Effects of glutamate receptor agonists and antagonists on Ca 2+ uptake in rat hippocampal slices lesioned by glucose deprivation or by kainate

Abstract

The functional relevance of presynaptic glutamate receptors in controlling presynaptic Ca 2+ influx and thereby transmitter release is unknown. To test if presynaptic Ca 2+ entry in the hippocampus is controlled by glutamate autoreceptors, we created a hippocampal slice preparation for investigation of presynaptic Ca 2+ signals with Ca 2+-sensitive microelectrodes after lesioning of neurons by glucose deprivation or kainate. Stratum radiatum and alveus stimulation-induced postsynaptic field potential components were irreversibly abolished in areas CA1 and CA3 of lesioned slices, whereas stratum radiatum stimulation still evoked afferent volleys. Repetitive stimulation of the stratum radiatum still induced decreases in extracellular Ca 2+ concentration. Repetitive stimulation of the alveus no longer induced decreases in extracellular Ca 2+ concentration, suggesting complete damage of pyramidal cells. The stratum radiatum stimulation-induced decreases in extracellular Ca 2+ concentration in lesioned slices were comparable to those elicited during application of the glutamate antagonists 6-cyano-7-nitroquinoxaline-2,3-dione and l-2-amino-5-phosphonovalerate. In lesioned slices the stimulus-induced presynaptic Ca 2+ influx was reversibly reduced by kainate, RS-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), N-methyl- d-aspartate and glutamate without effects on afferent volleys. The kainate and N-methyl- d-aspartate effects on presynaptic Ca 2+ signals were partly sensitive to 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline and l-2-amino-5-phosphonovalerate, respectively, while the AMPA effects were not significantly affected by 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline, suggesting involvement of a novel glutamate receptor subtype. The involvement of a novel glutamate receptor subtype was supported by our findings that ionotropic glutamate receptor agonists also reduce presynaptic Ca 2+ influx under conditions of blocked synaptic transmission by 6-cyano-7-nitroquinoxaline-2,3-dione and l-2-amino-5-phosphonovalerate. 1-Aminocyclopentane- trans-1,3-dicarboxylic acid had no significant effect on presynaptic Ca 2+ entry. Also, the presynaptic Ca 2+ influx was not influenced by the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione, 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(f)quinoxaline and l-2-amino-5-phosphonovalerate when applied alone. Low kainate concentrations (5 μM) reduced presynaptic Ca 2+ signals in area CA3 but not in area CA1, demonstrating the higher affinity of presynaptic kainate receptors on mossy fibre terminals.