Most excitatory synapses in the brain release glutamate. These synapses show a remarkable degree of plasticity, in which brief repetitive activation results in a long term potentiation or LTP. At most synapses LTP requires the activation of postsynaptic NMDA receptors. However, hippocampal mossy fiber synapses, exhibit a form of LTP that is independent of NMDA receptor activation. Our experiments during the past decade strongly suggest that mossy fiber LTP (mfLTP) is an entirely presynaptic form of plasticity. This grant will explore three separate, but overlapping, issues concerning the mechanisms underlying mfLTP. 1) The role of kainate receptors (KARs) in mfLTP will be explored, using KAR knockout mice and a novel KAR antagonist. Preliminary results suggest that KARs can control the threshold for the induction of mfLTP. 2) We have recently found that the hyperpolarization-activated nonselective cation current l-h mediates the expression of mfLTP. In our model we propose that the cAMP-dependent enhancement of l-h causes a depolarization of the mossy fibers which enhances transmitter release secondary to spike broadening. We will test this model with a number of experiments, a) If mfLTP is associated with a depolarization of the terminals, then the enhancement of synaptic transmission by K+ should occlude with mfLTP, b) We will record from mossy fiber boutons to directly determine if depolarization causes spike broadening and whether mfLTP is associated with a depolarization, c) Our model predicts that all synapses formed by dentate granule cells should show an LTP similar to that found at the mossy fiber to pyramidal cell synapse. However, others have reported that the synapses made by mossy fibers onto interneurons in s. lucidum do not show LTP. Thus we will analyze the properties of this synapse to understand why it fails to generate LTP. 3) We have preliminary results indicating that ambient extracellular adenosine exerts a profound tonic inhibition of mossy fiber transmitter release and, indeed, is necessary for mossy fibers to express both short-term and long-term plasticity. We will analyze the role that this tonic action has on the properties of mossy fiber synapses and why adenosine has such a pronounced and selective action on mossy fibers. These experiments will help elucidate the mechanisms involved in controlling transmitter release and more specifically how this control is involved in short term and long term synaptic plasticity. Our results should provide insight into the cellular mechanisms underlying learning and memory.
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