Postnatal development enhances the effects of cholinergic inputs on recruitment threshold and firing rate of rat oculomotor nucleus motoneurons.

Changes in the electrophysiological and morphological characteristics of motoneurons (Mns) of the oculomotor nucleus during postnatal development have been reported, however synaptic modifications that take place concurrently with postnatal development in these Mns are yet to be elucidated. We investigated whether cholinergic inputs exert different effects on the recruitment threshold and firing rate of Mns during postnatal development. Rat oculomotor nucleus Mns were intracellularly recorded in brain slice preparations and separated in neonatal (4-7 postnatal days) and adult (20-30 postnatal days) age groups. Stimulation of the medial longitudinal fasciculus evoked a monosynaptic excitatory potential in Mns that was attenuated with atropine (1.5 μM, a muscarinic antagonist). Mns were silent at their resting membrane potential, and bath application of carbachol (10 μM, a cholinergic agonist) induced depolarization of the membrane potential and a sustained firing rate that were more pronounced in adult Mns. Pharmacological and immunohistochemical assays showed that these responses were attributable to muscarinic receptors located in the membrane of Mns. In addition, compared to control Mns, carbachol-exposed Mns exhibited a higher firing rate in response to the injection of the same amount of current, and a decrease in the current threshold required to achieve sustained firing. These latter effects were more pronounced in adult than in neonatal Mns. In conclusion, our findings suggest that cholinergic synaptic inputs are already present in neonatal Mns, and that the electrophysiological effects of such inputs on recruitment threshold and firing rate are enhanced with the postnatal development in oculomotor nucleus Mns. We propose that cholinergic input maturation could provide a greater dynamic range in adult Mns to encode the output necessary for graded muscle contraction.