Question

What is the key role that NMDA receptors play in LTP induction and what two simultaneously occurring events are needed for LTP induction?

Short answer

Answer: The two simultaneously occurring events needed for LTP induction are: 1) presynaptic glutamate release, which binds to NMDA receptors on the postsynaptic neuron, and 2) significant postsynaptic depolarization, typically achieved due to increased activity at nearby AMPA receptors. These events interact with NMDA receptors by enabling them to act as coincidence detectors, allowing Ca2+ ions to enter the postsynaptic neuron only when both events are met simultaneously. This leads to the activation of NMDA receptors and the induction of LTP by strengthening the synaptic connection.

Step-by-step solution

Understanding NMDA Receptors and LTP

NMDA (N-methyl-D-aspartate) receptors are a type of glutamate receptor involved in the induction of long-term potentiation (LTP) in neurons. LTP is a long-lasting increase in synaptic strength that helps form memories. It is crucial to know the key role that NMDA receptors play in LTP induction before discussing the two events needed for its induction.

Identifying the Key Role of NMDA Receptors in LTP Induction

The key role of NMDA receptors in LTP induction is to act as a "coincidence detector" allowing calcium ions (Ca2+) to enter the postsynaptic neuron only when two conditions are met simultaneously. First, presynaptic glutamate release occurs, which binds to NMDA receptors. Second, sufficient postsynaptic depolarization is reached to dislodge Mg2+ ions blocking the receptor's ion channel. Thus, NMDA receptors enable detection of coincident pre- and postsynaptic neuronal activity, linking them to form lasting connections through LTP.

Event 1 - Presynaptic Glutamate Release

The first simultaneously occurring event needed for LTP induction is the release of the neurotransmitter glutamate by the presynaptic neuron. When the presynaptic neuron is activated due to an action potential, it releases glutamate into the synaptic cleft. Glutamate then binds to NMDA receptors on the postsynaptic neuron, but the ion channel remains closed due to Mg2+ ions' blockage.

Event 2 - Postsynaptic Depolarization

The second simultaneously occurring event needed for LTP induction is significant depolarization of the postsynaptic neuron's membrane. This membrane depolarization is typically achieved due to increased activity at nearby AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, which also bind glutamate and facilitate sodium (Na+) ions' influx, leading to depolarization. Once the postsynaptic membrane is sufficiently depolarized, Mg2+ ions blocking the NMDA receptor's ion channel are dislodged.

LTP Induction through NMDA Receptor Activation

Once both events (glutamate binding and significant postsynaptic depolarization) occur simultaneously, the Mg2+ ion blockage is removed, and the NMDA receptor becomes activated. This activation allows Ca2+ ions to flow into the postsynaptic neuron, triggering cascades of intracellular events that strengthen the synaptic connection, ultimately leading to the induction of LTP.