EXPANSION OF THE DENDRITIC TREE OF MOTONEURONS INNERVATING NECK MUSCLES OF THE ADULT CAT AFTER PERMANENT AXOTOMY

The morphologic characteristics of neck motoneurons with intact axons were compared with those of neck motoneurons that had been permanently axotomized for 11 to 17 weeks. Motoneurons were identified antidromic ally, intracellularly stained with horseradish peroxidase (HRP) and ex amined after reconstructions of their entire dendritic tree. Axotomize d motoneurons differed qualitatively and quantitatively from motoneuro ns with intact axons. The distal branches of axotomized motoneurons ex hibited two novel features: some gave rise to tangled appendages that exhibited growth cone-like specializations resembling lamellipodia and filopodia; others followed a meandering path and had unusually large diameters. These branches showed a discontinuous pattern of staining t hat a;as similar to the appearance of myelinated axons stained intra-a xonally with HRP. A quantitative analysis of the dendritic trees of 13 completely reconstructed dendritic trees (five axotomized motoneurons and eight motoneurons with intact axons) showed that total dendritic surface area, total dendritic length, and total number of branches inc reased 38, 34, and 215%, respectively, after axotomy. These measuremen ts were confirmed by comparing the sizes of a larger number of motoneu rons (16 axotomized and 21 intact), calcnlated on the basis of correla tions between dendritic tree size and proximal dendritic diameter. We conclude, therefore, that neck motoneurons, in contrast to other types of motoneurons, expand their dendritic trees after axotomy. It is sug gested that this expansion is a consequence of two mechanisms: one inv olves dendritic growth, possibly leading to new synaptic connections; the other causes a conversion of some dendrites into axons. (C) 1998 W iley-Liss, Inc.