The effects of aging and corticosterone levels on spatial learning.

Abstract

The effects of aging and corticosterone (CORT) injections on spatial learning were examined. Young (4-6mos.,Y), young-aged (23-25mos.,YA), and old-aged (31mos.,OA) male Fischer-344xBrown Norway hybrid rats (N = 80) received 6 pre-testing days in the Morris water task to determine baseline spatial learning performance. Half the animals in each group then received daily injections of CORT (2mg/100gms weight) for 15 days. During the last 6 injection days, all animals were again tested (post-testing) in the Morris task with the environment and goal location changed. Baseline, peak, and clearance plasma CORT levels were determined by CORT radioimmunoassay. During pre-testing, YA animals swam as directly to the goal as Y, but OA animals were impaired $\rm(p < .05).$ During post-testing, both YA and OA non-injected animals had significantly greater directional heading errors (DHE, $\rm p < .05)$ and flatter learning curves than Y (F = 4.22, p =.05 & F = 8.13, $\rm p < .01,$ respectively). Subtle age-related spatial learning deficits were identified when criteria for several learning measures were used to classify each YA and OA animal as unimpaired, altered, or impaired during pre- and post-testing. A more severe age-related spatial learning impairment was found in Fischer-344 rats than the hybrids. Among injected hybrids, OA performed as well as Y (F = 1.56, p = NS), but YA did not (F = 4.57, $\rm p < .05).$ No differences were found in the effects of restraint stress vs. CORT injections on DHE. Peak CORT levels moderately predicted DHE in Y and OA hybrids and accounted for 50% of DHE variance on days 3 and 2, respectively (Y: p =.07; OA: $\rm p < .01).$ A positive relationship exists between peak CORT level and DHE in OA animals, no relationship exists in YA animals, and an inverse relationship exists in Y. A range of peak CORT values influences learning. Young animals with high $(43{-}62\rm\mu g/dl)$ CORT levels performed as well as non-injected Y, but Y animals with low $(31{-}39\rm\mu g/dl)$ peak CORT levels were impaired (vs.high: F = 32.90, $\rm p < .01;$ vs. non-injected: F = 5.82, $\rm p < .05).$ Among OA animals, those with extra-low $(22{-}28\rm\mu g/dl)$ peak CORT levels exhibited early learning that was maintained across testing days. Thus, the way plasma CORT levels influence spatial learning shifts with age. Equating CORT level with types of stressors and identifying stressors beneficial to learning will enable nurses to assist elders in retaining or improving spatial learning abilities.