Two Channels, One Nociceptor
Pain-sensing neurons express nine sodium channel subtypes, but two dominate.
- 9: NaV subtypes in DRG neurons (NaV1.1–NaV1.9 family)
- SCN9A: NaV1.7 gene (chromosome 2q24)
- SCN10A: NaV1.8 gene (chromosome 3p22)
- Nociceptor: Shared cell type (peripheral sensory neuron)
A crowded axon
Nociceptors pack multiple sodium channel subtypes along one membrane.
Division of labor
Each channel shapes a different phase of the pain signal.
Why compare them
Selective blockade of one channel may relieve pain safely.
NaV1.7 Sets the Firing Threshold
NaV1.7 opens near resting potential, amplifying small generator signals.
- Low: Activation voltage (near resting membrane potential)
- No pain: Loss-of-function effect (congenital insensitivity to pain)
- Severe pain: Gain-of-function effect (erythromelalgia syndromes)
- Broad: Expression breadth (DRG, sympathetic, some CNS-adjacent)
Threshold amplifier role
NaV1.7 boosts weak stimuli toward the firing threshold.
Human genetic proof
SCN9A mutations cause either no pain or extreme pain.
The selectivity problem
NaV1.7 is expressed too widely for a clean drug target.
NaV1.8 Drives the Action Potential Upstroke
NaV1.8 resists inactivation, carrying most nociceptor spike current.
- Slow: Inactivation kinetics (stays open during depolarization)
- ~80%: Upstroke contribution (of nociceptor spike current)
- Resistant: Tetrodotoxin sensitivity (distinguishes it pharmacologically)
- Narrow: Expression breadth (peripheral sensory neurons mainly)
Slow-inactivating current
NaV1.8 keeps conducting through sustained depolarization.
TTX-resistant signature
A distinct pharmacology separates NaV1.8 from most NaV channels.
A tighter target profile
Narrower expression makes NaV1.8 easier to block selectively.
Peripheral Restriction Favors NaV1.8
Body-wide mapping shows NaV1.8 confined mostly to peripheral neurons.
- Wide: NaV1.7 tissue reach (DRG, olfactory, sympathetic ganglia)
- Narrow: NaV1.8 tissue reach (peripheral sensory neurons)
- None: CNS penetration needed (for peripheral-restricted target)
- Several: Off-target organs (NaV1.7) (incl. CNS-adjacent ganglia)
Mapping the body
Expression atlases show where each channel actually sits.
NaV1.7 spillover risk
CNS-adjacent expression raises central side-effect concerns.
NaV1.8 containment
Peripheral restriction limits drug exposure to pain fibers only.
Choosing the Selective Target
Drug selectivity determines how cleanly a compound spares the CNS.
- In trials: NaV1.8-selective candidates (e.g. suzetrigine class)
- Yes: Opioid-sparing goal (non-addictive analgesic mechanism)
- Minimal: CNS exposure (high selectivity) (peripheral restriction preserved)
- Significant: CNS exposure (broad drug) (crosses into central circuits)
Selectivity dial
Raising drug selectivity shifts binding toward the chosen channel.
Why NaV1.8 wins on safety
Peripheral restriction lowers central side-effect exposure.
Clinical translation
Selective NaV1.8 blockers are advancing as non-opioid analgesics.