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

  • Excitable Tissues and Central Nervous Physiology
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Abstract

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 (K B 2.5×104 s−1 M−1,P<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 μM 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. The alpha-cyano pyrethroid insecticide, cypermethrin, when applied at 10 nM concentration to this NMJ, caused a sustained 10-fold increase of transmitter quanta (240% of control), and blocked neurally evoked EPSCs after ca. 20 min (22°C). mEPSC frequency remained elevated for several hours after treatment with cypermethrin (ca. 3 Hz), before declining below control levels (0.26 Hz). A model based upon the frequency of mEPSC discharges, the quantal content of evoked responses, and the association constants of calcium to the four binding sites of calmodulin was used to investigate a possible role for this metalloprotein in both the spontaneous and evoked release of neurotransmitter. The probable lack of involvement of calmodulin in the mechanism of neurotransmitter release at the larval housefly neuromuscular junction is discussed in the light of this model, as are the implications of these findings for vesicular amino acid release in relation to pyrethroid insecticide action.

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Abbreviations

[Ca]:

ionic calcium concentration

CaM:

calmodulin

NMJ:

neuromuscular junction

EPSC:

excitatory postsynaptic current (prefix m denotes miniature EPSC)

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Authors and Affiliations

  1. Department of Pharmacology, University of Miami School of Medicine, P.O.B. 016189, 33101, Miami, FI, USA

    G. R. Seabrook

  2. Department of Zoology, University of Nottingham, University Park, NG7 2RD, Nottingham, UK

    I. R. Duce

  3. Wellcome Research Laboratories, Ravens Lane, HP4 2DY, Berkhamsted, Hertfordshire, UK

    S. N. Irving

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  1. G. R. Seabrook
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  2. I. R. Duce
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  3. S. N. Irving
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Seabrook, G.R., Duce, I.R. & Irving, S.N. Spontaneous and evoked quantal neurotransmitter release at the neuromuscular junction of the larval housefly,Musca domestica . Pflugers Arch. 414, 44–51 (1989). https://doi.org/10.1007/BF00585625

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  • DOI: https://doi.org/10.1007/BF00585625

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