Central effects of rat versus mouse leptin: ingestive behavior and adipose apoptosis.

Leptin decreases food intake, body weight and fat mass while sparing lean mass. Although mouse leptin (ML) and rat leptin (RL) are 95.9% identical, our impression from previous studies was that they were not equally potent in rats. Thus, in the present study, 0 microg (vehicle) or 10 microg of ML or RL was injected into the lateral ventricle of rats (eight per treatment) once a day for four consecutive days. Body temperature, body weight, food intake and water intake were measured daily. Intrascapular and perirenal brown fat pads, white fat tissues (retroperitoneal, epididymal and inguinal fat pads) and the gastrocnemius muscle were collected and weighed 24h after the last treatments. Body temperature was not affected by either ML or RL. Both ML and RL caused significant reductions in food intake (P<0.05), and there was no difference between them. ML and RL also similarly inhibited water intake on days 2 and 3 (P<0.05). By day 5 both ML- and RL-treated rats had lost weight (11.6 and 15.4 g, respectively), while vehicle-treated rats gained weight (6.8 g). Weights of retroperitoneal and epididymal fat pads, but not other tissues, were reduced similarly by both leptin treatments (P<0.05). However, an increased apoptotic response was detected in the retroperitoneal fat tissue of RL- but not ML-treated rats (P<0.05). These results suggest that RL is more effective than ML in inducing apoptosis in retroperitoneal fat tissue after i.c.v. injection in rats.