Brain Research, 489 (1989) 369-372 369 Elsevier BRE 23544 Stimulation of dopamine release in the rat neostriatum in vivo by activation of the voltage-sensitive sodium channel by scorpion venom neurotoxin D.J.S. Sirinathsinghji, R.P. Heavens and S.K. Sikdar Department of Neuroendocrinology, AFRC Institute of Animal Physiology and Genetics Research, Babraham, Cambridge (U.K.) (Accepted 21 February 1989) Key words: Scorpion toxin; Sodium channel; Tetrodotoxin; Calcium; EGTA; Dopamine; Push-pull; Neostriatum; Rat Scorpion venom neurotoxins open sodium channels and thus may enhance neurotransmitter release by increasing membrane permeability to sodium. This study carried out in vivo examined the effects of the scorpion Androctonus australis neurotoxin (ScAaTx) on the levels of dopamine (DA) in push-pull perfusates of the striatum of chloral hydrate-anaesthetised rats. ScAaTx (2.5, 5.0 and 10.0 ng//d) stimulated DA release in a dose-dependent manner. The release of DA induced by ScAaTx (10 ng/pl) was completely blocked when the brain site was perfused with Ca2÷-free CSF containing 2 mM EGTA or in the presence of TTX (10-5 M). These results indicate that the potent stimulatory effects of ScAaTx on neostriatal DA release in vivo are mediated via voltage-sensitive sodium channels. The release of a neurotransmitter by an action potential requires a change in the permeability of the nerve membrane to sodium, potassium and calcium. Neurotoxins have been extensively used to study the structure and function of the voltage-sensitive so- dium channel which plays a central role in the generation of action potentials 7. These neurotoxins have been classified in at least three groups accord- ing to their binding and pharmacological properties 7. They include the heterocyclic guanidines such as tetrodotoxin (Trx) and saxitoxin which bind at site I and cause a blockade of sodium flux 7,1°. A second class of lipid-soluble toxins include batrachotoxin and veratridine which bind at site 114,6 and cause a persistent activation of the sodium channel 8. A third group which include the polypeptide a-neurotoxins from scorpion venom and sea anemone nematocysts bind to type III site and act by slowing down or inhibiting sodium channel inactivation 7,11. Several in vitro studies 2 have employed these neurotoxins to study the ionic mechanisms and in particular the voltage-gated mechanisms involved in the release/uptake of neurotransmitters from nerve fibres. In the present study we used the push-pull perfusion technique to study in vivo the activational effects of the scorpion Androctonus australis neuro- toxin (ScAaTx) on the release of dopamine (DA) from the neostriatum of chloral hydrate-anaesthe- tised rats. In addition, we examined the effects of Ca2+-withdrawal from the perfusing medium on the release of DA induced by ScAaTx and also em- ployed TTX to determine if the effects of ScAaTx on neostriatal DA release are mediated via voltage- sensitive sodium channels. Adult male Wistar rats (250-280 g) were anaes- thetised with chloral hydrate (300 mg/kg b. wt., i.p. and supplemented with 90 mg/kg at 1 h intervals) and positioned in a Kopf stereotaxic instrument. Rectal temperature was monitored and maintained at 37 °C. A concentric push-pull cannula (consisting of an inner 30-gauge stainless-steel push-tube which protruded 0.9 mm beyond an outer 22-gauge stain- less-steel pull-tube) was then implanted into one striatum at the following coordinates9:2.0 mm Correspondence: D.J.S. Sirinathsinghji, AFRC Institute of Animal Physiology & Genetics Research Babraham, Cambridge CB2 4AT, U.K. 0006-8993/89/$03.50 ~ 1989 Elsevier Science Publishers B.V. (Biomedical Division)