Developmental changes in the electrophysiological properties of neonatal rat oculomotor neurons studied in vitro

The electrophysiological properties of oculomotor neurons were studied in neonatal rats aged 1-15 days. Action potentials were recorded from brainstem slices (frontal section) using the intracellular recording method at 35 degrees C. No significant age-dependent differences were detected in the resting potential (around -55 mV) and in the amplitude of the action potential (approximately 60 mV). However, the input resistance of oculomotor neurons declined with age from a mean of 60.8 M omega for rats 1-3 days old to 17.0 M omega for rats 14-15 days old. In addition, the duration of the action potential measured at the half-amplitude gradually decreased from 0.74 ms to 0.34 ms with increasing age. Increases were detected in the maximum rate of rise (from 117 V/s to 181 V/s) and the maximum rate of fall (from -67 V/s to -103 V/s) of the action potential. When long-lasting (500 ms) depolarizing current pulses were applied to oculomotor neurons, some neurons exhibited continuous repetitive discharge (i.e. tonic firing) while others showed transient discharge (phasic firing). The proportion of tonic-type neurons increased with age: the value was 9% for rats 1-5 days old, 37% for rats 6-10 days old and 54% for rats 11-15 days old. Concomitantly, the number of neurons showing a time-dependent inward rectification increased and the average maximum frequency of the discharge rose from 150 to 420 Hz, approximately, with age. Furthermore, it was found that the electrophysiological properties of oculomotor neurons of rats 14-15 days old were similar to those in adult rats. It is concluded that oculomotor neurons in neonatal rats show rapid alterations in their electrophysiological properties and that the ratio of tonic-type to phasic-type neurons changes during the early stages of development.