The GPCR/MOR internalization assay has allowed us to dissect the signaling pathways activated by estradiol. For example, in the arcuate nucleus, estradiol acts on a membrane estrogen receptor-α (ERα) to transactivate a metabotropic glutamate receptor, which phosphorylates a protein kinase-θ dependent signaling cascade responsible for lordosis circuit activation. Estradiol also regulates the levels of its cognate receptor on the membrane through modulating the trafficking of ERα to the membrane and internalization. The ERα is a classic nuclear estrogen receptor and requires a chaperone protein to be inserted into the cell membrane. Knock down of the chaperone protein caveolin-1 prevents ERα trafficking to the membrane and abrogates estradiol induced MOR activation and lordosis behavior. An intriguing and unexpected finding in these studies was the identification of an ERα splice variant, ERαΔ4 in addition to the full length ERα on the cell membrane. We are searching for the role of ERαΔ4.