Effect of forced running stress on behavior and on brain serotonin system in rats

This study investigated the effect of forced running for 10 days on rat behavioral activity and on the serotonergic (5-HT) system in the train. Twelve-week-old female rats were observed for daily spontaneous running activity and estrous cycle in vaginal smear, then the stress group was exposed to forced running for 10 days. After forced running for 10 days, rats showed some characteristics of stressful features such as suppression of spontaneous running activity, absence of estrous cycle in smear, adrenal weight gain and low body temperature. The concentrations of 5-HT were not altered in any region of the brain. However, the concentrations of 5-hydroxyindole acetic acid (5-HIAA) and the 5 HIAA/5-HT ratio were increased in frontal cortex, midbrain and pens after stress. The accumulation of 5-hydroxytryptophan (5-HTP) in the midbrain and pens after m-hydroxybenzylhydrazine (NSD-1015) injection, an inhibitor of aromatic l-amino acid decarboxylase, was greater in stress rats than that in non stressed control rats. 5-HT2A receptor density in frontal cortex was not altered after the stress. These results suggest that forced running for 10 days produces the elevation of pre-synaptic 5-HT synthesis and metabolism and that the pre-synaptic 5-HT activity may associate with animals behavioral insufficiency.