Estradiol Dose-Dependent Regulation of Membrane Estrogen Receptor-α, Metabotropic Glutamate Receptor-1a, and Their Complexes in the Arcuate Nucleus of the Hypothalamus in Female Rats

Sexual receptivity in the female rat is dependent on dose and duration of estradiol exposure. A 2 mu g dose of estradiol benzoate (EB) primes reproductive behavior circuits without facilitating lordosis. However, 50 mu g EB facilitates lordosis after 48 hours. Both EB doses activate membrane estrogen receptor-alpha (mER alpha) that complexes with and signals through metabotropic glutamate receptor-1a (mGluR1a). This mER alpha-mGluR1a signaling activates a multisynaptic lordosis-inhibiting circuit in the arcuate nucleus (ARH) that releases beta-endorphin in the medial preoptic nucleus (MPN), activating mu-opioid receptors (MOP). MPN MOP activation is maintained, inhibiting lordosis for 48 hours by 2 mu g EB, whereas 50 mu g EB at 48 hours deactivates MPN MOP, facilitating lordosis. We hypothesized that 50 mu g EB down-regulates ER alpha and mER alpha-mGluR1a complexes in the ARH to remove mER alpha-mGluR1a signaling. In experiment I, 48 hours after 2 mu g or 50 mu g EB, the number of ARH ER alpha-immunopositive cells was reduced compared with controls. In experiment II, compared with oil controls, total ARH ER alpha protein was decreased 48 hours after 50 mu g EB, but the 2 mu g dose was not. These results indicate that both EB doses reduced the total number of cells expressing ER alpha, but 2 mu g EB may have maintained or increased ER alpha expressed per cell, whereas 50 mu g EB appeared to reduce total ER alpha per cell. In experiment III, coimmunoprecipitation and Western blot revealed that total mER alpha and coimmunoprecipitated mER alpha with mGluR1a were greater 48 hours after 2 mu g EB treatment vs rats receiving 50 mu g EB. These results indicate 2 mu g EB maintains but 50 mu g EB down-regulates mER alpha-mGluR1a to regulate the lordosis circuit activity.