Recovery of locomotor activity in the adult chronic spinal rat after sublesional transplantation of embryonic nervous cells: specific role of serotonergic neurons.

Locomotor movements are programmed in a specialised neuronal network that is localised in the central nervous system and referred to as the central pattern generator (CPG) for locomotion. This CPG can be activated by pharmacological agents such as monoamines. The aim of the present study was to try to activate the CPGs by using cells that are supposed to release serotonin locally. Adult chronic spinal rats were injected with embryonic brainstem neurons within the spinal cord under a thoracic transection. This procedure resulted in a monoaminergic reinnervation of the lumbar enlargement. With the help of a specific neurotoxin for noradrenergic neurons (6-hydroxydopamine), it was possible to isolate the serotonergic system. After such transplantation of monoaminergic neurons and even with serotonergic neurons alone, a bilateral, alternating, rhythmic locomotor-like activity recovered in hindlimbs. Furthermore, this locomotor-like activity was clearly facilitated when the re-uptake of serotonin was blocked by zimelidine. Therefore, we conclude that transplanted embryonic serotonergic neurons are able to activate the CPG for locomotion.