## 4 Discussion
### Concentrations of CBZ and Its Metabolites in the Liver of Experimental Animals  
This study assessed the potential concentrations of CBZ and its metabolites in the liver of experimental rodents (common voles) exposed to CBZ concentrations detected in surface water. The exposure was conducted through the CBZ spiked drinking water of the CBZ and the As + CBZ groups. Furthermore, we sought to establish if the concentration level of this drug in the liver could initiate metabolic processes. These evaluations have become imperative because, at the concentrations quantified in surface waters, data regarding potential CBZ concentrations and metabolic pathways in non-target terrestrial mammals are scarce in the literature. However, such investigations have been extensively conducted on aquatic organisms [90; 91; 92]. Based on the findings from this study, the control and the As groups showed CBZ levels below the limit of detection (10 pg/g). These results were expected considering that these two experimental animal groups were not administered drinking water containing CBZ. On the other hand, significantly, the CBZ and As + CBZ groups documented CBZ concentrations above the detection limit. This indicates that CBZ exposure at environmentally relevant concentrations has the potential to accumulate within the liver of the model mammals, even though the observed concentration levels were very low. From the literature, CBZ accumulation in the liver induces the expression of CYP3A4 enzymes, with the predominant metabolic pathway being the formation of CBZ-10-11-epoxide as the primary metabolite, which is then followed by hydroxylation to CBZ-10,11-trans-dihydroxy [91; 93]. There were minor quantities of the primary metabolite, specifically CBZ 10, 11-epoxide detected, but very close to the limit of detection; thus, they were not considered in subsequent evaluations. Since the quantities of the primary metabolite of CBZ were close to the detection limit, we speculate that the concentration levels may not be enough to have triggered the increased expression of CYP3A4 enzymes. Therefore, the metabolisation of CBZ in this study cannot be ascertained. However, long-term exposure beyond that of this study may be conducted in the future to test if the bioavailable levels can induce the enzymatic process leading to CBZ metabolism. These results also indicate that the very high exposure dose of As in the As + CBZ group did not interact with CBZ to enhance the concentrations of the pharmaceutical. The Activity of the Antioxidant Enzyme Glutathione Activity (GPx) in the Blood Plasma of the Experimental Animal Groups  
Glutathione peroxidase (GPx) activities in the experimental animal groups were measured in enzyme unit (U) per millilitre (ml) of blood plasma. In mammals, GPx, together with catalase (CAT) and superoxide dismutase (SOD), constitute an enzymatic antioxidant defence system that curtails or offsets the toxic effects posed by ROS. Glutathione peroxidase is an important selenoenzyme responsible for catalysing the reduction of H2O2 and organic hydroperoxides to water or alcohol, respectively, as a way of lessening their toxicity [94]. Oxidative stress occurs when there is an excessive generation of ROS compared to the antioxidant defence system of the organism. This results in the development of aging and several diseases or pathological conditions [64; 95]. From the results, the GPx activities in the blood plasma of the control and the CBZ groups appeared to show proximate figures, suggesting the GPx responses of the control and the CBZ were similar. The results also appeared to show that the As group had the lowest activity, while the As + CBZ group appeared to have the highest activity. In this study, time-dependent GPx defence responses/activities were not conducted, but their measurement was conducted at the end of the experiment.