While leptin and insulin signaling in rodents appear to be much simpler than in humans, it appears that their signaling is improved when the rodents are fed a lower fat diet.
Obviosly the type of fat used is critical. I know omega-3 fatty acids and monounsaturated fats improve insulin signaling and probably leptin signaling as well. Saturated fats, trans fats and excess omega-6 polyunsaturated fats most likely are the real culprits here.