Silent synapse

In neuroscience, a silent synapse is one whose postsynaptic membrane contains NMDA receptors but no AMPA receptors. These synapses are so named because normal AMPA receptor-mediated signaling is not present, rendering the synapse inactive, or "silent", under typical conditions.

Contents

1 Synaptic transmission 2 Characteristics 3 Activation 4 Related topics 5 References

Synaptic transmission

Normal transmission across an excitatory synapse relies on the neurotransmitter glutamate, the glutamate-specific AMPA receptor (AMPAR), and calcium ions. When glutamate binds to AMPARs located on the postsynaptic membrane, they permit Ca²⁺ to enter the cell, eliciting a so-called excitatory postsynaptic potential (EPSP).

Silent synapses employ the same glutamate and calcium, but the AMPAR is replaced by the NMDA receptor (NMDAR). NMDAR is similar to AMPAR except that it requires strong postsynaptic depolarization to allow Ca²⁺ influx.

Characteristics

The stimulation of a silent synapse does not elicit EPSPs when the postsynaptic cell is clamped at -60 mV. Stimulation of a silent synapse will elicit EPSPs when the postsynaptic cell is held between +30 and +60 mV. Moreover, the EPSPs elicited with positive holding potentials can be completely blocked by APV, a selective NMDAR blocker.

Activation

Silent synapses can be activated via the insertion of AMPARs into the postsynaptic membrane, a phenomenon commonly called "AMPA receptor trafficking ".

When glutamate binds to a strongly-depolarized postsynaptic cell (e.g., during Hebbian LTP), Ca²⁺ quickly enters and binds to calmodulin. Calmodulin activates calcium/calmodulin-dependent protein kinase II (CaMKII), which — among other things — acts on AMPAR-containing vesicles near the postsynaptic membrane. CaMKII phosphorylates these AMPARs, which serves as a signal to insert them into the postsynaptic membrane. Once AMPARs are inserted, the synapse is no longer silent; activated synapses no longer require simultaneous pre- and postsynaptic activity in order to elicit EPSPs.

Related topics

• long-term potentiation
• Hebbian theory
• learning
• memory

References

• Isaac JT, Nicoll RA, Malenka RC. "Evidence for silent synapses: implications for the expression of LTP." Neuron. 1995 Aug;15(2):427-34. PMID 7646894