Central GABAA but not GABAB receptors mediate suppressive effects of caudal hindbrain glucoprivation on the luteinizing hormone surge in steroid-primed, ovariectomized female rats

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.

Glucosensing by GnRH neurons: inhibition by androgens and involvement of AMP-activated protein kinase

GnRH neurons integrate steroidal and metabolic cues to regulate fertility centrally. Central glucoprivation reduces LH secretion, which is governed by GnRH release, suggesting GnRH neuron activity is modulated by glucose availability. Here we tested whether GnRH neurons can sense changes in extracellular glucose, and whether glucosensing is altered by the steroids dihydrotestosterone (DHT) and/or estradiol (E). Extracellular recordings were made from GnRH neurons in brain slices from ovariectomized (OVX) mice ± DHT and/or E implants. Firing rate was reduced by a switch from 4.5 to 0.2 mm glucose in cells from OVX, OVX+E, and OVX+DHT+E mice, but not OVX+DHT mice. This suggests that androgens reduce the sensitivity of GnRH neurons to changes in extracellular glucose, but E mitigates this effect. Next we investigated potential mechanisms. In the presence of the ATP-sensitive potassium channel antagonist tolbutamide, glucosensing persisted. In contrast, glucosensing was attenuated in the presence of compound C, an antagonist of AMP-activated protein kinase (AMPK), suggesting a role for AMPK in glucosensing. The AMPK activator N1-(b-D-ribofuranosyl)-5-aminoimidazole-4-carboxamide (AICAR) mimicked the effect of low glucose and was less effective in cells from DHT-treated mice. The effect of DHT to diminish responses to low glucose and AICAR was abolished by blockade of fast synaptic transmission. Both AICAR and low glucose activated a current with a reversal potential near -50 mV, suggesting a nonspecific cation current. These studies indicate that glucosensing is one mechanism by which GnRH neurons sense fuel availability and point to a novel role for AMPK in the central regulation of fertility.

Sex steroid effects at target tissues: mechanisms of action

Our understanding of the mechanisms of sex hormone action has changed dramatically over the last 10 years. Estrogens, progestins, and androgens are the steroid hormones that modulate reproductive function. Recent data have shown that many other tissues are targets of sex hormones in addition to classical reproductive organs. This review outlines new advances in our understanding of the spectrum of steroid hormone ligands, newly recognized target tissues, structure-function relationships of steroid receptors, and, finally, their genomic and nongenomic actions. Sex-based specific effects are often related to the different steroid hormone mileu in men compared with women. Understanding the mechanisms of sex steroid action gives insight into the differences in normal physiology and disease states.