This chapter discusses the control of the ovarian cycle of the rats by the brain and its interaction with the anterior pituitary gland and ovaries. The ovarian cycle of the rats is characterized by a brief luteal phase, and the events of the cycle are largely under photoperiodic control. That is, the lighting periodicity plays a dominant role in the incidence and duration of the stages of the ovarian cycle. During the estrous cycle of the rat, three or more generations of corpora lutea can be present on the ovary from the immediately preceding ovulatory cycles whereas the preovulatory period of the estrous cycle is characterized by a growth of ovarian follicles and a concomitant enhanced secretion of estrogen. The secretion rate of estradiol into ovarian venous plasma is low on estrus, begins to rise significantly by late on metestrus through the morning of diestrus, and reaches peak concentrations by the afternoon of proestrus.

CHAPTER 43 – Neuroendocrine Control of the Ovarian Cycle of the Rat

Menopause signals the permanent end of menstrual cyclicity in a woman's life. Its impact reaches far beyond just the reproductive system. An understanding of the factors that interact and govern the process of aging in the reproductive system will help us to develop strategies for alleviating the negative aspects of menopause and may help us to better comprehend the process of biological aging.

https://science.sciencemag.org/content/sci/273/5271/67.full.pdf

Menopause: the aging of multiple pacemakers.

Neuroendocrinology of Reproduction

We have correlated catecholamine [CA; i.e. norepinephrine (NE), dopamine (DA), and epinephrine (E)] turnover rates in discrete hypothalamic nuclei and in the median eminence (ME), with concentration changes in ME LHRH and serum LH, FSH, PRL, estradiol, and progesterone levels at various times during proestrus and diestrous day 1 in 4-day cyclic rats. CA concentrations were measured with a radioenzymatic assay at 0, 60, and 120 min after ip injection of 400 mg/kg alpha-methyl-p-tyrosine, and rate constants and turnover rates were calculated. In a separate assay NE, DA, and E were separated by two-dimensional thin layer chromatography, and concentrations and turnover rates of CAs were calculated. The microdissected hypothalamic nuclei examined for NE turnover rates included the medial preoptic nucleus (MPN), suprachiasmatic nucleus (SCN), arcuate nucleus (AN), and ME. DA turnover rates also were measured in the MPN, ME, and AN. ME LHRH and serum hormone concentrations were measured by RIA. Between 0900--1200 h, proestrous serum estradiol was elevated, but other serum hormones were basal, and CA turnover rates in the brain were low. However, ME LHRH concentrations increased significantly between 0900--1200 h on proestrus. Between 1200--1500 h, serum LH, FSH, PRL, and progesterone levels increased and ME LHRH levels declined significantly; during this time interval (1200--1400 h), a significant rise in ME NE and DA turnover rates occurred. Between 1500--1700 h on proestrus, while serum gonadotropins were still rising toward peak concentrations, increased ME NE turnover rates were maintained, but increased NE turnover rates also were evident in MPN, SCN, and AN. During this same time interval (1500--1700 h), a marked decline in ME and AN DA turnover rates occurred, although such rates remained unchanged within the MPN. There were no corresponding changes in MPN E turnover rates at any of the time intervals studied. The increased turnover rates of ME NE coupled with the concomitant decline in ME LHRH levels and the rise in plasma LH and FSH levels suggest that increased NE release may be important in initiating preovulatory LH and FSH surges. These changes in brain neurotransmitters and serum hormones are not the result of a diurnal rhythm, since corresponding changes in CA turnover rates or serum gonadotropins did not occur between 0900--1100 h and 1500--1700 h diestrous day 1.

Catecholamine turnover rates in discrete hypothalamic areas and associated changes in median eminence luteinizing hormone-releasing hormone and serum gonadotropins on proestrus and diestrous day 1.

Neurokinin B and the hypothalamic regulation of reproduction

While LHRH evokes the release of both LH and FSH under a wide spectrum of experimental conditions, this hormone has not been shown to induce the discharge of FSH unaccompanied by LH release. As a consequence, the possibility that a separate FSHRH exists has not been discounted. These experiments were designed to demonstrate that LHRH is capable of promoting marked increases in plasma FSH concomitant with physiologically insignificant changes in plasma LH and, thus, that LHRH is also FSHRH. Seven groups of cyclic female rats received phenobarbital at 1200 h proestrus to block spontaneous preovulatory LH and FSH surges. These various groups then were subdivided according to whether phosphate-buffered saline (PBS), LHRH, or both was infused (iv). The experimental protocol consisted of infusing material for an initial 30 min (priming period), stopping the infusion for 30 min (rest interval), and again infusing material for an additional 210 min (maintenance period). When PBS was infused during the priming and...

Further Evidence that Luteinizing Hormone-Releasing Hormone also Is Follicle-Stimulating Hormone-Releasing Hormone

Temporal changes in medial basal hypothalamic LH-RH correlated with plasma LH during the rat estrous cycle and following electrochemical stimulation of the medial preoptic area in pentobarbital-treated proestrous rats.

George D. Barr

Papers

Further Evidence that Luteinizing Hormone-Releasing Hormone also Is Follicle-Stimulating Hormone-Releasing Hormone

Temporal changes in medial basal hypothalamic LH-RH correlated with plasma LH during the rat estrous cycle and following electrochemical stimulation of the medial preoptic area in pentobarbital-treated proestrous rats.