Chronically stressed adult male Balb C mice were submitted to the tail suspension test. Chronic immobilization stress (6 h/d for 14 consecutive days) induced a significant reduction in immobility time when compared to non-stressed controls. Pretreatment with LY 53857, a serotonin 5HT2 antagonist, and IPS 339, a selective beta-2 adrenoceptor blocker, reversed immobility time to the levels of non-stressed controls. Chronic administration of corticosterone (100 mg/kg for 7 d) did not modify immobility time as compared to saline treated controls. It is suggested that both serotonergic and adrenergic pathways in the brain may participate in the stress-induced changes occurring in the tail suspension test response and that corticosterone does not appear to play a role in this process.

This study was designed to verify whether fluoxetine (FL), a serotonin (5-HT) re-uptake inhibitor, would interfere with nortriptyline (NT), a biphasic U-shaped curvilinear dose-response relationship recently described in our laboratory. We associated 10 mg/kg NT or vehicle to 0, 5, 10, 20 and 40 mg/kg FL, in one group, and 10 mg FL or vehicle to 0, 5, 10, 20 and 40 mg/kg NT, in another group, 30 min before the tail suspension test (TST) in mice. Although we were not able to confirm a synergistic effect between FL and NT, FL-NT association seems to require higher doses of NT to block its own anti-immobility effect at high doses, thus widening NT effective antidepressant-like dose range in mice submitted to TST.

Mouse strain differences in immobility and in sensitivity to antidepressants have been observed in the forced swimming test (FST) and the tail suspension test (TST). However, the neurotransmitter systems and neural substrates that contribute to these differences remain unknown. To investigate the role of the hippocampal serotonin transporter (5-HTT), we measured baseline immobility and the immobility responses to fluoxetine (FLX) in the FST and the TST in male CD-1, C57BL/6, DBA and BALB/c mice. We observed strain differences in baseline immobility time, with CD-1 mice showing the longest and DBA mice showing the shortest. In contrast, DBA and BALB/c mice showed the highest sensitivity to FLX, whereas CD-1 and C57BL/6 mice showed the lowest sensitivity. Also we found strain differences in both the total 5-HTT protein level and the membrane-bound 5-HTT level (estimated by Vmax) as follows: DBA > BALB/c > CD-1 = C57BL/6. The uptake efficiency of the membrane-bound 5-HTT (estimated by 1/Km) was highest in DBA and BALB/c mice and lowest in CD-1 and C57BL/6 mice. A correlation analysis of subregions within the hippocampus revealed that immobility time was negatively correlated with Vmax and positively correlated with Km in the hippocampus. Therefore a higher uptake capacity of the membrane-bound 5-HTT in the hippocampus was associated with lower baseline immobility and greater sensitivity to FLX. These results suggest that alterations in hippocampal 5-HTT activity may contribute to mouse strain differences in the FST and the TST.

We studied the relationship of blood and brain levels of imipramine (IMI) and/or its active metabolite desipramine (DMI) to behavioural response, in the tail suspension test (TST). Compared to the control group, doses of 3.75, 7.5, 15, 30 and 60 mg/kg of IMI given intraperitoneally reduced immobility scores significantly. There were significant negative correlations between doses, brain levels of IMI, DMI, IMI + DMI, blood levels of IMI, DMI, IMI + DMI and the animal behaviour. Higher level of IMI and its metabolite in brain and in blood did not impair anti-immobility effect. These findings agree with most results found in men treated with IMI, and suggest a linear or sigmoid relationship between the antidepressant concentrations and its biological response.