Fibroblast Growth Factor 19 Increases the Excitability of Pre-Motor Glutamatergic Dorsal Vagal Complex Neurons From Hyperglycemic Mice

Endocrine FGFs and their signaling in the brain: Relevance for energy homeostasis

Since their discovery in 2000, there has been a continuous expansion of studies investigating the physiology, biochemistry, and pharmacology of endocrine fibroblast growth factors (FGFs). FGF19, FGF21, and FGF23 comprise a subfamily with attributes that distinguish them from typical FGFs, as they can act as hormones and are, therefore, referred to as endocrine FGFs. As they participate in a broad cross-organ endocrine signaling axis, endocrine FGFs are crucial lipidic, glycemic, and energetic metabolism regulators during energy availability fluctuations. They function as powerful metabolic signals in physiological responses induced by metabolic diseases, like type 2 diabetes and obesity. Pharmacologically, FGF19 and FGF21 cause body weight loss and ameliorate glucose homeostasis and energy expenditure in rodents and humans. In contrast, FGF23 expression in mice and humans has been linked with insulin resistance and obesity. Here, we discuss emerging concepts in endocrine FGF signaling in the brain and critically assess their putative role as therapeutic targets for treating metabolic disorders.

Systemic Glucose Regulation by a Hindbrain Inhibitory Circuit in a Mouse Model of Type 1 Diabetes

INTRODUCTION

Previous work showed that increasing the electrical activity of inhibitory neurons in the dorsal vagal complex (DVC) is sufficient to increase whole-body glucose concentration in normoglycemic mice. Here we tested the hypothesis that deactivating GABAergic neurons in the dorsal hindbrain of hyperglycemic mice decreases synaptic inhibition of parasympathetic motor neurons in the dorsal motor nucleus of the vagus (DMV) and reduces systemic glucose levels.

METHODS

Chemogenetic activation or inactivation of GABAergic neurons in the nucleus tractus solitarius (NTS) was used to assess effects of modulating parasympathetic output on blood glucose concentration in normoglycemic and hyperglycemic mice. Patch-clamp electrophysiology in vitro was used to assess cellular effects of chemogenetic manipulation of NTS GABA neurons.

RESULTS

Chemogenetic activation of GABAergic NTS neurons in normoglycemic mice increased their action potential firing, resulting in increased inhibitory synaptic input to DMV motor neurons and elevated blood glucose concentration. Deactivation of GABAergic DVC neurons in normoglycemic mice altered their electrical activity but did not alter systemic glucose levels. Conversely, stimulation of GABAergic DVC neurons in mice that were hyperglycemic subsequent to treatment with streptozotocin changed their electrical activity but did not alter whole-body glucose concentration, while deactivation of this inhibitory circuit significantly decreased circulating glucose concentration. Peripheral administration of a brain impermeant muscarinic acetylcholine receptor antagonist abolished these effects.

CONCLUSION

Disinhibiting vagal motor neurons decreases hyperglycemia in a mouse model of type 1 diabetes. This inhibitory brainstem circuit emerges as a key parasympathetic regulator of whole-body glucose homeostasis that undergoes functional plasticity in hyperglycemic conditions.