Transneuronal labelling of neurones projecting to forelimb motoneurones in cats performing different movements

Electrophysiologically identified propriospinal neurones (PNs) in the C3-C4 segments mediate disynaptic excitation and inhibition from higher centres to forelimb motoneurones. In behavioural experiments it was shown that C3-C4 PNs can mediate the command for target-reaching. We have investigated if the C3-C4 PNs which project monosynaptically to forelimb motoneurones are specifically active during target-reaching by using the technique of transneuronal uptake of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into last order neurones which appears to be activity-dependent. The nerve branches to the spino- and acromio-deltoid muscles were injected with WGA-HRP. Cats preoperatively trained, performed either target-reaching or walking movements. Transneuronally labelled neurones in C2 to C4, located ipsilaterally in laminae V-VIII and contralaterally in lamina VIII, were found in target-reaching cats but not in walking cats. The results show a functional specificity of the C3-C4 PNs and support the proposal of activity dependent transneuronal labelling. It is suggested that this method can be used as a tool for functional identification of last order neurones activated in different movements.