Systemic Glucoregulation by Glucose-Sensing Neurons in the Ventromedial Hypothalamic Nucleus (VMH)

The ventromedial hypothalamic nucleus (VMH) regulates glucose production in the liver as well as glucose uptake and utilization in peripheral tissues, including skeletal muscle and brown adipose tissue, via efferent sympathetic innervation and neuroendocrine mechanisms. The action of leptin on VMH neurons also increases glucose uptake in specific peripheral tissues through the sympathetic nervous system, with improved insulin sensitivity. On the other hand, subsets of VMH neurons, such as those that express steroidogenic factor 1 (SF1), sense changes in the ambient glucose concentration and are characterized as glucose-excited (GE) and glucose-inhibited (GI) neurons whose action potential frequency increases and decreases, respectively, as glucose levels rise. However, how these glucose-sensing (GE and GI) neurons in the VMH contribute to systemic glucoregulation remains poorly understood. In this review, we provide historical background and discuss recent advances related to glucoregulation by VMH neurons. In particular, the article describes the role of GE neurons in the control of peripheral glucose utilization and insulin sensitivity, which depend on mitochondrial uncoupling protein 2 of the neurons, as well as that of GI neurons in the control of hepatic glucose production through hypoglycemia-induced counterregulatory mechanisms.

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.

Co-localization of hypocretin-1 and leucine-enkephalin in hypothalamic neurons projecting to the nucleus of the solitary tract and their effect on arterial pressure

Experiments were done to investigate whether hypothalamic hypocretin-1 (hcrt-1; orexin-A) neurons that sent axonal projections to cardiovascular responsive sites in the nucleus of the solitary tract (NTS) co-expressed leucine-enkephalin (L-Enk), and to determine the effects of co-administration of hcrt-1 and D-Ala2,D-Leu5-Enkephalin (DADL) into NTS on mean arterial pressure (MAP) and heart rate. In the first series, in the Wistar rat the retrograde tract-tracer fluorogold (FG) was microinjected (50nl) into caudal NTS sites at which L-glutamate (0.25 M; 10 nl) elicited decreases in MAP and where fibers hcrt-1 immunoreactive fibers were observed that also contained L-Enk immunoreactivity. Of the number of hypothalamic hcrt-1 immunoreactive neurons identified ipsilateral to the NTS injection site (1207 ± 78), 32.3 ± 2.3% co-expressed L-Enk immunoreactivity and of these, 2.6 ± 1.1% were retrogradely labeled with FG. Hcrt-1/L-Enk neurons projecting to NTS were found mainly within the perifornical region. In the second series, the region of caudal NTS found to contain axons that co-expressed hcrt-1 and L-Enk immunoreactivity was microinjected with a combination of hcrt-1 and DADL in α-chloralose anesthetized Wistar rats. Microinjection of DADL into NTS elicited depressor and bradycardia responses similar to those elicited by microinjection of hcrt-1. An hcrt-1 injection immediately after the DADL injection elicited an almost twofold increase in the magnitude of the depressor and bradycardia responses compared to those elicited by hcrt-1 alone. Prior injections of the non-specific opioid receptor antagonist naloxone or the specific opioid δ-receptor antagonist ICI 154,129 significantly attenuated the cardiovascular responses to the combined hcrt-1-DADL injections. Taken together, these data suggest that activation of hypothalamic-opioidergic neuronal systems contribute to the NTS hcrt-1 induced cardiovascular responses, and that this descending hypothalamo-medullary pathway may represent the anatomical substrate by which hcrt-1/L-Enk neurons function in the coordination of autonomic-cardiovascular responses during different behavioral states.

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.

Sexual selection predicts brain structure in dragon lizards

Phenotypic traits such as ornaments and armaments are generally shaped by sexual selection, which often favours larger and more elaborate males compared to females. But can sexual selection also influence the brain? Previous studies in vertebrates report contradictory results with no consistent pattern between variation in brain structure and the strength of sexual selection. We hypothesize that sexual selection will act in a consistent way on two vertebrate brain regions that directly regulate sexual behaviour: the medial preoptic nucleus (MPON) and the ventromedial hypothalamic nucleus (VMN). The MPON regulates male reproductive behaviour whereas the VMN regulates female reproductive behaviour and is also involved in male aggression. To test our hypothesis, we used high-resolution magnetic resonance imaging combined with traditional histology of brains in 14 dragon lizard species of the genus Ctenophorus that vary in the strength of precopulatory sexual selection. Males belonging to species that experience greater sexual selection had a larger MPON and a smaller VMN. Conversely, females did not show any patterns of variation in these brain regions. As the volumes of both these regions also correlated with brain volume (BV) in our models, we tested whether they show the same pattern of evolution in response to changes in BV and found that the do. Therefore, we show that the primary brain nuclei underlying reproductive behaviour in vertebrates can evolve in a mosaic fashion, differently between males and females, likely in response to sexual selection, and that these same regions are simultaneously evolving in concert in relation to overall brain size.

Dorsomedial hindbrain catecholamine regulation of hypothalamic astrocyte glycogen metabolic enzyme protein expression: Impact of estradiol

The brain astrocyte glycogen reservoir is a vital energy reserve and, in the cerebral cortex, subject among other factors to noradrenergic control. The ovarian steroid estradiol potently stimulates nerve cell aerobic respiration, but its role in glial glycogen metabolism during energy homeostasis or mismatched substrate supply/demand is unclear. This study examined the premise that estradiol regulates hypothalamic astrocyte glycogen metabolic enzyme protein expression during normo- and hypoglycemia in vivo through dorsomedial hindbrain catecholamine (CA)-dependent mechanisms. Individual astrocytes identified in situ by glial fibrillary acidic protein immunolabeling were laser-microdissected from the ventromedial hypothalamic (VMH), arcuate hypothalamic (ARH), and paraventricular hypothalamic (PVH) nuclei and the lateral hypothalamic area (LHA) of estradiol (E)- or oil (O)-implanted ovariectomized (OVX) rats after insulin or vehicle injection, and pooled within each site. Stimulation [VMH, LHA] or suppression [PVH, ARH] of basal glycogen synthase (GS) protein expression by E was reversed in the former three sites by caudal fourth ventricular pretreatment with the CA neurotoxin 6-hydroxydopamine (6-OHDA). E diminished glycogen phosphorylase (GP) protein profiles by CA-dependent [VMH, PVH] or -independent mechanisms [LHA]. Insulin-induced hypoglycemia (IIH) increased GS expression in the PVH in OVX+E, but reduced this protein in the PVH, ARH, and LHA in OVX+O. Moreover, IIH augmented GP expression in the VMH, LHA, and ARH in OVX+E and in the ARH in OVX+O, responses that normalized by 6-OHDA. Results demonstrate site-specific effects of E on astrocyte glycogen metabolic enzyme expression in the female rat hypothalamus, and identify locations where dorsomedial hindbrain CA input is required for such action. Evidence that E correspondingly increases and reduces basal GS and GP in the VMH and LHA, but augments the latter protein during IIH suggests that E regulates glycogen content and turnover in these structures during glucose sufficiency and shortage.

Dynamics of progesterone and estrogen receptor alpha in the ventromedial hypothalamus

Cyclic fluctuations of estradiol and progesterone in females influence neuronal activity in the ventrolateral division of the ventromedial hypothalamic nucleus (VMNvl), through the activation of progesterone receptors (PRs) and estrogen receptors (ERs). The expression of ER and PR in the VMNvl is influenced by their cognate ligands and is a central upstream trigger in the pathway of VMNvl-dependent modulation of endocrine responses. By studying the role played by estradiol and progesterone in PR and ERa expression in the VMNvl along the estrous cycle and how the two receptors interact in the same neuron, we aim to evaluate the synergistic action of both ovarian hormones in the regulation of VMNvl activity. In animals at all phases of the estrous cycle, the number of VMN neurons expressing PR or ERa was estimated by stereological methods, and the percentage, and rostro-caudal distribution, of neurons simultaneously expressing both receptors was determined. The highest number of PR-immunoreactive neurons was seen at proestrus, and of ERa-immunoreactive neurons was seen at proestrus and metestrus. The ERa/PR co-localization is increased at caudal levels. Approximately half the neurons expressing PR co-express ERa, a proportion that stays constant along the estrous cycle. The percentage of ERa neurons co-expressing PR changes from 60% at proestrus to 40% at metestrus. Fluctuations in circulating ovarian hormone levels promote coordinated changes in PR and ERa expression and co-localization. This may be an important mechanism in the regulation of input relayed by the VMNvl, allowing a precise modulation of endocrine responses.

Norepinephrine Regulation of Ventromedial Hypothalamic Nucleus Astrocyte Glycogen Metabolism

The catecholamine norepinephrine (NE) links hindbrain metabolic-sensory neurons with key glucostatic control structures in the brain, including the ventromedial hypothalamic nucleus (VMN). In the brain, the glycogen reserve is maintained within the astrocyte cell compartment as an alternative energy source to blood-derived glucose. VMN astrocytes are direct targets for metabolic stimulus-driven noradrenergic signaling due to their adrenergic receptor expression (AR). The current review discusses recent affirmative evidence that neuro-metabolic stability in the VMN may be shaped by NE influence on astrocyte glycogen metabolism and glycogen-derived substrate fuel supply. Noradrenergic modulation of estrogen receptor (ER) control of VMN glycogen phosphorylase (GP) isoform expression supports the interaction of catecholamine and estradiol signals in shaping the physiological stimulus-specific control of astrocyte glycogen mobilization. Sex-dimorphic NE control of glycogen synthase and GP brain versus muscle type proteins may be due, in part, to the dissimilar noradrenergic governance of astrocyte AR and ER variant profiles in males versus females. Forthcoming advances in the understanding of the molecular mechanistic framework for catecholamine stimulus integration with other regulatory inputs to VMN astrocytes will undoubtedly reveal useful new molecular targets in each sex for glycogen mediated defense of neuronal metabolic equilibrium during neuro-glucopenia.

Inhibition of glycogen phosphorylase stimulates ventromedial hypothalamic nucleus AMP-activated protein kinase: Activity and neuronal nitric oxide synthase protein expression in male rats

The glucose polymer glycogen is a vital fuel reserve in the brain. The mediobasal hypothalamic energy sensor AMP‐activated protein kinase (AMPK) maintains glucostasis via neurotransmitter mechanisms that suppress [γ‐aminobutyric acid; GABA] or stimulate [nitric oxide; steroidogenic factor‐1 (SF1)] counter‐regulatory outflow. This study investigated whether glycogen‐derived fuel supply is a critical screened variable in ventromedial hypothalamic nucleus (VMN) monitoring of neuro‐metabolic stability during glucostasis and/or insulin (I)‐induced hypoglycemia. Adult male rats were pretreated by intra‐VMN infusion of the glycogen phosphorylase inhibitor 1,4‐dideoxy‐1,4‐imino‐D‐arabinitol (DAB) before sc vehicle or I injection. Western blot analyses of micropunch‐dissected VMN tissue from euglycemic animals showed DAB augmentation of phosphoAMPK (pAMPK), neuronal nitric oxide synthase (nNOS), and SF‐1, but not glutamate decarboxylase_65/67 (GAD) protein. Combinatory DAB/I treatment did not further enhance AMPK activity but significantly amplified nNOS expression relative to DAB alone. Hypoglycemic stimulation of corticosterone, but not glucagon release was prevented by DAB. Results imply that glycogen‐derived substrate fuel provision represses VMN AMPK activity and neurotransmitter signals of metabolic deficiency. Progressive augmentation of nNOS protein by DAB/I versus DAB/V intimates that “fuel‐inhibited” nitrergic neurons may exhibit increasing sensitivity to disrupted glycogen breakdown during glucoprivation versus glucostasis. nNOS and GAD reactivity to DAB/I, but not I implies that acute glycogen utilization during hypoglycemia may be sufficiently robust to avert effects on local metabolic sensory signaling. DAB/I upregulation of GAD alongside prevention of hypercorticosteronemia suggests that indicators of metabolic sufficiency may occur secondary to local compensatory adaptations to severe restriction of glucose‐derived energy.

Nuclear Thimet oligopeptidase is coexpressed with oestrogen receptor alpha in hypothalamic cells and regulated by oestradiol in female mice

Thimet oligopeptidase (EC 3.4.24.15; also called EP24.15 and TOP; referred to here as TOP) is a neuropeptidase involved in the regulation of several physiological functions including reproduction. Among its substrates is gonadotrophin-releasing hormone (GnRH), an important hypothalamic hormone that regulates the synthesis and release of oestradiol and facilitates female sexual behaviour. Using immunohistochemistry, we found that TOP is expressed in the nucleus of cells throughout the female mouse brain, and in high levels in steroid-sensitive regions of the hypothalamus, which is consistent with previous findings in male rats. Furthermore, dual-label immunofluorescence revealed that TOP and oestrogen receptor alpha (ERalpha) coexpress in several reproductively-relevant brain regions, including the medial preoptic area (mPOA), arcuate nucleus (ARC), ventrolateral portion of the ventromedial hypothalamic nucleus (VMNvl) and the midbrain central grey (MCG). Previous studies in rats have shown that oestradiol decreases hypothalamic TOP levels or activity, possibly potentiating the effects of GnRH. In the present study, analysis by immunohistochemistry revealed that oestradiol decreased TOP immunoreactivity in the VMNvl, whereas no differences were detected in the mPOA, ARC or median eminence. Overall, the present findings indicate that TOP is coexpressed with ERalpha, and oestradiol regulates TOP expression in a brain region-specific manner in female mice, providing neuroanatomical evidence that TOP may function in reproductive physiology and/or behaviour.

Neuroanatomical association of hypothalamic HSD2-containing neurons with ERα, catecholamines, or oxytocin: implications for feeding?

This study used immunohistochemical methods to investigate the possibility that hypothalamic neurons that contain 11-β-hydroxysteroid dehydrogenase type 2 (HSD2) are involved in the control of feeding by rats via neuroanatomical associations with the α subtype of estrogen receptor (ERα), catecholamines, and/or oxytocin (OT). An aggregate of HSD2-containing neurons is located laterally in the hypothalamus, and the numbers of these neurons were greatly increased by estradiol treatment in ovariectomized (OVX) rats compared to numbers in male rats and in OVX rats that were not given estradiol. However, HSD2-containing neurons were anatomically segregated from ERα-containing neurons in the Ventromedial Hypothalamus and the Arcuate Nucleus. There was an absence of OT-immunolabeled fibers in the area of HSD2-labeled neurons. Taken together, these findings provide no support for direct associations between hypothalamic HSD2 and ERα or OT neurons in the control of feeding. In contrast, there was catecholamine-fiber labeling in the area of HSD2-labeled neurons, and these fibers occasionally were in close apposition to HSD2-labeled neurons. Therefore, we cannot rule out interactions between HSD2 and catecholamines in the control of feeding; however, given the relative sparseness of the appositions, any such interaction would appear to be modest. Thus, these studies do not conclusively identify a neuroanatomical substrate by which HSD2-containing neurons in the hypothalamus may alter feeding, and leave the functional role of hypothalamic HSD2-containing neurons subject to further investigation.

Unilateral lesion increases oestrogen receptor α expression in the intact side of the ventromedial hypothalamic nucleus in ovariectomised rats

To determine the relationship between the right and left sides of the ventrolateral ventromedial hypothalamic nucleus (vlVMN) in regulating the expression of oestrogen receptor (ER)α, the unilateral vlVMN was lesioned and the number of ERα-immunoreactive cells and the ERα mRNA level in the intact side of the vlVMN and arcuate nucleus (ARC) were measured in ovariectomised rats. Twenty-four hours after lesioning, brain samples were collected for analysis of ERα expression by immunohistochemistry and the real-time reverse transcriptase-polymerase chain reaction. The number of ERα-immunoreactive cells in the intact side of the vlVMN but not the ARC in the unilateral lesioned group was significantly higher than that in the control or sham-lesioned group. Expression levels of ERα mRNA in the intact side of the vlVMN but not the ARC in unilateral lesioned rats were significantly higher than those in the sham-lesioned group. Of transcript variants with alternative 5'-untranslated regions (0S, 0N, 0, 0T and E1), the ERα 0 transcript level was significantly increased. These results indicate that unilateral damage of vlVMN induces an increase in ERα in the intact side by increasing ERα transcription in a promoter-specific manner. The findings also suggest the existence of new neuroendocrine control system between the right and left sides for the expression of ERα in the vlVMN.

Neuroestradiol regulation of ventromedial hypothalamic nucleus 5'-AMP-activated protein kinase activity and counterregulatory hormone secretion in hypoglycemic male versus female rats

Hypoglycemia activates the ultra-sensitive energy gauge 5′-AMP-activated protein kinase (AMPK) in ventromedial hypothalamic nucleus (VMN) gluco-regulatory neurons. The VMN is exemplified by high levels of expression of the enzyme aromatase, which converts testosterone to estradiol. This study examined the hypothesis that neuroestradiol imposes sex-dimorphic control of VMN AMPK activity during eu- and/or hypoglycemia. VMN tissue corresponding to distinct rostro-caudal segments was obtained by micropunch dissection from testes-intact male and estradiol-replaced ovariectomized female rats that were infused intracerebroventricularly with the aromatase inhibitor letrozole (Lz) before subcutaneous insulin (INS) injection. In euglycemic rats, Lz treatment elevated (male) or decreased (female) middle VMN phosphoAMPK content, with concurrent effects on total AMPK expression. Lz prevented hypoglycemic up-regulation of the mean pAMPK/AMPK ratio in rostral and middle segments of the male VMN, and significantly inhibited this proportion throughout the VMN of hypoglycemic female rats. Lz prevented glucagon secretion in hypoglycemic rats of each sex, and abolished hypoglycemic hypercorticosteronemia in males. Results show that neuroestradiol regulation of VMN AMPK activity during euglycemia is region-specific and gender-divergent, e.g. inhibitory in males versus stimulatory in females. Intra-VMN distribution of hypoglycemia-activated AMPK varies between sexes, but in each sex, locally-generated estradiol is critical for sensor reactivity to this stimulus. Coincident Lz attenuation of VMN AMPK and counter-regulatory hormone responses to hypoglycemia infers a possible cause-and-effect association. Further effort is needed to elucidate the cellular and molecular mechanisms that underlie sex-dimorphic neuroestradiol control of VMN total AMPK and phosphoAMPK expression during distinct metabolic states.

Effects of Ventromedial Hypothalamic Nucleus (VMN) Aromatase Gene Knockdown on VMN Glycogen Metabolism and Glucoregulatory Neurotransmission

The enzyme aromatase is expressed at high levels in the ventromedial hypothalamic nucleus (VMN), a principal component of the brain gluco-regulatory network. Current research utilized selective gene knockdown tools to investigate the premise that VMN neuroestradiol controls glucostasis. Intra-VMN aromatase siRNA administration decreased baseline aromatase protein expression and tissue estradiol concentrations and either reversed or attenuated the hypoglycemic regulation of these profiles in a VMN segment-specific manner. Aromatase gene repression down-regulated protein biomarkers for gluco-stimulatory (nitric oxide; NO) and -inhibitory (gamma-aminobutyric acid; GABA) neurochemical transmitters. Insulin-induced hypoglycemia (IIH) up- or down-regulated neuronal nitric oxide synthase (nNOS) and glutamate decarboxylase_65/67 (GAD), respectively, throughout the VMN. Interestingly, IIH caused divergent changes in tissue aromatase and estradiol levels in rostral (diminished) versus middle and caudal (elevated) VMN. Aromatase knockdown prevented hypoglycemic nNOS augmentation in VMN middle and caudal segments, but abolished the GAD inhibitory response to IIH throughout this nucleus. VMN nitrergic and GABAergic neurons monitor stimulus-specific glycogen breakdown. Here, glycogen synthase (GS) and phosphorylase brain- (GPbb; AMP-sensitive) and muscle- (GPmm; noradrenergic -responsive) type isoform responses to aromatase siRNA were evaluated. Aromatase repression reduced GPbb and GPmm content in euglycemic controls and prevented hypoglycemic regulation of GPmm but not GPbb expression while reversing glycogen accumulation. Aromatase siRNA elevated baseline glucagon and corticosterone secretion and abolished hypoglycemic hyperglucagonemia and hypercorticosteronemia. Outcomes document the involvement of VMN neuroestradiol signaling in brain control of glucose homeostasis. Aromatase regulation of VMN gluco-regulatory signaling of hypoglycemia-associated energy imbalance may entail, in part, control of GP variant-mediated glycogen disassembly.

Norepinephrine Regulation of Ventromedial Hypothalamic Nucleus Metabolic-Sensory Neuron 5'-AMP-Activated Protein Kinase Activity: Impact of Estradiol

The mediobasal hypothalamus (MBH) shapes the neural regulation of glucostasis by 5′-AMP-activated protein kinase (AMPK)-dependent mechanisms. Yet, the neurochemical identity and neuroanatomical distribution of MBH neurons that express glucoprivic-sensitive AMPK remain unclear. The neurotransmitters γ-aminobutyric acid (GABA) and nitric oxide (NO) act within the MBH to correspondingly inhibit or stimulate glucose counter-regulation. The current review highlights recent findings that GABA and NO, neurons located in the ventromedial hypothalamic nucleus (VMN), a distinct important element of the MBH, are direct targets of noradrenergic regulatory signaling, and thereby, likely operate under the control of hindbrain metabolic-sensory neurons. The ovarian hormone estradiol acts within the VMN to govern energy homeostasis. Discussed here is current evidence that estradiol regulates GABA and NO nerve cell receptivity to norepinephrine and moreover, controls the noradrenergic regulation of AMPK activity in each cell type. Future gains in insight on mechanisms underpinning estradiol’s impact on neurotransmitter communication between the hindbrain and hypothalamic AMPKergic neurons are expected to disclose viable new molecular targets for the therapeutic simulation of hormonal enhancement of neuro-metabolic stability during circumstances of diminished endogenous estrogen secretion or glucose dysregulation.

Possible involvement of the dopamine D2 receptors of ventromedial hypothalamus in the control of free- and scheduled-feeding and plasma ghrelin level in rat

OBJECTIVES

We investigated effect of the ventromedial hypothalamus (VMH) dopamine D2 receptor inhibition on food intake and plasma ghrelin following chronic free or scheduled meal with different caloric intakes.

METHODS

Male Wistar rats (220-250 g) were fed diets containing free (control) or three scheduled diets of standard, restricted and high-fat for 1 month. The animals stereotaxically received an intra VMH single dose of sulpiride (0.005 µg)/or saline (0.5 µL) before meal time. Thirty minutes later, food intake and circulating ghrelin were measured.

RESULTS

Sulpiride significantly reduced food intake and ghrelin concentration in freely fed and scheduled-standard diet (p<0.05), while increased food intake, with ghrelin level on fasted level in scheduled-restricted group (p<0.01) compared to control. Food intake and ghrelin concentration between scheduled-high fat and freely fed or scheduled-standard diets did not show significant changes.

CONCLUSIONS

The VMH D2 receptors are possibly involved in controlling scheduled eating behavior, depending on energy balance context.

Sex-Dimorphic Octadecaneuropeptide (ODN) Regulation of Ventromedial Hypothalamic Nucleus Glucoregulatory Neuron Function and Counterregulatory Hormone Secretion

Central endozepinergic signaling is implicated in glucose homeostasis. Ventromedial hypothalamic nucleus (VMN) metabolic monitoring governs glucose counter-regulation. VMN glucose-stimulatory nitric oxide (NO) and glucose-inhibitory γ-aminobutyric acid (GABA) neurons express the energy gauge 5'-AMP-activated protein kinase (AMPK). Current research addresses the premise that the astrocyte glio-peptide octadecaneuropeptide (ODN) imposes sex-dimorphic control of metabolic sensor activity and neurotransmitter signaling in these neurons. The ODN G-protein coupled-receptor antagonist cyclo(1-8)[DLeu5]OP (LV-1075) was administered intracerebroventricularly (icv) to euglycemic rats of each sex; additional groups were pretreated icv with the ODN isoactive surrogate ODN11-18 (OP) before insulin-induced hypoglycemia. Western blotting of laser-catapult-microdissected VMN NO and GABA neurons showed that hypoglycemia caused OP-reversible augmentation of phospho-, e.g., activated AMPK and nitric oxide synthase (nNOS) expression in rostral (female) or middle (male) VMN segments or ODN-dependent suppression of nNOS in male caudal VMN. OP prevented hypoglycemic down-regulation of glutamate decarboxylase profiles in female rat rostral VMN, without affecting AMPK activity. LV-1075 treatment of male, not female rats elevated plasma glucagon and corticosterone concentrations. Moreover, OP attenuated hypoglycemia-associated augmentation of these hormones in males only. Results identify, for each sex, regional VMN metabolic transmitter signals that are subject to endozepinergic regulation. Directional shifts and gain-or-loss of ODN control during eu- versus hypoglycemia infer that VMN neuron receptivity to or post-receptor processing of this stimulus may be modulated by energy state. In male, counter-regulatory hormone secretion may be governed principally by ODN-sensitive neural pathways, whereas this endocrine outflow may be controlled by parallel, redundant ODN-dependent and -independent mechanisms in female.

Neuroanatomical characterization of the Nmu-Cre knock-in mice reveals an interconnected network of unique neuropeptidergic cells

Neuromedin U (NMU) is an evolutionary conserved neuropeptide that has been implicated in multiple processes, such as circadian regulation, energy homeostasis, reward processing and stress coping. Although the central expression of NMU has been addressed previously, the lack of specific and sensitive tools has prevented a comprehensive characterization of NMU-expressing neurons in the brain. We have generated a knock-in mouse model constitutively expressing Cre recombinase under the Nmu promoter. We have validated the model using a multi-level approach based on quantitative reverse-transcription polymerase chain reactions, in situ hybridization, a reporter mouse line and an adenoviral vector driving Cre-dependent expression of a fluorescent protein. Using the Nmu-Cre mouse, we performed a complete characterization of NMU expression in adult mouse brain, unveiling a potential midline NMU modulatory circuit with the ventromedial hypothalamic nucleus (VMH) as a key node. Moreover, immunohistochemical analysis suggested that NMU neurons in the VMH mainly constitute a unique population of hypothalamic cells. Taken together, our results suggest that Cre expression in the Nmu-Cre mouse model largely reflects NMU expression in the adult mouse brain, without altering endogenous NMU expression. Thus, the Nmu-Cre mouse model is a powerful and sensitive tool to explore the role of NMU neurons in mice.