Effects of adrenalectomy on spatial memory performance and dentate gyrus morphology

Adrenalectomy (ADX) causes neuronal degeneration and cell loss in the dentate gyrus (DG) of the hippocampus. Since chemical or mechanical lesions of the DG are associated with impairments of spatial memory in rats, the effects of ADX on radial arm maze performance were evaluated. During 15 trials, where all 8 arms of the maze were baited, ADX rats were significantly impaired compared to sham operated controls (Shams). These trials were conducted 21-42 days post-ADX. Following these trials, time delays were instituted between the 4th and 5th choices, and ADX rats continued to show impaired performance. Daily intake of 3% saline was monitored in all rats and serum corticosterone (Cort) was measured. Saline consumption (ml/day) was higher in the ADX group (16.9 ± 1.6 in ADX vs. 1.3 ± 0.3 in Shams) and was negatively correlated with Cort level. serum Cort (% μg) differed between groups (0.6 ± 0.4 vs. 15.0 ± 2.3) and was negatively correlated with a greater number of maze errors, a measure of impaired performance. Cross sectional DG area was not reduced in ADX rats, and pyknotic cell number did not differ significantly between ADX and Sham animals. Moreover, pyknotic cell counts did not correlate with behavioral measures. These results lead to two conclusions: First, the recovery of accessory adrenal tissue in ADX rats, as indicated by the low levels of Cort, appears sufficient to suppress dentate granule neuron pyknosis, but may not be sufficient to suppress salt appetite. Secondly, the persistent deficits in spatial memory performance by ADX rats without the marked presence of pyknotic cells or reduction in DG size suggests that adrenal hormones themselves may exert activational effects on spatial memory.