large neurons A major advantage of some invertebrate model synapses is the large size of the neurons, allowing easy access with microelectrodes to pre-and postsynaptic cells. For instance, in the buccal ganglion of Aplysia californica two large cholinergic neurons contact the same postsynaptic neuron and form a chloride-dependent inhibitory synapse. With this preparation it is possible to microinject toxin into one neuron while the second neuron serves as a control for the excitability of the postsynaptic neuron (Tauc eta/., 1974; Poulain etal., 1986). This preparation is almost as sensitive to botulinum toxins as the vertebrate neuromuscular junction, and the mechanism of poisoning has been extensively investigated (Poulain et a/., 1989). Recently, toxins have also been shown to be active when microinjected into presynaptic compartments of the leech Retzius cell and the squid giant synapse (Bruns and Jahn, 1995, Bruns and Jahn, in preparation; Hunt et a/., 1994; Llinas etal., 1994). Finally, in a recent elegant study, TeTx-L chain was expressed in Drosophila using a promoter that contained the yeast UASGal regulatory-element. TeTx-L chain was thus dependent on the presence of the yeast GAL4 transcription factor. Toxin expression in embryonic neurons eliminated both synaptobrevin and the evoked neurotransmitter release at the neuromuscular junction (Sweeney et a/., 1995). Axonal outgrowth and synapse formation were not affected, suggesting an exclusive role of synaptobrevin in synaptic transmission. This approach has the advantage that numerous GAL4 enhancer-trap lines with different GAL4 expression patterns are available, allowing for the directed expression of TeTx L chain in selected cell types.
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