TPH1 and obesity due to melanocortin 4 receptor deficiency: To examine whether mast cell serotonin contributes to obesity and insulin resistance, mice lacking functional mast cells (KitW-sh/W-sh) were injected with saline (Kitsham) or in vitro-cultured bone marrow-derived mast cells (BMMCs) from Tph1+/+ (KitTph1+/+) or Tph1−/− (KitTph−/−) mice and fed a HFD (Fig. 2a).