Age-related changes of glutamate dehydrogenase in the rat hippocampus: an enzyme histochemical study

Enzyme histochemical techniques associated with microphotometry were used to evaluate the reactivity of glutamate dehydrogenase (GDH) in the hippocampus of young (3 month) adult (12 month) and old (26 month) male Sprague-Dawley rats. In young rats the highest level of enzymatic reactivity was found in the stratum oriens of the CA-1-CA-4 fields followed in descending order by the molecular layer of fascia dentata, and by the layer of mossy fibers. In all the layers investigated GDH reactivity was higher in adult that in young rats. The older animals exhibited lower GDH reactivity than adult rats in almost all of the layers and higher enzymatic reactivity than in young rats in the stratum oriens of the CA-1-CA-4 fields. The hippocampus is known to be involved in the acquisition of two-way avoidance learning and is thought to utilize glutamate as one of its main neurotransmitters. These data suggest a possible relationship between hippocampal glutamatergic transmission and impairment of acquisition of avoidance learning occurring with age. Moreover, the possibility that a decreased glutamate catabolism occurring in old rats may contribute to the neuronal loss described in the hippocampus of aged rats is discussed.