Postnatal developmental changes in the responses of mouse primary vestibular neurons to externally applied galvanic currents.

The ontogenesis of vestibular primary neuron sensitivity to depolarisation produced by galvanic current stimulations was studied in mouse inner ear explants maintained in vitro. Cathodal galvanic stimulations, which elicit an increase of the discharge frequencies, are assumed to act on the spike initiation site by depolarizing the neuron. The responses of neurons to galvanic currents at various developmental stages were recorded. The pattern of responses reflected the sensitivities of the neurons to depolarization. At birth, about 75% of the vestibular neurons responded weakly to high intensity galvanic currents thus indicating that they were able to generate action potentials. However, the very low gain of the response to the stimulation revealed the immaturity of the neurons at the spike generation site. Between the day of birth and the ninth postnatal day, an increase in the gain of the responses was observed, indicating the enhancement of the sensitivity of the vestibular neurons to the galvanic currents. This increase in sensitivity was more pronounced from the fourth postnatal day. The response of the neurons to galvanic stimulation increased gradually during postnatal development without reaching a plateau at postnatal day 9 indicating that a further physiological maturation occurs after this stage. These results are consistent with the morphological maturation of the vestibular primary afferents and with previous studies showing that the physiological maturation parallels myelination of the afferent fibers.