The effects of estradiol on gonadotropin-releasing hormone neurons in the developing mouse brain

General and Comparative Endocrinology 112, 356–363 (1998)
Article No. GC987134
The Effects of Estradiol on Gonadotropin-Releasing
Hormone Neurons in the Developing Mouse Brain

Matthew S. Grober,1 Greg M. Winterstein, Asif A. Ghazanfar,2
and Victor P. Eroschenko*

Department of Biological Sciences, *WAMI Medical Program, University of Idaho, Moscow, Idaho 83844-3051 The hypothalamic–pituitary–gonadal (HPG) axis plays a
The perinatal organization of the vertebrate brain critical role in the control of reproduction. Two key
plays a major role in determining adult reproductive hormonal components of the HPG axis are gonadal
function. Two brain regions that are involved in the steroids and gonadotropin-releasing hormone (GnRH).
control of reproductive behavior and physiology are Gonadal steroids are known to organize the development
the hypothalamus and the neurohypophysis, which, in of neural substrates which control adult reproductive
conjunction with the gonads, constitute the hypotha- behavior; GnRH is required for normal reproductive
lamic–pituitary–gonadal axis (HPG axis). Two impor- structure and function. The possibility that gonadal
tant hormonal components of the HPG axis are go- steroids may produce organizational changes in the
nadal steroids and gonadotropin-releasing hormone pattern of GnRH staining observed in the brain is
investigated through the use of injections of estradiol to
Gonadal steroids mediate developmental organiza- neonatal mice and subsequent GnRH immunocytochem-
tion of the neural and nonneural substrates that are istry at 2 months of age. Our results indicate that the
critical for adult reproductive function (Phoenix et al., number of GnRH-immunoreactive (GnRH-ir) cells is
1959). Experimental manipulation of testosterone re- normally lower in females than males. Estradiol did not
sults in changes in the expression of male-like external affect the number of GnRH-ir cells in females, but
genitalia and frequency of mounting behavior by significantly increased the number of GnRH-ir cells in
female offspring (Phoenix et al., 1959), the number of males, suggesting that early exposure to estradiol results
neurons in the spinal nucleus of the bulbocavernosus in masculinization of the GnRH axis of males.
in both males and females (Breedlove and Arnold, 1983), and anogenital distance (AGD) (Clemens et al., Key Words: GnRH; sexual differentiation; hypothalam-
1978). The sexually dimorphic nucleus of the preoptic ic–preoptic area; neonates; estradiol; immunocytochem-
area (SDN-POA) is one of several regions of the rodent istry; mouse; organization.
brain that is influenced during development by circu-lating gonadal steroids (Gorski et al., 1978). The SDN-POA is larger in males than females (Gorski et al.,1980), and injection with testosterone in early life 1To whom correspondence should be addressed at: Department of causes this region to increase in size in both gonadecto- Life Sciences, Arizona State University West, P.O. Box 37100, 4701 mized males and normal females (Rhees et al., 1990).
West Thunderbird Avenue, Phoenix, AZ 85069–7100. Fax: (602) The estrogen antagonist tamoxifen and the androgen 543–6073. E-mail: [email protected]
2Present address: Department of Neurobiology, Box 3209, Duke antagonist cyproterone acetate have been used to show University Medical Center, Durham, NC 27710.
the estrogenic mediation of sexual dimorphism in the All rights of reproduction in any form reserved.
SDN-POA (Do¨hler et al., 1984, 1986). More recently, ever, this analysis was qualitative and thus did not antisense oligonucleotides which block the production examine quantitative differences in the effect of estra- of the estrogen receptor have been used to show that in diol on the number of GnRH cells. Data from radioim- female rats SDN-POA size and parastrial nuclear size munoassays on brain tissue from neonatally estro- are mediated by estrogen and that estrogen influences genized rats suggest that early estrogen treatment the development of both lordosis and open field modifies the hypothalamic mechanism involved in the behavior (McCarthy et al., 1993). These data suggest release of LHRH (Hayashi et al., 1991). Two previous that aromatization of testosterone to estradiol is respon- studies have quantitatively examined GnRH cells in sible for several key components of the masculiniza- mice (Hoffman and Finch, 1986; Wray et al., 1989).
tion process. Thus, gonadal steroids have dramatic Wray et al. (1989) examined the progenitor cells that organizational effects on areas of the hypothalamus give rise to forebrain GnRH cells and provided counts and preoptic area that play a key role in the control of of GnRH cells from embryonic day 10.5 through reproduction. It is possible that the organizational adulthood. Hoffman and Finch (1986) looked at GnRH effects of gonadal steroids may also influence the cells during aging in a different strain of mice and development of the hypothalamic nuclei responsible found that onset of reproductive dysfunction did not correlate with a loss of GnRH forebrain cells. Presently, The gonadotropin-releasing hormones are a family no data are available on the effects of estradiol on the of decapeptide hormones found in all vertebrates development of the GnRH axis in mice. This study examined thus far and which represents an important tests the hypothesis that early postnatal estradiol link between the brain and reproduction. In rodents, treatment affects the number of GnRH-immunoreac- the majority of the GnRH-producing cells are in the tive (GnRH-ir) neurons in the preoptic area of mice of preoptic region of the hypothalamus (reviewed in both sexes during the first 2 months of development.
Silverman et al., 1994). Most of the GnRH cells arelocated amidst the diagonal band of Broca, the bednucleus of the stria terminalis, the preoptic region(periventricular, medial, and lateral), and the anterior MATERIALS AND METHODS
hypothalamus (Silverman et al., 1994). The majority ofGnRH cells located in the preoptic region project to the Animals
median eminence and are associated with the hypophy-seal portal system (Silverman et al., 1994). Gonadotro- Adult mice of ND4 Swiss Webster strain were pin-releasing hormone is primarily responsible for purchased from Simonsen Labs (Gilroy, CA), given causing the release of luteinizing hormone and follicle- food and water ad libitum, kept on a 12-h light–12-h stimulating hormone from the anterior pituitary. It is dark regimen, and allowed to breed. Within 24 h after also involved in the activation of lordosis in the female birth, the dams with their litters (housed together) (Moser and Mathiesen, 1996; Pfaff, 1973; Sakuma and were randomly assigned to different experimental Pfaff, 1980). Moreover, in both mice and humans groups. Immediately after group assignment, neonatal (reviewed in Silverman et al., 1994), improper develop- pups received the first of 14 daily intraperitoneal ment and or molecular regulation of the GnRH fore- injections (0.05 ml) of either sesame oil (control) or 10.0 brain nuclei results in hypogonadism and subsequent mg of 17␤-estradiol in sesame oil. All injections were infertility. Thus, early and proper development of the administered with a 27-gauge needle using filtered forebrain GnRH axis is a necessary requirement for sesame oil as a vehicle. The choice of estradiol dosage normal adult reproductive function in several mamma- was based on previous experiments where this dose induced significant morphological and biochemical Neonatal treatment of rats with estradiol decreases alterations in the reproductive organs of immature the number of cells stained for GnRH in the male and female mice (Eroschenko et al., 1995). We were notified increases the number of cells in the female within the as to the availability of brain tissue from these animals first 10 days of life (Elkind-Hirsch et al., 1981). How- after the study was completed, and the established All rights of reproduction in any form reserved.
experimental design places some limitations on our cific neuroanatomical landmarks including the merger of the right and left radiations of the frontal aspect of At 2 months of age (56–60 days), all treated mice the corpus callosum, anteriorly, to the appearance of were anesthetized, injected with 100 ml heparin (1000 the fornix and the disappearance of the anterior com- units/ml), and perfused with 0.9% sodium chloride missure, posteriorly. This forebrain area includes the and then with 4% paraformaldehyde in 0.1 M phos- medial and lateral preoptic areas, the medial and phate-buffered saline. Anogenital distance and body lateral septal nuclei, and the diagonal band of Broca.
weight were recorded before perfusion, whereas ovar- Total cell counts in the forebrain for each animal were ian weight was recorded after perfusion (testes weight tabulated. The GnRH-ir cells in some brain sections was not recorded). The brains were removed, blocked, were not visible because of poor staining. It is not clear sunk in 30% sucrose overnight at 4° C for cryoprotec- why some sections did not stain well, but poorly tion, sectioned at 50 µm in the coronal plane on a stained sections were distributed haphazardly among cryostat, and then stored in 0.1 M phosphate buffer brains and no brains were composed entirely of poorly (PB) at 4°C until the immunostaining was performed.
stained sections. Thus, GnRH-ir cell number in brainswith poorly stained sections would not be a reliableindicator of the total number of cells in those brains. In Immunocytochemistry
order to exclude brains with poorly stained sections, Immunocytochemistry was carried out on free-floating group averages were calculated from the four highest sections using culture plates. Briefly, the sections were values in each group. The top four brains were chosen rinsed twice in PB with 0.4% Triton X-100 (PBX) for 5 for two reasons: (1) one treatment group only had four min. The sections were then incubated for 1 h in the brains that showed robust staining for all sections and presoak solution (3% normal goat serum in PBX) at (2) to maintain equal sample sizes across all treat- room temperature on a shaker. The sections were then ments. This ensured that the staining quality was incubated with the primary antibody (monoclonal robust in all brains used in the analyses (Fig. 1).
antibody to GnRH, LR 132 (Park and Wakabayshi, Differences between the means for all groups were 1986), diluted 1:1000 in the presoak solution) overnight analyzed using analysis of variance (ANOVA). The (at least 16 h) at room temperature on a shaker. The statistical significance of the differences among the sections were rinsed twice in PBX for 5 min. Biotinyl- individual groups was analyzed using Fisher’s LSD.
ated secondary antibody (Kirkegaard & Perry Labora- Linear regression was used to evaluate the relationship tories, Gaithersburg, MD; KPL) was then applied for 1 between gonad size and body size (StatView 4.01 for h at room temperature on a shaker. The sections were the Macintosh, Abacus Concepts, Inc.). All data are rinsed twice with PBX for 5 min. Incubation of the presented as the mean Ϯ standard error of the mean.
sections with streptavidin–peroxidase (KPL) was at roomtemperature for 1 h. The sections were rinsed twicewith PBX for 5 min. DAB (diaminobenzidine), diluted according to the manufacturer’s instructions (KPL),was applied to the sections for 10 min. The sectionswere rinsed in PB to stop the DAB reaction, mounted Localization of GnRH-Producing Neurons
on slides, air dried, dehydrated, and coverslipped.
Our identification of GnRH-ir cells in the mouse forebrain (Fig. 1) is similar to previously published Cell Count Analysis
work (reviewed in Silverman et al., 1994). All GnRH-irneurons were located in forebrain regions that were We counted all stained cells in which a nucleus and included in the quantitative analyses.
one or more cell processes were visible. We includedthe second criterion to avoid double counting cells that Cell Counts
were cut through the plane of the nucleus and thuswould exhibit a nucleus on more than one section. The There were significant differences in the number of forebrain region examined was delineated using spe- GnRH-ir cells among the four treatment groups (Fig. 2; All rights of reproduction in any form reserved.
Characteristics of the mouse GnRH forebrain system. (A) Cross-section of the mouse brain. CC, corpus callosum; AC, anterior commissure; ON, optic nerve. Bar, 700 µm. The box indicates the area of B (not to scale). (B) A photomicrograph of representative GnRH-irneurons in the preoptic area of the mouse brain at 2 months of age. Bar, 100 µm. Arrow, GnRH-ir neurons shown in C. (C) Higher magnificationphotomicrograph of representative GnRH-ir neurons. Bar, 10 µm.
ANOVA, F ϭ 10.47, df ϭ 3, P Ͻ 0.01). The number of (369.5 Ϯ 16.56). Similarly, estradiol-treated males exhib- GnRH-ir cells in control males was 17% greater than in ited a significantly greater number of cells than estra- control females (statistically significant at P ϭ 0.057, diol-treated females (P Ͻ 0.001).
LSD). There were no significant differences in thenumber of GnRH cells between control females (307.5 Ϯ Somatic Characteristics
13.88 cells per brain) and estradiol-treated females(296.5 Ϯ 28.37; P ϭ 0.72) (Fig. 2). In contrast, estradiol- The body weight of the female mice was signifi- treated males have significantly more GnRH-ir neu- cantly increased by estradiol treatment (P Ͻ 0.01, Table rons (443.0 Ϯ 21.53; P ϭ 0.03) than control males 1). The male controls were, as expected, heavier than The effects of estradiol treatment on GnRH immunoreactivity in male and female mouse brains at 2 months of age. The columns and error bars represent the means and standard errors of the number of GnRH-ir neurons per brain (n ϭ 4), respectively. Different letters (a, b, c)indicate significant differences between the groups as determined using Fisher’s LSD test (P Ͻ 0.05).
All rights of reproduction in any form reserved.
Effects of Estradiol on Morphological Characteristics of Two-Month-Old Mice *Significant difference (P Ͻ 0.05) between control and estradiol treated mice. For all cells in the table, n ϭ 4.
the female controls (P Ͻ 0.001, Table 1). Anogenital is no statistically significant relationship. Thus, our distance was not affected by estradiol treatment in data suggest a sexual dimorphism in the number of either males or females (Table 1). However, the AGD GnRH-ir neurons in the preoptic area of the mouse data show the expected sexual dimorphism and are in brain. This pattern resembles observations in the rat, agreement with previously published values showing where males were reported to have a greater number males to have a greater AGD than females (Simon and of GnRH-ir neurons than females; however, this differ- Cologer-Clifford, 1991). Ovarian weight showed a ence was not quantified (Elkind-Hirsch et al., 1981). A nonsignificant decrease in estradiol-treated females second study on rats did not find a difference in the relative to control females (P ϭ 0.16, Table 1); however, number of GnRH cells between males and females, but only estradiol-treated females had cystic ovaries.
this study did not indicate the estrous state of thefemale animals (see discussion below) (Silverman etal., 1994).
Our data differ from two previous studies on mice in DISCUSSION
several ways. We counted fewer cells (300–400) com-pared to total GnRH cell numbers in Wray et al. (1989) Gonadal steroids affect the development and sexual (about 800) and Hoffman and Finch (1986) (about 650).
differentiation of all vertebrates that have been exam- While this is a dramatic difference, several sources of ined. Our results indicate that the number of preoptic variation may account for these differences. All three GnRH-ir cells in adult mice may be dependent upon studies used sections of different thickness (present the steroidal milieu during development. Since several study, 50µm; Wray et al., 12–14 µm; Hoffman and brain areas, including the suprachiasmatic nucleus, are Finch, 30µm) and employed different criteria for count- not affected by estrogens (Do¨hler et al., 1986), it is likely ing cells (we required a nucleus and at least one that the changes we observed in the number of neurite, Hoffman and Finch required a nucleus, and GnRH-ir cells are not a result of nonspecific effects of Wray et al. did not provide any specific criteria). The use of thinner sections and less conservative criteria The comparison between untreated males and fe- for cell counting would have increased the probability males suggests that the number of GnRH-ir neurons in of multiply counting cells, resulting in the higher cell mice may be sexually dimorphic. One explanation for our result is that gonadal steroids present in the Each of the studies utilized different strains of mice developing male and female differ, resulting in differ- which have been genetically isolated for many genera- ent numbers of GnRH-ir neurons at 2 months of age.
tions. Given that normal reproductive function can be An alternative explanation is that because male mice maintained with a very small number of GnRH cells are on average larger than female mice (Table 1), they (Silverman et al., 1994), there is no reason to expect may have a larger brain and hence a larger number of different strains to maintain similar cell numbers. An GnRH-ir neurons. However, regression analysis be- additional difference between the studies involves tween body weight and the number of GnRH-ir gonadal influences. Hoffman and Finch ovariecto- neurons, for the animals in our study, shows that there mized their mice, while Wray et al. did not. We were All rights of reproduction in any form reserved.
unable to include gonadectomized controls in this change in the AGD indicating no effects on androgen- study. However, Hoffman and Finch found that ovari- mediated characteristics. The increase in the number of ectomy did not significantly affect cell numbers in GnRH-ir neurons contrasts with prior data from the rat which showed a decrease in the number of GnRH-ir There are a number of factors that can affect the cells after treatment with estradiol in male rats (Elkind- amount of GnRH immunostaining observed in adult Hirsch et al., 1981). However, our data were collected at rodent brains. Changing gonadal hormone titers over 2 months of age while the data from the rat were the estrous cycle of the rat are correlated with GnRH collected before 10 days of age. After neonatal treat- content of the hypothalamus (Araki et al., 1975; Free- ment with estradiol, hypothalamic concentrations of man, 1988), and GnRH mRNA expression increases in GnRH are lower in early neonatal life (5–7 days) and late proestrus (Silverman et al., 1994). The number of higher later in development (60 days) when compared GnRH neurons that are stained in the brain of the rat is with those of the untreated control rat (Goomer et al., significantly lower in late estrus compared with the 1977). This could explain the differences between our remainder of the cycle (Ronnekleiv and Kelly, 1986; results and those of Elkind-Hirsch et al. (1981).
Silverman and Witkin, 1994), although at least one Since the male mouse brain contains active aro- study has shown that the number of neurons stained matase (Wozniak et al., 1992), it is possible that testos- for GnRH may not change during the estrous cycle of terone in normal males is aromatized to estradiol the rat (Marks et al., 1993). Low estradiol levels reduce causing an increase in the number of GnRH-ir neurons the release of GnRH in the female rat (Silverman et al., above that of normal females. This idea is consistent 1994), while a surge of estradiol precipitates the release with our data (and prior data, Elkind-Hirsch et al., of GnRH and subsequently the release of luteinizing 1981) showing a greater number of GnRH-ir neurons hormone from the pituitary (Kaynard et al., 1988; Krey in males. Taken together, these results suggest that this and Parsons, 1982). Thus, variation in GnRH cell sexual dimorphism may be mediated by the aromatiza- number may, or may not, occur over the course of the tion of testosterone to estradiol. However, it is known estrous cycle. Since the estradiol-treated female mice in that both 5␣-dihydrotestosterone and estradiol can our study were presumed to be in a state of persistent affect the number of GnRH-ir neurons in rats (Silver- vaginal estrus or diestrus, this variation should have a man et al., 1994) and frogs (Iela et al., 1994). Thus, it is minimal effect on the variation in GnRH cell number probably unlikely that aromatase alone is responsible for the difference in GnRH-ir neuron numbers.
Neonatal treatment with estradiol affects the uterus The exact mechanisms whereby estradiol mediates by causing a decrease in the number of estrogen GnRH cell function in the mammal brain are not receptors in the 2-month-old rat (Csaba and Incezefi- entirely clear. Several studies indicate that GnRH- Gonda, 1992). A similar effect may occur in the brain, producing neurons in a variety of mammalian species decreasing the estrogen receptor-binding capacity and do not possess estrogen receptors as adults (Herbison hence decreasing the adult sensitivity of GnRH neu- and Theodosis, 1992; Lehman and Karsch, 1993; Silver- rons to estradiol. Thus estradiol may have an activa- man et al., 1994). To our knowledge, there are no tional effect, but this would be masked by the de- existing studies that examine potential colocalization creased sensitivity of these neurons. Unfortunately, the of estrogen receptors in GnRH cells of preadult mam- design of this study prevents the testing of this hypoth- mals. Given that the GnRH gene in mice (Radovick et esis. Regardless, the lack of difference between the al., 1992) contains a region that binds the estrogen control and estradiol-treated females indicates that receptor (estrogen response elements), it is possible neonatal estradiol treatment does not appear to have that GnRH-producing neurons do express the estrogen an effect on the number of GnRH-producing neurons receptor during a specific developmental period. Tran- sient expression of estrogen receptors during develop- Males treated with estradiol show a significant ment has been shown in the rat cortex (Yokosuka et al., increase in the number of GnRH cells compared to 1995). Although the estrogen receptor has not been control males. At the same time, our data show no identified in vivo in adult GnRH cells, there are cell All rights of reproduction in any form reserved.
lines that do produce GnRH and may express the REFERENCES
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