The Journal of General Physiology, Vol 75, 381-402, Copyright © 1980 by The Rockefeller University Press
Fast electrical potentials arising from activation of metarhodopsin in the fly
B Minke and K Kirschfeld
The cellular origin and properties of fast electrical potentials arising
from activation of Calliphora photopigment were investigated. It was found
by intracellular recordings that only the corneal-negative M1 phase of fly
M potential arises in the photoreceptors' membrane. This M1 phase has all
the accepted characteristics of an early receptor potential (ERP). It has
no detectable latency, it survives fixation with glutaraldehyde, it is
linear with light intensity below pigment saturation, and it is linear with
the amount of metarhodopsin activated by light. The Calliphora ERP was
found, however, to be exceptional because activation of rhodopsin, which
causes the formation of metarhodopsin in 125 microsecond (25 degrees C),
was not manifested in the ERP. Also, the extracellularly recorded ERP was
not proportional to the rate of photopigment conversion. The
corneal-positive M2 phase of the M potential was found to arise from
second-order lamina neurons (L neurons). Intracellular recordings from
these cells showed a fast hyperpolarizing potential, which preceded the
normal hyperpolarizing transient of these cells. This fast potential
appeared only when metarhodopsin was activated by a strong flash. The data
indicate that the intracellularly recorded positive ERP, which arises from
activation of metarhodoposin, elicits a hyperpolarizing fast potential in
the second-order neuron. This potential is most likely the source of the
corneal-positive M potential.
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