Presynaptic NMDA Receptors Influence Ca 2+ Dynamics by Interacting with Voltage-Dependent Calcium Channels during the Induction of Long-Term Depression.

Spike-timing-dependent long-term depression (t-LTD) of glutamatergic layer (L)4-L2/3 synapses in developing neocortex requires activation of astrocytes by endocannabinoids (eCBs), which release glutamate onto presynaptic NMDA receptors (preNMDARs). The exact function of preNMDARs in this context is still elusive and strongly debated. To elucidate their function, we show that bath application of the eCB 2-arachidonylglycerol (2-AG) induces a preNMDAR-dependent form of chemically induced LTD (eCB-LTD) in L2/3 pyramidal neurons in the juvenile somatosensory cortex of rats. Presynaptic Ca 2+ imaging from L4 spiny stellate axons revealed that action potential (AP) evoked Ca 2+ transients show a preNMDAR-dependent broadening during eCB-LTD induction. However, blockade of voltage-dependent Ca 2+ channels (VDCCs) did not uncover direct preNMDAR-mediated Ca 2+ transients in the axon. This suggests that astrocyte-mediated glutamate release onto preNMDARs does not result in a direct Ca 2+ influx, but that it instead leads to an indirect interaction with presynaptic VDCCs, boosting axonal Ca 2+ influx. These results reveal one of the main remaining missing pieces in the signaling cascade of t-LTD at developing cortical synapses.