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Journal of Neuroscience, Vol 15, 7905-7915, Copyright © 1995 by Society for Neuroscience
Junctional and extrajunctional glutamate receptor channels in Drosophila embryos and larvae
K Nishikawa and Y Kidokoro
Gunma University School of Medicine, Maebashi, Japan.
Glutamate receptor channels in Drosophila embryos and larvae were examined
with the patch-clamp technique in various configurations. In the
cell-attached mode, only one type of channel was observed in the
extrajunctional region at any stages. The burst duration histogram was fit
with three exponentials. The burst duration of long component lengthened
with increasing glutamate concentration. In excised outside- out patches
the unitary channel current was 7.1 pA at -60 mV and direction of current
reversed at zero membrane potential. In contrast, junctional receptor
channels had different properties. In the whole- cell configuration,
spontaneous synaptic currents with steps on the falling phase were
observed. The step amplitudes had two discrete values of 9.4 and 18.5 pA at
-60 mV, due to openings of junctional glutamate receptor channels. Synaptic
currents changed amplitudes linearly with the membrane potential in the
negative potential range but nonlinearly above zero. With 1 mM glutamate in
the bath, synaptic currents were no longer observed. Instead, there were
single channel events with the current amplitude varying between 8 and 12
pA at -60 mV. Their long burst duration depended on glutamate concentration
indicating that they are glutamate receptor channel events. The
extrapolated reversal potential of these channel currents was around +12
mV. These junctional receptor channels were strictly localized at the
junction. Our findings suggest that the channel conversion mechanism in
Drosophila is different from that observed in vertebrates. Further close
examination of other intermediate steps during neuromuscular junction
formation is needed.
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