HIPPOCAMPAL ATP EVENTS ARE MANIPULATED BY ASTROCYTIC AND MICROGLIAL CAMP

Cyclic AMP is a second messenger that initiates intracellular signaling cascades in response to extracellular signals. In the hippocampus, forskolin stimulation of endogenous adenylyl cyclases strengthens synaptic connections. However, such pharmacological manipulation affects all cells in the tissue, making it impossible to distinguish contributions from different cell types, and the role of cAMP in glial cells, namely astrocytes and microglia, is still not clear. Astrocytes are responsible for the uptake and secretion of soluble factors, including glutamate, ATP, and cytokines. Microglia, the immune cells of the CNS, are responsible for synaptic pruning, neuroinflammation regulation, and phagocytosis. To explore the effects of cAMP in glial cells, we expressed the photoactivatable adenylyl cyclase PACmn to control cAMP signaling with blue light. This expression was achieved by adeno-associated virus injection, for astrocytes, or by using the newly developed PACmn mouse line, for microglia. One of the main gliotransmitters released from astrocytes is ATP. To ensure that we can detect virtually all extracellular ATP, we expressed GRAB-ATP on the surface of neurons using a synapsin promoter. In the absence of stimulation, spontaneous ATP release events occur in hippocampal slice cultures at a frequency of about 0.1 per minute. When cAMP is optogenetically elevated in astrocytes, the frequency of ATP events triple. Interestingly, increased microglial cAMP does not change the frequency of ATP events. Rather, the amount of ATP released in each event significantly increases. In summary, ATP events are modulated by both astrocytic and microglial cAMP, albeit differentially, suggesting different mechanisms of action.