NMDA Receptor Inhibition of L-Type Calcium Channels via ER Calcium Depletion and Activation of STIM1 in Cultured Hippocampal Neurons

Abstract

We have found that, in cultured hippocampal neurons, NMDA receptors (NMDAR) inhibit voltage-gated L-type Ca2+ channels through Ca2+ release from the endoplasmic reticulum (ER) and subsequent engagement of STIM1 with L channels. Here we employed laser spot photo-uncaging of glutamate near single dendritic spines combined with fluorescence Ca2+ imaging to investigate the relationships between stimulus frequency (glutamate uncaging) and Ca2+ release from the ER. Measurements of cytosolic Ca2+ were made with the genetically-encoded Ca2+ indicator, RGECO1, and simultaneously, measurements of [Ca2+]ER made with a genetically-encoded Ca2+ indicator targeted to the ER, D1ER. Using a laser pulse duration of 2 ms to photo-uncage glutamate near a single spine, simultaneous measurements of Ca2+cyto and [Ca2+]ER revealed that stimulation at 1 second intervals (1 Hz) for 60 seconds triggered a rapid rise in Ca2+cyto followed by release of Ca2+ from the ER. Spine stimulation at 6 second intervals (0.167 Hz) for 60 seconds elicited a large cytosolic Ca2+ transient, but no significant Ca2+ release from ER stores. At various stimulus frequencies, pharmacological analysis using the L-type Ca2+ channel blocker, nimodipine, uncovered a direct correlation between the magnitude of the nimodipine-sensitive, L-channel Ca2+ transient and Ca2+ release from the ER. Together, these results suggest that even though the lowest frequency stimulation is capable of generating a cytosolic Ca2+ signal, the NMDAR Ca2+ signal must contain the necessary details to activate L-channels by membrane depolarization and integrate with the L-channel Ca2+ signal to regulate Ca2+-induced Ca2+ release in a single dendritic spine. If this frequency dependence holds true for the activation of STIM1 to regulate L-channel, this work will have important implication in how we think about channel regulation.