Glucose sensing by GABAergic neurons in the mouse nucleus tractus solitarii

Abstract

Changes in blood glucose concentration alter autonomic function in a manner consistent with altered neural activity in brain regions controlling digestive processes, including neurons in the brain stem nucleus tractus solitarii (NTS), which process viscerosensory information. With whole cell or on-cell patch-clamp recordings, responses to elevating glucose concentration from 2.5 to 15 mM were assessed in identified GABAergic NTS neurons in slices from transgenic mice that express EGFP in a subset of GABA neurons. Single-cell real-time RT-PCR was also performed to detect glutamic acid decarboxylase (GAD67) in recorded neurons. In most identified GABA neurons (73%), elevating glucose concentration from 2.5 to 15 mM resulted in either increased (40%) or decreased (33%) neuronal excitability, reflected by altered membrane potential and/or action potential firing. Effects on membrane potential were maintained when action potentials or fast synaptic inputs were blocked, suggesting direct glucose sensing by GABA neurons. Glucose-inhibited GABA neurons were found predominantly in the lateral NTS, whereas glucose-excited cells were mainly in the medial NTS, suggesting regional segregation of responses. Responses were prevented in the presence of glucosamine, a glucokinase (GCK) inhibitor. Depolarizing responses were prevented when KATP channel activity was blocked with tolbutamide. Whereas effects on synaptic input to identified GABAergic neurons were variable in GABA neurons, elevating glucose increased glutamate release subsequent to stimulation of tractus solitarius in unlabeled, unidentified neurons. These results indicate that GABAergic NTS neurons act as GCK-dependent glucose sensors in the vagal complex, providing a means of modulating central autonomic signals when glucose is elevated.

Molecular and functional changes in glucokinase expression in the brainstem dorsal vagal complex in a murine model of type 1 diabetes

“…Despite this hypothalamic focus, abundant evidence indicates that brainstem dorsal vagal complex (DVC) plays a primary and critical role in glucose-sensitive modulation of plasma glucose and insulin levels, feeding and energy balance Zsombok and Smith 2009). Subsets of DVC neurons and synaptic terminals are glucose-sensitive (Balfour et al 2006;Boychuk et al 2015a;Browning 2013;Lamy et al 2014;Wan et al 2008), consistent with longstanding evidence that glucose-sensing neurons in this region regulates both feeding and blood glucose concentrations . Injection of a glucoprivic glucose analogue into the vagal complex, but not hypothalamic areas, increases both feeding and hyperglycemia in rats .…”

“…These glucose-sensitive neurons respond to either hypoglycemic or hyperglycemic conditions to elicit vagal efferent responses important to maintaining and reestablishing glucose homeostasis (Balfour et al 2006;Balfour and Trapp 2007;Boychuk et al 2015a;Lamy et al 2014). Thus, NTS neuron activity and responses to visceral and other synaptic input occurs in the context of local glucose concentration.…”

“…GCK is similarly important for glucose sensing in hypothalamic and other glucose sensing neurons, including in the DVC (Boychuk et al 2015b;Dunn-Meynell et al 2002), where mRNA is localized to neurons that exhibit electrophysiological sensitivity to glucose (Lamy et al 2014). In the NTS, these glucose-sensing neurons comprise a subset of inhibitory, GABAergic cells, which are usually either excited or inhibited by increasing glucose concentration in a GCK-dependent manner (Boychuk et al 2015a;Lamy et al 2014). Under conditions of normal glucose concentration, intracellular ATP is relatively low in these neurons, allowing KATP channels to remain open.…”

“…The importance of KATP channels in insulin secretion was established over 30 years ago (Ashcroft et al 1984), and these channels have more recently been shown to mediate most responses to glucose in neurons (Balfour et al 2006;Boychuk et al 2015a;Dunn-Meynell et al 2002). The KATP channel can be pharmacologically regulated by sulfonylurea drugs, which are widely used to treat type 2 diabetes; they work by binding to and closing the KATP channel, consequently depolarizing the cell membrane.…”

“…In NTS GABA neurons, blocking KATP channels prevented the excitatory effects of increased glucose concentration (Boychuk et al 2015a) (Lamy et al 2014;Shanley et al 2002). In either case responses of NTS GABA cells require GCK, implicating intracellular metabolic processing of glucose for both responses.…”