| Abstract: | Intracellular recordings of the spontaneous activity from mammalian spinal cord neurons in culture demonstrated different sensitivities of excitatory and inhibitory synaptic transmission for the action of tetanus toxin (Tetx) and botulinum toxin type A (Botx). The effects of Tetx and Botx on spontaneous and nerve-evoked transmitter release were compared under identical experimental conditions in experiments on in vitro poisoned mouse diaphragms. At 37 degrees C completely paralyzed endplates are characterized by a very low frequency of spontaneous miniature endplate potentials (m.e.p.p.s) and by a 100% failure to evoke endplate potentials (e.p.p.s) in response to single nerve stimuli. Striking differences in the action of both toxins have been observed when the very low transmitter release probabilities of paralyzed nerve-muscle preparations were increased by tetanic nerve stimulation and/or application of potent K+-channel blockers and/or by reduction of temperature to 25 degrees C. While Botx did not change the short latency between nerve impulse and postsynaptic response, Tetx produced a temporal dispersion of the quantal release suggesting that the toxins act at different sites in the chain of events that result in transmitter release. To find further evidence to support the different actions of the toxins the spontaneous transmitter release was studied in more detail. Tetx blocked preferentially the release of so-called large mode m.e.p.p.s without affecting the frequency of the small mode ones. In contrast, Botx strongly inhibited both the small and large mode m.e.p.p.s.(ABSTRACT TRUNCATED AT 250 WORDS) |
