EVIDENCE FOR AGE-DEPENDENT CHANGES IN MOTONEURON DENDRITIC MORPHOLOGYFOLLOWING NEONATAL NERVE-CRUSH IN THE RAT

Motoneurons supplying the extensor hallucis longus muscle of the rat w ere temporarily separated from the muscle by sciatic nerve-crush at fi ve days postnatally. Such treatment permanently alters the reflex resp onse of the affected motoneurons without the large-scale cell death as sociated with nerve-crush at birth. After reinnervation, the motoneuro ns were retrogradely labelled with cholera toxin subunit-B conjugated to horseradish peroxidase and the dendritic tree of each labelled cell was analysed. When compared to normal data, significantly higher leve ls of dendritic density were observed in the rostrodorsally orientated parts of the dendritic field. This was similar to that found previous ly for the same motor pool after nerve-crush at birth. However, in oth er parts of the field where a lower dendritic density was found after nerve-crush at birth, no change was seen after nerve-crush at five day s. These data present evidence for the influence of sensory afferents on the development of motoneuron dendrites. Taken together with the pr evious findings after nerve-crush al birth, we suggest that the differ ential dendritic changes caused by neonatal nerve lesion contribute to an imbalance in the pattern of excitatory and inhibitory inputs to th e motoneuron, which results either in cell death, or the abnormal acti vity seen in those motoneurons which survive.