Mitochondrial enzymes related to glutamate and GABA metabolism in the hippocampus of young and aged rats: a quantitative histochemical study.

Quantitative histochemistry (scanning microphotometry) was used to determine the activities of the mitochondrial enzymes NAD-linked isocitrate dehydrogenase (EC 1.1.1.41), L-glutamate dehydrogenase (EC 1.4.1.3) and GABA transaminase (EC 2.6.1.19) in various layers of the hippocampus (middle one third) of young (3-4 months old) and memory-impaired aged rats (28-30 months old). For comparison, determinations of cytochrome c oxidase (EC 1.9.3.1) as a marker for mitochondria and energy metabolism were also performed. The study showed that there was a layered reaction pattern in the hippocampus and that the cellular distribution and the levels of enzyme activity were different. However, the activities of the different enzymes (excepting GABA transaminase and cytochrome c oxidase) were significantly correlated in the hippocampus in both age groups. Age-dependent changes were only observed for NAD-linked isocitrate dehydrogenase and GABA transaminase (significant increases of activities in some layers of the hippocampus, preferentially in the terminal field of the perforant path). From the present study it is concluded that, 1. the enzymatic complement of mitochondria in neurons and glia depends upon layer specific metabolic processes of the hippocampus (also with respect to glutamatergic and GABAergic terminal fields) indicating a layer specific interaction of the enzymes studied to produce or catabolize glutamate and GABA, and 2. the age dependent changes of the studied enzymes are very restricted.