Frequency is somewhat less than rats found in the wild. If you give an injection of estrogen followed 6-8 hours later by an injection of progesterone, lordosis behavior is maximized. This can be greater than lordosis behavior found in proestrus. The highest frequency of sexual behavior occurs with estrogen followed by progesterone. Perhaps estrogen primes the system to facilitate the later progesterone injection.
Estrogen tends to cause a change in the electrical activity of the midbrain central gray.
These may be analogous to dendrites or axons in these animals. One way to change activity of the midbrain central gray is to administer an injection of steroids which increase the number of receptors it's associated with. Estrogen injections tend to promote the increased production of progesterone receptors in the midbrain central gray. Estrogen must be present in higher brain structures to cause lordosis behavior. Looks much like organizational effects prenatally.