Elsevier

NeuroToxicology

Volume 32, Issue 5, October 2011, Pages 509-517
NeuroToxicology

Same sex, no sex, and unaware sex in neurotoxicology

https://doi.org/10.1016/j.neuro.2010.09.005Get rights and content

Abstract

Males and females of virtually all species differ in how they respond to their environment. Because such differences exist in almost all biological realms, including disease patterns and therapeutic outcomes, they have evoked calls by various bodies to incorporate their assessment in research. Neurobehavioral indices pose special questions because, unlike outwardly visible markers, they are described by complex functional outcomes or subtle alterations in brain structure. These divergent responses arise because they are inscribed in the genome itself and then by endocrine mechanisms that govern sexual differentiation of the brain during development and operate throughout life. Other organ systems that exhibit sex differences include the liver, an important consideration for neurotoxicology because it may process many toxic chemicals differentially in males and females. Despite the scope and pervasiveness of sex differences, however, they are disregarded by much of neurotoxicology research. Males predominate in behavioral experiments, few such experiments study both sexes, some investigators fail to even describe the sex of their subjects, and in vitro studies tend to wholly ignore sex, even for model systems aimed at neurological disorders that display marked sex differences. The public is acutely aware of sex differences in behavior, as attested by its appetite for books on the topic. It closely follows debates about the proportion of women in professions that feature science and mathematics. Neurotoxicology, especially in the domain of laboratory research, will be hindered in its ability to translate its findings into human health measures if it assigns sex differences to a minor role. It must also be sensitive to how such debates are framed. Often, the differences evoking the most discussion are subtle in scope. They do not lend themselves to the typical analyses conducted by experimenters; that is, reliance on mean differences and null hypothesis testing.

Introduction

Chemical contaminants that are largely the contribution of industrial processes and their byproducts pervade the environment. Every human is exposed to a multiplicity of such agents and carries their signatures in their tissues. They include heavy metals, solvents, pesticides, cosmetics, plastic products, and other components contributed by a staggering array of manufacturing processes, manufactured items, and their wastes. They exert effects on virtually all organ systems and act through a wide variety of toxic mechanisms.

Researchers in toxicology and the environmental health sciences continue to produce a torrent of publications attesting to the health risks engendered by these agents. Despite this mountain of information, we remain relatively ignorant about one vital aspect: their differential impact on males and females. The Institute of Medicine (IOM) report, Does Sex Matter (Wizeman and Pardue, 2001) emphasized the need to examine differences between male and female responses in the research enterprise because of wide gaps between males and females in disease patterns, in response to therapeutic interventions such as drugs, and to external agents. For current neurotoxicology, the predominant question is how environmental chemicals modify brain function and behavior as a function of sex.

Questions about sex differences in disease, in environmental health, and in toxicology have come to assume an importance not accorded them a relatively short time ago. It was only in 1985 that NIH mandated the inclusion of women in clinical trials, followed by other government initiatives (Fig. 1). The Society for Women's Health Research was founded in 1990 in an effort to include women in major medical research studies and to address “…the need for more information about conditions affecting women exclusively, disproportionately, or differently than men.” The IOM report (p. 1–2) described its theme and rationale in this way:

…Over the past decade new discoveries in basic human biology have made it increasingly apparent that many normal physiological functions––and, in many cases, pathological functions––are influenced either directly or indirectly by sex-based differences in biology. This realization, however, has been slow in coming… Historically, the research community assumed that beyond the reproductive system, such differences do not exist or are not relevant. Still, …scientific evidence of the importance of sex differences throughout the life span abounds.

In 2005, the Scientific Group on Methodologies for the Safety Evaluation of Chemicals (SGOMSEC) held a workshop on Gender Differences and Human and Ecological Risk that extended the arguments in the IOM report to toxicology. The SGOMSEC report addressed in detail a number of topics especially pertinent to toxicology. These include sex-specific effects on germ cell mutagenesis, sex differences as an element in health risk assessment, and sex differences in how wildlife are affected by environmental chemicals. As in the IOM report, the authors stressed the fundamental role of sex at every level of biological organization. One of the issues seen as critical is the tactic, used in many investigations, to classify sex as a “confounder,” a tactic that essentially strips its significance from the analysis (Gochfeld, 2007).

Much the same lack of attention or even awareness is common in laboratory research. Wald and Wu (2010), reviewing ten research areas, entitled their article “Of Mice and Women” to underscore the persistent bias. Sometimes, It is only when regulations call for both sexes to be studied, as in cancer bioassays, that both males and females are included. In this instance, it would be ludicrous to not do so; the disparate vulnerabilities and biological substrates are too overwhelming, as in mammary cancers. Many other disciplines, however, often ignore sex differences. The IOM report (p. 1) remarks almost plaintively that, “…scientists have paid much less attention to the direct study of these differences at the basic cellular and molecular levels.” Allegedly mechanistic studies relying on in vitro models tend to ignore sex; few carry out such studies using a culture medium reproducing the in vivo environment of the intact organism. Take the example of how estrogen receptors are distributed in the brain. In weanling rats, females exhibited higher counts and densities of ERα than males (Schlenker and Hansen, 2006). In adult rats, Zhang et al. (2002) observed clear differences in the amount of ER-beta immunoreactivity between males and females. Given the number of additional, similar findings, and sex differences in circulating hormones, is it reasonable to study neuronal phenomena in vitro when the hormones governing sex differences are lacking in its milieu?

Sex differences in the response to environmental endocrine disruptors (EEDs) were one of the factors generating discussion about these chemical classes. Endocrine disruption surfaced as a critical element in environmental health research about 15 years ago. During 2009, 37 articles centered on endocrine disruptors were published in Environmental Health Perspectives, which publishes articles from many disciplines. They exemplify the scope of questions currently being addressed, ranging from in vitro, to whole animal, to epidemiological, to exposure assessment in approach. One of the subjects that EED research has highlighted is the difference between male and female sensitivity to environmental chemicals and the forms in which they might emerge. It feeds on questions that have risen to the apex of public curiosity and discussion, such as why girls, by some criteria (though not all), seem to be outperforming boys academically.

This review examines aspects of sex differences critical to neurotoxicology (cf., Becker et al., 2005). Although it centers on questions and studies that have irrupted into intense public debates, such as sex differences in mathematical talent, analogous questions are equally meaningful for laboratory research. One aim of this article is to highlight the connections between the two arenas and to encourage laboratory researchers to consider the implications of their work for the wider issues that prompt changes in policy.

Section snippets

The architecture of sex

Sex differences are hardly a new topic for science. Darwin (1871) addressed it in these terms (Chapter XXI):

It would be superfluous here to repeat what I have so lately said on the manner in which sexual selection has apparently acted on both the male and female side, causing the two sexes of man to differ in body and mind…

The IOM report distinguishes sex from gender, a distinction often overlooked in popular discourse and sometimes even in scientific writing. The report defines sex as a

Neurobehavioral indicators

The sex-based influence of environmental chemicals on endpoints such as reproductive structure and function is a fairly straightforward question. Brain anatomy and behavior offer far more subtle challenges. Anatomic and neurochemical sex differences are accompanied by, and in fact are the sources of, sex differences in behavior. Decades of research in neurobiology, especially with the availability of newer tools such as magnetic resonance imaging, has allowed us to peer into the human brain to

Clinical implications

Sex differences are ubiquitous in neurological disorders, with examples, taken from diverse sources in the literature, given in Fig. 4. The prevalence of ADHD, for example, is twice as high in males as in females according to NHANES, but an analysis of telephone interviews by Ramtekkar et al. (2010) showed lower male:female ratios than reported in some clinic-based studies, suggesting that females are underdiagnosed in the community, a “silent minority” so to speak (Berry et al., 1985). Also,

Animal behavior

The ubiquity of sex differences in human behavior, including clinical disorders, tells experimenters that neglecting such differences diminishes, or even erodes, the significance of laboratory research for the human condition. In many laboratory studies, though, sex may not even achieve the status of a mere confounder. It can be illuminating to survey the experimental literature to observe how frequently sex differences are ignored. Often, only one sex is studied, usually males, even for

Coda

It is puzzling, at least to this observer, to find such a lack of awareness, in much of neurotoxicology, of the degree to which sex differences pervade virtually every aspect of our research. It is as if we had denied a core principle of evolution. Yet, almost without exception, whenever we include both sexes in a study, we find that they differ in one or more dimensions. If they do not separate on the basis of some numerical index, they may differ in more qualitative terms. ADHD investigators,

Conflict of interest statement

I have no conflict of interest.

Acknowledgements

Preparation supported in part by grant 1RC2ES018736 from NIEHS, and EHS Center grant ES01247.

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