Changes in lateral septal nucleus neuron firing rate and coping with forced swim during gestation in the Wistar rat

The aim of the present study was to explore changes in immobility during the forced-swim test (FST) and in the firing rate of the lateral septal nucleus (LSN) during gestation (G) in the Wistar rat, Rattus norvegicus. In the latter test we also measured changes in plasma progesterone levels. In the FST we considered total time of immobility and also the angle formed by the longitudinal axis of the rat's body and the water surface during immobility. Total time of immobility progressively diminished along gestation at the expense of a decrease in immobility at the 0–30° angle. Immobility at 45 to 90° paralleled body weight gain, but locomotion in the open-field test did not change along gestation. The LSN firing rate increased progressively from day G1 to day G7, peaking significantly on G14 and decreasing to dioestrus levels by G19 and postpartum. Finally, plasma progesterone peaked significantly on G14 and G19, compared with dioestrus, G1, G7 and postpartum. Because clinically effective antidepressants as well as progesterone reduce immobility in FST and increase firing rate in the LSN, and both effects were observed during gestation but with different timing, we conclude that the smooth increase in plasma progesterone and the LSN firing rate peak at G14 may lead to additional changes that allow rats to cope with the stress represented by FST during the second third of gestation and postpartum.