Sexual differentiation of monoaminergic neurons--genetic or epigenetic?

Genotype-dependent sex differentiation of dopaminergic neurons in primary cultures of embryonic mouse brain

In order to investigate genetic factors that interfere with hormone-mediated sex differentiation of dopaminergic neurons, we raised sex-specific primary cultures from embryonic day 13 diencephalon (D) or mesencephalon (M) of three different strains of mice, NMRI, CBA/J, and BALBc/J. Part of the cultures were maintained for 6 or 13 days in vitro (DIV) in medium containing 17 beta-estradiol or testosterone. The cultures were analyzed for sex differences in numbers of tyrosine hydroxylase-immunoreactive neurons, endogenous dopamine (DA) levels, and specific uptake of [3H]DA. Previous results obtained with cultures of embryonic Sprague-Dawley rats had shown that these parameters develop sex-specific characteristics in the absence of sex differences in hormone environment. Similar steroid-independent sex differences as they occur in the rat were found in M cultures of NMRI but not in CBA and BALBc mice. Long-term sex steroid treatment did not affect any of the above parameters in any strain. It is concluded that cell-autonomous realization of the genetic sex of dopaminergic neurons depends on the genetic background.

Steroid imprinting and modulation of sexual dimorphism in the luteinizing hormone-releasing hormone neuronal system

1. Sex differences in the control of gonadotropin secretion and reproductive functions are a distinct characteristic in all mammalian species, including humans. Ovulation and cyclicity are among the most distinct neuroendocrine markers of female brain differentiation, along with sex behavioral traits that are also evident in different species. 2. The luteinizing hormone-releasing hormone (LHRH) neuronal system is the prime regulator of neuroendocrine events leading to ovulation and hormonal changes during the menstrual cycle and, as such, is the potential site where many of these sex differences may be expressed or, at the very least, integrated. However, until recently, no significant differences were seen in LHRH neurons between male and female brains, including cell number, pattern of distribution, and expression of message or peptide (LHRH) levels. 3. Recently, we reported that galanin (GAL), a brain-gut peptide, is coexpressed in LHRH neurons and that this coexpression is sexually dimorphic. When GAL is used as a marker for this neuronal system, it is clear that estradiol as well as progesterone profoundly affects the message and expression of the peptide and that this regulation, at least in rodents, is neonatally predetermined by gonadal steroid imprinting. 4. Changes in GAL expression and message can also be seen at puberty, during pregnancy and lactation, and in aging, all situations that affect the function of the LHRH neuronal system. Using an immortalized LHRH neuronal cell line (GT1) we have recently observed that these neurons express estrogen receptor (ER) and GAL and that estradiol can increase the expression of GAL, indicating functional activation of the endogenous ER.

Ontogeny of aromatase messenger ribonucleic acid and aromatase activity in the rat midbrain

Estrogen formation catalyzed by neural aromatase is crucial for the sexual differentiation of the brain. Ontogenic expression of aromatase mRNA and aromatase activity were studied in male and female rat midbrains. Aromatase mRNA was transiently expressed in both sexes showing maximum levels on postnatal day (P)2 and being absent on P20 and in adults. Developmental expression of aromatase mRNA preceded that of aromatase activity. These data demonstrate that the capacity for estrogen formation is present during a distinct phase of midbrain development. Our findings suggest an active role for estrogens in the differentiation of midbrain neurons.

Transcription of the Y chromosomal gene, Sry, in adult mouse brain

The Y chromosomal gene Sry encodes a putative transcription factor which appears to serve as a master switch initiating testicular development. Here we show that this gene is transcribed in hypothalamus, midbrain, and testis of adult male but not adult female mice. In contrast to its circular transcripts in adult testis, those in brain are linear and may be translated. We propose that Sry exerts a role in the regulation of sex differentiation of the mammalian nervous system.

Effects of sex and estrogen on tyrosine hydroxylase mRNA in cultured embryonic rat mesencephalon

In order to elucidate cellular events responsible for sex differentiation of the nigro-striatal system, we studied the influence of estrogen on the expression of tyrosine hydroxylase (TH) in sex-specific dissociated cell cultures of embryonic day 14 rat mesencephalon. Cultures were raised in the absence or presence of 17 beta-estradiol (10(-12) M) and hybridized with a [35S]oligonucleotide specific to TH. Cultured cells and tissues were probed for estrogen receptor (ER) transcripts by hemi-nested PCR. More TH mRNA containing cells were present in control cultures from female than from male donors. Estrogen treatment resulted in an up-regulation of TH expression in male cells only and induced a reversal of the sex difference in TH mRNA levels present in early control cultures. ER message was detectable in hypothalamic and uterine tissues but not in mesencephalic tissue or cultured cells. Estrogen exposure failed to induce ER expression in cultured mesencephalic cells. It is concluded that there are sex differences in TH mRNA expression of developing midbrain dopaminergic neurons which are independent of the steroid environment. Estrogen can up-regulate TH mRNA in a sex-specific fashion by modulating signal transduction mechanisms other than the classical nuclear receptor pathway.

Gender differences in the effects of haloperidol on avoidance conditioning in mice

Gender differences in the effects of haloperidol (0.075 mg/kg per day for 5 days) on avoidance conditioning were evaluated. We also studied performance of the subjects free of the drug and the acute effects of haloperidol in animals trained without drug 48 h after the last haloperidol administration. Latencies of escape and avoidance responses, number of nonresponses, escapes, avoidances, crossings during the adaptation period, crossings during intertrial intervals, and total crossings per minute were analyzed. This dosage impaired conditioning of the male animals but did not attain the same effects on females. Haloperidol did not deteriorate performance of the task when it had been learned previously without drug. The results confirm the existence of gender differences in haloperidol effects on avoidance conditioning in mice and suggest that these differences are related to the learning process and not only to the impairment of motor behavior characteristic of neuroleptic drugs.

Sexual differentiation of brain and behavior in quail and zebra finches: studies with a new aromatase inhibitor, R76713

In many species of vertebrates, major sex differences affect reproductive behavior and endocrinology. Most of these differences do not result from a direct genomic action but develop following early exposure to a sexually differentiated endocrine milieu. In rodents, the female reproductive phenotype mostly develops in the absence of early steroid influence and male differentiation is imposed by the early action of testosterone, acting at least in part through its central conversion into estrogens or aromatization. This pattern of differentiation does not seem to be applicable to avian species. In Japanese quail (Coturnix japonica), injection of estrogens into male embryos causes a permanent loss of the capacity to display male-type copulatory behavior when exposed to testosterone in adulthood. Based on this experimental result, it was proposed that the male reproductive phenotype is "neutral" in birds (i.e. develops in the absence of endocrine influence) and that endogenous estradiol secreted by the ovary of the female embryo is responsible for the physiological demasculinization of females. This model could be recently confirmed. Females indeed display a higher level of circulating estrogens that males during the second part of their embyronic life. In addition, treatment of female embryos with the potent aromatase inhibitor, R76713 or racemic vorozole which suppresses the endogenous secretion of estrogens maintains in females the capacity to display the full range of male copulatory behaviors. The brain mechanisms that control this sexually differentiated behavior have not been identified so far but recent data suggest that they should primarily concern a sub-population of aromatase-immunoreactive neurons located in the lateral parts of the sexually dimorphic preoptic nucleus. The zebra finch (Taeniopygia guttata) exhibits a more complex, still partly unexplained, differentiation pattern. In this species, early treatment with exogenous estrogens produces a masculinization of singing behavior in females and a demasculinization of copulatory behavior in males.(ABSTRACT TRUNCATED AT 400 WORDS)

Different neuronal and glial cell groups in corticomedial amygdala react differently to neonatally administered estrogen

The percentage of labeled neurons and glial cells in the phylogenetically older corticomedial part of the amygdala was investigated in control and estrogen-treated rats using [3H]thymidine autoradiography. Newborn, three-day-old female and male Wistar rats were treated with a single dose of 1 mg of estrogen and killed at the age of 10 days. The percentage of labeled neuronal and glial cells was determined by stereological methods in the medial, cortical and central nuclei, respectively. In treated male rats, the percentage of labeled neurons and glial cells in these nuclei was significantly greater on the 10th day of life as compared to controls, except for glial cells in the nucleus centralis. In treated female rats, the percentage of labeled neurons in all three nuclei was comparable to controls, while the percentage of labeled glial cells was increased in the nucleus medialis, decreased in the nucleus centralis and unchanged in the nucleus corticalis. Our results indicate clear sex- and region-specific differences in the reactivity of both neurons and glia to neonatally administered estrogen.

Sex dimorphic alterations in postnatal brain catecholamines after gestational morphine

The concentration of brain catecholamines was measured in the hypothalamus, preoptic area (POA), frontal cortex, cerebellum, and striatum of rats exposed in utero to morphine (5-10 mg/kg/twice daily) during gestation days 11-18. Prenatal morphine induced regionally specific, sexually dimorphic alterations in male and female norepinephrine (NE), and dopamine (DA) content at different postnatal ages. Prenatal morphine significantly increased NE content in the hypothalamus of both sexes at postnatal day (PND) 23. In the POA, on the other hand, morphine increased NE content in exposed males at PND 23 and in females at PND 33. In the cerebellum, the NE content of both sexes was significantly elevated at PND 45. In the striatum, NE content was increased by the prenatal morphine only in females at PND 16. The concentration of DA was also affected in a sexually dimorphic manner. At PND 16, prenatal morphine increased the levels of hypothalamic DA only in males, and it reduced the content of DA in female but not male POA. At PND 45, prenatal morphine increased DA in the hypothalamus of females and decreased it in males. In the cerebellum of 16-day-old morphine-exposed animals, DA levels were increased only in males; at PND 45, the levels of DA were still increased in males but had not changed in females. In the striatum, the DA content was reduced only in males at PND 16. Thus, prenatal morphine alters the development of both NE and DA neurotransmitter systems in the hypothalamus, POA, striatum, and cerebellum in a sexually dimorphic manner.

Sexual differentiation of brain and behavior in birds

It is currently accepted that most sex differences in brain and behavior do not result from direct genomic actions, but develop following early exposure to a sexually differentiated endocrine milieu. In Japanese quail (Coturnix japonica), in contrast to rodents, the male reproductive phenotype appears to develop in the absence of endocrine influence, and estradiol secreted by the ovary of the female embryo is responsible for the physiologic demasculinization of females. In zebra finches (Taeniopygia guttata), estrogens administered early in life demasculinize copulatory behavior in males, but masculinize the vocal control regions in the brain and singing behavior of females. It is difficult to understand how these behaviors differentiate given that normal untreated males sing and copulate in a male-typical manner, whereas females never show these behaviors. All attempts to resolve this paradox with experiments based on the rodent model of sexual differentiation have been unsuccessful. We propose that copulatory behavior in zebra finches is differentiated in a manner similar to what has been described in quail, but that novel approaches need to be considered to understand the differentiation of the telencephalic song control system. In particular, the possible involvement of afferent input that may differentiate in a steroid-dependent or -independent manner should be thoroughly tested.

Differentiation of hypothalamic GABAergic neurons in vitro: absence of effects of sex and gonadal steroids

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is involved in the control of sexually dimorphic brain functions, such as pituitary secretion and reproductive behavior. Hypothalamic GABAergic systems in vivo exhibit sexually dimorphic functional properties. Sexual dimorphisms in the rat brain are currently thought to be brought about by the organizational influence of gonadal steroids during the perinatal developmental period. The present study is concerned with the question of whether developing hypothalamic GABAergic neurons are primary targets of sex hormones. Since it is impossible to distinguish direct from indirect effects of experimental manipulations of the hormonal environment of the in vivo brain, sex-specific primary cultures raised from embryonic day 14 rat diencephalon and cultured for up to 8 days in vitro (DIV) were used as a model system. Effects of sex steroids were investigated on high affinity uptake of [3H]GABA. GABA transport was already mature at 3 DIV. [3H]GABA uptake was sensitive to inhibition by nipecotic acid and the transmitter was taken up by high affinity transport (Km = 15.2 microM). Immunocytochemical preparations demonstrated extensive networks of GABA-immunoreactive fibers at 8 DIV. Concomitantly with the outgrowth of neurites, there was a marked increase in maximum uptake velocity (Vmax). No differences could be detected regarding cell numbers or uptake kinetics between cultures from male and female donors. Neither cell numbers nor GABA uptake were affected by short- and long-term treatment with estradiol-17 beta or testosterone. It appears that hypothalamic GABAergic neurons in vitro do not develop sex differences in cell numbers or GABA transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Steroid-neuropeptide interactions that control reproductive behaviors in an amphibian

Investigations into the neuroendocrine regulation of reproductive behaviors in an amphibian (Taricha granulosa) reveal the same basic repertoire of chemical messengers as regulators of male behaviors in other vertebrates. These studies have identified seasonal neural interactions between gonadal steroids and neuropeptides that facilitate male courtship behavior. In addition, this species has served to elucidate how stress-induced suppression of courtship is mediated by corticosterone action through a neuronal membrane receptor and subsequent, rapid neurophysiological effects. These findings indicate that a principal mechanism by which steroids and neuropeptides control male reproductive behavior is the modulation of neural processing of specific sensory stimuli.

Social control of primary sex differentiation in the Midas cichlid

Sexual differentiation in teleost fishes is characteristically labile. The most dramatic form of sexual lability is postmaturational sex change, which is common among teleosts although rare or absent in other vertebrate taxa. In many cases this process is regulated by social cues, particularly dominance interactions. Here we show that in the Midas cichlid, Cichlasoma citrinellum, these same sorts of social interactions affect much earlier stages of sexual differentiation. In this species, males are larger than females. By manipulating relative size in juveniles, we show that this sex-based size difference does not arise from endogenous factors associated with sex. Rather, sex is determined by relative size as a juvenile. We argue that this mode of sex determination, which may be common among teleosts, is a heterochronic variant of postmaturational sex change, one in which some individuals are deflected from a default female trajectory before maturation, as a result of social signals. The size-advantage model, which specifies the optimal size for sex change in hermaphroditic species, can be extended to account for the decision whether to mature as a male or a female in the Midas cichlid.

Neonatal imprinting predetermines the sexually dimorphic, estrogen-dependent expression of galanin in luteinizing hormone-releasing hormone neurons

The incidence of colocalization of galanin (GAL) in luteinizing hormone-releasing hormone (LHRH) neurons is 4- to 5-fold higher in female than male rats. This fact and the finding that the degree of colocalization parallels estradiol levels during the estrous cycle suggest that GAL is an estrogen-inducible product in a subset of LHRH neurons. To analyze further this paradigm we evaluated the effects of gonadectomy and steroid replacement therapy in male and female rats. Ovariectomy resulted in a significant decrease in the number of cells colocalizing LHRH and GAL, whereas estradiol replacement to such animals restored the incidence of colocalization to that observed in controls. In males, however, estradiol treatment failed to enhance the incidence of colocalization of GAL and LHRH, indicating, therefore, that the colocalization of these peptides is gender-determined. This possibility--i.e., gender-specific determination of LHRH neurons coexpressing GAL--was evaluated by neonatal manipulation of hypothalamic steroid imprinting. As mentioned above, male rats did not respond to estrogen or testosterone by increasing GAL/LHRH colocalization as females did. Neonatally orchidectomized rats, whose hypothalami have not been exposed to testosterone during the critical period, when treated with estrogen in adulthood showed an increase in colocalization of GAL and LHRH similar to that seen in female animals. These observations indicate that the colocalization of LHRH/GAL is neonatally determined by an epigenetic mechanism that involves the testis. In summary, this sex difference in the incidence of colocalization of GAL and LHRH represents a unique aspect of sexual differentiation in that only certain phenotypic characteristics of a certain cellular lineage are dimorphic. The subpopulation of LHRH neurons that also produces GAL represents a portion of the LHRH neuronal system that is sexually differentiated and programed to integrate, under steroidal control, a network of LHRH neurons that could synchronize their activity to control the estrous cycle in rats.

The catecholaminergic system of the quail brain: immunocytochemical studies of dopamine beta-hydroxylase and tyrosine hydroxylase

The distribution of dopamine beta-hydroxylase and tyrosine hydroxylase, two key enzymes in the biosynthesis of catecholamines, was investigated by immunocytochemistry in the brain of male and female Japanese quail. Cells or fibers showing dopamine beta-hydroxylase and tyrosine hydroxylase immunoreactivity were considered to be noradrenergic or adrenergic, while all structures showing only tyrosine hydroxylase immunoreactivity were tentatively considered to be dopaminergic. The major dopaminergic and noradrenergic cell groups that have been identified in the brain of mammals could be observed in the Japanese quail, with the exception of a tuberoinfundibular dopaminergic group. The dopamine beta-hydroxylase-immunoreactive cells were found exclusively in the pons (locus ceruleus and nucleus subceruleus ventralis) and in the medulla (area of the nucleus reticularis). The tyrosine hydroxylase-immunoreactive cells had a much wider distribution and extended from the preoptic area to the level of the medulla. They were, however, present in larger numbers in the area ventralis of Tsai and in the nucleus tegmenti pedunculo-pontinus, pars compacta, which respectively correspond to the ventral tegmental area and to the substantia nigra of mammals. A high density of dopamine beta-hydroxylase- and tyrosine hydroxylase-immunoreactive fibers and punctate structures was found in several steroid-sensitive brain regions that are implicated in the control of reproduction. In the preoptic area and in the region of the nucleus accumbens-nucleus stria terminalis, immunonegative perikarya were completely surrounded by immunoreactive fibers forming basket-like structures. Given that some of these cells contain the enzyme aromatase, these structures may represent the morphological substrate for a regulation of aromatase activity by catecholamines. The dopamine beta-hydroxylase-immunoreactive fibers were also present in a larger part of the preoptic area of females than in males. This sex difference in the noradrenergic innervation of the preoptic area presumably reflects the sex difference in norepinephrine content in this region.

Sex steroids do not alter sex differences in tyrosine hydroxylase activity of dopaminergic neurons in vitro

In order to distinguish the effects of genetic sex from those of sex hormones on the sexual differentiation of dopaminergic neurons, catecholamine synthesis was studied in gender-specific cultures of embryonic day-14 rat diencephalon. In addition to embryos from normal dams, embryos were used whose mothers had been treated with the estrogen antagonist tamoxifen or the testosterone antagonist cyproterone acetate on days 12 and 13 of gestation. Cultures from embryos of untreated dams were fed daily with a medium containing 17 beta-estradiol or testosterone. After 10 days in vitro, cultures were immunostained for tyrosine hydroxylase and the accumulation of dihydroxyphenylalanine (DOPA) was measured in the presence of the DOPA decarboxylase inhibitor NSD 1015. Rates of DOPA synthesis, unlike the numbers of tyrosine hydroxylase-immunoreactive neurons, were markedly higher in female cultures under all experimental conditions. Treatment of dams with antisteroids prior to removal of the embryos had no influence on these results. Treatment of cultures with both steroids decreased DOPA formation in a dose-dependent manner without altering the sex difference. These results suggest that cultured diencephalic dopaminergic neurons develop sex differences in the activity of tyrosine hydroxylase. This sexual dimorphism is initiated independently on the activity of gonadal steroid hormones. Sex hormones exert an additional modulatory influence on the activity of the enzyme but do not abolish or reverse sex differences. Therefore, the concept of a purely epigenetic mode of sexual differentiation of the mammalian brain needs to be broadened to incorporate other mechanisms, such as the cell-autonomous fulfillment of a sex-specific genetic program.

Sex differences and estrous cycle-variations in the AF64A-induced cholinergic deficit in the rat hippocampus

The influence of gender and stage of the estrous cycle on the levels of acetylcholine, serotonin, and noradrenaline in the hippocampus and on the susceptibility of the cholinergic septo-hippocampal pathway to the neurotoxic effect of ethylcholine aziridinium (AF64A) was investigated in the rat. Levels of acetylcholine and serotonin were consistently higher in female rats during the stage of diestrus and proestrus than in age-matched male rats (p < 0.05). Across the estrous cycle the highest levels of acetylcholine and serotonin, coinciding with the lowest levels of noradrenaline, were measured on proestrus. Eight to 10 days after the bilateral intracerebroventricular injection of a submaximal dose of AF64A (1 nmol/ventricle) the decrease of acetylcholine in hippocampus was larger in females than in male rats. The reduction of acetylcholine was most pronounced in female rats that had received submaximal doses of AF64A on proestrus (42.7 +/- 3.4%), whereas in male rats, the corresponding decrease was 25.9 +/- 5.1% (p < 0.05). At a maximal dose of AF64A (2 nmole/ventricle), the sex-specific or cycle-dependent difference in the cholinotoxicity of AF64A vanished. The dose-dependent loss of acetylcholine was associated with a secondary dose-dependent decrease in the levels of serotonin and noradrenaline, but significant differences between male and female rats or stages of estrous cycle were not apparent. The present data provide evidence that adult female rats in general, and particularly females on proestrus, are more susceptible to the neurotoxic action of submaximal doses of AF64A than age-matched male rats.

Neuronal cell cultures: a tool for investigations in developmental neurobiology

The aim of this review is to describe environmental requirements for survival of neuronal cells in culture, and secondly to survey the complex interplay between hormones, neurotrophic factors, transport- and extracellular matrix- proteins, which characterize the developmental program of differentiating neurons. An overall reconsideration of the literature in this vast field is above the limits of the present paper; since progress and refinement in the techniques of neuronal cell cultures have paralleled the advancement in Developmental Neurobiology, we will run instead through the main steps which form the conceptual framework of neuronal cell cultures.

Sex differences of hypothalamic prolactin cells develop independently of the presence of sex steroids

There is evidence for a hypothalamic prolactin (PRL) system that expresses sexually dimorphic traits. The aim of this in vitro study is to gain an insight into the process of sexual differentiation of hypothalamic PRL cells. In particular, we wanted to determine whether sexual differentiation of these cells can occur independently of the surge of gonadal testosterone which, in the male rat embryo, takes place at embryonic day (E) 18 and is commonly believed to start the critical period of sexual differentiation of the brain. Gender-specific cell cultures were prepared from E 14 or E 17 rat diencephalon and raised in the absence of gonadal steroids. After 10 days in vitro, numbers of PRL-immunoreactive (IR) cells and PRL levels were quantified by immunocytochemistry and Western blotting, respectively. Numbers of PRL-IR cells and PRL levels were 2-3 times higher in cultures prepared from female than from male embryos of either age. It is concluded that sexual differentiation of hypothalamic PRL cells starts well before the generally acknowledged onset of the critical period and may proceed independently of the action of gonadal testosterone. Besides gonadal steroids, other mechanisms, such as cell-intrinsic realization of a sex-specific genetic program, may be responsible for initiating the development of sexually dimorphic neuronal phenotypes.

From an animal model of an attentional deficit towards new insights into the pathophysiology of schizophrenia

The paper presents an animal model of schizophrenic-like attentional deficit, consisting of an inability to ignore irrelevant stimuli. It is based on the paradigm of latent inhibition (LI), in which animals learn to ignore repeatedly presented stimuli not followed by meaningful consequences. In a series of experiments it was demonstrated that the capacity to ignore irrelevant stimuli is lost in rats treated with systemic or intra-accumbens injections of amphetamine, in normal volunteers given amphetamine, in high "psychosis-prone" persons, in acute schizophrenic patients and in untreated male adult rats that were raised until weaning under conditions of extremely restricted stimulation. In addition, LI is lost following the disruption of the hippocampal input to the nucleus accumbens. In all of the above conditions tested for antagonism by anti-psychotic drugs a loss of LI is reversed. On the basis of these results we propose an animal model which accommodates a neurodevelopmental dysfunction, hippocampal pathology, mesolimbic DA overactivity, vulnerability to stress, and gender differences, all of which have been postulated as factors in the pathophysiology of schizophrenia.

Sex differences in densities of dopaminergic fibers and GABAergic neurons in the prenatal rat striatum

On the basis of observations on dopaminergic neurons developing in gender-specific cultures of embryonic rat mesencephalon, we have hypothesized that as yet unknown sexual dimorphisms might be found in projection areas of dopaminergic neurons. Therefore we searched for possible sex differences in the striatum during the period when massive ingrowth of mesencephalic afferents occurs and the striatal gamma-aminobutyric acid (GABA)ergic neurons differentiate. Male and female rats of embryonic days (E) 16, 18, 20, and 21 were fixed by perfusion through the heart. Vibratome sections were cut from the striatal anlage and sequentially immunostained for GABA by the immunogold-silver technique and tyrosine hydroxylase (TH) by the avidin-biotin-peroxidase method. Ultrathin sections were scanned for numbers of GABA- and TH-immunoreactive (IR) elements. Densities of TH-IR axons as well as of GABA-IR cell body profiles progressed with time. Contacts between TH-IR axons and GABA-IR and immunonegative cells were observed as early as E-16, increasing in numbers toward later stages. Throughout prenatal development, female striata displayed higher densities of both TH-IR axon and GABA-IR cell body profiles than male ones. This is the first report of a distinct anatomical sex difference regarding two major components of a key center of motor control. Prenatal sexual differentiation of the striatum may lead to a sexually dimorphic extrapyramidal circuitry, the existence of which, in the adult, is suggested by experimental and clinical data.

Sexual dimorphism of the dopamine-beta-hydroxylase-immunoreactive neurons in the rat locus ceruleus

Sex differences in the noradrenaline synthesizing neurons of the locus ceruleus (LC) in rat brain were investigated immunocytochemically using an antibody to dopamine-beta-hydroxylase. Female adult rats contained a greater structural volume and average somatic area in the anterior intermediate region of the nucleus compared with males. Whether this difference is related to the endocrine status of the animals, and consequently a functionally distinct population of neurons, is yet to be determined.

Absence of sex differences in size of the genital ducts of the rat prior to embryonic day 15.5-16.0

Sexual dimorphisms of the rat brain are generally believed to be brought about by the presence of testosterone during a critical period starting at embryonic day (ED) 17/18. In contrast, sex differences of diencephalic and mesencephalic dopaminergic neurons were observed to develop in cell cultures raised from ED 14 rat brains. This was interpreted as evidence indicating that sexual differentiation of certain neural systems may occur independently of gonadal hormones. To substantiate this claim, it was felt necessary to examine the rat embryo for clues to a possible existence of sex differences in hormonal environment prior to ED 17. Morphometry was applied to compare the development of male and female Wolffian and Müllerian ducts, both primary targets of hormones secreted from the male gonad. Diameters of serially cross-sectioned Wolffian and Müllerian ducts were measured in rats of ED 15.0 to ED 16.5. Females had thicker Müllerian ducts from ED 15.5 on. The first step of differentiation in males was the widening of the lumen and a slight increase of the outer diameter of the Wolffian duct at ED 16.0. The size differences of both ducts were most obvious in the vicinity of the lower half of the gonad. Except in Wolffian ducts of ED 16.5, sex differences were absent in the caudal parts of the ducts. It appears that gonadal androgen and Müllerian inhibiting substance do not affect the development of their classical target organs prior to ED 16.0 and ED 15.5, respectively. Furthermore, the first effects are paracrine in nature. There is no evidence for sex differences in systemic androgen environment until ED 16.5.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex-specific schedule in steroid response of rhombencephalic catecholaminergic neurons in vitro

Morphological differentiation of tyrosine hydroxylase-immunoreactive neurons was investigated in dissociated cell cultures of rhombencephalon of male and female day 14 rat embryos grown in the presence or absence of sex steroids. Numbers of cells were counted and morphometrical measurements carried out of soma size and length of tyrosine hydroxylase-immunoreactive neurites (processes). Subtle sex differences in length of stained neurites, which were not yet present after 3 days in vitro, were observed after 6 days in cultures grown in the absence of sex steroids. Female tyrosine hydroxylase-immunoreactive neurites could be traced over longer distances than male ones. Daily treatment of cultures with testosterone or 17 beta-estradiol resulted in an increase of lengths of stained neurites of female neurons after 3 days and of male neurons after 6 days in vitro. Regarding cell numbers or soma size, there were no differences between genders or between controls and hormone-treated cultures. It is concluded that sex steroids promote the outgrowth of neurites from noradrenergic neurons within a gender-specific time frame. It appears that the critical period for developmental effects of sex steroids differs between males and females.