Mechanisms of Oestrogen-Progestin Interactions in the Regulation of Lordosis in Female Guinea Pigs

It has been known since the 1930s that the occurrence of sexual receptivity during the guinea pig oestrous cycle is regulated by the actions of oestradiol and progesterone (Young 1969). Work on the subcellular mechanisms of oestradiol action on sexual behaviour has steadily progressed through the years (McEwen et al. 1979). In the past 5 years substantial progress has also been made on the subcellular mechanisms of the interactions between oestradiol and progesterone on this behaviour. In this chapter we will focus on our work characterizing the cellular bases of the following three interactions between oestradiol and progesterone in the regulation of the expression of the lordosis posture in female guinea pigs: 1. Progesterone facilitates the expression of lordosis in oestrogen-primed guinea pigs. Although oestradiol alone can induce sexual behaviour in oestrogen-primed, ovariectomized guinea pigs, a progesterone injection results in a shorter latency to heat and a more consistent heat duration. Treatment with progesterone also allows a lower priming dose of oestradiol to be used (Dempsey et al. 1936). 2. Progesterone causes a period of refractoriness to further stimulation of lordosis by progesterone. This refractoriness has been referred to by a variety of terms depending upon the authors’ emphasis. It has been called the biphasic effect of progesterone, because progesterone first facilitates and then causes a failure to respond to progesterone (Zucker 1966). It has been called a refractory period because during this period guinea pigs are refractory to a progesterone injection (Morin 1977). It has also been called sequential inhibition to emphasize the fact that progesterone treatment follows oestradiol treatment (Powers and Moreines 1976). Finally, based on behavioural and biochemical experiments, we now view this influence of progesterone as desensitization to further stimulation by progesterone (Blaustein 1982a,b). 3. Oestradiol and progesterone interact to determine the duration of the period of sexual behaviour. Although there has been a question of the relative importance of each hormone in this regulation (Wallen and Thornton 1979), both hormones influence the duration of the period of sexual receptivity (Collins et al. 1938, Morin and Feder 1973).