Ionotropic glutamate receptor subtypes in the aged memory-impaired and unimpaired Long-Evans rat

The comparative quantitative autoradiographic distribution of ionotropic glutamate receptor subtypes were investigated in young adults (six months) and aged (24-25 months) cognitively impaired and unimpaired male Long-Evans rats. Aged rats were behaviorally characterized as either cognitively impaired or unimpaired based upon their performances in the Morris water maze task compared to the young adult controls. The status of the N-methyl-D-aspartate, [125I]dizocilpine maleate, [3H]kainate and amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA, [3H]AMPA) receptor binding sites were then established in these three subgroups of animals as a function of their cognitive performance in the Morris water maze task. The apparent densities of both N-methyl-D-aspartate/[125I]dizocilpine maleate and kainate binding sites were significantly decreased in various regions of the aged rat brain. Marked losses in [125I]dizocilpine maleate binding sites were observed in outer laminae of the frontal, parietal and temporal cortices, and the stratum radiatum of the CA3 subfield of the hippocampus. Interestingly, losses in [125I]dizocilpine maleate binding sites were generally most evident in the cognitively unimpaired aged subgroup, suggesting a possible inverse relationship between losses of this receptor subtype and cognitive performances in the Morris water maze task. The levels of [3H]kainate binding were most significantly diminished in various cortical and hippocampal areas as well as the striatum and septal nuclei of both groups of aged rats. In contrast, the apparent density of [3H]AMPA binding was increased in most hippocampal subfields and the superficial laminae of the occipital cortex of the cognitively impaired vs young adult rats. Changes in [3H]AMPA labeling failed to reach significance in the unimpaired cohort. Taken together, these results show that while losses in [3H]kainate binding were similar in both subgroups of aged rats, differences were seen with respect to cognitive status for both [125I]dizocilpine maleate/N-methyl-D-aspartate and [3H]AMPA binding sites. Decreases in [125I]dizocilpine maleate binding sites were mostly restricted to cortical areas of cognitively unimpaired rats, while increases in the AMPA binding subtype were seen in the memory-impaired subgroup. It would thus appear that changes in N-methyl-D-aspartate and AMPA receptor subtypes may be more critical than alterations in kainate binding sites for the emergence of the functional deficits seen in the aged cognitively impaired rat.