A possible alarm substance in the forced swimming test.

Rats were tested in a forced swimming paradigm under a number of conditions which showed that time of day of testing did not significantly affect duration of immobility and a shorter ten-minute test period was as effective as the traditional longer protocol which tests animals 24 hours after initial immersion. Prior foot shock and noise decreased the immobility response. In addition, swimming in a cylinder in which another rat had been swimming also decreased the immobility response. The latter was not due to fecal or urinary contamination but appeared to be due to an alarm substance released during immersion in the water. Animals were affected by this proposed substance whether they released it themselves or it was released by a stranger. The proposed substance was released after a 3.5-minute or longer swimming period.