The limbic system and food-anticipatory circadian rhythms in the rat: ablation and dopamine blocking studies

Rats behaviorally anticipate a fixed, daily opportunity to feed by entrainment of circadian oscillators that are physically separate from the light-entrainable circadian pacemaker that has been localized to the suprachiasmatic nucleus. Neural substrates mediating food-entrained rhythms are unknown. A variety of anatomical and functional observations suggest possible involvement of the limbic system and its dopaminergic component in the regulation of these rhythms. To test this hypothesis, the activity rhythms of rats bearing large, combined ablations of the hippocampus and amygdala or nucleus accumbens and medical forebrain anterior to the thalamus were examined under ad-lib feeding, 2 h daily feeding, and total food deprivation conditions. Some hippocampal-ablated rats showed alterations of free-running rhythms under ad-lib feeding, but none of the ablations impaired the rats' ability to anticipate daily feeding, or 'remember' the phase of feeding time during subsequent food deprivation. Additional groups of intact rats were treated with the dopamine antagonist haloperidol (0.3 mg/kg or 2.0 mg/kg) 30 min prior to daily feeding, but this also did not prevent the emergence of food-entrained rhythms. The limbic and dopamine systems do not appear to play a necessary role in the generation or entrainment of food-anticipatory circadian rhythms.