Putative acidic amino acid transmitters in the cerebellum. I. Depolarization-induced release

In the present investigation we studied the autoradiographic localization and the characteristics of the depolarization-induced release of acidic amino acids in in vitro rat cerebellar preparations. Light microscopy autoradiography of cerebellar slices preincubated in the presence of the non-metabolized glutamate analogue D-[3H]aspartate showed a large accumulation of radioactivity over glial cells, and very little labelling of the granule cells, whose putative neurotransmitter may be glutamate. In spite of its predominant localization in glia, D-[3H]aspartate (and [14C]glutamate) was released from cerebellar slices depolarized with high [K+] in a Ca2+-dependent way, and the release elicited by veratrine was prevented by TTX. These findings, together with the observation that freshly isolated or cultured glial cells did not show any Ca2+-dependent, depolarization-induced release of D-[3H]aspartate, suggest that the radioactive amino acid released from slices has a neuronal origin. The high [K+]-induced release of exogenous radioactive acidic amino acids from superfused cerebellar synaptosomal preparations exhibited, as best, a modest Ca2+-dependence, a result probably due to the existence of a substantial non-Ca2+-dependent release of the amino acid from glial fragments contaminating the preparation. However, both the K+-evoked release of endogenous glutamate, and that of [14C]glutamate previously synthesized from [14C]glutamine were largely Ca2+-dependent, suggesting that nerve endings are the main sites involved in the stimulus-coupled secretion. In the experiments in which synaptosomes had been prelabelled with [14C]glutamine, a study of the specific radioactivity of the glutamate released and of that present in synaptosomes at the beginning and at the end of superfusion period provided evidence in favour of a preferential release of the newly synthesized [14C]glutamate. In contrast to glutamate, endogenous aspartate was not released in a Ca2+-dependent manner, and the efflux of newly formed [14C]aspartate was only slightly potentiated by Ca2+, which suggests that glutamate and aspartate are not released from the same sites. Studies on preparations (slices and synaptosomes) from immature, 8-day-old cerebella showed that neither the K+-evoked release of D-[3H]aspartate, nor that of endogenous glutamate was Ca2+-dependent. In conclusion, the data presented are consistent with the proposition that glutamate has a neurotransmitter role in the cerebellum.U