Adult ovary transfer counteracts the callosal enlargement resulting from prepubertal ovariectomy

The effects of puberty and sex on adolescent white matter development: A systematic review

Adolescence, the transition between childhood and adulthood, is characterized by rapid brain development in white matter (WM) that is attributed in part to rising levels in adrenal and gonadal hormones. The extent to which pubertal hormones and related neuroendocrine processes explain sex differences in WM during this period is unclear. In this systematic review, we sought to examine whether there are consistent associations between hormonal changes and morphological and microstructural properties of WM across species and whether these effects are sex-specific. We identified 90 (75 human, 15 non-human) studies that met inclusion criteria for our analyses. While studies in human adolescents show notable heterogeneity, results broadly demonstrate that increases in gonadal hormones across pubertal development are associated with macro- and microstructural changes in WM tracts that are consistent with the sex differences found in non-human animals, particularly in the corpus callosum. We discuss limitations of the current state of the science and recommend important future directions for investigators in the field to consider in order to advance our understanding of the neuroscience of puberty and to promote forward and backward translation across model organisms.

Active feminization of the preoptic area occurs independently of the gonads in Amphiprion ocellaris

Sex differences in the anatomy and physiology of the vertebrate preoptic area (POA) arise during development, and influence sex-specific reproductive functions later in life. Relative to masculinization, mechanisms for feminization of the POA are not well understood. The purpose of this study was to induce sex change from male to female in the anemonefish Amphiprion ocellaris, and track the timing of changes in POA cytoarchitecture, composition of the gonads and circulating sex steroid levels. Reproductive males were paired together and then sampled after 3 weeks, 6 months, 1 year and 3 years. Results show that as males change sex into females, number of medium cells in the anterior POA (parvocellular region) approximately double to female levels over the course of several months to 1 year. Feminization of gonads, and plasma sex steroids occur independently, on a variable timescale, up to years after POA sex change has completed. Findings suggest the process of POA feminization is orchestrated by factors originating from within the brain as opposed to being cued from the gonads, consistent with the dominant hypothesis in mammals. Anemonefish provide an opportunity to explore active mechanisms responsible for female brain development in an individual with male gonads and circulating sex steroid levels.

The endocrine-brain-aging triad where many paths meet: female reproductive hormone changes at midlife and their influence on circuits important for learning and memory

Female mammals undergo natural fluctuations in sex steroid hormone levels throughout life. These fluctuations span from early development, to cyclic changes associated with the menstrual or estrous cycle and pregnancy, to marked hormone flux during perimenopause, and a final decline at reproductive senescence. While the transition to reproductive senescence is not yet fully understood, the vast majority of mammals experience this spontaneous, natural phenomenon with age, which has broad implications for long-lived species. Indeed, this post-reproductive life stage, and its transition, involves significant and enduring physiological changes, including considerably altered sex steroid hormone and gonadotropin profiles that impact multiple body systems, including the brain. The endocrine-brain-aging triad is especially noteworthy, as many paths meet and interact. Many of the brain regions affected by aging are also sensitive to changes in ovarian hormone levels, and aging and reproductive senescence are both associated with changes in memory performance. This review explores how menopause is related to cognitive aging, and discusses some of the key neural systems and molecular factors altered with age and reproductive hormone level changes, with an emphasis on brain regions important for learning and memory.

Trajectories and phenotypes with estrogen exposures across the lifespan: What does Goldilocks have to do with it?

This article is part of a Special Issue "Estradiol and cognition". Estrogens impact the organization and activation of the mammalian brain in both sexes, with sex-specific critical windows. Throughout the female lifespan estrogens activate brain substrates previously organized by estrogens, and estrogens can induce non-transient brain and behavior changes into adulthood. Therefore, from early life through the transition to reproductive senescence and beyond, estrogens are potent modulators of the brain and behavior. Organizational, reorganizational, and activational hormone events likely impact the trajectory of brain profiles during aging. A "brain profile," or quantitative brain measurement for research purposes, is typically a snapshot in time, but in life a brain profile is anything but static--it is in flux, variable, and dynamic. Akin to this, the only thing continuous and consistent about hormone exposures across a female's lifespan is that they are noncontinuous and inconsistent, building and rebuilding on past exposures to create a present brain and behavioral landscape. Thus, hormone variation is especially rich in females, and is likely the destiny for maximal responsiveness in the female brain. The magnitude and direction of estrogenic effects on the brain and its functions depend on a myriad of factors; a "Goldilocks" phenomenon exists for estrogens, whereby if the timing, dose, and regimen for an individual are just right, markedly efficacious effects present. Data indicate that exogenously-administered estrogens can bestow beneficial cognitive effects in some circumstances, especially when initiated in a window of opportunity such as the menopause transition. Could it be that the age-related reduction in efficacy of estrogens reflects the closure of a late-in-life critical window occurring around the menopause transition? Information from classic and contemporary works studying organizational/activational estrogen actions, in combination with acknowledging the tendency for maximal responsiveness to cyclicity, will elucidate ways to extend sensitivity and efficacy into post-menopause.

Corpus callosum size may predict late-life depression in women: a 10-year follow-up study

BACKGROUND

Recent research on late-life depression (LLD) pathophysiology suggests the implication of abnormalities in cerebral white matter and particularly in interhemispheric transfer. Corpus callosum (CC) is the main brain interhemispheric commissure. Hence, we investigated the association between baseline CC measures and risk of LDD.

METHODS

We studied 467 non-demented individuals without LLD at baseline from a cohort of elderly community-dwelling people (the ESPRIT study). LLD was assessed at year 2, 4, 7 and 10 of the study follow-up. At baseline, T1-weighted magnetic resonance images were manually traced to measure the mid-sagittal areas of the anterior, mid and posterior CC. Multivariate Cox proportional hazards models stratified by sex were used to predict LLD incidence over 10 years.

RESULTS

A significant interaction between gender and CC size was found (p=0.02). LLD incidence in elderly women, but not in men, was significantly associated with smaller anterior (HR 1.37 [1.05-1.79] p=0.017), mid (HR 1.43 [1.09-1.86] p=0.008), posterior (HR 1.39 [1.12-1.74] p=0.002) and total (HR 1.53 [1.16-2.00] p=0.002) CC areas at baseline in Cox models adjusted for age, education, global cognitive impairment, ischemic pathologies, left-handedness, white matter lesion, intracranial volume and past depression.

LIMITATIONS

The main limitation was the retrospective assessment of major depression.

CONCLUSION

Smaller CC size is a predictive factor of incident LLD over 10 years in elderly women independently of cognitive deterioration. Our finding suggests a possible role of CC and reduced interhemispheric connectivity in LLD pathophysiology. Extensive explorations are needed to clarify the mechanisms leading to CC morphometric changes in mood disorders.

An update on the cognitive impact of clinically-used hormone therapies in the female rat: models, mazes, and mechanisms

In women, ovarian hormone loss associated with menopause has been related to cognitive decline. Hormone therapy (HT) may ameliorate some of these changes. Understanding the cognitive impact of female steroids, including estrogens, progestogens, and androgens, is key to discovering treatments that promote brain health in women. The preclinical literature has presented elegant and methodical experiments allowing a better understanding of parameters driving the cognitive consequences of ovarian hormone loss and HT. Animal models have been a valuable tool in this regard, and will be vital to future discoveries. Here, we provide an update on the literature evaluating the impact of female steroid hormones on cognition, and the putative mechanisms mediating these effects. We focus on preclinical work that was done with an eye toward clinical realities. Parameters that govern the cognitive efficacy of HT, from what we know thus far, include but are not limited to: type, dose, duration, and route of HT, age at HT initiation, timing of HT relative to ovarian hormone loss, memory type examined, menopause history, and hormone receptor status. Researchers have identified intricate relationships between some of these factors by studying their individual effects on cognition. As of late, there is increased focus on studying interactions between these variables as well as multiple hormone types when administered concomitantly. This is key to translating preclinical data to the clinic, wherein women typically have concurrent exposure to endogenous ovarian hormones as well as exogenous combination HTs, which include both estrogens and progestins. Gains in understanding the parameters of HT effects on cognition provide exciting novel avenues that can inform clinical treatments, eventually expanding the window of opportunity to optimally enhance cognition and brain health in aging women. This article is part of a Special Issue entitled Hormone Therapy.

Suicidal behavior is associated with reduced corpus callosum area

BACKGROUND

Corpus callosum (CC) size has been associated with cognitive and emotional deficits in a range of neuropsychiatric and mood disorders. As such deficits are also found in suicidal behavior, we investigated specifically the association between CC atrophy and suicidal behavior.

METHODS

We studied 435 right-handed individuals without dementia from a cohort of community-dwelling persons aged 65 years and over (the ESPRIT study). They were divided in three groups: suicide attempters (n = 21), affective control subjects (AC) (n = 180) without history of suicide attempt but with a history of depression, and healthy control subjects (HC) (n = 234). T1-weighted magnetic resonance images were traced to measure the midsagittal areas of the anterior, mid, and posterior CC. Multivariate analysis of covariance was used to compare CC areas in the three groups.

RESULTS

Multivariate analyses adjusted for age, gender, childhood trauma, head trauma, and total brain volume showed that the area of the posterior third of CC was significantly smaller in suicide attempters than in AC (p = .020) and HC (p = .010) individuals. No significant differences were found between AC and HC. No differences were found for the anterior and mid thirds of the CC.

CONCLUSIONS

Our findings emphasize a reduced size of the posterior third of the CC in subjects with a history of suicide, suggesting a diminished interhemispheric connectivity and a possible role of CC in the pathophysiology of suicidal behavior. Further studies are needed to strengthen these results and clarify the underlying cellular changes leading to these morphometric differences.

Pubertal ovarian hormone exposure reduces the number of myelinated axons in the splenium of the rat corpus callosum

The size of the female rat corpus callosum decreases in response to pubertal ovarian hormone exposure, but the underlying changes in axonal composition have not been examined. In the current study, animals underwent ovariectomy or sham surgery at day 20, and the number of myelinated and unmyelinated axons were examined in young adulthood (2 months) using electron microscopy. Ovariectomized animals had a greater number of myelinated axons compared to intact animals, while total axon number was not affected. Ovarian hormone exposure seems to limit the number of axons that become myelinated in the splenium, while not affecting the number of axons crossing through the region.

Endogenous testosterone concentration, mental rotation, and size of the corpus callosum in a sample of young Hungarian women

In the present study brain laterality, hemispheric communication, and mental rotation performance were examined. A sample of 33 women were tested for a possible linear relationship of testosterone level and mental rotation with structural background of the brain. Subjects with a smaller splenial area of corpus callosum tended to have lower levels of testosterone (r =.37, p<.05). However, there were no significant differences in mean scores of mental rotation of object and hand between groups with high and low levels of testosterone. There was a significant difference in relative size of the 6th area (slice) of the corpus callosum between groups with good and poor scores on mental rotation of an object and also in relative size of the 4th and 5th slices of the corpus callosum between groups on mental rotation of the hand. The good and poor scorers' show different relations with the measures of the corpus. The mental rotation of hand was associated with the parietal areas of the corpus callosum, while the mental rotation of object was associated only with the occipital area. These observations suggest that higher testosterone levels may be associated with a larger splenial area, which represents an important connection between the parieto-occipitocortical areas involved in activation of mental images. Further srudy is encouraged.

Sex differences in the hippocampal dentate gyrus of the guinea-pig before puberty

The aim of the present research was to ascertain the presence of sex differences in the hippocampal dentate gyrus of the guinea-pig, a long-gestation rodent which gives birth to mature young and whose brain is at a more advanced stage of maturation at birth than that of the rat and mouse. The brains of neonatal (15-16 days old) and prepubescent (45-46 days old) male and female guinea pigs were Golgi-Cox stained. Granule cells were sampled from the upper (suprapyramidal) and lower (infrapyramidal) blade of the septal dentate gyrus and their dendritic tree and soma were measured. The analysis was conducted separately on granule cells with soma in the superficial (superficial granule cells) and deep (deep granule cells) half of the granule cell layer. Numerous sex differences were found in the upper blade of the dentate gyrus. Neonatal males had more dendritic branches than females in the innermost dendritic tree of both superficial and deep granule cells, but females had more branches over the middle/outer dendritic tree and a longer dendritic length. In prepubescent animals, the sex difference in the middle dendritic tree of the superficial granule cells changed direction, with males having more branches than females. In the deep granule cells, the sex differences were similar to those in neonatal animals. In both granule cell types, the dendritic length was similar in the two sexes. While no sex differences were found in dendritic spine density in neonatal animals, in prepubescent animals spine density was greater in females. In the lower blade the granule cells showed very few sex differences in both neonatal and prepubescent animals. This study shows wide dynamically changing sex differences in the granule cells located in the upper blade of the septal dentate gyrus, but almost no differences in the lower blade. These results demonstrate that sex differences are not ubiquitous in the dentate gyrus and suggest that the lower blade, unlike the upper blade, might be involved in non-sexually dimorphic behaviors.