BACKGROUND: The orbitofrontal cortex (OFC) is a brain region closely associated with emotion.5-hydroxytryptamine (5-HT) has been shown to be involved in human depression.OBJECTIVE: To investigate OFC actions and mechanisms of 5-HT and 5-HT1A receptor (5-HT1AR)in stress-induced depression.DESIGN, TIME AND SEI-rlNG: A randomized, controlled, animal experiment was performed at Laboratory of Neurobiology, College of Life Science, Shaanxi Normal University between May 2006 and March 2008.MATERIALS: 5-HT, p-chlorophenylalanine (PCPA, an inhibitor to tryptophan hydroxylase) andspiperone (5-HT1AR antagonist) were provided by Sigma, USA; rabbit anti-rat 5-HT1AR antibody was provided by Tlanjin Haoyang Biological Manufacture.METHODS: A total of 40 male Sprague Dawley rats, aged 3 months, were randomly divided into five groups: control, model, 5-HT, spiperone+5-HT, and PCPA, with 8 rats in each group. Except for control group, rats in the other four groups were used to establish depression models by forced swimming for 15 minutes. At 30 minutes before forced swimming test, 0.5μL of 5-HT (12.5μg/pL),PCPA (20μg/μL), spiperone (1.3 μg/μL)+5-HT (12.5μg/μL, 10 minutes later), and saline were respectively injected into the OFC of 5-HT, PCPA, spiperone+5-HT, and model groups, respectively.The control group received a saline microinjection into the OFC.MAIN OUTCOME MEASURES: Forced swimming and open field tests were employed to measure animal behaviors, and immunohistochemistry was used to analyze 5-HT1AR expression in the OFC,cingulate cortex, and piriform cortex.RESULTS: (1) Compared with the model group, 5-HT microinjection into the OFC prominently reduced immobility time in the forced swimming test and rearing in open field test (P<0.05);locomotion and grooming in open field test were increased, although there was no significance (P>0.05). Furthermore, following PCPA microinjection into the OFC (PCPA + forced swimming stress),immobility time in forced swimming test increased dramatically (P<0.01), locomotion and rearing inopen field test declined (P<0.05 and P<0.01, respectively). Compared with the 5-HT group,5-HT1AR antagonist (spiperone+5-HT+forced swimming stress) increased immobility time in forced swimming test (P<0.01), but decreased locomotion, rearing, and grooming in open field test.(2) Forced swimming stress markedly elevated 5-HT1AR expression in OFC, cingulate cortex, and piriform cortex (P<0.05 or P<0.01).CONCLUSION: 5-HT improved stress-induced depression, and 5-HT anti-depression effects are primarily achieved via 5-HT1AR. Stress-induced up regulation of 5-HT1AR expression might be a compensatory mechanism for decreased 5-HT expression.