Calcium Waves in Pyramidal Neurons

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Postsynaptic [Ca2+]i changes in pyramidal neurons in the hippocampus play an important role in the induction of various forms of synaptic plasticity, gene expression, and modulation of membrane conductances. All of these mechanisms can affect the behavior of these neurons in circuits involved in learning and memory. Therefore, a detailed understanding of these processes is important for understanding brain function. There are three clear sources of Ca2+ in pyramidal neurons that can be activated by synaptic mechanisms in the hippocampus: Ca2+ entry through NMDA receptors, entry through voltage-dependent Ca2+ channels, and Ca2+ release from internal stores. We will investigate the properties and functions of Ca2+ released from stores mediated by IP3 in pyramidal cells in slices from the CA1 region of the hippocampus in Sprague Dawley rats. The first set of experiments will examine the properties of newly discovered spontaneous elementary events ("puffs") in the main dendrites of these neurons. These local events can be modulated by membrane potential and mGluR mediated synaptic transmission and could have important signaling functions by themselves. The second set of experiments will examine the properties of these events in the oblique dendrites, soma, and axon. Previous experiments established that Ca2+ release waves, which are probably built from these events, are not found in these regions. We will try to understand the restricted spatial distribution of waves and more widespread distribution of elementary events. The location and time course of these events will be examined with high speed imaging and 2-photon microscopy. Stimulation with synaptic transmission will be supplemented with focal uncaging of extracellular glutamate and carbachol and intracellular IP3 and Ca2+ to achieve precise localization of signaling events in thick or thin dendritic regions. We will investigate the function of Ca2+ released from stores in several important physiological processes, with particular emphasis on the different consequences of Ca2+ released in different dendritic regions. One set of experiments will examine the role of Ca2+ release in the induction of plasticity of cell excitability. A second set of experiments will examine the role of the Ca2+ waves in suppressing synaptic inhibition onto pyramidal neurons mediated by endogenous cannabinoids.