Uptake of [3H]Glutamic acid in excitatory nerve endings: light and electronmicroscopic observations in the hippocampal formation of the rat.

Slices of the rat hippocampal formation were incubated in vitro with 2 μM [^3H]glutamic acid in Krebs' solution at 25°C for 10 min, fixed with 5% glutaraldehyde and processed for autoradiography. The highest concentrations of autoradiographic grains were found in the target zones of axons from the pyramidal cells of CA3/4 and in the target zones of mossy fibres from the granular cells in the area dentata. The mossy fibre boutons, which can be identified by their characteristic ultrastructure, were found to have a grain density six times that of non-axonal tissue elements (dendrites, glia and intercellular space). The corresponding figure-was four for boutons in the target zone of pyramidal axons. The overall grain density was 30–60% lower in the target zones of the perforant path axons (from the entorhinal area) than in those of the mossy fibres and pyramidal cell axons, but the grains were still concentrated over nerve endings and fibres. Consideration of ‘cross-firing’ effects led to the conclusion that boutons and axons probably contained a higher proportion of the radioactivity than is reflected in the grain counts (41–65%). Thus hypothetical grain analysis of the target zone of the medial perforant path suggested that axons and boutons, which constituted 20% of the section area, accounted for some 80% of the radioactivity. In a parallel study (Storm-Mathisen, 1977) the pyramidal and perforant path axon terminals have been shown to contain glutamate uptake sites, the uptake being dramatically reduced in their target zones following axotomy. The three systems of nerve endings found to contain glutamate uptake are all recognized excitatory systems. Only 4–6% of the autoradiographic grains occurred over glial elements which constituted 6–9% of the area of tissue sections. According to the hypothetical grain analysis 13 ± 5% of the radioactivity could be ascribed in glia.The findings show that high affinity glutamate uptake sites are concentrated in excitatory nerve endings, which may be ‘glutamergic’ according to other data. In the conditions used, such uptake may therefore be useful as a marker for glutamergic nerve endings.