Changes in neurofilament gene expression occur after axotomy of dorsal root ganglion neurons: an in situ hybridization study.

Neurofilaments (NFs) are predominant elements in large myelinated axons, where they are thought to serve the important function of maintaining axonal caliber. Previous studies have shown that changes in NF synthesis and axonal transport occur after axonal injury in rat dorsal root ganglion (DRG) cells. The resulting reduction in the NF supply to DRG axons is thought to be largely responsible for the observed decrease in axonal diameter in the proximal axonal stump after an injury. In the present study, we test the hypothesis that a change in NF gene expression precedes the changes in synthesis and transport of NF proteins. To address this hypothesis, the levels of mRNA encoding the 68-kilodalton (kd) neurofilament protein (NF68) in adult rat DRG neurons were assessed at different times after peripheral axotomy using in situ hybridization. For these studies we used a 35S-labeled cDNA probe to NF68. The levels of NF68 mRNA in sensory neurons located in ipsilateral fourth and fifth lumbar DRG at 1, 7, and 14 days after sciatic nerve crush were compared to those in normal DRG neurons using quantitative autoradiography. In large DRG neurons (greater than 1000 micron 2), the levels of NF68 mRNA were significantly reduced relative to normal at 1, 7, and 14 days after axotomy. Medium-sized cells (601-1000 micron 2) exhibited a reduction only at 14 days postinjury, and small-sized cells were not significantly affected. These findings indicate that larger DRG neurons which give rise to large myelinated sensory axons exhibit a change in NF gene expression after axonal injury. The observed changes in NF68 mRNA levels temporally precede changes in NF synthesis and transport in injured DRG cells. Thus, a change in NF gene expression may be an important component of an effective regenerative response and a critical step at which axonal caliber is regulated in injured neurons.