Postnatal Changes in Enzyme Activities in Synaptosomes Isolated from Hamster Optic Nerve Endings

Subsynaptosomal fractions isolated from optic terminal nuclei of adult and neonatal hamsters exhibited developmental changes in specific density, mitochondrial activity, and K+-stimulated, ouabain-inhibited p-nitrophenylphosphatase (K-pNPPase) activity around the time of eye opening. The specific activity of K-pNPPase was six- to sevenfold higher after eye opening (14-16 days postnatal). A significant proportion of high-specific- activity K-pNPPase was recovered from the lightest subsynaptosomal fraction at all ages. This fraction contained very little external membrane by galactose oxidase - NaB3H4 labeling, suggesting that it may represent an internal pool, possibly the axonally transported form of the enzyme. Synaptic mitochondrial cytochrome c. oxidase activity also approximately doubled in the period between 12 and 16 days. The specific density of the external membrane increased very slowly, banding at 1.0 M sucrose at 12 and 16 days, and at 1.2 M in adults. These maturational events may reflect increased energetic needs for optic nerve endings following eye opening.