RECOVERY OF NEUROFILAMENT EXPRESSION SELECTIVELY IN REGENERATING RETICULOSPINAL NEURONS

During regeneration of lamprey spinal axons, growth cones lack filopod ia and lamellipodia, contain little actin, and elongate much more slow ly than do typical growth cones of embryonic neurons. Moreover, these regenerating growth cones are densely packed with neurofilaments (NFs) . Therefore, after spinal hemisection the time course of changes in NF mRNA expression was correlated with the probability of regeneration f or each of 18 identified pairs of reticulospinal neurons and 12 cytoar chitectonic groups of spinal projecting neurons. During the first 4 we eks after operation, NF message levels were reduced dramatically in al l axotomized reticulospinal neurons, on the basis of semiquantitative in situ hybridization for the single lamprey NF subunit (NF-180). Ther eafter, NF expression returned toward normal in neurons whose axons no rmally regenerate beyond the transection but remained depressed in poo rly regenerating neurons. The recovery of NF expression in good regene rators was independent of axon growth across the lesion, because excis ion of a segment of spinal cord caudal to the transection site blocked regeneration but did not prevent the return of NF-180 mRNA. The early decrease in NF mRNA expression was not accompanied by a reduction in NF protein content. Thus the axotomy-induced loss of most of the axona l volume resulted in a reduced demand for NF rather than a reduction i n volume-specific NF synthesis. We conclude that the secondary upregul ation of NF message during axonal regeneration in the lamprey CNS may be part of an intrinsic growth program executed only in neurons with a strong propensity for regeneration.