Regulation of single quantal efficacy at the snake neuromuscular junction.

1. Postsynaptic responses to spontaneous quantal transmitter release have been compared among neuromuscular junctions in a thin snake muscle. For each junction the type, diameter, and input conductance, G(in) of the postsynaptic muscle fibre were determined. Particularly among fibres of a given type, G(in) was directly correlated with fibre diameter. 2. Miniature endplate potentials (MEPPs) were recorded intracellularly near endplates visualized with Nomarski optics. Mean MEPP amplitude decreased with increasing G(in) among fibres in one muscle. However, the dependence of mean amplitude upon G(in) was not ohmic, as would be expected if the underlying single quantal currents (miniature endplate currents, MEPCs) were of similar amplitude at all junctions. Instead, the relation between MEPPs and G(in) suggested that mean MEPC amplitudes, calculated as mean MEPP amplitude x G(in), increased with increasing G(in). 3. MEPCs were recorded directly using the two-microelectrode voltage clamp technique. Mean MEPC amplitudes depended systematically on G(in), again such that MEPCs were on average larger in fibres with higher G(in). 4. MEPCs were recorded extracellularly from small regions of endplates (underlying a few nerve terminal boutons). Amplitudes of MEPCs depended on G(in) or fibre diameter in the same manner as amplitudes of MEPCs recorded by intracellular voltage clamp. 5. When the anticholinesterase agent neostigmine was added to the bath, amplitude and duration of MEPPs, MEPCs, and extracellular MEPCs increased. However, the systematic dependence of mean MEPC amplitude on G(in) or fibre diameter remained. 6. Evoked subthreshold endplate potentials (EPPs) were recorded under conditions of low extracellular Ca2+. Endplate currents (EPP amplitudes x G(in)) were systematically larger in fibres with larger G(in), indicating regulation of evoked synaptic current in the muscle. The regulation was found to be due to a combination of increased quantal content and larger single quantal currents in larger (higher G(in)) fibres. 7. Synaptic size, assessed either by area of cholinesterase staining or number of terminal boutons, increased with increasing fibre diameter. Assuming that quantal content is proportional to synaptic size, this relation was sufficient to account for the observed increase in quantal content with increasing G(in) among fibres in the muscle, but was not alone sufficient to account for the observed regulation of evoked current. 8. It is concluded that the efficacy of individual transmitter quanta released at the snake neuromuscular junction is regulated such that large muscle fibres receive larger single quantal currents. Regulation of single quantal current contributes substantially to overall regulation of synaptic strength in the muscle.