Differential Regulation of Metabotropic Glutamate Receptor- and AMPA Receptor-Mediated Dendritic Ca2+Signals by Presynaptic and Postsynaptic Activity in Hippocampal Interneurons

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Abstract
Calcium plays a crucial role as a ubiquitous second messenger and has a key influence in many forms of synaptic plasticity in neurons. The spatiotemporal properties of dendritic Ca2+ signals in hippocampal interneurons are relatively unexplored. Here we use two-photon calcium imaging and whole-cell recordings to study properties of dendritic Ca2+ signals mediated by different glutamate receptors and their regulation by synaptic activity in oriens/alveus (O/A) interneurons of rat hippocampus. We demonstrate that O/A interneurons express Ca2+-permeable AMPA receptors (CP-AMPARs) providing fast Ca2+ signals. O/A cells can also coexpress CP-AMPARs, Ca2+-impermeable AMPARs (CI-AMPARs), and group I/II metabotropic glutamate receptors (mGluRs) (including mGluR1a), in the same cell. CI-AMPARs are often associated with mGluRs, resulting in longer-lasting Ca2+ signals than CP-AMPAR-mediated responses. Finally, CP-AMPAR- and mGluR-mediated Ca2+ signals demonstrate distinct voltage dependence and are differentially regulated by presynaptic and postsynaptic activity: weak synaptic stimulation produces Ca2+ signals mediated by CP-AMPARs, whereas stronger stimulation, or weak stimulation coupled with postsynaptic depolarization, recruits Ca2+ signals mediated by mGluRs. Our results suggest that differential activation of specific glutamate receptor-mediated Ca2+ signals within spatially restricted dendritic microdomains may serve distinct signaling functions and endow oriens/alveus interneurons with multiple forms of Ca2+-mediated synaptic plasticity. Specific activation of mGluR-mediated Ca2+ signals by coincident presynaptic and postsynaptic activity fulfills the conditions for Hebbian pairing and likely underlies their important role in long-term potentiation induction at O/A interneuron synapses.

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