第八脳神経系におけるイオンチャネル及びシナプス伝達の発達変化

Abstract

The functions of the eighth cranial nerve system changes markedly during the early postnatal period. The vestibulo-ocular reflex appears on postnatal day 2 in rats and its gain increases rapidly within 10 days. During the same period, the proportion of the regularly firing vestibular neurons increases, while that of the irregularly firing neurons decreases. The intrinsic firing properties of the isolated vestibular ganglion cells are heterogeneous, and the low-voltage activated K+ channels are critical to determining the pattern of the spike discharges. The firing patterns of these cells show considerable changes during the first 14 days of postnatal life. This developmental change might be regulated by some neurotrophic factors. At the brainstem auditory synapses, the Ca2+ channel types that trigger transmitter release switch from a mixture of P/Q-, N-, and R-types to predominantly the P/Q-types. This change is caused by replacement of the N-and R-type Ca2+ channels by P/Q-type calcium channels at the presynaptic nerve terminals. Since P/Q-type Ca2+ channels are more efficient for transmitter release, the developmental change of the Ca2+ channel types may contribute to high-fidelity synaptic transmission. Synaptic efficacy also changes during the same postnatal period at this synapse. The size of the readily releasable pool of synaptic vesicles increases, while the release probability concomitantly decreases. These developmental changes contribute to the formation of stable synapses at which only a small fraction of releasable synaptic vesicles is depleted during high-frequency tranmission. The eighth cranial nerve system changes its firing properties, ion channels and synaptic efficacy during the early postnatal period, towards establishing efficient synaptic function.