Molecular study of stress system in the European sea bass (Dicentrarchus labrax): cloning of different components and effects of essential oil of Lippia alba during stress situation

European sea bass (Dicentrarchus labrax) is one of the most important species in the Mediterranean aquaculture. In this context, fish are subjected to practices that activate the stress system and can adversely affect their health and welfare. To minimise the effects of stress on fish, investigators have begun to examine the use of natural products with anaesthetic properties that are more effective, safer and less expensive than the currently available synthetic drugs. The aim of the present study was to assess the effectiveness of the essential oil of Lippia alba (EOLA) to mitigate the stress response in D. labrax individuals disturbed by persecution. For this purpose, sea basses were subjected to 3 and 6 hours of stress procedure and sedated with two different concentrations of the EOLA, 25 and 50 μL L-1. Partial cDNA sequences of crhbp and trh genes were cloned and their mRNA expression, together with that of crh, pomc, star, nr3c1 and nr3c2, all stress-related genes of the hypothalamus-pituitary-interrenal axis, were analysed in different tissues. Results elucidated that the most conspicuous variations in gene expression were found out for crh at 6 hours, which pattern was inversely proportional to cortisol levels. This may indicate the existence of a negative feedback mechanism exerted by cortisol. Expression of mineralocorticoid and glucocorticoid receptors (nr3c1 and nr3c2) showed a trend that diverged in relation to the belonging tissue, but in hypothalamus it mirrored the variation of crh, highlighting again a role of cortisol in regulating gene expression during the stress response. Furthermore, an appreciable increment in trh mRNA occurred in every treatment after 3 hours, suggesting that this hormone can be involved in the stress response. The best concentration of EOLA in order to reduce stress is represented by 25 μL L-1. On the contrary, at 50 μL L-1, the oil can be itself a stressor and after 6 hours it loses its effectiveness and it degrades.