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

SIMPLE SUMMARY: The brain is a target for steroid hormones derived from the circulation or synthesized locally. Central estrogen receptors control neural functions that preserve whole-body energy stability and optimize blood levels of glucose, a principal metabolic fuel. Glucose insufficiency owing to insulin-induced hypoglycemia, an unremitting complication of type I diabetes mellitus management, can injure or destroy brain neurons. It is critical to understand how neuroestradiol may affect neural circuitries that initiate physiological and behavioral responses that rectify hypoglycemia. Current studies investigated whether estradiol generated by the enzyme aromatase acts within the key component of central nervous system regulation of glucose balance, the ventromedial hypothalamic nucleus, to regulate metabolic-sensitive neurochemical signaling. Here, state-of-the-art reagents for manipulation of in vivo aromatase gene expression were stereotactically delivered to that nucleus to investigate whether this enzyme protein is essential for optimal insulin-induced hypoglycemia-associated patterns of local neurotransmitter synthesis, involving microdissection-harvesting of pure glucose-stimulatory and glucose-inhibitory neuron cell samples and plasma glucose counter-regulatory hormone profiles. Results provide novel evidence that hypoglycemic adjustments in ventromedial nucleus neuroestradiol synthesis vary over the rostro-caudal length of that structure and that aromatase activity is required increased glucose-stimulatory and for decreased glucose-inhibitory transmission at distinct levels of this nucleus. ABSTRACT: 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.