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

Abstract

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 NEtreated animals. NE caused AMPK-independent down-regulation of alpha 2-, alongside Ccreversible augmentation of beta 1-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.

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

“…Additional studies corroborate these findings with proof that hypoglycemia stimulates VMN astrocyte AMPK and pAMPK profiles in female, but not male rats, and that these sex-dimorphic sensor activity responses are mediated by ERs [30]. Data reported by Ibrahim et al [24] show that AMPK is involved in the NE regulation of VMN astrocyte expression of plasma membrane estrogen receptor GPER and β 1 -AR protein expression, as well as in GS and GP profiles. Collectively, our research implicates AMPK in the noradrenergic control of VMN astrocyte glycogen synthesis and metabolism.…”

“…These cells likely detect the energetic sequelae of hypoglycemia as AMPK is activated in a lactate-reversible manner in this (but not other) hindbrain's catecholamine cell groups, alongside lactoprivic-dependent increases in hypothalamic NE activity [23]. The direct delivery of NE to the VMN alters metabolic transmitter marker proteins, e.g., nNOS and glutamate decarboxylase 65/67 (GAD) expression [24]. Interestingly, Cc pretreatment of the VMN normalizes local patterns of nNOS expression in NE-treated animals.…”

“…Due to the complex neurochemical heterogeneity of the VMN, studies focusing on individual neurotransmitter cell populations require high-neuroanatomical resolution dissection tools for the procurement of pure nerve cell samples. Using combinatory immunocytochemistry/laser-catapult microdissection for the discriminative harvesting of VMN NO and GABA neurons, our studies revealed that each population is likely a direct target for noradrenergic input as these nerve cells express alpha 1 -(α 1 -), alpha 2 -(α 2 -), and beta 1 -(β 1 -) adrenoreceptor (AR) proteins [24].…”

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

“…Characterized effector transmitters of energy insufficiency within the broader mediobasal hypothalamus include nitric oxide (NO) and γ-aminobutyric acid (GABA), which correspondingly stimulate or suppress counter-regulatory hormone output (Chan et al., 2006; Fioramonti et al., 2010; Routh et al., 2014). Studies focusing solely on the VMN, excluding other mediobasal hypothalamus components, for example, arcuate and tuberal nuclei, show that the catecholamine neurotransmitter norepinephrine (NE) regulates expression of biosynthetic enzyme protein markers of local glucoregulatory signaling, for example, neuronal nitric oxide synthase (nNOS) and glutamate decarboxylase 65/67 (GAD 65/67 ; Ibrahim et al., 2019). Selective sampling of VMN NO and GABA neurons using laser-catapult microdissection indicates that these cells are likely to be direct substrates for noradrenergic regulation as these cells express alpha 1 - (α 1 -), alpha 2 - (α 2 -), and beta 1 - (β 1 -) adrenergic receptor (AR) proteins (Ibrahim et al., 2019; Uddin et al., 2019).…”

Norepinephrine Regulation of Adrenergic Receptor Expression, 5’ AMP-Activated Protein Kinase Activity, and Glycogen Metabolism and Mass in Male Versus Female Hypothalamic Primary Astrocyte Cultures

“…Hypothalamic astrocytes express the ultrasensitive energy gauge 5'-AMP-activated protein kinase (AMPK) (Tamrakar and Briski, 2015). In male rats, NE regulation of hypothalamic astrocyte glycogen metabolic enzyme protein expression in vivo is AMPK-dependent (Ibrahim et al, 2019). A corollary objective of this work sought to examine the premise that NE controls astrocyte total AMPK protein expression and activity, and that such action may involve regulation of upstream stimulatory kinase (calcium/calmodulin-dependent protein kinase kinase-b) and inhibitory phosphatase (protein phosphatase-1) protein expression in one or both sexes.…”