Fish brain, sodium channels

Pyrethroid insecticides (deltamethrin, NRDC 157, cismethrin), DDT analogs ( p,j> -DDT, (>,j> -DDT, methoxychlor, EDO), and a DDT-pyrethroid hybrid compound (GH401) enhanced veratridine-dependent sodium uptake by mouse brain synaptosomes The effectiveness of these compounds in the sodium uptake assay was in good agreement with their acute mammalian toxicities. , -DDT also enhanced veratridine-dependent sodium uptake by fish brain synaptosomes These findings demonstrate the utility of ion flux assays to study interactions of insecticides with sodium channels in the central nervous system and to explore species differences in insecticide target site sensitivity ... 

Sodium uptake assay. Assays using mouse brain synaptosomes were performed as described previously (6., ), except that insecticides were introduced to resuspended synaptosomes in 0.2-0.4 yl of ethanol rather than as a residue in the incubation tube. This amount of ethanol improved the delivery of insecticides, thereby increasing the reproducibility of the assay, and had no measurable effect on veratridine-dependent sodium channel activation. These methods were also used for assays with fish brain membranes, except that all buffers were augmented with sucrose to give osmolarities equivalent to the 0 7 M sucrose used for membrane isolation. 

DDT significantly enhanced veratridine-dependent uptake at all veratridine concentrations above 10 pM, and maximum sodium uptake in the presence of both compounds was achieved at 50 pM, which is the approximate for veratridine alone These findings suggest that , -DDT notuonly enhances veratridine-dependent activation but also increases the potency of veratridine. This result contrasts with the interaction of deltamethrin and veratridine in mouse brain synaptosomes (Figure 1) where the insecticide enhanced activation but did not affect the potency of the activator The only previous report of insecticide-dependent potency shifts in sodium channel activation is in neuroblastoma cells, where pyrethroids increase the potency of batrachotoxin and dihydrograyanotoxin II as sodium channel activators, but do not alter the potency of veratridine (5). These experiments provide the basis for further studies to compare directly the selectivity and sensitivity of sodium channels from mammalian and fish brain in their interactions with insecticides ... 

Our preliminary results with fish brain preparations suggest that ion flux techniques may be valuable in studies of target site differences between species. We have demonstrated veratridine-stimulated, tetrodotoxin-sensitive sodium uptake in a vesicular preparation from fish brain, thus confirming the presence of functional sodium channels in this preparation. Our results with , -DDT in this system also agree well with the action of DDT analogs and pyrethroids in mouse brain assays. Further studies wih both preparations should allow the exploration of target site differences between mammals and fish that have been inferred from whole animal toxicity studies. 

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