Water maze performance and changes in serum corticosterone levels in zinc-deprived and pair-fed rats

The aims of the present study were (1) to evaluate the learning and short- and long-term memory of zinc-deprived (ZD) and pair-fed (PF) rats in a Morris water maze (MWM) and (2) to monitor the serum corticosterone levels of these rats before and after swimming. Young Sprague-Dawley rats (aged 27-31 days) consumed AIN-93G diet for 10 days, and then were separated into ad libitum control (CT), PF and ZD groups. The zinc content of the diet was 25-30 ppm (CT and PF) or <1 ppm (ZD). After 17 days on experimental diets, a MWM was used to test spatial cognition. Delayed-matching-to-place (DMP) test results indicate that both zinc deprivation and food restriction had no effect on short-term memory. The PF rats exhibited significantly impaired learning and thigmotaxia (i.e., wall hugging) in the learning test. The PF group also demonstrated less preference for the target zone in the first 15 s of the probing test. When the total 120 s of the probing test was considered, there were no differences in preference for the target zone, but thigmotaxia was greater in the PF than the CT group. The only behavioral change of the ZD group was thigmotaxia observed during the 120-s probing test following training, indicating the increment of anxiety. Morning basal corticosterone levels before swim training were significantly elevated in the PF group on Day 15 of dietary treatment, whereas a significant elevation of the basal corticosterone level in the ZD group was not statistically significant until Day 22. The data indicate an association between impaired learning, poor searching strategy and elevated corticosterone in the PF group. In contrast, the ZD rats showed normal cognitive performance but had elevated corticosterone and increased anxiety-like behavior (thigmotaxia).