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A new scorpion toxin (3751.8 Da) was isolated from the Buthus martensivenom, sequenced and chemically synthesized (sBmTX3). The A‐type current of striatum neurons in culture completely disappeared when 1 μM sBmTX3 was applied (Kd=54 nM), whereas the sustained K+current was unaffected. 125I‐sBmTX3 specifically bound to rat brain synaptosomes (maximum binding=14 fmol mg−1of protein, Kd=0.21 nM). A panel of toxins yet described as specific ligands for K+channels were unable to compete with 125I‐sBmTX3. A high density of 125I‐sBmTX3 binding sites was found in the striatum, hippocampus, superior colliculus, and cerebellum in the adult rat brain.

A new scorpion toxin (3751.8 Da) was isolated from the Buthus martensivenom, sequenced and chemically synthesized (sBmTX3). The A-type current of striatum neurons in culture completely disappeared when 1 μM sBmTX3 was applied ( Kd=54 nM), whereas the sustained K +current was unaffected. 125I-sBmTX3 specifically bound to rat brain synaptosomes (maximum binding=14 fmol mg −1of protein, Kd=0.21 nM). A panel of toxins yet described as specific ligands for K +channels were unable to compete with 125I-sBmTX3. A high density of 125I-sBmTX3 binding sites was found in the striatum, hippocampus, superior colliculus, and cerebellum in the adult rat brain.