Orexin-GABAAergic cross-talking events during lead-induced neurotoxicity of fish

Recent studies have focused their attention on cross-talking mechanisms between orexin (ORXR) and GABAA (GABAAR) receptors that are known to play a regulatory role on different physiological activities, although no information is available about their role during environmental toxicity especially in fish. In view of this, we exposed the ornate wrasses Thalassoma pavo for 96 hours to 1.6 mg/L of PbNO3 in combination with daily intraperitoneal administrations of GABAAR agonist (muscimol 0.1 μg/g body weight) or its antagonist (bicuculline 1 μg/g body weight) and data were compared to both Pb-exposed fish and controls, i.e. fish that was never exposed to any treatments. From behavioral analyses, it appeared that muscimol completely neutralized Pb-dependent enhancement of time spent swimming, as displayed by its great (p<0.01) capacity of reducing this behavior while bicuculline moderately (p<0.05) strengthened metal-induced feeding reductions. At molecular level, in situ hybridization revealed that Pb+muscimol treatment accounted for moderate reductions of ORXR mRNA levels in some encephalic regions such as the torus longitudinalis (-37%) whereas very great (p<0.001) up-regulations were detected in the corpus of the cerebellum (+91%). As expected, Pb+bicuculline induced a general downregulation in most brain areas. Interestingly, amino cupric silver stain technique showed a protective property of muscimol against Pb-mediated neurodegenerative mechanisms, that were instead strongly enhanced following bicuculline administration. Overall, these data supply important indications about GABAAR-ORXR cross-talking events during metal neurotoxicity of fish.