Hindbrain lactate regulates preoptic gonadotropin-releasing hormone (GnRH) neuron GnRH-I protein but not AMPK responses to hypoglycemia in the steroid-primed ovariectomized female rat

Expression of polyamines and its association with GnRH-I in the hypothalamus during aging in rodent model

GnRH-I and GnIH are the key neuropeptides that regulate the hypothalamic-pituitary-gonadal axis in mammals during aging. Polyamines are important aliphatic amines that are expressed in the brain and show variation with aging. The present study demonstrates evidence of variation in the level of expression of polyamines, GnRH-I and GnIH in the hypothalamus of female mice during aging. The study also suggests regulatory effects of polyamines over expression of the hypothalamic GnRH-I. The study shows a significant positive correlation between polyamines, its associated factors and GnRH-I along with significant negative correlation between polyamines, its associated factors and GnIH. This is the first study to report the effect of polyamines along with lactate or TNF-α or both on GnRH-I expression in GT1-7 cell line. TNF-α and lactate significantly decreased hypothalamic GnRH-I mRNA expression in GT1-7 cells when treated for 24 h. Polyamines (putrescine and agmatine) in contrast, significantly increased GnRH-I mRNA expression in GT1-7 cells when treated for 24 h. Also, polyamines increased GnRH-I mRNA expression when treated in presence of TNF-α or lactate thereby suggesting its neuro-protective role. This study also found 3809 differentially expressed genes through RNA-seq done between the hypothalamic GT1-7 cells treated with putrescine only versus TNF-α and putrescine. The present study suggests for the first time that putrescine treatment to TNFα-primed GT1-7 cells upregulates GnRH-I expression via regulation of several pathways such as calcium ion pathway, estrogen signaling, clock genes as well as regulating other metabolic process like neuronal differentiation and neurulation.

Lactate-Dependent Cross-Talk Between Astrocyte and GnRH-I Neurons in Hypothalamus of Aged Brain: Decreased GnRH-I Transcription

GnRH-I produced by hypothalamic neurosecretory cells is considered a master regulator of mammalian reproduction. Although GnRH-I transcription is well studied, the effect of ageing on transcriptional regulation of GnRH-I has not yet been explored. Here, we elucidate the effects of ageing on the metabolic environment like lactate level and TNF-α and how these affect GnRH-I transcription. Using pathway analysis of transcriptomic data, we found that lactate is upregulated in ageing astrocytes due to the downregulation of cellular respiration pathways possibly resulting in greater pyruvate concentration for lactate production. This lactate could then be shuttled into neurons where it would affect GnRH-I transcription. We showed that supra-physiological level of lactate in young mouse brain can mimic metabolic disturbances in the old brain and cause downregulation in GnRH-I transcription at a young age. In particular, we found upregulation of GnRH-I repressors in the young brain treated with high levels of lactate similar to old brain. Hence, this confirmed that aged metabolic environment can affect GnRH-I transcription even in the young brain. Further downstream analysis using the TRUST database showed NF-Kb signalling which lies downstream of both lactate and TNF-α as being capable of upregulating GnRH-I repressors. Since NF-Kb signalling has been shown in our study as well as others to be induced by TNF-α during ageing, it is likely that GnRH-I transcriptional regulation is mediated through these pathways. Thus, we formed a model for explaining the downregulation of GnRH-I transcription during ageing through differential expression of its TFs in an aged metabolic environment.

Impact of caudal hindbrain glycogen metabolism on A2 noradrenergic neuron AMPK activation and ventromedial hypothalamic nucleus norepinephrine activity and glucoregulatory neurotransmitter marker protein expression

The brain glycogen reserve is a source of oxidizable substrate fuel. Lactoprivic-sensitive hindbrain A2 noradrenergic neurons provide crucial metabolic-sensory input to downstream hypothalamic glucose-regulatory structures. Current research examined whether hindbrain glycogen fuel supply impacts A2 energy stability and governance of ventromedial hypothalamic nucleus (VMN) metabolic transmitter signaling. Male rats were injected into the caudal fourth ventricle (CV4) with the glycogen phosphorylase inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) prior to continuous intra-CV4 infusion of L-lactate or vehicle. Lactate reversed DAB suppression of A2 neuron AMPK protein and up-regulated phosphoAMPK profiles. A2 dopamine-β-hydroxylase expression was refractory to DAB, but elevated by DAB/lactate. Lactate normalized A2 estrogen receptor-alpha and GPER proteins and up-regulated estrogen receptor-beta levels in DAB-treated rats. VMN norepinephrine content was decreased by DAB, but partially restored by lactate. DAB caused lactate-reversible or -irreversible augmentation of VMN glycogen phosphorylase-brain (GPbb) and -muscle type (GPmm) variant profiles, and correspondingly up- or down-regulated VMN protein markers of glucose-stimulatory nitrergic and glucose-inhibitory γ-aminobutyric acid transmission. DAB did not alter plasma glucose, but suppressed or elevated circulating glucagon and corticosterone in that order. Results show that diminished hindbrain glycogen breakdown is communicated to the VMN, in part by NE signaling, to up-regulate VMN glycogen breakdown and trigger neurochemical signaling of energy imbalance in that site. DAB effects on GPmm, VMN glycogen content, and counter-regulatory hormone secretion were unabated by lactate infusion, suggesting that aside from substrate fuel provision rate, additional indicators of glycogen metabolism such as turnover rate may be monitored in the hindbrain.

Sex-specific acclimation of A2 noradrenergic neuron dopamine-β-hydroxylase and estrogen receptor variant protein and 5'-AMP-Activated protein kinase reactivity to recurring hypoglycemia in rat

Hindbrain estrogen receptors (ER) impose sex-dimorphic control of counter-regulatory hormone and hypothalamic glucoregulatory transmitter and glycogen metabolic responses to hypoglycemia. A2 noradrenergic neurons are estradiol- and metabolic-sensitive. Estradiol controls dopamine-beta-hydroxylase (DBH) protein habituation to recurrent insulin-induced hypoglycemia (RIIH) in females. Current research investigated the premise that sex-dimorphic patterns of A2 ER variant acclimation to RIIH correlate with differential A2 DBH and 5'-AMP-activated protein kinase (AMPK) adaptation to RIIH. A2 neurons were laser-catapult-microdissected from male and female rats after one or four insulin injections for Western blot analysis. A2 pAMPK and DBH levels were increased in males, but suppressed in females after single insulin dosing. ER-alpha (ERα) and -beta (ERβ) protein profiles were unaffected or decreased by acute hypoglycemia in each sex, whereas G protein-linked ER-1 (GPER) reactivity varied by sex. Antecedent hypoglycemia diminished basal A2 ERα/GPER and elevated ERβ content in each sex, yet reduced pAMPK and DBH levels in female rats only. Reintroduced hypoglycemia suppressed A2 ERβ levels in each sex, but altered DBH (↓), ERα (↓), and GPER (↑) levels in males only. Data document sex differences in A2 DBH adaptation to RIIH, e.g. a shift from positive-to-negative response in males versus loss of negative reactivity in females, as well as attenuated AMPK activation in both sexes. Between hypoglycemic episodes, A2 neurons in each sex likely exhibit diminished sensitivity to ERα/GPER signaling, but heightened receptivity to ERβ input. RIIH-induced changes in ERα and GPER expression in male but not female may contribute to DBH suppression (males) versus no change (females) relative to adapted baseline expression.

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.

Effects of estradiol on lactoprivic signaling of the hindbrain upon the contraregulatory hormonal response and metabolic neuropeptide synthesis in hypoglycemic female rats

BACKGROUND

Caudal dorsomedial hindbrain detection of hypoglycemia-associated lactoprivation regulates glucose counter-regulation in male rats. In females, estradiol (E) determines hypothalamic neuroanatomical and molecular foci of hindbrain energy sensor activation. This study investigated the hypothesis that E signal strength governs metabolic neuropeptide and counter-regulatory hormone responses to hindbrain lactoprivic stimuli in hypoglycemic female rats.

METHODS

Ovariectomized animals were implanted with E-filled silastic capsules [30 (E-30) or 300 μg (E-300)/mL] to replicate plasma concentrations at estrous cycle nadir versus peak levels. E-30 and E-300 rats were injected with insulin or vehicle following initiation of continuous caudal fourth ventricular L-lactate infusion.

RESULTS

Hypoglycemic hypercorticosteronemia was greater in E-30 versus E-300 animals. Glucagon and corticosterone outflow was correspondingly fully or partially reversed by hindbrain lactate infusion. Insulin-injected rats exhibited lactate-reversible augmentation of norepinephrine (NE) accumulation in all preoptic/hypothalamic structures examined, excluding the dorsomedial hypothalamic nucleus (DMH) where hindbrain lactate infusion either suppressed (E-30) or enhanced (E-300) NE content. Expression profiles of hypoglycemia-reactive metabolic neuropeptides were normalized (with greater efficacy in E-300 animals) by lactate infusion. DMH RFamide-related peptide-1 and -3, arcuate neuropeptide Y and kisspeptin, and ventromedial nucleus nitric oxide synthase protein responses to hypoglycemia were E dosage-dependent.

CONCLUSIONS

Distinct physiological patterns of E secretion characteristic of the female rat estrous cycle elicit differential corticosterone outflow during hypoglycemia, and establish both common and different hypothalamic metabolic neurotransmitter targets of hindbrain lactate deficit signaling. Outcomes emphasize a need for insight on systems-level organization, interaction, and involvement of E signal strength-sensitive neuropeptides in counter-regulatory functions.

Sex Differences and Role of Estradiol in Hypoglycemia-Associated Counter-Regulation

Vital nerve cell functions, including maintenance of transmembrane voltage and information transfer, occur at high energy expense. Inadequate provision of the obligate metabolic fuel glucose exposes neurons to risk of dysfunction or injury. Clinical hypoglycemia rarely occurs in nondiabetic individuals but is an unfortunate regular occurrence in patients with type 1 or advanced insulin-treated type 2 diabetes mellitus. Requisite strict glycemic control, involving treatment with insulin, sulfonylureas, or glinides, can cause frequent episodes of iatrogenic hypoglycemia due to defective counter-regulation, including reduced glycemic thresholds and diminished magnitude of motor responses. Multiple components of the body's far-reaching energy balance regulatory network, including the hindbrain dorsal vagal complex, provide dynamic readout of cellular energetic disequilibrium, signals that are utilized by the hypothalamus to shape counterregulatory autonomic, neuroendocrine, and behavioral outflow toward restoration of glucostasis. The ovarian steroid hormone 17β-estradiol acts on central substrates to preserve nerve cell energy stability brain-wide, thereby providing neuroprotection against bio-energetic insults such as neurodegenerative diseases and acute brain ischemia. The current review highlights recent evidence implicating estrogen in gluco-regulation in females by control of hindbrain metabolic sensor screening and signaling of hypoglycemia-associated neuro-energetic instability. It is anticipated that new understanding of the mechanistic basis of how estradiol influences metabolic sensory input from this critical brain locus to discrete downstream regulatory network substrates will likely reveal viable new molecular targets for therapeutic simulation of hormone actions that promote positive neuronal metabolic state during acute and recurring hypoglycemia.

Hindbrain estrogen receptor-beta antagonism normalizes reproductive and counter-regulatory hormone secretion in hypoglycemic steroid-primed ovariectomized female rats

Hindbrain dorsal vagal complex A2 noradrenergic signaling represses the pre-ovulatory luteinizing hormone (LH) surge in response to energy deficiency. Insulin-induced hypoglycemia augments A2 neuron adenosine 5'-monophosphate-activated protein kinase (AMPK) activity and estrogen receptor-beta (ERβ) expression, coincident with LH surge suppression. We hypothesized that ERβ is critical for hypoglycemia-associated patterns of LH secretion and norepinephrine (NE) activity in key reproduction-relevant forebrain structures. The neural mechanisms responsible for tight coupling of systemic energy balance and procreation remain unclear; here, we investigated whether ERβ-dependent hindbrain signals also control glucose counter-regulatory responses to hypoglycemia. Gonadal steroid-primed ovariectomized female rats were pretreated by caudal fourth ventricular administration of the ERβ antagonist 4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-3-yl]phenol (PHTPP) or vehicle before insulin injection at LH surge onset. Western blot analysis of laser-microdissected A2 neurons revealed hypoglycemic intensification of AMPK activity and dopamine-β-hydroxylase protein expression; the latter response was attenuated by PHTPP pretreatment. PHTPP regularized LH release, but not preoptic GnRH-I precursor protein expression in insulin-injected rats, and reversed hypoglycemic stimulation of glucagon and corticosterone secretion. Hypoglycemia caused PHTPP-reversible changes in NE and prepro-kisspeptin protein content in the hypothalamic arcuate (ARH), but not anteroventral periventricular nucleus. Results provide novel evidence for ERβ-dependent caudal hindbrain regulation of LH and counter-regulatory hormone secretion during hypoglycemia. Observed inhibition of LH likely involves mechanisms at the axon terminal that impede GnRH neurotransmission. Data also show that caudal hindbrain ERβ exerts site-specific control of NE activity in forebrain projection sites during hypoglycemia, including the ARH where prepro-kisspeptin may be a target of that signaling.

Sexual Motivation in the Female and Its Opposition by Stress

A well worked-out motivational system in laboratory animals produces estrogen-dependent female sex behavior. Here, we review (a) the logical definition of sexual motivation and (b) the basic neuronal and molecular mechanisms that allow the behavior to occur. Importantly, reproductive mechanisms in the female can be inhibited by stress. This is interesting because, in terms of the specificity of neuroendocrine dynamics in space and time, the two families of phenomena, sex and stress, are the opposite of each other. We cover papers that document stress effects on the underlying processes of reproductive endocrinology in the female. Not all of the mechanisms for such inhibition have been clearly laid out. Finally, as a current topic of investigation, this system offers several avenues for new investigation which we briefly characterize.