High-affinity GABA and glutamate transport in developing nerve ending particles.

The high-affinity transport of [3H]GABA and [14C]glutamate was measured in discrete nerve ending fractions isolated from the developing rat cortex, beginning on day 7 postpartum and prepared using discontinuous Ficoll-isotonic sucrose gradients. On day 7 the material sedimenting in the lightest gradient fractions contained the highest density of particles capable of high-affinity transport. With increasing age, the transport sites became progressively associated with more dense particles in the gradient. On day 14, the Vmax values for both [3H]GABA and [14C-A1glutamate transport in the most dense nerve ending fraction were significantly increased over the adult value. This transient increase in the Vmax values disappeared by day 21. While the Km values for both transport systems were constant in all fractions for the adult animal, this was not the case in the developing pup. The Km values in the lightest fractions were significantly greater than those in the more dense fractions, suggesting that different transport systems may be present during development. Electron micrographs of the various fractions were prepared from 7-day and adult animals. These data illustrate that the lightest fractions of the '7-day' gradient are enriched in profiles similar to developing nerve endings. With development, these light fractions become infiltrated with myelin, the immature nerve ending profiles disappear, and the bulk of the nerve endings are found in more dense fractions.