Goltash, S. | Search bioRxiv

bioRxiv2025

Altered excitability of dI3 neurons regulates hindlimb motor tone and locomotor recovery after spinal cord injury

Recovery of motor function after spinal cord injury is limited in mammals. Reactivation of locomotor circuits does occur, but primarily through the activation of sensorimotor pathways in the context of locomotor training. Previous investigations have shown that dI3 neurons, a developmentally-defined population of pre-motor, glutamatergic interneurons, are indispensable for this process. However, it remains unclear how dI3 neurons are recruited during locomotor recovery, and whether they could be leveraged to improve locomotor function following spinal cord injury. Herein, we investigated how the excitability of dI3 neurons influences locomotor behaviour and recovery after spinal cord injury. In T9-T10 transected mice, we found that acute chemogenetic silencing of dI3 neurons leads to immediate loss of hindlimb motor tone, and significant reduction in stepping during treadmill locomotion. Conversely, regular chemogenetic stimulation of dI3 neurons led to transient increases in hindlimb motor tone early after injury, but ultimately reduced hindlimb motor tone and locomotor recovery over the long term. These chronic changes resulting from dI3 neuron stimulation were associated with the absence of expression of the constitutive 5-HT2C-R isoform, potentially representing a homeostatic mechanism for the regulation of dI3 excitability following spinal cord injury. Given these findings, we hypothesized that dI3 stimulations effects on motor tone, while insufficient to drive locomotor function alone, may promote stepping improvements when locomotor rhythm-generating circuits are active. The addition of quipazine, a serotonergic agonist known to facilitate locomotor rhythmogenesis, in combination with dI3 stimulation, significantly improved locomotor function, while also mitigating the long-term reduction in treadmill stepping associated with dI3 stimulation alone. In aggregate, our results suggest that hyper-excitable dI3 neurons are involved in the maintenance of motor tone after spinal cord injury, possibly through a 5-HT2C-R-dependent mechanism, and further show that the selective stimulation of dI3 neurons could enhance the recovery of locomotor function following spinal cord injury.

neuroscience↗

bioRxiv2022

Homeostatic changes maintain the gain control of spinal motoneurones across the lifetime of C57BL/6J mice

Age-related changes in the excitability of spinal motoneurone have been observed in mouse models of neurodegenerative diseases affecting these neurones. How the excitability of spinal motoneurones change with healthy ageing in mice and how this compares with that seen in neurodegenerative diseases is unknown. Therefore, we performed in vivo intracellular recording from identified spinal motoneurones in C57BL/6 mice at three different ages (100, 300-400 and 600-750 days old). Behavioral tests confirmed a linear reduction in motor function across these ages (using the rotorod test). Significant differences were observed with respect to the features of individual somatic action potential with ageing including a decreased rate of rise and fall in aged mice. Surprisingly, the rate of rise of the action potential at the initial segment was altered in middle aged mice. Immunohistochemical labelling of the axon initial segment of the motoneurones confirmed structural changes occurring at middle age (decreased length and diameter) but returning to the earlier parameters in aged mice. To explore the effects on repetitive firing, this was tested across the age groups which showed surprising little difference as the mice aged, with a similar rheobase and I-f gain across all age groups (with the exception of a lower voltage threshold for action potential initiation in middle-aged mice). However, amplitudes of the after-hyperpolarization and the input resistance were both found to be significantly altered with age. We conclude that there are changes occurring in the intrinsic properties of spinal motoneurones that control their excitability over the lifetime of mice, although these do not develop in a linear fashion from young to old. We propose that these changes are homeostatic in nature and are able to compensate for one another to maintain a constant gain control across the lifetime.

neuroscience↗