Nav1.8 mediates peripheral-to-central nociceptive transmission independently of central presynaptic mechanisms in human DRG-spinal cord circuits

Using a novel intact human DRG-spinal cord circuit, this study demonstrates that the analgesic efficacy of the Nav1.8 blocker suzetrigine relies on inhibiting peripheral action potential propagation rather than central presynaptic mechanisms, suggesting that improved CNS penetration would not enhance its pain-relieving effects.

The Gist

Imagine your body's pain system as a massive, high-speed telephone network. When you stub your toe, a signal travels from your foot (the periphery) up a long wire (the nerve) to the central switchboard in your spine (the spinal cord). Once the signal hits the switchboard, it triggers a loud alarm bell (releasing a chemical called CGRP) that tells your brain, "Hey, we have a problem!"

For years, scientists have been trying to build a better "mute button" for this alarm system to treat chronic pain without using opioids. One promising candidate is a drug called Suzetrigine (brand name JOURNAVX), which targets a specific protein called Nav1.8. Think of Nav1.8 as the battery or the engine that keeps the electrical signal moving along the nerve wire.

However, there was a big mystery: Does this drug work by stopping the signal before it leaves the foot, or does it work by jamming the switchboard inside the spine? If it works at the switchboard, maybe we need to get more of the drug into the brain to make it stronger. If it only works at the foot, then getting it into the brain is a waste of time.

The Experiment: Building a Human "Test Track"

To solve this mystery, the researchers couldn't just use mice, because human nerves are different. Instead, they did something incredibly rare and difficult: they built a living, human pain circuit in a lab dish.

They took tissue from organ donors and created a setup that looked like a three-lane highway:

1. The Source: The Dorsal Root Ganglion (DRG), where the nerve cell bodies live (like the power plant).
2. The Wire: The nerve root connecting the source to the destination.
3. The Destination: The Spinal Cord, where the signal is supposed to arrive.

Crucially, they kept the "wire" connected between the source and the destination, preserving the natural path the pain signal takes.

The Test: Ringing the Bell

The researchers used a chemical called Capsaicin (the stuff in hot chili peppers) to "ring the bell."

• They applied Capsaicin to the "Source" (the DRG).
• This triggered the nerves to fire.
• They then measured if the "Alarm Bell" (CGRP) rang in the "Destination" (the Spinal Cord).

The Result: The bell rang loudly in the spinal cord, but not in the source. This proved that in a healthy human circuit, the pain signal travels all the way to the spine before releasing the alarm chemical.

The Big Discovery: Where the Mute Button Works

Now, they tested the drugs to see where they could stop the signal.

1. The "All-Purpose" Blocker (Bupivacaine):
When they applied a general anesthetic to the nerve wire, the signal stopped completely. The bell never rang. This confirmed the system was working.

2. The "Specific" Mute Button (Suzetrigine/Nav1.8 Inhibitor):
This is where it gets interesting. They tested the drug in two different places:

• Scenario A: Applying the drug to the Nerve Wire (DRG/Root).

  • Result: The signal stopped dead. The bell in the spinal cord stayed silent.
  • Analogy: It's like cutting the power to the telephone pole. The signal never even gets to the switchboard.

• Scenario B: Applying the drug directly to the Switchboard (Spinal Cord).

  • Result: The signal arrived, and the bell rang just as loudly as before. The drug did nothing to stop the release of the alarm chemical at the destination.
  • Analogy: It's like standing at the switchboard and trying to stop the bell by shouting at the machine. The machine doesn't care; the signal arrived, and the alarm went off.

The "Aha!" Moment

The researchers found that Nav1.8 is only the engine for the journey, not the trigger for the alarm.

• Nav1.8 is essential for the signal to travel along the wire from your foot to your spine.
• Nav1.8 is NOT involved in the final step of releasing the alarm chemical once the signal arrives at the spine.

What This Means for Patients

This is a huge deal for how we develop pain drugs.

For a long time, scientists wondered: "If we make a stronger version of Suzetrigine that gets deep into the brain and spinal cord, will it work even better?"

This study says No.

Because Nav1.8 doesn't do anything at the spinal switchboard, getting the drug into the brain won't make it more effective. In fact, trying to get it into the brain might just cause unnecessary side effects (like dizziness or confusion) without adding any pain relief.

The Takeaway:
The best way to use this drug is to focus on blocking the signal at the source (the nerves in your body), not at the destination. The drug works by stopping the message from being sent, not by silencing the receiver. This explains why the drug is effective for acute pain but suggests that for it to work even better, we need to ensure it reaches the nerves in your body, not necessarily your brain.

In short: You don't need to jam the switchboard; you just need to cut the wire.

Technical Summary

1. Problem Statement

Chronic pain affects a significant portion of the adult population, yet translating analgesic mechanisms from animal models to effective human therapies remains a major challenge. Nav1.8, a voltage-gated sodium channel selectively expressed in small-diameter sensory neurons, is a validated analgesic target. Suzetrigine (JOURNAVX), a selective Nav1.8 inhibitor, was recently FDA-approved for acute pain but shows modest efficacy in chronic pain conditions.

A critical mechanistic gap exists regarding the anatomical locus of Nav1.8 action in humans:

• Does Nav1.8 regulate action potential propagation along the axon from the periphery to the spinal cord?
• Does Nav1.8 regulate neurotransmitter release (specifically Calcitonin Gene-Related Peptide, CGRP) directly at the central presynaptic terminals within the spinal dorsal horn?
• Would improved Central Nervous System (CNS) penetration of Nav1.8 inhibitors enhance analgesic efficacy, or is the target strictly peripheral?

Current animal models cannot fully replicate human nociceptive circuitry, and dissociated human neuron cultures lack the anatomical continuity required to study directional signal propagation.

2. Methodology

The authors established a novel human organotypic nociceptive circuit assay using tissues from 21 organ donors. The study utilized three distinct preparation types to isolate specific mechanisms:

• Tissue Sources: Acute explants of adult human Dorsal Root Ganglia (DRG), spinal cord, and an intact DRG-spinal cord circuit preserving the dorsal root connection (ventral roots transected) to maintain directional peripheral-to-central signaling.
• Functional Readout: The release of CGRP, a neuropeptide abundant in TRPV1-positive nociceptors, was quantified using ELISA and Immunohistochemistry (IHC).
• Stimulation: Capsaicin (1–10 µM) was used to selectively activate TRPV1-positive afferents to evoke CGRP release.
• Pharmacological Interventions:
  • Suzetrigine: A selective Nav1.8 inhibitor (20 nM, 200 nM).
  • Bupivacaine: A broad-spectrum sodium channel blocker (100 µM).
  • Morphine: An opioid agonist used as a positive control for presynaptic inhibition.
• Experimental Design:
  • Isolated Spinal Cord: To test if Nav1.8 regulates presynaptic release directly.
  • Isolated DRG: To test somatic excitability.
  • Intact Circuit: Drugs were applied selectively to the DRG cell bodies, the nerve roots (dorsal roots), or the spinal cord to determine the specific anatomical site where Nav1.8 blockade inhibits signal transmission.
• Viability: Confirmed via MTS assays to ensure capsaicin-induced release was not due to tissue toxicity.

3. Key Contributions

• First Intact Human Pain Circuit Assay: The study successfully developed and validated a human ex-vivo model that preserves primary afferent anatomy and directional signaling, allowing for the direct testing of pharmacologic modulation in an intact human nociceptive circuit.
• Spatial Resolution of Nav1.8 Action: The study definitively mapped the functional role of Nav1.8 in human pain transmission, distinguishing between axonal propagation and presynaptic release.
• Mechanistic Insight into Suzetrigine: The work provides a structural basis for the clinical efficacy (and limitations) of suzetrigine, clarifying why systemic administration works but suggesting that CNS penetration may not be the limiting factor for efficacy.

4. Key Results

A. Validation of the Human Circuit Model

• Compartmentalized Release: In the intact DRG-spinal cord circuit, stimulation of the DRG with capsaicin triggered a robust (~1.8-fold) increase in CGRP release exclusively in the spinal cord compartment. No increase was detected in the DRG well, confirming that neuropeptide release is tightly compartmentalized to central terminals in intact adult human circuits.
• Viability: Capsaicin stimulation did not compromise tissue viability.

B. Nav1.8 is Essential for Peripheral Signal Propagation

• DRG/Root Blockade: When suzetrigine was applied to the DRG cell bodies or the dorsal nerve roots, it completely abolished capsaicin-evoked CGRP release in the spinal cord.
• Immunohistochemistry: Nav1.8 protein was robustly expressed in DRG neurons and dorsal roots (co-localizing with peripherin) but was absent in the spinal cord.
• Conclusion: Nav1.8 is critical for the propagation of action potentials from the periphery to the spinal cord.

C. Nav1.8 Does Not Regulate Central Presynaptic Release

• Spinal Cord Application: When suzetrigine was applied directly to isolated spinal cord slices (bypassing the DRG), it had no effect on capsaicin-evoked CGRP release, even at high concentrations (200 nM).
• Comparison with Controls:
  • Bupivacaine (broad Na+ blocker) applied to the spinal cord nearly abolished CGRP release (~70% reduction), indicating that other sodium channels (e.g., Nav1.7) or general ion channel blockade can inhibit terminal release.
  • Morphine significantly reduced release (~45%), consistent with presynaptic opioid inhibition of calcium-dependent exocytosis.
• Conclusion: Nav1.8 does not regulate neurotransmitter release at the central presynaptic terminals in the human spinal cord.

D. IHC Confirmation

Quantitative immunohistochemistry confirmed that while morphine and bupivacaine reduced dorsal horn CGRP staining intensity (indicating reduced release), suzetrigine produced no detectable change compared to capsaicin-only controls.

5. Significance and Implications

• Mechanism of Analgesia: The analgesic efficacy of Nav1.8 inhibitors like suzetrigine relies on blocking peripheral action potential propagation (in the DRG and nerve roots), not on inhibiting neurotransmitter release at the spinal cord.
• Drug Development Strategy: The findings suggest that improving CNS penetration of Nav1.8 inhibitors is unlikely to enhance their analgesic efficacy. Since Nav1.8 is not functionally involved in central presynaptic release, a drug that cannot cross the blood-brain barrier (or has low CNS penetration) is not at a mechanistic disadvantage for this specific target.
• Clinical Translation: The modest efficacy of suzetrigine in chronic pain settings may be due to insufficient drug concentration reaching the DRG cell bodies or proximal nerve roots systemically, rather than a failure to act centrally. Strategies such as perineural administration or developing inhibitors with better nerve bundle penetration could be more effective.
• Model Utility: This human organotypic circuit provides a critical platform for evaluating human-specific analgesic mechanisms, bridging the gap between animal models and clinical trials.

In summary, this study establishes that in human nociceptive circuits, Nav1.8 functions strictly as a gatekeeper for peripheral signal propagation to the spinal cord, and its blockade at the central terminal is ineffective. This redefines the therapeutic window for Nav1.8 inhibitors and guides future drug development strategies.