Postnatal development of cholinergic synapses on mouse spinal motoneurons.

Following birth, when mammals are relatively immobile, significant development of the motor system facilitates weight bearing and locomotion. Prominent cholinergic C-terminals develop on somata and proximal dendrites of spinal motoneurons during this time period. It is hypothesized that these terminals are essential in regulating motoneuron excitability and thus their development contributes to motor system maturation. Therefore, the development of pre- and postsynaptic components of the C-terminal synapse on motoneurons in mice during the early postnatal period was investigated. Fluorescence immunohistochemical studies revealed that developmental increases in punctate labeling of presynaptic cholinergic terminals, as visualized by vesicular acetylcholine transporter immunoreactivity (VAChT-IR) corresponded to the progressive expression and spatial restriction of immunoreactivity for the calcium channel subunit alpha(1)2.2 (N-type) located presynaptically and the muscarinic type 2 acetylcholine receptor situated postsynaptically. In addition, clustering of immunoreactivity for the potassium channel subunit K(V)2.1 occurred within the early postnatal period in concert and colocalized with the maturation of the C-terminals. The time course of development of these components of the C-terminal synapse corresponds to the maturation of the motor system that enables the animal to locomote in an adult-like fashion.