As a community, we have long been bewildered by the lack of insight as to what being 47XXY entails and in its place, find ourselves at the mercy of pathological directives that are totally unbecoming of our biological selves. The impact serves to accentuate the difficulties many have with adhering to such standardised treatments and worse, how pleas for help are of the silent kind by a population who mostly lack the language skillset to properly describe how they are feeling. Unlike the experience of a Butterfly, our metamorphosis through the use of exogenous (not natural) testosterone is anything but beautiful where the impact of ‘minimal’ virilisation makes the underlying issues of our biological differences all that more difficult to recognise, where many who have undergone such treatments (and survived mentally intact) often have a difficult time with doctors who only ever see an XY male person, even though the individual knows they are not. There’s ample research to indicate the only ‘benefit’ an XXY individual will get from testosterone is that it will cause their body to virilise, but even then not always, under normal circumstances with a different population this would usually serve to change tack and start again but, not so when our experiences are continuously compared with XY Males. Do you get the picture?

Author Tracy L. Bale, Sex Matters,

In 2015, the NIH required that all grants must consider sex as a biological variable(SABV) in their experimental plan. Not only were you required to consider SABV, but also to provide expected outcomes and relevant statistical analyses to test for potential sex differences. Yes, the time had come to shine a light on the importance of SABV. But let’s back up for a minute and review why SABV had been shelved for so long for most labs – when and how did the male become the control group? As reviewed by Beery and Zucker, studies across neuroscience by 2009 had focused on male-only outcomes 5.5 times more often than those that included females, and this number was unchanged from the previous 50 years. While it is unclear as to the specific reasons why the field of neuroscience became fearful of using females, the general assumption that females are somehow more ‘variable’ than males is a clear factor. While this myth has been put to rest by published meta-analyses in mice, rats and humans clearly showing that in many cases males are actually more variable in outcome measures relevant to neuropharmacology yet the fear lives on.

Based on published studies, somewhere in the late ’60s and early 70’s neuroscientists dramatically shifted their approach away from being sex inclusive, or at the very least, sex agnostic, to a significant male-bias. In 1969, 20% of studies using animal models reported examining males only, but by 1979 this number had jumped to 70%. Studies focusing on females or inclusive of both sexes had fallen to less than 10%. Whether this dramatic shift over a decade was produced by an increased awareness by the field for a possible role of gonadal hormones in relevant measures thus perpetuating the fear of variability, or if around this time journals began requiring authors to declare the sex of the species they were using, and thus it became more apparent as to the prevalence in neuroscience research in which the male was the now the standard control subject.

What has become clear in the last decade is the firm appreciation for SABV in
preclinical and clinical research is essential for our understanding of factors
contributing to disease risk and resilience. Studies focusing on examining sex
differences have demonstrated across the lifespan, from development to maturation and
aging, that males and females can differ significantly. From sex chromosomes that allow
for disparities in gene dosage and regulatory mechanisms to the important role of
gonadal hormones, our appreciation for the unique and highly mechanistic insight that
including SABV affords us has gained great attention. This special issue provides
coverage across areas most relevant to the Neuropsychopharmacology audience of the
current state of SABV and where sex differences have undoubtedly demonstrated
valuable insight. This is by no means a comprehensive review of all research in these
areas, but rather a selection of appropriate and representative examples to provide
guidance and reference for the inclusion of SABV as an essential component of
research in Neuropsychopharmacology.

Critical to emphasizing the importance of SABV inclusion is the warning labels all
too common in the pharmaceutical industry for the very real consequences of a
previous failure to include both sexes in clinical trials. Despite knowing that females are
at greater risk for a wide range of adverse drug reactions, limited inclusion of women in
randomized clinical trials have occurred in the recent past Results from the Physicians’ Health Study that included over 20,000 male physicians in the 1980s suggested that low dose aspirin was associated with a reduced risk of myocardial infarction (1989). However, decades later a follow-up study examining 39,000 women showed surprising outcomes in which low dose aspirin produced a greater risk of stroke and no overall effect on myocardial infarction risk in women (Ridker et al., 2005). In addition, the FDA recently approved labelling changes for dosing of the sleep drug, zolpidem (Ambien). The recommended starting dose was significantly reduced in women where the drug had a significantly slower rate of metabolism, resulting in higher drug concentrations and significant lasting effects not seen in men. To this point, of the ten prescription drugs removed from the U.S. market between 1997 and 2001, eight had greater adverse effects in women compared to men.

As to the biology and where sex differences originate, all cells have a sex, designated by the presence and dosage of X or Y chromosomes, which in most cases will be XX (female) or XY (male). However, gender is a designated societal determinant and traditionally described only in humans (Bachmann and Mussman, 2015). Importantly, the sexually dimorphic brain, similar to most sex differences, does not fall into a binary categorization – but rather is on a continuum or spectrum with each cell and each brain region comprised of varying degrees of ‘maleness’ and ‘femaleness’ based on an average report within each sex (Hines, 2005; Joel and McCarthy, 2016). Finally, the combination of genetic sex and gonadal hormones can promote dramatic changes in trajectory during important periods of brain development and maturation.

In this special issue, Green et al. begin at the most basic biological point of
deciphering sex differences – the DNA. The authors examine the rates and symptom
severity of neurodevelopmental and neuropsychiatric diseases in typically developing males (XY) and females (XX) and compare these with sex chromosome aneuploidies (SCA) (e.g., Turner syndrome (X0) and Klinefelter syndrome (XXY)) (Green et al., 2018). This review also discusses the consistent morphological and developmental differences in the SCA brain and their potential relationship with disease symptoms. What is clear from this intriguing discussion, is the vital importance of how many X or Y chromosomes each cell has, and that a loss (monosomy) of a sex chromosome is far more detrimental than a trisomy (XXY, XYY or XXX). Building on these chromosomal contributions to sex differences in brain structure and function, in an intriguing review and likely one of the
first to tackle a timely and important topic, Nguyen et al. discuss how known differences in the sexually dimorphic brain and the role of gonadal hormones inform decisions and outcomes for the transgender brain (Nguyen et al., 2018). This is an intriguing and timely topic and one that should be read by all clinicians and researchers interested in and working with diverse populations to appreciate the complexities of the sex and gender.

The next few reviews in this issue tackle the importance of developmental
windows in how sex differences in the brain may arise, and how they impact brain
function and disease risk across the lifespan. Margaret McCarthy, the world expert on sex differences in cellular processes key in brain development, reviews the role of microglia and the inflammatory signalling molecules responding to steroid hormones in shaping the sexually dimorphic brain (McCarthy, 2018). This review postulates that an increased risk for males for gestational perturbations and neurodevelopmental disorders may lie in an intersection of maleness with inflammatory processes involved in masculinization. Building on the development of the sexually dimorphic brain and the influence of gonadal hormones, Heck and Handa examine the development of
sex differences in the hypothalamic-pituitary-adrenal stress axis (Heck and Handa, 2018). Connecting these mechanistic components of immune activation with stress experience during gestation, Goldstein et al. then discuss the programming of sex differences in major depressive disorder and its comorbidity with cardio-metabolic disorders that again supports a contribution across the lifespan for disease risk and resilience that begins in these very early periods of development (Goldstein et al., 2018).

Childhood and the adolescent window of brain maturation are critical times for
appropriate wiring unique to the limbic system, especially that of the prefrontal cortex,
and for promotion of executive functioning and emotional regulation. Two reviews have
examined the importance of sex differences here. Kaczkurkin et al. review the evidence
for sex differences in brain structure, white matter organization, and perfusion during
these developmental periods, and use sex differences to discuss psychopathology,
focusing on the presentation of affective disturbance, psychosis, and ADHD (Kaczkurkin
et al., 2018). Focusing on executive functioning tasks in humans and animal studies,
Grissom and Reyes discuss the paucity of studies that have adequately examined sex
differences in this area, and how potential developmental trajectory differences for
males and females may confound studies examining attention, focus, and impulse
control (Grissom and Reyes, 2018). An additional behavioural measure that has become
a key feature in preclinical models for neurodevelopmental disorders, especially for
autism studies is social behaviour. Borland et al. review the current state of sex
differences in the presentation and development of social behaviours in humans and
animal models, and discuss the potential therapeutic efficacy of oxytocin in social
reward and treatment of social dysfunction.

Affective disorders, in general, have minimal sex differences in overall
presentation numbers prior to puberty, but during this hormonally dynamic period, significant differences dramatically emerge. Females present with an increased risk for
affective disturbance beginning in adolescence, and this sex difference continues into
adulthood across the remainder of the lifespan. Three reviews discuss potential points
of where these sex differences may arise. In the review by Rubinow and Schmidt, the
authors build a comprehensive framework for how sex differences in disease risk
involve both hormonal and chromosomal mechanisms (Rubinow and Schmidt, 2018). A
masterful discussion intertwines how dynamic neuroplasticity involved in key neural
networks interacts with genetic variants to provide risk or resilience to affective
disorders, such as depression and anxiety. Where sex differences arise in these arousal
circuits is a focus of the review by Bangasser et al., who discuss important factors
common to preclinical animal models and clinical research studies, largely examining
stress regulation by corticotropin-releasing factor (Bangasser et al., 2018).

Tackling sex differences in affective disorder risk from a therapeutic angle, LeGates et al. examine key factors underlying synaptic transmission and neuronal plasticity that may determine antidepressant drug efficacy (Legates et al., 2018). In addition to affective disorders, dramatic sex differences across pain and addiction neurocircuits have contributed to disparities in therapeutic drug treatment and addiction outcomes. Averitt et al. discuss significant differences in pain pathways, pain tolerance, and drug receptor pharmacology, all factors pertinent and vital to solving the current opioid crisis (Averitt et al., 2018). Examining reward circuits important to addiction, Becker and Chartoff build a conceptual blueprint detailing all levels, from the molecular and epigenetic, to the physiological and behavioural, that underlie sex differences in disease and substance abuse risk (Becker and Chartoff, 2018).

Finally, as we progress across the lifespan in this special issue, processes involved in aging become significant additional factors in neuropsychiatric disease, where women are at greater risk for dementia, and during the window of dynamic hormonal changes of the peripubertal transition women again present with a greater risk for affective disorders and schizophrenia (Bale and Epperson, 2015). One of the major peripheral systems thought to be contributing to this disparity is the immune system. Rainville and Hodes discuss the importance of known sex differences in the immune system where females typically have a more reactive immune response and examine how this may contribute to neural dysfunction, especially when added to an already taxed aging brain (Rainville and Hodes, 2018). One of the major mammalian brain structures contributing to dramatic sex differences in aging is the hippocampus. The hippocampus is vital to cognitive function, including learning, flexibility, and pattern separation. In our last review of this special issue, Yagi and Galea discuss the layers of known sex differences in hippocampal plasticity and neurogenesis, and how an appreciation of these differences may provide greater insight into mechanisms related to flexibility and aging.

While these timely reviews demonstrate valuable insight into the importance of
examining sex differences spanning the lifespan, additional work is needed. As our
environment becomes ever increasingly complex, understanding how sex differences
impact disease risk and resilience will only become more critical. For example, there is
a growing need for understanding sex differences in how social media is utilized and its
effects on the developing and maturing brain, from fragmented parental care to
adolescent screen addiction, all of which impact mental health (Baram and Bolton, 2018;
Casey et al., 2015). Importantly, this special issue highlights the causal and mechanistic
value in cases where significant sex differences are found, as well as when they’re not;
where sex differences may arise at one period of life and disappear in another. Lastly,
this special issue reinforces the message that males are not the control, and females
are not more variable. The value is in appreciating the difference. Sex matters.

Funding supplied by the National Institute of Health.