Lateral septal neuronal firing rate increases during proestrus-estrus in the rat.

Neuronal activity of the lateral septal nucleus (LSN) is related to motivational and hedonic behavior. Even though some changes in mood and anxiety during proestrus and pregnancy have been reported, the possible changes in the neuronal activity of the LSN through the phases of the estrous cycle are unknown. In the present study we explored the neuronal activity from the LSN using glass micropipettes (NaCl 1 M, and Evans blue 2.5%; 3-8 Mohms in 30 urethane (1 g/kg) anesthetized Wistar rats. Analysis of data included a total of 88 single-unit extracellular recordings taken from the LSN during proestrus (n = 22), estrus (n = 23), diestrus (n = 22), and metestrus (n = 21). The highest values of firing rate were found in proestrus, and the lowest in metestrus, F(3,84) = 3.78, p < 0.01. During estrous cycles, in the phase characterized by high plasma levels of estradiol and progesterone, i.e., proestrus-estrus, the neurons from the dorsal aspect of the LSN fired at significantly (p < 0.05) higher frequencies, shorter first-order intervals and a lower coefficient of variation than those in the phase characterized by lower levels of estradiol and progesterone (metestrus-diestrus). In another group of rats (n = 12), immobility in the forced-swim test was assessed. Consistently, a longer latency (p < 0.05) for the first period of immobility and a nonsignificant trend to a lowered total time in immobility were found in proestrus and estrus. It is concluded that the higher firing rate in neurons from the dorsal aspect of the LSN during proestrus-estrus, may be associated with an increased motivation to escape from a stressful situation.