Two types of motivation revealed by ibotenic acid nucleus accumbens lesions: dissociation of food carrying and hoarding and the role of primary and incentive motivation.

Rats with dopamine-depleting lesions of the nucleus accumbens (NAcc) no longer carry food and leave it in a refuge (carry-to-leave or hoard). The present study replicates this finding and extends it to rats with ibotenic acid lesions. The same rats, however, were found to carry food to a refuge to eat it (carry-to-eat). Thus, within a test session, control rats would first carry food to eat it, and when sated, carry the remaining food to leave it in the refuge. NAcc food-deprived rats carried food to eat it but when sated engaged mainly in exploratory behavior. When carrying-to-eat, NAcc rats modulated responses normally to food size and eating time, their movement speed and motor patterns were normal, and their eating times were normal. Nevertheless, they were slower to return for food after eating, and they did not show normal slowing of eating times with successively presented food pellets. The failure of NAcc rats to carry-to-leave when sated was not related to general inactivity. In activity tests, they were as active in the light, more active in the dark, and did not differ from control rats under amphetamine (1.25 and 2.5 mg/kg). Their impairments were also not related to damage to other forebrain related structures, as they displayed normal neocortical and hippocampal atropine-sensitive and atropine-resistant electroencephalograms (EEG). The finding that NAcc rats carry-to-eat when hungry but do not carry-to-leave when sated suggests two separate conclusions. First, food carrying can be dissociated into two separate actions with respect to neural control, carrying-to-eat and carrying-to-leave. Second, the selective involvement of the NAcc in only carrying-to-leave suggests that it is associated with recruiting actions in response to the secondary incentive features of food but is not required for behaviors associated with food consumption. Other structures are presumably activated by the primary drive-reducing features of food. The results are discussed in relation to two-factor theories of motivation.