Yes, there is a female and a male brain: Morphology versus functionality

Microglia signaling in health and disease - Implications in sex-specific brain development and plasticity

Microglia, the intrinsic neuroimmune cells residing in the central nervous system (CNS), exert a pivotal influence on brain development, homeostasis, and functionality, encompassing critical roles during both aging and pathological states. Recent advancements in comprehending brain plasticity and functions have spotlighted conspicuous variances between male and female brains, notably in neurogenesis, neuronal myelination, axon fasciculation, and synaptogenesis. Nevertheless, the precise impact of microglia on sex-specific brain cell plasticity, sculpting diverse neural network architectures and circuits, remains largely unexplored. This article seeks to unravel the present understanding of microglial involvement in brain development, plasticity, and function, with a specific emphasis on microglial signaling in brain sex polymorphism. Commencing with an overview of microglia in the CNS and their associated signaling cascades, we subsequently probe recent revelations regarding molecular signaling by microglia in sex-dependent brain developmental plasticity, functions, and diseases. Notably, C-X3-C motif chemokine receptor 1 (CX3CR1), triggering receptors expressed on myeloid cells 2 (TREM2), calcium (Ca2+), and apolipoprotein E (APOE) emerge as molecular candidates significantly contributing to sex-dependent brain development and plasticity. In conclusion, we address burgeoning inquiries surrounding microglia's pivotal role in the functional diversity of developing and aging brains, contemplating their potential implications for gender-tailored therapeutic strategies in neurodegenerative diseases.

Do Sex and Gender Have Separate Identities?

The largely binary nature of biological sex and its conflation with the socially constructed concept of gender has created much strife in the last few years. The notion of gender identity and its differences and similarities with sex have fostered much scientific and legal confusion and disagreement. Settling the debate can have significant repercussions for science, medicine, legislation, and people's lives. The present review addresses this debate though different levels of analysis (i.e., genetic, anatomical, physiological, behavioral, and sociocultural), and their implications and interactions. We propose a rationale where both perspectives coexist, where diversity is the default, establishing a delimitation to the conflation between sex and gender, while acknowledging their interaction. Whereas sex in humans and other mammals is a biological reality that is largely binary and based on genes, chromosomes, anatomy, and physiology, gender is a sociocultural construct that is often, but not always, concordant with a person' sex, and can span a multitude of expressions.

New perspectives on sex differences in learning and memory

Females have historically been disregarded in memory research, including the thousands of studies examining roles for the hippocampus, medial prefrontal cortex, and amygdala in learning and memory. Even when included, females are often judged based on male-centric behavioral and neurobiological standards, generating and perpetuating scientific stereotypes that females exhibit worse memories compared with males in domains such as spatial navigation and fear. Recent research challenges these dogmas by identifying sex-specific strategies in common memory tasks. Here, we discuss rodent data illustrating sex differences in spatial and fear memory, as well as the neural mechanisms underlying memory formation. The influence of sex steroid hormones in both sexes is discussed, as is the importance to basic and translational neuroscience of studying sex differences.

Bioactive Compounds for Customized Brain Health: What Are We and Where Should We Be Heading?

Many strides have been made in the field of nutrition that are making it an attractive field not only to nutrition professionals but also to healthcare practitioners. Thanks to the emergence of molecular nutrition, there is a better appreciation of how the diet modulates health at the cellular and molecular levels. More importantly, the advancements in brain imaging have produced a greater appreciation of the impact of diet on brain health. To date, our understanding of the effect of nutrients on brain health goes beyond the action of vitamins and minerals and dives into the intracellular, molecular, and epigenetic effects of nutrients. Bioactive compounds (BCs) in food are gaining a lot of attention due to their ability to modulate gene expression. In addition, bioactive compounds activate some nuclear receptors that are the target of many pharmaceuticals. With the emergence of personalized medicine, gaining an understanding of the biologically active compounds may help with the customization of therapies. This review explores the prominent BCs that can impact cognitive functions and mental health to deliver a potentially prophylactic framework for practitioners. Another purpose is to identify potential gaps in the literature to suggest new research agendas for scientists.

Feminism, gender medicine and beyond: a feminist analysis of "gender medicine"

The feminist women's health movement empowered women's knowledge regarding their health and battled against paternalistic and oppressive practices within healthcare systems. Gender Medicine (GM) is a new discipline that studies the effect of sex/gender on general health. The international society for gender medicine (IGM) was embraced by the FDA and granted funds by the European Union to formulate policies for medical practice and research.We conducted a review of IGM publications and policy statements in scientific journals and popular media. We found that while biological differences between men and women are emphasized, the impact of society on women is under- represented. The effect of gender-related violence, race, ethnic conflicts, poverty, immigration and discrimination on women's health is seldom recognized. Contrary to feminist practice, GM is practiced by physicians and scientists, neglecting voices of other disciplines and of women themselves.In this article we show that while GM may promote some aspects of women's health, at the same time it reaffirms conservative positions on sex and gender that can serve to justify discrimination and disregard the impact of society on women's lives and health. An alternative approach, that integrates feminist thinking and practices into medical science, practice and policies is likely to result in a deep and beneficiary change in women's health worldwide.

Beyond sex differences and a male-female continuum: Mosaic brains in a multidimensional space

In the last two decades, the 60 years old view that in utero exposure to testosterone irreversibly masculinizes the brain of males away from a default female form has been replaced by a complex scenario according to which sex affects the brains of both females and males via multiple mechanisms, which are susceptible to internal and external factors. These observations led to the "mosaic" hypothesis-the expectation that the degree of "maleness"/"femaleness" of different features within a single brain would not be internally consistent. Following a short review of the animal studies providing the basis of the mosaic hypothesis, I describe three studies conducted in humans that assessed internal consistency in regional volume, cortical thickness, and connectivity as revealed by magnetic resonance imaging (MRI); in neuronal numbers in the postmortem hypothalamus; and in changes in regional volume and cortical thickness (assessed with MRI) following exposure to extreme real-life stress. The conclusion from these studies, that human brains are largely composed of unique mosaics of female-typical and male-typical features, was supported by recent findings that the brain "types" typical of women are also typical of men, and vice versa. Lastly, I discuss criticism of the mosaic hypothesis and suggest replacing the framework of a male-female continuum with thinking about mosaic brains residing in a multidimensional space.

Input-selective adenosine A1 receptor-mediated synaptic depression of excitatory transmission in dorsal striatum

The medial (DMS) and lateral (DLS) dorsal striatum differentially drive goal-directed and habitual/compulsive behaviors, respectively, and are implicated in a variety of neuropsychiatric disorders. These subregions receive distinct inputs from cortical and thalamic regions which uniquely determine dorsal striatal activity and function. Adenosine A₁ receptors (A1Rs) are prolific within striatum and regulate excitatory glutamate transmission. Thus, A1Rs may have regionally-specific effects on neuroadaptive processes which may ultimately influence striatally-mediated behaviors. The occurrence of A1R-driven plasticity at specific excitatory inputs to dorsal striatum is currently unknown. To better understand how A1Rs may influence these behaviors, we first sought to understand how A1Rs modulate these distinct inputs. We evaluated A1R-mediated inhibition of cortico- and thalamostriatal transmission using in vitro whole-cell, patch clamp slice electrophysiology recordings in medium spiny neurons from both the DLS and DMS of C57BL/6J mice in conjunction with optogenetic approaches. In addition, conditional A1R KO mice lacking A1Rs at specific striatal inputs to DMS and DLS were generated to directly determine the role of these presynaptic A1Rs on the measured electrophysiological responses. Activation of presynaptic A1Rs produced significant and prolonged synaptic depression (A1R-SD) of excitatory transmission in the both the DLS and DMS of male and female animals. Our findings indicate that A1R-SD at corticostriatal and thalamostriatal inputs to DLS can be additive and that A1R-SD in DMS occurs primarily at thalamostriatal inputs. These findings advance the field's understanding of the functional roles of A1Rs in striatum and implicate their potential contribution to neuropsychiatric diseases.

Common and differential associations between levels of alcohol drinking, gender-specific neurobehaviors and mental distress in college students

BACKGROUND

Binge drinking is associated with poor academic behaviors and performance. Excessive alcohol drinking induces molecular changes and neurobehaviors that support use of other substances and alter cognitive functions. The purpose of this study was to compare neurobehaviors and academic effort among college students with low alcohol use with those of high alcohol consumption and build conceptual models that represent the integration of the different variables.

METHOD

College students from several U.S colleges were assessed through an anonymous online survey for alcohol use, academic performance, lifestyle factors and mental distress.

RESULTS

Our results depicted common neurobehaviors and differential responses to high alcohol use.

CONCLUSION

The common responses in young men and women with high alcohol use are reflective of a hyperactive limbic system. The different responses involve cognitive aptitudes, typically controlled by cortical regions and affected by levels of brain connectivity known to be dissimilar between men and women.

An improved sex-specific and age-dependent classification model for Parkinson's diagnosis using handwriting measurement

BACKGROUND AND OBJECTIVES

Diagnosis of Parkinson's with higher accuracy is always desirable to slow down the progression of the disease and improved quality of life. There are evidences of inherent neurological differences between male and females as well as between elderly and adults. However, the potential of such gender and age infomration have not been exploited yet for Parkinson's identification.

METHODS

In this paper, we develop a sex-specific and age-dependent classification method to diagnose the Parkinson's disease using the online handwriting recorded from individuals with Parkinson's (n = 37; m/f-19/18;age-69.3 ± 10.9yrs) and healthy controls (n = 38; m/f-20/18;age-62.4 ± 11.3yrs). A support vector machine ranking method is used to present the features specific to their dominance in sex and age group for Parkinson's diagnosis.

RESULTS

The sex-specific and age-dependent classifier was observed significantly outperforming the generalized classifier. An improved accuracy of 83.75% (SD = 1.63) with the female-specific classifier, and 79.55% (SD = 1.58) with the old-age dependent classifier was observed in comparison to 75.76% (SD = 1.17) accuracy with the generalized classifier.

CONCLUSIONS

Combining the age and sex information proved to be encouraging in classification. A distinct set of features were observed to be dominating for higher classification accuracy in a different category of classification.

Structural differences between male and female brains

Research based on structural magnetic resonance imaging (MRI) has revealed a number of sex differences in the anatomy of the human brain. The first part of this chapter presents an excerpt of these findings discriminating among effects on a global, regional, and local level. While findings are far from consistent and conclusive, there is general consensus with respect to sex-specific brain size, with male brains being bigger on average than female brains. So, the question arises as to whether any of the observed sex differences are merely driven by brain size. The second part of this chapter thus sheds light on a unique scientific attempt to discriminate between brain size effects and sex effects. The overarching goal of this chapter is to exemplify the variety of findings and to demonstrate that the presence, magnitude, and direction of significant sex differences strongly depend on the measurement applied. The assumption that sex differences are simply a by-product of brain size, rather than pure (size independent) sex effects has proven to be true for some but certainly not all findings. Therefore, when examining the possible sexual dimorphism of the brain, it is imperative to avoid oversimplification and generalization.

Sulcal pattern variation in extant human endocasts

Our knowledge of human brain evolution primarily relies on the interpretation of palaeoneurological evidence. In this context, an endocast or replica of the inside of the bony braincase can be used to reconstruct a timeline of cerebral changes that occurred during human evolution, including changes in topographic extension and structural organisation of cortical areas. These changes can be tracked by identifying cerebral imprints, particularly cortical sulci. The description of these crucial landmarks in fossil endocasts is, however, challenging. High-resolution imaging techniques in palaeoneurology offer new opportunities for tracking detailed endocranial neural characteristics. In this study, we use high-resolution imaging techniques to document the variation in extant human endocranial sulcal patterns for subsequent use as a platform for comparison with the fossil record. We selected 20 extant human crania from the Pretoria Bone Collection (University of Pretoria, South Africa), which were detailed using X-ray microtomography at a spatial resolution ranging from 94 to 123 μm (isometric). We used Endex to extract, and Matlab to analyse the cortical imprints on the endocasts. We consistently identified superior, middle and inferior sulci on the frontal lobe; and superior and inferior sulci on the temporal lobe. We were able to label sulci bordering critical functional areas such as Broca's cap. Mapping the sulcal patterns on extant endocasts is a prerequisite for constructing an atlas which can be used for automatic sulci recognition.

Sex differences and the neurobiology of affective disorders

Observations of the disproportionate incidence of depression in women compared with men have long preceded the recent explosion of interest in sex differences. Nonetheless, the source and implications of this epidemiologic sex difference remain unclear, as does the practical significance of the multitude of sex differences that have been reported in brain structure and function. In this article, we attempt to provide a framework for thinking about how sex and reproductive hormones (particularly estradiol as an example) might contribute to affective illness. After briefly reviewing some observed sex differences in depression, we discuss how sex might alter brain function through hormonal effects (both organizational (programmed) and activational (acute)), sex chromosome effects, and the interaction of sex with the environment. We next review sex differences in the brain at the structural, cellular, and network levels. We then focus on how sex and reproductive hormones regulate systems implicated in the pathophysiology of depression, including neuroplasticity, genetic and neural networks, the stress axis, and immune function. Finally, we suggest several models that might explain a sex-dependent differential regulation of affect and susceptibility to affective illness. As a disclaimer, the studies cited in this review are not intended to be comprehensive but rather serve as examples of the multitude of levels at which sex and reproductive hormones regulate brain structure and function. As such and despite our current ignorance regarding both the ontogeny of affective illness and the impact of sex on that ontogeny, sex differences may provide a lens through which we may better view the mechanisms underlying affective regulation and dysfunction.

Sex differences in hippocampal cognition and neurogenesis

Sex differences are reported in hippocampal plasticity, cognition, and in a number of disorders that target the integrity of the hippocampus. For example, meta-analyses reveal that males outperform females on hippocampus-dependent tasks in rodents and in humans, furthermore women are more likely to experience greater cognitive decline in Alzheimer's disease and depression, both diseases characterized by hippocampal dysfunction. The hippocampus is a highly plastic structure, important for processing higher order information and is sensitive to the environmental factors such as stress. The structure retains the ability to produce new neurons and this process plays an important role in pattern separation, proactive interference, and cognitive flexibility. Intriguingly, there are prominent sex differences in the level of neurogenesis and the activation of new neurons in response to hippocampus-dependent cognitive tasks in rodents. However, sex differences in spatial performance can be nuanced as animal studies have demonstrated that there are task, and strategy choice dependent sex differences in performance, as well as sex differences in the subregions of the hippocampus influenced by learning. This review discusses sex differences in pattern separation, pattern completion, spatial learning, and links between adult neurogenesis and these cognitive functions of the hippocampus. We emphasize the importance of including both sexes when studying genomic, cellular, and structural mechanisms of the hippocampal function.

Untangling the Gordian Knot of Human Sexuality

There is increasing interest in and tolerance of the lay public for variations in human sexuality. In contrast, the molecular biology that underlies gender identity, the development of gonadal and genital anatomy, and the factors that define sexual behavior is proving unexpectedly complex and is still incompletely understood. It is now evident that humans cannot be characterized as member of 1 of 2 clearly defined units: male or female. In fact, individuals exist on a continuum: those who do not conform unequivocally to the dyadic view of human sex in terms of anatomy, gender identity, and/or sexual behavior should be characterized as having variations in rather than disorders of sexual development. Such individuals can no longer be regarded as anomalies to be rejected, condemned, and, if possible, “corrected” either psychologically or anatomically.

A Key Characteristic of Sex Differences in the Developing Brain: Greater Variability in Brain Structure of Boys than Girls

In many domains, including cognition and personality, greater variability is observed in males than in females in humans. However, little is known about how variability differences between sexes are represented in the brain. The present study tested whether there is a sex difference in variance in brain structure using a cohort of 643 males and 591 females aged between 3 and 21 years. The broad age-range of the sample allowed us to test if variance differences in the brain differ across age. We observed significantly greater male than female variance for several key brain structures, including cerebral white matter and cortex, hippocampus, pallidum, putamen, and cerebellar cortex volumes. The differences were observed at both upper and lower extremities of the distributions and appeared stable across development. These findings move beyond mean levels by showing that sex differences were pronounced for variability, thereby providing a novel perspective on sex differences in the developing brain.

Principal component analysis identifies differential gender-specific dietary patterns that may be linked to mental distress in human adults

Independent reports describe the structural differences between the human male and female brains and the differential gender-susceptibility to mood disorders. Nutrition is one of the modifiable risk factors that has been reported to impact brain chemistry and affect mental health. Objectives: To study dietary patterns in adult men and women in relation to mental distress. Another aim was to develop evidence-based prototypes using System Dynamic Modeling methodology to better describe our findings. Methods: An anonymous internet-based survey was sent through social media platforms to different social and professional networks. Multivariate analyses were used for data mining. Data were stratified by gender and further by tertiles to capture the latent variables within the patterns of interest. Results: Mental distress in men associated with a consumption of a Western-like diet. In women, mental wellbeing associated with a Mediterranean-like diet and lifestyle. No other patterns in both genders were linked to mental distress. Based on the generated prototypes, men are more likely to experience mental wellbeing until nutritional deficiencies arise. However, women are less likely to experience mental wellbeing until a balanced diet and a healthy lifestyle are followed. In men, dietary deficiencies may have a profound effect on the limbic system; whereas dietary sufficiency in women may potentiate the mesocortical regulation of the limbic system. Discussion and conclusion: Our results may explain the several reports in the literature that women are at a greater risk for mental distress when compared to men and emphasize the role of a nutrient-dense diet in mental wellbeing.

Neonatal Sex Assignment in Disorders of Sex Development: A Philosophical Introspection

Management of ambiguous genitalia is highly controversial. This condition was known previously as intersex and presently as disorders of sex development (DSD). There is no consensus regarding the choice, timing and method of sex assignment in neonates with DSD. Consensus conferences could not unify the views of various stakeholders and third parties. This article philosophically examines the nature and origin of such controversies. Misconception, bias and conflicting priorities are identified as the three cardinal sources of controversies. Conceptual duality of sexes, confused notion of sex and gender, bias towards penetrative intercourse, conflict between utopian ideals and reality, unwillingness to compromise are identified as perpetuators of controversies. Suggestions are made regarding sex assignment in various types of DSD based on the understanding of published literature and the author's personal experience.

Sexually selected size differences and conserved sexual monomorphism of genital cortex

The mammalian somatosensory cortex shows marked species‐specific differences. How evolution in general and sexual selection in particular shape the somatosensory cortical body representation has not been delineated, however. Here we address this issue by a comparative analysis of genital cortex. Genitals are unique body parts in that they show sexual dimorphism, major changes in puberty and typically more pronounced species differences than other body parts (Hosken & Stockley, 2004). To study the evolution of genital cortex we flattened cortical hemispheres and assembled 104 complete body maps, revealed by cytochrome‐oxidase activity in layer 4 of 8 rodent and 1 lagomorph species. In two species, we also performed antibody stainings against vesicular glutamate transporter‐2, which suggested that cytochrome‐oxidase maps closely mirror thalamic innervation. We consistently observed a protrusion between hindlimb and forelimb representation, which in rats (Lenschow et al., 2016) corresponds to the penis representation in males and the clitoris representation in females. Consistent with the idea that this protrusion corresponds to genital cortex, we observed a size increase of this protrusion during puberty. Species differed in external genital sexual dimorphism, but we observed a sexual monomorphism of the putative genital protrusion in all species, similar to previous observations in rats. The relative size of the putative genital protrusion varied more than 3‐fold between species ranging from 0.5% of somatosensory cortex area in chipmunks to 1.7% in rats. This relative size of the genital protrusion co‐varied with relative testicle size, an indicator of sperm competition and sexual selection.

Gender-based normative values for pattern-reversal and flash visually evoked potentials under binocular and monocular stimulation in healthy adults

PURPOSE

To determine gender-based normative values for pattern-reversal (PR) and flash (F) visually evoked potentials (VEP) under binocular and monocular stimulation in healthy adults.

METHODS

Healthy adults (age ≥18 years) were recruited among university employees and students. Inclusion criteria were absence of abnormalities in fundoscopy, tracking ability, stereopsis and pupillary reflexes; best-corrected visual acuity ≤.00 logMAR; and refractive error (spherical equivalent) from -6.00 to +6.00. Exclusion criteria were previous intraocular surgery, systemic and/or neurological disorders. Binocular and monocular tests were performed according to International Society for Clinical Electrophysiology of Vision standards for PRVEP (reversal rate = 1.9 Hz, checkerboard stimuli 15' and 60' at 100% contrast) and FVEP (3 cd s/m², rate = 1 Hz). VEP parameters of amplitude (µV) and peak times (ms) were measured. Inter-ocular differences, inter-peak intervals (N135-N75) and binocular summation were determined.

RESULTS

Fifty-four subjects (28 females; mean age = 40.4 ± 13.7 years; median = 40.0 years) were included. Mean P100 latencies for 15' and 60' stimuli were, respectively, 94.6 ± 4.7 ms and 96.1 ± 4.2 for women. Mean values of P100 latency for men were 97.4 ± 4.9 for 15' and 97.7 ± 4.2 for 60' stimuli. Larger mean P100 for 15' checks was observed in women (12.8 ± 5.7 µV) than men (8.6 ± 2.5 µV) in PRVEP. Similar results were found for FVEP N2-P2 amplitudes (mean = 14.6 ± 4.9 for women and 9.8 ± 4.0 for men).

CONCLUSIONS

Gender-based normative values for PRVEP and FVEP were determined, with women disclosing higher responses than men for smaller stimuli in the visual pathway. The use of gender-based normative values in the analysis of clinical VEP data for diagnostic and therapeutic purposes is recommendable. Additional analysis including inter-peak intervals and binocular summation ratio might improve the diagnostic power of VEP.