Glutamate uptake into synaptic vesicles is driven by an electrochemical proton gradient formed across the membrane by a vacuolar H ؉ -ATPase. Chloride has a biphasic effect on glutamate transport, which it activates at low concentrations (2-8 mM) and inhibits at high concentrations (>20 mM). Stimulation with 4 mM chloride was due to an increase in the V max of transport, whereas inhibition by high chloride concentrations was related to an increase in K m to glutamate. Both stimulation and inhibition by Cl ؊ were observed in the presence of A23187 or (NH 4 ) 2 SO 4 , two substances that dissipate the proton gradient (⌬pH). With the use of these agents, we show that the transmembrane potential regulates the apparent affinity for glutamate, whereas the ⌬pH antagonizes the effect of high chloride concentrations and is important for retaining glutamate inside the vesicles. Selective dissipation of ⌬pH in the presence of chloride led to a significant glutamate efflux from the vesicles and promoted a decrease in the velocity of glutamate uptake. The H ؉ -ATPase activity was stimulated when the ⌬pH component was dissipated. Glutamate efflux induced by chloride was saturable, and half-maximal effect was attained in the presence of 30 mM Cl ؊ . The results indicate that: (i) both transmembrane potential and ⌬pH modulate the glutamate uptake at different levels and (ii) chloride affects glutamate transport by two different mechanisms. One is related to a change of the proportions between the transmembrane potential and the ⌬pH components of the electrochemical proton gradient, and the other involves a direct interaction of the anion with the glutamate transporter.

Polyamines are polycations present at high concentrations in the mammalian brain. We investigated the effect of an intrahippocampal infusion of spermidine, a polyamine agonist, immediately post-training on the inhibitory avoidance learning paradigm in rats. Bilateral intrahippocampal microinjection of spermidine (0.02-20 nmol) caused an increase in test step-down latencies at low concentrations. Administration of arcaine (0.002-0.2 nmol), an antagonist of the N-methyl-D-aspartate (NMDA) receptor polyamine binding site, did not modify the test step-down latencies. In contrast, co-administration of arcaine and spermidine completely reversed the facilitatory effect of spermidine on the test step-down latencies. These results provide evidence that polyamines may be involved in learning and memory modulation.

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