Volume 19, Issue 3 - Special Volume: Pathophysiology of Adipose Tissue: Effects of Steroid Hormones - Part D / Editors: Gérard S. Chetrite and Bruno Fève

Achievement of sexual maturation and maintenance of fertility in adulthood are functions that are sensitive to the metabolic status of the organism, particularly the magnitude of fat reserves. In this sense, the adipocyte-derived hormone, leptin, plays a major role in linking metabolic cues and the control of multiple neuroendocrine axes. The hypothalamus is a key site mediating leptin actions, including those involved in the modulation of the hypothalamus-pituitary-gonads (HPG) axis at different stages of development and in different environmental conditions. In the present review, we provide an update of the role of leptin in reproduction and discuss its interactions with neurons, neurotransmitters and downstream targets of the reproductive axis, with a special emphasis on the actions of leptin in the central nervous system. We hope this review will contribute to the understanding of the mechanisms whereby metabolic signals, especially leptin, influence the reproductive neuroendocrine axis modulating its activity in different nutritional states. Special attention will be given to recent advances in the identification of key hypothalamic sites and signaling pathways relevant to leptin’s action in reproductive control.

A sexual dimorphism exists in which increased abdominal and visceral adipose tissue (VAT) – found in women and marked by low sex hormone binding globulin (SHBG) and high bioavailable testosterone (BT) – is related to the metabolic risk profile. In men, increased BT is related to decreased abdominal obesity and a decrease in the metabolic risk profile. In women, race differences have been found in androgenic sex steroids including SHBG and BT as well as central fat distribution, creating inherently greater metabolic risk for certain populations. Estrogen and estrogen receptor isoforms play a role in fat deposition and distribution and may influence the changes that occur during the menopausal transition. Androgenic sex steroids serve a mediating role, influencing VAT accumulation and its associated metabolic risk factors while VAT also serves a mediating role influencing the androgenic sex steroid-metabolic risk relationship in women. Furthermore, androgenic sex steroids and VAT may independently contribute to the variance in several metabolic variables associated with cardiovascular disease, type 2 diabetes, and their antecedent conditions such as the metabolic syndrome. Race has been shown to modify the relationship between androgenic sex steroids and metabolic variables associated with risk for diabetes in Black and White women. Further research is warranted to examine the mechanisms involved in race differences. Total adiposity and central fat distribution in accordance with changes in the hormone and metabolic milieu influence breast cancer risk, which varies by race and menopausal status. These findings have broader implications for the study of health promotion/disease prevention in women.

Eating is a simple behavior with complex functions. The unconscious neuroendocrine process that stops eating and brings a meal to its end is called satiation. Energy homeostasis is mediated accomplished through the control of meal size via satiation. It involves neural integrations of phasic negative-feedback signals related to ingested food and tonic signals, such as those related to adipose tissue mass. Energy homeostasis is accomplished through adjustments in meal size brought about by changes in these satiation signals. The best understood meal-derived satiation signals arise from gastrointestinal nutrient sensing. Gastrointestinal hormones secreted during the meal, including cholecystokinin, glucagon-like peptide 1, and PYY, mediate most of these. Other physiological signals arise from activation of metabolic-sensing neurons, mainly in the hypothalamus and caudal brainstem. We review both classes of satiation signal and their integration in the brain, including their processing by melanocortin, neuropeptide Y/agouti-related peptide, serotonin, noradrenaline, and oxytocin neurons. Our review is not comprehensive; rather, we discuss only what we consider the best-understood mechanisms of satiation, with a special focus on normally operating physiological mechanisms.