Molecular aspects of sexual differentiation of the rodent brain

Therapeutic Interventions in Rat Models of Preterm Hypoxic Ischemic Injury: Effects of Hypothermia, Caffeine, and the Influence of Sex

Infants born prematurely have an increased risk of experiencing brain injury, specifically injury caused by Hypoxia Ischemia (HI). There is no approved treatment for preterm infants, in contrast to term infants that experience Hypoxic Ischemic Encephalopathy (HIE) and can be treated with hypothermia. Given this increased risk and lack of approved treatment, it is imperative to explore and model potential treatments in animal models of preterm injury. Hypothermia is one potential treatment, though cooling to current clinical standards has been found to be detrimental for preterm infants. However, mild hypothermia may prove useful. Caffeine is another treatment that is already used in preterm infants to treat apnea of prematurity, and has shown neuroprotective effects. Both of these treatments show sex differences in behavioral outcomes and neuroprotective effects, which are critical to explore when working to translate from animal to human. The effects and research history of hypothermia, caffeine and how sex affects these treatment outcomes will be explored further in this review article.

The Neurobiology of Behavior and Its Applicability for Animal Welfare: A Review

Simple Summary

Animal welfare is the result of physical and psychological well-being and is expected to occur if animals are free: (1) from hunger, thirst and malnutrition, (2) from discomfort, (3) from pain, (4) to express normal behavior, and (5) from fear and distress. Nevertheless, well-being is not a constant state but rather the result of certain brain dynamics underlying innate motivated behaviors and learned responses. Thus, by understanding the foundations of the neurobiology of behavior we fathom how emotions and well-being occur in the brain. Herein, we discuss the potential applicability of this approach for animal welfare. First, we provide a general view of the basic responses coordinated by the central nervous system from the processing of internal and external stimuli. Then, we discuss how those stimuli mediate activity in seven neurobiological systems that evoke innate emotional and behavioral responses that directly influence well-being and biological fitness. Finally, we discuss the basic mechanisms of learning and how it affects motivated responses and welfare.

Abstract

Understanding the foundations of the neurobiology of behavior and well-being can help us better achieve animal welfare. Behavior is the expression of several physiological, endocrine, motor and emotional responses that are coordinated by the central nervous system from the processing of internal and external stimuli. In mammals, seven basic emotional systems have been described that when activated by the right stimuli evoke positive or negative innate responses that evolved to facilitate biological fitness. This review describes the process of how those neurobiological systems can directly influence animal welfare. We also describe examples of the interaction between primary (innate) and secondary (learned) processes that influence behavior.

Ovarian hormones mediate running-induced changes in high fat diet choice patterns in female rats

Physical inactivity and increased consumption of energy dense, high fat (HF) foods often leads to a state of positive energy balance. Regular exercise can facilitate the maintenance of a healthy body weight and mediate changes in dietary selection. Past studies using a two-diet choice (chow vs. HF) and voluntary wheel running paradigm found that when a novel HF diet and wheel running are simultaneously introduced, male rats show complete and persistent HF diet avoidance whereas the majority of females show HF diet avoidance for a few days, but then revert to HF diet preference. Ovariectomy (OVX) appears to decrease preference for the HF diet bringing it closer to that of males. Given that estradiol but not progesterone mediates changes in food intake and energy balance, we hypothesized that estradiol signaling is required for the reversal of HF diet avoidance in female rats. Accordingly, Experiment 1 compared the persistency of running-induced HF diet avoidance in males, sham-operated females, and OVX rats with replacement of oil vehicle, estradiol benzoate (E), progesterone (P), or both (E + P). The number of wheel running rats that either avoided or preferred the HF diet varied with hormone treatment. The reversal of HF diet avoidance in running females and OVX E + P rats occurred more rapidly and frequently than male running rats. E + P but not E or P replaced OVX wheel running rats significantly reversed HF diet avoidance. OVX oil rats avoided HF diet to the same extent as male rats for the first 11 days of diet choice and then rapidly increased HF diet intake and began preferring it. This incomplete elimination of sex differences suggests that developmental factors or androgens might play a role in sustaining running-induced HF diet avoidance. Subsequently, Experiment 2 aimed to determine the role of androgens in the persistency of running-associated HF diet avoidance with sham-operated and orchiectomized (GDX) male rats. Both intact and GDX male running rats persistently avoided the HF diet to the same extent. Taken together, these results suggest that activational effects of ovarian hormones play a role in female specific running-induced changes in diet choice patterns. Furthermore, the activational effects of androgens are not required for the expression of HF diet avoidance in males.

Sex as a Biological Variable: Who, What, When, Why, and How

The inclusion of sex as a biological variable in research is absolutely essential for improving our understanding of disease mechanisms contributing to risk and resilience. Studies focusing on examining sex differences have demonstrated across many levels of analyses and stages of brain development and maturation that males and females can differ significantly. This review will discuss examples of animal models and clinical studies to provide guidance and reference for the inclusion of sex as an important biological variable relevant to a Neuropsychopharmacology audience.

Genetic dissection of neural circuits underlying sexually dimorphic social behaviours

The unique hormonal, genetic and epigenetic environments of males and females during development and adulthood shape the neural circuitry of the brain. These differences in neural circuitry result in sex-typical displays of social behaviours such as mating and aggression. Like other neural circuits, those underlying sex-typical social behaviours weave through complex brain regions that control a variety of diverse behaviours. For this reason, the functional dissection of neural circuits underlying sex-typical social behaviours has proved to be difficult. However, molecularly discrete neuronal subpopulations can be identified in the heterogeneous brain regions that control sex-typical social behaviours. In addition, the actions of oestrogens and androgens produce sex differences in gene expression within these brain regions, thereby highlighting the neuronal subpopulations most likely to control sexually dimorphic social behaviours. These conditions permit the implementation of innovative genetic approaches that, in mammals, are most highly advanced in the laboratory mouse. Such approaches have greatly advanced our understanding of the functional significance of sexually dimorphic neural circuits in the brain. In this review, we discuss the neural circuitry of sex-typical social behaviours in mice while highlighting the genetic technical innovations that have advanced the field.

Epigenetic and transgenerational reprogramming of brain development

Neurodevelopmental programming - the implementation of the genetic and epigenetic blueprints that guide and coordinate normal brain development - requires tight regulation of transcriptional processes. During prenatal and postnatal time periods, epigenetic processes fine-tune neurodevelopment towards an end product that determines how an organism interacts with and responds to exposures and experiences throughout life. Epigenetic processes also have the ability to reprogramme the epigenome in response to environmental challenges, such as maternal stress, making the organism more or less adaptive depending on the future challenges presented. Epigenetic marks generated within germ cells as a result of environmental influences throughout life can also shape future generations long before conception occurs.

Sex differences in behavioral outcome following neonatal hypoxia ischemia: insights from a clinical meta-analysis and a rodent model of induced hypoxic ischemic brain injury

Hypoxia ischemia (HI; reduced oxygen and/or blood flow to the brain) is one of the most common injuries among preterm infants and term infants with birth complications. Both populations show cognitive/behavioral deficits, including impairments in sensory, learning/memory, and attention domains. Clinical data suggests a sex difference in HI outcomes, with males exhibiting more severe cognitive/behavioral deficits relative to matched females. Our laboratory has also reported more severe behavioral deficits among male rats with induced HI relative to females with comparable injury (Hill et al., 2011a,b). The current study initially examined published clinical studies from the past 20years where long-term IQ outcome scores for matched groups of male and female premature infants were reported separately (IQ being the most common outcome measure). A meta-analysis revealed a female "advantage," as indicated by significantly better scores on performance and full scale IQ (but not verbal IQ) for premature females. We then utilized a rodent model of neonatal HI injury to assess sham and postnatal day 7 (P7) HI male and female rats on a battery of behavioral tasks. Results showed expected deficits in HI male rats, but also showed task-dependent sex differences, with HI males having significantly larger deficits than HI females on some tasks but equivalent deficits on other tasks. In contrast to behavioral results, post mortem neuropathology associated with HI was comparable across sex. These findings suggest: 1) neonatal female "protection" in some behavioral domains, as indexed by superior outcome following early injury relative to males; and 2) female protection may entail sex-specific plasticity or compensation, rather than a reduction in gross neuropathology. Further exploration of the mechanisms underlying this sex effect could aid in neuroprotection efforts for at-risk neonates in general, and males in particular. Moreover, our current report of comparable anatomical damage coupled with differences in cognitive outcomes (by sex) provides a framework for future studies to examine neural mechanisms underlying sex differences in cognition and behavior in general.

Sex differences in mechanisms and outcome of neonatal hypoxia-ischemia in rodent models: implications for sex-specific neuroprotection in clinical neonatal practice

Clinical findings show that male infants with hypoxic-ischemic injury (HI) fare more poorly than matched females on cognitive outcomes. Rodent models of neonatal hypoxia-ischemia support this difference, with data showing that perinatal brain injury leads to long-term behavioral deficits primarily in male rodents and in female rodents treated with early androgens. Results support the idea that sex-specific gonadal hormones may modulate developmental response to injury and dovetail with overwhelming evidence of developmental androgen effects on typical brain morphology and behavior. However, mechanisms underlying sex differences in response to early brain injury may be more complicated. Specifically, activation of cell death pathways in response to HI may also differ by sex. In females, the preferential activation of the caspase-dependent apoptotic pathway may actually afford greater protection, potentially due to the actions of X-linked inhibitor of apoptosis (XIAP) within this pathway. This contrasts the pattern of preferential activation of the caspase-independent pathway in males. While an integrated model of sex-specific hormonal and genetic modulation of response to early injury remains to be fully elucidated, these findings suggest that infants might benefit from sex-specific neuroprotection following HI injury.

Turner syndrome and sexual differentiation of the brain: implications for understanding male-biased neurodevelopmental disorders

Turner syndrome (TS) is one of the most common sex chromosome abnormalities. Affected individuals often show a unique pattern of cognitive strengths and weaknesses and are at increased risk for a number of other neurodevelopmental conditions, many of which are more common in typical males than typical females (e.g., autism and attention-deficit hyperactivity disorder). This phenotype may reflect gonadal steroid deficiency, haploinsufficiency of X chromosome genes, failure to express parentally imprinted genes, and the uncovering of X chromosome mutations. Understanding the contribution of these different mechanisms to outcome has the potential to improve clinical care for individuals with TS and to better our understanding of the differential vulnerability to and expression of neurodevelopmental disorders in males and females. In this paper, we review what is currently known about cognition and brain development in individuals with TS, discuss underlying mechanisms and their relevance to understanding male-biased neurodevelopmental conditions, and suggest directions for future research.

Reprint of "Early testosterone modulated sex differences in behavioral outcome following neonatal hypoxia ischemia in rats"

Hypoxia ischemia (HI; reduced blood oxygenation and/or flow to the brain) represents one of the most common injuries for both term and preterm/very low birth weight (VLBW) infants. These children experience elevated incidence of cognitive and/or sensory processing disabilities, including language based learning disabilities. Clinical data also indicate more substantial long-term deficits for HI injured male babies as compared to HI injured females. Previously, we reported significant deficits in rapid auditory processing and spatial learning in male rats with postnatal day 1 (P1), P7, or P10 HI injury. We also showed sex differences in HI injured animals, with more severe deficits in males as compared to females. Given these findings, combined with extant clinical data, the current study sought to assess a putative role for perinatal testosterone in modulating behavioral outcome following early hypoxic-ischemic injury in rats. Male, female, and testosterone-propionate (TP) treated females were subjected to P7 HI or sham surgery, and subsequently (P30+) underwent a battery of auditory testing and water maze assessment. Results confirm previous reports of sex differences following HI, and add new findings of significantly worse performance in TP-treated HI females compared to vehicle treated HI females. Post mortem anatomic analyses showed consistent effects, with significant brain weight decreases seen in HI male and TP-treated HI females but not female HI or sham groups. Further neuromorphometric analysis of brain structures showed that HI male animals exhibited increased pathology relative to HI females as reflected in ventricular enlargement. Findings suggest that neonatal testosterone may act to enhance the deleterious consequences of early HI brain injury, as measured by both neuropathology and behavior.

Sex differences in prenatal epigenetic programming of stress pathways

Maternal stress experience is associated with neurodevelopmental disorders including schizophrenia and autism. Recent studies have examined mechanisms by which changes in the maternal milieu may be transmitted to the developing embryo and potentially translated into programming of the epigenome. Animal models of prenatal stress have identified important sex- and temporal-specific effects on offspring stress responsivity. As dysregulation of stress pathways is a common feature in most neuropsychiatric diseases, molecular and epigenetic analyses at the maternal-embryo interface, especially in the placenta, may provide unique insight into identifying much-needed predictive biomarkers. In addition, as most neurodevelopmental disorders present with a sex bias, examination of sex differences in the inheritance of phenotypic outcomes may pinpoint gene targets and specific windows of vulnerability in neurodevelopment, which have been disrupted. This review discusses the association and possible contributing mechanisms of prenatal stress in programming offspring stress pathway dysregulation and the importance of sex.

Neonatal injections of methoxychlor decrease adult rat female reproductive behavior

Methoxychlor (MXC), a commonly used pesticide, has been labeled as an endocrine disruptor. To evaluate the impact of neonatal exposure to MXC on female reproduction, female Sprague-Dawley rats were given subcutaneous injections on postnatal days 1, 3, and 5. The injections contained 1.0mg MXC, 2.0mg MXC, 10 μg 17β-estradiol benzoate (positive control), or sesame oil (vehicle). The injections of MXC had no effect on anogenital distance or day of vaginal opening. Treatment with either 2.0mg MXC or estradiol significantly increased the total number of days with vaginal keratinization. Treatment with MXC had no effect on ability to exhibit a mating response as an adult female, although the high dose MXC (2.0) and the positive control (estradiol) animals demonstrated a decrease in degree of receptivity, a decrease in proceptive behavior and an increase in rejection behavior. These data suggest that higher doses of MXC given directly to pups during the neonatal period can act as an estrogen and alter aspects of the nervous system, impacting adult reproductive characteristics.

Early testosterone modulated sex differences in behavioral outcome following neonatal hypoxia ischemia in rats

Hypoxia ischemia (HI; reduced blood oxygenation and/or flow to the brain) represents one of the most common injuries for both term and preterm/very low birth weight (VLBW) infants. These children experience elevated incidence of cognitive and/or sensory processing disabilities, including language based learning disabilities. Clinical data also indicate more substantial long-term deficits for HI injured male babies as compared to HI injured females. Previously, we reported significant deficits in rapid auditory processing and spatial learning in male rats with postnatal day 1 (P1), P7, or P10 HI injury. We also showed sex differences in HI injured animals, with more severe deficits in males as compared to females. Given these findings, combined with extant clinical data, the current study sought to assess a putative role for perinatal testosterone in modulating behavioral outcome following early hypoxic-ischemic injury in rats. Male, female, and testosterone-propionate (TP) treated females were subjected to P7 HI or sham surgery, and subsequently (P30+) underwent a battery of auditory testing and water maze assessment. Results confirm previous reports of sex differences following HI, and add new findings of significantly worse performance in TP-treated HI females compared to vehicle treated HI females. Post mortem anatomic analyses showed consistent effects, with significant brain weight decreases seen in HI male and TP-treated HI females but not female HI or sham groups. Further neuromorphometric analysis of brain structures showed that HI male animals exhibited increased pathology relative to HI females as reflected in ventricular enlargement. Findings suggest that neonatal testosterone may act to enhance the deleterious consequences of early HI brain injury, as measured by both neuropathology and behavior.

Corepressors, nuclear receptors, and epigenetic factors on DNA: a tail of repression

The differential exposure to circulating steroid hormones during brain development can have lasting consequences on brain function and behavior; therefore, the tight control of steroid hormone action within the developing brain is necessary for the expression of appropriate sex-typical behavior patterns later in life. The restricted control of steroid hormone action at the level of the DNA can be accomplished through the recruitment of coregulatory complexes. Nuclear receptor action can either be enhanced by the recruitment of coactivator complexes or suppressed by the formation of corepressor complexes. Alternatively, the regulation of nuclear receptor-mediated gene transcription in the developing brain may involve a dynamic process of coactivator and corepressor function on DNA. It is likely that understanding how different combinations of coregulatory matrixes assembly on DNA will lead to further understanding of heterogeneous responses to nuclear receptor activation. We will discuss how coregulators influence gene transcription and repression, the role of chromatin-binding factors in the regulation of gene transcription, and their potential impact on brain development.

Nonenzymatic role of acetylcholinesterase in neuritic sprouting: regional changes in acetylcholinesterase and choline acetyltransferase after neonatal 6-hydroxydopamine lesions

Acetylcholinesterase (AChE) is postulated to play a nonenzymatic role in the development of neuritic projections. We gave the specific neurotoxin, 6-OHDA to rats on postnatal day (PN) 1, a treatment that destroys noradrenergic nerve terminals in the forebrain while producing reactive sprouting in the brainstem. AChE showed profound decreases in the forebrain that persisted in males over the entire phase of major synaptogenesis, from PN4 through PN21; in the brainstem, AChE was increased. Parallel examinations of choline acetyltransferase, an enzymatic marker for cholinergic nerve terminals, showed a different pattern of 6-OHDA-induced alterations, with initial decreases in both forebrain and brainstem in males and regression toward normal by PN21; females were far less affected. The sex differences are in accord with the greater plasticity of the female brain and its more rapid recovery from neurotoxic injury; our findings indicate that these differences are present well before puberty. These results support the view that AChE is involved in neurite formation, unrelated to its enzymatic role in cholinergic neurotransmission. Further, the results for choline acetyltransferase indicate that early depletion of norepinephrine compromises development of acetylcholine systems, consistent with a trophic role for this neurotransmitter.

Morphology and estrogen receptor alpha mRNA expression in the developing green anole forebrain

Sex differences in forebrain morphology arise during development and are often linked to hormonal changes. These dimorphisms frequently occur in regions related to reproductive behaviors. Little is known about the normal ontogeny of reproductive nuclei in the green anole lizard, including whether steroid hormones influence their development. To address this issue, brain region volume, cell density, soma size, and estrogen receptor alpha (ERalpha) mRNA expression were characterized in the preoptic area (POA), ventromedial amygdala (AMY), and ventromedial hypothalamus (VMH) of late embryonic and early post-hatchling anoles. In adulthood, the POA and AMY are associated with male-specific reproductive behaviors and the VMH is implicated in female receptivity. Although soma size decreased in all brain regions with age, brain region volume diminished only in the POA, with a transient sex difference appearing before hatching. Cell density increased with age only in the female AMY. ERalpha mRNA expression was up to four times greater in the developing VMH than POA and AMY, peaking in the VMH around the day of hatching. These results are consistent with the idea that estradiol may influence differentiation of the VMH in particular. However, other factors are likely important to the development of these three brain regions, some of which exert their effects at later developmental stages.

Sex differences and effects of neonatal aromatase inhibition on masculine and feminine copulatory potentials in prairie voles

Copulatory behaviors in most rodents are highly sexually dimorphic, even when circulating hormones are equated between the sexes. Prairie voles (Microtus ochrogaster) are monomorphic in their display of some social behaviors, including partner preferences and parenting, but differences between the sexes in their masculine and feminine copulatory behavior potentials have not been studied in detail. Furthermore, the role of neonatal aromatization of testosterone to estradiol on the development of prairie vole sexual behavior potentials or their brain is unknown. To address these issues, prairie vole pups were injected daily for the first week after birth with 0.5 mg of the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) or oil. Masculine and feminine copulatory behaviors in response to testosterone or estradiol were later examined in both sexes. Males and females showed high mounting and thrusting in response to testosterone, but only males reliably showed ejaculatory behavior. Conversely, males never showed feminine copulatory behaviors in response to estradiol. Sex differences in these behaviors were not affected by neonatal ATD, but ATD-treated females received fewer mounts and thrusts than controls, possibly indicating reduced attractiveness to males. In other groups of subjects, neonatal ATD demasculinized males' tyrosine hydroxylase expression in the anteroventral periventricular preoptic area, and estrogen receptor alpha expression in the medial preoptic area. Thus, although sexual behavior in both sexes of prairie voles is highly masculinized, aromatase during neonatal life is necessary only for females' femininity. Furthermore, copulatory behavior potentials and at least some aspects of brain development in male prairie voles are dissociable by their requirement for neonatal aromatase.

Epigenetic programming of phenotypic variations in reproductive strategies in the rat through maternal care

Studies across multiple organisms reveal considerable phenotypic variation in reproductive tactics. In some species, this variation is associated with maternal effects in which variation in maternal investment results in stable individual differences in reproductive function. Recent studies with the rat suggest that maternal effects can alter the function of neuroendocrine systems associated with female sexual behaviour as well as maternal behaviour. These maternal effects appear to be mediated by epigenetic modifications at the promoter for oestrogen receptor alpha (ERalpha) and subsequent effects on gene expression. The tissue-specific nature of such effects may underlie the co-ordinated variation in multiple forms of reproductive function, resulting in distinct reproductive strategies.

Maternal programming of sexual behavior and hypothalamic-pituitary-gonadal function in the female rat

Variations in parental care predict the age of puberty, sexual activity in adolescence and the age at first pregnancy in humans. These findings parallel descriptions of maternal effects on phenotypic variation in reproductive function in other species. Despite the prevalence of such reports, little is known about potential biological mechanisms and this especially true for effects on female reproductive development. We examined the hypothesis that parental care might alter hypothalamic-pituitary-ovarian function and thus reproductive function in the female offspring of rat mothers that vary pup licking/grooming (LG) over the first week postpartum. As adults, the female offspring of Low LG mothers showed 1) increased sexual receptivity; 2) increased plasma levels of luteinizing hormone (LH) and progesterone at proestrus; 3) an increased positive-feedback effect of estradiol on both plasma LH levels and gonadotropin releasing-hormone (GnRH) expression in the medial preoptic region; and 4) increased estrogen receptor α (ERα) expression in the anterioventral paraventricular nucleus, a system that regulates GnRH. The results of a cross-fostering study provide evidence for a direct effect of postnatal maternal care as well as a possible prenatal influence. Indeed, we found evidence for increased fetal testosterone levels at embryonic day 20 in the female fetuses of High compared to Low LG mothers. Finally, the female offspring of Low LG mothers showed accelerated puberty compared to those of High LG mothers. These data suggest maternal effects in the rat on the development of neuroendocrine systems that regulate female sexual behaviour. Together with studies revealing a maternal effect on the maternal behavior of the female offspring, these findings suggest that maternal care can program alternative reproductive phenotypes in the rat through regionally-specific effects on ERα expression.

Developmental neurotoxicity of low dose diazinon exposure of neonatal rats: effects on serotonin systems in adolescence and adulthood

The developmental neurotoxicity of organophosphate pesticides targets serotonin (5HT) systems, which are involved in emotional and appetitive behaviors. We exposed neonatal rats to daily doses of diazinon on postnatal days 1-4, using doses (0.5 or 2mg/kg) spanning the threshold for barely-detectable cholinesterase inhibition. We then evaluated the effects on 5HT(1A) and 5HT(2) receptors, and on the 5HT transporter in cerebral cortical regions and the brainstem in adolescence through adulthood. Diazinon evoked a lasting deficit in 5HT(1A) receptors in males only, whereas it caused a small but significant increase in 5HT transporters in females; neither effect showed a significant regional selectivity. This pattern differed substantially from that seen in earlier work with another organophosphate, chlorpyrifos, which at pharmacodynamically similar doses spanning the threshold for cholinesterase inhibition, evoked a much more substantial, global upregulation of 5HT receptor expression; with chlorpyrifos, effects on receptors were seen in females, albeit to a lesser extent than in males, and were also regionally distinct. The effects of diazinon were nonmonotonic, showing larger alterations at the lower dose, likely reflecting positive trophic effects of cholinergic stimulation once the threshold for cholinesterase inhibition is exceeded. Our results reinforce the idea that different organophosphates have fundamentally distinct effects on the developmental trajectories of specific neurotransmitter systems, unrelated to their shared action as cholinesterase inhibitors. The effects on 5HT circuits expand the scope of behavioral endpoints that need to be considered in evaluating the developmental neurotoxicity of organophosphates.

Endogenous oestradiol regulates progesterone receptor expression in the brain of female rat fetuses: what is the source of oestradiol?

Testosterone secreted by male testes during fetal development is aromatized to oestradiol (E(2)) or reduced to the androgen, dihydrotestosteorne (DHT), within specific tissues. The female brain is assumed to develop in the relative absence of gonadal steroid hormones, as the ovary is steroidogenically quiescent until later in postnatal life. However, the proximity of a female fetus to male littermates in utero can increase her exposure to testosterone, and thereby its metabolites. To date, it is has been difficult to dissociate the effects of male-derived E(2) from those of DHT on the developing female brain. In the present study, anogential distance (AGD) in females was used as an androgen-dependent bioassay, whereas progesterone receptor (PR) expression within the medial preoptic nucleus (MPN) was used as an E-dependent measure. Pregnant dams received the aromatase inhibitor, 1,4,6-androstatriene-3,17-dione (ATD), or vehicle from embryonic day 16 (ED16) to ED21. On ED22, AGD and PR-immunoreactivity (-ir) were measured in females that had zero, one, or two males (0-2M) or females that had three, four, or five males (3-5M) in the uterine horn. AGD was significantly greater in 3-5M females compared to 0-2M females, suggesting that male littermates are the source of androgenic exposure in the female fetus. ATD treatment significantly decreased PR-ir in the MPN, demonstrating E(2) regulation of PR. However, the total number of males in the uterine horn did not effect PR expression. There was no correlation between PR-ir and AGD, suggesting that these measures are influenced independently. Together, these results suggest that although male littermates provide a significant source of androgens to female fetuses, the amount of E(2) aromatized from male-derived testosterone may not be the only biologically relevant source of androgens or E(2). Alternative sources of E(2) may be essential in ensuring the normal development of the female brain.

The brain as the engine of sex differences in the organization of movement in rats

Sex differences in the kinematic organization of non-reproductive behavior are often relegated to byproducts of sex differences in body morphology. We review evidence showing not only that male and female rats organize their posture and stepping differently during a variety of actions, but that these differences arise from sex differences in the organization of movement in the central nervous system (CNS). Indeed, the expression and choice of sex-typical patterns of movement can be altered by CNS injury. The pattern of hormonal regulation of these sex differences is also not organized as commonly held theory would predict. As expected, males castrated shortly after birth are female-typical in their motor organization. Females ovariectomized at birth, however, are male-typical in their patterns of movement. Thus, female-typical patterns of movement organization are not the default form, but rather are dependent on the effects of gonadal steroids to feminize the developing CNS. The implications of these findings are discussed with regards to our understanding of the evolution of sex differences in CNS anatomy and behavior both for animals and humans.

Persistent cognitive alterations in rats after early postnatal exposure to low doses of the organophosphate pesticide, diazinon

BACKGROUND

Developmental neurotoxicity of organophosphorous insecticides (OPs) involves multiple mechanisms in addition to cholinesterase inhibition. We have found persisting effects of developmental chlorpyrifos (CPF) and diazinon (DZN) on cholinergic and serotonergic neurotransmitter systems and gene expression as well as behavioral function. Both molecular/neurochemical and behavioral effects of developmental OP exposure have been seen at doses below those which cause appreciable cholinesterase inhibition.

OBJECTIVES

We sought to determine if developmental DZN exposure at doses which do not produce significant acetylcholinesterase inhibition cause persisting cognitive deficits.

METHODS

Rats were exposed to DZN on postnatal days 1-4 at doses (0.5 and 2 mg/kg/d) that span the threshold for cholinesterase inhibition. They were later examined with a cognitive battery tests similar to that used with CPF.

RESULTS

In the T-maze DZN caused significant hyperactivity in the initial trials of the session, but not later. In a longer assessment of locomotor activity no DZN-induced changes were seen over a 1-hour session. Prepulse inhibition was reduced by DZN exposure selectively in males vs. females; DZN eliminated the sex difference present in controls. In the radial maze, the lower but not higher DZN dose significantly impaired spatial learning. This type of nonmonotonic dose-effect function has previously been seen with CPF as well. The lower dose DZN group also showed significantly greater sensitivity to the memory-impairing effects of scopolamine a muscarinic acetylcholine antagonist.

CONCLUSIONS

Neonatal DZN exposure below the threshold for appreciable cholinesterase inhibition caused persisting neurocognitive deficits in adulthood. The addition of some inhibition of AChE with a higher dose reversed the cognitive impairment. This non-monotonic dose-effect function has also been seen with neurochemical effects. Some of the DZN effects on cognition resemble those seen earlier for CPF, some differ. Our data suggest that DZN and CPF affect transmitter systems supporting memory function, differently, implying participation of mechanisms other than their common inhibition of cholinesterase.

Synthesis of estrogens in progenitor cells of adult fish brain: evolutive novelty or exaggeration of a more general mechanism implicating estrogens in neurogenesis?

In contrast to other vertebrates, in which the adult brain shows limited adult neurogenesis, teleost fishes exhibit an unparalleled capacity to generate new neurons as adults, suggesting that their brains present a highly permissive environment for the maintenance and proliferation of adult progenitors. Here, we examine the hypothesis that one of the factors permitting establishment of this favourable environment is estradiol. Indeed, recent data showed that radial glial cells strongly expressed one of two aromatase duplicated genes. Aromatase is the estrogen-synthesizing enzyme and this observation is of great interest, given that radial glial cells are progenitor cells capable of generating new neurons. Given the well-documented roles of estrogens on cell fate, and notably on cell proliferation, these data suggest that estradiol could be involved in maintaining and/or activating these progenitors. Examination of recent data in birds and mammals suggests that the situation in fish could well be an exaggeration of a more general mechanism implicating estrogens in neurogenesis. Indeed, there is accumulating evidence that estrogens are involved in embryonic, adult or reparative neurogenesis in other vertebrates, notably in mammals.

Environmental Programming of Phenotypic Diversity in Female Reproductive Strategies

Among invertebrates, certain hermaphroditic species reproduce sexually, but with no process of sexual differentiation. In such cases the brain is bisexual: Each member of the species develops male and female sexual organs and retains the capacity to express both male and female patterns of reproductive behavior. Members of such species can reproduce socially or alone. Mammals and many other species reproduce both sexually and socially, which requires an active process of sexual differentiation of reproductive organs and brain. The primary theme of this chapter is simply that this process admits to variation and thus individual differences in gender-specific patterns of reproductive function. The focus on this chapter is the often neglected variation in the development of reproductive function in the female mammal. The basic premise is that evolution has not defined any single, optimal reproductive phenotype, but rather encourages plasticity in specific reproductive traits among same sex members of the species that are derived from variations in the quality of the prevailing environment during development that are mediated by alterations in parent-offspring interactions. Thus, the variations in parental care that define the reproductive phenotype of the offspring are influenced by the quality of the environment (i.e., nutrient availability, predation, infection, population density, and so on).

Sexually dimorphic gene regulation in brain as a target for endocrine disrupters: developmental exposure of rats to 4-methylbenzylidene camphor

The developing neuroendocrine brain represents a potential target for endocrine active chemicals. The UV filter 4-methylbenzylidene camphor (4-MBC) exhibits estrogenic activity, but also interferes with the thyroid axis. We investigated effects of pre- and postnatal exposure to 4-MBC in the same rat offspring at brain and reproductive organ levels. 4-MBC (7, 24, 47 mg/kg/day) was administered in chow to the parent generation before mating, during gestation and lactation, and to the offspring until adulthood. mRNA of estrogen target genes involved in control of sexual behavior and gonadal functions was measured by real-time RT-PCR in ventromedial hypothalamic nucleus (VMH) and medial preoptic area (MPO) of adult offspring. 4-MBC exposure affected mRNA levels of ER alpha, progesterone receptor (PR), preproenkephalin (PPE) and insulin-like growth factor-I (IGF-I) in a sex- and region-specific manner. In order to assess possible changes in sensitivity of target genes to estrogens, offspring were gonadectomized on day 70, injected with estradiol (E2, 10 or 50 microg/kg s.c.) or vehicle on day 84, and sacrificed 6 h later. The acute induction of PR mRNA, and repression (at 6 h) of PPE mRNA by E2 was enhanced by 4-MBC in male and female VMH and female MPO, whereas male MPO exhibited reduced responsiveness of both genes. Steroid receptor coactivator SRC-1 mRNA levels were increased in female VMH and MPO. The data indicate profound sex- and region-specific alterations in the regulation of estrogen target genes at brain level. Effect patterns in baseline and E2-induced gene expression differ from those in uterus and prostate.

Play fighting in androgen-insensitivetfm rats: Evidence that androgen receptors are necessary for the development of adult playful attack and defense

The frequency of playful attack and the style of playful defense, are modifiable by gonadal steroids and change after puberty in male and female rats. The present study examined the play behavior exhibited by testicular feminized mutation (tfm)-affected males, who are insensitive to androgens but can bind estrogens aromatized from androgens, to determine the relative contributions of androgens and estrogens to the age-related changes in play behavior. tfm males did not exhibit a decrease in playful attack with age and were more likely to maintain the use of complete rotations, a juvenile form of playful defense, into adulthood. tfm males did however, show age related changes in the use of partial rotations and upright postures, two other forms of playful defense, that were similar to normal males. These data suggest that the development of play fighting and defense in males is dependent on both androgen- and estrogen-receptor-mediated effects.

Alpha7 nicotinic acetylcholine receptors targeted by cholinergic developmental neurotoxicants: nicotine and chlorpyrifos

Alpha7 nicotinic acetylcholine receptors (nAChRs) play a role in axonogenesis, synaptogenesis and synaptic plasticity, and are therefore potential targets for developmental neurotoxicants. We administered nicotine to neonatal rats during discrete periods spanning the onset and peak of axonogenesis/synaptogenesis, focusing on three brain regions with disparate distributions of cell bodies and neural projections: brainstem, forebrain and cerebellum. Nicotine treatment on postnatal days (PN) 1-4 had little or no effect on alpha7 nAChRs but treatment during the second (PN11-14) or third (PN21-24) weeks elicited significant decrements in receptor expression in brainstem and cerebellum, regions containing cell bodies that project to the forebrain. Exposure to chlorpyrifos, a neurotoxicant pesticide that acts partially through cholinergic mechanisms, also elicited deficits in alpha7 nAChRs during the second postnatal week but not the first week. For both nicotine and chlorpyrifos, the effects on alpha7 nAChRs were distinct from those on the alpha4beta2 subtype. Continuous prenatal nicotine exposure, which elicits subsequent, postnatal deficits in axonogenesis and synaptogenesis, also produced delayed-onset changes in alpha7 nAChRs, characterized by reductions in the forebrain and upregulation in the brainstem and cerebellum, a pattern consistent with impaired axonogenesis/synaptogenesis and reactive sprouting. Males were more sensitive to the persistent effects of prenatal nicotine exposure on alpha7 nAChRs, a pattern that mimics neurobehavioral deficits resulting from this treatment. The present findings reinforce the mechanistic involvement of alpha7 nAChRs in the actions of developmental neurotoxicants, and its biomarker potential for neuroteratogens that target neuritic outgrowth.

Androgen excess fetal programming of female reproduction: a developmental aetiology for polycystic ovary syndrome?

The aetiology of polycystic ovary syndrome (PCOS) remains unknown. This familial syndrome is prevalent among reproductive-aged women and its inheritance indicates a dominant regulatory gene with incomplete penetrance. However, promising candidate genes have proven unreliable as markers for the PCOS phenotype. This lack of genetic linkage may represent both extreme heterogeneity of PCOS and difficulty in establishing a universally accepted PCOS diagnosis. Nevertheless, hyperandrogenism is one of the most consistently expressed PCOS traits. Animal models that mimic fetal androgen excess may thus provide unique insight into the origins of the PCOS syndrome. Many female mammals exposed to androgen excess in utero or during early post-natal life typically show masculinized and defeminized behaviour, ovulatory dysfunction and virilized genitalia, although behavioural and ovulatory dysfunction can coexist without virilized genitalia based upon the timing of androgen excess. One animal model shows particular relevance to PCOS: the prenatally androgenized female rhesus monkey. Females exposed to androgen excess early in gestation exhibit hyperandrogenism, oligomenorrhoea and enlarged, polyfollicular ovaries, in addition to LH hypersecretion, impaired embryo development, insulin resistance accompanying abdominal obesity, impaired insulin response to glucose and hyperlipidaemia. Female monkeys exposed to androgen excess late in gestation mimic these programmed changes, except for LH and insulin secretion defects. In utero androgen excess may thus variably perturb multiple organ system programming and thereby provide a single, fetal origin for a heterogeneous adult syndrome.

Gestational dexamethasone treatment elicits sex-dependent alterations in locomotor activity, reward-based memory and hippocampal cholinergic function in adolescent and adult rats

Glucocorticoids are the consensus treatment for preventing respiratory distress syndrome in preterm infants but there is emerging evidence of subsequent neurobehavioral abnormalities, independent of somatic growth effects. Pregnant rats were given 0.2 mg/kg of dexamethasone, a dose commensurate with clinical use, on gestational days 17-19 and behavioral evaluations were made on the offspring in adolescence and adulthood. The dexamethasone groups had the same body weights as the controls but nevertheless displayed long-term, sex-selective alterations in locomotor and cognitive behaviors. In the figure-8 activity apparatus, dexamethasone treatment ablated the normal sex differences in locomotor activity by reducing values in females to the lower level typical of males; habituation of activity similarly was impaired in females, reducing the profile to match that of control males, while male rats in the dexamethasone group showed a partially feminized pattern of habituation. In the 8-arm radial maze, control rats displayed typical sex differences, with male rats performing more accurately than females. Dexamethasone treatment eliminated this normal dichotomy, delaying learning in males while improving performance in females to the level normally seen in control males. Finally, we assessed hippocampal [3H]hemicholinium-3 binding as a biomarker for cholinergic synaptic activity, and again found loss of sex differences in the dexamethasone group: values in males were increased to the higher levels typical of females. These results indicate that gestational treatment with dexamethasone obtunds the normal sex differences in neurochemistry and behavior that are typically seen in adolescence in adulthood, thus producing sex-selective alterations in activity, learning, and memory.

Alterations in central nervous system serotonergic and dopaminergic synaptic activity in adulthood after prenatal or neonatal chlorpyrifos exposure

Exposure to chlorpyrifos (CPF) alters neuronal development of serotonin (5HT) and dopamine systems, and we recently found long-term alterations in behaviors related to 5HT function. To characterize the synaptic mechanisms underlying these effects, we exposed developing rats to CPF regimens below the threshold for systemic toxicity, in three treatment windows: gestational days (GD) 17-20, postnatal days (PN) 1-4, or PN11-14. In early adulthood (PN60), we assessed basal neurotransmitter content and synaptic activity (turnover) in brain regions containing the major 5HT and dopamine projections. CPF exposure on GD17-20 or PN1-4 evoked long-term increases in 5HT turnover across multiple regions; the effects were not secondary to changes in neurotransmitter content, which was unaffected or even decreased. When the treatment window was shifted to PN11-14, there were no long-term effects. Dopamine turnover also showed significant increases after CPF exposure on GD17-20, but only when the dose was raised above the threshold for overt toxicity; however, hippocampal dopamine content was profoundly subnormal after exposures below or above the acute, toxic threshold, suggesting outright neurotoxicity. These results indicate that, in a critical developmental period, apparently nontoxic exposures to CPF produce lasting activation of 5HT systems in association with 5HT-associated behavioral anomalies.

Developmental exposure of rats to chlorpyrifos leads to behavioral alterations in adulthood, involving serotonergic mechanisms and resembling animal models of depression

Developmental exposure to chlorpyrifos (CPF) causes persistent changes in serotonergic (5HT) systems. We administered 1 mg/kg/day CPF to rats on postnatal days 1-4, a regimen below the threshold for systemic toxicity. When tested in adulthood, CPF-exposed animals showed abnormalities in behavioral tests that involve 5HT mechanisms. In the elevated plus maze, males treated with CPF spent more time in the open arms, an effect seen with 5HT deficiencies in animal models of depression. Similarly, in an anhedonia test, the CPF-exposed group showed a decreased preference for chocolate milk versus water. Developmental CPF exposure also has lasting effects on cognitive function. We replicated our earlier finding that developmental CPF exposure ablates the normal sex differences in 16-arm radial maze learning and memory: during acquisition training, control male rats typically perform more accurately than do control females, but CPF treatment eliminated this normal sex difference. Females exposed to CPF showed a reduction in working and reference memory errors down to the rate of control males. Conversely, CPF-exposed males exhibited an increase in working and reference memory errors. After radial-arm acquisition training, we assessed the role of 5HT by challenging the animals with the 5HT2 receptor antagonist ketanserin. Ketanserin did not affect performance in controls but elicited dose-dependent increases in working and reference memory errors in the CPF group, indicating an abnormal dependence on 5HT systems. Our results indicate that neonatal CPF exposures, classically thought to be subtoxic, produce lasting changes in 5HT-related behaviors that resemble animal models of depression.

Ontogenic expression patterns of several nuclear receptors and cytochrome P450 aromatases in brain and gonads of the Nile tilapia Oreochromis niloticus suggests their involvement in sex differentiation

Using semi-quantitative reverse transcriptase polymerase chain reaction we analyzed the ontogenic expression patterns of several nuclear receptors (estrogen receptors [ERalpha and beta], androgen receptors [ARalpha and beta], Ad4BP/SF-1 and Dax-1) and cytochrome P450 aromatases (brain and ovarian types) in whole brain and gonads of the Nile tilapia. ERalpha and beta transcripts were evident in both sexes with a high expression of ERalpha in females at 0 day after hatching (0 dah). ARalpha appeared early (0 dah) in males and while in females at 25 dah. Among the two types of cytochrome P450 aromatases, the expression of the brain type (bP450arom) but not the ovarian type (oP450arom) was evident from 0 to 90 dah in the whole brain of both males and females. Expression of Ad4BP/SF-1 in female brain began from 0 dah but in male brain at 5 dah. Expression of Dax-1 began at 0 dah and it was higher throughout in male brain than that of the female brain. In gonads, ERalpha and beta transcripts were evident in both sexes with slight variation. In females, both oP450arom and Ad4BP/SF-1 amplicons were evident at 15 dah. In males, although faint expressions of Ad4BP/SF-1 amplicons were evident at early duration of development, oP450arom did not appear until 90 dah. Conversely, expression of bP450arom was observed throughout in the developing testis with varied pattern while in developing ovary it was evident till 15 dah and reappeared only after 90 dah. Taken together, present results suggest that brain acts merely as a synchronizer in the sex differentiation process initiated by gonadal cues/factors in the Nile tilapia.

Sex Differences in Righting From Supine to Prone in Rats (Rattus norvegicus): A Masculinized Skeletomusculature Is Not Required

Previous research has shown that sex differences exist in the composition of lateral movements (E. F. Field, I. Q. Whishaw, & S. M. Pellis, 1996, 1997a, 1997b; see also records 1996-06132-009, 1997-05322-015, and 1997-04722-005). An unresolved question is whether sex differences are present in other movements, such as rotation around the longitudinal axis, and whether this difference is dependent on a feminine or masculine skeletomusculature. Female rats (Rattus norvegicus) first rotate their forequarters and then their hindquarters in the same direction. Male rats exhibit rotation of the hindquarters counter to the direction of forequarter rotation. Males with the testicular feminized mutation, who have a feminized skeletomusculature and masculinized central nervous system, are similar to male controls. This study provides evidence that sex differences in movement integration are not restricted to the lateral plane, are not solely due to sex differences in skeletomusculature, and thus are likely mediated by the central nervous system.

Sex-specific development of cortical monoamine levels in mouse

Several mental health disorders exhibit sex differences in monoamine levels associated with dimorphic cortical ontogeny. Studies in rodents support the notion that monoamines can profoundly modulate morphogenesis. Here, we show significant sex and hemisphere differences in BALB/cByJ mice on postnatal day 3 for dopamine (DA) and serotonin (5-TH), supporting the notion that sex differences in early monoaminergic ontogeny may result in dimorphic cortical development. Such sex differences may also influence differential behavioral and/or clinical outcomes.

Sexually dimorphic and estrogen-dependent expression of estrogen receptor beta in the ventromedial hypothalamus during rat postnatal development

The ventromedial hypothalamus (VMH) is a sexually dimorphic region of the brain related to female reproductive behavior. The effect of estrogen in the adult rat VMH is thought to be mediated predominantly via estrogen receptor (ER)alpha, because this receptor is expressed at considerably higher levels than ER beta. The present study revealed, using in situ hybridization and immunohistochemistry, that both ER beta mRNA and protein were expressed in the ventrolateral portion of the caudal VMH, at remarkably higher levels during early postnatal development than in adulthood. In addition, the expression was sexually dimorphic, with females having significantly more ER beta-immunoreactive (-ir) cells than males, between postnatal d 5 (P5) and P14, although the sex difference was not significant by P21. Double-label immunofluorescence revealed that 66% of ER beta-ir cells coexpressed ER alpha in the caudal VMH of the P5 female rat. Furthermore, neonatal treatment with E2 benzoate down-regulated ER beta mRNA in the female rat VMH at P5 and decreased VMH ER beta-ir cells during the period between P5 and P14. In contrast to females, no differences in expression of ER beta mRNA or protein were detected between control and E2 benzoate-treated males. These results suggest that estrogen is involved in regulating the sexually dimorphic expression of ER beta in the VMH during early postnatal development of the rat.

An environmental antiandrogen, vinclozolin, alters the organization of play behavior

During mammalian sexual differentiation, the androgens, testosterone and dihydrotestosterone are critical for the organization of the male phenotype. In rats, play behavior is sexually dimorphic. Administration of exogenous androgens during the perinatal period results in masculine-like play behavior of juveniles. Recently, there has been increasing concern about the potential for environmental endocrine-disrupting chemicals (EDCs) to alter sexual differentiation in mammals. One such EDC is the fungicide and androgen receptor (AR) antagonist, vinclozolin. We tested whether developmental exposure to an EDC could alter androgen-dependent behaviors such as play. To examine this possibility, neonatal male rats were injected from Postnatal Days (PND) 2 to 3 with corn oil, pharmacological antiandrogen flutamide (50 mg/kg/day) or vinclozolin (200 mg/kg/day); whereas neonatal females were treated either with corn oil or testosterone propionate (TP, 250 microg/kg/day). At PNDs 36-37, animals were observed for social play. Behaviors associated with general social activity, such as sniffing and dorsal contact, were unaffected by treatment or sex. However, play behavior in males treated with flutamide or vinclozolin was significantly reduced to near-female levels when compared to control males. Play behavior in females exposed to TP during the neonatal period was significantly increased when compared with control females. Hence, this study suggests that perinatal exposure to vinclozolin, an environmental antiandrogen, can alter androgen-dependent behavior, such as play, in the male rat.

Susceptibility to postnatal growth retardation induced by 5-AZA-2'-deoxycytidine in utero: gender specificity and correlation with reduced insulin-like growth factor 1

The DNA demethylating agent 5-AZA-2'-deoxyxytidine (5-AZA-CdR) alters gene expression in mice exposed during developmental stages and causes malformations and growth suppression. The aim of this study was to determine if 5-AZA-CdR-induced growth retardation is associated with alterations in energy metabolism or in serum IGF-1 levels. Mice were exposed in utero to 5-AZA-CdR at gestation day 10. At postnatal day 21, exposed pups were weaned and body weights recorded. At 3 months of age, reproductive capacity was studied. At 5 months old, after body weight was recorded mice were killed and serum was collected to determine serum glucose, corticosterone, and IGF-1 levels. The body weights of both treated males and females were reduced at weaning compared with controls, but by 5 months of age, only the male body weight was affected. Reproductive capacity of males and females was reduced with males being more affected. Levels of corticosterone and glucose were not altered. Serum IGF-1 levels were lower in males exposed in utero to 5-AZA-CdR when compared to controls, but not in females, and correlated significantly with body weights. Our data suggest that the decreased levels of IGF-1 associated with the treatment could be the cause of the observed growth retardation in the in utero-exposed mice. A gender dimorphic effect, where males are more affected, is evident.

Sex differences in rapid auditory processing deficits in ectopic BXSB/MpJ mice

Prior research with rodent models, performed predominantly in males, has demonstrated a significant association between focal neocortical malformations (e.g. ectopias and microgyria) and rate-specific auditory processing deficits. In the current study and consistent with prior findings, we report that ectopic male BXSB/MpJ mice exhibit impairments in detecting a two-tone oddball stimulus at short but not long inter-stimulus interval durations when compared to non-ectopic male littermates. However, ectopic female littermates showed no rapid auditory processing deficit when compared to non-ectopic females on this same task. Current results add growing support to: (1) an association between focal cortical malformations and impaired auditory processing in males; and (2) the existence of sex differences in the behavioral consequences of focal cortical malformations.

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.

Androgen receptors in the embryonic zebra finch hindbrain suggest a function for maternal androgens in perihatching survival

Bird embryos are exposed to maternal androgens deposited in the egg, but the role of these hormones in embryonic development and hatchling survival is unclear. To identify possible target organs, we used in situ hybridization to study the distribution of androgen receptor (AR) RNA in the developing zebra finch brain. The first brain expression domain of AR mRNA is in the hindbrain. From embryonic day 7 (E7) onward, when the hypoglossal motor nucleus (nXII) has just formed, there was AR mRNA expression in both its lingual (nXIIl) and its tracheosyringeal (nXIIts) parts, and this was the major site of hindbrain expression at all embryonic stages and in both sexes. From E8 onward, we also found AR mRNA in the supraspinal motor nucleus (nSSp), which innervates neck muscles. Furthermore, the syrinx, the target of the nXIIts, contained AR mRNA by E10, localized principally in the perichondria. Muscle was first evident in the syringeal region at E9, but no AR was detected in syringeal muscles until after hatching. The expression pattern of AR in the zebra finch embryo suggests that maternal androgens act via AR in the brainstem and syrinx to influence hatching as well as acoustic and visual components of food-begging behavior. Maternal androgens seem unlikely to function in the development of sexual dimorphisms in the zebra finch nXIIts and syrinx, insofar as these are not evident until between 10 and 20 days posthatching.

Sexual differentiation of astrocyte morphology in the developing rat preoptic area

The preoptic area is an important brain region controlling sex-typic behaviour and physiology, and astrocytes of this region are responsive to steroids perinatally. Utilizing glial fibrillary acidic protein immunocytochemistry, the morphology of astrocytes in the preoptic area of male and female rat pups was examined on the day of birth and on postnatal day 3. As early as the day of birth, astrocytes of the male preoptic area exhibit both significantly greater primary process length and number of primary processes, and these differences remain at postnatal day 3. Application of exogenous steroid to females suggested that gonadal steroids, in particular oestradiol, mediate the sex difference. Pups received 100 micro g of steroid on the day of birth and again on postnatal day 1, and astrocyte morphology was assessed on postnatal day 3. Both oestradiol and testosterone induced significant changes in process length and number compared to vehicle-treated controls. Astrocytes of oestradiol-treated females did not differ on PN3 from those of PN3-untreated males. Exposure to the nonaromatizable steroid, dihydrotestosterone, had no effect on any attribute of astrocyte morphology. This suggests the effects induced by testosterone are mediated by oestradiol following local aromatization of the steroid, and not through direct activation of the androgen receptor. Astrocytes are important in synapse formation and efficacy, and we hypothesize a role for astrocyte complexity and differentiation in the establishment of synaptic patterning.

Androgen imprinting of the brain in animal models and humans with intersex disorders: review and recommendations

PURPOSE

Psychosexual development, gender assignment and surgical treatment in patients with intersex are controversial issues in the medical literature. Some groups are of the opinion that gender identity and sexual orientation are determined prenatally secondary to the fetal hormonal environment causing irreversible development of the nervous system. We reviewed the evidence in animal and human studies to determine the possible role of early postnatal androgen production in gender development.

MATERIALS AND METHODS

An extensive literature review was performed of data from animal experiments and human studies. RESULTS Many animal studies show that adding or removing hormonal stimulus in early postnatal life can profoundly alter gender behavior of the adult animal. Human case studies show that late intervention is unable to reverse gender orientation from male to female. Most studies have not permitted testing of whether early gender assignment and treatment as female with suppression/ablation of postnatal androgen production leads to improved concordance of the gender identity and sex of rearing.

CONCLUSIONS

Animal studies support a role for postnatal androgens in brain/behavior development with human studies neither completely supportive nor antagonistic. Therefore, gender assignment in infants with intersex should be made with the possibility in mind that postnatal testicular hormones at ages 1 to 6 months may affect gender identity. A case-control study is required to test the hypothesis that postnatal androgen exposure may convert ambisexual brain functions to committed male behavior patterns.

Expression of the nuclear receptor coactivator, cAMP response element-binding protein, is sexually dimorphic and modulates sexual differentiation of neonatal rat brain

Recent studies indicate that the transcriptional activity of steroid receptors is governed by proteins called nuclear receptor coactivators. Using immunocytochemistry, we found that on the day of birth (postnatal d 0) males express higher levels of the nuclear receptor coactivator, cAMP response element binding protein-binding protein (CBP), within the ventromedial hypothalamus, medial preoptic area, and arcuate nucleus. Using Western immunoblots, we confirmed that males have higher levels of CBP on postnatal d 0, 1, and 5; however, there was no sex difference on postnatal d 11. To examine the functional role of CBP, we infused oligodeoxynucleotides that were antisense to CBP mRNA or a scrambled sequence as a control into the hypothalamus of female rats on postnatal d 0, 1, and 2. On postnatal d 1, all rats were injected with 100 microg testosterone propionate to both masculinize (increase male) and defeminize (decrease female) sexual behavior. Rats were ovariectomized in adulthood and tested for adult sexual behavior. Neonatal CBP antisense oligodeoxynucleotides treatment interfered with the defeminizing, but not the masculinizing, actions of testosterone. These results indicate that CBP expression in developing rat brain is sexually dimorphic and an important modulator for steroid hormone action.

Affective syndrome during perimenopause

Perimenopause is characterized by decreasing levels of estrogens and progesterone until gonadal secretion comes to a complete halt. There are still very different views and positions on the significance of the menopause. Physical, mental-vegetative and depressive symptoms during climacteric are different in each culture. Currently, there is some controversy as to whether or not there really is a rise in actual depression during the perimenopausal phase of woman's life. The observations from humans taken together indicate that depressive disorders, as defined in ICD-10, do not occur more frequently during perimenopause. In this context, the terms subthreshold depression and or subsyndromal depression are important, describing depressive symptoms which do not fulfil the complete clinical picture of a depressive episode, either because there are not enough symptoms or because they are not severe enough. The affected women still suffer, but until now not enough studies have been carried out on this particular area. In view of the complexity and relevance of the perimenopausal period in a woman's life, it is necessary to establish and maintain a network of treatment between the family physician, gynecologist, psychiatrist and or psychotherapist, as soon as significant depressive symptoms occur.