Whole-cell recordings of the light induced current in dissociated Drosophila photoreceptors: evidence for feedback by calcium permeating the light-sensitive channels

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Abstract
Tight seal whole-cell recordings of the light-induced current (LIC) were made from photoreceptors in dissociated Drosophila ommatidia. In dark-adapted cells dim light evokes discrete events (quantum bumps) 4-10 pA in amplitude at resting potential (-55 mV). Flash responses scale linearly with light intensity up to an intensity of at least 100 absorbed photons. The dependence of the LIC's reversal potential on [Ca$_{0}$] suggests that the channels are primarily permeable to Ca$^{2+}$ (P$_{\text{Ca}}$:P$_{\text{Na}}$ ca. 25:1). The current--voltage relation of the LIC is inwardly rectifying up to reversal potential (+ 10 mV in 0.5 mM Ca$_{0}$) and then becomes outwardly rectifying. The time to peak increases by up to five times over the range -80 to +60 mV. Raising [Ca$_{0}$] shortens the time to peak and lessens its dependence on holding potential. It is suggested that there is a sequential positive and negative feedback mediated by Ca$^{2+}$ ions entering the cell during the light response. In support of this hypothesis, raising [Ca$_{0}$] during the response first greatly enhances the LIC and then inhibits it. Hyperpolarizing the cell (which will increase the electromotive force for calcium) during the response also results in a transient increase in conductance followed by inhibition. The transient increase in particular can be so rapid (1-3 ms) that the site of action may be at the channel itself.