Sex differences and maturational changes in arcuate nucleus neuronal plasma membrane organization

Estrogen-Induced Hypothalamic Synaptic Plasticity and Pituitary Sensitization in the Control of the Estrogen-Induced Gonadotrophin Surge

Proper gonadal function requires coordinated (feedback) interactions between the gonads, adenohypophysis, and brain: the gonads elaborate sex steroids (progestins, androgens, and estrogens) and proteins (inhibin-activin family) during gamete development. In both sexes, the brain-pituitary gonadotrophin-regulating interaction is coordinated by estradiol through its opposing actions on pituitary gonadotrophs (sensitization of the response to gonadotrophin-releasing hormone [GnRH]) versus hypothalamic neurons (inhibition of GnRH secretion). This dynamic tension between the gonadotrophs and the GnRH cells in the brain regulates the circulating gonadotrophins and is termed reciprocal/negative feedback. In females, reciprocal/negative feedback dominates approximately 90% of the ovarian cycle. In a spectacular exception, the dynamic tension is broken during the surge of circulating estrogen that marks follicle and oocyte(s) maturation. The cause is an estradiol-induced disinhibition of the GnRH neurons that releases GnRH secretion to the highly sensitized pituitary gonadotrophs that in turn release the gonadotrophin surge (the estrogen-induced gonadotrophin surge [EIGS], also known as positive feedback). Studies during the past 4 decades have shown this disinhibition to result from estrogen-induced synaptic plasticity (EISP), including a reversible approximately 50% loss in arcuate nucleus synapses. The disinhibited GnRH secretion occurs during maximal gonadotroph sensitization and results in the EIGS. Specific immunoneutralization of estradiol blocks the EISP and EIGS. The EISP is accompanied by increases in insulinlike growth factor 1, polysialylated neural cell adhesion molecule, and ezrin, 3 proteins that the authors believe are the links between estrogen-induced astroglial extension and the EISP that releases GnRH secretion at the moment of maximal sensitization of the pituitary gonadotrophs. The result is the paradoxical surge of gonadotrophins at the peak of ovarian estrogen secretion and the triggering of ovulation. This enhanced understanding of the mechanics of gonadotrophin control clarifies elements of the involved feedback loops and opens the way to a better understanding of the neurobiology of reproduction.

Effects of perinatal overfeeding on mechanisms controlling food intake and body weight homeostasis

The prevalence of overweight and obesity in most developed countries has markedly increased during the last several decades. In addition to genetic, hormonal and metabolic influences, epigenetic environmental factors, such as fetal and neonatal nutrition, play a key role in the development of obesity. Interestingly, becoming overweight during critical developmental periods of fetal and/or neonatal life has been shown to continue throughout juvenile life into adulthood. In spite of this evidence, the specific biological mechanisms underlying this fetal/neonatal programming are not perfectly understood. However, it is clear that circulating hormones, such as insulin, leptin and ghrelin, play a critical role in the development and programming of hypothalamic circuits regulating food intake and bodyweight homeostasis.

Sexually dimorphic hormonal regulation of the gap junction protein, CX43, in rats and altered female reproductive function in CX43+/- mice

Astrocytic gap junctional communication is important in steroid hormone regulation of reproductive processes at the level of the hypothalamus, including estrous cyclicity and sexual behavior. We examined the effects of estradiol and progesterone on the abundance of the gap junctional protein, connexin 43 (CX43), which is highly expressed in astrocytes. Gonadectomized rats received hormone treatments that induce maximal sexual behavior and gonadotropin surges in females (estrogen for 48 h followed by progesterone, estrogen alone or progesterone alone). Control animals received vehicle (oil) injections. In the female rat preoptic area (POA), containing the gonadotropin-releasing hormone (GnRH) cell bodies, treatment with estrogen, progesterone or estrogen + progesterone significantly increased CX43 protein levels in immunoblots. In contrast, estrogen + progesterone significantly decreased CX43 levels in the male rat POA. This sexually dimorphic hormonal regulation of CX43 was not evident in the hypothalamus, which contains primarily GnRH nerve terminals. Treatment with estrogen + progesterone significantly decreased CX43 levels in both the male and female hypothalamus. To examine the role of CX43 in female reproductive function, we studied heterozygous female CX43 (CX43+/-) mice. Most mutant mice did not show normal estrous cycles. In addition, when compared to wild type females, CX43+/- mice had reduced lordosis behavior. These data suggest that hypothalamic CX43 expression is regulated by steroid hormones in a brain-region-specific and sexually dimorphic manner. Therefore, gap junctional communication in the POA and hypothalamus may be a factor regulating the estrous cycle and sexual behavior in female rodents.

Estrogens influence growth, maturation, and amyloid beta-peptide production in neuroblastoma cells and in a beta-APP transfected kidney 293 cell line

During development in vivo and in vitro, estrogens: a) increase brain excitability, particularly in limbic structures; b) are responsible for the maturation and cyclicity of limbic-hypothalamic interrelations; c) enhance myelinogenesis; and d) may act with NGF to stimulate neurite formation. In senescence, estrogen administration would improve memory in postmenopausal women. The absence or low levels of estrogens after menopause would increase prevalence of Alzheimer's dementia (AD) more in women than men, irrespective of age or ethnicity. In the present study, addition of 17-beta estradiol to cultured human neuroblastoma cells affected growth slightly, but stimulated cell maturation as shown by increased tyrosine hydroxylase activity. The extracellular deposition in brain tissue and around blood vessels of the amyloid beta-peptide (A beta), a 4.3 kD fragment of the larger integral membrane protein, beta-amyloid precursor protein (beta-APP), is considered an important characteristic of AD. We investigated whether 17-beta estradiol may influence the formation of the A beta (thus the abnormal accumulation of amyloid proteins) in neuroblastoma cells and in a beta-APP transfected human kidney 293 cell line. Two doses of 17 beta-estradiol were added to the cultures of both cell lines. Cells were grown until confluence, metabolically labeled with 35S-methionine, immunoprecipitated with the rabbit antiserum R1282, gel electrophoresed and autoradiographed in order to compare levels of A beta under the different estradiol concentrations. While in neuroblastoma cells, levels of A beta were only slightly reduced after estradiol and a dose-effect relationship with the hormone could not be demonstrated, in the 293 cells, A beta band intensity decreased as concentration of estradiol increased. These data support the role of estrogen in normal and abnormal brain metabolism and suggest potential hormonal interventions which may reduce or prevent the formation of amyloid deposits occur in AD.

Steroid metabolizing enzymes in pluripotential progenitor central nervous system cells: effect of differentiation and maturation

A novel in vitro system which allows extensive culturing of multipotential stem cells from mouse brain has made it possible to test whether enzymes that metabolize androgens and progestagens are present in undifferentiated central nervous system progenitor cells. Embryonic day 14 striatal cells were grown in the presence of either 20 ng/ml of epidermal growth factor (which prevents cell differentiation), or 2% fetal bovine serum (facilitating differentiation). Differentiation was complete by 35 days in vitro when the cell population comprised 86 +/- 2.0% astrocytes, 6 +/- 0.7% neurons 1.6 +/- 0.5% oligodendrocytes and 6.4 +/- 0.5% undifferentiated cells. No changes in the proportions of cell type were observed thereafter (38 and 45 days in vitro). 5 alpha-Reductive conversion (by 5 alpha-reductase) of testosterone and progesterone into dihydrotestosterone and dihydroprogesterone, and subsequent 3-alpha hydroxylation (by 3 alpha-hydroxysteroid dehydrogenase) to 3 alpha-diol and tetrahydroprogesterone were assayed in the cultures at 35, 38 and 45 days in vitro. Undifferentiated epidermal growth factor-treated cells (controls) formed about 10 times more dihydroprogesterone than dihydrotestosterone. Conversions of dihydrotestosterone and dihydroprogesterone, respectively, into 3 alpha-diol and tetrahydroprogesterone were very similar. In the fetal bovine serum-treated differentiating cells, 5 alpha-reductase converting progesterone increased at 38 days in vitro, and remained similarly elevated at 42 days in vitro (4 times). However, the conversion of testosterone into dihydrotestosterone remained at control levels up to 42 days in vitro when an increase was observed. 3 alpha-Hydroxysteroid dehydrogenase activity converting dihydroprogesterone but not dihydrotestosterone was increased at 38 and 42 days in vitro. These results show that undifferentiated central nervous system cells possess androgen and progestagen metabolizing enzymes which are strongly influenced by the cellular differentiation/maturation process.

Neuroplastic changes in the hypothalamic arcuate nucleus: the estradiol effect is accompanied by increased exoendocytotic activity of neuronal membranes

1. In the rat hypothalamic arcuate nucleus, estradiol induces coordinated changes in the number of axosomatic synapses, the amount of glial ensheathing, and the ultrastructure of the membrane of neuronal somas. In the present study we used conventional electron microscopy and freeze-fracture to examine cellular mechanisms responsible for the estradiol-induced changes at the membrane level. 2. In freeze-fracture replicas taken 10-60 min and 24 hr after injection of 17 beta-estradiol to adult ovariectomized females, it was found that there was a rapid increase in the number of exoendocytotic images that reached a plateau by 30 min. 3. In thin sections from animals injected 24 hr earlier we demonstrated a significant increase in coated vesicles in the periphery of the neurons and coated pits in the perikaryal membranes and decreased axosomatic synapses. 4. We conclude that these morphological alterations are signaling estrogen-induced transport and/or turnover of perikaryal membrane constituents and extracellular components which may affect interneuronal and neuroglial interactions.

A new organotypic culture method to study the actions of steroid hormones on the nervous system

A new organotypic culture method for growing slices of nervous system tissue, based on the use of hyaluronic acid as a growth supporting milieu, is described. This method allows cultures derived from either fetuses or newborns to grow and develop with markedly reduced amounts of added serum. Organotypic cultures from fetal rat hypothalamus were exposed to 17 beta estradiol and compared to control cultures exposed to the ethanol vehicle. When exposed to estradiol, cultures showed an outgrowth of thick nerve fibers that was accompanied by an elevation in the number of microtubules present in the neuronal processes, an increment in the number of synapses, and an increased morphological differentiation of synaptic terminals. Freeze-fracture analysis of neuronal membranes from estradiol-treated cultures revealed a significant increase in the number of exoendocytotic images and a decrease in the number of intramembranous particles. Estradiol's effects parallel those found in in vivo studies, indicating that hyaluronic acid-based organotypic cultures represent an appropriate model to study hormonal influences on the developing nervous system.

Regulation of arcuate nucleus synaptology by estrogen

Estrogen modulates the synaptology of the hypothalamic arcuate nucleus during sexual differentiation of the rat brain in both males and females. In males, testosterone of gonadal origin is converted to estrogen in the brain by an enzyme, aromatase, which is also present in females. The exposure of the male's hypothalamus to relatively high levels of estrogen (following a perinatal testosterone surge) leads to the development of a pattern of synaptogenesis which does not support an estrogen-induced gonadotrophin surge in the adult. In female rats, hypothalamic development occurs with permissively low levels of estrogen, enabling a midcycle estrogen-induced gonadotrophin surge and ovulation in adulthood. During adult reproductive life in female rats, circulating estrogen modulates the synaptology of the arcuate nucleus. The most physiological example of this is the 30-50% loss of axosomatic synapses following the preovulatory estrogen surge on diestrus-proestrus. Studies on post-synaptic membranes of the arcuate nucleus reveal sex differences in membrane organization and protein content which are estrogen-dependent. Estrogen apparently stimulates endocytosis of areas of post-synaptic membrane that are dense with small intramembranous protein particles, resulting in a reduction in the number of small intramembranous particles. This also appears to be the physiologic mechanism of neuronal changes in females during the estrus cycle. Repeated exposure to preovulatory levels of estrogen may lead to an age-related decline in reproductive capacity in female rats. Aging females lose the estrogen-induced gonadotrophin surge responsible for ovulation. This loss of function may result from a cumulative estrogen effect during the repeated ovarian cycles which results in a reorganization of the synaptology on which regulates the estrogen-induced gonadotrophin surge. The membrane organization of the senescent constant estrus aged female appears indistinguishable from the males. The hypothalamic circuits modulated by estrogen have yet to be delineated. However, recent research has shown that GABA, the monoamines, and several neuropeptides are participants in the estrogen-sensitive network which regulates GNRH secretion. In this regard, present work shows estrogen-induced changes in GABA and dopamine synapses in the arcuate nucleus.

Gonadal hormones as promoters of structural synaptic plasticity: cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.

Gonadal steroids as promoters of neuro-glial plasticity

Estradiol induces coordinated modifications in the extension of glial and neuronal processes in the arcuate nucleus of the hypothalamus of adult female rats. This hormonal effect results in natural fluctuations in the ensheathing of arcuate neurons by glial processes and these glial changes are linked to a remodelling of inhibitory GABAergic synapses during the estrous cycle. Hormonally induced glial and synaptic changes appear to be dependent on specific recognition or adhesion molecules on the neuronal and/or glial membranes.

Morphometric characterization of the arcuate nucleus neurons of the rat. A Golgi study

Six types of neurons were identified and characterized by the Golgi technique in the hypothalamic arcuate nucleus of the rat: non-ramified unipolar, ramified unipolar, non-ramified bipolar, ramified bipolar, small multipolar, and large multipolar. All had few spines, both somatic and dendritic spines. Characterization of the neuronal cytoarchitecture of the arcuate nucleus could be useful in developmental studies under specific experimental conditions.

Changes of rat striatal neuronal membrane morphology and steroid content during the estrous cycle

It is well documented that sex steroids affect striatal dopamine systems. However, the mechanism(s) of these hormonal effects in the striatum is still not well understood. We now report that gonadal steroid hormones during the estrous cycle affect the morphology and steroid hormone content of the rat striatum. Rats displaying at least two consecutive estrous cycles were included in this study as well as a group of female rats ovariectomized two weeks before being killed. The striatum was dissected from one half of each brain and used for morphological studies. From the other half of each brain, the striatum was dissected and steroid hormone concentrations in striatum and the remainder of the brain were determined. Tissues and serum concentrations of 17 beta-estradiol, progesterone and prolactin were measured by specific radioimmunoassays. Serum 17 beta-estradiol and prolactin concentrations peaked in proestrus, while progesterone was high in diestrus and proestrus. 17 beta-Estradiol levels were higher in the striatum than in the rest of the brain; both were also shown to fluctuate during the estrous cycle and with a pattern similar to that observed in serum. Progesterone serum levels showed a similar pattern of changes during the estrous cycle to progesterone concentrations in the striatum and the rest of the brain. The ultrastructure of the striatal dendritic membranes was studied by freeze-fracture. A significant difference in the content of intramembranous particles in dendritic shafts, which are mainly contacted by dopaminergic synapses, was found during the estrous cycle. The numerical density of large (greater than 10 nm) intramembranous particles was increased in diestrus I and II and in the afternoon of proestrus compared to estrus, the morning of proestrus and ovariectomized rats. In contrast, the numerical density of small (less than 10 nm) intramembranous particles was decreased in cycling animals compared to ovariectomized rats and fell in the afternoon of proestrus and then progressively increased in the following days to peak in the morning of proestrus. A negative correlation between steroid concentrations and small intramembranous particle density was observed, while the correlation was positive for large particles. No changes were observed in the membranes of dendritic spines, the main postsynaptic target for cortical afferents. In summary, this is the first report that concentrations of 17 beta-estradiol and progesterone in the striatum fluctuate during the estrous cycle. This is associated with estrous cycle-dependent changes of intramembranous particle density of striatal dendritic membranes. Our data therefore indicate that the striatum is a brain region hormonally modulated under physiological conditions.

Synaptogenic action of sex steroids in developing and adult neuroendocrine brain

Sex steroids exert potent influences on modulating neural development and neural circuit formation in both developing and adult sex steroid-sensitive neuroendocrine brain. During development, estrogen or aromatizable androgen can act as a neurotropic factor on neural tissues, stimulating axonal and dendritic growth and synapse formation. The development of sexual dimorphic synaptic organization may reflect sex steroid-modulating synaptogenesis in the hypothalamus and limbic system during the perinatal period. The onset of puberty also may be due, at least in part, to stimulation of synapse formation by estrogen in the hypothalamus. In adulthood, estrogen has a facilitatory effect on synapse formation in neural structures such as septum, hypothalamus and midbrain with or without brain lesions, and androgen plays a significant role in regulating synaptic remodeling in the androgen-sensitive spinal motoneuron pools. Thus, sex steroids seems to be critical from the developmental period to adulthood for organizing and reorganizing the neuronal circuitry driving neuroendocrine and behavioral functions.

The pre- and postnatal influence of hormones and neurotransmitters on sexual differentiation of the mammalian hypothalamus

A number of brain structures and a great number of brain functions have been shown to be sexually dimorphic. It has also been shown that development and differentiation of these structures and functions proceeds during a critical pre- and postnatal period of increased susceptibility, and is controlled by gonadal steroids and neurotransmitter substances. The brain of male and female mammals seems to be still undifferentiated before the period of increased susceptibility to gonadal steroids and neurotransmitters starts. Feminization of brain structure and functions, e.g., establishment of the cyclic LH-surge mechanism and the expression of lordosis behavior, seems to depend on the moderate interaction of estrogens with the developing nervous system. Defeminization and masculinization of brain functions seem to be established during interaction of the developing nervous system with androgens, which have to be converted, at least in part, into estrogens. Structural differentiation of the male brain, e.g., the sexually dimorphic nucleus of the preoptic area (SDN-POA), seems to be exclusively estrogen-dependent, during differentiation of male brain functions, however, estrogens may be supportive, rather than directive, to the primary action of androgens. The molecular mechanisms of sexual differentiation of the brain are not yet fully understood. It seems, however, that the priming action of gonadal steroids during the period of increased susceptibility is either mediated by neurotransmitters, or neurotransmitters modulate the priming action of gonadal steroids. In particular, the adrenergic, the serotoninergic, the cholinergic, and possibly the dopaminergic system were shown to have strong influences on sexual differentiation of brain structure and functions. In contrast to the great number of available studies on the influence of gonadal steroids on sexual differentiation of the brain, there are rather few studies available concerning the influence of neurotransmitter systems. The available results are partly contradictory, so that an interpretation must be done with caution and will leave plenty of room for speculation. Postnatal application of compounds which stimulate or inhibit adrenergic activity mainly affected the neural control of gonadotropin secretion, and had only minor influences on differentiation of behavior patterns. It seems, however, that adrenergic participation in the differentiation of the center for cyclic gonadotropin release is very complex and stimulatory and inhibitory components may operate simultaneously. Activation or inhibition of beta-adrenergic receptors during postnatal development was shown to impair the responsiveness of the center for cyclic gonadotropin release to gonadal steroids, and impairs the expression of ejaculatory behavior in male rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Sex difference in the bed nucleus of the stria terminalis of the human brain

A quantitative analysis of the volume of the darkly staining region of the posteromedial bed nucleus of the stria terminalis was performed on the brains of 26 age-matched male and female human subjects. We suggest the term "darkly staining posteromedial" component of the bed nucleus of the stria terminalis (BNST-dspm) to describe this sexually dimorphic region of the human brain. The volume of the BNST-dspm was 2.47 times greater in males than in females. This region in humans appears to correspond to an area of the bed nucleus of the stria terminalis in laboratory animals that exhibits volumetric and neurochemical sexual dimorphisms, concentrates gonadal steroids, and is anatomically connected to several other sexually dimorphic nuclei. Furthermore, the bed nucleus of the stria terminalis is involved in sexually dimorphic functions, including aggressive behavior, sexual behavior, and gonadotropin secretion, which are also influenced by gonadal steroids. Therefore, it is possible that in human beings as well, gonadal hormones influence the sexual dimorphism in the BNST-dspm and that this morphological difference, in part, underlies sexually dimorphic function.

Sexual differentiation of synaptic connectivity and neuronal plasma membrane in the arcuate nucleus of the rat hypothalamus

Plasma membranes of the hypothalamic arcuate neurons of the rat show a sexually dimorphic phenotype: the numerical density of intramembrane protein particles is greater in females. Male and female Sprague-Dawley rats, 10, 20 and 100 days old, were studied in order to determine whether sexual differentiation of the neuronal plasma membrane in the soma of arcuate neurons is associated with the establishment of sex differences in the pattern of axo-somatic synaptic contacts. Axo-somatic synapses were counted in thin sections of the arcuate nucleus and intramembrane particles were assessed in freeze-fracture replicas of the neuronal membrane. The number of synapses per length of perikaryal membrane increased from day 10 to day 20 in both sexes, reaching by 20 days values similar to those found on day 100. A sex difference in the number of synapses was observed only in 20-day-old and 100-day-old rats: neurons from females showed a greater number of presynaptic inputs than males (P less than 0.05). This sex difference was abolished by administration of testosterone propionate to 5-day-old females. Quantitative evaluation of freeze-fracture replicas of the arcuate neuronal perikarya revealed sex differences in the numerical density of intramembrane particles at all time points studied: neurons from females contained significantly more particles in their plasma membranes than neurons from males or androgenized females of the same age (P less than 0.001). These results indicate that sexual differentiation of the plasma membrane in neuronal somas precedes the establishment of sex differences in axo-somatic synapses. The results are compatible with a possible role of neuronal membranes in the sexual differentiation of synaptic connectivity.

Estradiol induces rapid remodelling of plasma membranes in developing rat cerebrocortical neurons in culture

Exo-endocytotic images and intramembrane particles were quantitatively assessed in freeze-fracture replicas from the plasma membrane of dispersed fetal rat cortical neurons (day 16 gestation) grown for 24 days in culture. The addition of 10(-10) M 17 beta-estradiol to the culture medium resulted in a significant increase in the numerical density of exo-endocytotic images within 1 min. A further increase of the number of exo-endocytotic images associated to a significant decrease in the number of intramembrane particles was observed in cells exposed for 10 min to 17 beta-estradiol. Similar results were observed when the cells were exposed to 17 beta-estradiol for 17 days. No effects on exo-endocytotic images and intramembrane particles were observed when 17 alpha-estradiol was added, instead of 17 beta-estradiol, to the cultures. These results indicate that physiological levels of 17 beta-estradiol can have rapid effects upon the ultrastructure of the neuronal membrane of developing cerebrocortical neurons.

Sex differences in plasma membrane concanavalin A binding in the rat arcuate neurons

Previous studies have shown that synaptic connections and organization of neuronal membranes are sexually dimorphic in the arcuate nucleus of developing and adult rats. These sex differences can be abolished by the perinatal androgenization of females. In this study the label-fracture method of Pinto da Silva and Kan was used in order to determine whether membrane sex differences are related to the glycoconjugates in neuronal plasma membranes. Six Sprague-Dawley female rats treated with testosterone on the day of birth, six control females injected with vehicle and six intact males were studied when they were 100 days old. The arcuate nucleus was dissected and incubated for 2 hours in a solution of 0.25 mg/ml concanavalin A, washed in buffer and incubated for 3 hours in a suspension of horseradish peroxidase-coated colloidal gold. Then, freeze-fracture replicas of the arcuate nucleus were prepared. Colloidal gold labeling was observed to be codistributed with intramembrane particles in the outer membrane face of the neuronal perikaryal plasma membrane. The numerical density of small (less than 10 nm) intramembrane particles and colloidal gold particles was significantly greater in control female membranes when compared to males or to androgenized females. The labeling was significantly reduced when the arcuate nucleus was incubated with concanavalin A in presence of 0.5 M methyl-alpha-manopyranoside. These findings indicate a sex difference in the density and distribution of glycoconjugates and intramembranous particles in the neuronal plasma membrane that is dependent on the perinatal levels of sex steroids and is concordant with, and could be the cause of, sex differences in the pattern of synaptic contacts.

Synaptic remodeling in the rat arcuate nucleus during the estrous cycle

Adult female rats showing regular vaginal cycles were studied in order to determine the number of axosomatic synapses in thin sections of the arcuate nucleus. The number of synapses per length of perikaryal membrane was significantly decreased in estrus, compared to other days of the estrous cycle (P less than 0.05). The reduction in the number of synapses in estrus was accompanied by a decrease in the percentage of the average length of perikaryal membrane covered by presynaptic terminals and by an increase in the percentage of membrane in close apposition of glial processes. Since the average perikaryal perimeter was not significantly changed during the estrous cycle, these results indicate a net decrease in the number of arcuate nucleus axosomatic synapses between proestrus and estrus, with a reinnervation of arcuate neurons between estrus and metestrus. These results suggest that there is a physiological synaptic turnover in the arcuate nucleus of the rat during the estrous cycle.

Early effects of gonadal steroids on the neuron number in the medial posterior region and the lateral division of the bed nucleus of the stria terminalis in the rat

This work investigates the possible existence of sex differences in the number of neurons in the medial posterior region (BNSTMp) and the lateral division (BNSTL) of the bed nucleus of the stria terminalis in the rat. These two zones of the bed nucleus of the stria terminalis belong, respectively, to the vomeronasal system (VNS), and to the main olfactory system (MOS). In the BNSTMp, males showed a greater number of neurons than females. Early postnatal (Day 1 after birth) orchidectomy in males, and androgenization in females, eliminated and reversed these differences. In the BNSTL, sexual dimorphism was restricted to its anterior region (BNSTLa). Females showed there a greater number of neurons than males. Male orchidectomy on Day 1 after birth increased the number of neurons, while female androgenization produced the opposite effect. The results obtained in this study support the hypothesis that the VNS is sexodimorphic, and suggest that sex differences exist in MOS, and that these differences are controlled by gonadal steroids during the perinatal period.

Sexual differentiation of the neuronal plasma membrane: neonatal levels of sex steroids modulate the number of exo-endocytotic images in the developing rat arcuate neurons

Exo-endocytotic images and intramembrane protein particles (IMP) were quantitatively assessed in freeze-fracture replicas from the plasma membrane of arcuate neurons of rats aged 0 (newborns), 10, 20 and 100 days postpartum. Membranes contained significantly (P less than 0.02) more IMPs in females than in males. Exo-endocytotic images were increased in newborn and 10-day-old males when compared to adult males or to developing females (48 +/- 6 vs 6 +/- 1 images/100 micron 2 in 10-day-old male and female rats, respectively). Androgenization of females with a single injection of testosterone propionate on the day of birth resulted in an increased number of exo-endocytotic images in developing animals (75 +/- 9 images/100 micron 2, 10-day-old rats) and in the abolishment of the sex differences in the number of IMPs.

The distribution of glial fibrillary acidic protein in the adult rat brain is influenced by the neonatal levels of sex steroids

Sex steroids during the perinatal period are able to modify the postnatal development of neurons within steroid-sensitive areas in the rat brain. This study was designed to test the possible influence of the early postnatal levels of sex steroids on the morphology of the astrocytes. The experimental manipulation of the neonatal levels of sex steroids was performed by the androgenization of females with a single injection of testosterone propionate and by the orchidectomy of males on the day of birth. Control females received a single injection of vehicle and control males were sham operated. All the animals were sacrificed at 3 months of age postnatally. The immunohistochemical distribution of the glial fibrillary acidic protein (GFAP), a marker of astrocytic filaments, was studied on coronal sections of the dorsal hippocampus, the globus pallidus and the hypothalamic arcuate nucleus. The number of GFAP immunoreactive cells, the number of GFAP immunoreactive primary processes per cell and the surface density of the GFAP immunoreactive material were evaluated. This morphometric evaluation revealed a decreased surface density of GFAP immunoreactive material in the hippocampus, globus pallidus and the ventral part of the arcuate nucleus of orchidectomized males when compared to control males. Sex differences in the distribution of GFAP immunoreactivity were detected in the hippocampus and globus pallidus. These differences were abolished by the androgenization of females. The number of GFAP immunoreactive cells was similar in all the experimental groups, indicating that the differences in surface density represent an effect of sex steroids on the growth of astrocytic processes rather than on the proliferation of astrocytes.

Neuronal membrane remodelling during the oestrus cycle: a freeze-fracture study in the arcuate nucleus of the rat hypothalamus

Freeze-fracture methodology was used to study the organization of the neuronal plasma membrane in the rat arcuate nucleus, an oestrogen sensitive area of the hypothalamus. The quantitative evaluation of freeze-fracture replicas of the perikarya, dendritic shafts and dendritic spines revealed that the numerical density of intramembranous particles varied during the ovarian cycle. The number of small (less than 10 nm) particles was decreased, and the number of large (greater than 10 nm) particles was increased, in the P-face of the perikaryal plasma membranes in prooestrus and oestrus when compared to dioestrus. This change was associated with a significant increase in the number of exo-endocytotic images in the perikaryal plasma membrane in prooestrus. Changes in intramembranous particles during the ovarian cycle were not detected in arcuate nucleus dendritic shafts and dendritic spines.

The cellular effects of estrogens on neuroendocrine tissues

Estrogen action on sensitive neurons in the rat diencephalon has been studied by morphologic techniques; evidence of estrogen action at every level is presented, including tracts, cells, circuitry and subcellular organelles. The demonstration in the arcuate nucleus of estrogen-induced synaptic remodelling, estrogen-induced postsynaptic membrane phenotypes, changes in intracellular membranes and rapid estrogen actions on neuronal endo-exocytosis indicates that cellular estrogen actions may underlie the neuronal control of reproduction.

Estrogen-induced synaptic remodelling in adult rat brain is accompanied by the reorganization of neuronal membranes

Adult cycling female rats were injected with estradiol valerate (2 mg/100 g body wt.), a treatment which has previously been shown to result in synaptic remodelling in the arcuate nucleus and constant vaginal estrus. During the 32 weeks following estrogen treatment, arcuate nucleus neuronal plasma membranes were quantitatively assessed for intramembrane particle (IMP) number and size using freeze-fracture techniques. Neuronal membranes from untreated cycling females, females injected with oil and untreated males were also studied. Untreated rats had dimorphic sexual phenotypes in membrane organization; female rats had more IMP than males, mainly due to greater numbers of small (less than 10 nm) particles. These sex differences were observed in perikarya and dendritic shafts, but not in dendritic spines. Following estrogen treatment the density of IMP in membranes from females decreased. The IMP changes were found only in neuronal perikarya and dendritic shafts, not in dendritic spines, and were mainly due to a massive decrease in the number of small (less than 10 nm) IMP which was only partially offset by an increase in the number of large (greater than or equal to 10 nm) IMP. Thus, by 32 weeks after estradiol valerate treatment, the number and size of IMP in neuronal membranes from females were not different from those seen in normal males. These results strengthen the idea that estradiol may affect the turnover of certain neuronal membrane components in sex-steroid sensitive areas of the brain.

Nuclear pores in rat hypothalamic arcuate neurons: sex differences and changes during the oestrous cycle

The numerical density of nuclear pores was assessed on freeze-fracture replicas of hypothalamic arcuate neurons from adult male and female rats. In females the nuclear pore density fluctuated during the oestrous cycle and was higher in oestrus than in pro-oestrus, metoestrus and dioestrus. Nuclear pore density in males and in metoestrus and dioestrus females was similar. The nuclear pore density in male rats was significantly less than that in oestrus and pro-oestrus females. The variation of the number of pores per unit nuclear volume and the total number of pores per nucleus was similar to the variation of the numerical density of nuclear pores. These results provide morphological evidence of cyclic changes in neuronal nucleocytoplasmic traffic during the ovarian cycle.

Rapid effects of gonadal steroids upon hypothalamic neuronal membrane ultrastructure

Freeze-fracture methodology was used to study rat hypothalamic arcuate nucleus (AN) neuronal plasma membrane organization following in vitro perfusion of brain slices with 17-beta-estradiol (17 beta E2) or other test compounds. Physiological levels (10(-10) M) of 17 beta E2 caused an increase in neuronal membrane exo-endocytotic pits within 1 min of perfusion. The increased density of pits was dose related, sustained at a constant rate during 10 min of perfusion, reverted to control values after perfusion with estradiol-free medium for 1 h, and was accompanied by an increased uptake of horseradish peroxidase by the arcuate nucleus in brain slices. The 17 beta E2-induced increase in exo-endocytotic pit density was blocked by tamoxifen (10(-8) M). Cholesterol (10(-10) M), 17-alpha-estradiol (10(-6) M) or dihydrotestosterone (10(-6) M) had no effect on exo-endocytotic pit density. Testosterone had about 50% the potency of 17 beta E2 in increasing exo-endocytotic pit density. These results indicate that physiological levels of 17 beta E2 can have rapid effects upon arcuate nucleus neuronal membrane ultrastructure.

Insulin modulates neuronal plasma membrane development in human fetal spinal cord neurons in culture

The number of intramembrane protein particles (IMP) in the protoplasmic face of the perikaryal plasma membrane was evaluated in neurons from 9 week-old and 12 week-old human fetal spinal cord in culture. An increased number of IMP was observed in membranes from 12 week-old fetal neurons when compared to membranes from 9 week-old fetal neurons. The addition of insulin (100-2500 microU/ml) to the culture media resulted in a significantly increased number of IMP in neuronal membranes. Incubation with glucose (1.5-6 mg/ml) did not modify the number of IMP and glucose did not potentiate the effect of insulin when both glucose (3 mg/ml) and insulin (500 microU/ml) were added to the culture media. The results suggest that insulin may modulate the development of neuronal membranes and that this effect is not mediated by an increased glucose utilization.