Cyclic Djungarian Hamsters,Phodopus campbelli, Lack the Progesterone Surge Normally Associated with Ovulation and Behavioral Receptivity*

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Abstract
Serum levels of 17.beta.-estradiol and progesterone were assayed at 4- to 5-h intervals across the estrous cycle in female Djungarian hamsters, Phodopus campbelli. The pattern of secretion for estradiol is similar to that described for the rat and the Syrian hamster, rising slowly from a baseline of 54 .+-. 2 pg/ml during the morning of estrus to a peak of 187 .+-. 16 pg/ml in the early afternoon of proestrus, then falling rapidly back to baseline levels. The pattern of progesterone secretion is significantly different from that of any estrous cycle previously described for a 4-day cyclic mammal. There is no evidence for a significant peak in serum progesterone levels associated with ovulation and receptivity. The highest levels of serum progesterone occur during the early afternoon of diestrous day 2 (8046 .+-. 1063 pg/ml). The lowest levels of serum progesterone occur in the early morning of proestrus (720 .+-. 125 pg/ml). During the period of sexual receptivity only 2504 .+-. 654 pg/ml progesterone are found in the serum. Ovariectomized females show behavioral receptivity after 48 h of treatment with 50 .mu.g estradiol benzoate/kg BW in sesame oil. Progesterone is not necessary for receptivity and will not facilitate receptivity when administered in conjunction with a subthreshold dose of estrogen. The presence of tubular ova confirms the time of ovulation to be a period of approximately 7 h between 2200 h on proestrus and 0500 h on estrus. Histological examination of ovarian sections from each of the 4 days of the estrous cycle shows follicular development to differ from that in the Syrian hamster in that the total number of follicles in the ovary is variable and low, and the ovulated follicles (new corpora lutea) are blood filled. Djungarian hamster primary follicles averaged 83 .+-. 2 .mu.m in diamter, and preovulatory follicles averaged 455 .+-. 17 .mu.m in diameter. The total number of intact follicles per ovary rose from 21 .+-. 5 on the evening of estrus to 73 .+-. 29 on the evening of diestrous day 2, then fell to 32 .+-. 26 by the evening of proestrus. Preovulatory follicles increased in number from 1.0 .+-. 0.6/ovary early on estrus to a maximum of 4 .+-. 1 on the evening of diestrus day 2. However, at ovulation a combined total of only 5.1 .+-. 0.8 ova are shed.