Differential innervation of specific motor neuron pools by serotoninergic fibers in the chick spinal cord.

A new technique in which cholera toxin subunit B conjugated to horseradish peroxidase is injected in chick muscles followed by perfusion with Zamboni's fixative for serotonin immunocytochemistry allows one to visualize immunoreactive fibers and retrogradely labelled motoneurons in alternate sections. Using these procedures, we have found that there is a differential innervation by serotoninergic fibers of motoneuron pools that project to specific muscles or muscle groups. Dense clusters of serotonin-positive fibers were located in the motoneuron pools of extensors of the hip joint consisting of the lateral iliotibialis, ischioflexorius, iliofibularis, accessorius and caudilioflexorius muscles.