Differential feeding behavior and neuronal responses to CCK in obesity-prone and -resistant rats

Deficits in satiation signals are strongly suspected of accompanying obesity and contributing to its pathogenesis in both humans and rats. One such satiation signal is cholecystokinin (CCK), whose effects on food intake are diminished in animals adapted to a high fat diet. In this study, we tested the hypothesis that diet-induced obese prone (OP) rats exhibit altered feeding and vagal responses to systemic (IP) administration of CCK-8 compared to diet-induced obese resistant (OR) rats. We found that CCK (4.0 microg/kg) suppressed food intake significantly more in OP than OR rats. To determine whether enhanced suppression of feeding is accompanied by altered vagal sensory responsiveness, we examined dorsal hindbrain expression of Fos-like immunoreactivity (Fos-Li) following IP CCK injection in OP and OR rats. After 4.0 microg/kg CCK, there were significantly more Fos-positive nuclei in the NTS of OP compared to OR rats. Treatment with 8.0 microg/kg CCK resulted in no significant difference in food intake or in Fos-Li between OP and OR rats. Also, we found that OP rats were hyperphagic on a regular chow diet and gained more weight compared to OR rats. Finally OP rats had decreased relative fat pad mass compared to OR rats. Collectively, these results show that OP rats exhibit a different behavioral and vagal neuronal responses to CCK than OR rats.