High affinity uptake of L-glutamate and γ-aminobutyric acid in Drosophila melanogaster

Abstract

Preparations having properties resembling those of synaptosomes have been isolated from whole fly homogenates of Drosophila melanogaster using ficoll gradient floatation technique. These have been characterized by marker enzymes and electron microscopy and binding of muscarinic antagenist 3H Quinuclidinyl benzilate. An uptake system for neurotransmitter, a-Aminobutyric acid has been demonstrated in these preparations. A high affinity uptake system for L-glutamate has also been studied in these subcellular fractions. This uptake of glutamate is transport into an osmotically sensitive compartment and not due to binding of glutamate to membrane components. The transport of glutamate has an obligatory requirements for either sodium or potassium ions. Kinetic experiments show that two transport systems, with K_m values 0.33×10^-6M and 2.0×10^-6, respectively, function in the accumulation of glutamate. ATP stimulates lower affinity transport of glutamate. Inhibition of glutamate uptake by L-aspartate but not by phenylalanine and tyrosine indicates that a common carrier mediates the transport of both glutamate and aspartate. β-N-oxalyl-L-β β-diamino propionic acid and kainic acid, both inhibitors of glutamate transport in mammalian brain preparations, strongly inhibited transport of glutamate in Drosophila preparations Comparison with uptake of a-aminobutyric acid and glutamate in isolated larval brain is presented to show that the synaptosome-like preparations we have isolated are rich in central nervous system derived structures, and presynaptic endings from neuromuscular junctions.