Questions: AMPA Receptors: Structure, Trafficking, and Function

Question 1

After strong stimulation that induces LTP at a synapse, synaptic strength increases over the following minutes. At the molecular level, this increased strength is primarily expressed by:

• A. Increased glutamate release from the presynaptic terminal
• B. More NMDA receptors inserted at the postsynaptic membrane
• C. Increased number of AMPA receptors at the postsynaptic density through exocytosis and lateral diffusion from extrasynaptic pools
• D. New AMPA receptors synthesized from scratch via upregulated gene expression

Answer: (C)

The expression of LTP is primarily postsynaptic and works by rapidly delivering existing AMPA receptors to the synapse surface, not by synthesizing new ones. CaMKII phosphorylates GluA1 subunits, promoting exocytosis of intracellular AMPAR vesicles and lateral diffusion of extrasynaptic receptors into the postsynaptic density. This can happen within minutes of LTP induction — far too fast for new gene expression. More AMPARs at the synapse means more ion channels open per glutamate release event, producing a larger excitatory postsynaptic potential. This is why AMPAR trafficking is called the expression mechanism of LTP.

Question 2

A neuron with AMPA receptors lacking the GluA2 subunit would be expected to show which property?

• A. Smaller currents per channel opening, because GluA2 enhances ion conductance
• B. Calcium-permeable AMPA channels, potentially contributing directly to plasticity signaling
• C. Complete loss of AMPAR function, since GluA2 is required for channel assembly
• D. Resistance to internalization during LTD, since GluA2 is required for endocytosis

Answer: (B)

GluA2 contains a unique edited arginine residue in the channel pore that blocks calcium permeation. When GluA2 is present (as in most mature synapses), AMPARs are calcium-impermeable and have linear current-voltage relationships. Without GluA2, the channel becomes calcium-permeable — and calcium is the critical second messenger that triggers CaMKII activation and downstream plasticity signaling. GluA2-lacking receptors are therefore more than just ion channels; they are signaling entry points. This is why the ratio of GluA2-containing to GluA2-lacking receptors at a synapse affects both the strength and the plasticity potential of that synapse.

Question 3

NMDA receptors are the primary expression mechanism of LTP because their activation directly produces the increased synaptic strength observed after potentiation.

• T. True
• F. False

Answer: False

NMDA receptors detect the conditions for plasticity — they are the coincidence detectors that require simultaneous pre- and postsynaptic activity to open and allow calcium influx. But they are not the expression mechanism. The increased synaptic strength is expressed by having more AMPA receptors at the postsynaptic membrane. NMDA receptor activation triggers the signaling cascades (CaMKII phosphorylation, etc.) that drive AMPAR delivery — but it is the additional AMPARs that produce the stronger response to subsequent glutamate release. NMDA receptors remain roughly constant; AMPA receptor number changes.

Question 4

Synaptic strength can be rapidly modulated by shuffling existing AMPA receptors to or from the postsynaptic membrane, without requiring synthesis of new receptor proteins.

• T. True
• F. False

Answer: True

This is the key insight about AMPAR trafficking as a plasticity mechanism. Neurons maintain intracellular pools of AMPARs in endosomal compartments and on extrasynaptic regions of the dendritic membrane. During LTP, these existing receptors are delivered to the postsynaptic density via exocytosis and lateral diffusion within minutes. During LTD, surface receptors are removed via endocytosis and stored or degraded. Because the cell does not need to wait for new transcription or translation, trafficking-based plasticity can occur on timescales of seconds to minutes — fast enough to encode memory-relevant changes in synaptic strength.

Question 5

Why is AMPA receptor trafficking described as the 'expression mechanism' of synaptic plasticity, and what distinct role do NMDA receptors play in this process?

Model answer: AMPA receptor trafficking is the expression mechanism because it is the physical change that makes a synapse stronger or weaker: more AMPARs at the synapse surface means larger excitatory postsynaptic currents in response to glutamate; fewer means smaller currents. The synapse's strength is literally set by AMPAR number. NMDA receptors play the complementary role of detection: they are coincidence detectors that open only when both glutamate is present (presynaptic activity) and the postsynaptic membrane is depolarized (postsynaptic activity). The resulting calcium influx activates CaMKII and other kinases that phosphorylate GluA1, triggering AMPAR delivery. NMDA receptors sense the signal for change; AMPA receptor trafficking executes the change.

The separation of detection (NMDA) from expression (AMPA) has important functional implications. It means that the same NMDA receptor mechanism can produce either LTP or LTD depending on the magnitude of calcium influx and the downstream cascades activated — high calcium activates kinases and drives AMPAR insertion (LTP), while low calcium activates phosphatases and drives AMPAR removal (LTD). This gives the synapse a graded, bidirectional plasticity mechanism controlled by a single calcium signal of varying amplitude.