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