Long-term consequences of early experience on adult avoidance learning in female rats: role of the dopaminergic system

Following our hypothesis that juvenile emotional and/or cognitive experience should affect learning performance at preweaning age as well as adulthood, the present study in female Wistar rats aimed to examine the impact of (i) avoidance training at preweaning age, (ii) exposure to repeated maternal separation, (iii) the combination of both, and (iv) the blockade of dopaminergic neurotransmission on adult two-way active avoidance learning in rats. We found that preweaning, i.e. three week old, rats were less capable of avoidance learning compared to adults. Our main findings revealed that preweaning avoidance training alone improved avoidance learning in adulthood. Furthermore, maternal separation alone also improved avoidance learning in preweaning and in adult rats, but this effect of maternal separation did not add up to the beneficial effect of preweaning avoidance training on adult learning. In addition, the pharmacological blockade of dopamine receptors during preweaning avoidance training via systemic application of haloperidol impaired preweaning avoidance performance in a dose-dependent manner. Testing the haloperidol-treated preweaning presumed "non-learners" as adults revealed that they still showed improved learning as adults. Taken together, our results strongly support the hypothesis that emotional as well as cognitive experience at preweaning age leaves an enduring "memory trace," which can facilitate learning in adulthood. Our pharmaco-behavioral studies suggest that unlike the adult brain, preweaning learning and memory formation is less dependent on dopaminergic mechanisms, which raises the intriguing question of possible alternative pathways.