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ABSTRACTThe Contribution of Serotonergic and Noradrenergic System to the Antidepressant-like Effect of Tramadol in the Unpredictable Chronic Mild Stress ModelTramadol is a centrally acting clinically effective analgesic which inhibitsnoradrenaline (NA) and serotonin (5-HT) reuptake. This study was planned toinvestigate the possible antidepressant-like effects of tramadol (20 mg/kg) in theunpredictable chronic mild stress (UCMS) model of depression in BALB/c miceand we also searched the participation of serotonergic and noradrenergic system.Desipramine was used like a positive control. Tramadol and desipramine reversedthe physical and behavioural abnormalities induced by the UCMS. Furthermore,the lesion of the dorsal raphe magnus (DRN) by 5, 7-dihydroxytryptamine (5,7-DHT) antagonized the antidepressant-like effects of tramadol and desipramine onthe coat state, in the splash test in stressed mice but not non-stressed mice.The results obtained by High Pressure Liquid Chromatography (HPLC)showed that level of the 5-HT and its metabolite 5-hydroxyindolacetic acid (5-HIAA) was reduced by the 5,7- DHT lesion in some brain regions. In contrast, thelevel of NA, Dopamine (DA), Homovanillic acid (HVA) and 3, 4dihydroxyphenylacetic acid (DOPAC) did not change by the 5, 7-DHT lesion.Moreover the UCMS regimen diminished the level of 5-HT, 5-HIAA, NA and DAin some brain regions.Furthermore, the contribution of the 5-HT1A receptors to theantidepressant-like effects of tramadol was searched by the 5-HT1A/1B receptorantagonist pindolol (10 mg/kg); we observed neither significant acceleration nordiminution by pindolol on the actions of desipramine and tramadol.In the last part, we measured the NA and its metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG) levels via HPLC in stressed mice. The chronictreatment with desipramine and/or tramadol significantly augmented MHPGand/or NA level in the locus coeruleus (LC), hypothalamus and hippocampus butnot in cerebellum. Moreover, desipramine and tramadol induced antidepressant-like effects in the UCMS model can be blocked by propranolol (non-selective β-adrenoreceptor antagonist, 5 mg/kg), ICI 118,551 (selective β2-adrenoreceptorantagonist, 2 mg/kg) and yohimbine (selective α2-adrenoreceptor antagonist, 2mg/kg).Taken together these results suggest that tramadol has an antidepressant-like effect in the UCMS model, which is mediated by both serotonergic andnoradrenergic system.Key Words: Unpredictable chronic mild stress, tramadol, serotonergic system,noradrenergic system, micexxvii