PMCH and parasitic infectious disease: Castration of male mice [19] or lipopolysaccharide administration [20] decrease the number of Hcrt expressing neurons whereas an agonist of the estrogen receptor α decreases the number of MCH expressing neurons [21] and parasitic infection with Trypanosoma brucei reduces both Hcrt and MCH cell numbers [22], indicating that the cell number of these hypothalamic populations may be modulated in response to an environmental challenge.