Norepinephrine control of ventromedial hypothalamic nucleus glucoregulatory neurotransmitter expression in the female rat: Role of monocarboxylate transporter function

Abstract

The ventromedial hypothalamic nucleus (VMN) is a critical component of the neural circuitry that regulates glucostasis. Astrocyte glycogen is a vital reserve of glucose and its oxidizable metabolite L-lactate. In hypoglycemic female rats, estradiol-dependent augmentation of VMN glycogen phosphorylase (GP) protein requires hindbrain catecholamine input. The research here investigated the premise that norepinephrine (NE) regulation of VMN astrocyte metabolism shapes local glucoregulatory neurotransmitter signaling in this sex. Estradiol-implanted ovariectomized rats were pretreated by intra-VMN administration of the monocarboxylate transporter inhibitor alphacyano-4-hydroxy-cinnamic acid (4CIN) or vehicle before NE delivery to that site. NE caused 4CIN-reversible reduction or augmentation of VMN glycogen synthase and phosphorylase expression. 4CIN prevented NE stimulation of gluco-inhibitory (glutamate decarboxylase 65/67) and suppression of gluco-stimulatory (neuronal nitric oxide synthase) neuron marker proteins. These outcomes imply that effects of noradrenergic stimulation of VMN astrocyte glycogen depletion on glucoregulatory transmitter signaling may be mediated, in part, by glycogen-derived substrate fuel provision. NE control of astrocyte glycogen metabolism may involve downregulated adrenoreceptor (AR), e.g. alpha 1 and alpha 2 , alongside amplified beta 1 AR and estrogen receptor-beta signaling. Noradrenergic hypoglycemia was refractory to 4CIN, implying that additional NE-sensitive VMN glucoregulatory neurochemicals may be insensitive to monocarboxylate uptake. Augmentation of circulating free fatty acids by combinatory NE and 4CIN, but not NE alone implies that acute hypoglycemia induced here is an insufficient stimulus for mobilization of these fuels, but is adequate when paired with diminished brain monocarboxylate fuel availability.

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

“…As NO neurons contain ER proteins (Uddin et al, 2019), intrinsic ER likely mediate estradiol stimulation of nNOS expression. Results affirm prior reports that in female rats, site-specific targeting of exogenous NE to the VMN elicits concomitant down-versus upregulation of glucostimulatory (nNOS) and glucoinhibitory (GAD) signaling marker proteins, which infers that VMN neuroenergetic stability is enhanced by NE (Mahmood et al, 2019). This assumption remains unverified as methodology to support analysis of single-nerve cell ATP content is currently unavailable; thus, definitive evidence that glucoregulatory transmitter responses to NE reflect a positive shift in energy state is yet to be attained.…”

“…Present data show that þ2 hr after intra-VMN NE administration, plasma glucose levels were measurable within the normal range, while circulating corticosterone and FFA concentrations were elevated. Previous observations of NE-associated mild hypoglycemia and unmodified FFA values at 1 hr posttreatment indicate that these specific responses adhere to distinctive temporal patterns, whereas hypercorticosteronemia is a uniform reaction, at least up to 2 hr posttreatment (Mahmood et al, 2019). Earlier studies showed that pharmacological inhibition of VMN GP activity by 1,4-dideoxy-1,4-imino-D-arabinitol did not alter euglycemic patterns of corticosterone secretion but prevented hypoglycemic hypercorticosteronemia (Alhamami et al, 2018).…”

“…Cell energy deficits amplify GPbb expression in vitro (Nadeau et al., 2018). We observed that exogenous NE upregulates VMN GPbb and GAD 65/67 and that these responses are abolished by lactate transport blockade (Mahmood et al., 2019). Our studies also show that hindbrain A2 noradrenergic lactoprivic signaling exerts opposite effects on VMN GPbb versus GPmm profiles in vivo (Briski and Mandal, 2019).…”

“…Astrocytes store glycogen as an energy reserve, which is maintained by incorporation or liberation of glucose residues by opposing actions of glycogen synthase (GS) and glycogen phosphorylase (GP; Stobart and Anderson, 2013). Exogenous NE regulates VMN astrocyte GS and net GP protein content (Ibrahim et al., 2019) and affects VMN glucoregulatory signaling through control of astrocyte supply of the oxidizable glycolytic end product L-lactate (Mahmood et al., 2019). Multiple GP isoforms are expressed in the brain, including muscle- (GPmm) and brain (GPbb) types, which differ according to regulation by phosphorylation versus adenosine monophosphate (AMP; Nadeau et al., 2018).…”

“…Our studies also show that hindbrain A2 noradrenergic lactoprivic signaling exerts opposite effects on VMN GPbb versus GPmm profiles in vivo (Briski and Mandal, 2019). Although female rat VMN astrocytes are directly sensitive to estradiol by virtue of ER expression (Mahmood et al., 2018, 2019), involvement of these ERs in noradrenergic regulation of GP isoforms is unclear. Current studies addressed the premise that VMN ERα and/or ERβ signaling may shape differential effects of NE on VMN GPbb versus GPmm protein expression and that ER control of glycogen metabolic enzyme profiles may correlate with observed noradrenergic regulation of nNOS or GAD.…”