Abstract
In these experiments, we examined the age-related differences in voltage sensitive sodium channel expression and concentration-dependent responses to pyrethroids in juvenile and adult rat brain tissue microtransplanted in Xenopus oocytes. Automated western blotting results indicate that adult neurolemma exhibited 2.5-fold higher level of expression of voltage sensitive sodium channels (VSSCs) compared with juvenile neurolemma when normalized to the housekeeping protein β-tubulin. The predominate isoform expressed in both tissues was Nav1.2 with both showing a significant difference from zero. Adult neurolemma, however, expressed 2.8-fold more Nav1.2 than juvenile and also expressed Nav1.6 at a higher level (2.2-fold). In addition, neurolemma tissue microtransplanted into Xenopus oocytes, which showed reconstituted native ion currents in the plasma membrane of oocytes that was sensitive to TTX and abolished by choline ion replacement, functionally demonstrating the presence of VSSC. Increasing concentrations of permethrin and deltamethrin exhibited concentration-dependent increases in TTX-sensitive current from both adult and juvenile tissues. Concentration-dependent response curves were analyzed using the equivalence test and the slopes of the curves were different (p < 0.05). VSSCs associated with juvenile neurolemma was up to 2.5X more sensitive to deltamethrin than VSSCs in adult neurolemma. In contrast, VSSCs from juvenile neurolemma were less sensitive than adult VSSCs at lower concentrations (0.6-0.8X) and more sensitive at higher concentrations (up to 2.4X). However, since the brain concentrations within the assay were higher than what is typically found in humans following realistic exposure levels, approximately 21- (deltamethrin) to 333- (permethrin) times below the threshold for response in rat neurolemma, age-related differences, if any, are not likely to be toxicologically relevant. This indicates that there is no difference seen in pyrethroid effects between juvenile and adults when exposed to normal levels of pyrethroid pesticides. This study provides new insight into addressing the advancement of new high through-out methods for toxicology studies as well addressing important toxicology questions regarding age dependent effects on VSSCs when exposed to pyrethroid pesticides.