Previous studies implicate the hypothalamic ventromedial nucleus (VMN) in glycemic control. Here, we report that selective inhibition of the subset of VMN neurons that express the transcription factor steroidogenic-factor 1 (VMN SF1 neurons) blocks recovery from insulin-induced hypoglycemia whereas, conversely, activation of VMN SF1 neurons causes diabetes-range hyperglycemia. Moreover, this hyperglycemic response is reproduced by selective activation of VMN SF1 fibers projecting to the anterior bed nucleus of the stria terminalis (aBNST), but not to other brain areas innervated by VMN SF1 neurons. We also report that neurons in the lateral parabrachial nucleus (LPBN), a brain area that is also implicated in the response to hypoglycemia, make synaptic connections with the specific subset of glucoregulatory VMN SF1 neurons that project to the aBNST. These results collectively establish a physiological role in glucose homeostasis for VMN SF1 neurons and suggest that these neurons are part of an ascending glucoregulatory LPBN→VMN SF1 →aBNST neurocircuit.glucoregulatory circuit | counter regulation | ventromedial nucleus | bed nucleus of the stria terminalis | hyperglycemia B ecause the brain relies exclusively on glucose as a fuel source, brain function is rapidly compromised when circulating glucose levels drop below the normal range. Consequently, hypoglycemia elicits a robust, integrated, and redundant set of counterregulatory responses (CRRs) that ensure the rapid and efficient recovery of plasma glucose concentrations into the normal range (1). Components of the CRR include increased secretion of the hormones glucagon, epinephrine, and glucocorticoids, inhibition of glucoseinduced insulin secretion, increased sympathetic nervous system (SNS) outflow to the liver, and increased food intake (1-3). Owing to this redundancy, recovery from hypoglycemia is difficult to block in normal humans and animal models, even when adrenal or glucagon responses are prevented. Only when multiple responses are blocked is the ability to recover from hypoglycemia significantly compromised (4). This arrangement is perhaps unsurprising, given the threat to survival posed by hypoglycemia.Although glucose sensing can occur at peripheral (e.g., neurons innervating the hepatic portal vein) as well as central sites (3, 5), the brain is the organ responsible both for transducing this information into effective glucose counterregulation and for terminating this response once euglycemia is restored. Of the many brain areas that have been investigated, the hypothalamic ventromedial nucleus (VMN) has emerged as potentially being both necessary and sufficient to elicit this powerful response. This assertion is based on evidence that, whereas electrical stimulation of the VMN activates the CRR and thereby raises circulating glucose levels (6), glucose infusion directly into the VMN can suppress the CRR during hypoglycemia (7) and thereby impair recovery of normal blood glucose levels (8). Moreover, two recent papers identified a circuit comprise...

Hypothalamic leptin action promotes negative energy balance and modulates glucose homeostasis, as well as serving as a permissive signal to the neuroendocrine axes that control growth and reproduction. Since the initial discovery of leptin 20 years ago, we have learned a great deal about the molecular mechanisms of leptin action. An important aspect of this has been the dissection of the cellular mechanisms of leptin signaling, and how specific leptin signals influence physiology. Leptin acts via the long form of the leptin receptor, LepRb. LepRb activation and subsequent tyrosine phosphorylation recruits and activates multiple signaling pathways, including STAT transcription factors, SHP2 and ERK signaling, the IRS-protein/PI3Kinase pathway, and SH2B1. Each of these pathways controls specific aspects of leptin action and physiology. Important inhibitory pathways mediated by suppressor of cytokine signaling (SOCS) proteins and protein tyrosine phosphatases (PTPases) also limit physiologic leptin action. This review summarizes the signaling pathways engaged by LepRb and their effects on energy balance, glucose homeostasis, and reproduction. Particular emphasis is given to the multiple mouse models which have been used to elucidate these functions in vivo.

SummaryLeptin is a critical regulator of metabolism, which acts on brain receptors (Lepr) to reduce energy intake and increase energy expenditure. Some of the cellular pathways mediating leptin’s anorectic actions are identified, but those mediating the thermogenic effects have proven more difficult to decipher. We define a population of neurons in the dorsomedial hypothalamic nucleus (DMH) containing the RFamide PrRP, which is activated by leptin. Disruption of Lepr selectively in these cells blocks thermogenic responses to leptin and causes obesity. A separate population of leptin-insensitive PrRP neurons in the brainstem is required, instead, for the satiating actions of the gut-derived hormone cholecystokinin (CCK). Global deletion of PrRP (in a loxSTOPlox-PrRP mouse) results in obesity and attenuated responses to leptin and CCK. Cre-recombinase-mediated reactivation of PrRP in brainstem rescues the anorectic actions of CCK, but reactivation in the hypothalamus is required to re-establish the thermogenic effect of leptin.

ObjectiveLeptin acts via its receptor (LepRb) on multiple subpopulations of LepRb neurons in the brain, each of which controls specific aspects of energy balance. Despite the importance of LepRb-containing neurons, the transcriptome and molecular identity of many LepRb subpopulations remain undefined due to the difficulty of studying the small fraction of total cells represented by LepRb neurons in heterogeneous brain regions. Here we sought to examine the transcriptome of LepRb neurons directly and identify markers for functionally relevant LepRb subsets.MethodsWe isolated mRNA from mouse hypothalamic and brainstem LepRb cells by Translating Ribosome Affinity Purification (TRAP) and analyzed it by RNA-seq (TRAP-seq).ResultsTRAP mRNA from LepRb cells was enriched for markers of peptidergic neurons, while TRAP-depleted mRNA from non-LepRb cells was enriched for markers of glial and immune cells. Genes encoding secreted proteins that were enriched in hypothalamic and brainstem TRAP mRNA revealed subpopulations of LepRb neurons that contained neuropeptide-encoding genes (including prodynorphin, Pdyn) not previously used as functional markers for LepRb neurons. Furthermore, Pdyncre-mediated ablation of Leprflox in Pdyn-expressing neurons (LepRbPdynKO mice) blunted energy expenditure to promote obesity during high-fat feeding.ConclusionsTRAP-seq of CNS LepRb neurons defines the LepRb neuron transcriptome and reveals novel markers for previously unrecognized subpopulations of LepRb neurons.

There are many different types of enteric neurons. Previous studies have identified the time at which some enteric neuron subtypes are born (exit the cell cycle) in the mouse, but the birthdates of some major enteric neuron subtypes are still incompletely characterized or unknown. We combined 5-ethynynl-2’-deoxyuridine (EdU) labeling with antibody markers that identify myenteric neuron subtypes to determine when neuron subtypes are born in the mouse small intestine. We found that different neurochemical classes of enteric neuron differed in their birthdates; serotonin neurons were born first with peak cell cycle exit at E11.5, followed by neurofilament-M neurons, calcitonin gene-related peptide neurons (peak cell cycle exit for both at E12.5-E13.5), tyrosine hydroxylase neurons (E15.5), nitric oxide synthase 1 (NOS1) neurons (E15.5) and calretinin neurons (P0). The vast majority of myenteric neurons had exited the cell cycle by P10. We did not observe any EdU+/NOS1+ myenteric neurons in the small intestine of adult mice following EdU injection at E10.5 or E11.5, which was unexpected as previous studies have shown that NOS1 neurons are present in E11.5 mice. Studies using the proliferation marker, Ki67, revealed that very few NOS1 neurons in the E11.5 and E12.5 gut were proliferating. However, Cre-lox-based genetic fate-mapping revealed a small sub-population of myenteric neurons that appears to express NOS1 only transiently. Together, our results confirm a relationship between enteric neuron subtype and birthdate, and suggest that some enteric neurons exhibit neurochemical phenotypes during development that are different from their mature phenotype.