Acute High Fat Diet‐Dependent Activation of Synaptic NMDA Receptors Requires Activation of Purinergic and Metabotropic Glutamate Receptors on Rat Dorsal Motor Nucleus of the Vagus Neurons

Abstract

Previous work from this lab has demonstrated that, in rats, restoration of caloric intake following acute high fat diet (aHFD) is dependent upon upregulated glutamatergic signaling in the dorsal motor nucleus of the vagus (DMV), which recruits previously silent synaptic NMDA receptors (NMDARs). We have further shown that NMDARs activation is driven by the activation of extrasynaptic NMDA receptors (NMDARex) in an astrocyte-dependent process, although the exact mechanism involved is still unknown. The current study was designed to test the hypothesis that activation of purinergic and/or metabotropic glutamate receptors (mGluR) by the gliotransmitters ATP and glutamate is responsible for activation of NMDARex and NMDARs. Whole cell patch clamp recordings were made from DMV neurons in thin brainstem slices prepared from male and female Sprague-Dawley rats fed either a control diet (14% kcal from fat) or following acute, 3-5 days, high fat diet (aHFD; 60% kcal from fat) exposure. The presence of activated NMDARs was assessed by the ability of the antagonist AP5 (25µM) to decrease the frequency of miniature excitatory synaptic currents (mEPSC) as well as action potential (AP) firing. The involvement of P2X receptors was investigated using the antagonist PPADS (10µM) and agonist αβMeATP (10µM) while the involvement of group I mGluR was investigated using the antagonist, AIDA (300µM). In aHFD DMV neurons, perfusion with AP5 decreased action potential firing in 8/9 neurons (1.2±0.1 vs 0.2±0.13 AP/sec; P<0.05) and decreased mEPSC frequency in 8/9 neurons (3.18±1.71 vs 2.11±1.12; P<0.05). Application of PPADS blocked the ability of AP5 to decrease AP firing in 6/10 neurons (1.8±0.18 vs 1.5±0.20 AP/sec; P>0.05; P<0.05 vs AP5 alone) and mEPSC frequency (2.57±2.10 vs 2.49±1.63; P>0.05; P<0.05 vs AP5 alone). In contrast, in control DMV neurons, application of the agonist, αβMeATP, uncovered the ability of AP5 to decrease AP firing (1.09±1.31 vs 1.56±1.12 AP/sec; P<0.05) although it did not uncover actions to decrease mEPSC frequency (1.94±0.82 vs 2.17±0.91 P>0.05). In contrast, application of the group 1 mGluR antagonist, AIDA, blocked the ability of AP5 to decrease mEPSC frequency in 7/10 aHFD DMV neurons (1.88±1.45 vs 2.18±1.42; P>0.05), although it failed to block the actions of AP5 to inhibit AP firing in 7/9 neurons (0.74±0.71 vs 0.39±0.53; P<0.05). Taken together, these results show that purinergic and metabotropic glutamatergic signaling are involved in activation of NMDARs on DMV neurons following aHFD exposure. The current study provides a potential mechanism by which caloric balance is restored following aHFD exposure. Specifically, aHFD-dependent release of the brainstem gliotransmitters ATP and glutamate are required for the activation of DMV extrasynaptic and synaptic NMDA receptors. This upregulates NTS-DMV glutamatergic signaling and alters the excitability of central vagal neurocircuits involved in the modulation of gastrointestinal functions including feeding behavior.