Thyroid hormone clearance in the paraventricular nucleus of male mice regulates lean mass and physical activity.

INTRODUCTION The actions of thyroid hormones (THs) in the central nervous system (CNS) are relevant to food intake and energy expenditure. TH receptors (TRs) exhibit high expression in brain areas modulating energy balance, including the arcuate, paraventricular (PVN), supraoptic, and ventromedial (VMH) hypothalamic nuclei. METHODS To examine the role of THs in the regulation of energy balance via action in specific hypothalamic nuclei of the adult mouse, we performed experiments of conditional inactivation of DIO3, the enzyme responsible for the clearance of THs, in the lateral hypothalamus (LH), and VMH and PVN hypothalamic nuclei. We accomplished DIO3 genetic inactivation via stereotaxic injection of the AAV-cre vector into adult mice homozygous for a "floxed" Dio3 allele. RESULTS Dio3 inactivation in the LH and VMH of males or females did not result in significant changes in body weight 8-weeks after injection. However, inactivation of Dio3 in the PVN resulted in increased body weight (both fat mass and lean mass) and locomotor activity, and decreased hypothalamic Mc4r expression in male, but not female mice. However, PNV-specific Dio3 KO did not cause hyperphagia. CONCLUSION These results suggest local TH action influences MC4R signaling and possibly other PVN-associated circuitries, with consequences for body composition and energy balance endpoints, but not for orexigenic pathways. They also support a regulatory role for PVN Dio3 in the central regulation of energy homeostasis in adult life.