Fasting and prolonged food restriction differentially affect GH secretion independently of GH receptor signaling in AgRP neurons

Litter sex composition influences plasma prolactin levels but not the melanin-concentrating hormone immunoreactive neurons in the medial preoptic area of late lactating Long-Evans rats

This study examines the influence of litter sex composition on melanin-concentrating hormone immunoreactive (MCH-ir) neurons in the ventromedial medial preoptic area (vmMPOA) and on plasma prolactin levels in lactating rats. MCH is a critical regulator of maternal behavior and displays sexual dimorphism within the MPOA, making it an important target for understanding neuroendocrine adaptations in lactation. Prolactin, a pivotal hormone in lactation and maternal care, was also assessed to elucidate its interaction with litter sex composition. Thirty lactating female rats were divided into five experimental groups based on litter sex composition: all-male (10 male pups), all-female (10 female pups), balanced control (five male and five female pups), predominantly male (seven male and three female pups), and predominantly female (three male and seven female pups). On post-partum day 19 (PPD19), the dams were euthanized for biological analysis. Blood samples were collected for plasma prolactin quantification, and the brains were processed to analyze MCH-ir neurons in the vmMPOA. Results showed no significant differences in food and water intake or the number of MCH-ir neurons in the vmMPOA among experimental groups. However, significant variation in prolactin levels was observed, with the all-male offspring group exhibiting the highest levels (mean prolactin level 23.9 ng/mL, p < .001), followed by the all-female group (20.3 ng/mL, p < .01), compared to the control group (14.3 ng/mL). Additionally, the all-male group showed a reduction in body weight gain. These results suggest that although litter sex composition does not alter the number of MCH-ir neurons in the vmMPOA, it significantly impacts maternal prolactin levels. This differential prolactin regulation may reflect distinct physiological demands or caregiving behaviors imposed by homogeneous litters, which could, in turn, influence maternal energy balance, lactation efficiency, and adaptive maternal responses. Understanding these sex-specific influences on maternal neuroendocrine function has important implications for comprehending maternal care dynamics and energy allocation during lactation.

GH-Releasing Hormone Neurons Regulate the Hypothalamic-Pituitary-Somatotropic Axis via Short-Loop Negative Feedback

Growth hormone (GH)-releasing hormone (GHRH) neurons are master regulators of GH secretion. However, the role of these cells in controlling pituitary GH secretion through short-loop negative feedback has not yet been fully clarified. Thus, GHRH-specific GH receptor (GHR) knockout (GHRHΔGHR) mice were generated, and possible consequences on GH secretion and body growth were determined. Approximately 60% of arcuate nucleus GHRH neurons exhibited GH-induced STAT5 phosphorylation, a marker of GHR-expressing cells. This response was practically eliminated in GHRHΔGHR mice. GHR ablation in GHRH-expressing cells increased body weight, lean mass, and naso-anal length in male and female mice without affecting fat mass. The higher body growth of GHRHΔGHR mice was associated with increases in GH secretion, mainly via higher pulsatile GH secretion and GH pulse amplitude. GHRHΔGHR female mice also showed increased GH pulse frequency and basal (non-pulsatile) secretion compared to control females. Liver Igf1 expression was increased only in GHRHΔGHR male mice. Mice carrying ablation of the insulin-like growth factor-1 (IGF-1) receptor (IGF1R) or both GHR and IGF1R in GHRH-expressing cells were generated. The increases in body growth and serum IGF-1 levels were significantly higher in GHRHΔGHR/IGF1R mice compared to GHRHΔGHR mice but similar to levels observed in GHRHΔIGF1R mice. Electrophysiological experiments showed no acute changes in the activity of GHRH neurons after GH or IGF-1 exposure. In conclusion, GH feeds back on GHRH cells to control the hypothalamic-pituitary-somatotropic axis. However, IGF1R signaling prevails over GHR as the primary signal sensed by GHRH neurons to regulate GH secretion.

Update on regulation of GHRH and its actions on GH secretion in health and disease

This review focuses on our current understanding of how growth hormone releasing hormone (GHRH): 1) stimulates GH release and synthesis from pituitary growth hormone (GH)-producing cells (somatotropes), 2) drives somatotrope proliferation, 3) is negatively regulated by somatostatin (SST), GH and IGF1, 4) is altered throughout lifespan and in response to metabolic challenges, and 5) analogues can be used clinically to treat conditions of GH excess or deficiency. Although a large body of early work provides an underpinning for our current understanding of GHRH, this review specifically highlights more recent work that was made possible by state-of-the-art analytical tools, receptor-specific agonists and antagonists, high-resolution in vivo and ex vivo imaging and the development of tissue (cell) -specific ablation mouse models, to paint a more detailed picture of the regulation and actions of GHRH.

Growth Hormone Receptor in Lateral Hypothalamic Neurons Is Required for Increased Food-Seeking Behavior during Food Restriction in Male Mice

Growth hormone (GH) action in the brain regulates neuroendocrine axes, energy and glucose homeostasis, and several neurological functions. The lateral hypothalamic area (LHA) contains numerous neurons that respond to a systemic GH injection by expressing the phosphorylated STAT5, a GH receptor (GHR) signaling marker. However, the potential role of GHR signaling in the LHA is unknown. In this study, we demonstrated that ∼70% of orexin- and leptin receptor (LepR)-expressing neurons in the LHA are responsive to GH. Male mice carrying inactivation of the Ghr gene in the LHA were generated via bilateral injections of an adeno-associated virus. In ad libitum-fed mice, GHR ablation in LHA neurons did not significantly change energy and glucose homeostasis. Subsequently, mice were subjected to 5 d of 40% food restriction. Food restriction decreased body weight, energy expenditure, and carbohydrate oxidation. These effects were similarly observed in control and LHAΔGHR mice. While food-deprived control mice progressively increased ambulatory/exploratory activity and food-seeking behavior, LHAΔGHR mice did not show hyperactivity induced by food restriction. GHR ablation in the LHA reduced the percentage of orexin neurons expressing c-Fos during food restriction. Additionally, an acute GH injection increased the expression of c-Fos in LHAORX neurons. Inactivation of Ghr in LepR-expressing cells did not prevent hyperactivity in food-deprived mice, whereas whole-brain Ghr knock-out mice showed reduced ambulatory activity during food restriction. Our findings indicate that GHR signaling in the LHA regulates the activity of orexin neurons and is necessary to increase food-seeking behavior in food-deprived male mice.

New findings on brain actions of growth hormone and potential clinical implications

Growth hormone (GH) is secreted by somatotropic cells of the anterior pituitary gland. The classical effects of GH comprise the stimulation of cell proliferation, tissue and body growth, lipolysis, and insulin resistance. The GH receptor (GHR) is expressed in numerous brain regions. Notably, a growing body of evidence indicates that GH-induced GHR signaling in specific neuronal populations regulates multiple physiological functions, including energy balance, glucose homeostasis, stress response, behavior, and several neurological/cognitive aspects. The importance of central GHR signaling is particularly evident when the organism is under metabolic stress, such as pregnancy, chronic food deprivation, hypoglycemia, and prolonged exercise. These particular situations are associated with elevated GH secretion. Thus, central GH action represents an internal signal that coordinates metabolic, neurological, neuroendocrine, and behavioral adaptations that are evolutionarily advantageous to increase the chances of survival. This review summarizes and discusses recent findings indicating that the brain is an important target of GH, and GHR signaling in different neuronal populations regulates essential physiological functions.

Adipocyte PI3K links adipostasis with baseline insulin secretion at fasting through an adipoincretin effect

Insulin-PI3K signaling controls insulin secretion. Understanding this feedback mechanism is crucial for comprehending how insulin functions. However, the role of adipocyte insulin-PI3K signaling in controlling insulin secretion in vivo remains unclear. Using adipocyte-specific PI3Kα knockout mice (PI3KαAdQ) and a panel of isoform-selective PI3K inhibitors, we show that PI3Kα and PI3Kβ activities are functionally redundant in adipocyte insulin signaling. PI3Kβ-selective inhibitors have no effect on adipocyte AKT phosphorylation in control mice but blunt it in adipocytes of PI3KαAdQ mice, demonstrating adipocyte-selective pharmacological PI3K inhibition in the latter. Acute adipocyte-selective PI3K inhibition increases serum free fatty acid (FFA) and potently induces insulin secretion. We name this phenomenon the adipoincretin effect. The adipoincretin effect operates in fasted mice with increasing FFA and decreasing glycemia, indicating that it is not primarily a control system for blood glucose. This feedback control system defines the rates of adipose tissue lipolysis and chiefly controls basal insulin secretion during fasting.

Experimental Autoimmune Encephalomyelitis Influences GH-Axis in Female Rats

Inflammation, demyelination, and axonal damage to the central nervous system (CNS) are the hallmarks of multiple sclerosis (MS) and its representative animal model, experimental autoimmune encephalomyelitis (EAE). There is scientific evidence for the involvement of growth hormone (GH) in autoimmune regulation. Previous data on the relationship between the GH/insulin like growth factor-1 (IGF-1) axis and MS/EAE are inconclusive; therefore, the aim of our study was to investigate the changes in the GH axis during acute monophasic EAE. The results show that the gene expression of Ghrh and Sst in the hypothalamus does not change, except for Npy and Agrp, while at the pituitary level the Gh, Ghrhr and Ghr genes are upregulated. Interestingly, the cell volume of somatotropic cells in the pituitary gland remains unchanged at the peak of the disease. We found elevated serum GH levels in association with low IGF-1 concentration and downregulated Ghr and Igf1r expression in the liver, indicating a condition resembling GH resistance. This is likely due to inadequate nutrient intake at the peak of the disease when inflammation in the CNS is greatest. Considering that GH secretion is finely regulated by numerous central and peripheral signals, the involvement of the GH/IGF-1 axis in MS/EAE should be thoroughly investigated for possible future therapeutic strategies, especially with a view to improving EAE disease.