The contribution of Shaker K+ channels to the information capacity of Drosophila photoreceptors

An array of rapidly inactivating voltage-gated K+ channels is distributed throughout the nervous systems of vertebrates and invertebrates(1-5). Although these channels are thought to regulate the excitability of neurons by attenuating voltage signals, their specific functions are often poorly understood. We studied the role of the prototypical inactivating K+ conductance, Shaker(6,7), in Drosophila photoreceptors(8,9) by recording intracellularly from wild-type and Shaker mutant photoreceptors. Here we show that loss of the Shaker K+ conductance produces a marked reductionin the signal-to-noise ratio of photoreceptors, generating a 50% decrease in the information capacity of these cells in fully light-adapted conditions. By combining experiments with modelling, we show that the inactivation of Shaker K+ channels amplifies voltage signals and enables photoreceptors to use their voltage range more effectively. Loss of the Shaker conductance attenuated the voltage signal and induced a compensatory decrease in impedance. Our results demonstrate the importance of the Shaker K+ conductance for neural coding precision and as a mechanism for selectively amplifying graded signals in neurons, and highlight the effect of compensatory mechanisms on neuronal information processing.