EXCITATORY AMINO-ACID RECEPTORS MEDIATE THE GLUTAMATE-INDUCED RELEASE OF GABA SYNTHESIZED FROM PUTRESCINE IN CULTURED-CELLS OF EMBRYONIC AVIAN RETINA

Cultured retina cells from chick embryos took up [H-3]putrescine and approx 10.8% of the incorporated amine was converted into [H-3]GABA. The putrescine-derived GABA accumulated in a pool that was released in the medium at a rate corresponding to 3.66% of the total [H-3]GABA in the cell at incubation intervals of 12 min. Treatment of cultures with L-glutamate (500 muM) promoted a 5-7 fold increase in the rate of [H-3]GABA efflux which was totally independent on the presence of calcium ions in the superfusing medium. (+)-5-Methyl-10, 11-dihydro-5h-Dibenzo(A,D)cyclohepten-5,10-Iminihydrogenmaleate (MK 801) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 100 muM, inhibited the glutamate evoked release of GABA by 78 and 73% respectively. N-methyl-D-aspartate (NMDA, 100 muM), elicited the release of putrescine-derived GABA only when magnesium ions were removed from the superfusing medium with 2 mM EGTA. In the presence of 1 mM MgCl2, NMDA was totally ineffective in inducing the release. As for glutamate, AMPA (R,S)-alpha-Amino-3-hydroxy-5-methyllisoxazole-4-propionicacid hydrobromide (100 muM) also induced the release of GABA synthesized from putrescine. Our data show that putrescine is an important source of GABA in the embryonic CNS and that GABA synthesized from putrescine can be released in the extracellular space when cells are stimulated by L-glutamate through the activation of excitatory amino acid (EAA) receptors.