Synaptic Plasticity: Mechanisms, Circuits, and Functional Roles

Definition and Fundamentals of Synaptic Plasticity

Etymology and General Definition:
- The term "plasticity" originates from the Greek word plastikos ( $\pi\lambda\alpha\sigma\tau\iota\kappa\acute{o}\varsigma$ ), which means "able to be molded".
- Brain plasticity refers broadly to all neural functions and mechanisms that involve change, including:
- Acquisition and refinement of motor and cognitive skills.
- Adaptation to and recovery from structural brain injury.
- Learning and memory storage.
- Plasticity occurs at all organizational levels of the brain, ranging from macro-level neural systems down to microcircuits and individual chemical synapses.
Glutamatergic Synaptic Strength:
- Synaptic strength is quantitatively defined as the amplitude of the postsynaptic potential (PSP) generated in response to a presynaptic action potential.
- At a glutamatergic synapse, synaptic strength is governed by two key parameters:
1. Presynaptic Parameter: The quantity of glutamate released into the synaptic cleft per action potential.
2. Postsynaptic Parameter: The number and functional responsiveness of postsynaptic glutamatergic receptors (e.g., AMPA and NMDA receptors).
- Synaptic Plasticity: Refers to any activity-dependent modification in synaptic strength, achieved through changes in presynaptic neurotransmitter release, postsynaptic receptor density, or both.

Taxonomy of Synaptic Plasticity

Activity-dependent changes in synaptic strength are classified into three primary operational forms based on induction conditions, temporal persistence, and physiological roles:

Short-Term Plasticity (STP):
- Trigger: Induced during repeated activation of any chemical synapse.
- Duration: Transient changes lasting from milliseconds to seconds.
- Function: Subserves fast neural computations, gain control, dynamic filtering, and temporal processing in local microcircuits.
Long-Term Plasticity (LTP / LTD):
- Trigger: Results from strong, frequent, repeated stimulation patterns (e.g., high-frequency tetanus or spike-timing-dependent protocols).
- Duration: Persistent modifications lasting minutes, hours, days, weeks, or longer.
- Function: Forms the cellular substrate for learning, memory trace formation, and long-term memory consolidation.
Homeostatic Plasticity:
- Trigger: Results from sustained, prolonged alterations in overall network activity levels (e.g., chronic hyper-excitation or activity deprivation).
- Duration: Adaptive modifications lasting minutes, hours, or days.
- Function: Maintains overall network excitability within stable, healthy physiological boundaries to prevent runaway excitation or quiescence.

Mechanisms of Short-Term Synaptic Plasticity: Depression and Facilitation

Short-term synaptic plasticity (STP) is divided into two main forms based on the presynaptic initial release probability ( $P_r$ ):

Short-Term Depression (STD):
- Presynaptic State: High initial release probability ( $P_r$ ).
- Molecular Mechanism: Upon repetitive action potential stimulation, synaptic vesicles in the Readily Releasable Pool (RRP) are exocytosed faster than the rate at which new vesicles can be mobilized and docked. This depletion of the RRP reduces the number of vesicles available for subsequent release.
- Electrophysiological Profile: Evoked postsynaptic potentials or currents show a progressive decay in amplitude across successive action potentials in a train.
Short-Term Facilitation (STF):
- Presynaptic State: Low initial release probability ( $P_r$ ).
- Molecular Mechanism: When presynaptic action potentials arrive at high frequencies, residual free calcium ( $\text{Ca}^{2+}$ ) accumulates within the presynaptic axon terminal/bouton because calcium influx exceeds the rate of calcium clearance and buffering. This residual $\text{Ca}^{2+}$ summates with incoming $\text{Ca}^{2+}$ from subsequent action potentials, raising local intracellular $\text{Ca}^{2+}$ concentration and dramatically elevating the neurotransmitter release probability ( $P_r$ ) for subsequent pulses.
- Electrophysiological Profile: Evoked postsynaptic potentials or currents exhibit a progressive increase in amplitude across successive action potentials in a train.

Computational Filtering Functions of Short-Term Plasticity

Short-term synaptic plasticity acts as a frequency-dependent dynamic filter that shapes action potential output relative to the postsynaptic spike threshold:

Static Synapse (No Plasticity):
- Every presynaptic input elicits an identical postsynaptic potential amplitude above threshold.
- Functions as a pure relay without temporal filtering.
Short-Term Depression (STD) as a Low-Pass Filter:
- Postsynaptic responses are maximal for initial inputs and rapidly attenuate during high-frequency trains.
- Spike Output: Only the first inputs in a stimulus train reach the spike threshold to trigger postsynaptic action potentials.
- Filtering Role: Functions as a low-pass filter that preferentially transmits low-frequency or short-lived, transient (phasic) signals while suppressing continuous high-frequency inputs.
Short-Term Facilitation (STF) as a High-Pass Filter:
- Postsynaptic responses start sub-threshold and progressively build up during continuous stimulation.
- Spike Output: Single isolated action potentials fail to trigger postsynaptic spikes; only sustained high-frequency input trains accumulate sufficient facilitation to cross the spike threshold.
- Filtering Role: Functions as a high-pass filter that preferentially transmits high-frequency, sustained (tonic) signals while filtering out isolated background noise or sporadic action potentials.

Filtering mechanisms of short-term plasticity

Biological and Physiological Systems Expressing Short-Term Plasticity

Crayfish Neuromuscular Systems:
- Research on crayfish motor pathways (Jackman & Regehr, 2017) demonstrates how STP is tuned to specific motor outputs:
- Tail-flip Escape Reflex Muscles: Innervated by motor axon synapses exhibiting pronounced Short-Term Depression (STD). This produces an immediate, maximal muscular contraction for rapid escape, which then depresses upon repeated stimulation.
- Postural Control Muscles: Innervated by motor axon synapses exhibiting strong Short-Term Facilitation (STF). This enables gradual, sustained muscle tension during ongoing motor firing to maintain body posture.

Crayfish motor axon responses

Cortical Recurrent Microcircuits:
- Recurrent excitatory synapses between neocortical pyramidal neurons predominantly express Short-Term Depression (STD) (Markram et al., 1998; Loebel et al., 2002).
- Synaptic STD regulates overall cortical network dynamics:
- Duration of Network Bursts: Directly restricted and terminated by the rate of synaptic depression across excitatory connections.
- Frequency of Network Bursts: Constrained by the kinetic rate of synaptic recovery from vesicle depletion (RRP replenishment rate).

Target-Specific Short-Term Plasticity and Disynaptic Cortical Circuits

Target-Specific Synaptic Tuning:
- A single presynaptic neuron can express different forms of STP at different axon terminal branches, depending on the identity of the postsynaptic target cell (Markram et al., 1998; Silberberg & Markram, 2007).
- Pyramidal Cell to Basket Cell (PC $\rightarrow$ BC): Exhibits Short-Term Depression (STD).
- Pyramidal Cell to Martinotti Cell (PC $\rightarrow$ MC): Exhibits Short-Term Facilitation (STF).

Target-specific short-term plasticity in cortical circuits

Temporal Recruitment of Postsynaptic Targets:
- In response to an identical high-frequency action potential train from a presynaptic pyramidal neuron:
- Basket Cells (BC) are recruited rapidly at the onset of firing due to high initial release probability and STD, delivering fast perisomatic inhibition.
- Martinotti Cells (MC) display a delayed recruitment profile; initial inputs produce small sub-threshold responses, but ongoing high-frequency firing facilitates synaptic responses, eventually driving the Martinotti cell above firing threshold.
Pyramidal-Martinotti-Pyramidal Disynaptic Inhibitory Circuit:
- Circuit Connectivity: Pyramidal Cell 1 (PC1) $\rightarrow$ Martinotti Cell (MC) $\rightarrow$ Pyramidal Cell 2 (PC2) apical dendrite.
- Functional Mechanism: Functions as a comparison or competition circuit. As PC1 fires a train of action potentials, the PC1-to-MC facilitating synapse recruits the Martinotti cell. Once activated, the Martinotti cell releases GABA onto the apical dendrites of neighboring pyramidal neurons (including PC2).
- Physiological Outcome: The most strongly activated pyramidal neuron silences neighboring pyramidal neurons by providing targeted disynaptic feedback and lateral inhibition onto their apical dendrites.

Pyramidal to Martinotti disynaptic inhibitory circuit

Molecular Mechanisms of Asynchronous Neurotransmitter Release

Synchronous vs. Asynchronous Neurotransmitter Release:
- Synchronous Release (SR): Highly time-locked neurotransmitter exocytosis occurring within $\sim 1\,\text{ms}$ of presynaptic action potential arrival. Driven by brief, high-concentration localized calcium microdomains triggering low-affinity synaptotagmins (e.g., $\text{Syt}_{\text{LA}}$ such as Synaptotagmin 1 or 2).
- Asynchronous Release (AR): A transient, activity-dependent form of STP characterized by delayed, temporally dispersed vesicle exocytosis occurring tens to hundreds of milliseconds ( $>10\,\text{ms}$ ) after action potentials. Driven by persistent, low-concentration residual calcium ([\text{Ca}^{2+}]_i \sim 0.1\,\text{\mu M} to low micromolar range) acting on high-affinity synaptotagmins (e.g., $\text{Syt}_{\text{HA}}$ such as Synaptotagmin 7).

Calcium dynamics underlying synchronous vs asynchronous neurotransmitter release

Presynaptic Calcium Clearance Dynamics:
- Following voltage-gated calcium channel (VGCC) opening during an action potential, intracellular calcium levels are gradually restored via:
1. Spatial diffusion away from active zones.
2. Endogenous cytosolic calcium buffering.
3. Active extrusion via the Plasma Membrane $\text{Ca}^{2+}$ -ATPase (PMCA) pump (ATP-dependent).
4. Active exchange via the Sodium-Calcium Exchanger (NCX) ( $\text{Na}^+/\text{Ca}^{2+}$ antiporter).
- At times $>10\,\text{ms}$ post-spike train, residual calcium lingering in the terminal continues to activate high-affinity synaptotagmins to maintain asynchronous release.
SNARE Complex and Vesicle Fusion Cascade:
- Proteins Involved:
- Vesicle membrane: Synaptobrevin (VAMP) and Synaptotagmin (containing C2A and C2B calcium-binding domains).
- Target plasma membrane: SNAP-25, Syntaxin, and phosphatidylinositol 4,5-bisphosphate ( $\text{PtdIns}(4,5)\text{P}_2$ ).
- Fusion Steps:
1. In the resting state (absence of $\text{Ca}^{2+}$ ), the C2B domain of Synaptotagmin binds weakly to $\text{PtdIns}(4,5)\text{P}_2$
2. Upon $\text{Ca}^{2+}$ binding, SNARE proteins (Synaptobrevin, SNAP-25, Syntaxin) zipper/twist together tightly, forcing the C2 domains to penetrate the target lipid bilayer.
3. Formation of a hemifusion intermediate state.
4. Opening and dilation of the aqueous fusion pore for neurotransmitter release.
5. Vesicle retrieval via endocytosis.

Molecular mechanisms of synaptotagmin and SNARE-mediated vesicle fusion

Physiological Role of Asynchronous Release in Circuit Protection

Asynchronous Release at Pyramidal-to-Martinotti (PC-MC) Synapses:
- High-frequency firing trains (e.g., $100\,\text{Hz}$ or $70\,\text{Hz}$ ) in pyramidal neurons induce prominent asynchronous release at PC-MC synapses (Deng et al., 2020).
- Asynchronous release prolongs Martinotti cell depolarization and sustained firing beyond the duration of the presynaptic stimulus, maintaining a long-lasting stream of inhibitory postsynaptic currents (IPSCs) onto pyramidal dendrites.
Temporal Precision and Jitter:
- Fast-Spiking Interneurons (e.g., Basket Cells): Mediates fast disynaptic IPSPs with high temporal precision and low jitter (jitter window approximately $-2\,\text{ms}$ to $+2\,\text{ms}$ ).
- Martinotti / CCK+ Interneurons: Mediates slow disynaptic IPSPs characterized by significant temporal jitter (jitter window spanning $-5\,\text{ms}$ to $+6\,\text{ms}$ ) as a direct consequence of asynchronous release at the input synapse.
Network Protection Against Over-Excitation:
- Asynchronous release acts as an endogenous protective shield against hyper-excitability and seizure development (Rozov et al., 2019; Manseau et al., 2010):
- At baseline/moderate firing rates, network populations operate with tight millisecond temporal synchronization via synchronous neurotransmitter release.
- Under conditions of excessive excitation or threatening hyper-synchrony, high-frequency spike trains shift synaptic release from synchronous to asynchronous mode.
- This transition scatters neurotransmitter release over extended time windows, breaking up hypersynchronous neuronal firing and desynchronizing the network to prevent epileptiform activity.

Key Summary and Fundamental Takeaways

Short-Term Synaptic Plasticity Overview:
- Ubiquitously observed across chemical synapses during repeated activation.
- Manifests as transient changes on a millisecond-to-second time frame, specialized for fast real-time computations and network gating.
Short-Term Depression (STD):
- Characterized by declining synaptic strength due to depletion of the Readily Releasable Pool (RRP).
- High initial release probability ( $P_r$ ).
- Functions as a low-pass filter, selectively transmitting low-frequency or short-lived (phasic) signals.
Short-Term Facilitation (STF):
- Characterized by increasing synaptic strength driven by presynaptic residual $\text{Ca}^{2+}$ accumulation.
- Low initial release probability ( $P_r$ ).
- Functions as a high-pass filter, selectively transmitting high-frequency or sustained (tonic) signals.
Target-Specific Circuit Organization:
- Cortical pyramidal neurons express target-specific STP: facilitating (STF) to Martinotti cells and depressing (STD) to Basket cells.
- Dictates distinct temporal recruitment of inhibitory cell populations and mediates apical dendritic lateral inhibition in Pyramidal-Martinotti-Pyramidal microcircuits.
Asynchronous Neurotransmitter Release:
- Activity-dependent, prolonged mode of transmitter release untethered to exact spike timing.
- Mediated by residual presynaptic $\text{Ca}^{2+}$ acting on high-affinity calcium sensors like Synaptotagmin 7.
- Extrusion regulated by PMCA pumps and NCX exchangers.
- Serves a critical protective function in desynchronizing neural networks during states of excessive excitation to prevent runaway network toxicity.