Early postnatal stimulation influences passive avoidance behaviour of adult rats.

In this study, the effects of stimulation on either postnatal days 1 to 7 or 21 to 27 on the passive avoidance reaction (PAR) of 3-month-old rats were examined. Animals received tactile or visual stimulation or tactile-visual stimulation for 10 min each day, and were trained at the beginning of the 4th month of life in a step-through apparatus using a footshock of 0.5 mA. Memory retention was measured 24, 48, 72, 96, and 120 h after the acquisition trial. Step-through latencies to enter the dark compartment and the total duration of stay in the illuminated compartment were recorded up to 200 s. Rats that received tactile or a combined tactile-visual stimulation during the 1st postnatal week displayed significantly longer PAR latencies and a longer duration of stay in the illuminated compartment compared to unstimulated control animals. Visual stimulation during the postnatal days 1 to 7 and 21 to 27 resulted in a longer duration of stay in the illuminated compartment. This effect, however, was more pronounced when stimulation was applied during the 1st postnatal week. Rats that received tactile stimulation during the 4th postnatal week showed decreased PAR performance for all measured parameters when compared to animals that received stimulation during the 1st postnatal week. Furthermore, combined tactile-visual stimulation during the 4th postnatal week led to a reduced duration of stay in the illuminated compartment when compared to the stimulation during the 1st postnatal week. These findings can be attributed to the higher degree of plasticity and to a heightened sensitivity to various stimuli in the 1st postnatal week. The results suggest that tactile, visual or combined tactile-visual stimulation have a long-lasting effect on the ability of adult rats to cope with stressful tasks.