1.6 Toxic Effects of Paracetamol on Fish
Realistic levels of exposure to paracetamol seem to significantly activate the antioxidant response in fish, mechanistically similarly to what was described for other
taxa. In fact, fish species such as Oncorhynchus mykiss (rainbow trout) were shown
to be extremely responsive to paracetamol in ecologically relevant levels, as
described by Ramos et al. [68]. This study showed that acute and chronic exposure
of these trouts to paracetamol resulted in the activation of glutathione peroxidase,
glutathione reductase, and glutathione-S-transferases that was not however efficient
enough to prevent the establishment of oxidative damage, reflected by a significant
increase of lipid peroxidation. It is important to stress that such results, especially
those obtained following the chronic exposure, were attained at low, realistic levels
of exposure, increasing the ecological relevance of the entire set of results. On the
contrary, individuals of the European eel (Anguilla anguilla) seemed to be more
refractory to paracetamol, as demonstrated by Nunes et al. [27]. Despite the occurrence of biological responses, none of the tested metabolic or oxidative stress biomarkers signaled the occurrence of significant modifications after a 48 h exposure to
ecologically relevant levels of paracetamol. Indeed, these conditions were not able to
alter catalase or glutathione-S-transferase levels, suggesting the absence of an
antioxidant response. The combination of low levels (albeit relevant) + short duration of duration may not have been enough to attain the conditions required to
establish such a condition of oxidative stress. However, cholinesterasic activity of
exposed fish was significantly impaired, suggesting a new, previously unsuspected
manifestation of toxicity by paracetamol: neurotoxicity due to the direct denaturation
of enzymatic forms by ROS resulting from paracetamol metabolism. This possibility
opens new possibilities, considering that neurotoxic effects may be in direct relationship with behavioral alterations, which are always of extreme ecological relevance. In fact, this finding was again suggested as a main toxicological mechanism
underlying the effects observed in the neotropical freshwater fish species Phaloceros
harpagos, as shown by Matus et al. [28]. In this study, authors reported a significant
behavioral alteration, namely for fish exposed to the highest levels of paracetamol
(80 mg L
À1 ), which showed an altered preference in terms of scototaxis, i.e.,
preference for dark/light compartments in the aquaria. Fish exposed to such high
levels of paracetamol were more prone to place themselves in a light area, which is a
nonnatural behavior, considering that dark areas provide better refuge from predators. Despite being attained at high and thus non-ecologically relevant level of
paracetamol, this altered trend evidenced the association between alteration in
cholinesterasic activity (reported in previous studies) and behavioral modifications.
The acute and chronic effects of paracetamol on cholinesterases of fish were also
demonstrated by Pereira et al. [69]. In addition to antioxidant and metabolic
responses (viz., with the increase of glutathione-S-transferases activity, after acute
exposure), cholinesterasic activity was significantly increased in exposed fish. This
was assumed as a surprising result, since the most frequently used effect criteria
involving cholinesterases are their inhibition, not their increase. Among others,
Ecotoxicological Effects of the Drug Paracetamol: A Critical Review of Past. . .
139
Realistic levels of exposure to paracetamol seem to significantly activate the antioxidant response in fish, mechanistically similarly to what was described for other
taxa. In fact, fish species such as Oncorhynchus mykiss (rainbow trout) were shown
to be extremely responsive to paracetamol in ecologically relevant levels, as
described by Ramos et al. [68]. This study showed that acute and chronic exposure
of these trouts to paracetamol resulted in the activation of glutathione peroxidase,
glutathione reductase, and glutathione-S-transferases that was not however efficient
enough to prevent the establishment of oxidative damage, reflected by a significant
increase of lipid peroxidation. It is important to stress that such results, especially
those obtained following the chronic exposure, were attained at low, realistic levels
of exposure, increasing the ecological relevance of the entire set of results. On the
contrary, individuals of the European eel (Anguilla anguilla) seemed to be more
refractory to paracetamol, as demonstrated by Nunes et al. [27]. Despite the occurrence of biological responses, none of the tested metabolic or oxidative stress biomarkers signaled the occurrence of significant modifications after a 48 h exposure to
ecologically relevant levels of paracetamol. Indeed, these conditions were not able to
alter catalase or glutathione-S-transferase levels, suggesting the absence of an
antioxidant response. The combination of low levels (albeit relevant) + short duration of duration may not have been enough to attain the conditions required to
establish such a condition of oxidative stress. However, cholinesterasic activity of
exposed fish was significantly impaired, suggesting a new, previously unsuspected
manifestation of toxicity by paracetamol: neurotoxicity due to the direct denaturation
of enzymatic forms by ROS resulting from paracetamol metabolism. This possibility
opens new possibilities, considering that neurotoxic effects may be in direct relationship with behavioral alterations, which are always of extreme ecological relevance. In fact, this finding was again suggested as a main toxicological mechanism
underlying the effects observed in the neotropical freshwater fish species Phaloceros
harpagos, as shown by Matus et al. [28]. In this study, authors reported a significant
behavioral alteration, namely for fish exposed to the highest levels of paracetamol
(80 mg L
À1 ), which showed an altered preference in terms of scototaxis, i.e.,
preference for dark/light compartments in the aquaria. Fish exposed to such high
levels of paracetamol were more prone to place themselves in a light area, which is a
nonnatural behavior, considering that dark areas provide better refuge from predators. Despite being attained at high and thus non-ecologically relevant level of
paracetamol, this altered trend evidenced the association between alteration in
cholinesterasic activity (reported in previous studies) and behavioral modifications.
The acute and chronic effects of paracetamol on cholinesterases of fish were also
demonstrated by Pereira et al. [69]. In addition to antioxidant and metabolic
responses (viz., with the increase of glutathione-S-transferases activity, after acute
exposure), cholinesterasic activity was significantly increased in exposed fish. This
was assumed as a surprising result, since the most frequently used effect criteria
involving cholinesterases are their inhibition, not their increase. Among others,
Ecotoxicological Effects of the Drug Paracetamol: A Critical Review of Past. . .
139