Abstract
Merkel discs, located in skin touch domes and whisker hair follicles, are tactile end organs essential for environmental exploration, social interaction, and tactile discrimination. Recent studies from our group and two others have shown that mechanical stimulation excites Merkel cells via Piezo2 channel activation to subsequently activate sensory neural pathways. We have further shown that mechanical stimulation leads to the release of 5-HT from Merkel cells to synaptically transmit tactile signals to whisker afferent nerves. However, a more recent study using skin touch domes has raised the possibility that Merkel discs are adrenergic synapses. It was proposed that norepinephrine is released from Merkel cells upon mechanical stimulation to subsequently activate β2 adrenergic receptors on Merkel disc nerve endings leading to nerve impulses. In the present study, we examined effects of norepinephrine and β2 adrenergic receptor antagonist ICI 118,551 on Merkel disc mechanoreceptors in mouse whisker hair follicles. We show that norepinephrine did not directly induce impulses from Merkel disc mechanoreceptors. Furthermore, we found that ICI 118,551 at 50 μM inhibited voltage-gated Na+ channels and suppressed impulses of Merkel disc mechanoreceptors, but ICI 118,551 at 1 μM had no effects on the impulse. These findings challenge the hypothesis of Merkel discs being adrenergic synapses.
Highlights
Merkel discs, located in skin touch domes and whisker hair follicles, are tactile end organs essential for environmental exploration, social interaction, and tactile discrimination
We have recently shown that Merkel discs in whisker hair follicles are serotonergic synapses [8], but a more recent study has demonstrated that Merkel discs in skin touch domes are adrenergic synapses [9]
Adapting type 1 (SA1) impulses could be recorded following mechanically probing whisker hair follicles (Fig. 1a, top trace), which indicated the activation of Merkel disc mechanoreceptors
Summary
Merkel discs, located in skin touch domes and whisker hair follicles, are tactile end organs essential for environmental exploration, social interaction, and tactile discrimination. Main In both touch domes and whisker hair follicles, Merkel cells and their associated afferent endings form synaptic-like structures called Merkel discs [1–3]. We have recently shown that Merkel discs in whisker hair follicles are serotonergic synapses [8], but a more recent study has demonstrated that Merkel discs in skin touch domes are adrenergic synapses [9].
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