Selective adaptation of male rats to repeated social encounters and experimental manipulations

The objective of the present study was to determine how rats adapt to repeated encounters with an unfamiliar male or group of males and to experimental manipulations. Male rats were instrumented with radiotelemetry transmitters to monitor heart rate (HR), mean arterial blood pressure (MAP), and movements in the home cage as indices of arousal and stress. Groups of eight singly housed rats were exposed in their home cages to encounters with an unfamiliar rat or a group of three rats of the same strain, age, and sex for a period of 18 h (1600 to 1000 h) on four consecutive days. On separate occasions, the instrumented rats were given a subcutaneous injection of sterile saline in the animal room or were transported from the animal room to the laboratory and then injected at 1000 h on four consecutive days. Exposure to an unfamiliar rat or a group of 3 rats induced significant (P < 0.05) increases in HR, MAP, and movement in the first 30 min of exposure, and these responses partially but significantly (P < 0.05) adapted by the second or third day. Nocturnal movement in the home cage also was significantly (P < 0.05) increased on the first night and partially reduced on subsequent nights, but nocturnal HR responses were absent or significantly blunted. Nocturnal MAP was not increased. Subcutaneous injection in the animal room or after transport to the laboratory produced significant (P < 0.05) increases in HR, MAP, and movement in the cage 30 min after the injection, and these responses did not adapt over the four days of the experiment. We concluded that male Sprague-Dawley rats adapted quickly, albeit partially, to social encounters but did not adapt to repeated injections, suggesting selective adaptation depending on the type of stimulus used.