Release of amino acid neurotransmitters in different cortical areas of conscious adult and aged rats.

The aim of the present study was to investigate whether or not the levels of amino acid neurotransmitters change during the normal process of aging in the cerebral cortex. In vivo push-pull perfusions were performed in four different areas of the cortex of young (3-4 months) and aged (24-26 months) rats: medial prefrontal cortex, sulcal prefrontal cortex, parieto-temporal cortex and occipital cortex. Extracellular levels of Asp, Glu, Ser, Gln, and Gly were analyzed by HPLC-fluorimetric detection. Aspartate, glutamate, and serine showed no differences between young and aged animals in any of the four cortical areas studied. However, in the aged rats, the levels of glutamine, a precursor of acidic amino acid neurotransmitters, were significantly increased in medial prefrontal cortex, sulcal prefrontal cortex, and parieto-temporal cortex. No changes in glutamine were detected in occipital cortex. These results suggest that the extracellular levels of Asp and Glu in the cerebral cortex do not change during the normal process of aging possibly due to functional compensations made by intact neurons and/or astrocytes. In addition, the increase of glutamine in some areas of the cerebral cortex could be indicative of glial (astrocytes) proliferation as a result of neuronal degeneration during the normal process of aging.