Developmental changes in the serotoninergic innervation of hindlimb extensor motoneurons in neonatal rats

The postnatal development of quadriceps femoris motoneurons (Q-MNs) and serotonin (5-HT) nerve terminals in rat spinal cord were studied using retrograde neurotracing techniques combined with 5-HT immunohistochemistry. We attempted to elucidate the 5-HT-ergic innervation to the Q-MNs by counting the number of 5-HT-immunoreactive varicosities that were in close apposition to the Q-MNs. The following results were obtained: (1) Q-MNs possessed, at birth, few if any very short dendrites. The size of these somata was relatively uniform and small. During postnatal periods lasting from 1 to 30 days, the mean cell size of Q-MNs increased with the development of dendrites. From 5 to 14 days after birth, in particular, cell size increased markedly. (2) 5-HT-immunopositive fibers were, at birth, already observed in the ventral horn of the lumbar spinal cord. The density of these fibers increased gradually with age. (3) At birth, only a few 5-HT terminals and varicosities showed close apposition with about half the Q-MNs examined. At 5-days postnatally, such close apposition was found in all Q-MNs. By the first two postnatal weeks, Q-MNs grew quickly and the 5-HT innervation to the Q-MNs appeared to have been established. Based on these results, the significance of 5-HT innervation to developing Q-MNs is discussed in relation to the postnatal development of motor function.