Effects of avermectins on neurite outgrowth in differentiating mouse neuroblastoma N2a cells

Avermectins (AVMs) are macrocyclic lactone compounds that have been widely used as parasiticides in veterinary and human medicine and as pesticides in agriculture and horticulture. The multidrug resistance transporter, P-glycoprotein (P-gp), is associated with the efflux transport of AVMs and other drugs across the blood-brain and placental barrier, and plays an important role in attenuating the neurotoxicity and developmental toxicity of AVMs. In this study, the mouse neuroblastoma N2a cell line was used to investigate the neurotoxicity of two AVM derivatives: abamectin (ABM) and doramectin (DOR). We found that both these compounds caused significant dose-dependent inhibition of neurite growth in differentiating N2a cells. In addition, Western blotting analysis showed that ABM and DOR significantly inhibited the expression of not only P-gp but also the cytoskeletal proteins, beta-actin and beta-tubulin. This suggests ABM and DOR may inhibit neurite growth by down-regulating the expression of P-gp and cytoskeletal proteins. Furthermore, knockdown of P-gp expression by RNA interference in N2a cells reduced neurite growth even in the absence of ABM and DOR, and reduced it even more in the presence of low levels of these compounds. These results suggest that even subcytotoxic levels of ABM and DOR can be neurotoxic in differentiating cells and that this neurotoxicity may, at least in part, be the result of the down-regulation of P-gp and cytoskeletal proteins.