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 the identification of drugs most potent on nociceptors with no effects on non-nociceptors (as a proxy for unwanted side effects on CNS 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 an unbiased and efficient assessment of a drug effect on nociceptors (targeted neurons) and non-nociceptors (non-targeted neurons). We confirmed the link between the oscillatory profile and nociceptors; and the slow-decay profile and non-nociceptors using three transgenic mouse lines of known pain phenotypes. We used the assay to show that blockers for Nav1.7 and Nav1.8 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.