Overconsumption of high-fat diets is one of the strongest contributing factors to the rise of obesity rates. Orexin neurons are known to be activated by a palatable high-fat diet and mediate the activation of the mesolimbic reward pathway, resulting in further food intake. While short-term exposure to a high-fat diet is known to induce synaptic plasticity within the mesolimbic pathway, it is unknown if such changes occur in orexin neurons. To investigate this, 3-week-old male rats were fed a palatable high-fat western diet (WD) or control chow for 1 week and then in vitro patch clamp recording was performed. In the WD condition, an activity-dependent long-term depression (LTD) of excitatory synapses was observed in orexin neurons, but not in chow controls. This LTD was presynaptic and depended on postsynaptic metabotropic glutamate receptor 5 (mGluR5) and retrograde endocannabinoid signalling. WD also increased extracellular glutamate levels, suggesting that glutamate spillover and subsequent activation of perisynaptic mGluR5 may occur more readily in the WD condition. In support of this, pharmacological inhibition of glutamate uptake was sufficient to prime chow control synapses to undergo a presynaptic LTD. Interestingly, these WD effects are transient, as extracellular glutamate levels were similar to controls and LTD was no longer observed in orexin neurons after 4 weeks of WD. In summary, excitatory synapses to orexin neurons become amenable to LTD under a palatable high-fat diet, which may represent a homeostatic mechanism to prevent overactivation of these neurons and to curtail high-fat diet consumption.

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N-methyl-D-aspartate receptors (NMDARs) assemble as functionally diverse heterotetramers. Incorporation of the GluN3A subunit into NMDARs alters conventional NMDAR properties by reducing both magnesium sensitivity and calcium permeability. GluN1 together with GluN3A can also form functional receptors that lack a glutamate binding site and instead serve as excitatory glycine receptors (eGlyRs). GluN3A expression is high in early development but naturally declines to low levels in most brain regions by adulthood. Interestingly, GluN3A expression remains elevated in the CA1 of the adult ventral hippocampus (VH), but not in the dorsal hippocampus (DH). The DH and VH are now well-understood to play very different functional roles, with the DH being primarily involved in cognitive functions and the VH in emotional processing. Why GluN3A persists in the adult VH, and the impact its presence has on glutamatergic neurotransmission in the VH is currently unknown. Here, we show that GluN3A remains elevated both at synaptic and extrasynaptic locations in the adult VH, assembling as GluN1/GluN2/GluN3A NMDARs with reduced magnesium sensitivity, as well as GluN1/GluN3A eGlyRs. By comparing various synaptic properties in the DH and VH of wild-type (WT) and GluN3A knockout (KO) mice, we demonstrate that GluN3A persistence in the VH attenuates glutamate release, limits postsynaptic calcium influx through NMDARs, and reduces the magnitude of NMDAR-dependent long-term potentiation. In comparison, GluN3A KO had relatively little effect on these same properties in the DH. In all, our data demonstrate that GluN3A persistence in the VH represents a key modulator of VH excitability and therefore may play a central role in emotional processing.

Objective: High-fat diets (HFD) are thought to disrupt energy homeostasis to drive overeating and obesity. However, weight loss resistance in individuals with obesity suggests that homeostasis is intact. This study aimed to reconcile this difference by systematically assessing body weight (BW) regulation under HFD.Methods: Male C57BL/6 N mice were fed diets with varying fat and sugar in different durations and patterns. BW and food intake were monitored.Results: BW gain was transiently accelerated by HFD (≥40%) prior to plateauing. The plateau was consistent regardless of starting age, HFD duration, or fat/sugar content.Reverting to a low-fat diet (LFD) caused transiently accelerated weight loss, which correlated with how heavy mice were before the diet relative to LFD-only controls.Chronic HFD attenuated the efficacy of single or repetitive dieting, revealing a defended BW higher than that of LFD-only controls.Conclusions: This study suggests that dietary fat modulates the BW set point immediately upon switching from LFD to HFD. Mice defend a new elevated set point by increasing caloric intake and efficiency. This response is consistent and controlled, suggesting that hedonic mechanisms contribute to rather than disrupt energy homeostasis. An elevated floor of the BW set point after chronic HFD could explain weight loss resistance in individuals with obesity.

Hypothalamic inflammation reduces appetite and body weight during inflammatory diseases, while promoting weight gain when induced by high-fat diet (HFD). How hypothalamic inflammation can induce opposite energy balance outcomes remains unclear. We found that prostaglandin E 2 (PGE 2 ), a key hypothalamic inflammatory mediator of sickness, also mediates diet-induced obesity (DIO) by activating appetite-promoting melanin-concentrating hormone (MCH) neurons in the hypothalamus in rats and mice. The effect of PGE 2 on MCH neurons is excitatory at low concentrations while inhibitory at high concentrations, indicating that these neurons can bidirectionally respond to varying levels of inflammation. During prolonged HFD, endogenous PGE 2 depolarizes MCH neurons through an EP2 receptor-mediated inhibition of the electrogenic Na + /K + -ATPase. Disrupting this mechanism by genetic deletion of EP2 receptors on MCH neurons is protective against DIO and liver steatosis in male and female mice. Thus, an inflammatory mediator can directly stimulate appetite-promoting neurons to exacerbate DIO and fatty liver.