Food- and light-entrained circadian rhythms in rats with hypocretin-2-saporin ablations of the lateral hypothalamus

Daily feeding schedules can entrain circadian rhythms of food-anticipatory activity in mammals. The site of the circadian oscillators that drive food-entrained rhythms is unknown. Lateral hypothalamic (LH) neurons containing hypocretins (Hcrt1 and 2, also known as orexin A and B) regulate feeding, energy metabolism and arousal state, raising the possibility that they may also participate in the entrainment of activity rhythms by a daily mealtime. To examine this, Hcrt neurons in rats were ablated by LH injections of Hcrt2 conjugated to the ribosome-inactivating protein saporin. To assess photic entrainment and masking, drinking activity was recorded continuously in LD 12:12 for ∼6 weeks, in DD for 48 h, and in LD 2:2 for 24 h. To assess food-entrainment, drinking and food cup activity were recorded for 4–7 weeks during which food was restricted to a 3-h daily meal beginning 6 h after lights-on. Lesions were assessed by immunocytochemistry or inspection of Nissl stained sections. Hcrt cell depletion ranged from 0 to 100%. Lesions were associated with hypophagia, hypodypsia and weight loss. Despite reduced mean daily drinking, there was no significant effect on the shape or amplitude of the circadian waveforms in LD, LD 2:2 or DD at ∼6 weeks after surgery. All rats exhibited drinking or food cup activity in anticipation of the daily meal, indicative of circadian entrainment. These results indicate that the Hcrt system modulates ingestive behaviors but does not play a necessary role in the entrainment or expression of food-anticipatory circadian rhythms.