Age-related changes in glutamate release in the CA3 and dentate gyrus of the rat hippocampus

Abstract

The present studies employed a novel microelectrode array recording technology to study glutamate release and uptake in the dentate gyrus, CA3 and CA1 hippocampal subregions in anesthetized young, late-middle aged and aged male Fischer 344 rats. The mossy fiber terminals in CA3 showed a significantly decreased amount of KCl-evoked glutamate release in aged rats compared to both young and late-middle-aged rats. Significantly more KCl-evoked glutamate release was seen from perforant path terminals in the DG of late-middle-aged rats compared young and aged rats. The DG of aged rats developed an increased glutamate uptake rate compared to the DG of young animals, indicating a possible age-related change in glutamate regulation to deal with increased glutamate release that occurred in late-middle age. No age-related changes in resting levels of glutamate were observed in the DG, CA3 and CA1. Taken together, these data support dynamic changes to glutamate regulation during aging in subregions of the mammalian hippocampus that are critical for learning and memory.

Fig. 1

Fast Green (Sigma–Aldrich) staining in the hippocampal formation confirming placement of the microelectrode/micropipette assembly in the dendritic tress of the CA1 and CA3 subregions (upper panel) and the DG subregion (lower panel).

Fig. 2

Mean tonic glutamate concentration in the trisynaptic circuit during aging. Aging did not significantly alter tonic glutamate levels in the DG (F(2,34) = 0.19, p = 0.83), CA3 (F(2,34) = 0.19, p = 0.83) or CA1 (F(2,34) = 0.65, p = 0.53).

Fig. 3

Representative glutamate release signals following local application of KCl (50 nL, 70 mM, isotonic, pH 7.4) in the trisynaptic circuit of F344 rats during aging. Each symbol in the traces is an individual glutamate measurement (2 Hz). Inset signals show reproducibility of evoked-glutamate release signals. (Right) Bar graphs represent the mean maximum amplitude of glutamate signals across the trisynaptic circuit. Terminals in CA3 and CA1 had significantly less glutamate release compared to terminals in the DG in the 18- and 24-month groups. (Bottom) Graphs represent the mean max. amplitude of glutamate signals across age groups. Each symbol is an individual animal. The perforant path terminals in the DG of late-middle aged rats released more glutamate compared to young (p < 0.05) and aged (p < 0.05) rats. The mossy fiber terminals in the CA3 of aged rats released less glutamate compared to young (p < 0.05) and late-middle aged (p < 0.05) animals.

Fig. 4

Mean glutamate uptake rate (µM/s) following local application of exogenous glutamate (100 µM in 0.9% saline, pH 7.4) in the trisynaptic circuit F344 rats during aging. Uptake rate was significantly faster in the aged DG compared to young rats (5.5 ± 0.7 µM/s vs. 3.3 ± 0.4 µM/s, p < 0.05). Within the aged trisynaptic circuit, uptake rates were significantly different showing faster glutamate uptake rate in the DG when compared to the other hippocampal subregions (CA3: 3.7 ± 0.5 µM/s, p < 0.05; CA1: 3.9 ± 0.4 µM/s, p < 0.05).

Fig. 5

Comparison of amplitude-matched signals in the DG showing faster clearance of glutamate in aged animals. Each symbol is an individual glutamate measurement (2 Hz). Arrow indicates local application of exogenous glutamate (100 µM in 0.9% saline, pH 7.4). (Inset) KCl-evoked glutamate signals from perforant path terminals in the DG of young and aged rats. Aged rats have a faster first order rate of glutamate clearance (0.29 ± 0.03 s⁻¹ vs. 0.21 ± 0.02 s⁻¹; t-test p = 0.036). Dashed line indicates predicted maximum amplitude of glutamate signal in an aged animal without increased glutamate clearance capacity.