Biphasic effects of the antiserotonergic methysergide on lordosis in rats

As also reported by other workers, the antiserotonergic drug methysergide was found to facilitate lordotic responding in estrogen primed, ovariectomized rats. A second dose of methysergide 24 hr after the first, however, failed to produce any increment in responding. Animals received daily estrogen injections in order to maintain a relatively constant level of priming. After several days of methysergide, a progesterone injection facilitated lordosis to the same degree as in controls receiving only saline and estrogen. When a second injection of progesterone was given 24 hr later, however, the animals failed to respond. In contrast, saline controls with this estrogen paradigm responded equally well to both progesterone injections. These results are discussed in terms of their bearing on possible serotonergic and non-serotonergic mechanisms by which progesterone may control lordosis.