Electrophysiological properties of rat phrenic motoneurons during perinatal development

Past studies determined that there is a critical period at approximately embryonic day (E) 17 during which phrenic motoneurons (PMNs) undergo a number of pivotal developmental events, including the inception of functional recruitment via synaptic drive from medullary respiratory centers, contact with spinal afferent terminals, the completion of diaphragm innervation, and a major transformation of PMN morphology. The objective of this study was to test the hypothesis that there would be a marked maturation of motoneuron electrophysiological properties occurring in conjunction with these developmental processes. PMN properties were measured via whole cell patch recordings with a cervical slice-phrenic nerve preparation isolated from perinatal rats. From E16 to postnatal day 1, there was a considerable transformation in a number of motoneuron properties, including 1) 10-mV increase in the hyperpolarization of the resting membrane potential, 2) threefold reduction in the input resistance, 3) 12-mV increase in amplitude and 50% decrease duration of action potential, 4) major changes in the shapes of potassium- and calcium-mediated afterpotentials, 5) decline in the prominence of calcium-dependent rebound depolarizations, and 6) increases in rheobase current and steady-state firing rates. Electrical coupling among PMNs was detected in 15-25% of recordings at all ages studied. Collectively, these data and those from parallel studies of PMN-diaphragm ontogeny describe how a multitude of regulatory mechanisms operate in concert during the embryonic development of a single mammalian neuromuscular system.