Feeding behavior drastically changes from suckling to chewing during the postnatal period. Thus, it is possible that the firing properties of jaw-closing motoneurons (JCMNs), one of the final common pathways in the oro-motor system, are also altered during the postnatal period. We examined the repetitive firing properties of JCMNs in developing rats from an early postnatal stage to a post-weaning stage using intracellular recording techniques. Firing frequency adaptation was observed within the

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... first four spikes in a spike train. The neurons were classified into two groups according to the firing patterns subsequent to the first four spikes: neurons with a constant or slightly decrementing firing pattern (type I) and neurons with an incrementing firing pattern (type II). The percentage of type I neurons increased with age. In more than 50% of the neurons in each age group except for the group at 10-15 days of age, an extra spike was produced from the post-spike afterdepolarization (ADP) of the 1st spike in a spike train, leading to a high firing rate for the first inter-spike interval (1st ISI). The incidence of an extra spike also increased with age. The firing rate for the steady-state was high in JCMNs, even at 4-9 days of age, and approximately 50% of the neurons at this age fired at higher than 50 Hz. These findings demonstrate that JCMNs can fire at a high rate, even in the early postnatal period, and the firing rate for the 1st ISI further increases with age. These changes may contribute to the transition from suckling to chewing.