Q. To examine how a certain hormone affects food intake and weight growth, a researcher is undertaking an animal study. Two groups of mice are used in the study; one group receives the hormone agonist, while the other receives a placebo. The results show that animals given the agonist consumed more food and gained weight than mice given a placebo. These findings point to a potential role for the hormone in stimulating food intake and weight growth in mice, which may have implications for treating metabolic diseases like obesity. Which of the following hormone's would have the most direct effect on hunger stimulation without other effects?