Ultrafast optical recording reveals distinct capsaicin-induced ion dynamics along single nociceptive neurite terminals in vitro

Pain signals are detected by terminals of nociceptive peripheral fibers situated among the keratinocytes and epithelial cells. Despite being key structures for pain-related stimuli detection and transmission, little is known about the functional organization of terminals. This is mainly due to their minute size, rendering them largely inaccessible by conventional experimental approaches. Here, we report the implementation of an ultrafast optical recording approach for studying cultured neurite terminals, which are readily accessible for assay manipulations. Using this approach, we were able to study capsaicin-induced calcium and sodium dynamics in the nociceptive processes, at a near-action potential time resolution. The approach was sensitive enough to detect differences in latency, time-to-peak, and amplitude of capsaicin-induced ion transients along the terminal neurites. Using this approach, we found that capsaicin evokes distinctive calcium signals along the neurite. At the terminal, the signal was insensitive to voltage-gated sodium channel blockers, and showed slower kinetics and smaller signal amplitudes, compared with signals that were measured further up the neurite. These latter signals were mainly abolished by sodium channel blockers. We propose this ultrafast optical recording approach as a model for studying peripheral terminal signaling, forming a basis for studying pain mechanisms in normal and pathological states.