The comparison of immobility time in experimental rat swimming models

Rat swimming models have been used in studies about stress and depression. However, there is no consensus about interpreting immobility (helplessness or adaptation) in the literature. In the present study, immobility time, glucose and glycogen mobilization, corticosterone and the effect of desipramine and diazepam were investigated in two different models: swimming stress and the forced swimming test. Immobility time was lower in swimming stress than in the forced swimming test. Both swimming models increased corticosterone levels in comparison with control animal levels. Moreover, swimming stress induced higher corticosterone levels than the forced swimming test did [F(2,14) = 59.52; p < 0.001]. Liver glycogen content values differed from one another (swimming stress < forced swimming test < control), [F(2,17) = 32.08; p < 0.001]. The glycogen content values in the gastrocnemius [F(2,16) = 11.35; p = 0.026] and soleus [F(2,16) = 8.68; p = 0.006] muscles were lower during swimming stress in comparison with the forced swimming test and control. The immobility time was recorded and measured in another group treated with desipramine and diazepam in two protocols: a single session of forced swimming test or swimming stress and two sessions (pre- and retest) of forced swimming model or swimming stress. Desipramine decreased the immobility time in the forced swimming test in both the single [F(2,25) = 20.63; p < 0.0001] and retest [F(2,37) = 7.28; p = 0.002] swimming session, without changes in the swimming stress model. Diazepam increased the immobility time in the swimming stress but not in the forced swimming test during the single [F(2,26) = 11.24; p = 0.0003] and retest sessions [F(2,38) = 4.17; p = 0.02]. It was concluded that swimming stress and the forced swimming test induced different behavior, hormonal and metabolic responses and represented different situations to the animal.