Early experiences and the development of emotional learning systems in rats

Bridget L Callaghan, Rick Richardson

Research first reported nearly 50 years ago demonstrated that infant and young animals (including humans) exhibit profoundly faster rates of forgetting (i.e., infantile amnesia) than do adults. In addition to these differences in retention, more recent research has shown that inhibition of fear learning is also very different in infancy than in adulthood. Specifically, extinction of fear early in life is much more resistant to relapse than is extinction later in life. Both of these findings suggest that young animals should be especially resilient to the emergence of mental health disorders, which appears to be at odds with the view that early-life experiences are particularly important for the development of later psychopathologies (such as anxiety disorders) and with the finding that the majority of anxiety disorders first emerge in adolescence or childhood. This apparent paradox might be resolved, however, if exposure to chronic stress early in life affects the maturation of the fear retention and extinction systems, leading to a faster transition to the adult form of each (i.e., long-lasting fear memories and relapse-prone extinction). In several recent studies we have found exactly this pattern; that is, infant rats exposed to maternal-separation stress exhibit adult-like fear and extinction learning early in development. Further, we have demonstrated that some of these effects can be mimicked by exposing the mother to the stress hormone corticosterone in their drinking water (in lieu of the separation procedure). These findings suggest that early-life exposure to stress and stress hormones may act as a general signal that can alter the developmental trajectory of emotional systems and potentially place animals at greater risk for the development of anxiety. The implications of these recent findings for our understanding of the developmental origins of health and disease, and for enhancing preventative and therapeutic treatments across the lifespan, are considered.