Abstract
Shaker K(+)-channels are one of several voltage-activated K(+)-channels expressed in Drosophila photoreceptors. We have shown recently that Shaker channels act as selective amplifiers, attenuating some signals while boosting others. Loss of these channels reduces the photoreceptor information capacity (bits s(-1)) and induces compensatory changes in photoreceptors enabling them to minimize the impact of this loss upon coding natural-like stimuli. Energy as well as coding is also an important consideration in understanding the role of ion channels in neural processing. Here, we use a simple circuit model that incorporates the major ion channels, pumps and exchangers of the photoreceptors to derive experimentally based estimates of the metabolic cost of neural information in wild-type (WT) and Shaker mutant photoreceptors. We show that in WT photoreceptors, which contain Shaker K(+)-channels, each bit of information costs approximately half the number of ATP molecules than each bit in Shaker photoreceptors, in which lack of the Shaker K(+)-channels is compensated by increased leak conductance. Additionally, using a Hodgkin-Huxley-type model coupled to the circuit model we show that the amount of leak present in both WT and Shaker photoreceptors is optimized to both maximize the available voltage range and minimize the metabolic cost.
