Investigating the safety factor at an invertebrate neuromuscular junction.
Fidelity of synaptic transmission is essential at the neuromuscular junction (NMJ). To ensure that transmission does not fail, vertebrate motoneurons often release more neurotransmitter than is required for muscle contraction. This safety factor allows some loss of synaptic function without failure of muscle contraction. It is not known whether a similar mechanism operates at the invertebrate neuromuscular junction. In our study of the Drosophila NMJ, we find that glutamate receptor mutants can exhibit a substantial decrease in synaptic function while maintaining muscle contraction. The persistence of neuromuscular function in these mutants is not explained by synaptic facilitation, temporal summation of high frequency stimuli, or a hyperpolarizing shift in the activation range of muscle calcium channels. Instead, the attenuated synaptic response is sufficient to drive muscle contraction. Quantitative analysis of the decrease in synaptic transmission in these mutants implies that at the wild-type NMJ there is an approximately five- to ninefold excess in released transmitter. Hence, the presence of a synaptic safety factor is a conserved feature of neuromuscular organization in both invertebrates and vertebrates.
Marrus
Scott B
SB
Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
DiAntonio
Aaron
A
D000200:Action Potentials / Q000235:genetics / Q000528:radiation effects; D000818:Animals; D030801:Animals, Genetically Modified; D002118:Calcium / Q000378:metabolism; D004330:Drosophila; D029721:Drosophila Proteins / Q000172:deficiency* / Q000235:genetics; D004558:Electric Stimulation / Q000379:methods; D005260:Female; D066298:In Vitro Techniques; D007448:Invertebrates; D007814:Larva / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology / Q000528:radiation effects; D009119:Muscle Contraction / Q000502:physiology / Q000528:radiation effects; D009433:Neural Inhibition / Q000235:genetics / Q000528:radiation effects; D009469:Neuromuscular Junction / Q000502:physiology* / Q000528:radiation effects; D018377:Neurotransmitter Agents / Q000378:metabolism; D018408:Patch-Clamp Techniques / Q000379:methods; D017470:Receptors, Glutamate / Q000172:deficiency* / Q000235:genetics; D009435:Synaptic Transmission / Q000502:physiology*
D029721:Drosophila Proteins; D018377:Neurotransmitter Agents; D017470:Receptors, Glutamate; C483908:glutamate receptor III, Drosophila; D002118:Calcium