Predominance of alpha1D subunit in L-type volttage-gated Ca2+ channels of hair cells in the chicken's cochlea

The voltage-gated [Ca.sup.2+] channels that effect tonic release of neurotransmitter from hair cells have unusual pharmacological properties: unlike most presynaptic [Ca.sup.2+] channels, they are sensitive to dihydropyridines and therefore are L-type. To characterize these [Ca.sup.2+] channels, we investigated the expression of L-type [[Alpha].sub.1] subunits in hair cells of the chicken's cochlea. In PCRs with five different pairs of degenerate primers, we always obtained [[Alpha].sub.1D] products, but only once an [[Alpha].sub.1C] product and never an [[Alpha].sub.1S] product. A full-length [[Alpha].sub.1D] mRNA sequence was assembled from overlapping PCR products: the predicted amino acid sequence of the [[Alpha].sub.1D] subunit was about 90% identical to those of the mammalian [[Alpha].sub.1D] subunits. In situ hybridization confirmed that the [[Alpha].sub.1D] mRNA is present in hair cells. By using a quantitative PCR assay, we determined that the [[Alpha].sub.1D] mRNA is 100-500 times more abundant than the [[Alpha].sub.1C] mRNA. We conclude that most, if not all, voltage-gated [Ca.sup.2+] channels in hair cells contain an [[Alpha].sub.1D] subunit. Furthermore, we propose that the [[Alpha].sub.1D] subunit plays a hitherto undocumented role at tonic synapses.