Astrocytes are found throughout the brain and play well documented physiological roles. Emerging roles for astrocytes include signaling to and from neurons and regulation of local blood flow. Certain astrocyte functions are correlated with or regulated by cytosolic calcium transients, which are a physiological signal. During the previous grant cycle, we developed and used a membrane tethered genetically encoded calcium indicator called Lck-GCaMP3 with the aim of measuring astrocyte calcium transients. Using Lck-GCaMP3 we made the serendipitous discovery of a novel calcium signal in astrocytes due to transmembrane fluxes mediated by TRPA1 ion channels. Moreover, our ongoing experiments show that TRPA1 mediated calcium fluxes give rise to frequent and highly localized near membrane calcium signals that contribute significantly to the resting calcium levels of astrocytes not only within a single astrocyte, but also in a network of astrocytes and neurons in cell cultures, as well as in acute brain slices. Our preliminary data also show that pharmacological block or genetic deletion of TRPA1 channels reduced inhibitory synapse efficacy onto interneurons and long term synaptic potentiation of Schaffer collateral synapses onto pyramidal neurons. We have three specific aims with which we seek to further extend these findings, test novel hypotheses and evaluate the function of astrocyte TRPA1 mediated calcium signals.
In Aim 1 we will study near membrane calcium signals in astrocytes within hippocampal slices.
In Aim 2 we will employ a variety of methods to systematically evaluate why blocking astrocyte TRPA1 channels or buffering astrocyte calcium levels below rest reduces inhibitory synapse efficacy onto interneurons, but not pyramidal neurons in the stratum radiatum (s.r.) of the hippocampus.
In Aim 3 we will study how long-term potentiation (LTP) is reduced by blocking TRPA1 channels. By completing these experiments we will provide new information on the function of a novel astrocyte calcium signal. This information will contribute significantly to our understanding of astrocytes in neuronal networks and allow us and others to test novel hypotheses on the roles of near membrane Ca2+ signals in astrocyte-neuron signaling, and lay the foundations for determining if TRPA1 channels are valid drug targets in brain disorders that involve astrocytes.
We will study the physiological properties and functional roles of a novel type of calcium signal in astrocytes of the brain. Our data will provide new information to explore the roles of astrocytes in the normal healthy brain and in diseases of the nervous system, including the processes that lead to the development of neurological disorders. Our work is also highly relevant to all forms of brain damage.
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