Motoneuron development after deafferentation. I. Dorsal rhizotomy does not alter growth in the spinal nucleus of the bulbocavernosus (SNB)

Abstract

The spinal nucleus of the bulbocavernosus (SNB) and the dorsolateral nucleus (DLN) are sexually dimorphic motor nuclei in the rat lumbar spinal cord. During postnatal development, SNB and DLN motoneurons grow substantially in measures of soma size, dendritic length, and radial dendritic extent. SNB motoneurons exhibit a biphasic pattern of dendritic growth, where there is an initial period of exuberant growth followed by a period of retraction to mature lengths by 7 weeks. In this experiment, we examined whether primary afferent input to the SNB nucleus was necessary for the normal postnatal growth of SNB motoneurons. We partially deafferented the SNB via unilateral dorsal rhizotomy of lumbosacral dorsal roots in male rats at 1 week of age. Using cholera toxin horseradish peroxidase (BHRP) to visualize SNB motoneurons, we examined SNB motoneuron morphology at 4 and 7 weeks of age. SNB motoneurons in rhizotomized males developed normally; measures of dendritic length in rhizotomized males were typically exuberant at 4 weeks of age, and declined significantly to mature lengths by 7 weeks of age. In addition, dorsal rhizotomy did not alter the development of SNB motoneuron soma size or radial dendritic extent. These results are discussed in reference to sensorimotor connections in the SNB, the extent of the deafferentation, and dendrodendritic interactions.