Rats and mice are "reflex ovulators". That is, they ovulate in
response to vaginal stimulation during mating. Input from the vagina
directly causes release of prolactin, which in turn initiates the
secretion of ovarian progesterone to prepare the uterus for pregnancy.
Experiments in which the sensory nerve from the vagina was severed
prevented this prolactin release. The more matings the female engages in,
the greater the ammount of prolactin and, consequently, of progesterone.
Also, prolactin secretion follows a circadian rhythm in many animals,
including rodents, so this high level of prolactin may be sustained until
the animal begins to cycle again. Dopamine does have a suppressive effect
on prolactin levels, but in the rat I do not know exactly what mechanism
governs dopaminergic regulation. I am currently working on answering this
question in mares, but data is strictly "preliminary" at this point.
Hope this helps!
ldavison at ukcc.uky.edu