Characterization of the corticotropin-releasing hormone family in largemouth bass and their expression profiles in response to feeding and acute and chronic heat stress.

Elevated cortisol levels reduce appetite and inhibit growth in fish, a process mediated by corticotropin-releasing hormone (CRH) family members. However, the specific role of CRH members in appetite regulation in largemouth bass (Micropterus nigricans) remains unclear. In this study, we identified the CRH family members in bass and analysed their expression profiles in response to feeding, acute heat stress and chronic heat stress conditions. Five CRH members (Crha, Crhb, Uts1, Ucn2 and Ucn3) were identified in bass. These genes were highly expressed in the brain and intestinal tissues. Within 24 h of feeding, hypothalamic expression of chra decreased initially, reaching a minimum at 3 h post-feeding, then increased. In contrast, crhb, uts1 and ucn3 expression rose initially, peaking at 3 h, then declined. Ucn2 expression declined significantly throughout a 24-h period. During 24-h acute heat stress, serum cortisol levels and hypothalamic expression of crhb, uts1 and ucn3 increased significantly, peaking after 3 h at 33°C, then declining after heating ceased. Conversely, crha and ucn2 expression decreased. After 8 weeks of chronic heat stress, serum cortisol levels were significantly higher in the 33°C group than in the 25°C group. Hypothalamic expression of crhb and ucn3 significantly upregulated, ucn2 was downregulated, while crha and uts1 remained unchanged. Taken together, the coordinated upregulation of hypothalamic crhb and ucn3 with elevated serum cortisol levels under both acute and chronic heat stress, coupled with their expression patterns, is consistent with a potential anorexigenic role for these peptides. Conversely, the downregulation of ucn2 under these conditions aligns with a potential orexigenic function. This study provides insights for future breeding of heat-tolerant varieties of largemouth bass.