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

Neuronal activity of the lateral septal nucleus (LSN) is related to motivat ional and hedonic behavior. Even though some changes in mood and anxiety du ring proestrus and pregnancy have been reported, the possible changes in th e 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 LS N using glass micropipettes (NaCl 1 M, and Evans blue 2.5%; 3-8 M Omega) 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 lo west 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 interva ls and a lower coefficient of variation than those in the phase characteriz ed by lower levels of estradiol and progesterone (metestrus-diestrus). In a nother group of rats (n = 12), immobility in the forced-swim test was asses sed. Consistently, a longer latency (p < 0.05) for the first period of immo bility and a nonsignificant trend to a lowered total time in immobility wer e found in proestrus and estrus. It is concluded that the higher firing rat e in neurons from the dorsal aspect of the LSN during proestrus-estrus, may be associated with an increased motivation to escape from a stressful situ ation. (C) 2000 Elsevier Science Inc. All rights reserved.