Figure 1.

High-capacity runners (HCR) were more responsive to the ability of intra-ventromedial hypothalamic (VMH) melanocortin receptor activation to enhance activity-related energy expenditure (EE). Low-capacity runners (LCR) and HCR were given intra-VMH microinjections of the mixed melanocortin receptor agonist Melanotan II (MTII; gray bars) or vehicle (Veh; black bars). Over 4 hours, with a significant interaction where HCR responded more than LCR, intra-VMH MTII significantly increased free-moving EE (A), VO₂ (B), and physical activity (D), while decreasing respiratory exchange ratio (RER) similarly in HCR and LCR (C). (E–H) Intra-VMH MTII also induced changes in EE and RER in rats walking on a treadmill at 7 meters/min for 30 min. MTII increased activity-associated EE (E) and VO₂ (F), and decreased RER (G) in HCR and LCR. HCR showed lower overall walking-induced RER, while intra-VMH MTII produced a larger RER decrease in LCR than HCR (G), whereas HCR were more responsive to MTII-induced enhancement of activity EE (H). *within group, MTII treatment significantly different from veh; **HCR significantly different from LCR within treatment; p<0.05. (N=10)