13 KiB
13 KiB
Detailed Temporal Dynamics of Postsynaptic Response and Plasticity
From glutamate binding to structural consolidation, with concentration changes, receptor trafficking, and calcium signaling across timescales
Baseline State (Resting Spine)
Time: Continuous
Postsynaptic [Ca²⁺]: ~50-100 nM
Membrane Potential (Vₘ): -70 mV
AMPARs in PSD: 10-20 receptors (GluA1/GluA2 heteromers)
NMDARs in PSD: 5-10 receptors (GluN1/GluN2B)
Mg²⁺ block of NMDARs: ~80% at -70 mV
CaMKII state: Mostly inactive (α:β ≈ 3:1 ratio)
PSD-95 clusters: ~300 molecules per PSD
PHASE 1: FAST TIMESCALE (0-100 ms) - RECEPTOR ACTIVATION
0.0-0.2 ms: Glutamate Arrival and Binding
Presynaptic glutamate release (~5000 molecules)
↓
Diffusion across 20 nm synaptic cleft (t ≈ 0.1 ms)
↓
**Glutamate concentration in cleft:**
- Peak: 1-3 mM at PSD surface
- Rapid clearance by EAATs (t½ ≈ 1 ms)
↓
**Simultaneous binding to:**
1. **AMPARs (ionotropic, fast):**
- 2 glutamate molecules bind per channel
- Binding Kd ≈ 500 µM
- Channel opens in ~0.2 ms
2. **NMDARs (ionotropic, slow):**
- Requires glutamate + glycine/D-serine
- Binding Kd ≈ 1-5 µM
- Mg²⁺ block prevents opening at rest
3. **mGluRs (metabotropic):**
- Group I mGluRs (mGluR1/5)
- G-protein coupled, slower signaling
0.2-2.0 ms: AMPAR-Mediated Depolarization
**For each open AMPAR:**
- Conductance: 8-12 pS (single channel)
- Reversal potential: 0 mV
- **Na⁺ influx:** ~3000 ions/channel/ms
- K⁺ efflux: ~1000 ions/channel/ms
**Net effect at spine head:**
Without other inputs: EPSP amplitude = 0.5-2 mV
With 20 AMPARs open: Current = 10-30 pA
Depolarization to Vₘ ≈ -60 mV
1.0-5.0 ms: NMDAR Activation (if depolarized)
**Requirement:** Vₘ > -40 mV to relieve Mg²⁺ block
**Coincidence detection window:** 5-10 ms
If depolarized (from AMPARs or bAP):
↓
Mg²⁺ expelled from NMDAR channel
↓
**NMDAR opens with characteristic:**
- Slow kinetics (τrise ≈ 10 ms, τdecay ≈ 50-100 ms)
- High Ca²⁺ permeability (PCa/PNa ≈ 10:1)
- **Single channel Ca²⁺ influx:** ~5000 Ca²⁺ ions/ms
↓
**Local [Ca²⁺] in spine head:**
- Baseline: 100 nM
- With NMDAR activation: **→ 1-10 µM**
- With NMDAR + bAP coincidence: **→ 10-30 µM**
5.0-50 ms: Calcium Dynamics and Clearance
**Calcium sources in spine:**
1. NMDARs (main source for plasticity)
2. Voltage-gated Ca²⁺ channels (VGCCs) from bAP
3. Internal stores (IP₃R, RyR)
**Calcium buffers in spine:**
- Calbindin-D28K (Kd ≈ 200 nM)
- Parvalbumin (Kd ≈ 10 nM)
- Calmodulin (Ca²⁺ sensor, Kd ≈ 1-10 µM)
**Clearance mechanisms:**
1. Plasma Membrane Ca²⁺ ATPase (PMCA):
- High affinity (Kd ≈ 100 nM)
- Slow: clears ~30 Ca²⁺/sec per pump
2. Sodium-Calcium Exchanger (NCX):
- Low affinity (Kd ≈ 1 µM)
- Fast: 3 Na⁺ in, 1 Ca²⁺ out
3. SERCA pumps into ER:
- If spine has smooth ER
4. Mitochondrial uptake (larger spines):
- MCU (mitochondrial Ca²⁺ uniporter)
- Kd ≈ 10-20 µM
**Result:**
- 90% Ca²⁺ cleared in 50-100 ms
- Returns to baseline [Ca²⁺] in 200-500 ms
PHASE 2: MEDIUM TIMESCALE (100 ms - 10 sec) - SIGNALING CASCADES
Calcium-Decoded Plasticity Decision
**The "Calcium Rule":**
[Ca²⁺] amplitude × duration → plasticity direction
**Thresholds:**
- LTD: 1-5 µM sustained (100 ms - 1 sec)
- LTP: >10 µM brief (10-50 ms)
- LTP requires **rapid rise** (d[Ca²⁺]/dt > 0.5 µM/ms)
LTD Pathway (Moderate Ca²⁺)
[Ca²⁺] = 1-5 µM for >100 ms
↓
Calcium binds Calmodulin (CaM)
↓
**Activates Calcineurin (CaN, PP2B):**
- Phosphatase, Kd ≈ 0.5 µM Ca²⁺
- Activated at lower [Ca²⁺] than CaMKII
↓
CaN dephosphorylates Inhibitor-1
↓
**Releases inhibition of Protein Phosphatase-1 (PP1)**
↓
PP1 dephosphorylates:
1. GluA1 at S845 → increases endocytosis
2. Stargazin → reduces AMPAR synaptic retention
3. Other targets promoting AMPAR removal
↓
**Result: AMPAR internalization begins in 30-60 sec**
LTP Pathway (High Ca²⁺)
[Ca²⁺] > 10 µM with rapid rise
↓
Calcium binds Calmodulin (CaM)
↓
**Activates Ca²⁺/Calmodulin Kinase II (CaMKII):**
- 12-subunit holoenzyme
- Each subunit has autoinhibitory domain
- Requires Ca²⁺/CaM binding to activate
↓
**Autophosphorylation at T286:**
- First subunit phosphorylates neighbor
- Creates Ca²⁺-independent activity
- **Molecular switch:** stays active after Ca²⁺ clears
↓
**Active CaMKII translocates to PSD:**
- Binds to NR2B subunit of NMDAR
- Binds to α-actinin (actin linker)
- Becomes structural component of PSD
PHASE 3: SLOW TIMESCALE (10 sec - 10 min) - RECEPTOR TRAFFICKING
LTD Execution (1-10 minutes)
**Clathrin-mediated endocytosis:**
PP1 activity → GluA1 S845 dephosphorylated
↓
Increased binding to AP2 adaptor complex
↓
**Clathrin coats form at spine periphery (t ≈ 1-2 min)**
↓
AMPARs internalized via endocytosis
↓
**Vesicles transported to early endosomes**
↓
Receptors either:
1. Recycled back to surface (silent synapses)
2. Degraded in lysosomes (long-term LTD)
↓
**By 10 min:**
- 30-50% reduction in surface AMPARs
- EPSP amplitude decreases proportionally
LTP Execution (1-10 minutes)
**Rapid AMPAR insertion:**
CaMKII phosphorylates:
1. **Stargazin (TARP γ-2) at S9:**
- Increases binding to PSD-95
- **Traps AMPARs in PSD** (Kd improves 10×)
2. **SynGAP (RasGAP):**
- Phosphorylation inhibits Ras inactivation
- Increases ERK/MAPK signaling
↓
**Exocytosis of AMPARs:**
1. From recycling endosomes (Rab11-dependent)
2. From intracellular pools
3. **Insertion at extrasynaptic sites first**
↓
**Lateral diffusion into PSD:**
- AMPARs diffuse in membrane (D ≈ 0.1 µm²/s)
- Phosphorylated Stargazin binds PSD-95
- **Trapped in PSD for minutes-hours**
↓
**By 10 min:**
- 50-100% increase in surface AMPARs
- EPSP amplitude increases 50-200%
Phosphorylation State Changes
**AMPAR modifications during LTP:**
- **GluA1 S831:** Phosphorylated by CaMKII/PKC
→ Increases single channel conductance (γ from 8→12 pS)
- **GluA1 S845:** Phosphorylated by PKA
→ Increases open probability (Po from 0.8→0.95)
- **GluA2 S880:** Phosphorylated by PKC
→ Regulates binding to GRIP/ABP vs PICK1
PHASE 4: METABOLIC SUPPORT (10 min - 2 hours) - PROTEIN SYNTHESIS
Local Translation in Spine
**Trigger:**
1. CaMKII activation
2. mGluR activation
3. BDNF-TrkB signaling
**Pathways:**
1. **mTOR pathway:**
- PI3K → Akt → mTORC1
- Phosphorylates 4E-BP, releases eIF4E
- **Initiates cap-dependent translation**
2. **MAPK pathway:**
- Ras → Raf → MEK → ERK
- Phosphorylates translation factors
↓
**Dendritic mRNA translation begins (t ≈ 20-30 min):**
Key mRNAs locally translated:
1. **CaMKIIα** - more kinase molecules
2. **GluA1** - new AMPAR subunits
3. **Arc/Arg3.1** - regulates AMPAR trafficking
4. **PSD-95** - scaffolding protein
5. **Homer1a** - regulates mGluR signaling
↓
**New proteins synthesized locally:**
- Concentration increases over 1-2 hours
- Replaces initial plasticity with stable changes
Retrograde Signaling Synthesis
**For LTP:**
Ca²⁺ → activates nNOS (neuronal nitric oxide synthase)
↓
**NO synthesis from arginine:**
- Diffusion constant: ~3300 µm²/s
- Half-life: ~1-5 seconds
- Diffuses 10-20 µm to presynaptic terminal
↓
**BDNF synthesis and release:**
- Transcription begins in 30 min
- Release occurs 1-2 hours post-induction
PHASE 5: STRUCTURAL CONSOLIDATION (2 hours - 24 hours)
Actin Cytoskeleton Remodeling
**Spine enlargement (LTP):**
Active CaMKII → phosphorylates **Profilin**
↓
Profilin binds actin monomers → promotes polymerization
↓
**Rho GTPase activation:**
- Rac1 activated → promotes actin branching (via Arp2/3)
- Cdc42 activated → promotes filopodia formation
↓
**Actin polymerization in spine head:**
- F-actin increases 2-3×
- Spine volume increases over 1-3 hours
↓
**PSD expansion:**
- More space for AMPARs
- More PSD-95 scaffolding
↓
**By 6 hours:** Spine volume increased 50-100%
Nuclear Signaling and Gene Expression
**Signals reach nucleus (1-3 hours):**
1. **CaMKIV translocation:**
- Activated by Ca²⁺ in dendrites
- Translocates to nucleus when phosphorylated
2. **MAPK/ERK translocation:**
- Activated at synapse
- Travels to nucleus (active transport)
3. **CREB phosphorylation:**
- At S133 by CaMKIV/PKA/RSK
- Recruits CBP/p300 coactivators
↓
**Transcriptional activation (3-6 hours):**
Early genes (IEGs):
- c-Fos, c-Jun, Egr1/Zif268
Late genes (plasticity-related):
- **BDNF** (brain-derived neurotrophic factor)
- **GluA1** (AMPAR subunit)
- **CaMKIIα**
- **Arc**
- **Homer1a**
↓
**New proteins synthesized in soma (6-12 hours)**
↓
**Transport to dendrites (12-24 hours)**
↓
**Incorporation into spine (24+ hours)**
PHASE 6: VERY SLOW TIMESCALE (Days - Weeks) - STRUCTURAL STABILITY
Spine Maturation and Stabilization
**Day 1-7:**
- **PSD thickening:** from 30 nm → 50 nm
- **AMPAR subtype switch:**
GluA2-lacking (Ca²⁺-permeable) → GluA2-containing
(Occurs over days via subunit replacement)
- **Synaptic adhesion molecules:**
Neuroligin-Neurexin complexes stabilize contact
**Week 1-4:**
- **Spine shape changes:**
Thin → Mushroom (LTP)
Mushroom → Thin (LTD)
- **Presynaptic coordination:**
Active zone aligns with expanded PSD
- **Perisynaptic astrocyte processes:**
Enwrap mature synapse for metabolic support
Homeostatic Scaling
**Days 2-7:**
If overall neuron firing rate changes significantly:
↓
**Global scaling mechanisms:**
1. **TNFα signaling:** from astrocytes
2. **BDNF level changes**
↓
All synapses on neuron scaled up or down
↓
**AMPAR number adjusted** while relative differences maintained
↓
**Preserves signal-to-noise ratio** of individual synapses
COMPLETE LTP TIMELINE EXAMPLE
Induction (Seconds)
T=0 ms: Presynaptic glutamate release
T=10 ms: bAP arrives at spine (coincidence)
T=15 ms: [Ca²⁺] peaks at 25 µM
T=50 ms: Ca²⁺ clears to 1 µM
T=1 sec: CaMKII autophosphorylated (T286)
T=10 sec: CaMKII translocates to PSD
Early Expression (Minutes)
T=1 min: AMPARs inserted (from recycling endosomes)
T=2 min: EPSP amplitude increases 100%
T=5 min: Stargazin phosphorylated, AMPARs trapped
T=10 min: Early LTP established
Protein Synthesis-Dependent Phase (Hours)
T=30 min: Local translation begins (CaMKIIα, GluA1)
T=1 hour: BDNF transcription initiated
T=2 hours: Spine volume begins increasing
T=3 hours: New proteins from local synthesis incorporated
T=6 hours: Spine volume increased 60%
Late Maintenance (Days)
T=12 hours: New proteins from soma arrive
T=24 hours: Structural changes stabilized
T=48 hours: GluA2 subunits replace GluA1 homomers
T=7 days: Mature mushroom spine established
CALCIUM SIGNALING THRESHOLDS SUMMARY
| [Ca²⁺] Range | Duration | Sensor | Outcome |
|---|---|---|---|
| < 0.5 µM | Any | None | Baseline signaling |
| 0.5-1 µM | >1 sec | Calcineurin | Weak LTD |
| 1-5 µM | 100 ms-1 sec | Calcineurin | Strong LTD |
| 5-10 µM | Brief (<50 ms) | CaMKII | Weak LTP |
| >10 µM | Brief (<50 ms) | CaMKII | Strong LTP |
| >20 µM | Any | Calpain | Pathological, spine damage |
KEY BIOLOGICAL PRINCIPLES
- Spine as Biochemical Compartment:
- Neck resistance (50-500 MΩ) restricts diffusion
- Allows independent [Ca²⁺] signaling in each spine
- Enables synapse-specific plasticity
- Kinetic Competition:
- Calcineurin activates faster at low [Ca²⁺] (Kd ≈ 0.5 µM)
- CaMKII requires higher [Ca²⁺] but has positive feedback
- Winner-takes-all decision based on [Ca²⁺] time course
- Energy Requirements:
- Each AMPAR insertion: ~1000 ATP
- CaMKII autophosphorylation: 1 ATP/subunit
- Protein synthesis: ~4 ATP/amino acid
- ATP supplied by astrocyte lactate
- Timescale Coupling:
- Fast (ms): Receptor activation
- Medium (min): Trafficking existing proteins
- Slow (hours): Making new proteins
- Very slow (days): Structural changes
This postsynaptic timeline shows how a brief glutamate signal triggers a cascade of events across multiple timescales, converting transient electrical activity into lasting structural and functional changes that underlie learning and memory.