Voltage-gated Na+ channel activation induces both action potentials in utricular hair cells and brain-derived neurotrophic factor release in the rat utricle during a restricted period of development
Chabbert, C.; Mechaly, I.; Sieso, V.; Giraud, P.; Brugeaud, A.; Lehouelleur, J.; Couraud, F.ço.; Valmier, J.; Sans, A.
Journal of Physiology 553(Part 1): 113-123
ISSN/ISBN: 0022-3751 PMID: 12963806 DOI: 10.1113/jphysiol.2003.043034
The mammalian utricular sensory receptors are commonly believed to be non-spiking cells with electrical activity limited to graded membrane potential changes. Here we provide evidence that during the first post-natal week, the sensory hair cells of the rat utricle express a tetrodotoxin (TTX)-sensitive voltage-gated Na+ current that displays most of the biophysical and pharmacological characteristics of neuronal Na+ current. Single-cell RT-PCR reveals that several alpha-subunit isoforms of the Na+ channels are co-expressed within a single hair cell, with a major expression of Nav1.2 and Nav1.6 subunits. In neonatal hair cells, 30 % of the Na+ channels are available for activation at the resting potential. Depolarizing current injections in the range of the transduction currents are able to trigger TTX-sensitive action potentials. We also provide evidence of a TTX-sensitive activity-dependent brain-derived neurotrophic factor (BDNF) release by early post-natal utricle explants. Developmental analysis shows that Na+ currents decrease dramatically from post-natal day 0 (P0) to P8 and become almost undetectable at P21. Concomitantly, depolarizing stimuli fail to induce both action potential and BDNF release at P20. The present findings reveal that vestibular hair cells express neuronal-like TTX-sensitive Na+ channels able to generate Na+-driven action potentials only during the early post-natal period of development. During the same period an activity-dependent BDNF secretion by utricular explants has been demonstrated. This could be an important mechanism involved in vestibular sensory system differentiation and synaptogenesis.