A conspecific attenuates prolactin responses to open-field exposure in rats

Acute exposure to a novel environment, such as an open field, generally results in a prolactin surge, while several days of exposure to the open field is often characterized by a decline in prolactin. As exposure to the open field is a psychological stressor, altering the animal's interpretation of the event should alter prolactin levels. In the present study, juvenile male and female rats were habituated to the open field for 1 or 5 days prior to testing in the chamber alone or with a same-sex conspecific. Levels of prolactin were measured across all rats, and play (pins) was recorded for animals tested with a conspecific. Five days of habituation to the chamber resulted in lower levels of prolactin and more play than 1 day of habituation. Across both conditions of habituation, testing with a conspecific caused lower levels of prolactin than testing alone. In addition, play and prolactin were negatively correlated. The presence of a conspecific in a stressful situation may have reduced stress by altering the animal's negative interpretation of the open field. Further, as the intensity of the social interaction increased (more play), prolactin levels decreased.