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Acceptors for BoNT have been detected autoradiographically on the terminal membrane of motor nerves at a density of approximately 150/micron2 and shown to mediate toxin internalization, a process deemed essential for its inhibition of transmitter release. DTX, a protein with pronounced central neurotoxicity, was shown to induce convulsive states in hippocampal slices from guinea-pig. Synaptic transmission was facilitated and spontaneous epileptiform activity produced in intact cell populations. Voltage clamp analysis of hippocampal neurones revealed that DTX specifically attenuated a transient voltage-dependent K+ conductance (A-current) and this could account for the excitatory effects observed. Proteinaceous acceptors with high affinity for DTX were identified on brain synaptosomal membranes and found to contain a 65 000 Mr polypeptide. Their location in rat brain regions was established and contrasted with that of the binding sites for beta-bungarotoxin. These findings indicate the usefulness of DTX as a probe for a protein associated with one variety of K+ channel while the larger subunit of BoNT was found to interact with a membraneous component that resides at cholinergic nerve terminals and, hence, is likely to have a unique role.