Publications

Abstract

Weight loss inhibits thyrotropic function and reduces metabolic rate, thereby contributing to weight regain. Under negative energy balance there is an increase in the hypothalamic expression of both neuropeptide Y (NPY) and agouti related peptide (AgRP), the endogenous antagonist of melanocortin 4 (MC4) receptors. Both NPY and MC4 receptor antagonism reduce thyrotropic function centrally, but it is not known whether these pathways operate by similar or distinct mechanisms. We compared the time-course of effects of acute or chronic intracerebroventricular (ICV) administration of NPY (1.2nmol acute bolus, or 3.5nmol/day for 6days) or the MC4 receptor antagonist HS014 (1.5nmol bolus, or 4.8nmol/day) on plasma concentrations of thyroid stimulating hormone (TSH) or free thyroxine (T4) in male rats pair-fed with vehicle-infused controls. These doses equipotently induced hyperphagia in acute studies, reduced latency to feed, and increased white adipose tissue mass after 6days of infusion. Acute central NPY but not HS014 administration significantly reduced plasma TSH concentrations within 30-60min and plasma free T4 levels within 90-120min. These inhibitory effects were sustained for up to 5-6days of continuous NPY infusion. HS014 induced a transient decrease in plasma free T4 levels that was observed only after 1-2days of continuous ICV infusion. While both NPY and HS014 significantly increased corticosteronemia within an hour after ICV injection, the effect of NPY was significantly more pronounced and was sustained for up to 4days of administration. Both NPY and HS014 significantly decreased the brown adipose tissue protein levels of uncoupling protein-3. We conclude that central NPY and MC4 antagonism decrease thyrotropic function via partially distinct mechanisms with different time courses, possibly involving glucocorticoid effects of NPY. MC4 receptor antagonism increases adiposity via pathways independent of increased food intake or changes in circulating concentrations of TSH, free T4 or corticosterone.