Melanocortin regulation of histaminergic neurons via perifornical lateral hypothalamic melanocortin 4 receptors

Do POMC neurons have a sweet tooth for leptin? Special issue: Role of nutrients in nervous control of energy balance

Coordinated detection of changes in metabolic state by the nervous system is fundamental for survival. Hypothalamic pro-opiomelanocortin (POMC) neurons play a critical role in integrating metabolic signals, including leptin levels. They also coordinate adaptative responses and thus represent an important relay in the regulation of energy balance. Despite a plethora of work documenting the effects of individual hormones, nutrients, and neuropeptides on POMC neurons, the importance for crosstalk and additive effects between such signaling molecules is still underexplored. The ability of the metabolic state and the concentrations of nutrients, such as glucose, to influence leptin's effects on POMC neurons appears critical for understanding the function and complexity of this regulatory network. Here, we summarize the current knowledge on the effects of leptin on POMC neuron electrical excitability and discuss factors potentially contributing to variability in these effects, with a particular focus on the mouse models that have been developed and the importance of extracellular glucose levels. This review highlights the importance of the metabolic "environment" for determining hypothalamic neuronal responsiveness to metabolic cues and for determining the fundamental effects of leptin on the activity of hypothalamic POMC neurons.

Histaminergic regulation of food intake

Histamine is a biogenic amine that acts as a neuromodulator within the brain. In the hypothalamus, histaminergic signaling contributes to the regulation of numerous physiological and homeostatic processes, including the regulation of energy balance. Histaminergic neurons project extensively throughout the hypothalamus and two histamine receptors (H1R, H3R) are strongly expressed in key hypothalamic nuclei known to regulate energy homeostasis, including the paraventricular (PVH), ventromedial (VMH), dorsomedial (DMH), and arcuate (ARC) nuclei. The activation of different histamine receptors is associated with differential effects on neuronal activity, mediated by their different G protein-coupling. Consequently, activation of H1R has opposing effects on food intake to that of H3R: H1R activation suppresses food intake, while H3R activation mediates an orexigenic response. The central histaminergic system has been implicated in atypical antipsychotic-induced weight gain and has been proposed as a potential therapeutic target for the treatment of obesity. It has also been demonstrated to interact with other major regulators of energy homeostasis, including the central melanocortin system and the adipose-derived hormone leptin. However, the exact mechanisms by which the histaminergic system contributes to the modification of these satiety signals remain underexplored. The present review focuses on recent advances in our understanding of the central histaminergic system's role in regulating feeding and highlights unanswered questions remaining in our knowledge of the functionality of this system.

Impact of the Central Histaminergic and Melanocortin Systems on Leptin-Induced Hypophagia in Neonatal Layer Chicken

The present study aimed to assess the probable impact of the central histaminergic and melanocortin systems on leptin-induced hypophagia in neonatal layer chickens. In experiment 1, the chickens received intracerebroventricular (ICV) injections of the control solution, 250 nmol of α-FMH, 10 µg of leptin, and α-FMH+leptin. Experimental groups 2-8 were injected the same as experiment 1. Nonetheless, the chickens in experiments 2-8 received ICV injections of 300 nmol of chlorpheniramine (H₁ receptor antagonist), 82 nmol of famotidine (H₂ receptor antagonist), 300 nmol of thioperamide (H₃ receptor antagonist), 0.5 nmol of SHU9119 (M₃/M₄ receptors antagonist), 0.5 nmol of MCL0020 (M₄ receptor antagonist), 30 µg of astressin-B (CRF₁/ CRF₂ receptors antagonist), and 30 µg of astressin2-B (CRF₂ receptor antagonist), instead of α-FMH, respectively. Food was provided for the birds immediately following the injection, and 30, 60, and 120 min after the injection, cumulative food intake (g) was measured. The findings pointed out that the ICV injection of leptin diminished food intake in neonatal chickens (P<0.05). The co-administration of M₃/M₄ receptor antagonist+leptin significantly decreased the hypophagic effect of leptin (P<0.05). A significant decrease was also detected in the hypophagic effect of leptin following the co-administration of the M₄ receptor antagonist and leptin (P<0.05). Moreover, the co-injection of the antagonists of CRF₁/CRF₂ receptors and leptin significantly mitigated the hypophagic effect of leptin (P<0.05). The co-injection of CRF₂ receptor antagonist and leptin led to a decrease in the hypophagic effect of leptin. As evidenced by the results of the current study the hypophagic effect of leptin is mediated by the receptors of H₁, H₃, M₃/M₄, and CRF₁/CRF₂ in neonatal layer chicken.

Coordination of metabolism, arousal, and reward by orexin/hypocretin neurons

Orexin/hypocretin neurons located in the lateral hypothalamus play a critical role in the maintenance of arousal and contribute to the regulation of multiple homeostatic and behavioral processes. In this issue of the JCI, Tan and Hang et al. report that feeding a high-fat diet to mice compromised the function of the orexin system, leading to impairments in reward-seeking and active coping mechanisms. The researchers observed changes at the cellular and circuit levels suggesting that reduced excitability of orexin neurons affects behavior through induction of a hypoarousal state.

Antifibrotic and Anti-Inflammatory Actions of α-Melanocytic Hormone: New Roles for an Old Player

The melanocortin system encompasses melanocortin peptides, five receptors, and two endogenous antagonists. Besides pigmentary effects generated by α-Melanocytic Hormone (α-MSH), new physiologic roles in sexual activity, exocrine secretion, energy homeostasis, as well as immunomodulatory actions, exerted by melanocortins, have been described recently. Among the most common and burdensome consequences of chronic inflammation is the development of fibrosis. Depending on the regenerative capacity of the affected tissue and the quality of the inflammatory response, the outcome is not always perfect, with the development of some fibrosis. Despite the heterogeneous etiology and clinical presentations, fibrosis in many pathological states follows the same path of activation or migration of fibroblasts, and the differentiation of fibroblasts to myofibroblasts, which produce collagen and α-SMA in fibrosing tissue. The melanocortin agonists might have favorable effects on the trajectories leading from tissue injury to inflammation, from inflammation to fibrosis, and from fibrosis to organ dysfunction. In this review we briefly summarized the data on structure, receptor signaling, and anti-inflammatory and anti-fibrotic properties of α-MSH and proposed that α-MSH analogues might be promising future therapeutic candidates for inflammatory and fibrotic diseases, regarding their favorable safety profile.

Electrophysiological Properties of Genetically Identified Histaminergic Neurons

Histaminergic neurons of the tuberomammillary nucleus (TMN) are important regulators of behavioral and homeostatic processes. Previous work suggested that histaminergic neurons exhibit a characteristic electrophysiological signature, allowing for their identification in brain slice preparations. However, these previous investigations focused on neurons in the ventral subregion of the TMN of rats. Consequently, it remains unclear whether such electrophysiological properties extend to mice, including other subregions of the TMN, and the potential for differences between males and females. To further characterize the electrophysiological properties of histaminergic neurons, we performed whole-cell patch-clamp recordings on transgenic mice expressing Cre recombinase in histidine decarboxylase (HDC)-expressing cells; the sole enzyme for histamine synthesis (Hdc-cre::tdTomato). Despite similarities with the electrophysiological properties reported in rats, we observed considerable variability in mouse HDC neuron passive membrane properties, action potential firing, and intrinsic subthreshold active membrane properties. Overall, the electrophysiological properties of HDC neurons appeared similar across subregions of the TMN, consistent with a lack of topographical organization in this nucleus. Moreover, we found no obvious sex differences in the electrical excitability of HDC neurons. However, our data reveal a diversity in the electrophysiological properties of genetically identified histaminergic neurons from mice not previously appreciated from rat studies. Thus, these data highlight the utility of mouse genetics to target the widespread histaminergic neuronal population within the TMN and support the idea that histaminergic neurons are a heterogeneous neuronal population.