Temporal Interrelationships Among Circulating Levels of Estradiol, Progesterone and LH During the Rat Estrous Cycle: Effects of Exogenous Progesterone¹

Abstract

Serum progesterone (P) levels display a circadian rhythm of varying amplitude during the estrous cycle. This rhythm is characterized by lowest levels of the day around noon and maximal levels at 1600 or 2300 hr. Amplitude was smallest on estrus and progressively greater on diestrus II, diestrus I and proestrus. Serum estradiol (E 2 ) concentrations declined gradually through estrus till 0800 hr of diestrus I. Thereafter, levels increased rapidly to peak at noon, and returned later to near nadir at 2300 hr of diestrus I. During diestrus II and morning of proestrus serum E 2 concentrations increased in a biphasic pattern; a moderate increase till 1800 hr of diestrus II was followed by relatively rapid rise to peak levels at 0800 hr of proestrus. A reciprocal relationship between circulating levels of E 2 and P during the estrous cycle was also detected. The elevations in serum P levels during estrus, diestrus I and proestrus appeared to coincide with a gradual reduction in serum E 2 levels. In addition to the surge of serum LH in the afternoon of proestrus small increases in serum concentration of LH over the base line value were noted at 0800 hr of diestrus I and at 1600 hr of estrus.The influence of experimentally altering the circulating levels of P on endogenous levels of E 2 was next examined. Administration of P at 0800 hr of diestrus I blocked the afternoon rise in serum E 2 ; the suppressive effects persisted till 1800 hr of diestrus II. Similarly, injection of P at 0800, 1800 or 2300 hr of diestrus II suppressed the rise in E 2 normally observed at proestrus. These observations suggest that circadian variations in peripheral serum P concentrations during the estrous cycle may constitute a regulatory mechanism for the observed daily fluctuations in serum E 2 . (Endocrinology 95: 1711(Endocrinology 95: , 1974

Cyclical Variations in the Abundance of Leptin Receptors, but not in Circulating Leptin, Correlate with NPY Expression during the Oestrous Cycle

“…As previously reported oestradiol levels vary through the cycle with peak levels in pro-oestrus and lower levels at the other stages [30, 31]. Oestradiol levels at pro-oestrus were significantly higher than at di-oestrus (140.31 ± 11.29 pmol/l pro-oestrus versus 104.13 ± 19.94 pmol/l di-oestrus, p < 0.05).…”

Influence of Uterine Inflammation on the Estrous Cycle in Rats

“…Hence, we suspect that a factor other than endotoxin extends diestrus. Kalra et al [11] and Kaneko et al [12] reported that the administration of progesterone to rats at proestrus inhibited the secretion of estrogen, which triggers the ovulatory LH surge, and prevented ovulation. As P 4 concentration was high, with a two to three fold increase in peak concentration compared to control rats on the day following infusion, we assume that the inoculation into the uterus stimulated progesterone secretion and suppressed ovulation.…”

Variations in the Hypothalamic-Pituitary-Adrenal Response to Stress during the Estrous Cycle in the Rat

“…During diestrous 1 (Di 1) systemic levels of estrogen, prolactin (PRL), and the gonadotropins luteinizing hormone (LH) and follicle-sumulating honnone (FSH) are low (Kalra and Kalra, 1974). Plasma progesterone levels are slightly clevated, produced by corpora lutea ceUs of the ovary.…”

“…It has been shown that progesterone secretion from the corpora lutea acts locally 10 limit follicular secretion of estrogen. Injecting progesterone into animaIs on Di II, thus preventing a faU in progesterone circulaLng levels, delays the onset of ovulation (KaIra and Kalra, 1974), while administration of progesterone antisera advances the onset of ovulation (Kaneko et aL, 1986). In the Long-Evans rat the extended life of the corpora luteum is observed by higher levels of progesterone during Di II.…”