Hormones and the Fetus: Volume 1: Production, Concentration and Metabolism During Pregnancy

By F. A. Kincl and J. R. Pasqualini

Written by the world's two leading researchers in steroid biochemistry this volume describes the ways in which hormonal concentration is regulated during pregnancy. It is a comprehensive account of how biosynthesis, metabolism and inter-compartmental transport are related to the concentration of each hormone found in placental, fetal and maternal compartments. There is also an introductory chapter on hormonal mechanism of action.

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 22, 2013
• ISBN: 9781483285382

Book preview

HORMONES and the FETUS

Volume I

JORGE R. PASQUALINI

C.N.R.S. Steroid Hormone Research Unit, Foundation for Hormone Research, 26 Boulevard Brune, 75014 Paris, France

FRED A. KINCL

The College of Staten Island, The City University of New York, Staten Island, NY 10301, U.S.A.

Table of Contents

Cover image

Title page

Inside Front Cover

Copyright

Preface

Chapter 1: Hormonal Mechanisms in Reproduction

Publisher Summary

INTRODUCTION

1 THE HYPOTHALAMO-HYPOPHYSEAL-OVARIAN AXIS

3 PLASMA GONADAL HORMONE LEVELS DURING THE ESTRUS CYCLE IN DIFFERENT ANIMAL SPECIES

4 PROSTAGLANDINS

5 THE OVIDUCT

6 PREGNANCY

Chapter 2: Biosynthesis and Metabolism of Different Hormones in the Fetal and Placental Compartments

Publisher Summary

INTRODUCTION

1 POLYPEPTIDE HORMONES

2 STEROID HORMONES

CONCLUSIONS

Chapter 3: Hormone Production and Concentrations During Pregnancy in Humans and in Other Mammalian Species

Publisher Summary

INTRODUCTION

A HUMANS

B GESTATION IN ANIMALS

CONCLUSIONS

Chapter 4: Transfer of Hormones Between the Fetal, Placental and Maternal Compartments

Publisher Summary

INTRODUCTION

1 TRANSFER OF STEROID HORMONES AND PRECURSORS

2 POLYPEPTIDE HORMONES

3 THYROID HORMONES

Chapter 5: Hormonal Changes Preceding Parturition

Publisher Summary

1 INTRODUCTION

2 PRIMATES

3 ARTIODACTYLA

4 RODENTIA

5 OXYTOCIN

6 CONCLUSIONS

Appendix I: The Nomenclature, Structure and Physical Properties of Main Pregnancy Hormones

Appendix II: Trivial Names and Nomenclature of Steroids Used in This Book

Index

Inside Front Cover

Other Pergamon publications of related interest

KUMAR, S. and RATHI, M. Perinatal Medicine

Journal of Steroid Biochemistry

Copyright

Copyright © 1985 Pergamon Press Ltd.

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers.

First edition 1985

Library of Congress Cataloging in Publication Data

Pasqualini, Jorge R.

Hormones and the fetus.

(Pergamon international library of science, technology, engineering, and social studies)

Includes index.

1. Obstetrical endocrinology. 2. Placental hormones. I. Kincl, Fred A. II. Title. III. Series. [DNLM: 1. Hormones – Physiology. 2. Hormones – Biosynthesis. 3. Maternal-fetal exchange. 4. Reproduction. 5. Fetus. W 210.5 P284h]

RG558.5.P37 1984 612′.647 84-344

British Library Cataloguing in Publication Data

Pasqualini, Jorge R.

Hormones and the fetus. – (Pergamon international library)

Vol. 1

1. Fetus – Growth 2. Hormones

I. Title II. Kincl, Fred A.

612′.647 RG613

ISBN 0-08-019708-6

Printed and bound in Great Britain by

William Clowes Limited, Beccles and London

Preface

It has been known for many decades that various hormones play a major role in the development of the mammalian embryo, from the moment of conception. The last decade has seen an ever increasing amount of published original papers, review articles, symposia and books related to hormone biosynthesis, production, concentrations in various tissues and metabolism both in the maternal and the fetal compartments. Yet, there are many pitfalls if we try to correlate the vast amount of available data. The underlying principles are no doubt common to all mammalian species: progesterone, the hormone of pregnancy, is present in ever increasing amounts in most species during gestation. Other hormones, such as those from the thyroid, the group of growth hormones and insulin, and gonadotropins are needed at various times during embryogenesis. As parturition nears, estrogens, prostaglandins and other tropic substances become present in increasing concentrations or appear for the first time.

The hormonal milieu which bathes the fetus is the product of a maternal-placental-fetal interaction. In general, large molecular hormones (glycoproteins and polypeptides) do not cross the placental barrier and the fetus must depend upon its own or the placental contribution.

Low molecular substances (steroids) cross the placenta and maternal contribution plays a pivotal role. In most species the fetus lacks specific enzymes and is dependent upon the placental and maternal compartments to supply the necessary precursors. In others the fetus does possess the enzymatic systems for the biosynthesis but quantitatively the contribution of placental precursors is needed. Thus the different hormones present in the fetus are the product of equilibrium exchange of the hormones between the fetus, the placenta and the mother.

Hormones and the Fetus has been designed to explore the influences which shape the development of the fetus. It is directed to those who wish to become acquainted with the recent advances and problems of fetal endocrinology and provides review material to workers in the field. It will be published in three volumes. In the First volume we treat the qualitative and quantitative parameters of different hormones in the three compartments in various mammalian species, the transfer between the compartments and qualitative and quantitative changes preceding parturition. Volume II will include the binding of different hormones to fetal plasma proteins, the presence of receptor macromolecules in the cytosol and in the nucleus of the diverse fetal tissues and placenta and their correlation with hormonal activity. Information concerning different aspects of sexual differentiation and fetal neuroendocrinology will also be included in this volume. The series will be completed with Volume III which will include different hormone-dependent pathologies and the effects of different hormones and anti-hormones in the fetus and newborns.

In this volume we first survey the broad field of hormonal mechanisms operative during the control of female sex functions. This is followed by a discussion of the biosynthesis and metabolism of hormones in the fetal and placental compartments. Most information found in this chapter is based on studies in humans; references to other mammalian species were included only to illustrate major differences. Chapter 3 describes qualitative and quantitative measurements of different hormones in the three compartments in humans, and in other mammalian species. Chapter 4 covers the transfer of hormones between the various compartments. The volume closes with the discussion of hormonal changes preceding, and during, parturition. The Appendixes include a list of trivial and scientific names and the structure and physical properties of hormones.

The authors wish to thank their many friends and colleagues who gave considerable assistance and support during the preparation of the manuscript. Drs M.R. Henzl (Palo Alto, CA, U.S.A.) and E. Patek (Stockholm, Sweden) provided several microphotographs used in Chapter 1. Dr. J.J. Haberman (Weston, CT, U.S.A.) read and edited Chapter 1 and Dr. C. Sumida (Paris, France) portions of other chapters. Those who read various sections of the manuscript and offered criticism and aid include Drs W.R. Allen (Cambridge, U.K.), K. Brown-Grant (St. John’s, Canada), L.A. Ciaccio (Staten Island, N.Y., U.S.A.), E. Gurpide (New York, U.S.A.), J.-Å. Gustafsson (Stockholm, Sweden), J.C. Hoffmann (Honolulu, HI, U.S.A.), D.C. Johnson (Kansas City, KA, U.S.A.), S. Kawashima (Tokyo, Japan), C. Martin (Nedlands, W. Australia), J. Pedersen (Copenhagen, Denmark), W.B. Quay (Galveston, TX, U.S.A.) and R.V. Short (Edinburgh, U.K.).

We would like to express our thanks to Ms S. MacDonald (Paris, France) for her efficient cooperation in the preparation of the manuscript and Mr. M.J. Richardson (Senior Publishing Manager) and the staff of Pergamon Press for their valuable contribution to the publication of this book.

Chapter 1

Hormonal Mechanisms in Reproduction

Publisher Summary

This chapter examines the unity in hormonal mechanisms in reproduction of different species, while focusing on the functions of the ovary and the uterus, and the hormonal relationships that initiate and maintain reproductive processes. The chain of events that results in fertility depends on balanced interaction of the humoral products of the hypothalamus, gonadotropic hormones secreted by the anterior pituitary gland, and the ovarian steroid hormones. It has been found that many mammals exhibit a characteristic and often disagreeable odor that depends on intact gonadal function. Odors have been implicated not only in the recognition of sexual partners and in sexual arousal but also in the performance of the sexual act and in care of the young. Evidence concerning interdependence between female pheromones and olfactory stimulation and behavioral changes in the male has been documented. Specialized olfactory receptors in most mammals capture these pheromones and stimulate reproductive stimulation. These stimuli initiate the complex cycle of reproductive preparedness in a female subject. The whole cycle of ovulation—the creation and release of the ovarian steroids estrogens and progesterone in the follicle growth of the ovaries, the maturing of the ovaries signaled by the presence of the corpus luteum—is a complex one.

CONTENTS

INTRODUCTION

1 THE HYPOTHALAMO-HYPOPHYSEAL-OVARIAN AXIS

1.1 The Regulatory Function of the Central Nervous System

1.1.1 Olfactory stimulation

1.1.1.1 Female pheromones

1.1.1.2 Male pheromones

1.1.2 Photic stimulation

1.2 The Hypothalamus

1.2.1 Effects on the pituitary gland

1.2.2 Tissue and plasma concentrations of GnRH

1.2.3 Other effects

1.3 The Pituitary

1.3.1 FSH and LH

1.3.1.1 Plasma levels

1.3.2 Prolactin

1.3.2.1 Plasma levels

1.4 Feedback Mechanism

1.4.1 Facilitative action

1.4.1.1 Trigger mechanism

1.4.2 Inhibitory action

1.5 The Hormone Receptors

2 THE OVARY AND THE UTERUS

2.1 Follicle Formation, Ovulation and the Corpus Luteum

2.1.1 Follicle growth

2.1.2 Ovulation

2.1.3 The corpus luteum

2.2 Ovarian Steroidogenesis

2.2.1 Estrogens

2.2.2 Progesterone

2.2.3 Estrogen – progesterone synergism

3 PLASMA GONADAL HORMONE LEVELS DURING THE ESTRUS CYCLE IN DIFFERENT ANIMAL SPECIES

3.1 Patterns of Ovarian Steroids in Plasma and Production Rates During the Human Menstrual Cycle

3.1.1 Pregnenolone and derivatives

3.1.2 Progesterone and derivatives

3.1.3 Estrogens

3.1.3.1 Estradiol, estrone and their sulfates

3.1.3.2 Urinary excretion

3.1.3.3 Estriol

3.1.4 Catechol estrogens

3.1.5 Androgens

3.1.6 Steroids in human endometrial and myometrial tissues

4 PROSTAGLANDINS

4.1 The Luteolytic Effect

5 THE OVIDUCT

5.1 Ova Transport

5.2 Tubal Secretion

6 PREGNANCY

6.1 Implantation

6.2 Gestation

6.2.1 The placenta

6.2.2 Hormonal influences

6.3 Birth and Lactation

REFERENCES

INTRODUCTION

The science of endocrinology is less than a hundred years old. During this brief period, and particularly during the last 40 years, we have come to understand many of the intricate biological mechanisms and hormonal balances that regulate reproductive processes. This understanding has been made possible primarily by advances in molecular biology and the development of experimental techniques and highly sensitive analytical methods. Nevertheless, many aspects still remain obscure.

Most body functions are directed towards a single goal, namely the preservation of the species, and animals have evolved a wide range of biological mechanisms to overcome the problems of perpetuating their kind. Some animals ovulate spontaneously, for example, primates, ungulates and many rodents; in others ovulation is induced, for example the ferret and the rabbit; in other species the reproductive cycle is timed by a biological clock.

Species have evolved differing modes of regulating fertility as adaptations to particular ecological niches. There are some 4000 species of mammals in the world and our knowledge of mammalian reproductive physiology is based on observations from a mere handful. There is a tendency to assume that all mammals conform to some basic reproductive plan. However, there are vast differences between species, and as yet we are only in a position to make generalizations based on the few underlying principles they hold in common.

The endocrine function of the ovary was recognized at the beginning of this century, leading to the search for the active substances. In 1900 Knauer’s classic experiment demonstrated that ovarian transplants could prevent atrophy of the uterus in ovariectomized rabbits. In 1901 and 1903 the German gynecologist L. Fraenkel showed that the corpus luteum was responsible for maintaining pregnancy in rabbits. In 1910 two French biologists, Bouin and Ancel, reported that the corpus luteum brought about a secretory transformation of rabbit endometrium. The search for an estrogenic (estrus-inducing) compound in extracts of ovaries, and the pregnancy-maintaining or pro-gestatio (progesterone) substance from corpus luteum, was launched.

Adler, in 1912, prepared a potent, non-identified ovarian extract which exhibited the characteristics of estrogens: administered to immature guinea pigs, it produced estrus and opening of the vagina. The cyclic variations in vaginal smears and cornification of the vaginal epithelium during estrus were first described in 1917 by Stockard and Papanicolaou. In 1923 Allen and Doisy suggested that characteristic changes in the degree of cornification of the lining of the vagina in spayed mice could be used as a biological test for the measurement of estrogenic potency. The assay was easy to reproduce, the reproduction was readily recognized and the results could be evaluated statistically. This procedure was soon being used to quantify the estrogenic potency of biological extracts, and as an index in separation procedures and the preparation of pure estrogenic material. The technique was used to test the estrogenic potency of material from a variety of sources and eventually resulted in the isolation of pure hormones.

The first steroid hormone for which a structure was established was estrone. In 1927 Ascheim and Zondek found that the urine of pregnant women was unusually rich in estrogenic compounds. This became a major source for the isolation and purification of estrone which was isolated in 1929 and 1930 by independent groups, headed by Doisy in the United States and Butenandt in Germany. The determination of its structure was completed 2 years later and was chiefly the work of Marrian and Butenandt. A second hormone, estriol, was also isolated from human urine by Marrian in 1930. It was not until 5 years later, in 1936, that the principal estrogenic hormone, estradiol, was isolated in pure form by Doisy and MacQuorquodale, using 1000 kg of pig ovaries.

The search for progesterone was also taking place at this time. The isolation and structure of the pure substance was reported independently by four groups of investigators in 1934: Allen and Wintersteiner in the United States, Slotta and his co-workers and Butenandt’s group in Germany, and Hartmann and Wettstein in Switzerland. The original biological test developed by Corner and Allen in 1929 was soon replaced by the simpler assay method devised by Clauberg (1930), which directly measured the effect of progesterone on the endometrium.

One of the aims of this chapter is to emphasize what is known of the unity in hormonal mechanisms in reproduction while pointing out the complexity of a process which is still not entirely understood. Attention will necessarily be focused on the functions of the ovary and the uterus, and the hormonal relationships that initiate and maintain reproductive processes. There is not space here to discuss fully the behavioral changes that precede fertilization, and that take place during pregnancy and persist beyond parturition. At the present time, we can barely perceive the hormonal influences which trigger, or which may be affected by, such changes.

1 THE HYPOTHALAMO-HYPOPHYSEAL-OVARIAN AXIS

The chain of events that results in fertility depends on balanced interaction of the humoral products of the hypothalamus, gonadotropic hormones secreted by the anterior pituitary gland, and the ovarian steroid hormones. Hormonal and nervous stimuli cause the production of a polypeptide substance, gonadotropin-releasing hormone (GnRH), in the hypothalamus. This, in turn, evokes the production and/or the release of gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary, which act on the gonad, regulating follicular growth, ovulation, and the synthesis of steroids. The gonadal hormones trigger cell division in target tissues (uterus and vagina) and stimulate and inhibit, via a feedback mechanism, the release of hormones secreted via the brain-pituitary axis.

Cyclical changes are easily followed by monitoring alterations in the composition of vaginal and/or uterine (endometrial) epithelium. In primates the changes are referred to as the menstrual cycle and in all other species, whether cyclic or intermittent, as estrus. As an example let us review the changes that take place during the 4–5 day estrus cycle of the rat. The shortest pro-estrus period lasts about 12 hours and is characterized by the appearance of small, nucleated cells in the vaginal epithelium. During this stage Graafian follicles form in the ovary, estrogen secretion increases and the endometrium begins to proliferate. In laboratory animals maintained on regular light-dark schedules, pro-estrus heralds ovulation, which takes place within 18 hours. The females are receptive during late pro-estrus, or early estrus. If mating is desired males are introduced at this time. About 24 hours following the appearance of nucleated cells the vaginal epithelium has undergone cornification – the characteristic sign of estrus – with the cells now large and flat. At this time the graafian follicles rupture and estrogen synthesis is maximal. Di-estrus marks the appearance of large numbers of leucocytes in a vaginal lavage, and the cornified cells rapidly decrease in number and soon disappear. During this time (about 48 hours) corpora lutea have formed, the secretion of progesterone is maximal and the endometrium, in preparation for the acceptance of the fertilized ovum, becomes secretory. If fertilization has taken place, the corpora lutea continue to secrete progesterone and a secretory endometrium is maintained. If fertilization has not taken place the endometrium regresses (menstruation in primates) and the cycle is repeated.

In seasonally ovulating animals (for example the bitch) both pro-estrus and estrus may last for a number of days, during which time the animal is sexually receptive. Ovulation usually occurs toward the end of estrus.

1.1 The Regulatory Function of the Central Nervous System

Neurosecretory changes, triggered by environmental stimuli, control or modulate reproductive processes. Internal as well as external stimuli recorded through special receptor cells are transmitted through neural pathways to regions of the hypothalamus. Hormones acting within the body to maintain a homeostatic balance, or distributed via the external environment (pheromones) are indispensable to maintain, and frequently to initiate, the reproductive process (Fig. 1.1). Of the external stimuli, olfactory and photic stimulation are the most important in this respect. In seasonally breeding vertebrate species, photoperiod changes are required, and in cyclically ovulating species, photic stimulation exerts a major modulatory influence.

Fig. 1.1 Pathways by which Environmental Stimuli Affect Reproductive Responses

Stimuli arriving through the optic and olfactory pathways and other receptors (such as external genitalia) communicate (catecholamines) with the hypothalamic neurons which respond by releasing a polypeptide hormone (RH). Carried by a vascular system to the pituitary, RH triggers a release of pituitary hormones (FSH and LH) which in turn stimulate the production of steroid hormones by the gonads. Feedback mechanism (reverse arrows) is exerted by the target gland hormones at the pituitary and hypothalamus and by the pituitary trophic hormones at the hypothalamus (see also Figure 1.2 and text).

1.1.1 Olfactory stimulation

Mammalian species communicate female sexual receptivity in a variety of ways. The male may respond not only to the visual mating patterns of the female but also to specific olfactory stimuli, which are particularly intense during the pre-ovulatory phase of the estrous cycle. Pheromone is the term applied to the hormone-like substances that affect the development, behavior or reproduction of members of the same species, but usually by members of the opposite sex. The route of communication is usually olfactory. The existence of pheromones was originally established in insects, but evidence has now accumulated on their importance in other animal groups, including mammals. For instance, chemicals elaborated in the vagina, excreted in the urine or secreted from cutaneous glands are all examples of the message that triggers sexual arousal in the male (for a review see Shorey, 1976). The chemical nature of many pheromones has been established (most are low molecular weight aliphatic compounds possessing hydroxyl or carboxyl groups, and are usually highly volatile) (Table 1.1).

TABLE 1.1

Representative Chemicals that Serve as Pheromones in Various Species

The ability of mammalian olfactory epithelium to react to volatile chemical stimuli is well documented. We can safely assume that specialized receptors respond to sexual odors, and that the responses are communicated through pathways that exist between the olfactory bulb and the hypothalamus (Barraclough and Cross, 1963; Scott and Pfaffmann, 1967).

In most species these attracting hormones are specific. In others, a single chemical entity may be used by sibling species, but habitat preference, temporal distribution and differing diurnal cycles help to ensure reproductive isolation. Secondary sex pheromone chemicals may also frequently be elaborated. Many male mammals also exhibit a characteristic and often disagreeable odor which depends on intact gonadal function. Pigs, lambs and calves are often castrated for this reason. Odors have been implicated not only in the recognition of sexual partners and in sexual arousal but also in the performance of the sexual act and in care of the young.

1.1.1.1 Female pheromones.

Evidence concerning an interdependence between female pheromones and olfactory stimulation and behavioral changes in the male has been documented in several species. Rams, for example, show preference in approaching ewes in heat (Kelley, 1937) and this response can be abolished by olfactory bulb ablation (Lindsay, 1965). The mating behavior of male hamsters can also be eliminated by olfactory bulb removal (Murphy and Schneider, 1970). Changes in vaginal secretion of volatile substances during the estrus cycle have been noted in cows (Hart et al., 1946), hamsters (Murphy and Schneider, 1970), dogs (Beach and Merari, 1970) and rhesus monkeys (Michael and Keverne, 1968, 1970). Modifications in the intensity and pleasantness of human vaginal secretions during the menstrual cycle have also been reported (Doty et al., 1975). The importance of olfaction in mating behavior is less pronounced in other mammalian species. Olfactory bulb removal causes no interruption of the estrus cycle of rats (Orbach and Kling, 1966) and does not prevent mating, birth, and nursing of the young in rabbits (Brooks, 1937; Sawyer, 1956) and guinea pigs (Donovan and Kopřiva, 1965).

Pheromones released by females may result in a suppression of estrus and/or synchronization of cycles in field mice (Bronson and Marsden, 1964), sheep (Schinckel, 1954) and goats (Shelton, 1960). It has been speculated that similar mechanisms may also be operative in women (Graham and McGrew, 1980). Perception of olfactory stimuli may also be required to identify offspring. Abnormal maternal behavior in mice, resulting in the destruction of offspring, can be eliminated by removal of the olfactory bulb (Gandelman et al., 1971).

1.1.1.2 Male pheromones.

Odors released by males have been shown to elicit endocrine responses in females (Schinckel et al., 1954). Female mice respond to a pheromone in the urine of male mice by increasing the release of gonadotropins, which can result in precocious puberty (Castro, 1967; Vandenbergh, 1969; Teague and Bradley, 1978) and an increase in litter size (Beilharz, 1968). The estrus cycles in mice are shorter and tend to become synchronized (Whitten, 1956; Marsden and Bronson, 1964) if the females have had their cycles suppressed by grouping before pairing.

Bruce (1960) observed that the normal course of pregnancy in recently mated laboratory mice was interrupted when they were exposed to strange males. Pregnancy can be blocked, however, no later than 5–6 days following mating, i.e., before implantation. Pregnancy blockage by strange males was also demonstrated in other species of mice (Eleftheriou et al., 1962; Bronson and Eleftheriou, 1963; Chipman and Fox, 1966; Clulow and Clarke, 1968), voles (Clulow and Langford, 1971; Stehn and Richmond, 1975) and lemmings (Mallory and Brooks, 1978). The pregnancy block results from luteal function failure (Dominic, 1970) and in changes in the secretion patterns of LH, prolactin and progesterone which lead to the beginning of a new ovulatory cycle (Chapman et al., 1970). Parkes and Bruce (1962) have demonstrated that the block is transmitted through the urine and have thus established that this action depends upon a pheromone. Castrated males do not secrete the pheromone (Bruce, 1965), indicating that the secretion of the pregnancy-blocking chemical is androgen-dependent; its production can be elicited in castrated males using testosterone (Dominic, 1965). Receptors for pheromone-mediated endocrine response are present in the vomeronasal organ (Reynolds and Keverne, 1979) and the pregnancy block is most likely mediated through increased prolactin secretion (Bellringer et al., 1980). It has been suggested that the difference in pheromones in various Mus subspecies is linked genetically. Whitten (1973) has shown that whereas it was possible to bring about pregnancy block in BALB/c and SJL female mice, the response in F1 hybrids of these species was markedly reduced.

1.1.2 Photic stimulation

Changes in the intensity and duration of light are of importance in regulating gonadal functions. In many species breeding takes place in the spring when the amount of light begins to increase; in others, during the autumn, when the days shorten. Even in cyclically ovulating species, an increase or decrease in the amount of light may modify reproductive functions. Reproductive activity coincides with the optimal season of the year: birth and weaning occur during spring and early summer months. In the northern hemisphere, beaver, rabbit, raccoon, cat and woodchuck, species which have short gestation periods, breed early in the year and their young are born between March and May. Animals with long gestation periods (fallow deer, horse and donkey) breed late in the spring or summer and their young are born the following spring. Ermine, badger, roe deer, harbor seal, and armadillo mate in the summer. Their gestation periods, of intermediate lengths, are prolonged by delayed implantation so that the young can be born in the following spring.

In regularly ovulating species the duration of light and dark periods modulates the cyclicity of the events. In temperate regions of the world most species of murid rodents, for example, have a restricted breeding season which is regulated photoperiodically (Baker and Ranson, 1932). Dempsey et al. (1934) found that behavioral estrus in guinea pigs occurred only in darkness and that the time of estrus tended to shift with varying day lengths so that estrus behavior always occurred in the dark. Snell et al. (1940) found that in the mouse, estrus also occurred in the dark and that a reversal of the light-dark schedule resulted in a similar change in the time of the onset of estrus. Everett and Sawyer (1949; 1950) determined that the release of ovulating hormones in the rat maintained on a fixed light-dark regimen could be blocked by barbiturates given on the day of pro-estrus several hours prior to the dark period. When measured from the mid-point of dark, the critical period for the blockage of ovulation occurred 14 and 16 hours later, regardless of the length of the photoperiod. Continuous light or darkness may also affect reproduction. Constant exposure of rodents to light may result in persistent estrus while exposure to continuous darkness is usually less disturbing (Hoffman, 1974).

An important organ involved in photic regulation is the pineal gland, which was recognized by Galen as a separate organ nearly 2000 years ago. Cases of delayed puberty have been attributed to tumors of the pineal body, whereas decreased function of this gland, resulting from a teratoma, was stated to be the reason for precocious puberty (Kitay and Altschule, 1954). Lerner et al. (1960) isolated a substance, melatonin (an amine closely related to serotonin), which caused aggregation of melanin in amphibian melanocytes, resulting in lightening of the skin; however, it is not clear whether the pineal gland participates in pigmentation in this species. Melatonin is synthesized in the pineal in response to the neurotransmitter norepinephrine; the levels of norepinephrine decline as light activates photoreceptors in the retina and increase when the sympathetic nervous system is stimulated. The tracts involved are the inferior accessory optic tract and direct retinohypothalamic connections; the fibers of these tracts terminate within the supra-optic nucleus, which may be the location of the biological clock. In addition to melatonin the gland may contain other indoleamines (Wurtman et al., 1968) and polypeptides of undetermined structure (Reiter and Vaughan, 1977; Ebels and Benson, 1978; Rosenblum et al., 1979); of these, arginine vasotocin is the only compound identified so far (Pavel, 1978).

The pineal gland exerts varying influences on endocrine function, depending on species, dosage and timing of melatonin administration. In albino rats, removal of the gland within 24 hours of birth has no inhibitory influence on reproduction in either sex, maternal behavior and lactation (Kincl and Benagiano, 1967). Minor changes include earlier vaginal opening, increases in body weight, advanced bone calcification and an increase in corticosterone concentration in peripheral plasma (Henzl et al., 1970). Others have noted that in pinealectomized female rats, the weight of ovaries was increased (Simonnet et al., 1951) and reported an abnormal incidence of cornified vaginal smears (Chu et al., 1964). Wurtman et al. (1964) reported that a daily injection of 1–20 μg of melatonin delayed vaginal introitus, while Ebels and Prop (1965) failed to demonstrate the suppressive effect of melatonin on gonadal weights, or on estrus. Blake (1976) could find no effect of the pineal gland on the timing or magnitude of LH, FSH or prolactin release at pro-estrus, the length of the estrus cycle, or LH release in ovariectomized white rats. The hypothesis that pinealectomy in ewes would prevent photoperiodic entrainment of the annual reproductive cycle could not be substantiated (Roche et al., 1970).

In male Syrian (golden) hamster, a light period shorter than about 12 hours will induce gonadal atrophy (Reiter and Fraschini, 1969), ovulation ceases in females and the uteri become involuted (Hoffman and Reiter, 1966); these changes are presumably an adaptation to periods of hibernation. Reiter (1968) reported that the dark-induced regression of the testes and accessory sex organs in the male can be counteracted by pinealectomy or sympathetic denervation of the pineal gland. Darkness-induced testis involution can be prevented by exogenous melatonin given in low doses (Hoffman, 1973; Reiter et al., 1975a, b). Under different conditions (higher dose regimen) exogenous melatonin has been shown to induce testicular atrophy (Goldman et al., 1979). These paradoxical results have not yet been satisfactorily explained. Reiter (1980) has concluded that in the golden hamsters (and possibly also in some other seasonally reproducing rodents), the pineal may be important in adjusting the level of reproductive activity to season changes in environmental conditions. Natural populations of Syrian hamsters are presumably reproductively competent during the spring and summer and sexually incompetent during other seasons. In the absence of the pineal gland, the sexual organs (may) continue to produce sizeable gametes regardless of the season. Thus the role of the pineal is to restrict breeding to a very carefully defined portion of the biometeorological cycle.

Kincl et al. (1970) and Quay (1972) reported that changes in the activity rhythm of white rats in response to changing photoperiods showed a dependence on the pineal gland. In the presence of the pineal gland, rats of both sexes adjust their motor activity slowly to changing photoperiods, regardless of the qualitative nature of the change. Rats whose epiphysis cerebri have been removed within 24 hours of birth adapt more rapidly to such changes. Similar responses may also be operative in diurnal species. These results suggest that the pineal gland may help to adjust the activity of animals in response to changing photoperiods. In the absence of such homeostasis, rapid light changes may expose animals to asynchrony with light-dependent circadian cycles.

1.2 The Hypothalamus

It was known in the early 1930s that estrogenic extracts caused ovarian atrophy in females and gonadal atrophy in males (Moore and Price, 1932). Hohlweg and Junkmann (1932) demonstrated that low levels of estrogens stimulated ovulation and deduced nervous control of the pituitary. They coined the term das Sexualcentrum to describe the area in the brain which controls reproductive functions. Many research workers have subsequently confirmed these observations. Harris in England (1948; 1955) contributed to the concept that the anterior pituitary is controlled by a hormonal mechanism involving the hypophysial portal system. The Hungarian school (Szentagothai et al., 1968) introduced the concept of a hypophysiotropic area in the hypothalamus to designate the area capable of supporting the secretory condition of implanted pituitary tissue by locally produced stimulatory hormone(s). The search for the elusive hormone involved culminated in the isolation and identification of a polypeptide (GnRH) by two teams led by Guillemin and Schally for which they were awarded the Nobel prize in Physiology and Medicine in 1977. The content of GnRH in the hypothalamus is very low; Schally’s group used 60,000 pig hypothalami to isolate 800 µg of the purified hormone. The nature of the releasing hormone (GnRH) isolated from hypothalami of porcine origin has been established (Matsuo et al., 1971) and its structure has been confirmed by synthesis. There appears to be only one RH; the pituitary responds by secreting both FSH and LH when stimulated by GnRH (Sandow et al., 1975). The demonstration of increased secretion of both gonadotropins following administration of a synthetic decapeptide has been shown in several species, including man (Borreman et al., 1975; Thompson et al., 1976). It has yet to be established whether the neurohormone is transported within the neurons complexed to neurophysins (the cysteine-rich carrier protein present in neurosecretory cells shown to bind oxytocin and vasopressin) or whether other mechanisms are operative. The close similarity of the chemical structures of the posterior pituitary hormones (octapeptides) to that of the releasing hormone (decapeptide) would appear to favor this possibility.

The peptidergic neurons responsible for releasing GnRH have been identified by immunohistochemical methods in the median eminence of the rat (Pelletier et al., 1974), the basomedial hypothalamus (Ramirez et al., 1975), the pre-optic area (Estes et al., 1977) and localized in neurosecretory granules in synaptosomes (Barnea et al., 1975; Bennett et al., 1975; Styne et al., 1977). The location of GnRH releasing hormone is shown schematically in Fig. 1.2. Ependymal elements connecting the floor of the third ventricle and the portal vessels are thought to be the transmission loci. These cells possess microvilli on the surface facing towards the ventricle. They have abundant microfilaments and microtubules on the surface facing the walls of the portal capillaries of the median eminence; axons incorporating dopamine originating in the hypothalamus seem to make synaptic contacts and interaction with acetylcholine appears to be likely (Baker et al., 1975; Kizer et al., 1976). Despite some controversy relative to the function of dopamine and noradrenaline in controlling gonadotrophic function, the existence of neural control of the pituitary-gonadal function is now well established.

Fig. 1.2 Hypothalamo-Pituitary-Ovarian Axis Showing Regulation of Steroidogenesis and Feedback Mechanisms

AHA, anterior hypothalamic area; ARC: arcuate nucleus; ME, median eminence; OC, optic chiasma; POA, preoptic area; PG, pituitary gland; stippled area, hypophysiotropic area; the solid arrows indicate the direction of blood flow in the hypophysial portal system; the open arrows show possible sites of feedback mechanism; the stippled arrows the effects of gonadotrophins on steroid hormone production. E2, estradiol; P, progesterone. Possible effect of gonadotropins feedback on GnRH synthesis and/or release shown schematically (see text).

1.2.1 Effects on the pituitary gland

Pituitary cells vary in sensitivity to GnRH and their degree of sensitivity is mediated by levels of gonadal hormones. The evidence for this is fairly conclusive. In the ewe, for example, the pituitary can respond to injected GnRH during the anestrous season by releasing LH. Greater responses, however, are found in animals pretreated with estrogen (Reeves et al., 1971). In rats, the amount of gonadotropins in plasma was found to vary according to the state of the cycle when synthetic GnRH was injected; the greatest sensitivity was noted when the animals were in pro-estrus (Aiyer et al., 1974). Ovariectomy was found to reduce the amount of LH released and injections of estradiol and progesterone produced an increase (Fink and Aiyer, 1974). However, increasing the estrogen dose will result in a negative feedback (vide infra).

1.2.2 Tissue and plasma concentrations of GnRH

GnRH is distributed throughout the central nervous system. Wheaton et al. (1975) have studied the tissue concentration of the different areas of the hypothalamus and found significant differences which are illustrated in Fig. 1.3. Chiappa and Fink (1977) reported an increase in the mean hypothalamic content from about 7000 pg/hypothalamus found in the morning of pro-estrus to 8500 pg/hypothalamus late in the pro-estrus cycle. Wilber et al. (1976) report in male rats the highest concentration in the hypothalamus (205 ng/mg wet weight), followed by the pituitary (167 ng/mg), the midbrain (84 ng/mg), and the cerebellum (32 ng/mg); the concentration in the cerebral cortex is less than 10 ng/mg.

Fig. 1.3 Concentration of Gonadotropin Releasing Hormone (GnRH) in Areas of Rat Hypothalamus

The values represent the average concentration of GnRH contained in sections of 400 μm using a radioimmunoassay method.

CA: commissura anterior; OC: optic chiasma; ARC: arcuate nucleus; AHA: nucleus anterior; HVM: nucleus ventromedialis; POA: preoptic area; NMM: nucleus mamillaris medialis; SC: nucleus suprachiasmaticus. From Wheaton et al. (1975).

The hormone is released intermittently (Carmel et al., 1976). In the rhesus monkey the concentration of GnRH peaks at intervals of 1 to 3 hours (200–800 pg/ml plasma). In ovariectomized monkeys only pulsed administration of GnRH, 1 µg/min for 6 minutes every hour, resulted in sustained recovery of gonadotropin secretion (Belchetz et al., 1978; Nakai et al., 1978). In sheep, GnRH spikes occur at frequent intervals (1.5–6 hours) throughout the cycle. In contrast, the LH surge happens only once during the cycle, further proof that the sensitivity of the pituitary to the hypothalamic hormone is dependent upon the steroid hormone milieu within the gland. Clearly the quantity and temporal patterns of LH release occur in response to the sex steroids bound to the gonadotrophic cells (Castro-Vazquez and McCann, 1975). The existence of gonadal hormone receptors in pituitary glands has been suggested by McEwen and co-workers (1970).

A reliable radioimmunoassay for GnRH in biological fluids was developed only in the late 70s; hence any data prior to about 1977 pertaining to plasma levels must be viewed with caution. Concentration in humans is about 20 pg/ml plasma, in both men and women (Teuwissen et al., 1978; Patschen et al., 1979). Aksel and Glass (1979) found no discernible pattern of plasma concentration during the menstrual cycle (Table 1.2) The biological half-life of GnRH is short. Rosenblum and Schlaff (1976) report t1/2α to be 2.2 min and t1/2β 37.3 min; at this time more than 90% of all material has been removed from plasma. Saito et al. (1975) report that in men the hormone disappears from the peripheral circulation within 1 to 3 min.

TABLE 1.2

Peripheral Plasma Levels of Gonadotropin Releasing Hormone (GnRH) in Women During the Menstrual Cycle

From Aksel and Glass (1979).

A pre-ovulatory surge in GnRH levels in stalk plasma has been observed by Neill et al. (1977) in rhesus monkeys (104 ± 34 pg/ml), while follicular phase levels were 23 ± 5 pg/ml, but the data await confirmation. In rabbits, when ovulation was induced by intravenous injection of copper acetate solution, GnRH stalk plasma concentrations were 30–40 pg/ml (Tsou et al., 1977). In rats, the pro-estrus concentration is about 15 pg/ml. This level increases to about 55 pg/ml prior to the LH surge and the concentrations persist into diestrus (Sarkar et al., 1976; Eskay et al., 1977).

1.2.3 Other effects

GnRH may directly effect the growing follicle. Hsueh and Erickson (1979) reported that GnRH inhibited the in vitro FSH-induced increase in estrogen and progesterone production by ovarian granulosa cells of the rat. Synthetic GnRH analogs also inhibit FSH-induced changes in ovarian function in hypophysectomized rats (White and Nicoll, 1979).

1.3 The Pituitary

Under the influence of hypothalamic releasing hormone the anterior part of the pituitary gland elaborates two hormones, which stimulate the growth and secretion of the gonads of both sexes. Custom has described these hormones based on their activity in the female: the follicle stimulating hormone (FSH) dominates during follicular growth, and the luteinizing hormone (LH) peaks at ovulation. The chorionic gonadotropins (CG), produced by the syncytiotrophoblastic cells of the placenta, are very similar to LH in their biological function and immunological properties.

1.3.1 FSH and LH

FSH and LH are glycoproteins, of which virtually all of our knowledge has come from studies on mammalian pituitaries. Avian, reptilian and piscine gonadotropins have received scant attention. Chemical characterization of FSH and LH isolated from snapping turtles has revealed a remarkable close relationship to bovine FSH and LH, indicating that the hormones from both species have a common distant phylogenetic origin (Papkoff et al., 1973).

The major anatomical event related to the changing gonadotropin milieu is the growth and maturation of the ovarian follicle and ova expulsion (ovulation). Once ovulation occurs, the ruptured follicle is converted into a corpus luteum, which is maintained for the remaining portion of the menstrual cycle.

FSH and LH are produced by specific pituitary cells, where their presence can be demonstrated by light and electron microscopy (Tougard et al., 1971; 1973). Both have been measured in the pituitary, pituitary stalk blood, peripheral plasma, ovaries and urine. FSH appears to increase the number of follicles in their early stages of development and LH exerts its effect on enlarging follicles (Lindner et al., 1974). The hormones are also involved in the control of the production of steroid hormones (steroidogenesis). Many in vitro and in vivo studies have demonstrated the influence of LH on the incorporation of acetate and other intermediates into ovarian cholesterol – a precursor in the biosynthesis of progesterone and estradiol.

1.3.1.1 Plasma levels.

The patterns of FSH and LH secretion have been the subjects of numerous studies. Among the cyclically ovulating species with relatively long cycles the human female has been studied most extensively (see Ross et al., 1970; Vande Wiele et al., 1970; Yen et al., 1975). Most investigators report that serum FSH levels rise during menstruation and are higher early in the proliferative phase. The concentration then declines to a pre-ovulatory nadir just before the midcycle LH surge, and rises again to a second peak roughly coincidental with the LH level. The FSH peak is not as dramatic as that seen for LH. During the luteal phase FSH levels decline, but less consistently than the LH levels.

The declining levels of serum FSH in the late follicular phase suggest that once follicular growth has begun, continued maturation requires only low levels of stimulation. A similar situation may prevail during the luteal phase since both FSH and LH levels tend to fall during that period. Serum concentration of LH reaches its nadir at the end of the cycle, i.e., one or two days prior to menstrual bleeding. The levels then begin to increase slowly during the proliferative phase. At midcycle there is a sharp rise in LH concentration (the LH peak) to approximately eight times the average proliferative phase levels. During the luteal phase, a gradual fall in the base-line concentration may be interrupted by one or more lesser peaks (Fig. 1.4). The patterns obtained by radioimmunoassay techniques agreed well with those obtained by bioassay procedures, except that LH levels measured by bioassay exceeded those estimated by radioimmunoassay (Romani et al., 1977). The LH peak lasts approximately 48 hours (Thorneycroft et al., 1974) and is preceded by an increase in estradiol concentration. Following ovulation, progesterone in plasma begins to rise; if pregnancy does not occur, the levels decline rapidly and the breakdown of the endometrium and menstruation follow (vide infra).

Fig. 1.4 Daily Plasma FSH and LH Values During the Normal Human Menstrual Cycle

The concentration of LH and FSH in peripheral plasma has been plotted by assigning day 0 to correspond to a theoretical day of ovulation. After Rosemberg et al. (1974).

In animals with a short cycle the LH surge takes place within a few hours (Fig. 1.5). Proestrus plasma levels in the rat have been observed to rise from 200 ng/ml plasma LH to over 1500 ng/ml within about 3 hours and remain at this level for about 6 hours before they decline (Blake, 1976). In animals ovulating seasonally (ewes) concentrations of LH and FSH begin to increase prior to estrus (Wheeler et al., 1977). Peak plasma levels of both hormones, in various mammalian species, are shown in Table 1.3.

TABLE 1.3

Average Peak Concentrations of FSH and LH in Peripheral Plasma During a Menstrual or Estrus Cycle of Various Species Determined by Radioimmunoassays

Variation in hormone levels can often be due to the type of antiserum used, frequency of measurement, duration of initial rise and duration of the subsequent plateau (see also Appendix I).

Fig. 1.5 The Pattern of Hormones Circulating in the Blood of Cycling Rats GnRH concentrations were determined in the portal vein blood. The small arrows show the time of the LH surge and the large arrows the time of ovulation. The numbers on the abscissa indicate the time of day in terms of a 24-hour clock. Redrawn after Smith et al. (1975) and Sarker et al. (1976).

Since the metabolic clearance rates of the two gonadotropic hormones do not change significantly during the cycle and are not altered by large differences in concentration, changes in serum levels must be presumed to reflect changes in pituitary secretion. This is supported by determination of FSH and LH contents in pituitaries. Pituitary FSH content is significantly diminished during the early proliferative phase and after ovulation, when the values in plasma are increasing. In contrast, the pituitary content of LH is high during the pre-ovulatory and post-ovulatory stages of the cycle, when LH blood levels are low. During ovulation, the pituitary content is decreased, reflecting the ovulatory surge of LH (Fig. 1.6).

Fig. 1.6 Inverse Relation Between the Human Pituitary Content and Serum Levels of Luteinizing Hormone (LH) Strongly Suggesting a Release of Stored Hormone Following the Releasing Hormone Stimulus

The inverse relation is shown between the content of LH in human pituitaries (average of 35 observations determined by bioassay) and average serum concentration as reported in several publications. The LH activity was variable during the cycle; there was a decrease during the early follicular phase, followed by a sharp rise of plasma LH at the time when the pituitary store of LH was decreasing. The FSH activity in the pituitary was fairly constant. The day 0 corresponds to a theoretical day of ovulation, based on histological evaluation of the ovaries and endometrium. After Bischoff et al. (1969).

Specificity of gonadotropin action is predicted by the presence of binding sites on the membranes of target cells (vide infra). The binding of human LH to granulosa cells increases 35-fold as the follicle enlarges, although cell size remains constant. The affinity for the hormones does not change as the cells mature, indicating that the number of receptor sites increases with maturation (Kammerman and Ross, 1975) (see Vol. II).

1.3.2 Prolactin

Prolactin (lactogenic hormone, mammotrophin, PRL) is a polypeptide hormone similar in structure to growth hormone and to placental lactogen. The hormone stimulates many vertebrate functions related to growth and metabolism, osmoregulation, reproduction, and parental care. Earlier investigators considered that the corpus luteum was maintained by prolactin. This hormone has been shown to be luteotropic only in some murine species (rat), but demonstrates no steroidogenic stimulating activity for most mammals, and the term luteotrophin is no longer used to describe this hormone.

Prolactin secretion from the pituitary is provoked by suppressing neurons located in the lateral pre-optic area which produce a PRL-inhibiting-factor. Thus, in male rats prolactin plasma concentrations fall almost immediately following injection of crude hypothalamic extracts. Neurotensin and substance P peptides, isolated from hypothalamic extracts, have been identified as the agents which cause plasma PRL elevation (Rivier et al., 1977). Evidence has been presented of the existence of a human PRL-releasing-factor (Hagen et al., 1976), but its nature awaits elucidation.

Secretion of excessive amounts of PRL results in inhibition of ovulation (hyperprolactinemia), although it is not clear why high PRL levels result in anovulation. PRL hypersecretion can be decreased by a synthetic analog of an ergot alkaloid, bromocriptine mesylate. This compound has been used under clinical conditions to successfully bring about ovulation in hyperprolactinemia and in the treatment of galactorrhea.

1.3.2.1 Plasma levels.

In non-pregnant females, prolactin levels fluctuate hourly and diurnal and nocturnal peaks can be distinguished; the greatest concentration appears during the early morning hours (about 11 ng/ml) and smaller peaks occur in the afternoon (see Wolstenholme and Knight, 1972). There is a progressive increase during the late follicular phase with maximum values concomitant with the LH peak (Vekemans et al., 1977). Plasma concentrations are elevated during pregnancy and lactation (see Chapter 3).

1.4 Feedback Mechanism

The stimulatory action of pituitary hormones results in the gonadal production of estradiol and progesterone. These, in turn, exert inhibitory influences on the hypothalamo-pituitary axis. It has been shown in numerous animal studies that the major sites of action are the hypothalamic centers controlling the releasing hormones and, consequently, the production and/or release of gonadotropins by the anterior pituitary. The inhibition is specific; there is no evidence of a similar action by gonadal hormones on other hormones of the adenohypophysis or neurohypophysis. Inhibitory and facilitative actions of estrogens were already described by Moore and Price (1932) and by Hohlweg and Junkmann (1932), and have since been confirmed by many others (for reviews see Everett, 1966; Kincl, 1971, 1972). The dual role of gonadal hormones is probably best described as the small doses stimulate large doses inhibit rule already formulated by endocrinologists in the 1930s.

1.4.1 Facilitative action

Systemic administration of small doses of estrogens to immature female rats typically advances the onset of puberty and induces a fall in pituitary LH and a rise in plasma LH levels. In the ovariectomized rhesus monkey with hypothalamic lesions leading to abolished endogenous GnRH production and low circulating LH and FSH levels, estradiol administration is followed by a discharge of these hormones (Nakai et al., 1978). These results suggest that in the monkey estradiol feedback takes place at the level of the pituitary gland. In most mammalian species an estrogen surge precedes the LH peak and ovulation (vide infra).

Under certain conditions, progesterone may stimulate ovulation. In rats, the administration of progesterone will advance ovulation by 24 hours; in rabbits it facilitates ovulation and increases LH plasma levels. The success of stimulation may be dependent on previous exposure to either endogenous or exogenous sources of estrogen. It is thought that progesterone may facilitate LH release by lowering the hypothalamic activation threshold to neural stimuli (Sawyer, 1952). In some animals (sheep) both estradiol and progesterone are involved in triggering the LH surge (Hauger et al., 1977). In humans LH (and episodic PRL) release can be induced by progesterone (Rakoff et al., 1977); this implies endogenous GnRH release and reduction of the inhibitory effect of dopamine (vide infra).

1.4.1.1 Trigger mechanism.

It has been stressed above, and elsewhere in this book, that steroid plasma levels (estradiol, progesterone, 17-hydroxyprogesterone) rise prior to the LH surge, and that administration of these hormones in concentrations comparable to those found in the preovulatory phase will induce a release of either FSH and/or LH, depending on species and conditions. Such observations implicate steroid hormones and both the hypothalamus and the pituitary. Other hormones (neurotransmitters, prolactin and prostaglandins) appear to be involved in the hypothalamic-pituitary interaction.

In the rat, dopaminergic (DA) nerve terminals may be inhibitory and noradrenergic (NE) stimulatory in respect to GnRH secretion control; the finding that during pro-estrus DA turnover is lower and NE higher points to the inhibitory action of estrogens on dopamine release (Fuxe et al., 1976). Other neurotransmitters may also be involved as shown by the observation that acetylcholine stimulates (Simonovic et al., 1974) and serotonin inhibits (McCann and Moss, 1975) the release of GnRH from the median eminence. The injection of γ-aminobutyric acid (Ondo, 1974) into the third ventricle will stimulate LH release.

Other observations suggest that either the release or the post-junctional action of the neurotransmitters is regulated by brain prostaglandins (see also Section 4); PGE2 enhances the release of GnRH (Eskay et al., 1975; Ojeda et al., 1975) and of LH and FSH (Harms et al., 1973; Batta et al., 1974). Additional evidence that prostaglandins may play a mediating role is provided by the observation that ovulation can be blocked by inhibitors of prostaglandin biosynthesis (Orczyk and Behrman, 1972).

In the pituitary gland progesterone amplifies the estrogen-augmented pituitary sensitivity to GnRH (Lasley et al., 1975), which results in the LH surge, possibly by mediating the number of receptor binding molecules in the gonadotropin-producing cells (see Vol. II).

Despite the evidence cited, the mechanism(s) which trigger GnRH release and the LH surge is unknown. Fig. 1.7 illustrates events that take place prior to and following the LH surge and suggest a possible mechanism triggering the reaction.

Fig. 1.7 A Speculative Presentation of the Events Resulting in the Development of Cyclic LH Release

. Prostaglandins (PG) stimulated by E2 may act directly on GnRH neurons contributing to GnRH synthesis and/or release. The pituitary gonadotrophs’ capacity to respond may be influenced by increases in E2 and P levels. The final event, LH secretion, is the result of the GnRH surge. In the above drawing, the ordinate indicates quantitative changes and the abscissa the time sequence.

1.4.2 Inhibitory action

Although a treatment of short duration may result in stimulating pituitary output, steroids given in high enough amounts inhibit the synthesis and/or release of gonadotropins and consequently the growth of gonads. Prolonged treatment with estrogens causes characteristic changes in pituitary histology, including degranulation of basophils, acidophils, and chromophobes, many of which are enlarged. This results in the enlargement of the pituitary gland (which is anyway larger in females than in males), atrophy of the gonads and the arrest of follicular development. Progesterone acts in a similar fashion. Given in large doses, and for prolonged periods, it will inhibit ovulation. Rabbits, sheep and cows are especially sensitive to progesterone. Inhibition of ovulation in rabbits was noted by Makepeace and co-workers in 1937 (for a review, see Zeilmaker, 1971).

The ovulation-inhibiting effects of gonadal hormones, particularly in the induced ovulators, are mediated through the central nervous system (Everett, 1952; Kawakami and Sawyer, 1959; Döcke et al., 1975) and the pituitary gland (Döcke and Dörner, 1965; Kingsley and Bogdanove, 1973; Bogdanove et al., 1975; Sawyer, 1975). Evidence for this is supported by observations that synthetic progestagens inhibit ovulation in rabbits only if ovulation is induced by mating but not when induced directly by stimulating the ovaries with human chorionic gonadotropin (Kincl, 1963). Reports indicating that estrogens and progestagens may inhibit ovulation by a direct action on the ovary have not been substantiated. It has been shown that ovulation is not inhibited in hypophysectomized rats in which follicle formation has been stimulated by the administration of FSH and ovulation by hCG (Rudel and Kincl, 1966).

It has been proposed that changes in feedback sensitivity are responsible for the onset of puberty in a number of species, including man. It is thought that the control centers in very young animals are more sensitive to steroid