Cholinergic and serotonergic excitation of ascending commissural neurons in the thoraco-lumbar spinal cord of the neonatal mouse

Locomotion requires the coordination of the two sides of the spinal cord-a function fulfilled by commissural neurons. Ascending commissural neurons (aCNs) are known to be rhythmically active during locomotion, and mice lacking a population of aCNs display uncoupling between the left and right hemicords during locomotion. Acetylcholine (ACh) applied to the isolated spinal cord commonly produces left-right alternation, with co-contraction of ipsilateral flexor and extensor motoneuron groups. In this study, aCNs were examined in the neonatal mouse spinal cord after retrograde labeling with a fluorescent dextran. The axons of these cells crossed in the ventral commissure with many crossing in the same transverse plane as the cell body. For cells located in lamina VII and VIII, ACh (10-50 mu M) depolarized 92% (13/14) of the cells tested. ACh depolarized and increased the excitability of aCNs in the presence of a decrease in input resistance. ACh was without significant effect on afterhyperpolarization amplitude or voltage threshold of action potential initiation. In those cells sensitive to application of ACh, 90% (9/10 cells) were also depolarized by 5HT (10-50 mu M). Application of 5HT significantly increased the input resistance of these cells, and this effect was likely responsible for the observed increase in excitability, because significant effects on the afterhyperpolarization and voltage threshold were again not detected. The high proportion of aCNs excited by both ACh and 5HT suggests that direct activation of aCNs by these two neurotransmitters contributes to the production of a bilaterally coordinated locomotor-like rhythm in the isolated spinal cord.