Maturation of persistent and hyperpolarization-activated inward currents shapes the differential activation of motoneuron subtypes during postnatal development

• Published in: eLife Vol. 10
• Main Authors: Sharples, Simon A, Miles, Gareth B
• Format: Journal Article
• Language: English
• Published: England eLife Science Publications, Ltd 16.11.2021; eLife Sciences Publications Ltd; eLife Sciences Publications, Ltd
• Subjects: Analysis; Animals; Animals, Newborn; Cell Size; Electrophysiological Phenomena; Electrophysiology; Female; Hyperpolarization; intrinsic properties; Male; Maturation; Mice; Mice, Inbred C57BL; motoneuron; Motor neurons; Motor Neurons - physiology; Motor units; Nervous System - growth & development; Neuroscience; Patch-Clamp Techniques; Physiological aspects; postnatal development; Principal components analysis; recruitment; size principle; Spinal cord; United Kingdom; mouse; size principle; recruitment; neuroscience; motoneuron; intrinsic properties; spinal cord; postnatal development
• ISSN: 2050-084X; 2050-084X
• DOI: 10.7554/eLife.71385

The size principle underlies the orderly recruitment of motor units; however, motoneuron size is a poor predictor of recruitment amongst functionally defined motoneuron subtypes. Whilst intrinsic properties are key regulators of motoneuron recruitment, the underlying currents involved are not well defined. Whole-cell patch-clamp electrophysiology was deployed to study intrinsic properties, and the underlying currents, that contribute to the differential activation of delayed and immediate firing motoneuron subtypes. Motoneurons were studied during the first three postnatal weeks in mice to identify key properties that contribute to rheobase and may be important to establish orderly recruitment. We find that delayed and immediate firing motoneurons are functionally homogeneous during the first postnatal week and are activated based on size, irrespective of subtype. The rheobase of motoneuron subtypes becomes staggered during the second postnatal week, which coincides with the differential maturation of passive and active properties, particularly persistent inward currents. Rheobase of delayed firing motoneurons increases further in the third postnatal week due to the development of a prominent resting hyperpolarization-activated inward current. Our results suggest that motoneuron recruitment is multifactorial, with recruitment order established during postnatal development through the differential maturation of passive properties and sequential integration of persistent and hyperpolarization-activated inward currents.