Diabetes induces GABA receptor plasticity in murine vagal motor neurons

Abstract

Autonomic dysregulation accompanies type-1 diabetes, and synaptic regulation of parasympathetic preganglionic motor neurons in the dorsal motor nucleus of the vagus (DMV) is altered after chronic hyperglycemia/hypoinsulinemia. Tonic gamma-aminobutyric acid A (GABA A ) inhibition prominently regulates DMV neuron activity, which contributes to autonomic control of energy homeostasis. This study investigated persistent effects of chronic hyperglycemia/hypoinsulinemia on GABA A receptor-mediated inhibition in the DMV after streptozotocin-induced type-1 diabetes using electrophysiological recordings in vitro, quantitative (q)RT-PCR, and immunohistochemistry. Application of the nonspecific GABA A receptor agonist muscimol evoked an outward current of significantly larger amplitude in DMV neurons from diabetic mice than controls. Results from application of 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP), a ␦-subunit agonist, suggested that GABA A receptors containing ␦-subunits contributed to the enhanced inducible tonic GABA current in diabetic mice. Sensitivity to THIP of inhibitory postsynaptic currents in DMV neurons from diabetic mice was also increased. Results from qRT-PCR and immunohistochemical analyses indicated that the altered GABAergic inhibition may be related to increased trafficking of GABA A receptors that contain the ␦-subunit, rather than an expression change. Overall these findings suggest increased sensitivity of ␦-subunit containing GABA A receptors after several days of hyperglycemia/ hypoinsulinemia, which dramatically alters GABAergic inhibition of DMV neurons and could contribute to diabetic autonomic dysregulation.

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

“…Type I diabetes is characterized by uncontrolled hyperglycemia due to reduced insulin secretion from pancreatic beta cells. Synaptic and other cellular responses in the dorsal vagal complex are altered in models of type 1 diabetes, even after normalizing glucose concentration (Zsombok et al, 2011, Browning, 2013, Blake and Smith, 2014, Bach et al, 2015, Boychuk et al, 2015b). Vagal reflexes are often blunted during chronic hyperglycemia, and altered vagal function may contribute to diabetes-associated visceral dysfunction (Saltzman and McCallum, 1983, Undeland et al, 1998), suggesting that chronically-elevated glucose alters responsiveness of neurons in the dorsal vagal complex.…”

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

“…Subsets of these neurons, and GABA neurons in particular, are known to be glucose-sensitive , Balfour et al, 2006, Wan and Browning, 2008, Browning, 2013, Lamy et al, 2014, Boychuk et al, 2015a. Several physiological aspects of central vagal circuitry are altered functionally in diabetes (Zsombok et al, 2011, Browning, 2013, Blake and Smith, 2014, Bach et al, 2015, Boychuk et al, 2015b, consistent with modified parasympathetic regulation of the viscera concurrent with the disease (Saltzman and McCallum, 1983). Glucose sensing in NTS neurons involves GCK, which catalyzes the conversion of glucose to glucose-6 phosphate in neurons and other cells (Balfour et al, 2006, Briski et al, 2009, resulting in increased ATP/ADP ratio.…”

“…This model involves injecting animals systematically with STZ, which kills the insulin-producing pancreatic β-cells, resulting in chronic hyperglycemia in conjunction with hypoinsulinemia (Craighead 1980). These studies revealed that chronically increased blood glucose concentrations in the STZ model of type 1 diabetes is accompanied by altered neurotransmitter function in the DVC (Bach et al 2015;Blake and Smith 2014;Boychuk et al 2015b;Zsombok et al 2011). These effects persisted for hours (at least) after glucose was standardized to control levels in vitro.…”

“…Paired with hepatic gluconeogenesis, the process by which amino acids are metabolized to generate glucose in hepatocytes, this process is important in both hepatic and pancreatic control of blood glucose levels. 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).…”

“…Synaptic and other cellular responses in the dorsal vagal complex are altered in models of type 1 diabetes, even after normalizing glucose concentration (Zsombok et al, 2011, Browning, 2013, Blake and Smith, 2014, Bach et al, 2015, Boychuk et al, 2015b.…”