Electrophysiological property and chemical sensitivity of primary afferent neurons that innervate rat whisker hair follicles

Abstract

Whisker hair follicles are sensory organs that sense touch and perform tactile discrimination in animals, and they are sites where sensory impulses are initiated when whisker hairs touch an object. The sensory signals are then conveyed by whisker afferent fibers to the brain for sensory perception. Electrophysiological property and chemical sensitivity of whisker afferent fibers, important factors affecting whisker sensory processing, are largely not known. In the present study, we performed patch-clamp recordings from pre-identified whisker afferent neurons in whole-mount trigeminal ganglion preparations and characterized their electrophysiological property and sensitivity to ATP, serotonin and glutamate. Of 97 whisker afferent neurons examined, 67% of them are found to be large-sized (diameter ≥45 µm) cells and 33% of them are medium- to small-sized (diameter <45 µm) cells. Almost every large-sized whisker afferent neuron fires a single action potential but many (40%) small/medium-sized whisker afferent neurons fire multiple action potentials in response to prolonged stepwise depolarization. Other electrophysiological properties including resting membrane potential, action potential threshold, and membrane input resistance are also significantly different between large-sized and small/medium-sized whisker afferent neurons. Most large-sized and many small/medium-sized whisker afferent neurons are sensitive to ATP and/or serotonin, and ATP and/or serotonin could evoke strong inward currents in these cells. In contrast, few whisker afferent neurons are sensitive to glutamate. Our results raise a possibility that ATP and/or serotonin may be chemical messengers involving sensory signaling for different types of rat whisker afferent fibers.

Serotonergic transmission at Merkel discs: modulation by exogenously applied chemical messengers and involvement of Ih currents

“…Studies have shown the presence of vesicular glutamate transporters in Merkel cells (Maksimovic et al . ) and expression of P2X receptors in whisker afferent neurons (Ikeda and Gu ). Furthermore, we have recently shown that 5‐HT receptors and P2X receptors are co‐expressed in the majority of large‐sized whisker afferent neurons of the rats (Ikeda and Gu ).…”

“…) and expression of P2X receptors in whisker afferent neurons (Ikeda and Gu ). Furthermore, we have recently shown that 5‐HT receptors and P2X receptors are co‐expressed in the majority of large‐sized whisker afferent neurons of the rats (Ikeda and Gu ). The lack of effects on SA1 responses by glutamate, ATP, and other tested candidates may suggest that their receptors are not present at the postsynaptic sites in Aβ‐afferent terminals of Merkel discs.…”