Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

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Prolactin: Structure, Function, and Regulation of Secretion

Publisher Summary This chapter discusses the development of follicles in the mammalian ovary. The unresolved issues in follicular development are focused. Folliculogenesis culminates in the production of fully ripe, preovulatory follicles visible to the naked eye as large bulges on the surface of the ovary. Each ripe follicle contains thousands of highly differentiated cells. This complex, functional miniature organ arises from the handful of cells that constitute a simple primordial follicle, a structure so small that it is invisible at the lower magnifications of a light microscope. All regulatory influences can only permit or prevent cells from completing the full maturation process; they cannot change the course of differentiation. A plethora of modulating influences act as permissive inducers, impeding or propelling the committed follicular cells through the process of clonal expansion. As each follicle progresses through its program of limited clonal expansion and maturation, its cells proliferate more and more rapidly. With every passing generation, the proliferative potential of the granulosa and theca cells continues to diminish, while the state of maturation increases.

Development of Follicles in the Mammalian Ovary

Introduction * Hormonal and Nonhormonal Basis of Maternal Behavior * Experiential Factors Influencing Maternal Behavior * Motivational Models of the Onset and Maintenance of Maternal Behavior and Maternal Agression * Neuroanatomy of Maternal Behavior * Neurochemistry and Molecular Biology of Maternal Behavior * Paternal Behavior * Neural Basis of Parental Behavior Revisited * Human Implications

The Neurobiology of Parental Behavior

In the adult castrated male rat, exposure to inescapable, intermittent electroshocks inhibited the pulsatile pattern of luteinizing hormone release and markedly lowered its plasma concentrations. The central administration of the corticotropin-releasing factor (CRF) antagonist alpha-helical ovine CRF residues 9 to 41 reversed the inhibitory action of stress. Neither its peripheral injection, nor the intraventricular injection of the inactive CRF analog des-Glu to Arg ovine CRF was effective. These results suggest that endogenous CRF may mediate some deleterious effects of noxious stimuli on reproduction.

https://science.sciencemag.org/content/sci/231/4738/607.full.pdf

Stress-induced inhibition of reproductive functions: role of endogenous corticotropin-releasing factor

Microscopic evaluation of the types of cells present in vaginal smears has long been used to document the stages of the estrous cycle in laboratory rats and mice and as an index of the functional status of the hypothalamic-pituitary-ovarian axis. The estrous cycle is generally divided into the four stages of proestrus, estrus, metestrus, and diestrus. On cytological evaluation, these stages are defined by the absence, presence, or proportion of 4 basic cell types as well as by the cell density and arrangement of the cells on the slide. Multiple references regarding the cytology of the rat and mouse estrous cycle are available. Many contemporary references and studies, however, have relatively abbreviated definitions of the stages, are in reference to direct wet mount preparations, or lack comprehensive illustrations. This has led to ambiguity and, in some cases, a loss of appreciation for the encountered nuances of dividing a steadily moving cycle into 4 stages. The aim of this review is to provide a detailed description, discussion, and illustration of vaginal cytology of the rat and mouse estrous cycle as it appears on smears stained with metachromatic stains.

https://journals.sagepub.com/doi/pdf/10.1177/0192623315570339

Vaginal Cytology of the Laboratory Rat and Mouse Review and Criteria for the Staging of the Estrous Cycle Using Stained Vaginal Smears

Groups of female rats were decapitated at 3-hr intervals throughout 4-day estrous cycles and the plasma of each rat was assayed for LH, FSH, prolactin, progesterone and estradiol-17β. Radioimmunoassays were used to measure these hormones, except for progesterone which was determined by the competitive protein binding assay. All five hormones exhibited a peak concentration in plasma on the day of proestrus. In addition, progesterone was elevated from 0900 hr of metestrus to 0900 hr of diestrus, while plasma prolactin showed a brief rise at 1500 hr of estrus. The elevated level of estradiol on the day of proestrus followed a different pattern than the other four hormones. Estradiol concentration began to rise late on metestrus, reached a peak at noon of proestrus, and fell before peak levels of the other hormones were reached. The preovulatory elevation of LH, prolactin and progesterone occurred between noon and midnight of proestrus, while the peak of FSH lasted from noon of proestrus to noon of estrus. (E...

Plasma Concentration of LH, FSH, Prolactin, Progesterone and Estradiol-17β Throughout the 4-Day Estrous Cycle of the Rat

Blood was collected following decapitation of 280 Sprague-Dawley rats with 4- day estrous cycles, which were killed at 3 hr intervals from 3 AM of proestrus to midnight of day 4 of gestation (day after finding of sperm in the vaginal smear was considered day 1 of gestation). Groups of rats also were killed at 30 min intervals from 2–7 PM of proestrus. Plasma was stored frozen until analysed by radioimmunoassay for prolactin. The plasma concentrations of prolactin showed a distinct semicircadian rhythm during the first 4 days of gestation. Peaks of prolactin were observed late in the periods of light and dark with nadirs at noon and between 9 PM and midnight. The proestrus peak of prolactin was present, but extended 12 hr longer than the interval reported for unmated rats. A peak of prolactin also was observed on the afternoon of estrus. (Endocrinology 90: 1125, 1972)

Semicircadian Rhythm in Plasma Levels of Prolactin During Early Gestation in the Rat

Rats were killed by decapitation at 3-h intervals during a 48-h delay of ovulation induced by sodium pentobarbital, as well as during the ensuing delayed proestrus, estrus and the first 4 days of gestation. Control animals were killed at the same intervals following injections of vehicle. Blood was collected and analyzed for LH, FSH, prolactin, progesterone and estradiol-17β to determine if alterations in hormonal levels could account for the abnormal embryonic development which follows delayed ovulation. Hormonal concentrations in plasma were measured by radioimmunoassay except for progesterone, which was determined by competitive protein binding. Embryos were examined to verify the occurrence of abnormal development. Rate of oocyte maturation was studied in serial sections of ovaries from all animals killed at 30-min intervals on the afternoon of proestrus. Oocytes remained in meiotic arrest during the 48-h delay of ovulation, but resumed maturation at the expected time on the afternoon of the preovulat...

W. E. Collins

Papers

Plasma Concentration of LH, FSH, Prolactin, Progesterone and Estradiol-17β Throughout the 4-Day Estrous Cycle of the Rat

Semicircadian Rhythm in Plasma Levels of Prolactin During Early Gestation in the Rat

Altered Secretion of Gonadotropins and Steroids Resulting from Delayed Ovulation in the Rat