Central administration of melanocortin ligands has been used as a critical technique to study energy homeostasis. While intracerebroventricular (ICV) injection is the most commonly used method during these investigations, intrathecal (IT) injection can be equally efficacious for the central delivery of ligands. Importantly, intrathecal administration can optimize exploration of melanocortin receptors in the spinal cord. Herein, we investigate comparative IT and ICV administration of two melanocortin ligands, the synthetic MTII (Ac-Nle-c[Asp-His-DPhe-Arg-Trp-Lys]-NH2) MC4R agonist and agouti-related peptide [AGRP(87-132)] MC4R inverse agonist/antagonist, on the same batch of age-matched mice in TSE metabolic cages undergoing a nocturnal satiated paradigm. To our knowledge, this is the first study to test how central administration of these ligands directly to the spinal cord affects energy homeostasis. Results showed, as expected, that MTII IT administration caused a decrease in food and water intake and an overall negative energy balance without affecting activity. As anticipated, IT administration of AGRP caused weight gain, increase of food/water intake, and increase respiratory exchange ratio (RER). Unexpectantly, the prolonged activity of AGRP was notably shorter (2 days) compared to mice given ICV injections of the same concentrations in previous studies (7 days or more).1-4 It appears that IT administration results in a more sensitive response that may be a good approach for testing synthetic compound potency values ranging in nanomolar to high micromolar in vitro EC50 values. Indeed, our investigation reveals that the spine influences a different melanocortin response compared to the brain for the AGRP ligand. This study indicates that IT administration can be a useful technique for future metabolic studies using melanocortin ligands and highlights the importance of exploring the role of melanocortin receptors in the spinal cord.
Bibliographical noteFunding Information:
Animal studies were performed by D.N.A, M.M.L., and S.L.W. Experimental design is contributed to C.J.L. and D.N.A. Data statistics were performed by D.N.A. and A.M.G. Analysis and interpretations were performed by D.N.A., C.J.L., and C.H.-L. The manuscript was written by D.N.A. with contributions by all authors. Funding These experiments were supported by NIH Grants R01DK091906 and R01DK108893 (C.H.-L.). This work was also supported by a 2017 Engebretson Drug Design & Development Grant from the College of Pharmacy at the University of Minnesota (C.H.-L). C.J.L. was provided support from the University of Minnesota Doctoral Dissertation Fellowship. C.J.L. was provided additional support by the University of Minnesota College of Pharmacy Olsteins Graduate Fellowship. Notes The authors declare no competing financial interest.