Presynaptic Spike Timing-Dependent Long-Term Depression in the Mouse Hippocampus

Area of Science:

• Neuroscience
• Synaptic Plasticity
• Molecular Biology

Background:

• Spike-timing-dependent plasticity (STDP) is crucial for learning and memory.
• Hippocampal STDP mechanisms and functions remain incompletely understood.
• Investigating STDP in the hippocampus is vital for understanding memory processes.

Purpose of the Study:

• To investigate the induction requirements for hippocampal spike-timing-dependent long-term potentiation (t-LTP) and long-term depression (t-LTD).
• To elucidate the mechanisms underlying t-LTP and t-LTD at CA3-CA1 synapses in the developing mouse hippocampus.
• To differentiate the roles of NMDA receptor subtypes and intracellular signaling cascades in hippocampal plasticity.

Main Methods:

• Electrophysiological recordings in young mouse hippocampus (P12-P18).
• Induction of t-LTP and t-LTD using paired pre- and postsynaptic activity at 0.2 Hz.
• Pharmacological manipulation of NMDA receptor subunits (GluN2A-D), metabotropic glutamate receptors, phospholipase C, IP3 receptors, endocannabinoids, and CB1 receptors.
• Analysis of presynaptic mechanisms using fluctuation analysis, paired-pulse ratio, and MK801-induced depression.

Main Results:

• Both t-LTP and t-LTD were induced at CA3-CA1 synapses at low frequency (0.2 Hz).
• t-LTP requires postsynaptic ionotropic NMDA receptors (GluN2A/B), while t-LTD does not rely on postsynaptic NMDA receptors but is blocked by GluN2C/D antagonists.
• t-LTD induction involves metabotropic glutamate receptors, phospholipase C, IP3-mediated Ca2+ release, endocannabinoid synthesis, CB1 receptor activation, astrocytic signaling, and presynaptic calcineurin.
• t-LTD is expressed presynaptically, impacting neurotransmitter release.

Conclusions:

• Hippocampal CA3-CA1 synapses exhibit both NMDA receptor-dependent t-LTP and t-LTD during development.
• Distinct NMDA receptor subtypes mediate t-LTP and t-LTD.
• A presynaptic form of hippocampal t-LTD, similar to neocortical plasticity, was identified.
• Understanding these distinct plasticity mechanisms is key to hippocampal function in memory formation.