Abstract P324: Role of GABAergic Signaling in AgRP Neurons in the Dipsogenic and Metabolic Responses to Brain Angiotensin

Abstract

Obesity is a major risk factor for development of hypertension, and evidence supports a role for the suppression of resting metabolic rate (RMR) in the development and maintenance of obesity. Angiotensin II (ANG) AT1A receptors, localized to cells expressing the leptin receptor and Agouti-related peptide (AgRP) within the arcuate nucleus of the hypothalamus (ARC), are involved in transcriptional control of AgRP and γ-aminobutyric acid (GABA) synthetic genes ( Gad65 , Gad67 , Slc32a1 ) within the ARC, as well as the integrative control of thermogenic sympathetic nerve activity (SNA) and RMR responses in vivo to various stimuli including leptin and high fat diet. These data led us to hypothesize that AT1A functions to suppress GABA signaling in AgRP neurons, ultimately to disinhibit SNA and RMR. Treatment of cultured mouse hypothalamic N43/5 cells (which express endogenous Agtr1a , AgRP and Gad67 ) with 100 nM ANG for 1 hour (n=6) resulted in reduced AgRP (0.57-fold of control, p=0.03) and Gad67 (0.72-fold of control, p=0.09) mRNA. Mice with targeted disruption of Slc32a1 (vesicular GABA transporter; VGAT) in AgRP neurons (VGAT AgRP-KO mice) were generated on a mixed background. Compared to littermates (male n=9-17, female n=7-8), VGAT AgRP-KO mice (male n=8-13, female n=2-3) at 8 weeks of age exhibited normal body mass (genotype p=ns, sex p<0.05, interaction p=ns), no changes in food intake (genotype p=ns, sex p=ns, interaction p=ns), but a sex-dependent genotype effect on fluid intake was observed (male con 4.5±0.3 vs KO 5.1±0.4; fem con 5.1±0.4 vs KO 3.7±0.6 mL/d; interaction p<0.05). A preliminary study of male mice instrumented for chronic infusion of ANG (5 ng/hr, 21 days icv) was also performed. This caused modest increases in drinking (con+aCSF n=7, 3.3±0.8, con+ANG n=4, 5.2±1.0, KO+ANG n=3, 7.9±1.1 mL/d, p<0.05) but no change in energy efficiency (11.2±2.9, 11.3±3.5, 10.1±4.1 mg/kcal), indicating no change in energy expenditure, possibly due to the mixed background of this strain. We conclude that ANG acts at AgRP cells of the ARC to suppress GABA and AgRP signaling, and this contributes to energy and fluid homeostasis in a complex, strain- and sex-dependent manner.