MECHANISM UNDERLYING RAPID ASTROCYTIC MORPHOLOGY CHANGES AFTER LONG-TERM POTENTIATION IN THE CA1 HIPPOCAMPAL REGION

Induction of long-term potentiation (LTP) of CA3-CA1 Schaffer collaterals results in a rapid withdrawal of fine astrocytic processes from potentiated synapses. This leads to an increased escape of synaptically released glutamate into extrasynaptic space. The signalling pathway controlling this withdrawal remains to be fully understood. Previous pharmacological experiments suggested the Na⁺-K⁺-2Cl^­--cotransporter 1 (NKCC1), which is involved in cell volume regulation and ion homeostasis, to be part of the signalling cascade. Further likely candidates are the Rho GTPase RhoA, which is known to control cytoskeletal dynamics, and the downstream Rho-associated kinase ROCK1. We therefore tested the effect of astrocyte-specific knockouts of these candidates on the withdrawal of perisynaptic astrocytic processes in acute hippocampal slices.
We started by reproducing our previous observations that induction of LTP by high frequency stimulation in CA3-CA1 Schaffer collaterals leads to astrocytic process withdrawal. The astrocytic remodelling was quantified by calculating the change of the tissue volume fraction occupied by astrocytic processes expressing EGFP. We then tested the effect of astrocyte-specific knockouts of the signalling candidates by conditionally expressing Cre recombinase under the control of the GLAST promoter in RhoA fl/fl and ROCK1 fl/fl and the ALDH1L1 promoter in NKCC1 fl/fl mouse lines. All three manipulations prevented LTP-associated astrocytic morphology changes without having a significant effect on baseline synaptic transmission and LTP. We conclude that NKCC1-RhoA-ROCK1 signalling is responsible for activity-dependent rapid astrocyte morphology changes and are now testing the functional consequences for subsequent synaptic plasticity and learning.