Acute High‐Fat Diet Induced Regulation of Caloric Intake is Dependent on Activation of Extrasynaptic NMDA Receptors in Dorsal Motor Nucleus of the Vagus Neurons
Authors: Clyburn Courtney , Travagli R. Alberto , Browning Kirsteen N. 
Published: April 2020

During HFD exposure, both humans and experimental animal models undergo a 24hr hyperphagic period before caloric balance is restored within 3–5 days. We have demonstrated previously that regulation is associated with activation of synaptic NMDA receptors (NMDA‐Rsyn) on neurons of the dorsal motor nucleus of the vagus (DMV), which increases motoneuron excitability and efferent control of gastric functions (PMID: 29368945). Previous studies have suggested that activation of extrasynaptic NMDA‐R(ex) may cause a sufficient local depolarization to remove the Mg2+ block on NMDA‐Rsyn, allowing for their activation. The purpose of this study is to test the hypothesis that, following acute HFD exposure, increased activation of DMV NMDA‐Rsyn occurs subsequent to activation of NMDA‐Rex, and is responsible for the regulation of caloric intake.

Whole‐cell patch clamp recordings were made from gastric‐projecting DMV neurons in thin (300μm) brainstem slices of Sprague‐Dawley rats, 4–6 weeks of age. Rats were fed a control or HFD (14%, 60% kcal from fat, respectively) for 3–5 days prior to experimentation. The effects of the NMDA‐Rex antagonist, memantine (30μM), and the glutamate uptake inhibitor, dihydrokainate (DHK; 30μM), on NMDAsyn‐mediated miniature excitatory postsynaptic currents (mEPSCs) and DMV neuronal excitability (action potential firing; AP) were assessed. The ability of 4th ventricular application of memantine or DHK (50 and 20pmol/2μL, respectively) to attenuate or uncover the inhibitory effects of the glutamate antagonist, kynurenic acid (Kyn A, 100pmol/60nL) on gastric tone and motility were recorded in anesthetized rats. Implantation of indwelling 4th ventricular cannulae allowed for chronic administration of memantine (50pmol/2mL); food intake was measured twice daily for 10 days prior to, and throughout, HFD exposure in freely moving rats, to assess the role of NMDA‐Rex on the homeostatic regulation of caloric intake.

Following acute HFD, memantine prevented the inhibitory actions of the NMDA‐Rsyn antagonist AP5 (25mM) on mEPSC frequency and AP firing rate in DMV neurons. In control rats, however, increasing extracellular glutamate concentration with DHK uncovered this inhibitory effect pf AP5. Brainstem microinjection of memantine attenuated the glutamate‐mediated decrease in gastric tone and motility observed following HFD exposure, while DHK uncovered this effect in control rats. Chronic application of memantine attenuated the homeostatic regulation of caloric intake observed following acute HFD exposure (for all data and statistics, see Table
).

These data suggest that the acute HFD‐induced increase in NMDA‐Rsyn activation occurs following NMDA‐Rex activation, although the source of this glutamate remains to be elucidated. This mechanism is also responsible for the restoration of caloric balance. Understanding these mechanisms and, importantly, how they are lost following chronic HFD exposure, may provide a mechanistic understanding hyperphagia and weight gain as well as identify potential therapeutic targets for the treatment of obesity.

Support or Funding Information

NIH DK111667 to KNB NIH F31 118833 to CC

Memantine + AP5 in HFD

DHK + AP5 in CTL

In vitro

mEPSC Frequency

96±6.2% of baseline, N=6

73±6.7% of baseline
, N=13

AP firing rate

111±11.0% of baseline, N=9

73±10.9% of baseline
, N=15

Memantine+ KynA in HFD

DHK + KynA in CTL

Antrum

Corpus

Antrum

Corpus

In vivo

Gastric motility

−4±8.2% of baseline, N=6

−1±7.5% of baseline, N=6

−53±4.6% of baseline
, N=6

−44±9.2% of baseline
, N=6

Gastric tone

−6±10.8 mg, N=6

−15±18.0 mg, N=6

−86±17.3 mg
, N=6

−123±34.23mg
, N=6

4th Ventricular Memantine (AUC)

4th Ventricular PBS (AUC)

Caloric Intake

251±22.9
, N=6

178±16.2, N=6

*P<0.05 compared to controls or baseline from Students T‐Test