Differential effects of melatonin on plasma levels of thyroxine and triiodothyronine levels in the air-breathing fish, Clarias gariepinus, during breeding and quiescent periods

Melatonin and osmoregulation in fish: A focus on Atlantic salmon Salmo salar smoltification

Part of the life cycle of several fish species includes important salinity changes, as is the case for the sea bass (Dicentrarchus labrax) or the Atlantic salmon (Salmo salar). Salmo salar juveniles migrate downstream from their spawning sites to reach seawater, where they grow and become sexually mature. The process of preparation enabling juveniles to migrate downstream and physiologically adapt to seawater is called smoltification. Daily and seasonal variations of photoperiod and temperature play a role in defining the timing of smoltification, which may take weeks to months, depending on the river length and latitude. Smoltification is characterised by a series of biochemical, physiological and behavioural changes within the neuroendocrine axis. This review discusses the current knowledge and gaps related to the neuroendocrine mechanisms that mediate the effects of light and temperature on smoltification. Studies performed in S. salar and other salmonids, as well as in other species undergoing important salinity changes, are reviewed, and a particular emphasis is given to the pineal hormone melatonin and its possible role in osmoregulation. The daily and annual variations of plasma melatonin levels reflect corresponding changes in external photoperiod and temperature, which suggests that the hormonal time-keeper melatonin might contribute to controlling smoltification. Here, we review studies on (i) the impact of pinealectomy and/or melatonin administration on smoltification; (ii) melatonin interactions with hormones involved in osmoregulation (e.g., prolactin, growth hormone and cortisol); (iii) the presence of melatonin receptors in tissues involved in osmoregulation; and (iv) the impacts of salinity changes on melatonin receptors and circulating melatonin levels. Altogether, these studies show evidence indicating that melatonin interacts with the neuroendocrine pathways controlling smoltification, although more information is needed to clearly decipher its mechanisms of action.

Toxicological effects of clofibric acid and diclofenac on plasma thyroid hormones of an Indian major carp, Cirrhinus mrigala during short and long-term exposures

In the present investigation, the toxicity of most commonly detected pharmaceuticals in the aquatic environment namely clofibric acid (CA) and diclofenac (DCF) was investigated in an Indian major carp Cirrhinus mrigala. Fingerlings of C. mrigala were exposed to different concentrations (1, 10 and 100μgL(-1)) of CA and DCF for a period of 96h (short term) and 35 days (long term). The toxic effects of CA and DCF on thyroid hormones (THs) such as thyroid stimulating hormone (TSH), thyroxine (T4) and triiodothyronine (T3) levels were evaluated. During the short and long-term exposure period TSH level was found to be decreased at all concentrations of CA (except at the end of 14(th) day in 1 and 10μgL(-l) and 21(st) day in 1μgL(-l)) whereas in DCF exposed fish TSH level was found to be increased when compared to control groups. T4 level was found to be decreased at 1 and 100μgL(-l) of CA exposure at the end of 96h. However, T4 level was decreased at all concentrations of CA and DCF during long-term (35 days) exposure period. Fish exposed to all concentrations of CA and DCF had lower level of T3 in both the treatments. These results suggest that both CA and DCF drugs induced significant changes (P<0.01 and P<0.05) on thyroid hormonal levels of C. mrigala. The alterations of these hormonal levels can be used as potential biomarkers in monitoring of pharmaceutical drugs in aquatic organisms.

Effect of alpha-tocopherol on antioxidant enzyme activities and lipid peroxidation in rainbow trout (Oncorhynchus mykiss)

In this study, the effect of alpha-tocopherol on the antioxidant capacity of rainbow trout (Oncorhynchus mykiss) was determined. For this purpose, activities of antioxidant enzymes such as catalase (CAT), peroxidase (POD) and glutathione reductase (GSSG-Rx) were investigated. Enzyme activities were measured at 0 (control), 1, 3 and 5 h after alpha-tocopherol injection. In addition, the levels of malondialdehyde (MDA) as a lipid peroxidation marker were determined in the erythrocytes. The results showed that alpha-tocopherol significantly activated the CAT, POD and GSSG-Rx enzymes as compared with the enzyme activities found in the controls (p < 0.05). However, MDA levels were significantly decreased by alpha-tocopherol treatment (p < 0.05). The results suggest that alpha-tocopherol may have a pro-oxidant tendency at a high dose and cause mild oxidative stress which could modulate signal transduction cascades, redirect gene expression, and influence many cellular responses such as proliferation, differentiation, and reproduction. For this reason, alpha-tocopherol should be used carefully in all applications in relation to fish.

Effect of vitamin E on carbonic anhydrase enzyme activity in rainbow trout (Oncorhynchus mykiss) erythrocytes in vitro and in vivo

Considering that the excessive usage of vitamin E causes hypervitaminosis and thus reduces blood erythrocyte concentrations, therefore it is worth studying how its pharmacological dosage affects the activity of carbonic anhydrase (CA) enzyme found in erythrocytes of rainbow trout (Oncorhynchus mykiss) in vitro and in vivo. Vitamin E inhibited CA enzyme and the IC50 value of the vitamin was 0.039 mM in vitro. Similarly, it was seen that vitamin E inhibited CA enzyme activity after the first hour following vitamin E injections in vivo. The activities of CA in groups of trout given vitamin E injection were measured at 1, 3 and 5 h and the corresponding activities were found to be 772.7 +/- 290.5 (P < 0.05), 1286.4 +/- 378.2 and 1005.7 +/- 436.1 enzyme units (EU) g Hb(-1). The difference over the control was significant (P < 0.05) in the first hour and insignificant at 3 and 5 h (P > 0.05). The activity of CA in the control, which did not contain vitamin E, was determined as 1597.7 +/- 429.0 EU g Hb(-1).