Leptin-inhibited PBN neurons enhance counter-regulatory responses to hypoglycemia in negative energy balance

Hypoglycemia initiates the counter regulatory response (CRR), in which the sympathetic nervous system, glucagon, and glucocorticoids restore glucose to appropriate concentrations. During starvation, low leptin restrains energy utilization, enhancing long-term survival. To ensure shortterm survival during hypoglycemia in fasted animals, the CRR must overcome this energy-sparing program and nutrient depletion. Here, we identify in mice a previously unrecognized role for leptin and a population of leptin-regulated neurons that modulate the CRR to meet these challenges. Hypoglycemia activates leptin receptor (LepRb) and cholecystokinin (CCK)expressing neurons of the parabrachial nucleus (PBN), which project to the ventromedial hypothalamic nucleus. Leptin inhibits these cells and Cckcre-mediated ablation of LepRb enhances Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:http://www.nature.com/authors/editorial_policies/license.html#terms Correspondence: Martin G Myers, Jr., MD, PhD, Departments of Internal Medicine and Molecular and Integrative Physiology, University of Michigan, 1000 Wall St; 6317 Brehm Tower, Ann Arbor, MI 48105, Phone: 734-647-9515, Fax: 734-232-8175, [email protected]; Lora K. Heisler, PhD, Rowett Institute of Nutrition and Health, University of Aberdeen, Aberdeen, UK AB21 9SB, Tel: 01224 437446, Fax: 01224 437300, [email protected]. Author contributions: JNF and CMP produced the data in Figs. 1-5 and Supplemental Figs. 1-10, with the exception of the data in Figures 1e-I (produced by PBG and LS) and measurements of catecholamines, which were performed by PM, J-MTW and RTK. MG and MR helped produce Supplemental Figs 2, 5, and 10. JCJ aided with Figures 2-4 and with animal genotyping and husbandry. GD’A, ASG, and LKH performed the experiments in Fig. 6. Adenoviral tracers were produced by AKS and CJR. Experimental design, interpretation, and manuscript preparation were led by MGM, LKH, JNF, CMP, ASG and DPO. Europe PMC Funders Group Author Manuscript Nat Neurosci. Author manuscript; available in PMC 2015 June 01. Published in final edited form as: Nat Neurosci. 2014 December ; 17(12): 1744–1750. doi:10.1038/nn.3861. E uope PM C Fuders A uhor M ancripts E uope PM C Fuders A uhor M ancripts the CRR. Inhibition of PBN LepRb cells blunts the CRR, while their activation mimics the CRR in a CCK-dependent manner. PBN LepRbCCK neurons represent a crucial component of the CRR system, and may represent a therapeutic target in hypoglycemia. Hypoglycemia and glucoprivation (which mimics low glucose availability by interfering with cellular glucose metabolism) activate a neurohormonal counter-regulatory response (CRR) that stimulates the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS) to promote glucose release into the bloodstream1,2. The SNS also acts on pancreatic islets to promote glucagon release and suppress insulin secretion3. The CRR serves to restore normoglycemia and protect the brain/body from damage due to hypoglycemia. An appropriately robust CRR is crucial to prevent cognitive impairment, unconsciousness, or even death when blood glucose levels fall too low. This response is especially crucial to counteract insulin-induced hypoglycemia (IIH) in diabetic patients, for whom the risk of hypoglycemia (especially during the night and other periods of fasting) represents the most serious limitation to achieving tight glycemic control2,4. Defining the neural systems that mediate and modulate the CRR to hypoglycemia will reveal mechanisms that represent potential targets for the prevention and therapy of this life-threatening complication of insulin therapy. Diminished nutritional reserves present a particularly severe challenge to mounting an appropriate CRR to hypoglycemia; not only does fasting predispose to hypoglycemia5 and deplete stores of glycogen and gluconeogenic substrates, but also negative energy balance initiates a neuroendocrine starvation response–decreasing overall SNS tone and initiating energy-sparing changes in endocrine function6. Teleologically, it would thus make sense to deploy a more robust CRR in the face of depleted energy stores, although such a system has not been described previously. The energy-conserving response to starvation results in large part from decreased circulating concentrations of the adipose-derived hormone leptin, which is produced in proportion to fat stores6-9. In general, low leptin signals the insufficiency of energy reserves to decrease energy utilization and promote hunger, along with other adaptations to cope with decreased energy availability, including alterations in anxiety, motivation, locomotor activity, glucose homeostasis and a wide range of other behavioral and physiologic parameters10-14. Here, we test the hypothesis that low leptin also enhances the CRR to permit an appropriately robust response to hypoglycemia in the context of decreased nutritional reserves.