Effect of maternal presence on the fear response to an unfamiliar environment as measured by heart rate in rats as a function of age

Abstract

Rats 16 to 60 days of age were placed in an unfamiliar environment either alone or in the presence of an anesthetized lactating dam. Rats of all ages show a dramatic increase in fear, as measured by heart rate (HR), when placed alone in an unfamiliar environment. Adult rats, however, show a rapid decline in heart rate during the course of a 90‐min session, whereas 16‐day‐old rats show no significant decrease in HR during the same period. These results suggest that the adult rats become less fearful of the test environment over time, but that the 16‐day‐olds maintain a high level of fear throughout the 90‐min test period. Twenty‐three‐ and 30‐day‐old rats show an adultlike pattern of adaptation to the unfamiliar environment, i.e., their heart rates return partially but not completely to baseline levels within the 90‐min session. When an anesthetized lactating dam is present in the test environment, a strikingly different pattern of results is obtained. Heart rate in the 16‐day‐old rat is immediately reduced to near baseline levels whereas in the young adult (60‐day‐old) rat the presence of the anesthetized dam has no effect on either initial HR or rate of adaptation. Again the 23‐ and 30‐day‐old rats showed an intermediate pattern of adaptation. In two follow‐up experiments the effects of adding (Expt. 2) or removing (Expt. 3) an anesthetized dam on the 16‐day‐old rats' response to an unfamiliar environment was examined. Both experiments showed that animals were more fearful when the dam was absent from the environment than when it was present.