Pre-pubertal gonadectomy and the social consequences of acute ethanol in adolescent male and female rats

It has previously been shown that pre-pubertal or adult gonadectomy (GX) increases ethanol intake in male rats. This study examined whether this sex-selective increase reflects a GX-induced maintenance in males of more adolescent-typical responsiveness to ethanol characterized by enhanced sensitivity to positive (e.g., socially facilitating) and a decreased sensitivity to adverse (e.g., socially inhibitory) effects of ethanol. Male and female Sprague-Dawley rats were pre-pubertally GX, sham (SH)-operated, or non-manipulated (NM) at postnatal day (P) 25. During the late adolescent transition into adulthood (P48 — baseline day), rats were given a saline injection, placed alone into a familiar test apparatus for 30 min and then exposed for 10 min to an unfamiliar partner of the same age and sex. On the following day (P49), similar testing occurred after administration of 0.5, 0.75, 1.0 or 1.25 g/kg ethanol. At baseline, GX males and females displayed higher levels of social activity (especially adolescent-typical play and contact behavior) than SH and NM animals, with GX females displaying greater social activity than GX males. Neither males nor females demonstrated social facilitation at lower ethanol doses, regardless of hormonal status. Whereas the social inhibitory effects of higher doses of ethanol were similar across groups among females, SH males were less sensitive than both GX and NM males to ethanol-induced social inhibition. These results suggest that enhanced ethanol intake in GX males is not related to alterations in sensitivity to ethanol's social inhibitory effects. GX, however, results in retention of adolescent-typical social behaviors, with older GX adolescent rats resembling early adolescents in exhibiting elevated social activity—particularly play and contact behavior.     

How environment and genes shape the adolescent brain

This article is part of a Special Issue “Puberty and Adolescence”.     

哺乳动物成体神经元的再生:内、外环境因子的作用

哺乳动物成体神经元的再生现象是最近三十年才被科学家们所认识并逐渐接受的。随着科研方法与实验技术的发展,在成年哺乳动物的一些特定脑区,比如海马齿状回(Dentate gyrus of the hippocampus)、室下区(Subventricular zone)和杏仁核(Amygdala)中发现了新生细胞。研究表明,内外环境因子可影响成体神经元的再生。具体表现在环境多样性、自主活动、有益社会交往、短日光照、化学刺激以及诸如5—羟色胺和脑源性神经营养因子等神经递质水平的增加,都会促进新生细胞的增生或存活。而负面社会交往及应激激素皮质酮对成体神经元的再生有抑制和降低作用。研究还表明,根据种和性别的差异,类脂醇激素对成体神经元的再生起到促进或抑制作用。最新的实验证实新生细胞在成体中具有显著功能[动物学报49(2)：151—162,2003]。     

Gonadal Influences on the Sexual Differentiation of Monoamine Oxidase Type A and B Activities in the Rat Brain

Abstract: The sex-dependent differentiation of monoamine oxidase (MAO) in the hypothalamus of 60-day-old, Charles River rats was found to involve only type A (MAO-A), and not type B (MAO-B) enzyme. In vivo inhibition of type A by clorgyline, and type B by (−)deprenyl, however, tended to decrease the specific activity of both types of MAO to a smaller extent in the female than in the male hypothalamus. When masculinization was prevented by neonatal administration of estradiol (E) to males, hypothalamic MAO-A and MAO-B activities increased in both control and MAO-inhibited rats. Androgenization of females, however, had little effect on the MAO activity. Whereas the effects of neonatal estrogenization were attributable neither to a direct influence of E nor to a sexual difference in the peripheral clearance of the MAO-inhibitor used, single, high doses of steroids to adult, but not to newborn rats, did acutely affect the kinetics of MAO-A. The activity of MAO-A was also decreased by high concentrations of E or TS in vitro. The imprinting for patterns of hypothalamic MAO-A and MAO-B in the two sexes results, probably, from genetic predetermination. Neonatal changes in the homeostasis of gonadal hormones may result in type-MAO nonspecific effects in adulthood, whereas the short-term effects of high concentrations of steroids may be selective for the A form.     

Sexual dimorphism in the adolescent brain: Role of testosterone and androgen receptor in global and local volumes of grey and white matter

Here we examined sex differences in the volumes of grey and white matter, and in grey-matter “density,” in a group of typically developing adolescents participating in the Saguenay Youth Study (n = 419; 12-18 years). In male adolescents, we also investigated the role of a functional polymorphism in androgen-receptor gene (AR) in moderating the effect of testosterone on volumes of grey and white matter and grey-matter density. Overall, both absolute and relative volumes of white matter were larger in male vs. females adolescents. The relative grey-matter volumes were slightly larger in female than male adolescents and so was the grey-matter density in a large number of cortical regions. In male adolescents, functional polymorphism of AR moderated the effect of testosterone on relative white- and grey-matter volumes. Following a discussion of several methodological and interpretational issues, we outline future directions in investigating brain-behavior relationships vis-à-vis psychopathology.     

Sexual differentiation of the rodent hypothalamus: hormonal and environmental influences

Brain sexual differentiation is a complex developmental phenomenon influenced by the genetic background, sex hormone secretions and environmental inputs, including pollution. The main hormonal drive to masculinize and defeminize the rodent brain is testosterone secreted by the testis. The hormone does not influence sex brain differentiation only in its native configuration, but it mostly needs local conversion into active metabolites (estradiol and DHT) through the action of specific enzymatic systems: the aromatase and 5alpha-reductase (5alpha-R), respectively. This allows the hormone to control target cell gene expression either through the estrogen (ER) or the androgen (AR) receptors. The developmental profile of testosterone metabolizing enzymes, different in the two sexes, is therefore of the utmost importance in affecting the bioavailability of the steroids active in brain differentiation. Widely diffused pollutants, like polychlorinated biphenyls (PCBs) are able to affect the production and/or action of testosterone metabolites, exerting detrimental influences on reproduction and sex behavior. The main studies performed in our and other laboratories concerning the pattern of expression and the control of the enzymatic systems involved in brain androgen action and metabolism are shortly reviewed. Some recent data on the influence exerted by PCBs on these metabolic systems are also reported.     

Effects of stressors in adolescence on learning and memory in rodent models

This article is part of a Special Issue "Puberty and Adolescence".     

Individual variation and the endocrine regulation of behaviour and physiology in birds: a cellular/molecular perspective

Investigations of the cellular and molecular mechanisms of physiology and behaviour have generally avoided attempts to explain individual differences. The goal has rather been to discover general processes. However, understanding the causes of individual variation in many phenomena of interest to avian eco-physiologists will require a consideration of such mechanisms. For example, in birds, changes in plasma concentrations of steroid hormones are important in the activation of social behaviours related to reproduction and aggression. Attempts to explain individual variation in these behaviours as a function of variation in plasma hormone concentrations have generally failed. Cellular variables related to the effectiveness of steroid hormone have been useful in some cases. Steroid hormone target sensitivity can be affected by variables such as metabolizing enzyme activity, hormone receptor expression as well as receptor cofactor expression. At present, no general theory has emerged that might provide a clear guidance when trying to explain individual variability in birds or in any other group of vertebrates. One strategy is to learn from studies of large units of intraspecific variation such as population or sex differences to provide ideas about variables that might be important in explaining individual variation. This approach along with the use of newly developed molecular genetic tools represents a promising avenue for avian eco-physiologists to pursue.     

New tricks by an old dogma: Mechanisms of the Organizational / Activational Hypothesis of steroid-mediated sexual differentiation of brain and behavior

The hormonal regulation of sexual behavior has been the topic of study for over 50 years and yet controversies persist regarding the importance of early versus late events and the identity of the critical neural and cellular substrates. We have taken a mechanistic approach toward the masculinizing actions of the gonadal steroid estradiol, as a means to understand how organization of the neuroarchitechture during a perinatal sensitive period exerts enduring influences on adult behavior. We have identified important roles for prostaglandins, FAK and paxillin, PI3 kinase and glutamate, and determined that cell-to-cell signaling is a critical component of the early organizational events. We have further determined that the mechanisms mediating different components of sexual behavior are distinct and regionally specific. The multitude of mechanisms by which the steroid estradiol, exerts divergent effects on the developing nervous system provides for a multitude of phenotypes which can vary significantly both within and between the sexes.     

Aggressive behavior in female golden hamsters: development and the effect of repeated social stress

In male golden hamsters, agonistic behavior matures during puberty, changing from play fighting to adult-like aggression. In addition, this transition is accelerated by repeated social subjugation early in puberty. However, little is known about the development of agonistic behavior in females. In the present study, we compared the development of agonistic behavior in male and female golden hamsters. Furthermore, we also tested the effects of repeated social subjugation on the development of agonistic behavior during puberty. Hamsters were tested for agonistic behavior in the presence of a smaller intruder at different intervals during puberty. Several observations were made. First, the frequency of attacks remained stable in females, while varying in males. Second, the transition from play fighting to adult-like aggression occurred at earlier time periods in females than in males. Finally, a clear transitional period marked by attacks focused on the flanks was observable in males around mid-puberty. However, this transitional period was not apparent in females. In addition, juvenile females were exposed to aggressive adult males or females. In both cases, repeated exposure to stress had no statistically significant effect on the development of agonistic behavior. After 2 weeks of subjugation, exposure to aggressive adults had no effect on serum cortisol levels, indicating that juvenile females habituate to repeated social stress. These data show significant sex differences in the development of agonistic behavior and adaptation to repeated stress in juvenile golden hamsters.     

Steroid-induced sexual differentiation of the developing brain: multiple pathways, one goal

Hormone exposure, including testosterone and its metabolite estradiol, induces a myriad of effects during a critical period of brain development that are necessary for brain sexual differentiation. Nuclear volume, neuronal morphology, and astrocyte complexity are examples of the wide range of effects by which testosterone and estradiol can induce permanent changes in the function of neurons for the purpose of reproduction in adulthood. This review will examine the multitude of mechanisms by which steroid hormones induce these permanent changes in brain structure and function. Elucidating how steroids alter brain development sheds light on how individual variation in neuronal phenotype is established during a critical period.     

Sexual differentiation in the developing mouse brain: contributions of sex chromosome genes

Neural sexual differentiation begins during embryogenesis and continues after birth for a variable amount of time depending on the species and brain region. Because gonadal hormones were the first factors identified in neural sexual differentiation, their role in this process has eclipsed investigation of other factors. Here, we use a mouse with a spontaneous translocation that produces four different unique sets of sex chromosomes. Each genotype has one normal X‐chromosome and a unique second sex chromosome creating the following genotypes: XY^*x, XX, XY^*, XX^Y*. This Y^* mouse line is used by several laboratories to study two human aneuploid conditions: Turner and Klinefelter syndromes. As sex chromosome number affects behavior and brain morphology, we surveyed brain gene expression at embryonic days 11.5 and 18.5 to isolate X‐chromosome dose effects in the developing brain as possible mechanistic changes underlying the phenotypes. We compared gene expression differences between gonadal males and females as well as individuals with one vs. two X‐chromosomes. We present data showing, in addition to genes reported to escape X‐inactivation, a number of autosomal genes are differentially expressed between the sexes and in mice with different numbers of X‐chromosomes. Based on our results, we can now identify the genes present in the region around the chromosomal break point that produces the Y^* model. Our results also indicate an interaction between gonadal development and sex chromosome number that could further elucidate the role of sex chromosome genes and hormones in the sexual differentiation of behavior.     

The organizational hypothesis and final common pathways: Sexual differentiation of the spinal cord and peripheral nervous system

In honor of the 50th anniversary of the “organizational hypothesis,” this paper reviews work on sexual differentiation of the spinal cord and peripheral nervous system. Topics considered include the spinal nucleus of the bulbocavernosus, the ejaculation center, the cremaster nucleus, sensory and autonomic neurons, and pain. These relatively simple neural systems offer ample confirmation that early exposure to testicular hormones masculinizes the nervous system, including final common pathways. However, I also discuss findings that challenge, or at least stretch, the organizational hypothesis, with important implications for understanding sex differences throughout the nervous system.     

Relationships of serum thyroid hormones and follicle-stimulating hormone concentrations to Sertoli cell differentiation during the first wave of spermatogenesis in euthyroid ram lambs

The main purpose of this study was to determine if temporal relationships exist between serum concentrations of free fractions of thyroxin (fT₄) and triiodothyronine (fT₃), follicle-stimulating hormone (FSH) levels, and Sertoli cell differentiation in euthyroid ram lamb testes. Additionally, testicular thyroid hormone (TH) receptors (TRs) were identified using immunohistochemistry and Western blot analysis. Weekly testicular biopsies and jugular blood samples were collected from 12 ram lambs over the 9 weeks of study. Hormone concentrations and the numbers of dividing Sertoli cells per seminiferous tubule (ST) area were analyzed relative to chronological age of animals and the two distinctive stages of Sertoli cell differentiation: (a) tight junction/ST lumen formation and (b) the onset of support mechanisms for the development of multiple germ cell types (presence of primary spermatocytes in >95% STs). Circulating FSH concentrations increased (p < 0.05) immediately after first detection of ST lumen and reached a nadir (p < 0.05) just prior to the end of the first wave of spermatogenesis. A decline in both fT₄ and fT₃ levels (p < 0.05) occurred after Sertoli cells had formed the ST lumen and began supporting germ cell differentiation. There was a positive correlation between the numbers of proliferating Sertoli cells and serum fT₄ (r = 0.51, p < 0.001) and fT₃ (r = 0.52, p < 0.001) concentrations. TRs were expressed throughout the study period; however, prior to the formation of ST lumen, two isoforms were detected while only one TR isoform was present by the end of the first wave of spermatogenesis. Overall, the exit of Sertoli cells from the cell cycle that presages their final differentiation begins when THs and FSH levels are high, suggesting a permissive role of these hormones in the maturation of STs in prepubertal ram lambs.     

Perinatal visceral events and brain mechanisms involved in the development of mother-young bonding in sheep

In sheep the onset of maternal responsiveness and the development of the mutual mother-young bond are under the combined influence of hormonal and visceral somatosensory stimulations. These stimuli are provided in the mother by parturition (via steroids and vaginocervical stimulation) and in the neonate by the first suckling episodes (via cholecystokinin and oro-gastro-intestinal stimulation). In addition, each partner relies on specific chemosensory stimulation for reciprocal attraction: amniotic fluids for the mother, colostrum for the young. In the ewe parturition activates several brain structures to respond specifically to sensory cues emanating from the young. The main olfactory bulbs undergo profound neurophysiological changes when exposed to offspring odors at parturition. Additional activations in the hypothalamus - preoptic area - and the amygdala - medial and cortical nuclei - also contribute to maternal responsiveness and memorization of lamb odors. In the neonate, post-ingestive stimulations activate the brain stem via vagal afferents. Like in the ewe, several regions of the hypothalamus and the amygdala respond to colostrum ingestion suggesting common ground for the integrative neural processes involved in early learning and bonding. This leads to rapid visual and auditory recognition in both partners although olfaction remains important in the ewe to display selective nursing. It is concluded that the biological basis for the development of maternal and filial bonding in sheep presents striking similarities.     

The CNTF/LIF signaling pathway regulates developmental programmed cell death and differentiation of rod precursor cells in the mouse retina in vivo

Natural cell death is critical for normal development of the nervous system, but the extracellular regulators of developmental cell death remain poorly characterized. Here, we studied the role of the CNTF/LIF signaling pathway during mouse retinal development in vivo. We show that exposure to CNTF during neonatal retinal development in vivo retards rhodopsin expression and results in an important and specific deficit in photoreceptor cells. Detailed analysis revealed that exposure to CNTF during retinal development causes a sharp increase in cell death of postmitotic rod precursor cells. Importantly, we show that blocking the CNTF/LIF signaling pathway during mouse retinal development in vivo results in a significant reduction of naturally occurring cell death. Using retroviral lineage analysis, we demonstrate that exposure to CNTF causes a specific reduction of clones containing only rods without affecting other clone types, whereas blocking the CNTF/LIF receptor complex causes a specific increase of clones containing only rods. In addition, we show that stimulation of the CNTF/LIF pathway positively regulates the expression of the neuronal and endothelial nitric oxide synthase (NOS) genes, and blocking nitric oxide production by pre-treatment with a NOS inhibitor abolishes CNTF-induced cell death. Taken together, these results indicate that the CNTF/LIF signaling pathway acts via regulation of nitric oxide production to modulate developmental programmed cell death of postmitotic rod precursor cells.     

Developmental plasticity in neural circuits controlling birdsong: Sexual differentiation and the neural basis of learning

In many species of passerine songbirds, males learn their song during defined periods of life. Female song in often reduced or absent, as are the brain regions controlling song. Sexual differences in the brain arise because of the action of sex steroids, which trigger the formation of some neural pathways (especially the pathway from the higher vocal center to the robust nucleus) and prevent the atrophy of others in males. These neural changes occur during periods of developmental song learning and can recur during periods of learning in adult birds. The process of learning is correlated with major increases or decreases in the number of neurons in specific neuronal populations, suggesting that the formation or loss of specific neural pathways regulates the ability to learn. Species differences in sexual differentiation and learning allow informative cross-species comparisons of neural structure and behavior. © 1992 John Wiley & Sons, Inc.     

Sexual differentiation of the brain: genes,estrogen, and neurotrophic factors

Based on evidence obtained during the past 50 years, the current hypothesis to explain the sexual dimorphism of structure and function in the brain of vertebrates maintains that these differences are produced by the epigenetic action of gonadal hormones. However, evidence has progressively accumulated suggesting that genetic mechanisms controlling sexual-specific neuronal characteristics precede, or occur in parallel with, hormonal effects.1. In cultures of hypothalamic neurons taken from gestation day 16 (GD16) embryos, treatment of sexually segregated cultures with estradiol (E2) induces axon growth in neurons from male neurons, but not from female neurons. In these cultures treatment with E2 increased the levels of tyrosine kinase type B (TrkB) and insulin-like growth factor I (IGF-I) receptors in male but not in female neurons. This and other sex differences cannot be explained by differences in hormonal environment, because the donor embryos were obtained when gonadal secretion of steroids is just beginning, before the perinatal surge of testosterone that determines development of the male brain beginning at GD17/18.2. The response to estrogen is contingent upon coculture with heterotopic glia (mostly astrocytes) from a target region (amygdala) harvested from same-sex fetuses at GD16, whereas in the presence of homotopic glia or in cultures without glia, E2 had no effect. It was concluded that the axogenic effect of E2 depends on interaction between neurons and glia from a target region and that neurons from fetal male donors appear to mature earlier than neurons from females, a differentiated response that takes place prior to divergent exposure to gonadal secretions.3. The effects of target and nontarget glia-conditioned media (CM) on the E2-induced growth of neuronal processes of hypothalamic neurons obtained from sexually segregated fetal donors were also studied. Estrogen added to media conditioned by target glia modified the number of primary neurites and the growth of axons of hypothalamic neurons of males but not of females.4. Neither the Type III steroidal receptor blocker tamoxifen nor Type I antiestrogen ICI 182,780 prevented the axogenic effects of the hormone. Estradiol made membrane-impermeable by conjugation to a protein of high molecular weight (E2-BSA) preserved its axogenic capacity, suggesting the possibility of a membrane effect responsible for the action of E2.5. Western blot analysis of the tyrosine kinase type A (TrkA), type B (TrkB), type C (TrkC), and insulin-like growth factor (IGF-I R) receptors in extracts from homogenates of cultured hypothalamic neurons showed that in cultures of male-derived neurons grown with E2 and CM from target glia, the amounts of TrkB and IGF-I R increased notably. Densitometric quantification showed that these cultures had more TrkB than cultures with CM alone or E2 alone. On the contrary, in cultures of female-derived neurons, the presence of CM alone induced maximal levels of TrkB, which were not further increased by E2; female-derived neurons in all conditions did not contain IGF-I R. Levels of TrkC were not modified by any experimental condition in male- or female-derived cultures and Trk A was not found in the homogenates.These results are compared with similar data from other laboratories and integrated in a model for the confluent interaction of estrogen and neurotrophic factors released by glia that may contribute to the sexual differentiation of the brain.     

The 13th J. A. F. Stevenson memorial lecture. Sexual differentiation of the brain: possible mechanisms and implications

The mammalian brain appears to be inherently feminine and the action of testicular hormones during development is necessary for the differentiation of the masculine brain both in terms of functional potential and actual structure. Experimental evidence for this statement is reviewed in this discussion. Recent discoveries of marked structural sex differences in the central nervous system, such as the sexually dimorphic nucleus of the preoptic area in the rat, offer model systems to investigate potential mechanisms by which gonadal hormones permanently modify neuronal differentiation. Although effects of these steroids on neurogenesis and neuronal migration and specification have not been conclusively eliminated, it is currently believed, but not proven, that the principle mechanism of steroid action is to maintain neuronal survival during a period of neuronal death. The structural models of the sexual differentiation of the central nervous system also provide the opportunity to identify sex differences in neurochemical distribution. Two examples in the rat brain are presented: the distribution of serotonin-immunoreactive fibers in the medial preoptic nucleus and of tyrosine hydroxylase-immunoreactive fibers and cells in the anteroventral periventricular nucleus. It is likely that sexual dimorphisms will be found to be characteristic of many neural and neurochemical systems. The final section of this review raises the possibility that the brain of the adult may, in response to steroid action, be morphologically plastic, and considers briefly the likelihood that the brain of the human species is also influenced during development by the hormonal environment.     

A developmental study of serotonin-immunoreactive neurons in the embryonic brain of the Marbled Crayfish and the Migratory Locust: Evidence for a homologous protocerebral group of neurons

It is well established that the brains of adult malacostracan crustaceans and winged insects display distinct homologies down to the level of single neuropils such as the central complex and the optic neuropils. We wanted to know if developing insect and crustacean brains also share similarities and therefore have explored how neurotransmitter systems arise during arthropod embryogenesis. Previously, Sintoni et al. (2007) had already reported a homology of an individually identified cluster of neurons in the embryonic crayfish and insect brain, the secondary head spot cells that express the Engrailed protein. In the present study, we have documented the ontogeny of the serotonergic system in embryonic brains of the Marbled Crayfish in comparison to Migratory Locust embryos using immunohistochemical methods combined with confocal laser-scan microscopy. In both species, we found a cluster of early emerging serotonin-immunoreactive neurons in the protocerebrum with neurites that cross to the contralateral brain hemisphere in a characteristic commissure suggesting a homology of this cell cluster. Our study is a first step towards a phylogenetic analysis of neurotransmitter system development and shows that, as for the ventral nerve cord, traits related to neurogenesis in the brain can provide valuable hints for resolving the much debated question of arthropod phylogeny.