Double duty for sex differences in the brain

Sex differences in viewing sexual stimuli: an eye-tracking study in men and women

Men and women exhibit different neural, genital, and subjective arousal responses to visual sexual stimuli. The source of these sex differences is unknown. We hypothesized that men and women look differently at sexual stimuli, resulting in different responses. We used eye tracking to measure looking by 15 male and 30 female (15 normal cycling (NC) and 15 oral contracepting (OC)) heterosexual adults viewing sexually explicit photos. NC Women were tested during their menstrual, periovulatory, and luteal phases while Men and OC Women were tested at equivalent intervals, producing three test sessions per individual. Men, NC, and OC Women differed in the relative amounts of first looks towards, percent time looking at, and probability of looking at, defined regions of the pictures. Men spent more time, and had a higher probability of, looking at female faces. NC Women had more first looks towards, spent more time, and had a higher probability of, looking at genitals. OC Women spent more time, and had a higher probability of, looking at contextual regions of pictures, those featuring clothing or background. Groups did not differ in looking at the female body. Menstrual cycle phase did not affect women's looking patterns. However, differences between OC and NC groups suggest hormonal influences on attention to sexual stimuli that were unexplained by subject characteristic differences. Our finding that men and women attend to different aspects of the same visual sexual stimuli could reflect pre-existing cognitive biases that possibly contribute to sex differences in neural, subjective, and physiological arousal.

Paternal behavior influences development of aggression and vasopressin expression in male California mouse offspring

Parental care has been demonstrated to have important effects on offspring behavioral development. California mice (Peromyscus californicus) are biparental, and correlational evidence suggests that pup retrieving by fathers has important effects on the development of aggressive behavior and extra-hypothalamic vasopressin systems. We tested whether retrievals affected these systems by manipulating paternal retrieval behavior between day 15 and 21 postpartum. Licking and grooming behavior affect behavioral development in rats, so we also experimentally reduced huddling and grooming behavior by castrating a subset of fathers. Experimentally increasing the frequency of paternal pup retrieving behavior decreased attack latency in resident-intruder in both male and female adult offspring, whereas experimental reduction of huddling and grooming had no effect. In a separate group of male offspring, we examined vasopressin immunoreactivity (AVP-ir) in two regions of the posterior bed nucleus of the stria terminalis (BNST): the dorsal fiber tracts (dBNST) and the ventral cell body-containing region (vBNST). Experimentally increasing retrievals led to an apparent shift in AVP-ir distribution. Specifically, offspring from the high retrieval group had more AVP-ir than offspring from the sham retrieval group in the dBNST, whereas the opposite was observed in the vBNST. Experimental reduction of paternal grooming was associated with increased AVP-ir in the paraventricular nucleus and also increased corticosterone and progesterone, similar to observed effects of maternal grooming on HPA function. This study provides further evidence that paternal behavior influences the development of aggression and associated neural substrates.

Recollections of one's own past: the effects of aging and gender on the neural mechanisms of episodic autobiographical memory

Episodic autobiographical recollection is the most complex form of human memory. It relies on interactions between episodic memory, associated emotions, and a sense of self-continuity along the time axis of one's personal life history. Evidence exists that autobiographical memory performance as well as its underlying brain mechanisms are influenced by genetic, physiological, psychological, situational, and social-cultural factors. In particular, age (normal cognitive aging as well as age of memories, as defined by the time interval elapsed since information encoding) and gender affect both the performance level and the neural substrates of autobiographical recollection. In this review, studies concerned with aging and gender effects on autobiographical memory are discussed with reference to other age- and gender-related influences on human cognition, as well as clinical data on demented patients. Both age and gender act upon the functional hemispheric lateralization of autobiographical recollection and the prefrontal, hippocampal and parahippocampal engagement in information processing. On the performance level, re-collective qualities such as episodic detail and emotional intensity of autobiographical memories are modulated by both factors. Although the effects of aging and gender on human brain function are built upon different genetic and physiological mechanisms, they influence at least in part the same neurofunctional and behavioral dimensions of autobiographical recollection. Interestingly, age- and gender-related specificities in the neural mechanisms of autobiographical recollection need not be reflected on the performance level.

Direct Regulation of Adult Brain Function by the Male-Specific Factor SRY

The central dogma of mammalian brain sexual differentiation has contended that sex steroids of gonadal origin organize the neural circuits of the developing brain. Recent evidence has begun to challenge this idea and has suggested that, independent of the masculinizing effects of gonadal secretions, XY and XX brain cells have different patterns of gene expression that influence their differentiation and function. We have previously shown that specific differences in gene expression exist between male and female developing brains and that these differences precede the influences of gonadal hormones. Here we demonstrate that the Y chromosome-linked, male-determining gene Sry is specifically expressed in the substantia nigra of the adult male rodent in tyrosine hydroxylase-expressing neurons. Furthermore, using antisense oligodeoxynucleotides, we show that Sry downregulation in the substantia nigra causes a statistically significant decrease in tyrosine hydroxylase expression with no overall effect on neuronal numbers and that this decrease leads to motor deficits in male rats. Our studies suggest that Sry directly affects the biochemical properties of the dopaminergic neurons of the nigrostriatal system and the specific motor behaviors they control. These results demonstrate a direct male-specific effect on the brain by a gene encoded only in the male genome, without any mediation by gonadal hormones.

Sexual differentiation of central vasopressin and vasotocin systems in vertebrates: different mechanisms, similar endpoints

Vasopressin neurons in the bed nucleus of the stria terminalis and amygdala and vasotocin neurons in homologous areas in non-mammalian vertebrates show some of the most consistently found neural sex differences, with males having more cells and denser projections than females. These projections have been implicated in social and reproductive behaviors but also in autonomic functions. The sex differences in these projections may cause as well as prevent sex differences in these functions. This paper discusses the anatomy, steroid dependency, and sexual differentiation of these neurons. Although the final steps in sexual differentiation of vasopressin/vasotocin expression may be similar across vertebrate species, what triggers differentiation may vary dramatically. For example, during development, estrogen masculinizes vasopressin expression in rats but feminizes its counterpart in Japanese quail. Apparently, nature consistently finds a way of maintaining sex differences in vasopressin and vasotocin pathways, suggesting that the function of these differences is important enough that it was conserved during evolution.

Sexual dimorphism in the vasopressin system: Lack of an altered behavioral phenotype in female V1a receptor knockout mice

Previous findings with an AVP V1a receptor knockout mouse (V1aRKO) demonstrate a significant role for this receptor in anxiety-like behavior in males. Here we report the lack of anxiety-like effects of the null mutation in female mice. V1aRKO females performed normally on all tests for anxiety-like behavior. This sex difference may be due to the sexual dimorphism in the extra-hypothalamic vasopressin system, with males having significantly more vasopressin fibers in this system.

The vertebrate social behavior network: evolutionary themes and variations

Based on a wide variety of data, it is now clear that birds and teleost (bony) fish possess a core "social behavior network" within the basal forebrain and midbrain that is homologous to the social behavior network of mammals. The nodes of this network are reciprocally connected, contain receptors for sex steroid hormones, and are involved in multiple forms of social behavior. Other hodological features and neuropeptide distributions are likewise very similar across taxa. This evolutionary conservation represents a boon for experiments on phenotypic behavioral variation, as the extraordinary social diversity of teleost fish and songbirds can now be used to generate broadly relevant insights into issues of brain function that are not particularly tractable in other vertebrate groups. Two such lines of research are presented here, each of which addresses functional variation within the network as it relates to divergent patterns of social behavior. In the first set of experiments, we have used a sexually polymorphic fish to demonstrate that natural selection can operate independently on hypothalamic neuroendocrine functions that are relevant for (1) gonadal regulation and (2) sex-typical behavioral modulation. In the second set of experiments, we have exploited the diversity of avian social organizations and ecologies to isolate species-typical group size as a quasi-independent variable. These experiments have shown that specific areas and peptidergic components of the social behavior network possess functional properties that evolve in parallel with divergence and convergence in sociality.

Gender differences in the functional neuroanatomy of emotional episodic autobiographical memory

Autobiographical memory is based on interactions between episodic memory contents, associated emotions, and a sense of self-continuity along the time axis of one's life. The functional neuroanatomy subserving autobiographical memory is known to include prefrontal, medial and lateral temporal, as well as retrosplenial brain areas; however, whether gender differences exist in neural correlates of autobiographical memory remains to be clarified. We reanalyzed data from a previous functional magnetic resonance imaging (fMRI) experiment to investigate gender-related differences in the neural bases of autobiographical memories with differential remoteness and emotional valence. On the behavioral level, there were no significant gender differences in memory performance or emotional intensity of memories. Activations common to males and females during autobiographical memory retrieval were observed in a bilateral network of brain areas comprising medial and lateral temporal regions, including hippocampal and parahippocampal structures, posterior cingulate, as well as prefrontal cortex. In males (relative to females), all types of autobiographical memories investigated were associated with differential activation of the left parahippocampal gyrus. By contrast, right dorsolateral prefrontal cortex was activated differentially by females. In addition, the right insula was activated differentially in females during remote and negative memory retrieval. The data show gender-related differential neural activations within the network subserving autobiographical memory in both genders. We suggest that the differential activations may reflect gender-specific cognitive strategies during access to autobiographical memories that do not necessarily affect the behavioral level of memory performance and emotionality.

Co-localization and distribution of corticotrophin-releasing hormone, arginine vasopressin and enkephalin in the paraventricular nucleus of sheep: A sex comparison

The paraventricular nucleus (PVN) is integral to regulation of the hypothalamo-pituitary-adrenal (HPA) axis and contains cells producing corticotrophin-releasing hormone (CRH), arginine vasopressin (AVP) and enkephalin. We used immunohistochemistry to map these peptides and to resolve the extent of co-localization within PVN cells in intact and gonadectomized male and female sheep. Immunoreactive (ir) CRH, AVP and enkephalin cells were mapped in two rams and two ewes at 180 mum intervals throughout the rostro-caudal extent of the PVN. Similar distributions of AVP-ir cells occurred in both sexes whereas CRH-ir and enkephalin-ir cells extended more rostrally in rams. In groups (n=4) of intact and gonadectomized sheep of both sexes, co-localization and distribution of neuropeptides was influenced by sex and gonadectomy. Males had more AVP and CRH cells than females. Intact animals had more AVP cells than gonadectomized animals. There were no differences between groups in the number or percentage of cells that stained for both CRH and AVP or in the number of cells that stained for both CRH and enkephalin. Differences were observed in the percentage of enkephalin cells that contained CRH with males having a greater percentage of co-localized cells than did females. Differences were also observed in the number and percentage of cells that stained for both enkephalin and AVP; the number of cells that stained for both neuropeptides was greater in males than in females and greater in intact animals than in gonadectomized animals. Differences were observed in the percentage of AVP cells that contained enkephalin, and in the percentage of enkephalin cells that contained AVP with males having a greater percentage of co-localized cells than did females. We conclude that sex and gonadal status affect peptide distribution in the PVN of the sheep which may provide an anatomical basis for sex differences in HPA axis responses to stress.

The genetic basis for sex differences in human behaviour: role of the sex chromosomes

The nature of the mechanisms underlying observed sex differences in human behaviour continues to be debated. This review concentrates on the thesis that genes on the sex chromosomes other than those directly controlling sex determination, and whose functions are, at least in part, independent from hormonal influences, play a significant role in determining gender differences in behaviour. To provide an adequate basis for examining this issue, the current understanding of the nature of sex determination, differences in behaviour and the influences of sex hormones are evaluated. The possible contribution to behavioural differences of those X-linked genes which escape inactivation, or which may be subjected to imprinting, is discussed. The review concludes with a summary of the genetic basis for two sexually disparate types of behaviour.

Minireview: Sex differences in adult and developing brains: compensation, compensation, compensation

Despite decades of research, we do not know the functional significance of most sex differences in the brain. We are heavily invested in the idea that sex differences in brain structure cause sex differences in behavior. We rarely consider the possibility that sex differences in brain structure may also prevent sex differences in overt functions and behavior, by compensating for sex differences in physiological conditions, e.g. gonadal hormone levels that may generate undesirable sex differences if left unchecked. Such a dual function for sex differences is unlikely to be restricted to adult brains. This review will entertain the possibility that transient sex differences in gene expression in developing brains may cause permanent differences in brain structure but prevent them as well, by compensating for potentially differentiating effects of sex differences in gonadal hormone levels and sex chromosomal gene expression. Consistent application of this dual-function hypothesis will make the search for the functional significance of sex differences more productive.

Behavioral and physiological responses to anabolic-androgenic steroids

Anabolic-androgenic steroids (AAS) are synthetic derivatives of testosterone originally designed for therapeutic uses to provide enhanced anabolic potency with negligible androgenic effects. Although AAS continue to be used clinically today, the medical benefits of low therapeutic doses of AAS stand in sharp contrast to the potential health risks associated with the excessive doses self-administered not only by elite athletes and body builders, but by a growing number of recreational users, including adolescent boys and girls. The deleterious effects of AAS on peripheral organs and the incidence of altered behaviors in AAS abusers have been well documented in a number of excellent current reviews for clinical populations. However, a comparable synthesis of nonclinical studies has not been made. Our purpose in this review is to summarize the literature for animal models of the effects of supraphysiological doses of AAS (e.g. those that mimic human abuse regimes) on behaviors and on the neural circuitry for these behaviors. In particular, we have focused on studies in rodents that have examined how AAS alter aggression, sexual behaviors, anxiety, reward, learning, and locomotion and how AAS alter the expression and function of neurotransmitter systems and other signaling molecules that underlie these behaviors.

Sexually dimorphic neurocalcin expression in the developing zebra finch telencephalon

Differential display RT-PCR was used on RNA isolated from the zebra finch telencephalon to identify gene products potentially involved in its development, including the sexually dimorphic nuclei responsible for song learning and production. A cDNA identified only in juvenile females was cloned and sequenced. It shares homology with neurocalcin, a calcium binding protein. Northern blots indicated three neurocalcin species. A 10.6 kb transcript was present in males and most females throughout development and in adulthood. Smaller 6.2 and 3.3 kb species were detected almost exclusively in females and primarily between posthatching days 18-25. In situ hybridization, using a probe that identified all three mRNA species, indicated a broad distribution in the telencephalon of both sexes, with particularly high levels in the song nucleus RA. Across regions examined, neurocalcin expression was enhanced in females compared to males, probably reflecting the presence of the two smaller transcripts. However, within RA, neurocalcin expression was statistically equivalent between the sexes. These data indicate that calcium signaling via neurocalcin may be involved in telencephalic development, but suggest that sexually dimorphic expression of this gene exists on a level too general to specifically regulate masculine or feminine development of song control regions. Neurocalcin might: broadly influence functional differentiation, including areas that are not morphologically distinct between the sexes; be a benign consequence of general dimorphisms, such as those due to sex chromosomes; or involve a compensatory mechanism, which allows function of the juvenile female telencephalon to equal that of males, despite fundamental physiological differences.

Salivary hormone concentrations in mothers and fathers becoming parents are not correlated

A time- and date-matched set of saliva samples (N = 229) from nine couples first expecting, and then caring for, their first child were used to test whether hormone changes in the father could be predicted by the hormonal status of the mother. Testosterone, cortisol, and estradiol were quantified from saliva. Neither testosterone nor estradiol concentration was correlated within couples before or after the birth, although there was a positive correlation for cortisol concentration in the mother and father before the birth. As the hormone that might be influenced by chemical signals, that already played a similar role in men and women, and that had been empirically linked to paternal behavior, cortisol concentration was also compared with sex steroid concentrations. The mother's cortisol concentration was positively correlated to the father's testosterone concentration, and the mother's testosterone concentration was positively correlated with the father's cortisol concentration. However, both effects were similar in magnitude to the cortisol to cortisol correlation, and all could parsimoniously be explained by similar responses to a shared environment. Thus, this analysis rejects parallels in peripheral hormone concentrations of estradiol, testosterone, and cortisol in mothers and fathers. However, the available data were not able to test or reject hypotheses about local neuroendocrine homology, nor to control for masking effects of other hormonal demands on men and women, nor to determine the relative importance of shared environment versus mother-father signaling.

Neuroendocrine control of a sexually dimorphic behavior by a few neurons of the pars intercerebralis in Drosophila

In Drosophila, locomotor activity is sexually dimorphic and the brain area controlling this dimorphism has been mapped. The neurons of the pars intercerebralis (PI) have been suggested to participate in such differences between males and females. However, the precise physical nature of the dimorphism, the identity of the PI neurons involved, and the nature of the neuronal signal coding the dimorphism remain unknown. In this study, we used a video-tracking paradigm to characterize further the pattern of locomotor activity in Drosophila. We show that the number of activity/inactivity periods (start/stop bouts) is also sexually dimorphic, and that it can be genetically feminized in males. Moreover, the transplantation of PI neurons from a female, or of feminized PI neurons from a donor male into a receiver wild-type male is sufficient to induce the feminization of locomotor behavior, confirming that this tiny cluster of approximately 10 neurons is directly responsible for the sexual dimorphism in locomotor activity. Finally, feeding males with fluvastatin, a juvenile hormone (JH) inhibitor, also led to start/stop feminization, and this effect is reversible by the simultaneous application of methoprene, a JH analog, suggesting the existence of a neuroendocrine control, by JH, of such behavioral dimorphism.

Sex differences in brain and behavior: emphasis on nicotine, nitric oxide and place learning

Although males and females are unmistakably different, the recognition of sex as a key variable in science and medicine is considered a revolution in some circles. Sex differences transcend reproductive functions, are evident in the structural and functional organization of the brain, and are reflected in group differences in cognitive abilities and behavior. Males and females have different neural organizational patterns for information processing and different strategies in problem solving. Research on sex differences not only provides descriptive data, but also allows us to elucidate mechanisms that underlie our behavior. In this review, sex differences in the central actions of nicotine (an addictive substance) and nitric oxide, and performance on active avoidance and place learning tasks are discussed as examples, and biobehavioral approaches relating to these topics are presented.

Sex differences in the brain of goldfish: gonadotropin-releasing hormone and vasotocinergic neurons

The differences between male and female behaviors are reflected in sexual dimorphism of brain structures and are found throughout the nervous system in a variety of vertebrates. The present study examined neurons immunolabeled for gonadotropin-releasing hormone and arginine vasotocin in the brain of the goldfish Carassius auratus to determine if these neurons are sexually dimorphic. There was no sex difference or influence of sex steroids on the neuronal volume and optical density of staining of arginine vasotocin neurons. Similarly, gonadotropin-releasing hormone neurons of the terminal nerve and midbrain tegmentum did not differ between sexually mature males, females and maturing females replaced with sex steroids with respect to distribution, numbers, optical density of staining, or gross morphology. In maturing females, testosterone specifically recruited additional preoptic gonadotropin-releasing hormone neurons to equal those in sexually mature individuals. Since estrogen had no effect, the influence of testosterone on gonadotropin-releasing hormone neuronal numbers appears to be independent of aromatization. Specifically, the preoptic gonadotropin-releasing hormone neuronal size was significantly larger in sexually mature males than females. 11-Ketotestosterone-replacement to ovariectomized maturing females induced male-typical secondary characters and male-type courtship behavior but did not masculinize the preoptic gonadotropin-releasing hormone neuronal size. Our results show that the sexually dimorphic preoptic gonadotropin-releasing hormone neuronal size is determined by factors (genetic) other than gonadal steroids. Further, we propose the hypothesis that phenotypic and behavioral sex differences need not be accompanied by structural differences in gonadotropin-releasing hormone and arginine vasotocin in the brain.

Hormonal changes in mammalian fathers

Known and hypothesized relationships between steroid (estradiol, testosterone, and cortisol) and peptide (oxytocin, vasopressin, and prolactin) hormones and the expression of mammalian paternal behavior are reviewed. Emphasis is placed on newly emerging animal models, including nonhuman primates and men, with elaborate paternal behavior repertoires. Currently available data are broadly consistent with a working hypothesis that the expression of parental behavior will involve homologous neuroendocrine circuits in male and females. Understanding the neuroendocrinology of paternal behavior is an emerging research opportunity in behavioral neuroscience.

The adolescent brain and age-related behavioral manifestations

To successfully negotiate the developmental transition between youth and adulthood, adolescents must maneuver this often stressful period while acquiring skills necessary for independence. Certain behavioral features, including age-related increases in social behavior and risk-taking/novelty-seeking, are common among adolescents of diverse mammalian species and may aid in this process. Reduced positive incentive values from stimuli may lead adolescents to pursue new appetitive reinforcers through drug use and other risk-taking behaviors, with their relative insensitivity to drugs supporting comparatively greater per occasion use. Pubertal increases in gonadal hormones are a hallmark of adolescence, although there is little evidence for a simple association of these hormones with behavioral change during adolescence. Prominent developmental transformations are seen in prefrontal cortex and limbic brain regions of adolescents across a variety of species, alterations that include an apparent shift in the balance between mesocortical and mesolimbic dopamine systems. Developmental changes in these stressor-sensitive regions, which are critical for attributing incentive salience to drugs and other stimuli, likely contribute to the unique characteristics of adolescence.

Sexual dimorphism in numbers of vasotocin-immunoreactive neurons in brain areas associated with reproductive behaviors in the roughskin newt

Vasotocin (VT) and vasopressin control many endocrine and neuroendocrine functions, including the regulation of reproductive behaviors. In the roughskin newt (Taricha granulosa), VT administration can enhance courtship behaviors in males and egg-laying behaviors in females. This study used immunohistochemistry to investigate whether there are sex differences in VT in specific brain areas, and whether these differences persist in nonbreeding animals. Numbers of VT immunoreactive (ir) cell bodies were counted in males and females collected in February, April, June, and August. Radioimmunoassay of plasma samples confirmed that testosterone and 5alpha-dihydrotestosterone concentrations were higher in males than females, and that 17beta-estradiol concentrations were higher in females than males. In 11 brain areas, no sexual or seasonal differences in the number of VTir cells were found. But in 3 brain regions-the bed nucleus of the stria terminalis (BNST), the nucleus amygdalae dorsolateralis (AMYG), and the anterior preoptic area (aPOA)-there were significantly greater numbers of VTir cells in males than in females, and these differences did not change seasonally. In the aPOA, an area important to male sex behaviors, the sexual dimorphism in VTir was particularly pronounced. In four brain regions, there were significantly greater numbers of VTir cells in females than males, but only in specific seasons. In April-collected (breeding) animals, more VTir cells were found in females than in males in the populations of VT cells within the pars dorsalis hypothalami and ventromedial hypothalamus, brain regions frequently associated with stress responses and female mating behaviors. In August-collected (nonbreeding) animals, more VTir cells were found in females than in males, in the region of the bed nucleus of the decussation of the fasciculus lateralis telencephali and in the nucleus visceralis superior, nucleus isthmi region. Significantly greater numbers of VTir cells were observed in the magnocellular preoptic area of males and females collected in February. These results indicate that the functional interactions between gonadal steroid hormones and VT are complex and appear to involve site-, sex-, and season-specific regulatory mechanisms. Furthermore, it seems likely that populations of VT neurons in the BNST, AMYG, and aPOA are involved in regulating male-specific behaviors, and that the VT neurons in the pars dorsalis hypothalami/ventromedial hypothalamus may be involved in female-specific behaviors.

Functional sex differences ('sexual diergism') of central nervous system cholinergic systems, vasopressin, and hypothalamic-pituitary-adrenal axis activity in mammals: a selective review

Sexual dimorphism of the mammalian central nervous system (CNS) has been widely documented. Morphological sex differences in brain areas underlie sex differences in function. To distinguish sex differences in physiological function from underlying sexual dimorphisms, we use the term, sexual diergism, to encompass differences in function between males and females. Whereas the influence of sex hormones on CNS morphological characteristics and function of the hypothalamic-pituitary-gonadal axis has been well-documented, little is known about sexual diergism of CNS control of the hypothalamic-pituitary-adrenal (HPA) axis. Many studies have been conducted on both men and women but have not reported comparisons between them, and many animal studies have used males or females, but not both. From a diergic standpoint, the CNS cholinergic system appears to be more responsive to stress and other stimuli in female than in male mammals; but from a dimorphic standpoint, it is anatomically larger, higher in cell density, and more stable with age in males than in females. Dimorphism often produces diergism, but age, hormones, environment and genetics contribute differentially. This review focuses on the sexual diergism of CNS cholinergic and vasopressinergic systems and their relationship to the HPA axis, with resulting implications for the study of behavior, disease, and therapeutics.