Frank P. M. Kruijver

Sex differences in the distribution of androgen receptors in the human hypothalamus

The present study reports for the first time the distribution of androgen receptor immunoreactivity (AR‐ir) in the human hypothalamus of ten human subjects (five men and five women) ranging in age between 20 years and 39 years using the antibody PG21. Prolonged postmortem delay (72:00 hours) or fixation time (100 days) did not influence the AR‐ir. In men, intense nuclear AR‐ir was found in neurons of the horizontal limb of the diagonal band of Broca, in neurons of the lateromamillary nucleus (LMN), and in the medial mamillary nucleus (MMN). An intermediate nuclear staining was found in the diagonal band of Broca, sexually dimorphic nucleus of the preoptic area, paraventricular nucleus, suprachiasmatic nucleus, ventromedial nucleus, and infundibular nucleus, whereas weaker labeling was found in the bed nucleus of the stria terminalis, medial preoptic area, dorsal and ventral zones of the periventricular nucleus, supraoptic nucleus, and nucleus basalis of Meynert. In most brain areas, women revealed less staining than men. In the LMN and the MMN, a strong sex difference was found. Cytoplasmic labeling was observed in neurons of both sexes, although women showed a higher variability in the intensity of such staining. However, no sex differences in AR‐ir were observed in the bed nucleus of the stria terminalis, the nucleus basalis of Meynert, or the islands of Calleja. Species differences and similarities of the AR‐ir distribution are discussed. The present results suggest the participation of androgens in the regulation of various hypothalamic processes that are sexually dimorphic. J. Comp. Neurol. 425:422–435, 2000.

Sex hormone receptors are present in the human suprachiasmatic nucleus.

The suprachiasmatic nucleus (SCN) is the clock of the brain that orchestrates circadian and circannual biological rhythms, such as the rhythms of hormones, body temperature, sleep and mood. These rhythms are frequently disturbed in menopause and even more so in dementia and can be restored in postmenopausal women by sex hormone replacement therapy (SHRT). Although it seems clear, both from clinical and experimental studies, that sex hormones influence circadian rhythms, it is not known whether this is by a direct or an indirect effect on the SCN. Therefore, using immunocytochemistry in the present study, we investigated whether the human SCN expresses sex hormone receptors in 5 premenopausal women and 5 young men. SCN neurons appeared to contain estrogen receptor-α (ERα), estrogen receptor-β (ERβ) and progesterone receptors. Median ratings of ER immunoreactivity per individual and per gender group revealed a statistically significantly stronger nuclear ERα expression pattern in female SCN neurons (p < 0.05). No significant sexual dimorphic tendency was observed for nuclear ERβ (p > 0.1) and progesterone receptors (p > 0.7). These data seem to support previously reported functional and structural SCN differences in relation to sex and sexual orientation and indicate for the first time that estrogen and progesterone may act directly on neurons of the human biological clock. In addition, the present findings provide a potential neuroendocrine mechanism by which SHRT can act to improve or restore SCN-related rhythm disturbances, such as body temperature, sleep and mood.

Estrogen receptor‐α distribution in the human hypothalamus in relation to sex and endocrine status

The present study reports the first systematic rostrocaudal distribution of estrogen receptor‐α immunoreactivity (ERα‐ir) in the human hypothalamus and its adjacent areas in young adults. Postmortem material taken from 10 subjects (five male and five female), between 20 and 39 years of age, was investigated. In addition, three age‐matched subjects with abnormal levels of estrogens were studied: a castrated, estrogen‐treated 50‐year‐old male‐to‐female transsexual (T1), a 31‐year‐old man with an estrogen‐producing tumor (S2), and an ovariectomized 46‐year‐old woman (S8). A strong sex difference, with more nuclear ERα‐ir in women, was observed rostrally in the diagonal band of Broca and caudally in the medial mamillary nucleus. Less robust sex differences were observed in other brain areas, with more intense nuclear ERα‐ir in men, e.g., in the sexually dimorphic nucleus of the medial preoptic area, paraventricular nucleus, and lateral hypothalamic area, whereas women had more nuclear ERα‐ir in the suprachiasmatic nucleus and ventromedial nucleus. No nuclear sex differences in ERα were found, e.g., in the central part of the bed nucleus of the stria terminalis. In addition to nuclear staining, ERα‐ir appeared to be sex‐dependently present in the cytoplasm of neurons and was observed in astrocytes, plexus choroideus, and other non‐neuronal cells. ERα‐ir in T1, S2, and S8 suggested that most of the observed sex differences in ERα‐ir are “activational” (e.g., ventromedial nucleus/medial mamillary nucleus) rather than “organizational.” Species similarities and differences in ERα‐ir distribution and possible functional implications are discussed. J. Comp. Neurol. 454:115–139, 2002.

Estrogen‐receptor‐β distribution in the human hypothalamus: Similarities and differences with ERα distribution

This study reports the first systematic rostrocaudal distribution of estrogen receptor beta immunoreactivity (ERβ‐ir) in the human hypothalamus and adjacent areas in five males and five females between 20–39 years of age and compares its distribution to previously reported ERα in the same patients. ERβ‐ir was generally observed more frequently in the cytoplasm than in the nucleus and appeared to be stronger in women. Basket‐like fiber stainings, suggestive for ERβ‐ir in synaptic terminals, were additionally observed in various areas. Men showed more robust nuclear ERβ‐ir than women in the medial part of the bed nucleus of the stria terminalis, paraventricular and paratenial nucleus of the thalamus, while less intense, but more nuclear, ERβ‐ir appeared to be present in, e.g., the BSTc, sexually dimorphic nucleus of the medial preoptic area, diagonal band of Broca and ventromedial nucleus. Women revealed more nuclear ERβ‐ir than men of a low to intermediate level, e.g., in the suprachiasmatic, supraoptic, paraventricular, infundibular, and medial mamillary nucleus. These data indicate potential sex differences in ERβ expression. ERβ‐ir expression patterns in subjects with abnormal hormone levels suggests that there may be sex differences in ERβ‐ir that are “activational” rather than “organizational” in nature. Similarities, differences, potential functional, and clinical implications of the observed ERα and ERβ distributions are discussed in relation to reproduction, autonomic‐function, mood, cognition, and neuroprotection in health and disease. J. Comp. Neurol. 466:251–277, 2003.

Structural and Functional Sex Differences in the Human Hypothalamus

Sex differences in the brain may be the basis not only for sex differences in reproduction, gender identity (the feeling of being male or female), and sexual orientation (heterosexuality vs homosexuality), but also for the sex difference in prevalence of psychiatric and neurological diseases ( Swaab and Hofman, 1995 ). In this brief article we discuss a few examples of structural and functional sex differences in the human brain.

Early social stress in female guinea pigs induces a masculinization of adult behavior and corresponding changes in brain and neuroendocrine function.

This study was undertaken to investigate, in guinea pigs, the effects of pre- and early postnatal social stress on the functioning of hormonal-, autonomic-, behavioral-, and limbic-brain systems. Dams had either lived in groups with a constant composition (i.e. stable social environment) or in groups with changing compositions, that means every 3 days two females were transferred from one group to another (i.e. unstable social environment). The subjects studied were female offspring of dams who had either lived in a stable social environment during pregnancy and lactation (i.e. control daughters, CF) or in an unstable social environment during this period of life (i.e. early stressed daughters, SF). After weaning, each five groups of CF and SF, consisting of two females each, were established. The spontaneous behavior of the females was recorded, blood samples were taken to determine cortisol, testosterone, dehydroepiandrosterone, dehydroepiandrosterone sulfate and estrogen levels, the adrenals were prepared to determine tyrosinehydroxylase (TH) activities and the brains to investigate the distribution of sex hormone receptors. SF showed not only a behavioral and endocrine masculinization, but also an upregulation of androgen receptor and estrogen receptor-alpha in the medial preoptic area and the nucleus arcuatus of the hypothalamus, the nucleus paraventricularis of the thalamus, and the CA1 region of the hippocampus. These findings corresponded with distinctly elevated serum-concentrations of testosterone and increased activities of the adrenal TH. In conclusion, early social stress caused by an unstable social environment induces in female guinea pigs a permanent behavioral masculinization that is accompanied by changes in the endocrine and autonomic system as well as by changes in the distribution of sex hormone receptors in the limbic system.

Sex differences in the hypothalamus in the different stages of human life

Quite a number of structural and functional sex differences have been reported in the human hypothalamus and adjacent structures that may be related to not only reproduction, sexual orientation and gender identity, but also to the often pronounced sex differences in prevalence of psychiatric and neurological diseases. One of the recent focuses of interest in this respect is the possible beneficial effect of sex hormones on cognition in Alzheimer patients. The immunocytochemical localization of estrogen receptors (ER) alpha, beta and androgen receptors has shown that there are indeed numerous targets for sex hormones in the adult human brain. Observations in the infundibular nucleus have, however, indicated that in this brain area the hyperactivity resulting from a lack of estrogens in the menopause seems to protect females against Alzheimer changes, in contrast to males. It is thus quite possible that estrogen replacement therapy may, in these brain areas, lead to inhibition of neuronal metabolism and thus to the same proportion of Alzheimer changes as are observed in men. Knowledge about the functional sex differences in the brain and the effect of sex hormones on neuronal metabolism may thus provide clues not only for the possible beneficial effects of these hormones (e.g., on cognition or hypertension), but also on possible central side effects of estrogen replacement therapy.

Sexual Differentiation of the Human Hypothalamus

Functional sex differences in reproduction, gender and sexual orientation and in the incidence of neurological and psychiatric diseases are presumed to be based on structural and functional differences in the hypothalamus and other limbic structures. Factors influencing gender, i.e., the feeling to be male or female, are prenatal hormones and compounds that change the levels of these hormones, such as anticonvulsants, while the influence of postnatal social factors is controversial. Genetic factors and prenatal hormone levels are factors in the determination of sexual orientation, i.e. heterosexuality, bisexuality or homosexuality. There is no convincing evidence for postnatal social factors involved in the determination of sexual orientation. The period of overt sexual differentiation of the human hypothalamus occurs between approximately four years of age and adulthood, thus much later than is generally presumed, although the late sexual differentiation may of course be based upon processes that have already been programmed in mid-pregnancy or during the neonatal period. The recently reported differences in a number of structures in the human hypothalamus and adjacent structures depend strongly on age. Replication of these data is certainly necessary. Since the size of brain structures may be influenced by premortem factors (e.g. agonal state) and postmortem factors (e.g. fixation time), one should not only perform volume measurements, but also estimate a parameter that is not dependent on such factors as, i.e., total cell number of the brain structure in question. In addition, functional differences that depend on the levels of circulating hormones in adulthood have been observed in several hypothalamic and other brain structures. The mechanisms causing sexual differentiation of hypothalamic nuclei, the pre- and postnatal factors influencing this process, and the exact functional consequences of the morphological and functional hypothalamic differences await further elucidation.

Androgens and male behavior

Sexual differentiation into a male or a female includes sexual differentiation of the brain. The paradigm of mammalian sexual differentiation is that in the presence of androgens (normally produced by the fetal testis) a male brain differentiation occurs, while in the absence of androgens (normal in females) a female brain differentiation follows. In the human there is a sex-dimorphism in gender identity/role, sexual orientation, sexual functioning, and in non-sexual functions, such as spatial ability, and verbal fluency. Inasmuch these properties can be studied in other mammals the effects of androgens are solidly demonstrable. In the human the evidence for androgen effects is equally plausible, evident from observations in subjects with errors in the process of sexual differentiation and in morphological studies of brain structures presumably related to these properties. But clinical observations show compellingly that other, largely unidentified, factors may modulate, or even override the effects of androgens.

Galanin neurons in the intermediate nucleus (InM) of the human hypothalamus in relation to sex, age and gender identity.

The intermediate nucleus (InM) in the preoptic area of the human brain, also known as the sexually dimorphic nucleus of the preoptic area (SDN‐POA) and the interstitial nucleus of the anterior hypothalamus‐1 (INAH‐1) is explored here. We investigated its population of galanin‐immunoreactive (Gal‐Ir) neurons in relation to sex, age, and gender identity in the postmortem brain of 77 subjects. First we compared the InM volume and number of Gal‐Ir neurons of 22 males and 22 females in the course of aging. In a second experiment, we compared for the first time the InM volume and the total and Gal‐Ir neuron number in 43 subjects with different gender identities: 14 control males (M), 11 control females (F), 10 male‐to‐female (MtF) transsexual people, and 5 men who were castrated because of prostate cancer (CAS). In the first experiment we found a sex difference in the younger age group (<45 years of age), i.e., a larger volume and Gal‐Ir neuron number in males and an age difference, with a decrease in volume and Gal‐Ir neuron number in males > 45 years. In the second experiment the MtF transsexual group presented an intermediate value for the total InM neuron number and volume that did not seem different in males and females. Because the CAS group did not have total neuron numbers that were different from the intact males, the change in adult circulating testosterone levels does not seem to explain the intermediate values in the MtF group. Organizational and activational hormone effects on the InM are discussed. J. Comp. Neurol. 519:3061–3084, 2011.