Decrease of uptake and exchange of neurotransmitter amino acids after depletion of their synaptosomal pools

Synaptosomes prelabeled at 37 degrees C with radioactive amino acids (GABA, glutamate, glycine, taurine, alpha-aminoisobutyric acid, phenylalanine, leucine) and then washed at 0 degrees C on Millipore filters (DAWP 02500) lost 60-70% of the accumulated radioactivity. The loss was similar with exogenous tritiated GABA and glutamate, and with [14C]GABA and [14C]glutamate metabolically derived from [14C]glucose. In contrast, radioactive norepinephrine, dopamine and 5-hydroxytryptamine were almost totally retained by cold shocked synaptosomes. After pretreatment with reserpine and nialamide the loss of norepinephrine became significantly greater (about 25%). The uptake of radioactive GABA, glutamate and clycine after cold shock was about 50% reduced, whereas that of radioactive biogenic amines was less affected (reduction of 22% for norepinephrine, 29% for 5-hydroxytryptamine and 35% for dopamine). The loss of amino acids and the reduction of uptake could be minimized by performing the cold shock in hypertonic conditions. In synaptosomes prelabeled with [3H]GABA, a good correlation was observed among magnitude of amino acid pool depletion induced by cold shock or by 56 mM KCl, decrease of subsequent accumulation of [14C]GABA, and decrease of [14C]-GABA-stimulated [3H]GABA release (homoexchange).