Effects on tactile transmission by serotonin transporter inhibitors at Merkel discs of mouse whisker hair follicles

The Merkel disc is a main type of tactile end organs formed by Merkel cells and Aβ-afferent endings as first tactile sensory synapses. They are highly abundant in fingertips, touch domes, and whisker hair follicles of mammals and are essential for sensory tasks including social interaction, environmental exploration, and tactile discrimination. We have recently shown that Merkel discs use serotonin to transmit tactile signals from Merkel cells to Aβ-afferent endings to drive slowly adapting type 1 impulses on the Aβ-afferent nerves. This raises a question as whether the serotoninergic transmission at Merkel discs may be regulated by serotonin transporters and whether serotonin transporter inhibitors may affect the tactile transmission. Here, we made recordings from whisker afferent nerves of mouse whisker hair follicles and tested the effects of monoamine transporter inhibitors on slowly adapting type 1 impulses. We show that methamphetamine, a monoamine releasing facilitator and reuptake inhibitor, elicited spontaneous impulses as well as increased the numbers of slowly adapting type 1 impulses elicited by whisker hair deflections. S-duloxetine, a potent inhibitor of transporters of serotonin and norepinephrine, and fluoxetine, a selective inhibitor of serotonin transporters, both also increased the numbers of slowly adapting type 1 impulses. Prolonged treatment of whisker hair follicles with methamphetamine abolished most of slowly adapting type 1 impulses. Furthermore, the treatment of whisker hair follicles with methamphetamine resulted in serotonin release from whisker hair follicles. Taken together, our results suggest that serotonin transporters play a role in regulating tactile transmission at Merkel discs.