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

Abstract

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.

Contributions of h- and Na+/K+ Pump Currents to the Generation of Episodic and Continuous Rhythmic Activities

“…Importantly, our model is critically dependent on the dynamics of h-current, with I h being largely active at the silent interepisode interval, slowly deactivating over the duration of an episode, and subsequently producing episode termination. In line with our model, a resting I h is expressed in a subset of spinal motoneurons during later stages of postnatal development in mice (Sharples and Miles, 2021), in addition to glutamatergic rhythm-generating interneurons (dINs) of Xenopus tadpoles, where it opposes and mitigates the hyperpolarizing influence of I Pump on the membrane potential (Picton et al, 2018). Furthermore, multiple modulators, including dopamine and serotonin, have been reported to depolarize the activation voltage of I h in neurons (Kjaerulff and Kiehn, 2001;Ballo et al, 2010), and I h can be active near the rest potential.…”

“…Swimming episodes in tadpoles have been linked to the activity-dependent Na + /K + ATPase pump current (I Pump ; Zhang and Sillar, 2012;Zhang et al, 2015), which interacts with hyperpolarizationactivated cation current (I h ) and A-type K + current (I KA ) in excitatory rhythm-generating interneurons (Zhang et al, 2015;Picton et al, 2018). I h plays an important role in the functioning of rhythmic networks in both invertebrate (Angstadt and Calabrese, 1989;Peck et al, 2006) and vertebrate systems (Takahashi, 1990;Thoby-Brisson and Ramirez, 2000;Kjaerulff and Kiehn, 2001;Butt et al, 2002;Smith and Perrier, 2006;Picton et al, 2018;Sharples and Miles, 2021).…”

Changes in Sensorimotor Connectivity to dI3 Interneurons in Relation to the Postnatal Maturation of Grasping

“…descending pathways continue their establishment of connections with spinal circuits-a process that begins in later embryonic stages (Branchereau et al, 2000), sensory afferent inputs onto spinal circuits are refined through both sprouting and pruning (Fitzgerald et al, 1994;Granmo et al, 2008), the connectivity of some spinal circuits become more specific (Wilson et al, 2004;Bui et al, 2013;Sonner and Ladle, 2013), and the intrinsic properties of spinal neurons continue to mature postnatally (Jiang et al, 1999;Smith and Brownstone, 2020;Sharples and Miles, 2021).…”

In vitro longitudinal lumbar spinal cord preparations to study sensory and recurrent motor microcircuits of juvenile mice

“…The identification of neuronal populations based on the expression of specific transcription factors (Jessell, 2000), allowed to selectively target or manipulate spinal neurons, prompting ground-breaking work into the understanding of spinal networks controlling movement. All these achievements and improvements are somewhat diminished by the 'elephant in the room' among spinal cord electrophysiologists: in vitro recordings from the spinal cord were mostly performed in embryonic and neonatal preparations, when motoneurons and motor circuits are only partially developed (Clarac et al, 2004;Sharples & Miles, 2021;Smith & Brownstone, 2020;Whelan, 2003). In vitro spinal cord preparations, and especially recordings from motoneurons, from late postnatal mice, have been challenging: motoneurons are very sensitive to anoxia and the thick layer of white matter enveloping the spinal cord prevents sufficient exchange of oxygenated solution within the tissue, leading to motoneuron death.…”

“…Despite technical difficulties (Jensen et al, 2021;Manuel, 2021;Manuel & Zytnicki, 2021), sharp recordings can be used to record membrane and firing properties, but they are not suitable for voltage clamp recordings, thus making it difficult to characterize synaptic conductances in an intact preparation. Recently, researchers have been able to successfully extend the age of in vitro motoneuron recordings from mice, to their juvenile period (P14-30), by tweaking the composition of the solutions used, relying on a very fast dissection and slicing at cold temperatures (Bhumbra & Beato, 2018;Sharples & Miles, 2021;Smith & Brownstone, 2020). These studies have been done in transverse spinal cord slices, an useful preparation to study motoneuron physiology, but that largely ablates motoneuron dendritic arborization and pre-motor networks (Mousa & Elbasiouny, 2021).…”