Physiological Mechanisms for the Metabolic Control of Reproduction Chapter 35 Physiological Mechanisms for the Metabolic Control of Reproduction © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 35.1 Schematic presentation of the regulatory roles of key hormonal signals, originating from different metabolic tissues, involved in the integrative control of food intake, energy balance, and reproductive function, including puberty. Paradigmatic examples of hormones with proven roles in the integrative control of food intake and the reproductive axis are presented, including factors from the adipose tissue (leptin: an inhibitory signal for food intake and a stimulatory or permissive signal for puberty and reproduction), pancreas (insulin; the same profile as leptin in terms of actions on food intake and reproduction), and gastrointestinal tract (ghrelin: a stimulatory signal for food intake and an inhibitory signal for the reproductive axis). These peripheral signals do not primarily target GnRH neurons, but rather operate via neuronal afferents that are either stimulatory (light gray) or inhibitory (dark gray). In addition to actions at central levels, inhibitory and stimulatory actions of these metabolic hormones directly at the pituitary (not depicted) and the gonads have been described. CNS: central nervous system; WAT: white adipose tissue; GIT: gastrointestinal tract. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 35.2 Impact of conditions of metabolic stress on the hypothalamic KISS1 system and its functional consequences in pubertal female rats. In the left panels (A), the suppressive effect of short-term fasting on the number of kisspeptin-immunoreactive neurons in the ARC is shown. In the right panels (B), the consequences of kisspeptin replacement in terms of pubertal onset in a model of chronic subnutrition (FR: a 30% reduction in daily food ration) during puberty is illustrated. This protocol of partial food deprivation caused a 30% decrease in body weight (BW) gain and prevented vaginal opening (VO), an external sign of puberty. However, repeated intracerebral injections of kisspeptin 10, in the face of a persistent reduction in BW, was sufficient to rescue VO in >60% of the animals; see the accumulated percentage of VO in the experimental groups on PND37 in the inset of the lower graph of Panel (B). In addition, potent gonadotropic and estrogenic responses were evoked by kisspeptin 10 in all treated females (data not shown). PND: postnatal day (age); ND: not detectable; d: day. Source: Taken from Refs 4,81, with modifications. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 35.3 Potential mechanisms for transmitting leptin actions onto GnRH neurons, as deduced from expression and functional analyses in rodents and other mammals. Since GnRH neurons are devoid of leptin receptors, the actions of the adipose hormone must be conveyed via afferents that are sensitive to leptin effects. These may include (1) POMC neurons, which seem to play a key role in the joint transmission of leptin and insulin effects to GnRH neurons; (2) kisspeptin (Kiss1) neurons, which are sensitive to changes in leptin inputs and operate as major regulators of GnRH neurons; and (3) other neuronal populations, including intermediate neurons (of as-yet-unknown nature), in the vicinity of Kiss1 or GnRH neurons, and neurons in the ventral premammillary nucleus (PMV). Note that fragmentary neuroanatomical and functional data suggest the existence of potential connections between some of these neuronal populations (indicated by dotted-line projections). Such connections may help to explain the fact that leptin appears to operate predominantly in an indirect manner on some of those neuronal subsets (such as Kiss1 neurons). Some of the putative transmitters for these neuronal circuits are depicted. In any event, it is noted that other neuronal populations in the ARC, such as neuropeptide Y (NPY) and agouti-related peptide (AgRP) neurons, are known to be sensitive to leptin effects and have been reported to interplay with POMC, GnRH, and (probably) Kiss1 neurons. However, their roles in transmitting leptin actions to Kiss1 and GnRH neurons need further clarification and, for sake of simplicity, are not depicted in this figure. Likewise, other circuits (such as those originating from the lateral hypothalamus) are not represented either. POA: preoptic area; PMV: ventral premammillary nucleus; Kp: kisspeptins; NKB: neurokinin B; αMSH: alpha melanocyte stimulating hormone; NO: nitric oxide; GLU: glutamate; WAT: white adipose tissue. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 35.4 Effects of intracerebroventricular injection of the agonists of NKB (senktide; 600 pmol/rat) and dynorphin (U-50; 1 nmol/rat) on LH secretion. Adult female rats, subjected to a standard protocol of ovariectomy and physiological estradiol replacement (as described in Ref. 139), were used in these experiments. Saline-injected females (0.9% saline) served as controls. Blood samples were taken before (0 min) and at 20, 60, and 120 min after compound administration. In addition to time-course profiles, the integral secretory responses to the agonists or vehicle were calculated using the trapezoidal rule and depicted in the inset. These hormonal responses illustrate the opposite effects of NKB (stimulatory) and DYN (inhibitory) on GnRH neurons, as indirectly estimated by its surrogate marker, LH secretion. Statistically significant differences are denoted by * (P < 0.05) and ** (P < 0.01). Source: Taken from Ref. 139, with modifications, and from our unpublished data. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 35.5 Characterization of the putative roles of NKB in the control of puberty onset in female rats. The left panels (A) show hypothalamic expression levels of the genes encoding NKB (Tac2) and its receptor (Tacr3) along postnatal maturation and during the pubertal transition. These data illustrate a significant increase in the expression of both hypothalamic genes during the transition between the infantile (<PND21) and juvenile (>PND21) stages. The right panels (B) illustrate the effects of repeated central injections of the NKB agonist, senktide, on puberty onset and LH responses in female rats subjected to chronic subnutrition (FR: reduction of 30% in the daily food ration) during the pubertal transition. This regimen of feeding restriction, which induced a drop of ∼30% in body weight (BW), was sufficient to cause pubertal arrest, as manifested by lack of vaginal opening (VO) and lowering of basal LH levels in vehicle-treated animals. In this model, repeated central injections of senktide did not change BW but was able to rescue VO in >50% of animals and to induce potent LH responses. Comparison with data from Figure 35.2(B) demonstrate that such effects are remarkably similar to those evoked by kisspeptin replacement in food-restricted pubertal female rats and, together with other results summarized in the section Other Products of KISS1 Neurons in the Metabolic Control of Reproduction: Role of KNDy Neuropeptides, indirectly suggest that NKB cooperates with kisspeptins in the metabolic control of puberty onset. S11: ribosomal protein S11, used as internal control for qPCR assays; PND: postnatal day (age). Source: Taken from Ref. 140 with modifications. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition

FIGURE 35.6 Tentative model for the potential role of brain mammalian target of rapamycin (mTOR) signaling in the metabolic control of puberty. Leptin, as a key peripheral metabolic signal, is known to modulate hypothalamic mTOR signaling, which mediates part of its regulatory actions in energy homeostasis. In the same vein, experimental evidence suggests that central mTOR may participate in conveying at least part of the actions of leptin on the reproductive brain. This action is likely to involve the regulatory effect of mTOR on Kiss1 expression, as a major regulatory afferent to GnRH neurons, since blockade of brain mTOR activity not only delayed puberty but also suppressed Kiss1 mRNA levels in key hypothalamic nuclei, such as the ARC. All in all, the pharmacological and expression data gathered to date strongly suggest that rapamycin-sensitive mTORC1 signaling may operate, in conjunction with other energetic sensors such as AMPK, as a transmitting hub that provides a putative molecular link between peripheral signals and central neuropeptide effectors in the metabolic control of the HPG axis. For further details, see the text. Source: Taken from Ref. 87, with modifications. © 2015, Elsevier, Inc., Plant and Zeleznik, Knobil and Neill's Physiology of Reproduction, Fourth Edition