The effects of early stress and undernutrition on the behavior of young adult rats and the correlations between behavioral and brain parameters

Abstract

Control and chronic undernourished newborn rat pups were exposed to cold stress for 3 min daily or left undisturbed on Days 2–11. On Days 25–29 (preweaning) and at 90 days of age, stressed and non-stressed pups in each group were tested in a cross-maze and a T-maze, and their preference for novel and social stimulation and activity levels were measured. At 90 days only, another group of such pups was trained to learn an active avoidance response to electric shock. The results on maze performance show that prior to weaning, stressed controls (SC) and nonstressed undernourished (NSU) pups exhibited shorter response latencies, and a greater preference for social stimulation than the non-stressed controls (NSC). At 90 days, SC and NSU again exhibited shorter latencies, and higher activity levels, and also a greater preference for novel stimulation than the NSC animals. In these parameters, there were no significant differences between stressed undernourished (SU) and NSU animals. In original avoidance learning, undernourished animals exhibited a slower rate of acquisition learning than controls. However, in reversal learning, NSU animals made significantly more errors than the SU animals. There were no significant differences between the two control groups and the SU group. These results show that early cold stress alone can have the same behavioral effects as early undernutrition alone and that early stress can have a significant reversing effect on the learning performance of previously undernourished adult animals. Statistical correlations between behavioral parameters (X- and T-maze behavior and active avoidance learning) and crude brain parameters (weight, cell number (DNA), cell density and protein) were investigated in the above animals. It was found that, in general, behavioral parameters whose high values appear to be of advantage for the animal (e.g. number of entries or social time in mazes and percent correct moves in the original and reversal learning) had significant positive correlations with weight, cell number (DNA), and protein content, at least in cerebral cortex and cerebellum, and negative correlations with cortical cell density. Conversely, parameters whose high values are of disadvantage (latency in mazes, or total trials to criterion and total errors in learning) generally had significant negative correlations with brain weight, cell number, and protein content, and positive with cortical cell density. Age, stress or undernutrition had no effect on these results. It is concluded that, statistically, even these crude brain parameters may be significantly correlated with behavior. That is, animals tend to perform better if their cortical cells are further apart and if their brain weight, cell number (DNA), and protein content are higher.