Jacob Welch, 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. Plasticity can either strengthen or weaken synapses, known as long-term potentiation (LTP) or long-term depression (LTD) respectively. While many forms of plasticity are NMDA-dependent, recently endocannabinoids were identified to mediate several new forms of hippocampal synaptic plasticity through the CB1 and TRPV1 receptors. However, research has demonstrated a non-CB1/TRPV1-dependent endocannabinoid synaptic plasticity in the hippocampus. Several potential candidate receptors that bind the endocannabinoid anandamide have been identified. These are among the orphan G-protein coupled receptors (GPRs) whose distribution in the brain and/or function is less well known. GPR55 is of particular interest as it activates second messenger systems. Using quantitative RT-PCR, electrophysiological and memory behavioral tasks we examined hippocampal GPR55 expression and function. GPR55 is 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 significant enhancement of LTP in brain slices. This LPI effect was not noted in GPR55 knock-out (KO) mice, which exhibit significantly (p < 0.05) smaller LTP (146%) than wildtype (WT) (181%). GPR55 also appears to increase release probability (Sylantyev et al., PNAS, 2013), denoting a presynaptic role. Paired-pulse ratios are now being analyzed between GPR55 KO and WT mice to confirm this finding; however we did not note a change in EPSCs in CA1 in response to 2μM LPI. These data suggest GPR55 is expressed and physiologically relevant in the hippocampus. Because enhanced LTP is usually associated with better memory performance in rodents, this provides a potential target to enhance the cellular mechanism associated with memory formation.