An unbiased and efficient assessment of excitability of sensory neurons for analgesic drug discovery

Alleviating chronic pain is challenging, due to lack of drugs that effectively inhibit nociceptors without off-target effects on motor or central neurons. Dorsal root ganglia (DRG) contain nociceptive and non-nociceptive neurons. Drug screening on cultured DRG neurons, rather than cell lines, allows for the identification of drugs most potent on nociceptors with no effects on non-nociceptors (as a proxy for unwanted side effects on central nervous system and motor neurons). However, screening using DRG neurons is currently a low-throughput process, and there is a need for assays to speed this process for analgesic drug discovery. We previously showed that veratridine elicits distinct response profiles in sensory neurons. Here, we show evidence that a veratridine-based calcium assay allows for an unbiased and efficient assessment of a drug effect on nociceptors (targeted neurons) and non-nociceptors (nontargeted neurons). We confirmed the link between the oscillatory profile and nociceptors, and the slow-decay profile and non-nociceptors using 3 transgenic mouse lines of known pain phenotypes. We used the assay to show that blockers for Na(v)1.7 and Na(v)1.8 channels, which are validated targets for analgesics, affect non-nociceptors at concentrations needed to effectively inhibit nociceptors. However, a combination of low doses of both blockers had an additive effect on nociceptors without a significant effect on non-nociceptors, indicating that the assay can also be used to screen for combinations of existing or novel drugs for the greatest selective inhibition of nociceptors.