Temporal sequence of changes in electrophysiological properties of oculomotor motoneurons during postnatal development

The temporal sequence of changes in electrophysiological properties during postnatal development in different neuronal populations has been the subject of previous studies. Those studies demonstrated major physiological modifications with age, and postnatal periods in which such changes are more pronounced. Until now, no similar systematic study has been performed in motoneurons of the oculomotor nucleus. This work has two main aims: first, to determine whether the physiological changes in oculomotor nucleus motoneurons follow a similar time course for different parameters; and second, to compare the temporal sequence with that in other neuronal populations. We recorded the electrophysiological properties of 134 identified oculomotor nucleus motoneurons from 1 to 40 days postnatal in brain slices of rats. The resting membrane potential did not significantly change with postnatal development, and it had a mean value of −61.8 mV. The input resistance and time constant diminished from 82.9–53.1 MΩ and from 9.4–4.9 ms respectively with age. These decrements occurred drastically in a short time after birth (1–5 days postnatally). The motoneurons’ rheobase gradually decayed from 0.29–0.11 nA along postnatal development. From birth until postnatal day 15 and postnatal day 20 respectively, the action potential shortened from 2.3–1.2 ms, and the medium afterhyperpolarization from 184.8–94.4 ms. The firing gain and the maximum discharge increased with age. The former rose continuously, while the increase in maximum discharge was most pronounced between postnatal day 16 and postnatal day 20. We conclude that the developmental sequence was not similar for all electrophysiological properties, and was unique for each neuronal population.