Endocannabinoids and GABAergic Control of Plasticity

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DESCRIPTION (provided by applicant): Cannabinoids, the bioactive ingredients in marijuana and hashish, affect the brain by acting at specific, membrane bound. G-protein-coupled receptors (CB1s). Endogenous ligands for CB1 exist (endocannabinoids), but information on receptors, receptor localization, endogenous agonists and associated second-messenger systems does not reveal the detailed workings of the endocannabinoid system, i.e., the cells that release endocannabinoids and the cells that respond to them. At the cellular level almost nothing is known about endocannabinoid release or its functional roles. Depolarization of hippocampal CA 1 pyramidal neurons releases endocannabinoids, which bind to CB 1s on presynaptic terminals of a sub-class of interneurons synapsing on them. Activation of CB1s causes a transient decrease in GABA release. This process, called DSI (depolarization-induced suppression of inhibition), represents a unique means for detecting the release and studying the actions of endocannabinoids. Using electrophysiological and optical recording techniques on hippocampal slices from normal rats and mice, and from genetically altered mice, we will test the hypothesis that neuronal excitability is governed by the endocannabinoid system. Details of the functional roles of endocannabmoids in CAl DSI remain unclear. Moreover, endocannabinoid receptors exist in other parts of the brain, and it is not known if they perform the same functions everywhere. The dentate gyrus (DG) differs from CA 1 in many ways, including neuronal circuitry and mechanisms of plasticity, such as long-term potentiation (LTP). CB 1s in the DG are associated with the same class of interneurons as in CAl. The DG is an ideal model for testing the generality of the endocannabinoid hypothesis. The DG is a gateway to the hippocampus from extrahippocampal areas and so understanding how plasticity is controlled there will be especially important. Finally, a major scientific and clinical problem is "tolerance" to the application of exogenous cannabinoids. It is not known if tolerance to exogenous cannabinoids affects the endocannabinoid system. Our Specific Aims are to test the hypotheses: 1. That endocannabinoids are necessary and sufficient to mediate DSI. 2. That endocannabinoid release facilitates NMDAR-dependent LTP induction in CAl. 3. That mediation of DSI is a function of endocannabinoids in dentate gyrus. 4. That endocannabinoid release facilitates induction of LTP in dentate gyrus. 5. That the development of tolerance to exogenous cannabinoids affects the endocannabinoid system.