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.

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The presence of functional cannabinoid CB 2 receptors in the CNS has provoked considerable controversy over the past few years. Formerly considered as an exclusively peripheral receptor, it is now accepted that it is also present in limited amounts and distinct locations in the brain of several animal species, including humans. Furthermore, the inducible nature of these receptors under neuroinflammatory conditions, in contrast to CB 1 , makes them attractive targets for the development of novel therapeutic approaches. In fact, the undesired psychoactive effects caused by CB 1 activation have largely limited the clinical use of cannabinoid-related compounds that act on these receptors. In this review some recent findings on the antiinflammatory properties of CB 2 receptors are presented, as well as new perspectives that have been obtained based on studies of human postmortem brain samples. In addition, various working hypotheses are also proposed and discussed.

Increasing evidence supports the idea of a beneficial effect of cannabinoid compounds for the treatment of multiple sclerosis (MS).However, most experimental data come from animal models of MS. We investigated the status of cannabinoid CB 1 and CB 2 receptors and fatty acid amide hydrolase (FAAH) enzyme in brain tissue samples obtained from MS patients. Areas of demyelination were identified and classified as active, chronic, and inactive plaques. CB 1 and CB 2 receptors and FAAH densities and cellular sites of expression were examined using immunohistochemistry and immunofluorescence. In MS samples, cannabinoid CB 1 receptors were expressed by cortical neurons, oligodendrocytes, and also oligodendrocyte precursor cells, demonstrated using double immunofluorescence with antibodies against the CB 1 receptor with antibodies against type 2 microtubule-associated protein, myelin basic protein, and the platelet-derived growth factor receptor-␣, respectively. CB 1 receptors were also present in macrophages and infiltrated T-lymphocytes. Conversely, CB 2 receptors were present in T-lymphocytes, astrocytes, and perivascular and reactive microglia (major histocompatibility complex class-II positive) in MS plaques. Specifically, CB 2 -positive microglial cells were evenly distributed within active plaques but were located in the periphery of chronic active plaques. FAAH expression was restricted to neurons and hypertrophic astrocytes. As seen for other neuroinflammatory conditions, selective glial expression of cannabinoid CB 1 and CB 2 receptors and FAAH enzyme is induced in MS, thus supporting a role for the endocannabinoid system in the pathogenesis and/or evolution of this disease.