cannabinoids ⌬ 9 -tetrahydrocannabinol (THC) and cannabidiol (CBD) both have immunosuppressive effects; although some effects of THC are mediated by the CB2 receptor, CB2 binds CBD weakly. In examining the effects of THC and CBD on microglial proliferation, we found that these compounds potently inhibit [ 3 H]thymidine incorporation into a murine microglial cell line with no effect on cell cycle. Because adenosine agonists have antiinflammatory effects, and because uptake of adenosine is a primary mechanism of terminating adenosine signaling, we tested the hypothesis that CBD is immunosuppressive because it enhances endogenous adenosine signaling. In vivo treatment with a low dose of CBD decreases TNF␣ production in lipopolysaccharide-treated mice; this effect is reversed with an A2A adenosine receptor antagonist and abolished in A2A receptor knockout mice. These studies demonstrate that CBD has the ability to enhance adenosine signaling through inhibition of uptake and provide a non-cannabinoid receptor mechanism by which CBD can decrease inflammation.adenosine ͉ lipopolysaccharide ͉ tetrahydrocannabinol ͉ thymidine ͉ tumor necrosis factor-␣ T he marijuana-derived cannabinoids ⌬ 9 -tetrahydrocannabinol (THC) and cannabidiol (CBD) each have immunosuppressive effects (1) and are currently in clinical trials for treatment of multiple sclerosis (2). Although the CB 2 cannabinoid receptor partially mediates the antiinflammatory effects of THC (3), CBD does not bind well to the known cannabinoid receptors (4); this low affinity results in the inability of CBD to produce the subjective ''high'' and cognitive effects that are characteristic of marijuana and THC (5, 6). To date, the mechanism by which CBD decreases inflammation is unknown, although micromolar concentrations of CBD have been shown to inhibit lipoxygenase activity (7).In addition to multiple sclerosis, CBD has shown promise in several rodent models of inflammation. Oral treatment with CBD decreases edema and hyperalgesia in a rat paw model of carrageenan-induced inflammation (8). A single dose of CBD also decreases serum TNF␣ production in lipopolysaccharide (LPS)-treated mice (9). CBD improves arthritis symptoms and joint pathology in murine collagen-induced arthritis while inhibiting IFN-␥ production and lymph node cell proliferation, as measured by [ 3 H]thymidine incorporation (9). Treatment with THC also decreases proliferation of lymph node cells as well as splenocytes (10).Given the potential of THC and CBD to decrease proliferation in immune cells, we initially examined the effects of several natural and synthetic cannabinoids on microglial cell proliferation by using [ 3 H]thymidine incorporation. Microglia are the resident immune cells of the brain, and their proliferation has been linked to a number of neurodegenerative diseases (11). However, CBD and THC decreased [ 3 H]thymidine incorporation into EOC-20 microglial cells with no effect on cell cycle. Further experiments demonstrate that this decrease in [ 3 H]thymidine incor...