Enhanced NMDA receptor-mediated modulation of excitatory neurotransmission in the dorsal vagal complex of streptozotocin-treated, chronically hyperglycemic mice.

Eva C Bach,Bret N Smith,Katalin Cs Halmos,Leonardo Barbosa Moraes Resstel

doi:10.1371/journal.pone.0121022

Abstract

A variety of metabolic disorders, including complications experienced by diabetic patients, have been linked to altered neural activity in the dorsal vagal complex. This study tested the hypothesis that augmentation of N-Methyl-D-Aspartate (NMDA) receptor-mediated responses in the vagal complex contributes to increased glutamate release in the dorsal motor nucleus of the vagus nerve (DMV) in mice with streptozotocin-induced chronic hyperglycemia (i.e., hyperglycemic mice), a model of type 1 diabetes. Antagonism of NMDA receptors with AP-5 (100 μM) suppressed sEPSC frequency in vagal motor neurons recorded in vitro, confirming that constitutively active NMDA receptors regulate glutamate release in the DMV. There was a greater relative effect of NMDA receptor antagonism in hyperglycemic mice, suggesting that augmented NMDA effects occur in neurons presynaptic to the DMV. Effects of NMDA receptor blockade on mEPSC frequency were equivalent in control and diabetic mice, suggesting that differential effects on glutamate release were due to altered NMDA function in the soma-dendritic membrane of intact afferent neurons. Application of NMDA (300 μM) resulted in greater inward current and current density in NTS neurons recorded from hyperglycemic than control mice, particularly in glutamatergic NTS neurons identified by single-cell RT-PCR for VGLUT2. Overall expression of NR1 protein and message in the dorsal vagal complex were not different between the two groups. Enhanced postsynaptic NMDA responsiveness of glutamatergic NTS neurons is consistent with tonically-increased glutamate release in the DMV in mice with chronic hyperglycemia. Functional augmentation of NMDA-mediated responses may serve as a physiological counter-regulatory mechanism to control pathological disturbances of homeostatic autonomic function in type 1 diabetes.

Highlights

The dorsal vagal complex of the caudal brainstem, including the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus nerve (DMV), is a center for integrating neural and humoral signals regulating parasympathetic output to the viscera, including the digestive system
NMDA receptors are typically located on neuronal postsynaptic membranes, but they have been identified on presynaptic terminals, where they modulate the release of GABA and glutamate [17,18,19]
The relative effect of AP-5 on mEPSC frequency in neurons from control mice (31.12 ± 5.67% decrease; n = 9) was not different from that observed in neurons from hyperglycemic mice (31.90 ± 6.04% decrease; n = 10; p>0.05; Fig. 3D). These results indicated that preNMDA receptors modulate glutamate release in the DMV to a similar degree in both control and hyperglycemic mice

Summary

Introduction

The dorsal vagal complex of the caudal brainstem, including the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus nerve (DMV), is a center for integrating neural and humoral signals regulating parasympathetic output to the viscera, including the digestive system. Upon activation by glutamate (in the presence of glycine), NMDA receptors typically contribute to membrane depolarization and Ca2+-dependent signaling cascades by increasing the conductance of Na+ and Ca2+ [16]. NMDA receptors are typically located on neuronal postsynaptic membranes, but they have been identified on presynaptic terminals, where they modulate the release of GABA and glutamate [17,18,19]. NMDA receptor function in the dorsal vagal complex is critical for homeostatic regulation of vagal activity and visceral function

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Results

Conclusion