Research clearly demonstrates that astrocytes in situ sense neurotransmitters released during neuronal activity, and release neuroactive molecules in response to specific stimuli. What remains unclear is the level of astrocytic receptor activation under physiological conditions as well as the cellular and behavioral consequences of activating astrocytic receptors. During the past funding period we developed a system that enables us to uncage IP3 in astrocytic somata while recording the activity of adjacent CA1 pyramidal neurons using patch clamp electrodes. We find that uncaging IP3 in a small region of the astrocyte soma with a 15ms pulse of UV light leads to a Ca2+ wave that propagates throughout its fine processes but not into adjacent astrocytes. The generation of a Ca2+ wave within a single astrocyte leads to a 52% increase (p>.03) in the frequency and a 20% (p<.04) increase in the amplitude of AMP A receptor (AMPAR) mediated mEPSCs; Group 1 mGluR antagonists block these increases. These findings have led to the following model: 1) Increases in astrocytic Ca2+ lead to glutamate release that then activates presynaptic and postsynaptic Group 1 mGluRs, 2) Activating presynaptic Group 1 mGluRs leads to an increase in presynaptic Ca2+, an increase in the frequency of spontaneous glutamate release and a consequential increase in the frequency of postsynaptic AMPAR-mediated sEPSCs, 3) Activating postsynaptic Group 1 mGluRs activates signaling cascades that lead to the potentiation of AMPAR currents in CA1 pyramidal neurons. Experiments described in Specific Aim 1 are designed to test predictions made by this working model. The similar array of neurotransmitter receptors expressed by neurons and astrocytes has made it difficult to determine the role of astrocytic receptors and signaling cascades in synaptic transmission and behavior. We have developed a transgenic mouse model that enables us to selectively activate astrocytic G-protein coupled receptors (GPCRs). This transgenic model will be used to study the role of astrocytic receptors in synaptic transmission in situ. Experiments described in Specific Aim 2 are designed to use transgenic lines expressing unique GPCRs in astrocytes to examine the influence of astrocytic signaling cascades on synaptic transmission at the Schaffer colIateral-CA1 synapse. To date, it has not been possible to study the effect of activating single axons on astrocytic signaling cascades within the astrocytic syncytium in situ. This is a severe limitation given that, under physiological conditions, it is unlikely that all astrocytic microdomains are activated simultaneously.
In Specific Aim 3 we propose to study discrete points of axonal-astrocytic interaction while activating individual axons. - Our long-term goal is to understand the role of astrocytes in synaptic transmission and animal behavior. The studies described in this proposal will provide new insight into the mechanisms whereby astrocytes influence synaptic transmission as well as the functional readout of astrocytic participation in synaptic transmission. Further, the development of transgenic models that enable us to selectively activate astrocytic signaling cascades in situ and in vivo will allow us in future studies to address the role of astrocytic signaling systems in behavior.
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