SUMMARY
Spayed female hamsters were given a 150 μg pellet of oestradiol s.c. (day 0). After s.c. placement of a 200 μg pellet of progesterone 7 days later, all animals displayed oestrous behaviour. Treatment with 200 μg progesterone suspended in propylene glycol or sesame oil, on days 4, 5 or 6 also induced an immediate period of heat but subsequent placement of a 200 μg pellet of progesterone on day 7 did not always result in oestrous behaviour. The animals did not show the lordosis response after placement of the progesterone pellet on day 7 when 200 μg progesterone in sesame oil had been given on either day 5 or 6. In contrast, when propylene glycol was employed as the vehicle, only injection on day 6 blocked oestrous behaviour on day 7.
When the same amount of progesterone in propylene glycol was administered on day 5, as injections of 50 μg each over 12 h or more, progesterone in propylene glycol was more effective in blocking oestrous behaviour on day 7. The injection on day 5 of 100 μg progesterone in propylene glycol as four injections of 25 μg over 12 h did not block oestrous behaviour on day 7. The injection of 50 μg progesterone in propylene glycol on four successive nights induced a period of sexual receptivity after each injection. These observations indicate that the duration of the insensitive condition, developing as a result of a period of progesterone-facilitated heat, is a function of the amount of hormone causing the initial receptive period, as well as the duration of progesterone stimulation.
The role of hypothalamic progestin receptors in the activation and maintenance of sexual receptivity in female rats and guinea pigs