The Journal of General Physiology, Vol 79, 361-385, Copyright © 1982 by The Rockefeller University Press
Light-induced reduction in excitation efficiency in the trp mutant of Drosophila
B Minke
In the transient receptor potential (trp) mutant of Drosophila, the
receptor potential appears almost normal in response to a flash but quickly
decays to baseline during prolonged illumination. Photometric and early
receptor potential measurements of the pigment suggest that the pigment is
normal and that the decay of the trp response during illumination does not
arise from a reduction in the available photopigment molecules. However,
there is reduction in pigment concentration with age. Light adaptation
cannot account for the decay of the trp response during illumination: in
normal Drosophila a dim background light shortens the latency and rise time
of the response and also shifts the intensity response function (V-log I
curve) to higher levels of light intensity with relatively little reduction
in the maximal amplitude (Vmax) of response. In the trp mutant, a dim
background light or short, strong adapting light paradoxically lengthens
the latency and rise time of the response and substantially reduces Vmax
without a pronounced shift of the V-log I curve along the I axis. The
effect of adapting light on the latency and V-log I curve seen in trp are
associated with a reduction in effective stimulus intensity (reduction in
excitation efficiency) rather than with light adaptation. Removing
extracellular Ca+2 reduces light adaptation in normal Drosophila, as
evidenced by the appearance of "square" responses to strong illumination.
In the trp mutant, removing extracellular Ca+2 does not prevent the decay
of the response during illumination.
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