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The ATP-dependent uptake of l-glutamate into synaptic vesicles has been well characterized, implicating a key role for synaptic vesicles in glutamatergic neurotransmission. In the present study, we provide evidence that vesicular glutamate uptake is selectively inhibited by the pep-tide-containing halogenated ergot bromocriptine. It is the most potent inhibitor of the agents tested; the IC5o was de-termined to be 22 μM. The uptake was also inhibited by other ergopeptines such as ergotamine and ergocristine, but with less potency. Ergots devoid of the peptide moiety, however, such as ergonovine, lergotrile, and methysergide, had little or no effect. Although bromocriptine is known to elicit dopaminergic and serotonergic effects, its inhibitory effect on vesicular glutamate uptake was not mimicked by agents known to interact with dopamine and serotonin receptors. Kinetic data suggest that bromocriptine competes with glutamate for the glutamate binding site on the glutamate trans-locator. It is proposed that this inhibitor could be useful as a prototype probe in identifying and characterizing the vesicular glutamate translocator, as well as in developing a more specific inhibitor of the transport system.

The ATP-dependent glutamate uptake system in synaptic vesicles prepared from mouse cerebellum was characterized, and the levels of glutamate uptake were investigated in the cerebellar mutant mice, staggerer and weaver, whose main defect is the loss of cerebellar granule cells, and the nervous mutant, whose main defect is the loss of Purkinje cells. The ATP-dependent glutamate uptake is stimulated by low concentrations of chloride, is insensitive to aspartate, and is inhibited by agents known to dissipate the electrochemical proton gradient. These properties are similar to those of the glutamate uptake system observed in the highly purified synaptic vesicles prepared from bovine cortex. The ATP-dependent glutamate uptake system is reduced by 68% in the staggerer and 57–67% in the weaver mutant; these reductions parallel the substantial loss of granule cells in those mutants. In contrast, the cerebellar levels of glutamate uptake are not altered significantly in the nervous mutant, which has lost Purkinje cells, but not granule cells. In view of evidence that granule cells are glutamatergic neurons and Purkinje cells are GABAergic neurons, these observations support the notion that the ATP-dependent glutamate uptake system is present in synaptic vesicles of glutamatergic neurons.