Spontaneous and evoked quantal neurotransmitter release at the neuromuscular junction of the larval housefly, Musca domestica.

The release of neurotransmitter was monitored at the neuromuscular junctions of larval housefly ventrolateral muscles 6a and 7a, using intracellular recording, and a loose patch clamp technique to isolate discrete release sites. Transmitter release occurred spontaneously and could also be evoked by neural stimuli. Spontaneous discharges consisted of events which were randomly distributed in time and of bursts of temporally ordered events. Evoked and spontaneous release occurred in a quantal manner. The quantal content of evoked excitatory postsynaptic currents (EPSCs) was dependent upon the extracellular calcium concentration, increasing with a 3.8 power dependency. The relationship between the quantal content of a response and extracellular calcium concentration was offset by the presence of magnesium in the bathing saline. The rates of decay of miniature EPSCs (mEPSCs) and EPSCs were also found to increase with extracellular calcium concentration, consistent with a non-diffusion limited block of the glutamate receptor-channel complex by calcium ions (KB 2.5 x 10(4) s-1 M-1, P less than 0.01). The frequency of random mEPSCs (0.26 +/- 0.32 Hz, n = 24 cells) was independent of the extracellular calcium concentration. Random mEPSCs were not inhibited by 1 microM tetrodotoxin which blocked mEPSC bursts and neurally evoked responses. EPSCs evoked during mEPSC bursts had a significantly lower quantal content than those EPSCs recorded from the same nerve terminal between bursting, indicating that both of these forms of release recruited quanta from a common pool of transmitter. Following a neurally evoked EPSC the mEPSC frequency was potentiated severalfold, this delayed release was influenced by EPSCs with large quantal contents evoked in saline containing elevated calcium concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)