Synaptic organization in cochlear inner hair cells deficient for the CaV1.3 (α1D) subunit of L-type Ca2+ channels

Abstract

Cochlear inner hair cells (IHCs) release neurotransmitter onto afferent auditory nerve fibers in response to sound stimulation. Normal development and function of inner hair cells require the expression of α subunit 1.3 forming L-type voltage-gated Ca2+ channel (CaV1.3).Here, we used immunohistochemistry and reverse transcription-polymerase chain reaction to study the synaptic organization and expression of large conductance Ca2+-activated potassium channels in IHCs of mice lacking the CaV1.3 Ca2+ channel (CaV1.3−/−). Despite the near complete block of evoked afferent synaptic transmission, hair cell ribbon synapses were formed and remained preserved for at least 4 weeks after birth. Moreover, these “silent” afferent synapses held major components of the synaptic machinery such as Bassoon, Piccolo, and CSP. Hence, the block of exocytosis might be solely attributed to the lack of Ca2+ influx through CaV1.3 channels. Later on, CaV1.3 deficient IHCs subsequently lost their afferent synapses. This was probably due to a secondary degeneration of the postsynaptic spiral ganglion neurons.In line with a prolonged efferent synaptic transmission onto CaV1.3 deficient IHCs, which normally ceases around onset of hearing, we found juxtaposed immunoreactive spots of efferent presynaptic synaptophysin and postsynaptic (IHCs) small conductance Ca2+-activated potassium channels (SK channels) up to six weeks after birth. Finally, we show a substantial reduction of mRNA for the α subunit of the large conductance Ca2+-activated potassium channel (BK) in the apical cochlea, suggesting a reduced transcription of its gene in CaV1.3 deficient IHCs. CaV1.3 deficient IHCs lacked the apical spot-like immunoreactivity of clustered BK channels, which normally contribute to the temporal precision of hair cell afferent synaptic transmission.In summary, these data indicate that the CaV1.3 channels are crucially involved in regulation of the expression of BK and SK channels. CaV1.3 channels seem not to be essential for ribbon synapse formation, but are required for the maintenance of ribbon synapses and spiral ganglion neurons.