Effects of acute exposure to 2-phenoxyethanol, clove oil, MS-222, and metomidate on primary and secondary stress responses in Senegalese sole (Solea senegalensis Kaup 1858)

The present study investigated possible stress effects in juvenile Solea senegalensis caused by exposure for 5, 10, 20 and 30min to the anaesthetic agents 2-phenoxyethanol (500mgL⁻¹), clove oil (80mgL⁻¹), MS-222 (200mgL⁻¹) and metomidate (6mgL⁻¹). The effects were also assessed 24h after the longest exposure (30min), and after killing the fish with a sharp blow to the head. Levels of cortisol, haematocrit, haemoglobin, glucose and lactate were measured in the fish as indicators of stress. The plasma concentrations of cortisol increased significantly after 20min exposure to 2-phenoxyethanol, and after 20 and 30min exposure to clove oil and MS-222. The greatest increases occurred after 30min exposure to clove oil and MS-222, and 20min exposure to 2-phenoxyethanol. With metomidate, significant increases in cortisol levels were only observed after 10min exposure, and the magnitude of the increases was much lower than after exposure to the other anaesthetics. None of the anaesthetic agents induced significant changes in the haematocrit or haemoglobin levels. By contrast, the concentration of glucose increased significantly after 10, 20 and 30min exposure to MS-222, after 20 and 30min exposure to clove oil and after 30min exposure to 2-phenoxyethanol and to metomidate. The concentrations of lactate increased significantly after 20 and 30min exposure to clove oil and to MS-222, and after 30min exposure to 2-phenoxyethanol and to metomidate. In all cases the levels of the stress indicators returned to control values (5min exposure), 24h after 30min exposure to the anaesthetics. The results indicate that exposure to the anaesthetics induces a stress effect, characterised by stimulation of the hypothalamus–pituitary–interrenal (HPI) axis and to a much lesser extent, the sympathetic nervous system-associated chromaffin tissue.