Postnatal development of hypoglossal motoneuron intrinsic properties

This review has provided evidence that marked changes are occurring in ionic currents present in upper airway motoneurons during the early postnatal period. Our results have shown that the density of the LVA Ca2+ current decreases during this period, and this probably reflects a reduced expression of the Ca2+ channel responsible for this current, the so-called T-type channel. These results help to explain the changes in burst firing behavior of HMs during the early postnatal period. We have shown that the fraction of HMs exhibiting burst firing behavior was the greatest among HMs just at or after birth, and disappeared by 10 days of age (Viana et al, 1993). The LVA Ca2+ current contributes to this firing behavior. In contrast to the reduction in the LVA Ca2+ current density with postnatal development, there is an apparent increase in Ih current density during this period. The increase in Ih provides a basis for a number of differences in the electrophysiological properties of adult versus neonate HMs. These include a striking depolarizing sag and overshoot during and immediately after application of hyperpolarizing current pulses in adult HMs. It is of interest that rebound depolarization following hyperpolarization can be observed in neonatal HMs even though there is little Ih present. This response probably reflects the activation of a LVA Ca2+ current. Other differences in neonate versus adult HMs also are in part probably due to differences in Ih current density. Since Ih is active at normal resting membrane potential (approximately -70 mV), Ih may contribute to the lower input resistance of adult compared with neonatal HMs (Haddad et al, 1990; Núñez-Abades et al, 1993; Viana et al, 1994), and the lower apparent membrane resistivity of older HMs (Viana et al 1994). The larger Ih in the adult may be a factor in the shorter spike afterhyperpolarization observed in adult versus neonatal HMs (Viana, et al, 1994). This may be a consequence of the greater amount of Ih activated during the afterhyperpolarization in adult HMs. The larger Ih in adult HMs may also contribute to differences in how synaptic inputs are integrated. For example, inhibitory inputs which hyperpolarize the membrane potential may have their effect lessened due to Ih activation with hyperpolarization. Thus in adult HMs Ih may weaken prolonged or strong hyperpolarizations that occur in response to inhibitory synaptic inputs, while depolarizing responses arising from excitatory synaptic inputs may not be compromised. In contrast, neonatal HMs, which lack a substantial Ih current, do not have the stabilizing influence upon membrane potential that is due to Ih. Therefore, these cells may be more susceptible to such inhibitions. In conclusion, this chapter has described the changes that take place in two ionic currents during postnatal development, and how they contribute to distinct subthreshold and firing properties of neonatal and adult motoneurons.