Selective neuronal accumulation of omega-amino acids in the rabbit retina

The uptakes of radioactive glycine, beta-alanine, GABA, delta-aminovaleric acid and epsilon-aminocaproic acid were studied in the rabbit retina in vivo and in vitro. Autoradiography demonstrated that the amino acids were mainly taken up and retained by certain cells of amacrine type. In the neurons, the omega-amino acids seem to be stored in a protected fashion. The uptake of delta-aminovaleric acid and epsilon-aminocaproic acid was temperature dependent, saturable, and of high-affinity type (with Km for delta-aminovaleric acid = 3.02 X 10(-5) M and for epsilon-aminocaproic acid 9.38 X 10(-5) M). The uptake of both delta-aminovaleric acid and epsilon-aminocaproic acid was competitively inhibited by GABA, but not by glycine or a number of other amino acids regarded as possible neurotransmitters. The uptakes of delta-aminolevulinic acid, taurine and alpha-amino-isobutyric acid were studied for comparison, but no selective neuronal uptake was observed. It is concluded that there are certain amacrine cells which selectively accumulate delta-amino acids such as beta-alanine, GABA, delta-aminovaleric acid and epsilon-aminocaproic acid. It seems possible that the mechanism accumulating GABA may also work with the other omega-amino acids. This opens the possibility that the omega-amino acids may serve as 'false neurotransmitters'.