Abstract 114: Metabolic Adaptation During Obesity Is Associated With G Protein Switching By The Angiotensin At1a Receptor In Agrp Neurons

Abstract

Obesity represents the single greatest impediment to continued improvements in hypertension and cardiovascular disease. Prolonged obesity is associated with “Resting Metabolic Rate (RMR) adaptation” (ie, reduced RMR relative to body composition), and RMR adaptation is proposed to explain resistance to weight loss / weight maintenance after obesity is established. Previously we showed that angiotensin II (ANG) Agtr1a (AT1A) receptors expressed in Agouti-related peptide (AgRP) neurons of the hypothalamic arcuate nucleus (ARC) are required for integrative control of RMR. We hypothesize that changes in AT1A signaling in AgRP cells underlie the development of RMR adaptation. ARC slices from mice expressing tdTomato in AgRP cells ( Agrp -Cre x Ai9) were used for cell-attached voltage clamp recording of AgRP neurons. ANG inhibited ~33% of AgRP neurons via an AT1A-dependent mechanism (deemed a “Type 1i” response, which was inhibitable by losartan (LOS) or genetic deletion of AT1A; Firing rate; aCSF:0.85 ± 0.03 vs ANG:0.20 ± 0.04 vs ANG+LOS:0.91 ± 0.11 Hz, p<0.05) that was also blocked by the Gαi inhibitor, pertussis toxin (PTX), but not by the Gαq inhibitor, BIM-46187 (Δ Firing rate: PTX: -0.39 ± 0.09 Hz, p<0.05 vs BIM: 0.10 ± 0.09 Hz). After 10 weeks of high-fat feeding, roughly half of Type 1 cells exhibited a spontaneous switch from the ANG/AT1A-mediated inhibitory (Type 1i) response to an ANG/AT1A-mediated stimulatory response (“Type 1s”; Firing rate; aCSF: 0.86 ± 0.03 vs ANG:1.10 ± 0.12 vs ANG+LOS:0.82 ± 0.03 Hz, p<0.05), which was blocked by BIM-46187, but not by PTX (Δ Firing rate: PTX: 0.07 ± 0.06 Hz vs BIM: 0.49 ± 0.06 Hz, p<0.05). AAV delivery of Cre-inducible DREADDs into the ARC of lean Agtr1a -Cre mice was then used to study the effects of Gαi vs Gαq signaling in Type 1 AgRP cells upon RMR control. Activation of the Gαi-coupled hM4Di DREADD in AT1A-expressing cells of the ARC was sufficient to increase RMR (+25% from baseline, n=8-10, p<0.05), but activation of a Gαq-coupled DREADD had no effect. We conclude (i) that the AT1A receptor is expressed by a unique subtype of AgRP neuron; (ii) that inhibitory AT1A-Gαi signaling in the Type 1 AgRP cell stimulates RMR; and (iii) that loss of AT1A-Gαi coupling in these cells is sufficient to explain obesity-associated RMR adaptation.