Hypothalamic Pomc Neurons Innervate the Spinal Cord and Modulate the Excitability of Premotor Circuits

From Mammals to Insects: Exploring the Genetic and Neural Basis of Eating Behavior

Obesity and anorexia are life-threatening diseases that are still poorly understood at the genetic and neuronal levels. Patients suffering from these conditions experience disrupted regulation of food consumption, leading to extreme weight gain or loss and, in severe situations, death from metabolic dysfunction. Despite the development of various behavioral and pharmacological interventions, current treatments often yield limited and short-lived success. To address this, a deeper understanding of the genetic and neural mechanisms underlying food perception and appetite regulation is essential for identifying new drug targets and developing more effective treatment methods. This review summarizes the progress of past research in understanding the genetic and neural mechanisms controlling food consumption and appetite regulation, focusing on two key model organisms: the fruit fly Drosophila melanogaster and the mouse Mus musculus. These studies investigate how the brain senses energy and nutrient deficiency, how sensory signals trigger appetitive behaviors, and how food intake is regulated through interconnected neural circuits in the brain.

Intraperitoneal administration of α-melanocyte stimulating hormone (α-MSH) suppresses food intake and induces anxiety-like behavior via the brain MC4 receptor-signaling pathway in goldfish

α-Melanocyte stimulating hormone (α-MSH) is a peptide hormone released from the intermediate lobe of the pituitary which regulates body pigmentation. In addition to the pituitary, α-MSH is also produced in the midbrain, and exerts both anorexigenic and an anxiogenic actions. Acyl ghrelin and cholecystokinin are peripheral hormones derived from the digestive tract which affect the brain to control food intake and feeding behavior in vertebrates. In the present study, hypothesizing that plasma α-MSH may also stimulate the brain and exert central effects, we examined whether peripherally administered α-MSH affects food intake and psychomotor activity using a goldfish model. Intraperitoneal (IP) administration of α-MSH at 100 pmol g-1 body weight (BW) reduced food consumption and enhanced thigmotaxis. These α-MSH-induced actions were blocked by intracerebroventricular administration of HS024, an antagonist of the melanocortin 4 receptor (MC4R), at 50 pmol g-1 BW, whereas these actions were not attenuated by pretreatment with an IP-injected excess amount of capsaicin, a neurotoxin that destroys primary sensory (vagal and splanchnic) afferents, at 160 nmol g-1 BW. Transcripts for the MC4R showed higher expression in the diencephalon in other regions of the brain. These results suggest that, in goldfish, IP administered α-MSH is taken up by the brain, and also acts as anorexigenic and anxiogenic factor via the MC4R signaling pathway.

Advanced neurobiological tools to interrogate metabolism

Engineered neurobiological tools for the manipulation of cellular activity, such as chemogenetics and optogenetics, have become a cornerstone of modern neuroscience research. These tools are invaluable for the interrogation of the central control of metabolism as they provide a direct means to establish a causal relationship between brain activity and biological processes at the cellular, tissue and organismal levels. The utility of these methods has grown substantially due to advances in cellular-targeting strategies, alongside improvements in the resolution and potency of such tools. Furthermore, the potential to recapitulate endogenous cellular signalling has been enriched by insights into the molecular signatures and activity dynamics of discrete brain cell types. However, each modulatory tool has a specific set of advantages and limitations; therefore, tool selection and suitability are of paramount importance to optimally interrogate the cellular and circuit-based underpinnings of metabolic outcomes within the organism. Here, we describe the key principles and uses of engineered neurobiological tools. We also highlight inspiring applications and outline critical considerations to be made when using these tools within the field of metabolism research. We contend that the appropriate application of these biotechnological advances will enable the delineation of the central circuitry regulating systemic metabolism with unprecedented potential.

Perforated Patch Clamp Recordings in ex vivo Brain Slices from Adult Mice

Intracellular signaling pathways directly and indirectly regulate neuronal activity. In cellular electrophysiological measurements with sharp electrodes or whole-cell patch clamp recordings, there is a great risk that these signaling pathways are disturbed, significantly altering the electrophysiological properties of the measured neurons. Perforated-patch clamp recordings circumvent this issue, allowing long-term electrophysiological recordings with minimized impairment of the intracellular milieu. Based on previous studies, we describe a superstition-free protocol that can be used to routinely perform perforated patch clamp recordings for current and voltage measurements.

Increased body weight in mice with fragile X messenger ribonucleoprotein 1 (Fmr1) gene mutation is associated with hypothalamic dysfunction

Mutations in the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene are linked to Fragile X Syndrome, the most common monogenic cause of intellectual disability and autism. People affected with mutations in FMR1 have higher incidence of obesity, but the mechanisms are largely unknown. In the current study, we determined that male Fmr1 knockout mice (KO, Fmr1-/y), but not female Fmr1-/-, exhibit increased weight when compared to wild-type controls, similarly to humans with FMR1 mutations. No differences in food or water intake were found between groups; however, male Fmr1-/y display lower locomotor activity, especially during their active phase. Moreover, Fmr1-/y have olfactory dysfunction determined by buried food test, although they exhibit increased compulsive behavior, determined by marble burying test. Since olfactory brain regions communicate with hypothalamic regions that regulate food intake, including POMC neurons that also regulate locomotion, we examined POMC neuron innervation and numbers in Fmr1-/y mice. POMC neurons express Fmrp, and POMC neurons in Fmr1-/y have higher inhibitory GABAergic synaptic inputs. Consistent with increased inhibitory innervation, POMC neurons in the Fmr1-/y mice exhibit lower activity, based on cFOS expression. Notably, Fmr1-/y mice have fewer POMC neurons than controls, specifically in the rostral arcuate nucleus, which could contribute to decreased locomotion and increased body weight. These results suggest a role for Fmr1 in the regulation of POMC neuron function and the etiology of Fmr1-linked obesity.

Loss of Agrp1 in zebrafish: Effects on the growth and reproductive axis

Loss of agouti related neuropeptide (AgRP) does not lead to overt phenotypes in mammals unless AgRP neurons are ablated. In contrast, in zebrafish it has been shown that Agrp1 loss of function (LOF) leads to reduced growth in Agrp1 morphant as well as Agrp1 mutant larvae. Further, it has been shown that multiple endocrine axes are dysregulated upon Agrp1 LOF in Agrp1 morphant larvae. Here we show that adult Agrp1 LOF zebrafish show normal growth and reproductive behavior in spite of a significant reduction in multiple related endocrine axes namely reduced expression in pituitary growth hormone (gh) follicle stimulating hormone (fshb) as well as luteinizing hormone (lhb). We looked for compensatory changes in candidate gene expression but found no changes in growth hormone and gonadotropin hormone receptors that would explain the lack of phenotype. We further looked at expression in the hepatic and muscular insulin-like growth factor (Igf) axis which appears to be normal. Fecundity as well as ovarian histology also appear largely normal while we do see an increase in mating efficiency specifically in fed but not fasted AgRP1 LOF animals. This data shows that zebrafish can grow and reproduce normally in spite of significant central hormone changes and suggests a peripheral compensatory mechanism additional to previously reported central compensatory mechanisms in other zebrafish neuropeptide LOF lines.

Islet MC4R Regulates PC1/3 to Improve Insulin Secretion in T2DM Mice via the cAMP and β-arrestin-1 Pathways

Melanocortin-4 receptor (MC4R) plays an important role in energy balance regulation and insulin secretion. It has been demonstrated that in the pancreas, it is expressed in islet α and β cells, wherein it is significantly correlated with insulin and glucagon-like peptide-1 (GLP-1) secretion. However, the molecular mechanism by which it regulates islet function is still unclear. Therefore, in this study, our aim was to clarify the signaling and target genes involved in the regulation of insulin and GLP-1 secretion by islet MC4R. The results obtained showed that in islet cells, the expression of prohormone convertase 1/3 (PC1/3), which is correlated with islet GLP-1 and insulin secretion, increased significantly under the action of the MC4R agonist, NDP-α-MSH, but decreased under the action of the MC4R antagonist, AgRP. Additionally, we observed that to exert their regulatory functions in the islets, cAMP and β-arrestin-1 acted as important signaling mediators of MC4R, and compared with control islets, the cAMP, PKA, and β-arrestin-1 levels corresponding to NDP-α-MSH-treated islets were significantly elevated; however, in AgRP-treated islets, their levels decreased significantly. Islets treated with the PKA inhibitor, H89, and the ERK1/2 inhibitor, PD98059, also showed significant decreases in PC1/3 expression level, indicating that the cAMP and β-arrestin-1 pathways are significantly correlated with PC1/3 expression. These findings suggest that islet MC4R possibly affects PC1/3 expression via the cAMP and β-arrestin-1 pathways to regulate GLP-1 and insulin secretion. These results provide a new theoretical basis for targeting the molecular mechanism of type 2 diabetes mellitus.