Magnesium-dependent block of the light-activated and trp-dependent
conductance in Drosophila photoreceptors.
Hardie, Roger C, and Mart H Mojet.
Cambridge University, Department of Zoology, Downing St, Cambridge CB2 3EJ
UK.
APStracts 2:0232N, 1995.
SUMMARY AND CONCLUSIONS
1. The effect of Mg2+ on the light-sensitive conductance in Drosophila
photoreceptors was examined using whole-cell voltage clamp recordings from
dissociated ommatidia. In wild type (WT) photoreceptors, at resting potential
(-70 mV), Mgo2+ reduces response amplitude by up to ca. 4 fold in the presence
of normal (1.5 mM) Cao2+ and by up to 20-fold in the absence of Cao2+. The
Mg2+ concentration required for 50% maximum block (K1/2) was ca. 1 mM with 1.5
mM Cao2+ and ca. 280 (M in Ca2+-free Ringer's. 2. The Mg2+ block was largely
relieved in photoreceptors of the transient receptor potential mutant (trp):
the maximum block being only ca. 2-fold with a K1/2 of ca. 4 mM in both Ca2+-
free and 1.5 mM Cao2+. 3. The Mg2+ block in WT, but not in trp, was strongly
voltage dependent, being relieved by both hyperpolarization and
depolarization. The Mg2+ block in WT also resulted in slower response kinetics
due to the associated decrease in Ca2+ influx 4. Noise analysis indicates
that, with normal Cao2+, the Mg2+ block in WT is associated with a ca. 10-fold
reduction in effective single channel conductance at resting potential. 5. The
results support the hypothesis that the trp gene encodes a subunit of a light-
sensitive channel which is required for sensitivity to block by Mg2+. The
concentration and voltage dependence of the Mg2+ block suggest it plays an
important physiological role in determining the gain, kinetics and signal-to-
noise of transduction.
Received 5 May 1995; accepted in final form 3 August 1995.
APS Manuscript Number J305-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 14 August 1995.