V3 interneuron subpopulations in the mouse spinal cord undergo distinctive postnatal maturation processes

- The maturation process of spinal V3 interneurons is studied during the first 3 weeks of birth.
- The ventral and dorsal V3 subpopulations are physiologically distinguishable at postnatal day (P) 0.
- The physiological properties of V3 subpopulations mature separately from P0 to P21.
- Both V3 subpopulations progress through multiple phases during maturation.

Mice develop weight-bearing locomotion within the first 2-3 weeks of birth, a period during which motoneurons (MNs) and interneurons (INs) that control locomotor activities undergo rapid maturation. In this study, we investigate the maturation of two subpopulations of V3 INs in the mouse spinal cord during this period. To do this, we conducted whole-cell patch-clamp recordings of tdTomato fluorescent protein-expressing spinal V3 INs from Sim1Cre/+;tdTom mice at post-natal day (P) 0, P4, P9 and P14 and compared their properties to those at P21. Combining electrophysiology with computational analyses, we show that dorsal and ventral V3 subpopulations are physiologically distinct at birth, but the electrophysiological properties of V3 INs change significantly during the first three post-natal weeks. We further reveal that there are multiple developmental phases of both V3 subpopulations during the maturation process. The different developmental trajectories of physiological properties also coincide with changes in an animal's locomotor behavior. These properties likely reflect the differential functions of V3 subpopulations in maturing spinal locomotor circuits.