SYNAPTIC VESICULAR LOCALIZATION AND EXOCYTOSIS OF L-ASPARTATE IN EXCITATORY NERVE-TERMINALS - A QUANTITATIVE IMMUNOGOLD ANALYSIS IN RAT HIPPOCAMPUS

To elucidate the role of aspartate as a signal molecule in the brain, its localization and those of related amino acids were examined by lig ht and electron microscopic quantitative immunocytochemistry using ant ibodies specifically recognizing the aldehyde-fixed amino acids. Rat h ippocampal slices were incubated at physiological and depolarizing [K] before glutaraldehyde fixation. At normal [K+], aspartate-like and g lutamate-like immunoreactivities were colocalized in nerve terminals f orming asymmetrical synapses on spines in stratum radiatum of CA1 and the inner molecular layer of fascia dentata (i.e., excitatory afferent s from CA3 and hilus, respectively). During K+ depolarization there wa s a loss of aspartate and glutamate from these terminals. Simultaneous ly the immunoreactivities strongly increased in glial cells. These cha nges were Ca2+ dependent and tetanus toxin-sensitive and did not compr ise taurine-like immunoreactivity. Adding glutamine at CSF concentrati on prevented the loss of aspartate and glutamate and revealed an enhan cement of aspartate in the terminals at moderate depolarization. In hi ppocampi from animals perfused with glutaraldehyde during insulin-indu ced hypoglycemia (to combine a strong aspartate signal with good ultra structure) aspartate was colocalized with glutamate in excitatory term inals in stratum radiatum of CA1. The synaptic vesicle-to-cytoplasmic matrix ratios of immunogold particle density were similar for aspartat e and glutamate, significantly higher than those observed for glutamin e or taurine. Similar results were obtained in normoglycemic animals, although the nerve terminal contents of aspartate were lower. The resu lts indicate that aspartate can be concentrated in synaptic vesicles a nd subject to sustained exocytotic release from the same nerve endings that contain and release glutamate.