Orexinergic responses to environmental stressors in the zebrafish gut.

The hypothalamic hypocretin (hcrt)/orexinergic (OX) system is known to modulate sleep/wake cycles, arousal, and feeding. While orexinergic circuits have been well-described in the central nervous system, further research is needed to delineate whether gut-associated hcrt/OX signaling represents a conserved feature of vertebrate physiology. The goal of this study was to determine the presence and localization of hypocretin and its receptor in the zebrafish intestine and examine the impact of feeding state and exposure to chronic early life stress. A multimodal visualization and characterization approach was utilized which included a combination of in situ hybridization, quantitative real-time PCR, histological techniques and reporter lines. Expression was observed in the gut mucosa with morphological and anatomic evidence suggesting that epithelial cells are a major source of gut orexin in zebrafish. Feeding state was shown to modulate hcrt abundance and localization within mucosal enterocytes as well as along the vertical base-to-tip axis of the intestinal villar ridges. Gut hcrt expression appears to be positively correlated with the time of feeding. Moreover, exposure to chronic early life stress was found to negatively correlate with gut orexin expression. Altogether our data suggests that suggest gut hypocretin transcription is environmentally responsive with potential implications for the maintenance of physiological homeostasis.