The correlation between cortical neuron maturation and neurofilament phosphorylation: a developmental study of phosphorylated 200 kDa neurofilament protein in cat visual cortex.

Neurofilament proteins are major constituents of the neuronal cytoskeleton. When phosphorylated, neurofilament proteins are thought to be involved in slowing down the slower component of axonal transport, thereby increasing neurofilament stability and conferring resistance to proteolysis. In order to investigate the correlation between phosphorylated neurofilament and plasticity of the cat visual cortex, we applied a monoclonal antibody, which recognizes only the phosphorylated form of neurofilament protein, to kitten visual cortex. This antibody appeared to recognize only axons. There was a progressive increase of phosphorylated neurofilament-immunoreactive axons in the cortical layers and white matter during development. In the visual cortex of young kittens, phosphorylated neurofilament immunoreactive processes were mostly long, fine, and continuous axons. However, in kittens older than 2 months of age, phosphorylated neurofilament immunoreactivity was characterized by short, punctae-like staining. The immunoreactive axons were most dense in deep cortical layers V and VI initially, followed at later ages by layers IVa and IVb, and finally in layer IVc and in the superficial cortical layers of adult animals. This developmental laminar pattern of distribution was not affected by early input restriction and coincides with electrophysiological evidence of early neuronal maturation. In addition, the phosphorylated neurofilament immunoreactivity can also be found in the dendrites and perikaya of degenerating neurons. These results suggest that phosphorylated neurofilament can be used as an index of functional maturity in the kitten visual cortex and also as a marker for neurodegeneration.