Effects on low threshold mechanoreceptors in whisker hair follicles by 5-HT, Cd(2+), tetraethylammonium, 4-aminopyridine, and Ba(2+)

Low threshold mechanoreceptors (LTMRs) are important for environmental exploration, social interaction, and tactile discrimination. Whisker hair follicles are mechanical sensory organs in non-primate mammals that are functionally equivalent to human fingertips. Several functional types of LTMRs have been identified in rodent whisker hair follicles, including rapidly adapting (RA), slow adapting type 1 (SA1), and slowly adapting type 2 (SA2) LTMRs. Properties of these LTMRs have not been fully characterized. In the present study, we have used pressure-clamped single-fiber recording technique to record impulses of RA, SA1, and SA2 LTMRs in mouse whisker hair follicles, and tested effects of 5-HT, Cd(2+), tetraethylammonium (TEA), 4-aminopyridine (4-AP), and Ba(2+) on the LTMR impulses. We show that 5-HT at 2 mM suppresses SA1 impulses but has no effects on RA and SA2 impulses. Cd(2+) at 100 μM suppresses both SA1 and SA2 impulses but has no effects on RA impulses. TEA at 10 mM has no effects on RA and SA1 impulses but increased SA2 impulses. However, TEA at 1 mM and 200 μM decreases SA2 impulses. 4-AP at 1 mM suppresses both SA1 and SA2 impulses but has no effects on RA impulses. Ba(2+) at 5 mM increases both RA and SA1 impulses but suppresses SA2 impulses. Collectively, RA, SA1, and SA2 LTMRs show distinct pharmacological properties, suggesting that these LTMRs may use different mechanisms to tune their mechanical signaling.