Effects of Tityus serrulatus scorpion venom and its toxin TsTX-V on neurotransmitter uptake in vitro

Scorpion neurotoxins targeting the Nav channel can be classified into two classes: α- and β-neurotoxins and are reported as highly active in mammalian brain. In this work, we evaluate the effects of Tityus serrulatus venom (Ts venom) and its α-neurotoxin TsTX-V on γ-aminobutyric acid (GABA), dopamine (DA) and glutamate (Glu) uptake in isolated rat brain synaptosomes. TsTX-V was isolated from Ts venom by ion exchange chromatography followed by reverse-phase (C18) high-performance liquid chromatography. Neither Ts venom nor TsTX-V was able to affect 3H-Glu uptake. On the other hand, Ts venom (0.13 μg/mg) significantly inhibited both 3H-GABA and 3H-DA uptake (∼ 50%). TsTX-V showed IC50 values of 9.37 μM and 22.2 μM for the inhibition of 3H-GABA and 3H-DA uptake, respectively. These effects were abolished by pre-treatment with tetrodotoxin (TTX, 1 μM), indicating the involvement of voltage-gated Na+ channels in this process. In the absence of Ca2+, and at low Ts venom concentrations, the reduction of 3H-GABA uptake was not as marked as in the presence of Ca2+. TsTX-V did not reduce 3H-GABA uptake in COS-7 cells expressing the GABA transporters GAT-1 and GAT-3, suggesting that this toxin indirectly reduces the transport. The reduced 3H-GABA uptake by synaptosomes might be due to rapid cell depolarization as revealed by confocal microscopy of C6 glioma cells. Thus, TsTX-V causes a reduction of 3H-GABA and 3H-DA uptake in a Ca2+-dependent manner, not directly affecting GABA transporters, but, in consequence of depolarization, involving voltage-gated Na+ channels.