Prolactin Levels in Adult Rats Following Equivalent Exposure to Either the Open Field or Restraint and the Open Field

Prolactin is a reliable measure of both physical and psychological stress in rats. In addition, stress can be quantified by activity, with rats showing increased activity under many stressful conditions. In the present study, rats were (1) exposed to an open-field for 10 min, followed by exposure to a novel, circular open-field for 10 min (controls) or (2) restrained for 10 min followed by 10 min in the circular open field. Both male and female animals in the restraint condition reared and ambulated more than rats in the control condition. Conversely, rats in the restraint group froze less than those in the control group. Thus, restraint prior to exposure to the open field did cause more stress than mere exposure to the open field as measured by activity. Prolactin levels were not altered by treatment condition. Perhaps the stress of being moved from one novel open field to another was enough to initiate a new surge of prolactin in the control animals so that their levels were peaking after 10 min of stress and were therefore indistinguishable from those in the restraint condition