Immobilization stress increases serotonin turnover in the extended brain regions in the rat.

Male Wistar rats were exposed to immobilization stress for 30 to 120 min. Changes in the levels of serotonin (5-HT) and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA) were determined in six brain regions. Immobilization stress virtually did not affect 5-HT levels at any time with the exception of a significant increase in 5-HT levels in the amygdala at 30 min. In contrast, within 30 min, immobilization stress caused significant increases in 5-HIAA levels in the hypothalamus, amygdala, hippocampus, cerebral cortex and pons+med. obl. Significant increases in 5-HIAA levels were observed at 60 min in the hippocampus and at 60 and 120 min in the pons+med. obi., while 5-HIAA returned to control levels in the other regions at both 60 and 120 min. After an i. p. injection of probenecid at 200 mg/kg, the stressed rats showed significantly higher 5 -HIAA levels in the hypothalamus, thalamus, amygdala, hippocampus, cerebral cortex and pons+med. obi. within 30 min as compared to those in the non-stressed rats, however, these increases were continued in the thalamus and hippocampus even at 60 and 120 min. Plasma corticosterone levels were significantly increased at 30 min and were manitained at the same levels for up to 120 min of stress. These results suggest that immobilization stress causes increases in 5-HT turnover in many brain regions including the hypothalamus and that the increases in most regions, in particular, in the hypothalamus and amygdala, occur rapidly and transiently.