Social isolation effects on the "behavioral despair" forced swimming test: effect of age and duration of testing.

Social loss is considered to be one of the major precipitants of depression. Prior work with the Porsolt forced swimming test (FST) has failed to demonstrate increases in despair-like immobility as a result of prior social isolation in adult animals. In the present work, increased immobility was observed in young Swiss Webster mice that had been socially isolated for 24 h prior to a 15-minute FST. The effect was not apparent until after the first five minutes of testing. The increase in immobility as a result of social isolation was apparent in 17-21-day-old animals but not in 26-30-day-old ones. Control experiments indicated that the increase in immobility was not due to the slightly higher weight loss of the socially isolated animals. Administration of reserpine (0.25 mg/kg) induced a marginal increase in immobility in the youngest animals but decreased immobility at later ages. These data suggest that the mouse only exhibits a short period of time during early development where social isolation can promote despair-like immobility in the FST and suggest that analyses of depressive processes which result from social variables may be best studied during a limited age range in this species.