DEVELOPMENTAL INCREASES IN EXPRESSION OF NEUROFILAMENT MESSENGER-RNA SELECTIVELY IN PROJECTION NEURONS OF THE LAMPREY CNS

Neurofilaments of the sea lamprey are unique in being homopolymers of a single subunit (NF-180). Digoxigenin-labeled RNA probes complementar y to NF-180 were used to determine the distribution and timing of expr ession of neurofilament message in the brain and spinal cord of the la mprey. In the brainstem, detection of NF-180 mRNA was restricted to ne urons with axons projecting to the spinal cord or the periphery. The m ajority of brainstem neurons, whose axons project locally, did not exp ress NF-180 within the detection limits of this technique. NF-180-posi tive neurons included cells with a wide range of axon diameters, sugge sting neurofilament mRNA expression was linked to axon length rather t han caliber. To further evaluate this hypothesis, expression was studi ed in animals of different developmental stages between larvae and adu lts. In younger (shorter) larvae, the large Mauthner and rhombencephal ic Muller cells did not express NF-180 mRNA, even though their axons a re among the largest caliber in the animal and extend the entire lengt h of the spinal cord. In contrast, many other reticulospinal neurons, whose axons are smaller in diameter than those of the Muller and Mauth ner cells, expressed NF-180 message throughout larval development. Fur thermore, neurons of the cranial motor nuclei did not express NF-180 u ntil later developmental stages and the extraocular motor neurons did not label until metamorphosis. Therefore, while detectable neurofilame nt mRNA expression in the lamprey is restricted to neurons with long a xons, its expression in this population of neurons appears to be devel opmentally regulated by factors still not determined. It is postulated that need for NF message is determined by a balance between the volum e of axon to be filled and the rate of turnover of NF in that axon. (C ) 1996 Wiley-Liss, Inc.