Food motivation in hypothalamic hyperphagic rats reexamined

Abstract

The food motivation of rats made hyperphagic with ventromedial hypothalamic (VMH) knife cuts was examined in bar pressing tests on variable interval (VI) and fixed ratio (FR) schedules. Compared to normal rats, nondeprived VMH rats did not bar press reliably more for food during brief tests on a VI schedule, but did respond significantly more when prefed just prior to VI testing, and when tested on FR schedules. VMH rats also bar pressed more than controls during 24 hr/day tests on a FR-64 schedule. During 24 hr/day tests on a VI schedule, in which they were rewarded with entire meals, VMH rats overate by taking large meals, but they did not bar press more than controls to obtain these meals. When prevented from eating large meals the VMH rats continued to overeat by increasing their meal frequency and total VI responses, but their responses/meal again did not differ from controls. Food-deprived VMH rats did not respond more than did controls during VI or FR tests, although they ate more when food was freely available. The results indicate that under ad lib conditions VMH rats are motivated to eat larger or more frequent meals than do normal rats, although their motivation to initiate any given meal is not elevated, but that under conditions of food deprivation and reduced food accessibility VMH rats are not more motivated to eat than are normal animals.