Effect of Agouti-Related Protein in Regulation of the Hypothalamic-Pituitary-Thyroid Axis in the Melanocortin 4 Receptor Knockout Mouse

• Published in: Endocrinology (Philadelphia) 2004-11, Vol.145 (11), p.4816-4821
• Main Authors: Fekete, Csaba, Marks, Daniel L, Sarkar, Sumit, Emerson, Charles H, Rand, William M, Cone, Roger D, Lechan, Ronald M
• Format: Article
• Language: English
• Subjects: Adipose tissue; Adipose tissue (brown); Adipose Tissue - anatomy & histology; Adipose Tissue - drug effects; Agouti-Related Protein; Animals; Biological and medical sciences; Body Weight - drug effects; Cerebrospinal fluid; Eating - drug effects; Food intake; Fundamental and applied biological sciences. Psychology; Hypothalamic-pituitary-thyroid axis; Hypothalamo-Hypophyseal System - drug effects; Hypothalamo-Hypophyseal System - physiology; Hypothalamus; Hypothyroidism; Injections, Intraventricular; Intercellular Signaling Peptides and Proteins; Inverse agonists; Leptin; Male; Melanocortin receptors; Mice; Mice, Inbred C57BL; Mice, Knockout; mRNA; Neuropeptides; Organ Size; Paraventricular Hypothalamic Nucleus - drug effects; Paraventricular Hypothalamic Nucleus - physiology; Paraventricular nucleus; Pituitary; Pituitary-Adrenal System - drug effects; Pituitary-Adrenal System - physiology; Proteins; Proteins - pharmacology; Receptor, Melanocortin, Type 4 - genetics; Receptors; RNA, Messenger - metabolism; Satiety; Thyroid; Thyroid gland; Thyroid Gland - drug effects; Thyroid Gland - physiology; Thyrotropin - genetics; Thyrotropin-releasing hormone; Thyroxine; Thyroxine - blood; Ventricles (cerebral); Vertebrates: endocrinology
• ISSN: 0013-7227; 1945-7170
• DOI: 10.1210/en.2004-0476

Agouti-related protein (AGRP) is thought to be one of the neuropeptides mediating the effects of leptin on appetite and satiety. The central administration of AGRP not only stimulates food intake, but also inhibits the hypothalamic-pituitary-thyroid axis (HPT) axis, closely replicating the central hypothyroid state induced by fasting. AGRP binds as an endogenous antagonist or inverse agonist of the central melanocortin receptors but has also been hypothesized to have melanocortin receptor-independent effects. Thus, we determined whether the central effects of AGRP on the HPT axis are altered in mice with selective deletion of the melanocortin 4 receptor (MC4-R). AGRP or artificial cerebrospinal fluid was administered daily into the lateral ventricle of adult, male MC4-R knockout and wild-type (WT) mice for 3 d. AGRP significantly increased the cumulative food intake and weight of white and brown adipose tissue, suppressed circulating levels of T4 [control vs. AGRP in WT (μg/dl): 4.54 ± 0.16 vs. 3.87 ± 21], and inhibited proTRH mRNA content in the hypothalamic paraventricular nucleus of WT mice (control vs. AGRP in WT (density units ± sem): 4.65 ± 0.50 vs. 2.47 ± 0.17). In contrast, no significant effects of AGRP were observed in any of these parameters in the MC4-R knockout mice. These data suggest that AGRP signaling to TRH hypophysiotropic neurons in the paraventricular nucleus is primarily mediated by the MC4-R and therefore, binding to the MC3-R or other putative AGRP receptors may have only a minor role.