cAMP-dependent insulin modulation of synaptic inhibition in neurons of the dorsal motor nucleus of the vagus is altered in diabetic mice

Abstract

Pathologies in which insulin is dysregulated, including diabetes, can disrupt central vagal circuitry, leading to gastrointestinal and other autonomic dysfunction. Insulin affects whole body metabolism through central mechanisms and is transported into the brain stem dorsal motor nucleus of the vagus (DMV) and nucleus tractus solitarius (NTS), which mediate parasympathetic visceral regulation. The NTS receives viscerosensory vagal input and projects heavily to the DMV, which supplies parasympathetic vagal motor output. Normally, insulin inhibits synaptic excitation of DMV neurons, with no effect on synaptic inhibition. Modulation of synaptic inhibition in DMV, however, is often sensitive to cAMP-dependent mechanisms. We hypothesized that an effect of insulin on GABAergic synaptic transmission may be uncovered by elevating resting cAMP levels in GABAergic terminals. We used whole cell patch-clamp recordings in brain stem slices from control and diabetic mice to identify insulin effects on inhibitory neurotransmission in the DMV in the presence of forskolin to elevate cAMP levels. In the presence of forskolin, insulin decreased the frequency of inhibitory postsynaptic currents (IPSCs) and the paired-pulse ratio of evoked IPSCs in DMV neurons from control mice. This effect was blocked by brefeldin-A, a Golgi-disrupting agent, or indinavir, a GLUT4 blocker, indicating that protein trafficking and glucose transport were involved. In streptozotocin-treated, diabetic mice, insulin did not affect IPSCs in DMV neurons in the presence of forskolin. Results suggest an impairment of cAMP-induced insulin effects on GABA release in the DMV, which likely involves disrupted protein trafficking in diabetic mice. These findings provide insight into mechanisms underlying vagal dysregulation associated with diabetes.

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 (Oomura et al, 1974, 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.…”

“…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.…”

“…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.…”