Bradley Prince, Brigham Young University
Life Sciences
Learning and memory occur due to adaptive brain changes in response to our environment. These changes are mediated by synaptic plasticity, particularly within the hippocampus, where spatial and declarative memories occur. Plasticity can either strengthen or weaken synapses, known as long-term potentiation (LTP) or long-term depression respectively. While many forms of synaptic plasticity are N-methyl-D-Aspartate receptor-dependent, recently endocannabinoids were identified to mediate several new forms of hippocampal synaptic plasticity. Endocannabinoids bind to receptors such as cannabinoid receptor 1 (CB1) and transient receptor potential vanilloid 1 (TRPV1), and mediate several forms of plasticity, including in the hippocampus. However, new research has demonstrated a non-CB1/TRPV1-dependent endocannabinoid synaptic plasticity in the hippocampus. While the receptor(s) involved is currently unknown, several potential candidate receptors that bind the endocannabinoid anandamide have been identified. These are orphan G-protein coupled receptors (GPRs) whose distribution in the brain and/or function is unknown. GPR55 is of particular interest as it activates second message systems, including increasing intracellular calcium. Using quantitative RT-PCR, electrophysiological and memory behavioral tasks we examined hippocampal GPR55 expression and function. GPR55 is indeed expressed in hippocampus of both rats and mice. Cellular expression is currently being examined and appears to be rare in interneurons and more likely expressed by pyramidal cells. Interestingly, application of the GPR55 agonist LPI (2 µM) to wild-type mice demonstrates a decrease of LTD in brain slices. This LPI effect was not noted in GPR55 knock-out mice in the presence of LPI. This data suggest GPR55 is physiologically relevant in the hippocampus. This is the first direct evidence we are aware of that a novel endocannabinoid receptor directly effects hippocampal LTD. Because neurodegeneration that affects memory is typically associated with an increase in LTD, this provides a potential target to slow the advance of diseases such as Alzheimer’s.