Questions: Glutamatergic Signaling and Receptors

Question 1

A postsynaptic neuron is at its resting membrane potential and receives a weak glutamatergic input. AMPA receptors open and a small depolarization occurs — but not enough to remove the Mg²⁺ block from NMDA receptors. Will calcium enter the cell through NMDA receptors?

• A. Yes, because glutamate has bound to the NMDA receptor and that is sufficient to open it
• B. No, because the Mg²⁺ block remains in the NMDA channel pore at near-resting potentials
• C. Yes, but with a delay proportional to the intensity of the glutamate signal
• D. No, because AMPA receptors competitively occupy the glutamate-binding site on NMDA receptors

Answer: (B)

NMDA receptors are coincidence detectors: glutamate binding is necessary but not sufficient. At resting potential, a Mg²⁺ ion physically plugs the channel pore. This voltage-dependent block is only relieved when the membrane is sufficiently depolarized — typically by concurrent AMPA receptor activation (or other depolarizing input). Without strong enough depolarization, NMDA channels stay blocked regardless of how much glutamate is present. Option A is the key misconception: NMDA receptors are NOT simply high-affinity glutamate receptors.

Question 2

Which property of NMDA receptors makes them the molecular basis of Hebbian synaptic plasticity?

• A. They mediate fast sodium influx, producing large depolarizations more rapidly than AMPA receptors
• B. They require simultaneous presynaptic glutamate release and postsynaptic depolarization to conduct calcium, detecting coincident activity
• C. They activate G-proteins that trigger second-messenger cascades modulating gene expression
• D. They desensitize slowly, allowing sustained calcium entry during repeated stimulation

Answer: (B)

Hebb's postulate states that synapses strengthen when pre- and postsynaptic neurons fire together. The NMDA receptor implements this physically: it only conducts when glutamate is present (presynaptic release) AND the membrane is depolarized (postsynaptic activity). If only one condition is met, the channel stays closed. The calcium influx when both conditions are met triggers the molecular cascades — CaMKII activation, AMPA receptor insertion — that strengthen the synapse. Option C describes metabotropic glutamate receptors, not NMDA.

Question 3

If glutamate is applied at sufficiently high concentration, NMDA receptors will open even when the postsynaptic membrane is at resting potential.

• T. True
• F. False

Answer: False

The Mg²⁺ block is voltage-dependent, not concentration-dependent. At resting potential (~−70 mV), the electrochemical gradient holds the Mg²⁺ ion firmly in the channel pore regardless of how much glutamate is bound. Only membrane depolarization (typically to around −40 mV or above) provides the electrostatic force to expel the Mg²⁺ block. This is what makes NMDA receptors genuine coincidence detectors rather than simple high-affinity receptors.

Question 4

Calcium influx through NMDA receptors provides the intracellular signal that triggers long-term changes in synaptic strength, including long-term potentiation.

• T. True
• F. False

Answer: True

When the coincidence condition is met (glutamate + depolarization), NMDA receptors pass calcium ions in addition to sodium. Calcium is a powerful second messenger that activates CaMKII and other kinases, which phosphorylate existing AMPA receptors (increasing their conductance) and trigger trafficking of additional AMPA receptors to the postsynaptic membrane. This potentiation of AMPA-mediated transmission is the cellular substrate of long-term potentiation (LTP) and a leading model for how learning and memory are encoded.

Question 5

Why are NMDA receptors described as 'coincidence detectors,' and how does this property enable synaptic plasticity?

Model answer: NMDA receptors require two simultaneous conditions to open: (1) glutamate must be bound (indicating presynaptic activity) and (2) the postsynaptic membrane must be sufficiently depolarized to expel the Mg²⁺ block (indicating postsynaptic activity). A single condition alone is insufficient. This AND-gate logic detects coincident pre- and postsynaptic firing. When both conditions are met, NMDA channels conduct calcium, which activates kinase cascades that strengthen the synapse — implementing Hebb's rule at the molecular level.

This is why NMDA receptor-dependent plasticity is associative: pairing a weak (subthreshold) input with a strong (depolarizing) input causes the weak input's synapse to strengthen, because the strong input provides the depolarization needed to open the NMDA channels at the weak synapse. This cellular property underlies classical conditioning and forms the basis for modern theories of learning and memory in the hippocampus and cortex.