Journal: Journal of Neurophysiology Title: Analyses of the Facilitatory Effect of Orexin on Eating and Masticatory Muscle Activity in Rats Running Title: Effects of Orexin on Eating Behavior Authors

26 The orexins (Orexin-A and orexin-B) are neuropeptides which are secreted from 27 neurons in the lateral hypothalamus, which participate in the regulation of feeding 28 behavior. It remains to be determined, however, how the orexins exert their effects on 29 feeding behavior, including masticatory movements. To this end, we analyzed food 30 intake behavior and masticatory muscle activity using video-analysis and 31 electromyography (EMG)-recording methods. The results showed that the cumulative 32 food intake over 4 hrs was larger in rats intraventricularly injected with either orexin-A 33 or orexin-B than saline-injected control rats. The latency to eat and the feeding time for 34 a fixed amount of pellets were shortened by injections of orexins in a dose-dependent 35 manner, with a more potent effect by orexin-A than orexin-B. The shorter feeding time 36 corresponded to a decreased number of chewing cycles. EMG recordings from both the 37 digastric and masseter muscles showed two distinct patterns of bursts corresponding to 38 the gnawing and chewing phases. After the injection of orexin-A, the magnitude of the 39 bursts became larger in both phases in the masseter muscle, the burst duration became 40 longer in the chewing phase in the masseter muscle, and the inter-burst interval became 41 shortened in the gnawing phase in both muscles. Consequently, the burst frequency in 42 the chewing phase was increased in the digastric muscle, and conversely, reduced in the 43 masseter muscle. These results suggest that the orexin-A-induced facilitatory feeding 44 behavior is characterized by a dynamic jaw-opener activity which opens the mouth 45 rapidly and powerful jaw-closer activity for crushing the increased amount of food 46 taken into the mouth. The possible involvement of orexin-A in binge eating disorder is 47 discussed. 48