Melatonin as an Ameliorative Agent Against Cadmium- and Lead-Induced Toxicity in Fish: an Overview

Diverse anthropogenic activities and lack of knowledge on its consequences have promoted serious heavy metal contaminations in different aquatic systems throughout the globe. The non-biodegradable nature of most of these toxic heavy metals has increased the concern on their possible bioaccumulation in aquatic organisms as well as in other vertebrates. Among these aquatic species, fish are most sensitive to such contaminated water that not only decreases their chance of survivability in the nature but also increases the probability of biomagnifications of these heavy metals in higher order food chain. After entering the fish body, heavy metals induce detrimental changes in different vital organs by impairing multiple physiological and biochemical pathways that are essential for the species. Such alterations may include tissue damage, induction of oxidative stress, immune-suppression, endocrine disorders, uncontrolled cell proliferation, DNA damage, and even apoptosis. Although uncountable reports have explored the toxic effects of different heavy metals in diverse fish species, but surprisingly, only a few attempts have been made to ameliorate such toxic effects. Since, oxidative stress seems to be the underlying common factor in such heavy metal–induced toxicity, therefore, a potent and endogenous antioxidant with no side effect may be an appropriate therapeutic solution. Apart from summarizing the toxic effects of two important toxicants, i.e., cadmium and lead in fish, the novelty of the present treatise lies in its arguments in favor of using melatonin, an endogenous free radical scavenger and indirect antioxidant, in ameliorating the toxic effects of heavy metals in any fish species. Graphical Abstract The protective role of melatonin against lead (Pb) and cadmium (Cd) induced toxicities in fish. After entering the body of any fish, Pb and Cd induce generation of harmful free radicals that induce oxidative stress within the target cell/tissue/organ which in turn modulates the gene expression of different important genes responsible for maintaining the antioxidant defense system and inflammatory pathways, thus causing diverse complications in the physiology of the aquatic organism. Melatonin, on the other hand, defends the cell/tissue (marked as an inhibitory symbol) by scavenging these unwanted free radicals and indirectly modulating the antioxidant system to restore the intracellular oxidative load, thus restricting the propagation of the toxicity cascade (marked as cross symbols) and protecting different organs of the fish from harmful toxic effects triggered by Pb or Cd.