Hindbrain A2 noradrenergic neuron adenosine 5'-monophosphate-activated protein kinase activation, upstream kinase/phosphorylase protein expression, and receptivity to hormone and fuel reporters of short-term food deprivation are regulated by estradiol

Effects of Short-Term Food Deprivation on Catecholamine and Metabolic-Sensory Biomarker Gene Expression in Hindbrain A2 Noradrenergic Neurons Projecting to the Forebrain Rostral Preoptic Area: Impact of Negative versus Positive Estradiol Feedback

Hindbrain A2 noradrenergic neurons assimilate estrogenic and metabolic cues. In female mammals, negative- versus positive-feedback patterns of estradiol (E) secretion impose divergent regulation of the gonadotropin-releasing hormone (GnRH)-pituitary-gonadal (HPG) neuroendocrine axis. Current research used retrograde tracing, dual-label immunocytochemistry, single-cell laser-microdissection, and multiplex qPCR methods to address the premise that E feedback modes uniquely affect metabolic regulation of A2 neurons involved in HPG control. Ovariectomized female rats were given E replacement to replicate plasma hormone levels characteristic of positive (high-E dose) or negative (low-E dose) feedback. Animals were either full-fed (FF) or subjected to short-term, e.g., 18-h food deprivation (FD). After FF or FD, rostral preoptic area (rPO)-projecting A2 neurons were characterized by the presence or absence of nuclear glucokinase regulatory protein (nGKRP) immunostaining. FD augmented or suppressed mRNAs encoding the catecholamine enzyme dopamine-beta-hydroxylase (DβH) and the metabolic-sensory biomarker glucokinase (GCK), relative to FF controls, in nGKRP-immunoreactive (ir)-positive A2 neurons from low-E or high-E animals, respectively. Yet, these transcript profiles were unaffected by FD in nGKRP-ir-negative A2 neurons at either E dosage level. FD altered estrogen receptor (ER)-alpha and ATP-sensitive potassium channel subunit sulfonylurea receptor-1 gene expression in nGKRP-ir-positive neurons from low-E, but not high-E animals. Results provide novel evidence that distinct hindbrain A2 neuron populations exhibit altered versus unaffected transmission to the rPO during FD-associated metabolic imbalance, and that the direction of change in this noradrenergic input is controlled by E feedback mode. These A2 cell types are correspondingly distinguished by FD-sensitive or -insensitive GCK, which correlates with the presence versus absence of nGKRP-ir. Further studies are needed to determine how E signal volume regulates neurotransmitter and metabolic sensor responses to FD in GKRP-expressing A2 neurons.

Nutrient infusion in the dorsal vagal complex controls hepatic lipid and glucose metabolism in rats

Hypothalamic regulation of lipid and glucose homeostasis is emerging, but whether the dorsal vagal complex (DVC) senses nutrients and regulates hepatic nutrient metabolism remains unclear. Here, we found in rats DVC oleic acid infusion suppressed hepatic secretion of triglyceride-rich very-low-density lipoprotein (VLDL-TG), which was disrupted by inhibiting DVC long-chain fatty acyl-CoA synthetase that in parallel disturbed lipid homeostasis during intravenous lipid infusion. DVC glucose infusion elevated local glucose levels similarly as intravenous glucose infusion and suppressed hepatic glucose production. This was independent of lactate metabolism as inhibiting lactate dehydrogenase failed to disrupt glucose sensing and neither could DVC lactate infusion recapitulate glucose effect. DVC oleic acid and glucose infusion failed to lower VLDL-TG secretion and glucose production in high-fat fed rats, while inhibiting DVC farnesoid X receptor enhanced oleic acid but not glucose sensing. Thus, an impairment of DVC nutrient sensing may lead to the disruption of lipid and glucose homeostasis in metabolic syndrome.

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DVC oleic acid infusion lowers hepatic secretion of VLDL-TG in chow but not HF rats

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Inhibition of ACSL in the DVC negates lipid sensing

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DVC glucose infusion lowers hepatic glucose production in chow but not HF rats

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Inhibition of FXR in the DVC enhances oleic acid but not glucose sensing in HF rats

Immunohistochemical evidence of P2X7R, P2X4R and CaMKK2 in pyramidal neurons of frontal cortex does not align with Alzheimer's disease

Alzheimer's disease (AD) is an incurable neurodegenerative condition resulting in progressive cognitive decline. Pathological features include Aβ plaques, neurofibrillary tangles, neuroinflammation and neuronal death. Purinergic receptors 7 and 4 (P2X7R and P2X4R) and calcium/calmodulin-dependent kinase kinase 2 (CaMKK2) are implicated in neuronal death. We used immunohistochemistry to investigate the distribution of these proteins in neurones from frontal cortex of donors (n = 3/group; aged 79-83 years) who died with and without AD. Neurones were identified morphologically and immunoperoxidase staining was achieved using commercial antibodies. Immunoreactive neurones were counted for each protein by 2-3 raters blinded to the diagnoses. We observed no differences in percentages of P2X7R, P2X4R or CaMKK2 positive neurones (p = 0.2-0.99), but sections from individuals with AD had marginally fewer neurones (p = 0.10). Hence P2X7R, P2X4R or CaMKK2 appear to be expressed in neurones from older donors, but expression does not associate with AD.

Central Type II Glucocorticoid Receptor Regulation of Ventromedial Hypothalamic Nucleus Glycogen Metabolic Enzyme and Glucoregulatory Neurotransmitter Marker Protein Expression in the Male Rat

The ventromedial hypothalamic nucleus (VMN) glucoregulatory neurotransmitters γ-aminobutyric acid (GABA) and nitric oxide (NO) signal adjustments in glycogen mobilization. Glucocorticoids control astrocyte glycogen metabolism in vitro. The classical (type II) glucocorticoid receptor (GR) is expressed in key brain structures that govern glucostasis, including the VMN. Current research addressed the hypothesis that forebrain GR regulation of VMN glycogen synthase (GS) and phosphorylase (GP) protein expression correlates with control of glucoregulatory transmission. Groups of male rats were pretreated by intracerebroventricular (icv) delivery of the GR antagonist RU486 or vehicle prior to insulin-induced hypoglycemia (IIH), or were pretreated icv with dexamethasone (DEX) or vehicle before subcutaneous insulin diluent injection. DEX increased VMN GS and norepinephrine-sensitive GP-muscle type (GPmm), but did not alter metabolic deficit-sensitive GP-brain type (GPbb) expression. RU486 enhanced GS and GPbb profiles during IIH. VMN astrocyte (MCT1) and neuronal (MCT2) monocarboxylate transporter profiles were up-regulated in euglycemic and hypoglycemic animals by DEX or RU486, respectively. Glutamate decarboxylase65/67 and neuronal nitric oxide synthase (nNOS) proteins were both increased by DEX, yet RU486 augmented hypoglycemic nNOS expression patterns. Results show that GR exert divergent effects on VMN GS, MCT1/2, and nNOS proteins during eu- (stimulatory) versus hypoglycemia (inhibitory); these findings imply that up-regulated NO transmission may reflect, in part, augmented glucose incorporation into glycogen and/or increased tissue lactate requirements. Data also provide novel evidence for metabolic state-dependent GR regulation of VMN GPmm and GPbb profiles; thus, GABA signaling of metabolic stability may reflect, in part, stimulus-specific glycogen breakdown during eu- versus hypoglycemia.

Estrogen Receptor Involvement in Noradrenergic Regulation of Ventromedial Hypothalamic Nucleus Glucoregulatory Neurotransmitter and Stimulus-Specific Glycogen Phosphorylase Enzyme Isoform Expression

Norepinephrine (NE) directly regulates ventromedial hypothalamic nucleus (VMN) glucoregulatory neurons and also controls glycogen-derived fuel provision to those cells. VMN nitric oxide (NO) and γ-aminobutyric acid (GABA) neurons and astrocytes express estrogen receptor-alpha (ERα) and ER-beta (ERβ) proteins. Current research used selective ERα (1,3Bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]-1H-pyrazole dihydrochloride) or ERβ (4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-3-yl]phenol) antagonists to address the premise that these ERs govern basal and/or NE-associated patterns of VMN metabolic neuron signaling and astrocyte glycogen metabolism. Both ERs stimulate expression of the enzyme marker protein neuronal nitric oxide synthase, not glutamate decarboxylase_65/67. NE inhibition or augmentation of neuronal nitric oxide synthase and glutamate decarboxylase_65/67 profiles was ER-independent or -dependent, respectively. In both neuron types, VMN ERβ activity inhibited baseline alpha1- (α₁-) and/or alpha2- (α₂-)adrenergic receptor (AR) expression, but ERα and -β signaling was paradoxically crucial for noradrenergic upregulation of α₂-AR. NE inhibited glycogen synthase expression and exerted opposite effects on VMN adenosine monophosphate-sensitive glycogen phosphorylase (GP)-brain type (stimulatory) versus NE-sensitive GP muscle (inhibitory) via ERα or -β activity. Results document unique ERα and ERβ actions on metabolic transmitter and AR protein expression in VMN nitrergic versus GABAergic neurons. ER effects varied in the presence versus absence of NE, indicating that both neuron types are substrates for estradiol and noradrenergic regulatory interaction. NE-dependent ER control of VMN GP variant expression implies that these signals also act on astrocytes to direct physiological stimulus-specific control of glycogen metabolism, which may in turn influence GABA transmission.

Hindbrain Estrogen Receptor Regulation of Ventromedial Hypothalamic Glycogen Metabolism and Glucoregulatory Transmitter Expression in the Hypoglycemic Female Rat

Neural substrates for estrogen regulation of glucose homeostasis remain unclear. Female rat dorsal vagal complex (DVC) A2 noradrenergic neurons are estrogen- and metabolic-sensitive. The ventromedial hypothalamic nucleus (VMN) is a key component of the brain network that governs counter-regulatory responses to insulin-induced hypoglycemia (IIH). Here, the selective estrogen receptor-alpha (ERα) or -beta (ERβ) antagonists MPP and PHTPP were administered separately to the caudal fourth ventricle to address the premise that these hindbrain ER variants exert distinctive control of VMN reactivity to IIH in the female sex. Data show that ERα governs hypoglycemic patterns of VMN astrocyte glycogen metabolic enzyme, e.g. glycogen synthase and phosphorylase protein expression, whereas ERβ mediates local glycogen breakdown. DVC ERs also regulate VMN neurotransmitter signaling of energy sufficiency [γ-aminobutyric acid] or deficiency [nitric oxide, steroidogenic factor-1] during IIH. Neither hindbrain ER mediates IIH-associated diminution of VMN norepinephrine (NE) content. Both ERs oppose hypoglycemic hyperglucagonemia, while ERβ contributes to reduced corticosterone output. Outcomes reveal that input from the female hindbrain to the VMN is critical for energy reserve mobilization, metabolic transmitter signaling, and counter-regulatory hormone secretion during hypoglycemia, and that ERs control those cues. Evidence that VMN NE content is not controlled by hindbrain ERα or -β implies that these receptors may regulate VMN function via NE-independent mechanisms, or alternatively, that other neurotransmitter signals to the VMN may control local substrate receptivity to NE.

Hindbrain estrogen receptor regulation of ventromedial hypothalamic glycogen metabolism and glucoregulatory transmitter expression in the hypoglycemic male rat

Estrogen receptor-alpha (ERα) and -beta (ERβ) occur in key elements of the brain gluco-homeostatic network in both sexes, including the hindbrain dorsal vagal complex (DVC), but the influence of distinct receptor populations on this critical function is unclear. The ventromedial hypothalamic nucleus (VMN) maintains glucose balance by integrating nutrient, endocrine, and neurochemical cues, including metabolic sensory information supplied by DVC A2 noradrenergic neurons. Current research utilized the selective ERα and ERβ antagonists MPP and PHTPP to characterize effects of DVC ERs on VMN norepinephrine (NE) activity and metabolic neurotransmitter signaling in insulin-induced hypoglycemic (IIH) male rats. Data show that ERβ inhibits VMN glycogen synthase and stimulates phosphorylase protein expression, while attenuating hypoglycemic augmentation of glycogen content. Furthermore, both ERs attenuate VMN glucose concentrations during IIH. Hypoglycemic up-regulation of nitric oxide (NO) and brain-derived neurotrophic factor (BDNF) signaling was correspondingly driven by ERα or -β, whereas GABA and steroidogenic factor-1 were respectively suppressed independently of ER input or by ERβ. IIH intensified VMN NE accumulation by ERβ-dependent mechanisms, but did not alter NE levels in other gluco-regulatory loci. ERβ amplified the magnitude of insulin-induced decline in blood glucose. Both ERs regulate corticosterone, but not glucagon secretion during IIH and oppose hypoglycemic diminution of circulating free fatty acids. These findings identify distinguishing versus common VMN functions targeted by DVC ERα and -β. Sex differences in hypoglycemic VMN NE accumulation, glycogen metabolism, and transmitter signaling may involve, in part, discrepant regulatory involvement or differential magnitude of impact of these hindbrain ERs.

Norepinephrine regulation of ventromedial hypothalamic nucleus metabolic transmitter biomarker and astrocyte enzyme and receptor expression: Impact of 5' AMP-activated protein kinase

The ventromedial hypothalamic energy sensor AMP-activated protein kinase (AMPK) maintains glucostasis via neurotransmitter signals that diminish [γ-aminobutyric acid] or enhance [nitric oxide] counter-regulation. Ventromedial hypothalamic nucleus (VMN) 'fuel-inhibited' neurons are sensitive to astrocyte-generated metabolic substrate stream. Norepinephrine (NE) regulates astrocyte glycogen metabolism in vitro, and hypoglycemia intensifies VMN NE activity in vivo. Current research investigated the premise that NE elicits AMPK-dependent adjustments in VMN astrocyte glycogen metabolic enzyme [glycogen synthase (GS); glycogen phosphorylase (GP)] and gluco-regulatory neuron biomarker [glutamate decarboxylase_65/67 (GAD); neuronal nitric oxide synthase (nNOS); SF-1] protein expression in male rats. We also examined whether VMN astrocytes are directly receptive to NE and if noradrenergic input regulates cellular sensitivity to the neuro-protective steroid estradiol. Intra-VMN NE correspondingly augmented or reduced VMN tissue GAD and nNOS protein despite no change in circulating glucose, data that imply that short-term exposure to NE promotes persistent improvement in VMN nerve cell energy stability. The AMPK inhibitor Compound C (Cc) normalized VMN nNOS, GS, and GP expression in NE-treated animals. NE caused AMPK-independent down-regulation of alpha₂-, alongside Cc-reversible augmentation of beta₁-adrenergic receptor protein profiles in laser-microdissected astrocytes. NE elicited divergent adjustments in astrocyte estrogen receptor-beta (AMPK-unrelated reduction) and GPR-30 (Cc-revocable increase) proteins. Outcomes implicate AMPK in noradrenergic diminution of VMN nitrergic metabolic-deficit signaling and astrocyte glycogen shunt activity. Differentiating NE effects on VMN astrocyte adrenergic and estrogen receptor variant expression suggest that noradrenergic regulation of glycogen metabolism may be mediated, in part, by one or more receptors characterized here by sensitivity to this catecholamine.

Hindbrain 5'-Adenosine Monophosphate-activated Protein Kinase Mediates Short-term Food Deprivation Inhibition of the Gonadotropin-releasing Hormone-Luteinizing Hormone Axis: Role of Nitric Oxide

Hindbrain-derived stimuli restrain the gonadotropin-releasing hormone (GnRH)-pituitary luteinizing hormone (LH) reproductive neuroendocrine axis during energy insufficiency. Interruption of food intake, planned or unplanned, is emblematic of modern life. This study investigated the premise that the hindbrain energy sensor 5'-adenosine monophosphate-activated protein kinase (AMPK) inhibits reproductive neuroendocrine function in short term, e.g. 18-h food-deprived (FD) estradiol (E)-implanted ovariectomized female rats. Intra-caudal fourth ventricular administration of the AMPK inhibitor Compound C (Cc) reversed FD-induced inhibition of rostral preoptic (rPO) GnRH protein expression and LH release in animals given E to replicate proestrus (high-E dose-, but not metestrus (low-E dose)-stage plasma steroid levels. FD caused Cc-reversible augmentation or diminution of preoptic norepinephrine (NE) activity in high- versus low-E rats, respectively, and AMPK-independent reductions in hypothalamic NE accumulation in the latter. Nitric oxide (NO) and kisspeptin are key stimulatory signals for the preovulatory LH surge. Here, FD inhibited rPO neuronal nitric oxide synthase protein expression in high-, but not low-E-dosed animals. Lateral ventricular delivery of the NO donor 3-morpholinosydnonimine (SIN-1) reversed inhibitory GnRH and LH responses to FD in high-E rats, and normalized rPO Vglut2, anteroventral periventricular KiSS1, and dorsomedial hypothalamic RFRP-3 mRNA and/or protein profiles. Data show that FD curtails reproductive neuroendocrine outflow by hindbrain AMPK-dependent mechanisms in the presence of peak estrous cycle E levels. Results indicate that neural networks linking this sensor to GnRH neurons likely involve NO signaling, which may function upstream of one or more neurotransmitters identified here by SIN-1-reversible inhibitory responses to FD.

Rebound Feeding in the Wake of Short-Term Suspension of Food Intake Differs in the Presence of Estrous Cycle Peak versus Nadir Levels of Estradiol

BACKGROUND

Short-term interruption of feeding is ordinary in modern life but negatively impacts appetite control and body weight. Estradiol (E) imposes long-term inhibitory tonus on food consumption; however, E influence on energy repletion secondary to food deprivation (FD) is unclear. This study investigated the hypothesis that E signal strength regulates hyperphagic responses to FD of varying duration.

METHODS

Ovariectomized female rats were implanted with E-containing silastic capsules (30 [E-30] or 300 μg [E-300]/mL) to replicate plasma concentrations at cycle nadir versus peak levels.

RESULTS

Data show that food intake was increased equally in E-30 and E-300 rats after 12 hours of food deprivation (FD-12); yet, FD of 18 hours (FD-18) amplified refeeding by E-300 versus E-30. Caudal fourth ventricular administration of the 5'-monophosphate-activated protein kinase (AMPK) inhibitor compound C (Cc) did not modify FD-induced hyperphagia in E-30 (regardless of FD interval) or E-300 animals exposed to FD-12, but diminished refeeding after FD-18 in E-300 rats. Cc-reversible hyperglycemia occurred in refed FD-18 groups. Serum insulin was resistant to FD-12 plus refeeding, but was elevated by AMPK-dependent mechanisms in refed E-300 FD-18 rats; equivalent Cc-insensitive decrements in circulating leptin occurred in all FD groups.

CONCLUSION

Current results show that estrous cycle peak, but not baseline, E levels engage hindbrain AMPK signaling to intensify hyperphagia in response to prolongation of FD. Observations of hindbrain AMPK-dependent hyperglycemia, alongside elevated insulin secretion, in refed rats exposed to FD-18 implicate this sensor in insulin resistance mechanisms of glucose partitioning in response to this metabolic imbalance.