ORX neuroreceptor system and HSP90 are linked to recovery strategies against copper toxicity in Thalassoma pavo

Fish are particularly sensitive to copper (Cu) because although it is an essential metal, it becomes dangerous for aquatic ecosystems, thus accounting for physiological alterations. In this study, we investigated Cu effects on neurobehavioral activities of Thalassoma pavo and, above all, its recovery strategies by evaluating behavioral disturbances, neurodegeneration, and expression of heat shock protein (HSP)90 and orexin receptor (ORXR). Ornate wrasses exposed to nominal sublethal higher (1.07 mg/l) and lower (0.25 mg/l) concentrations of CuCl₂·H₂O mostly reduced swimming and feeding activities along with inducing abnormal behaviors in a dose-dependent manner. Concomitantly, ORXR transcripts were mostly upregulated in the diffuse nucleus of the inferior lobe (NDLI, + 142%) and the corpus of the cerebellum (CCe, + 243%), whereas HSP90 was downregulated only in lateral part of the dorsal telencephalon (Dl, -35%), the nucleus glomerulosus (NG; -40%), and the optic tectum (OT; -33%). Interestingly, when fish were transferred to Cu-free water, some behaviors were promptly rescued, especially in fish previously exposed to the lower Cu concentration. This rescuing tendency was confirmed by evident reductions of argyrophilic signals in Dl (-67%), NG (-31%), and OT (-42%) of fish exposed to 0.25 mg/l Cu. Moreover, transcriptional events of both ORXR and HSP90 were further upregulated in order to orchestrate a reactivation of behavioral and neuronal functions. Overall, this study highlights, for the first time, new neuronal strategies against environmental adverse conditions involving both ORXergic system and HSP90 as key elements that may assure, at least in part, protection and recovery processes against toxic agents.