Environmental and tactile stimulation modulates the neonatal handling effect on adult rat spatial memory

Handling of rat pups promotes their adult cognitive performance. However, new data suggest that individual components of the handling procedure, like exposure to novelty or tactile stimulation, have distinct lasting effects on behaviour. In this study we examined the interaction of early novelty exposure with a varying amount of tactile stimulation on spatial recognition memory and corticosterone secretion of adult male and female rats. A split litter design was used and the experimental animals were also compared to animal facility reared controls. The experiment was conducted in two phases. In the first phase, we examined the effect of novel or home environment during the 15-min of neonatal handling, following 10 back-strokes. Tactile stimulation of 10 back-strokes combined with novelty exposure, enhanced novel arm discrimination in a Y-maze task in adult rats of both sexes compared to their siblings that stayed at home, as well as to the animal facility reared controls. In the second phase, additional back-stroking (total of 20 back-strokes) reduced the Y-maze performance of males neonatally exposed to novelty, while the same treatment enhanced the performance of their siblings that stayed at home. Basal corticosterone levels, determined 1 week post-Y-maze, were significantly increased only in the novelty exposed/10 back-stroked females compared to same sex non-handled controls. In contrast, 10 back-strokes combined with the home cage environment increased corticosterone in males. Increase to 20 back-strokes reversed the impact of neonatal environment on corticosterone levels. These data suggest that the nature and intensity of the individual components of a mild early life manipulation, like handling, are critical in modifying aspects of adult memory performance and basal adrenocortical function.