Time course of estrogen and progesterone induced lordosis in mice

Estrogen and progesterone regulate sexual receptivity by acting through a neuronal circuitry within the hypothalamus. Initially, estrogen inhibits lordosis behavior by inducing u-opioid receptor internalization. Estrogen also stimulates the expression of neuroprogesterone and progesterone receptors, through which progesterone acts to release the inhibition on lordosis. Therefore, the time course of hormone administration in female rodents is critical to elicit sexual receptivity. To understand this effect, ovariectomized female mice were grouped in one of the three time groups: 24 hours, 16 hours, and 48 hours between estrogen and progesterone treatment. Animals were treated with estrogen and progesterone, tested behaviorally, and sacrificed in order to perform immunohistochemistry for progesterone and u-opioid receptor expression. The 16- and 48-hour time group demonstrated significantly reduced lordosis quotients in comparison to the 24-hour group. No significant differences in progesterone receptor nor u-opioid receptor expression were found in either group, although the 16-hour group did present an upward trend in progesterone receptor expression. These findings highlight the temporal dependence of estrogen’s action in the hypothalamus which eventually facilitate sexual receptivity