ANALYSIS OF THE DISTRIBUTION OF GLYCINE AND GABA IN AMACRINE CELLS OFTHE DEVELOPING RABBIT RETINA - A COMPARISON WITH THE ONTOGENY OF A FUNCTIONAL GABA TRANSPORT-SYSTEM IN RETINAL NEURONS

The objectives of this study were to (1) determine whether the glycine rgic and GABAergic amacrine cells in the developing rabbit retina were neurochemically distinct at birth, (2) determine if the ratio of GABA ergic to glycinergic amacrine cells was constant during development, ( 3) determine whether the capacity to take up a GABA analogue was restr icted to GABAergic neurons, and (4) whether initiation of GABA transpo rt into GABAergic neurons preceded the presence of a content of GABA i n these neurons. We have used a novel strategy to immunolocalize a non -endogenous GABA analogue, gamma-vinyl GABA, which is taken up into ne urons by a GABA transporter. Examination of serial semithin resin-embe dded sections of neonatal rabbit retinae that had been immunolabelled for glycine, GABA or gamma-vinyl GABA revealed that at 1 day postnatum , 60% of amacrine cells contain glycine but not GABA and did not accum ulate gamma-vinyl GABA, which is similar to the percentage of glyciner gic amacrine cells in the adult retina. The vast majority of the remai ning amacrine cells contained GABA and many also transported gamma-vin yl GABA; however, a significant number of GABA-containing cells failed to accumulate gamma-vinyl GABA suggesting that possession of a conten t of GABA did not have to be preceded by, or be concomitant with, the presence of a GABA transport system. By 10 days postnatum, over 99% of GABA-containing amacrine cells also transported gamma-vinyl GABA indi cating their functional maturity. Analysis of the horizontal cells rev ealed no evidence for uptake of gamma-vinyl GABA, but another GABA ana logue, diaminobutyric acid, which is a substrate both for the neuron-a ssociated GABA transporter and the glial GABA transporter, was accumul ated into some horizontal cells at 21 days postnatum, a time point whe n these cells also contain endogenous GABA. We conclude that amacrine cells are committed to being GABAergic or glycinergic at, or prior to birth, and that in some amacrine cells, expression of a content of GAB A may occur prior to the capacity to transport GABA. Conversely, in so me ganglion cells transport of gamma-vinyl GABA may precede a content of GABA.