The expression of GAP-43 and synaptophysin in the developing rat retina.

We have undertaken a detailed study of the expression of GAP-43 and synaptophysin immunoreactivity in the developing postnatal rat retina. We found that these two 'presynaptic' proteins have quite different expression patterns. GAP-43 was first expressed in the optic nerve and the optic fiber layer of the retina, where it disappeared between the 8th and 16th postnatal day. From the 5th postnatal day on, GAP-43 also appeared in the inner plexiform layer, where it was present in three distinct bands. This expression changed little in postnatal development and persisted in the adult retina. GAP-43 was not detected in the outer plexiform layer of the retina. Synaptophysin was absent from the optic nerve and optic fibers at all postnatal stages. It was first expressed in the developing outer plexiform and, with reduced intensity, in the outer nuclear layer between postnatal days 2 and 5. In the inner plexiform layer, synaptophysin could be first detected between postnatal days 8 and 12. The intensity of staining increased during postnatal development in both plexiform layers. The developmental sequence of synaptophysin expression can be correlated with the maturation of presynaptic terminals of photoreceptors and bipolar cells. The rather complex pattern of GAP-43 expression is not easily compatible with a single model of GAP-43 function, and suggests diverse functions of this molecule in the retina.