Synaptic transport of endocannabinoids in the brain

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Project Summary The endocannabinoid (eCB) system plays a key role in regulating synaptic function in the brain. Dysfunction of eCB signaling contributes to numerous psychiatric and neurological disorders including anxiety, depression, and autism. Consequently, the development of treatments for disorders involving eCB dysfunction requires a thorough understanding of the mechanisms regulating eCB signaling in the brain. It is well-established that physiological and/or pathological activation of postsynaptic neurons leads to the biosynthesis and release of the eCB 2-arachidonoylglycerol (2-AG). Once released, 2-AG traverses the synaptic cleft and activates cannabinoid receptors located on presynaptic axon terminals, which mediate its behavioral and physiological effects. Although considerable progress has been made in elucidating how 2-AG signaling controls synaptic function and behavioral outputs, the mechanism(s) governing synaptic 2-AG transport remains unknown, highlighting a major gap in our fundamental understanding of 2-AG signaling in the brain. The lipophilic nature of 2-AG limits its diffusion across the synapse, suggesting the existence of a carrier(s) that facilitates 2-AG transport to permit cannabinoid receptor activation. Identification of a synaptic 2-AG carrier would not only greatly enhance our basic understanding of 2-AG signaling but could also lead to the discovery of a new therapeutic target(s) to treat disorders involving eCB dysfunction. To that end, our group has recently identified fatty acid binding proteins (FABPs) as intracellular carriers for eCBs. In this application, we will build upon this progress and test the novel hypothesis that FABP5, secreted by astrocytes, functions as a synaptic carrier that is essential for 2-AG signaling in multiple brain areas. In Aim 1, we will employ complementary pharmacological and genetic approaches to test the hypothesis that FABP5 mediates retrograde 2-AG transport at inhibitory and excitatory synapses in the hippocampus and ventral tegmental area, brain areas involved in cognitive and emotional regulation. In Aim 2, we will employ our novel FABP5Flox/Flox mice to delineate the roles of astrocytic and neuronal FABP5 in controlling synaptic 2-AG transport. Aim 3 will characterize the contributions of intracellular and secreted FABP5 in mediating 2-AG transport at hippocampal and ventral tegmental area synapses. Successful completion of this proposal will position FABP5 as a synaptic carrier for 2-AG at central synapses, which will greatly enhance our basic understanding of eCB signaling and may facilitate the development of future therapeutics targeting disorders involving eCB dysfunction.