A lumpers versus splitters approach to sexual differentiation of the brain

Abstract

Over 50 years of rigorous empirical attention to the study of sexual differentiation of the brain has produced sufficient data to reveal fundamental guiding principles, but has also required the generation of new hypotheses to explain non-conforming observations. An early emphasis on the powerful impact and essential role of gonadal steroids is now complemented by an appreciation for genetic contributions to sex differences in the brain. The organizing effects of early steroid hormones on reproductively relevant brain regions and endpoints are largely dependent upon neuronal aromatization of androgens to estrogens. The effect of estradiol is mediated via estrogen receptors (ER). The presence or absence of ER can restrict hormone action to select cells and either prevent or invoke cell death. Alternatively, ER activation can initiate signaling cascades that induce cell-to-cell communication and thereby transduce organizational steroid effects to large numbers of cells. However, the specific details by which cell death and cell-to-cell communication are achieved appear to be locally, even cellularly, unique and specific to that particular subpopulation. As the field moves forward the increasingly specific and detailed elucidation of mechanism challenges us to generate new guiding principles in order to gain a holistic understanding of how the brain develops in males and females.

Figure 1. Phases in scientific discovery

Most scientific discoveries begin with the observation and description of a fundamental phenomenon, thereby forming the basis for an initial hypothesis. Once accepted as true, the hypothesis becomes a tenet which over time becomes a dogma. This stimulates challenges in the form of new observations which result in a refining and/or expanding of the initial hypothesis and associated dogma. Eventually, the focus of the field turns to mechanism, which mostly reconfirms the initial dogma but also provides a new set of observations upon which new hypotheses can be based. The field of sexual differentiation of the brain is currently in this period of scientific discovery as we make great inroads into determining the mechanism of steroid hormone action while also forming new hypotheses regarding the importance of genetic and epigenetic variables.

Figure 2. A Lumpers versus Splitters approach to sexual differentiation of the brain

Studies on the sexual differentiation of the brain have been going on for 50+ years, providing a rich and complex collection of findings. Several fundamental principles arise from the dogma of early organizational effects of steroids determining adult reproductive physiology and sexual behavior. These can be lumped into broad categories. But elucidation of mechanism reveals highly unique region specific cellular pathways that underlie the organizational actions of steroids. Moreover, examination of sex differences outside of the context of reproduction reveals many new and possibly fundamental principles that both induce and reduce sex differences in brain and behavior. The relative lumping and splitting of various components is fluid and likely to change with additional new discoveries and reinterpretation of old ones. The colors used to designate each major category under Lumping is then used to designate the related set of individual findings under Splitting. New colors under Splitting indicates areas that are so recent as to not yet be associated with any fundamental concepts.

Figure 3. Complex cellular cascades mediate organizational effects of steroids

In the preoptic area, estradiol up regulates the synthesis of prostaglandin E2 (PGE2), which binds to EP2&4 receptors found on neurons and astrocytes. The EP2&4 receptors are positively linked to adenylate cyclase and the production of cAMP, thereby activating protein kinase A (PKA), which is associated with the actin scaffolding in the head and neck of dendritic spines. Activated PKA phosphorylates the GluR1 subunit of the AMPA receptor, which causes it to move into the cellular membrane and promotes the development and stabilization of the post-synaptic density of the dendritic spine. In the absence of phosphorylated GluR1, the AMPA receptor is not maintained in the membrane and dendritic spine synapses collapse or fail to form. In the photomicrographs on the left, POA neurons grown in vitro are visualized by red staining for the neuron specific microtubule associated protein (MAP-2) and phosphorylated GluR1 is visualized as green. The combination of MAP-2 and phosphorylated GluR1 is visualized as yellow. Cells that appear entirely green are presumptive astrocytes. Cells in the top panel were treated with vehicle while those in the bottom panel were treated with PGE2 two hours prior. The intensity and amount of colocalization of phosphorylated GluR1 and MAP-2 is increased and the movement of GluR1 to the membranes of astrocytes is readily apparent (photo courtesy of Katherine Lenz). This combined effect of PGE2 on neurons and glial is speculated to be the result of cell-to-cell communication and to be the basis for organizationally determined sex differences in dendritic spine density and astroglial morphology.

Figure 4. Cell genesis may become a new fundamental principle of sexual differentiation of the brain

There are almost twice as many future neurons being born in the developing hippocampus of male rat pups compared to their female littermates. Treating females with estradiol increases the rate of neurogenesis to that of males, while treating males with either an estrogen receptor antagonist or aromatase inhibitor decreases the rate to that of females [97]. This sex difference is notably different than the well established fundamental principle of sex differences in cell death in select nuclei during the perinatal period. A second example of a sex difference in cell genesis is illustrated here and is unusual in several ways. First is that a sex difference in the overall tone of endocannabinoids mediates the sex difference. Females have higher levels of the degradation enzymes, FAAH and MAGL, and as a result have lower resting levels of the endocannabinoids, 2-AG and anandamide. There is also a higher rate of cell proliferation in the medial amygdala of females during the early neonatal period. Raising the endocannabinoid tone by either inhibiting FAAH and MAGL or supplying exogenous endocannabinoids, reduces the rate of proliferation in females to that of males, but has no effect on males. Lastly, most of the new cells being born during the perinatal period will differentiate into neurons, but a small population that accounts for the observed sex difference will become astrocytes, and as a result females have more astrocytes in the medial amygdala compared to males [89].