Site-specific induction of Fos immunoreactivity in preoptic and hypothalamic NADPH-positive neurons during glucoprivation

Sex differences in ventromedial hypothalamic nucleus glucoregulatory transmitter biomarker protein during recurring insulin-induced hypoglycemia

Recurring insulin-induced hypoglycemia (RIIH) in males correlates with maladaptive glucose counter-regulatory collapse and acclimated expression of ventromedial hypothalamic nucleus (VMN) nitric oxide (NO) and γ-aminobutyric acid (GABA) metabolic transmitter biomarkers, e.g., neuronal nitric oxide synthase (nNOS) and glutamate decarboxylase_65/67 (GAD). Hindbrain noradrenergic neurons innervate the VMN, where norepinephrine regulates nNOS and GAD expression. Current research investigated the hypothesis that antecedent hypoglycemia (AH) exposure causes sex-dimorphic habituation of VMN glucoregulatory biomarker proteins between and/or during serial hypoglycemic bouts, and that hindbrain catecholaminergic (CA) signaling may control sex-specific adaptation of one or more of these proteins. Data show that upon recovery from AH, females exhibit CA-mediated reductions in baseline VMN nNOS, GAD, steroidogenic factor-1 (SF-1), and brain-derived neurotrophic factor (BNDF) expression compared to euglycemic profiles. In males, however, AH caused 6-OHDA-insensitive suppression of only basal SF-1 levels in the VMN. VMN transmitter protein acclimation to RIIH was sex-contingent, as differential nNOS, GAD, SF-1, and BDNF responses to a single vs final bout of hypoglycemia occur in males, whereas females show acclimated reactivity of GAD and SF-1 only to renewed hypoglycemia. CA-mediated and -independent habituation of distinctive VMN protein profiles occurred in each sex. Further research is necessary to evaluate, in each sex, effects of altered baseline VMN metabolic neurotransmitter signals on glucose homeostasis as well as non-metabolic functions under the control of those neurochemicals. It would also be insightful to learn if and how sex-contingent habituation of VMN transmitter responses to hypoglycemia contribute to sex-dimorphic patterns of glucose counter-regulation during RIIH.

Paraventricular Dynorphin A Neurons Mediate LH Pulse Suppression Induced by Hindbrain Glucoprivation in Female Rats

Malnutrition suppresses reproductive functions in mammals, which is considered to be mostly due to the inhibition of pulsatile gonadotropin-releasing hormone (GnRH)/gonadotropin secretion. Accumulating evidence suggests that kisspeptin neurons in the arcuate nucleus (ARC) play a critical role in the regulation of pulsatile GnRH/gonadotropin release. The present study aimed to examine if the hypothalamic dynorphin A (Dyn) neurons mediate the suppression of GnRH/luteinizing hormone (LH) pulses during malnutrition. Ovariectomized rats treated with a negative feedback level of estradiol-17β-treated (OVX+E2) were administered with intravenous (iv) or fourth cerebroventricle (4V) 2-deoxy-D-glucose (2DG), an inhibitor of glucose utilization, to serve as a malnutrition model. Central administration of a Dyn receptor antagonist blocked the iv- or 4V-2DG-induced suppression of LH pulses in OVX+E2 rats. The 4V 2DG administration significantly increased the number of Pdyn (Dyn gene)-positive cells co-expressing fos in the paraventricular nucleus (PVN), but not in the ARC and supraoptic nucleus (SON), and the iv 2DG treatment significantly increased the number of fos and Pdyn-co-expressing cells in the PVN and SON, but decreased it in the ARC. The E2 treatment significantly increased Pdyn expression in the PVN, but not in the ARC and SON. Double in situ hybridization for Kiss1 (kisspeptin gene) and Oprk1 (Dyn receptor gene) revealed that around 60% of ARC Kiss1-expressing cells co-expressed Oprk1. These results suggest that the PVN Dyn neurons, at least in part, mediate LH pulse suppression induced by the hindbrain or peripheral glucoprivation, and Dyn neurons may directly suppress the ARC kisspeptin neurons in female rats.

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.

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.

Caudal fourth ventricular administration of the AMPK activator 5-aminoimidazole-4-carboxamide-riboside regulates glucose and counterregulatory hormone profiles, dorsal vagal complex metabolosensory neuron function, and hypothalamic Fos expression

This study investigated the hypothesis that estrogen controls hindbrain AMP-activated protein kinase (AMPK) activity and regulation of blood glucose, counterregulatory hormone secretion, and hypothalamic nerve cell transcriptional status. Dorsal vagal complex A2 noradrenergic neurons were laser microdissected from estradiol benzoate (E)- or oil (O)-implanted ovariectomized female rats after caudal fourth ventricular (CV4) delivery of the AMPK activator 5-aminoimidazole-4-carboxamide-riboside (AICAR), for Western blot analysis. E advanced AICAR-induced increases in A2 phospho-AMPK (pAMPK) expression and in blood glucose levels and was required for augmentation of Fos, estrogen receptor-α (ERα), monocarboxylate transporter-2, and glucose transporter-3 protein in A2 neurons and enhancement of corticosterone secretion by this treatment paradigm. CV4 AICAR also resulted in site-specific modifications in Fos immunolabeling of hypothalamic metabolic structures, including the paraventricular, ventromedial, and arcuate nuclei. The current studies demonstrate that estrogen regulates AMPK activation in caudal hindbrain A2 noradrenergic neurons during pharmacological replication of energy shortage in this area of the brain, and that this sensor is involved in neural regulation of glucostasis, in part, through control of corticosterone secretion. The data provide unique evidence that A2 neurons express both ERα and -β proteins and that AMPK upregulates cellular sensitivity to ERα-mediated signaling during simulated energy insufficiency. The results also imply that estrogen promotes glucose and lactate uptake by these cells under those conditions. Evidence for correlation between hindbrain AMPK and hypothalamic nerve cell genomic activation provides novel proof for functional connectivity between this hindbrain sensor and higher order metabolic brain loci while demonstrating a modulatory role for estrogen in this interaction.

Review of the neuroanatomic landscape implicated in glucose sensing and regulation of nutrient signaling: immunophenotypic localization of diabetes gene Tcf7l2 in the developing murine brain

Genetic variants in the transcription factor 7-like 2(Tcf7l2) gene have been found to confer a significant risk of type 2 diabetes and attenuated insulin secretion. Based on its genomic wide association Tcf7l2 is considered the single most important predictor of diabetes to date. Previous studies of Tcf7l2 mRNA localization in the adult brain suggest a putative role of Tcf7l2 in the CNS regulation of energy homeostasis. The present study further characterizes the immunophenotypic distribution of peptide expression in the brains of Tcf7l2 progeny during developmental time periods between E12.5 and P1. Tcf7l2(-/-) is lethal beyond P1. Results show that while negligible TCF7L2 expression is found in the developing brains of Tcf7l2(-/-)mice, TCF7L2 protein is relatively widespread and robustly expressed in the brain by E18.5 and exhibits specific expression within neuronal populations and regions of the brain in Tcf7l2(+/-) and Tcf7l2(+/+) progeny. Strong immunophenotypic labeling was found in the diencephalic structure of the thalamus that suggests a role of Tcf7l2 in the development and maintenance of thalamic activity. Strongly expressed TCF7L2 was localized in select hypothalamic and preoptic nuclei indicative of Tcf7l2 function within neurons controlling energy balance. Definitive neuronal staining for TCF7L2 within nuclei of the brain stem and circumventricular organs extends TCF7L2 localization within autonomic neurons and its potential integration with autonomic function. In addition robust TCF7L2 expression was found in the tectal and tegmental structures of the superior and inferior colliculi as well as transient expression in neuroepithelium of the cerebral and hippocampal cortices of E16 and E18.5. Patterns of TCF7L2 peptide localization when compared to the adult protein synthetic chemical/anatomical landscape of glucose sensing exhibit a good correlational fit between its expression and regions, nuclei, and pathways regulating energy homeostasis via integration and response to peripheral endocrine, metabolic and neuronal signaling. TCF was also found co-localized with peptides that regulate energy homeostasis including AgRP, POMC and NPY. TCF7l2, some variants of which have been shown to impair GLP-1-induced insulin secretion, was also found co-localize with GLP-1 in adult TCF wild type progeny. Impaired Tcf7l2-mediated neural regulation may contribute to the risk and/or underlying pathophysiology of type 2 diabetes that has found high expression in genomic studies of Tcf7l2 variants.

Peripheral ghrelin participates in the glucostatic signaling mediated by the ventromedial and lateral hypothalamus neurons

Employing immunohistochemistry techniques, we examined the c-fos expression in different hypothalamic areas, when plasma glucose levels were modified by the administration of insulin and 2-deoxyglucose (2-DG) respectively. Subsequently, the hypoglycemia produced by an injection of insulin significantly increased feeding concomitant to higher c-fos expression in the arcuate nucleus (ARC), paraventricular nucleus (PVN), dorsomedial hypothalamus (DMH) and lateral hypothalamus (LH), while no statistical changes in the ventromedial hypothalamus (VMH) were found. Also, the glucopenia induced by 2-DG administration produced similar stimulatory effects on appetite and the neuronal activity affecting all the hypothalamic areas studied, including the VMH. The peripheral blockade of the orexigenic hormone ghrelin with a specific antibody (AGA) significantly decreased food intake as induced from acute hypoglycemia and glucopenia. Curiously, the conjoint AGA and insulin or 2-DG administration produced a differential effect on the hypothalamic neurons analyzed, by increasing the number of c-fos positive neurons in the ARC, PVN and DMH, but not in the VMH and LH. This outcome suggests an interactive effect of the glucostatic pathways involving these two areas with the ghrelin signaling.

Induction of Fos immunoreactivity labeling in rat forebrain metabolic loci by caudal fourth ventricular infusion of the monocarboxylate transporter inhibitor, alpha-cyano-4-hydroxycinnamic acid

Caudal fourth ventricular (CV4) infusion of the monocarboxylate transporter inhibitor, alpha-cyano-4-hydroxycinnamic acid (4CIN), causes hyperglycemia coincident with Fos expression in the hindbrain nucleus tractus solitarius, a rare central source of metabolic deficit signaling. The present studies examined the hypothesis that hindbrain lactoprivic signaling activates central autonomic pathways that regulate systemic glucostasis by examining the effects of this drug treatment paradigm on patterns of Fos expression in forebrain structures that integrate sensory input from metabolic sensors and coordinate motor responses to energy shortages. Two hours after CV4 infusion of graded doses of 4CIN or vehicle alone, adult female rats were sacrificed by transcardial perfusion and sections through the telencephalic and diencephalic metabolic loci were processed for Fos immunoreactivity (-ir). Fos labeling of the hypothalamic paraventricular (PVH), dorsomedial (DMH), and ventromedial (VMH) nuclei was significantly elevated, relative to the vehicle-treated controls, in response to the lowest dose of 4CIN, e.g. 10 microg/animal. Treatment with higher doses of 4CIN (25 or 50 microg) further augmented numbers of Fos-ir-positive neurons in these structures, and also elicited staining of the bed nuclei of the stria terminalis (BST), medial preoptic (MPN), arcuate (ARH), supraoptic (SO), and anterior hypothalamic nuclei (AHN), and lateral hypothalamic area (LHA). Mean numbers of Fos-immunolabeled neurons in the ARH, DMH, LHA, AHN, MPN, and SO were not different between animals infused with 25 versus 50 microg 4CIN, whereas neuronal labeling in the VMH, BST, and PVH was significantly greater in the high- versus the middle-dose groups. The present data show that pharmacological inhibition of lactate uptake within the caudal hindbrain results in dose-dependent neuronal Fos immunoexpression within characterized forebrain components of the central metabolic circuitry, and that these patterns of neuronal transcriptional activation parallel observed drug effects on blood glucose levels. These results suggest that lactoprivic signaling by metabolic 'sensing' neurons in the caudal hindbrain initiates central neural mechanisms that control systemic energy availability, and that local lactate-'sensitive' neurons are connected neuroanatomically with principal higher-order autonomic metabolic loci that regulate glucostasis.

Septopreoptic mu opioid receptor mediation of hindbrain glucoprivic inhibition of reproductive neuroendocrine function in the female rat

Central glucostasis is a critical monitored variable in neuroendocrine regulation of pituitary LH secretion. Glucoprivic signals originating within the caudal hindbrain suppress LH. Septopreoptic mu opioid receptors (mu-R) function within neural pathways maintaining basal LH levels and mediate the effects of diverse physiological stimuli on hormone release. To identify potential sites in the septopreoptic area where ligand neuromodulatory actions may occur in response to hindbrain glucoprivic signaling, the present studies evaluated the distribution of mu-R-immunoreactive (-ir) neurons in the septopreoptic area that are genomically activated in response to caudal fourth ventricular (CV4) delivery of the glucose antimetabolite, 5-thioglucose (5TG). The effects of lateral ventricular pretreatment with the selective mu-R antagonist, d-Phe-Cys-Tyr-d-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), on LH secretory and GnRH neuronal transcriptional responses to hindbrain glucoprivation were also evaluated. Estradiol benzoate- and progesterone-primed, ovariectomized female rats were treated by CV4 administration of 5TG or the vehicle, saline, at the onset of the afternoon LH surge. The inhibitory effects of hindbrain glucoprivation on mean plasma LH levels as well as colabeling of rostral preoptic GnRH neurons for Fos-ir were attenuated in animals pretreated by lateral ventricular delivery of CTOP. Dual immunocytochemical labeling for septopreoptic mu-R-ir and Fos-ir demonstrated a robust induction of Fos expression by receptor-positive neurons within discrete septopreoptic sites in response to CV4 5TG, a genomic response that was diminished by CTOP pretreatment. The current studies provide novel evidence for the transcriptional activation of neuroanatomically characterized, mu-R-expressing neurons by decreased hindbrain glucose utilization and show that the functional status of mu-R is critical for maximal induction of the Fos stimulus-transcription cascade in these cells by central glucoprivic signaling. The finding that receptor antagonist-mediated suppression of this genomic response is correlated with increased reproductive neuroendocrine output supports a role for these discrete mu-R-expressing neuron populations as substrates for ligand regulatory effects on the GnRH-pituitary LH axis during neuroglucopenia.

Nitric oxide production in hypothalamus of 2-deoxy-D-glucose-treated and food deprived mice

Nitric oxide (NO) has been suggested to be involved in the regulation of food intake. In the present study, NO metabolite (nitrite and nitrate, NOx) levels in the hypothalamus were determined in hyperphagic mice. In normal mice, NOx levels were higher in the hypothalamus than those in frontal cortex. Although 2-deoxy-D-glucose (2-DG) is known to induce hyperphagia by inhibiting glucose utilization, it did not affect NOx levels in the hypothalamus of mice. NOx concentration in the hypothalamus decreased in 48 h-food deprived mice. In the frontal cortex, neither 2-DG nor food deprivation affected NOx levels. These results suggest that NO production in the hypothalamus does not increase in 2-DG-elicited hyperphagia and that food deprivation reduces hypothalamic NO, probably by inhibiting NO synthase.

Food intake and neuronal activation after acute 2DG treatment are attenuated during lactation

In the present study, we compared the ability of acute peripheral 2-deoxy--glucose (2DG) treatment to induce food intake and increase immediate early gene expression in lactating versus virgin female rats. In Experiment 1, virgin and lactating rats were treated intraperitoneally with either saline or 2DG (400 mg/kg) and their food intake was compared across the next 6 h. In Experiment 2, lactating and virgin rats were given saline or 2DG, sacrificed 1 h later, and their brains were processed for Fos-like immunocytochemistry (FLI). The average number of cells expressing Fos protein within different brain regions was compared among the different groups. Statistical analyses of the data from Experiment 1 show that 2DG produces an increase in food intake in virgin rats, but not in lactating rats. These data correlate with the results from Experiment 2, where 2DG treatment resulted in an increase in FLI within the caudal ventrolateral medulla (cVLM), the paraventricular nucleus of the hypothalamus (PVN), and the supraoptic nucleus of the hypothalamus (SON) of cycling females. In lactating rats, however, 2DG failed to increase FLI in these regions. Together, these results show that the 2DG-induced food intake response is attenuated during lactation and this attenuation is reflected in the activation of neuronal groups that are thought to participate specifically in the food intake response to glucoprivation. Processes mediating this differential response are discussed in terms of the hormonal and metabolic changes that are characteristic of lactation.

Co-distribution of Fos- and mu opioid receptor immunoreactivity within the rat septopreoptic area and hypothalamus during acute glucose deprivation: effects of the mu receptor antagonist CTOP

Mu opioid receptors occur throughout the brain, but central sites where ligand neuromodulatory effects occur during glucopenia have not been identified. The present studies investigated whether septal, preoptic, and hypothalamic neurons that express immunoreactivity for this receptor are transcriptionally activated in response to the glucose antimetabolite, 2-deoxy-D-glucose (2DG), and if intracerebroventricular (icv) administration of the selective mu receptor antagonist, CTOP, modifies this functional response to glucose substrate imbalance. Neurons labeled for mu receptor-immunoreactivity (-ir) were observed in the lateral septal nucleus (LS), medial septum (MS), anterior division of the stria terminalis (BSTa), median preoptic nucleus (MEPO), medial preoptic nucleus (MPN), parastrial nucleus (PS), anterior hypothalamic periventricular nucleus (PVa), and lateral hypothalamic area (LPO). 2DG injection (400 mg/kg i.p.) resulted in co-labeling of mu receptor-positive neurons in the LS, MS, BSTa, MEPO, PVa, and LPO for nuclear Fos-ir. Icv delivery of CTOP decreased mean numbers of co-labeled neurons in the LS, MS, BSTa, and MEPO. These results provide evidence for transactivational effects of glucopenia on mu opioid receptor-expressing neurons within the septum, preoptic area, and hypothalamus, and suggest that the functional status of these receptors within discrete septopreoptic sites may be critical for maximal glucoprivic induction of the Fos stimulus-transcription cascade within local cells. These results thus support the view that the neural loci described above may serve as substrates for regulatory effects of mu opioid receptor ligands on central compensatory activities during acute glucose deprivation.