RESUMO
Signal transmission by sensory auditory and vestibular hair cells relies upon Ca2+-dependent exocytosis of glutamate. The Ca2+ current in mammalian inner ear hair cells is predominantly carried through Ca V 1.3 voltage-gated Ca2+ channels. Despite this, Ca V 1.3 deficient mice (Ca V 1.3-/- ) are deaf but do not show any obvious vestibular phenotype. Here, we compared the Ca2+ current (I Ca ) in auditory and vestibular hair cells from wild-type and Ca V 1.3-/- mice, to assess whether differences in the size of the residual I Ca could explain, at least in part, the two phenotypes. Using 5 mM extracellular Ca2+ and near-body temperature conditions, we investigated the cochlear primary sensory receptors inner hair cells (IHCs) and both type I and type II hair cells of the semicircular canals. We found that the residual I Ca in both auditory and vestibular hair cells from Ca V 1.3-/- mice was less than 20% (12-19%, depending on the hair cell type and age investigated) compared to controls, indicating a comparable expression of Ca V 1.3 Ca2+ channels in both sensory organs. We also showed that, different from IHCs, type I and type II hair cells from Ca V 1.3-/- mice were able to acquire the adult-like K+ current profile in their basolateral membrane. Intercellular K+ accumulation was still present in Ca V 1.3-/- mice during I K,L activation, suggesting that the K+-based, non-exocytotic, afferent transmission is still functional in these mice. This non-vesicular mechanism might contribute to the apparent normal vestibular functions in Ca V 1.3-/- mice.
RESUMO
KEY POINTS: Outer hair cells (OHCs) enhance the sensitivity and the frequency tuning of the mammalian cochlea. Similar to the primary sensory receptor, the inner hair cells (IHCs), the mature functional characteristics of OHCs are acquired before hearing onset. We found that OHCs, like IHCs, fire spontaneous Ca2+ -induced action potentials (APs) during immature stages of development, which are driven by CaV 1.3 Ca2+ channels. We also showed that the development of low- and high-frequency hair cells is differentially regulated during pre-hearing stages, with the former cells being more strongly dependent on experience-independent Ca2+ action potential activity. ABSTRACT: Sound amplification within the mammalian cochlea depends upon specialized hair cells, the outer hair cells (OHCs), which possess both sensory and motile capabilities. In various altricial rodents, OHCs become functionally competent from around postnatal day 7 (P7), before the primary sensory inner hair cells (IHCs), which become competent at about the onset of hearing (P12). The mechanisms responsible for the maturation of OHCs and their synaptic specialization remain poorly understood. We report that spontaneous Ca2+ activity in the immature cochlea, which is generated by CaV 1.3 Ca2+ channels, differentially regulates the maturation of hair cells along the cochlea. Under near-physiological recording conditions we found that, similar to IHCs, immature OHCs elicited spontaneous Ca2+ action potentials (APs), but only during the first few postnatal days. Genetic ablation of these APs in vivo, using CaV 1.3-/- mice, prevented the normal developmental acquisition of mature-like basolateral membrane currents in low-frequency (apical) hair cells, such as IK,n (carried by KCNQ4 channels), ISK2 and IACh (α9α10nAChRs) in OHCs and IK,n and IK,f (BK channels) in IHCs. Electromotility and prestin expression in OHCs were normal in CaV 1.3-/- mice. The maturation of high-frequency (basal) hair cells was also affected in CaV 1.3-/- mice, but to a much lesser extent than apical cells. However, a characteristic feature in CaV 1.3-/- mice was the reduced hair cell size irrespective of their cochlear location. We conclude that the development of low- and high-frequency hair cells is differentially regulated during development, with apical cells being more strongly dependent on experience-independent Ca2+ APs.
