Metabotropic NMDA receptor function is required for NMDA receptor-dependent long-term depression

Area of Science:

• Neuroscience
• Synaptic Plasticity

Background:

• NMDA receptor (NMDAR) activation is central to synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD).
• Existing models propose that the magnitude of calcium (Ca²⁺) influx through NMDARs dictates whether LTP or LTD occurs.

Purpose of the Study:

• To investigate the precise role of NMDAR activation, ion flow, and calcium dynamics in inducing LTD.
• To challenge the established view linking Ca²⁺ influx levels directly to LTD induction.

Main Methods:

• Experimental manipulation of NMDAR ligand binding.
• Assessment of ion flow through NMDARs.
• Measurement of postsynaptic Ca²⁺ levels.
• Use of Ca²⁺ chelators to modulate basal Ca²⁺ concentrations.
• Electrophysiological recordings of synaptic transmission.

Main Results:

• Ligand binding to NMDARs is sufficient to trigger LTD, irrespective of ion flow or postsynaptic Ca²⁺ rise.
• Basal Ca²⁺ levels are permissively required for LTD.
• Modulating basal Ca²⁺ levels, but not maintaining them, blocks LTD and promotes synaptic potentiation.
• These findings highlight the role of basal Ca²⁺ in controlling NMDAR-dependent LTD and synaptic transmission.

Conclusions:

• Metabotropic actions of NMDARs can induce synaptic weakening without requiring an increase in postsynaptic calcium.
• This challenges and expands the known mechanisms underlying synaptic plasticity and learning.
• Basal calcium homeostasis plays a critical regulatory role in synaptic plasticity.