Those of us who have required some minor dental work in the past will be familiar with the main problem of routinely-used local anesthetics: they are not terribly selective. In addition to blocking the transmission of pain signals, locally administered drugs such as Lidocaine indiscriminately block non-nociceptive neurons as well, including motor and tactile neurons.

Lidocaine’s lack of selectivity is hardly surprising given the drug’s mechanism of action: it works by blocking voltage-gated sodium (NaV) channels, which are of course vital for the propagation of action potentials. In a paper recently published in Nature, Binshtok and co-workers describe a rather novel and clever strategy for selectively inhibiting only the NaV channels expressed on nociceptive neurons. To do this, they used a quaternary derivative of Lidocaine, QX-314, which is usually ineffective as a local anesthetic because it is not membrane permeable. Lidocaine-related anesthetics can only block NaV channels from the inside of the cell, and so must readily cross the plasma membrane in order to be active. QX-314 cannot enter the cell, and thus cannot inhibit NaV. However, another ion channel chiefly expressed in small diameter nociceptive neurons, TRPV1, is known to have a particularly wide channel pore capable of transporting relatively large molecules across the membrane, including organic molecules such as N-methyl-D-glucamine (NMDG) and the dye FM1-43. The authors hypothesized that QX-314 might be able to enter the neuron through activated TRPV1 channels, and subsequently block NaV channels and pain transmission. The idea being that this way only pain-transmitting nociceptors would be blocked by QX-314.

Anyone with a penchant for spicy food will be familiar with the function of TRPV1 receptors, which confer our sensitivity to the chili pepper ingredient, capsaicin. TRPV1 receptors are polymodal cation-permeable channels that open in response to a diverse array of noxious stimuli, including high temperature, acid, ethanol, spider venom toxins among others.

Using voltage clamp electrophysiology, the authors measured the amplitude of NaV currents evoked by depolarization of the cell membrane in isolated rat dorsal root ganglion (DRG) neurons. As expected, application of QX-314 alone to the bath solution had no effect on NaV currents evoked in DRGs. However, when capsaicin (used to activate and open the TRPV1 channels) and QX-314 were co-applied, the amplitude of the NaV currents were significantly reduced, while the NaV currents of large diameter, capsaicin-insensitive neurons were unaffected. The authors then carried out current clamp experiments where they showed that co-application of capsaicin and QX-314 significantly disrupted the initiation of action potentials in nociceptive neurons, consistent with the effect of inhibiting NaV channels.

Turning their attention to the whole animal, the authors tested whether the co-administration of capsaicin and QX-314 could selectively block pain transmission in vivo. They injected capsaicin and QX-314, separately or together, into the rat hindpaw, and tested the effects of these drugs on two withdrawal models of pain. First, they looked at how much force could be applied to the paw with a microfilament (von Frey filament) before the rat withdrew it. They discovered that 1 hour after co-administration of QX-314 and capsaicin, there was a significant and long-lasting increase in the amount of force that the rat could tolerate before withdrawing its paw. Next, the authors tested the effect of drug administration on the latency of withdrawal in response to a noxious heat stimulus to the hindpaw. Again, 1 hour after co-administration of capsaicin and QX-314 there was a significant and persistent increase in the latency of paw withdrawal, suggesting that the animal could tolerate higher temperatures before feeling discomfort. When applied alone, QX-314 had no effect on either of these responses. Capsaicin on the other hand actually decreased the rat’s tolerance for both mechanical and thermal stimuli, consistent with sensitizing nociception.

The next question to address was whether motor neurons were affected by the co-administration of capsaicin and QX-314. To test this, the drugs were injected separately or together in close proximity to the sciatic nerve, the main nerve branch extending into the leg. Consistent with the previous set of experiments, the presence of both QX-314 and capsaicin significantly increased the subject rat’s tolerance to both mechanical and thermal pain stimuli. However, the presence of these drugs had only a negligible affect on locomotion, showing that motor function was not substantially impaired. In contrast, application of Lidocaine severely disrupted locomotion in addition to suppressing pain transmission, as would be expected for a drug that effectively blocks all neuronal activity.

In summary, the authors of this paper succeeded in selectively delivering anesthetic to pain transmitting neurons, and in doing so significantly reduced the side effects usually associated with NaV-blocking drugs.

Discussion Questions:

Q1. Is this procedure therapeutically viable?
The time taken to establish a therapeutic effect is arguably only a minor drawback. The co-administration of QX-314 and capsaicin does admittedly take in excess of 30 min to be effective. In contrast, Lidocaine is usually maximally effective within a shorter time period of 5 to 15 minutes. However, an extra 20 minutes or so might be a small price to pay to avoid the indignities of the drooling, tongue lolling, rubber-faced side effects.

Q2. Could a similar procedure be used to attain specificity with regard to the application of other types of drugs to other types of cell?
The significance of these data arguably extends beyond the use of local anesthetics. TRPV1 receptors are not the only ion channels that are known to conduct large molecules across the membrane. Certain members of the adenosine 5’-triphosphate (ATP)-sensitive P2X receptor channels have been known to do so as well. Therefore, it is certainly plausible that the strategy described by Binshtok and co-workers could be applied in the delivery of other intracellularly-acting drugs to different cell types.