Labelling of midlumbar neurones projecting to cat hindlimb motoneurones by transneuronal transport of a horseradish peroxidase conjugate.

When wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) is injected into a hindlimb nerve it is first transported to motoneuronal somata and then, transneuronally, to interneurones in several spinal segments. However, the distribution of the labelled interneurones has been found to be dependent on the experimental conditions. Interneurones of the S1, L7, L6 and L5 segments were labelled in all the preparations used in this study while interneurones of the L4 and L3 segments were labelled only in some of them. The labelling of the L3-L4 interneurones was found when the animals were awake and active (after injection of WGA-HRP under short lasting anaesthesia) or when the contralateral pyramid was stimulated in animals which remained anaesthetized during the whole survival period. Since the transneuronal transport of WGA-HRP is enhanced by synaptic activity, these results are taken to indicate that L3 and L4 interneurones operate as last order neurones of neuronal pathways which subserve centrally initiated movements, in particular those activated by the corticospinal tract fibres.