Capsaicin and nicotine both activate a subset of rat trigeminal ganglion neurons

Nicotine and capsaicin produce many similar physiological responses that include pain, irritation, and vasodilation. To determine whether neuronal nicotine acetylcholine receptors (nAChR) are present on capsaicin-sensitive neurons, whole cell patch-clamp recordings were performed on rat trigeminal ganglion cells. It was found that similar to 20% of the total number of neurons tested was activated by both 100 mu M nicotine and 1 mu M capsaicin. Other subsets of neurons were activated by only one of these compounds, whereas a fourth subset was not activated by either compound. At -60 mV, the magnitude of the capsaicin-activated currents was about three times larger than the magnitude of the nicotine-activated currents. The current-voltage relationship of the nAChR exhibited marked rectification, such that for voltages greater than or equal to 0 mV the current was essentially zero. In contrast, the current-voltage relationship of the capsaicin-activated current was ohmic from +/-60 mV. These data indicate the existence of subsets of capsaicin-sensitive afferent neurons.