SEXUAL RECEPTIVITY
1. What hormone is required for the initiation of sexual receptivity in all animals?
2. How does progesterone enhance receptivity in some domestic animal species?
3. What domestic animal species require estrogen synergism with progesterone?
If copulation is to occur near ovulation, the female must be receptive to the male.
Initiation of sexual receptivity in all animals requires estrogen derived from the antral follicles. Also, in some species (e.g., bitch, ewe, sow, cow), progesterone acts synergistically with estrogen for manifestation of receptivity. Neurons associated with a “sex center” are located diffusely in the hypothalamus and are critical in initiating the mechanisms of sexual behavior as a response to hormones. It seems that progesterone (tonic levels) acts as a primer for the hypothalamic sexual centers so that estrogen becomes effective. During the postpartum (after parturition) period in some cows and sows, the low progesterone concentration fails to prime the sexual centers of the hypothalamus, and the animals are not sexually receptive at the time of the first postpartum ovulation. In sheep, the priming of the hypothalamus with progesterone is essential, after their seasonal anestrus, before sexual receptivity is manifested. Accordingly, ewes do not show sexual receptivity in conjunction with the first ovulation of the breeding season.During proestrus in the bitch, when estrogen levels increase, sexual receptivity is absent although they might be sexually attractive. It is only when the LH surge occurs near ovulation that sexual receptivity occurs. Preovulatory progesterone from the LH surge (luteinized granulosa cells) can be sufficient to prime the hypothalamus. Before proestrus, a long period of sexual inactivity (anestrus) occurs, during which progesterone levels are either low or nonexistent.
Some evidence has shown that GnRH has a role in the manifestation of sexual receptivity. Injection of GnRH without estrogen has been found to cause sexual posturing in some animals. Also, the onset of sexual receptivity is correlated closely with the preovulatory LH surge as caused by GnRH release.
Progesterone is not synergistic with estrogen in manifesting sexual receptivity in the doe, queen, and mare.
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