Presynaptic and postsynaptic effects of the venom of the Australian tiger snake at the neuromuscular junction.

1. Crude venom (TSV) from the Australian tiger snake (Notechis scutatus scutatus) has both presynaptic and postsynaptic effects at the neuromuscular junctions of toads.2. TSV (50 mug/ml) rapidly blocked indirectly elicited muscle twitches without affecting the compound action potential in the sciatic nerve or twitches elicited by direct stimulation.3. Low concentrations of the venom (1-10 mug/ml) reduced the amplitude of miniature endplate potentials (m.e.p.ps) and inhibited the depolarization of muscle fibres normally caused by carbachol. It was concluded that a fraction of the venom binds to acetylcholine receptors.4. The frequency of m.e.p.ps was at first increased by TSV at a concentration of 1 mug/ml. Occasional, high frequency ;bursts' of m.e.p.ps were recorded in some preparations. The mean frequency of m.e.p.ps appeared to fall after several hours in the venom.5. The quantal content of endplate potentials (e.p.ps) was reduced by the venom. With low concentrations (1 mug/ml), an initial increase in quantal content was often seen. When the quantal content was markedly depressed there was no parallel reduction in the amplitude of nerve terminal spikes recorded extracellularly, though a later fall in size and slowing of time course was often seen.6. There was evidence that TSV eventually changed the normal Poisson characteristics of the spontaneous release of quanta and this may be correlated with electronmicroscopic changes in nerve terminals.7. Tiger snake antivenene counteracted the postsynaptic, but not the presynaptic effects of TSV when they had developed.