Modulation of Glucose Production by Central Insulin Requires IGF-1 Receptors in AgRP Neurons

Similar to insulin, central administration of insulin-like Growth Factor-1 (IGF-1) can suppress hepatic glucose production (HGP), but it is unclear if this effect is mediated via insulin receptors (InsRs) or IGF-1 receptors (IGF-1Rs) in the brain. To this end, we utilized pharmacologic and genetic approaches in combination with hyperinsulinemic-euglycemic clamps to decipher the role of these receptors in mediating central effects of IGF-1 and insulin on HGP. In rats, we observed that intracerebroventricular (ICV) administration of IGF-1 or insulin markedly increased the glucose infusion rate (GIR) by >50% and suppressed HGP (P<0.001). However, these effects were completely prevented by preemptive ICV infusion with an IGF-1R and InsR/IGF-1R Hybrid (HybridRs) blocking antibody. Likewise, ICV infusion of the InsR antagonist, S961, which also can bind HybridRs, interfered with the ability of central insulin, but not IGF-1 to increase the GIR. Furthermore, hyperinsulinemic clamps in mice lacking IGF-1Rs in AgRP neurons revealed ~30% reduction in the GIR in KO animals, which was explained by an impaired ability of peripheral insulin to completely suppress HGP (P<0.05). Signaling studies further revealed an impaired ability of peripheral insulin to trigger ribosomal S6 phosphorylation or PIP3 production in AgRP neurons lacking IGF-1Rs. In summary, these data suggest that attenuation of IGF-1Rs signaling in the MBH, and specifically in AgRP neurons, can phenocopy impaired regulation of HGP as previously demonstrated in mice lacking InsRs in these cells, suggesting a previously unappreciated role for IGF-1Rs and/or HybridRs in the regulation of central insulin/IGF-1 signaling on glucose metabolism.