Impaired melanocortin pathway function in Prader-Willi syndrome gene-Magel2 deficient mice.

Prader-Willi Syndrome (PWS) is a neurodevelopmental disorder causing social and learning deficits, impaired satiety and severe childhood obesity. Genetic underpinning of PWS involves deletion of a chromosomal region with several genes, including MAGEL2, which is abundantly expressed in the hypothalamus. Of appetite regulating hypothalamic cell types, both AGRP and POMC-expressing neurons contain Magel2 transcripts but the functional impact of its deletion on these cells has not been fully characterized. Here, we investigated these key neurons in Magel2-null mice in terms of the activity levels at different energy states as well as their behavioral function. Using cell type specific ex vivo electrophysiological recordings and in vivo chemogenetic activation approaches we evaluated impact of Magel2 deletion on AGRP and POMC-neuron induced changes in appetite. Our results suggest that POMC neuron activity profile as well as its communication with downstream targets is significantly compromised, while AGRP neuron function with respect to short term feeding is relatively unaffected in Magel2 deficiency.

MCH Neuron Activity Is Sufficient for Reward and Reinforces Feeding.

BACKGROUND: Melanin-concentrating hormone (MCH)-expressing neurons have been implicated in regulation of energy homeostasis and reward, yet the role of their electrical activity in short-term appetite and reward modulation has not been fully understood. OBJECTIVES: We investigated short-term behavioral and physiological effects of MCH neuron activity manipulations. METHODS: We used optogenetic and chemogenetic approaches in Pmch-cre transgenic mice to acutely stimulate/inhibit MCH neuronal activity while probing feeding, locomotor activity, anxiety-like behaviors, glucose homeostasis, and reward. RESULTS: MCH neuron activity is neither required nor sufficient for short-term appetite unless stimulation is temporally paired with consumption. MCH neuronal activation does not affect short-term locomotor activity, but inhibition improves glucose tolerance and is mildly anxiolytic. Finally, using two different operant tasks, we showed that activation of MCH neurons alone is sufficient to induce reward. CONCLUSIONS: Our results confirm diverse behavioral/physiological functions of MCH neurons and suggest a direct role in reward function.

Inactivation of Magel2 suppresses oxytocin neurons through synaptic excitation-inhibition imbalance.

Prader-Willi and the related Schaaf-Yang Syndromes (PWS/SYS) are rare neurodevelopmental disorders characterized by overlapping phenotypes of high incidence of autism spectrum disorders (ASD) and neonatal feeding difficulties. Based on clinical and basic studies, oxytocin pathway defects are suggested to contribute disease pathogenesis but the mechanism has been poorly understood. Specifically, whether the impairment in oxytocin system is limited to neuropeptide levels and how the functional properties of broader oxytocin neuron circuits affected in PWS/SYS have not been addressed. Using cell type specific electrophysiology, we investigated basic synaptic and cell autonomous properties of oxytocin neurons in the absence of MAGEL2; a hypothalamus enriched ubiquitin ligase regulator that is inactivated in both syndromes. We observed significant suppression of overall ex vivo oxytocin neuron activity, which was largely contributed by altered synaptic input profile; with reduced excitatory and increased inhibitory currents. Our results suggest that dysregulation of oxytocin system goes beyond altered neuropeptide expression and synaptic excitation inhibition imbalance impairs overall oxytocin pathway function.

Stress integration by an ascending adrenergic-melanocortin circuit.

Stress is thought to be an important contributing factor for eating disorders; however, neural substrates underlying the complex relationship between stress and appetite are not fully understood. Using in vivo recordings from awake behaving mice, we show that various acute stressors activate catecholaminergic nucleus tractus solitarius (NTSTH) projections in the paraventricular hypothalamus (PVH). Remarkably, the resulting adrenergic tone inhibits MC4R-expressing neurons (PVHMC4R), which are known for their role in feeding suppression. We found that PVHMC4R silencing encodes negative valence in sated mice and is required for avoidance induced by visceral malaise. Collectively, these findings establish PVHMC4R neurons as an effector of stress-activated brainstem adrenergic input in addition to the well-established hypothalamic-pituitary-adrenal axis. Convergent modulation of stress and feeding by PVHMC4R neurons implicates NTSTH → PVHMC4R input in stress-associated appetite disorders.

Adrenergic modulation of melanocortin pathway by hunger signals.

Norepinephrine (NE) is a well-known appetite regulator, and the nor/adrenergic system is targeted by several anti-obesity drugs. To better understand the circuitry underlying adrenergic appetite control, here we investigated the paraventricular hypothalamic nucleus (PVN), a key brain region that integrates energy signals and receives dense nor/adrenergic input, using a mouse model. We found that PVN NE level increases with signals of energy deficit and decreases with food access. This pattern is recapitulated by the innervating catecholaminergic axon terminals originating from NTSTH-neurons. Optogenetic activation of rostral-NTSTH → PVN projection elicited strong motivation to eat comparable to overnight fasting whereas its inhibition attenuated both fasting-induced & hypoglycemic feeding. We found that NTSTH-axons functionally targeted PVNMC4R-neurons by predominantly inhibiting them, in part, through α1-AR mediated potentiation of GABA release from ARCAgRP presynaptic terminals. Furthermore, glucoprivation suppressed PVNMC4R activity, which was required for hypoglycemic feeding response. These results define an ascending nor/adrenergic circuit, NTSTH → PVNMC4R, that conveys peripheral hunger signals to melanocortin pathway.

NTS Catecholamine Neurons Mediate Hypoglycemic Hunger via Medial Hypothalamic Feeding Pathways.

Glucose is the essential energy source for the brain, whose deficit, triggered by energy deprivation or therapeutic agents, can be fatal. Increased appetite is the key behavioral defense against hypoglycemia; however, the central pathways involved are not well understood. Here, we describe a glucoprivic feeding pathway by tyrosine hydroxylase (TH)-expressing neurons from nucleus of solitary tract (NTS), which project densely to the hypothalamus and elicit feeding through bidirectional adrenergic modulation of agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons. Acute chemogenetic inhibition of arcuate nucleus (ARC)-projecting NTSTH neurons or their target, AgRP neurons, impaired glucoprivic feeding induced by 2-Deoxy-D-glucose (2DG) injection. Neuroanatomical tracing results suggested that ARC-projecting orexigenic NTSTH neurons are largely distinct from neighboring catecholamine neurons projecting to parabrachial nucleus (PBN) that promotes satiety. Collectively, we describe a circuit organization in which an ascending pathway from brainstem stimulates appetite through key hunger neurons in the hypothalamus in response to hypoglycemia.