Questions: Glutamatergic Excitation: Information Transfer and Synaptic Plasticity

Question 1

A postsynaptic neuron is strongly hyperpolarized by inhibitory input when a presynaptic glutamatergic neuron fires and releases glutamate. What happens at the synapse?

• A. Both AMPA and NMDA receptors open normally, since glutamate has been released
• B. AMPA receptors open and produce an EPSP, but NMDA receptors remain blocked by Mg²⁺ and do not conduct
• C. NMDA receptors open because glutamate is present, while AMPA receptors require postsynaptic depolarization to open
• D. Neither receptor type opens because the inhibitory input prevents neurotransmitter release from the presynaptic terminal

Answer: (B)

AMPA receptors are ligand-gated and open when glutamate binds, regardless of membrane potential — they produce a fast EPSP by admitting Na⁺. NMDA receptors also bind glutamate, but at resting or hyperpolarized potentials, Mg²⁺ physically blocks the channel pore. The Mg²⁺ block is only relieved when the postsynaptic membrane is already depolarized. Since the neuron is hyperpolarized, NMDA receptors remain blocked despite glutamate binding. This is the coincidence-detection mechanism in action. Option C reverses the requirements — it is NMDA, not AMPA, that requires postsynaptic depolarization.

Question 2

Why does excitotoxic neuronal death specifically implicate NMDA receptors rather than AMPA receptors?

• A. NMDA receptors are far more numerous than AMPA receptors at excitatory synapses, making them statistically more important
• B. NMDA receptors are located exclusively in brain regions most vulnerable to ischemia
• C. NMDA receptors conduct Ca²⁺ in addition to Na⁺, and excessive Ca²⁺ influx activates destructive intracellular enzymes
• D. AMPA receptors automatically inactivate during ischemia, leaving NMDA as the only active glutamate-gated channel

Answer: (C)

The key distinction is ion selectivity. Standard AMPA receptors admit primarily Na⁺ with little Ca²⁺. NMDA receptors, when open, pass substantial Ca²⁺ in addition to Na⁺. It is this Ca²⁺ influx that triggers excitotoxicity: cytosolic Ca²⁺ overload activates proteases, lipases, and endonucleases, generates free radicals, and initiates apoptotic pathways that kill the neuron. During stroke or injury, uncontrolled glutamate release forces NMDA channels open continuously, flooding cells with toxic Ca²⁺. Memantine's therapeutic action — partial NMDA blockade — is designed precisely to reduce this pathological Ca²⁺ entry.

Question 3

The NMDA receptor's requirement for both glutamate binding and postsynaptic depolarization means it acts as a coincidence detector linking presynaptic and postsynaptic activity.

• T. True
• F. False

Answer: True

This is the defining functional property of the NMDA receptor. It conducts only when two conditions are simultaneously met: glutamate is released from the presynaptic terminal (signaling presynaptic activity) and the postsynaptic membrane is depolarized enough to relieve the Mg²⁺ block (signaling postsynaptic activity, typically from nearby AMPA receptor activation). By requiring both, the NMDA receptor detects correlated pre- and postsynaptic firing, implementing Hebb's rule at the molecular level. The resulting Ca²⁺ influx initiates the cascade that produces long-term potentiation.

Question 4

AMPA receptors are the primary molecular triggers of long-term potentiation (LTP) because they are more abundant at excitatory synapses than NMDA receptors.

• T. True
• F. False

Answer: False

NMDA receptors, not AMPA receptors, are the primary molecular triggers of LTP. Their coincidence-detection property allows them to detect correlated firing and admit Ca²⁺, which initiates the intracellular cascade that inserts more AMPA receptors into the postsynaptic membrane. AMPA receptors are the endpoint of LTP expression — more AMPA receptors means a stronger synapse — but the TRIGGER is NMDA receptor activation and the resulting Ca²⁺ influx. The misconception conflates abundance and downstream expression with mechanistic causation.

Question 5

Explain why the NMDA receptor is called a 'coincidence detector,' and why this property makes it the key molecular substrate for Hebbian synaptic plasticity.

Model answer: The NMDA receptor requires two simultaneous conditions to open: glutamate binding (signaling that the presynaptic neuron fired) and relief of the Mg²⁺ pore block by postsynaptic depolarization (signaling that the postsynaptic neuron is already active). Neither condition alone is sufficient. This makes the receptor detect the coincidence of pre- and postsynaptic activity. Hebbian plasticity says synapses should strengthen when pre- and postsynaptic neurons fire together ('neurons that fire together wire together'). The NMDA receptor implements this rule: it admits Ca²⁺ precisely under the conditions that should produce lasting synaptic change, and the Ca²⁺ influx triggers insertion of more AMPA receptors, permanently increasing synaptic strength (LTP).

The elegance of the NMDA receptor is that its biophysical properties — dual ligand-gating and voltage-dependent Mg²⁺ block — directly encode Hebb's rule without requiring any higher-level coordination. The same receptor that enables fast excitatory signaling also detects the pattern of activity that should produce lasting change. This dual role makes glutamatergic synapses both the wiring medium of neural circuits and the mechanism of their modification.