Orexin-A/hypocretin-1 Immunoreactivity in the Lateral Hypothalamus is Reduced in Genetically Obese but not in Diet-induced Obese Mice

Abstract

The mechanisms that link diet and body weight are not fully understood. A diet high in fat often leads to obesity, and this in part is the consequence of diet-induced injury to specific hypothalamic nuclei. It has been suggested that a diet high in fat leads to cell loss in the lateral hypothalamus, which contains specific populations of neurons that are essential for regulating energy homoeostasis; however, we do not know which cell types are affected by the diet. We studied the possibility that high-fat diet leads to a reduction in orexin-A/hypocretin-1 (Hcrt1) and/or melanin-concentrating hormone (MCH) immunoreactivity in the lateral hypothalamus. We quantified immuno-labeled Hcrt1 and MCH cells in brain sections of mice fed a diet high in fat for up to 12 weeks starting at 4 weeks of age and found that this diet did not modify the number of Hcrt1- or MCH-immunoreactive neurons. By contrast, there were fewer Hcrt1- (but not MCH-) immunoreactive cells in genetically obese db/db mice compared to wild-type mice. Non-obese, heterozygous db/+ mice also had fewer Hcrt1-immunoreactive cells. Differences in the number of Hcrt1-immunoreactive cells were only a function of the db genotype but not of diet or body weight. Our findings show that the lateral hypothalamus is affected differently in the db genotype and in diet-induced obesity, and support the idea that not all hypothalamic neurons involved in energy balance regulation are sensitive to the effects of diet.

An anatomically distinct subpopulation of orexin neurons project from the lateral hypothalamus to the olfactory bulb

“…An average of 225 orexin‐A‐expressing neurons were present in the ipsilateral hypothalamus in each preparation following quantification of alternate sections separated by 160 μm. Our counts are consistent with previous reports of ∼1000 orexin‐A neurons in each hypothalamus hemisphere when considering our counting frequency (every fourth section) in the anterior–posterior axis (Berteotti et al., 2021; Black et al., 2018; González & Prehn, 2018; Obukuro et al., 2010). This suggests that our estimate of the relative proportion of orexin‐A‐expressing neurons that innervate the olfactory bulb is based on a reasonable estimate of the total population of orexin‐A neurons.…”

“…It remains unclear whether the hypothalamic pathway to the main olfactory bulb is exclusively orexinergic, and if not, what proportion of the pathway orexin neurons comprise. Although the total number of orexin‐expressing neurons is relatively small (Berteotti et al., 2021; Black et al., 2018; González & Prehn, 2018; Obukuro et al., 2010), a current model proposes that they are a heterogeneous population (Sagi et al., 2021). It remains unknown whether the orexinergic input to the main olfactory bulb reflects a distinct subpopulation of the overall orexin population.…”

Orexin-A and endocannabinoids are involved in obesity-associated alteration of hippocampal neurogenesis, plasticity, and episodic memory in mice

“…Since this sequence of events may appear to contrast with other studies showing a linear correlation between memory ability and neurogenesis 13 , 14 , we here specifically studied the phenomenon of neurogenesis-dependent LTP 15 , 18 to unravel this discrepancy in a model of aberrant AHN such as that provided by obese ob/ob and HFD mice. These effects have not been investigated before neither in ob/ob nor HFD mouse models of obesity nor in obese db/db leptin receptor knockout mice, which exhibit AHN alterations similar to those of ob/ob mice 24 despite their reduction of OxA levels in the lateral hypothalamus 66 , 67 .…”

International Union of Basic and Clinical Pharmacology CXIV: Orexin Receptor Function, Nomenclature and Pharmacology

“…Orexin neurons are found in the perifornical, dorsal and lateral hypothalamic areas, as determined in several mammalian species using antibodies against PPO, detection of PPO mRNA, labeling of the orexin neurons with the EGFP gene under the Hcrt promoter (Li et al, 2002) and tracing of the orexin neurons from target areas (more recent studies: Ciriello et al, 2013;Ciriello and Caverson, 2014;Garcia-Garcia et al, 2013;Yokota et al, 2016; reviewed in Sakurai, 2007;Kukkonen, 2013;Sakurai et al, 2021) In humans, the number of orexin neurons is estimated at 80 000 at most and just a few thousand or hundreds in rats and mice (Peyron et al, 1998;Harrison et al, 1999;Fronczek et al, 2005;Thannickal et al, 2000;Fronczek et al, 2007;Thannickal et al, 2009;Obukuro et al, 2010;Black et al, 2018;Gonzalez and Prehn, 2018;Berteotti et al, 2021) (see also IV.C.1. and IV.D.…”

“…For instance, a recent very high impact paper reports about 20 pg/ml in mice (McAlpine et al, 2019), which corresponds to 5.6 pmol/l and total of 6.2 fmol (the plasma volume = 25 g × 80 µl blood/g × 0.55 = 1.1 ml). The reported numbers of orexin neurons in the mouse brain are quite variable, e.g., ranging between 440 and 2300 (Obukuro et al, 2010;Black et al, 2018;Gonzalez and Prehn, 2018;Berteotti et al, 2021). Assuming there were 540 neurons (the median of these studies), every neuron would have to produce 0.011 fmol of PPO to reach this a plasma concentration.…”