Extracellular serotonin in the striatum increased after immobilization stress only in the nighttime

The release of serotonin (5-HT) in the striatum and the motor activity of rats given the immobilization stress were measured using in vivo microdialysis techniques and locomotion counts. Serotonin release in the striatum did not change in the daytime and nighttime, but motor activity in the nighttime was significantly higher than in the daytime. Serotonin release was not significantly increased during immobilization stress in the daytime or nighttime. In the nighttime, however, after the end of stress, 5-HT release was significantly increased from 0.002 ng/dialysate to 0.47 ng/dialysate. The motor activities were also significantly increased after the stress in the nighttime. These results suggest that the effects of immobilization stress on serotonin release in the striatum were different in the daytime and nighttime. In the nighttime, serotonin release in the striatum and the motor activities increased not during but after immobilization stress.