Proceedings of The Physiological Society
University of Cambridge (2008) Proc Physiol Soc 11, PC103
Biophysical model of Drosophila photoreceptor
Z. Song1,2, D. Coca1, S. Billings1, M. Juusola2
1. Department of Automatic Control and System Engineering, University of Sheffield, Sheffield, United Kingdom. 2. Department of Biomedical Science, University of Sheffield, Sheffield, United Kingdom.
To gain understanding how complex bio-molecular interactions govern the conversion of light stimuli into voltage responses in the fly eye, we generated a simplified biophysical model of a Drosophila photoreceptor that included photo-sensitive and photo-insensitive membrane. Photoreceptors transform images falling on the eyes into electrical signals and transmit that information toward the brain for updating neural representations of the visual world. Unlike in mammalian retina, where rods and cones are specialized for dim and bright vision, respectively, Drosophila photoreceptors can reliably transform the combined range of nocturnal and diurnal intensity changes into electrical responses(1). However, relatively little is known about dynamic interactions that enable such a powerful adaptation. By constructing a mathematical model of a Drosophila photoreceptor, based on experimentally measured parameters, we aim to learn more how feedback interactions within and between the photo-sensitive and photo-insensitive membrane provide the necessary gain control mechanisms for light-adaptation. It is believed that photo-transduction cascade, which translates light-quanta into light current (i.e. trans-membrane current-responses), happens mainly in the photo-sensitive part (rhadomere) of fly photoreceptors, whereas the photo-insensitive membrane helps to convert light current into a well-defined voltage response. Accordingly, there are two parts in our model. The dynamics of light current are simulated by a simplified cascade model of the known biochemical interactions within rhadomere, giving rise to realistic light current responses. These signals then drive a model of the photo-insensitive cell body, which uses Hodgkin-Huxley-formalism to approximate the dynamics of the known voltage-gated ion-channels(2). In order to understand the relative roles of the different parts in the phototransduction cascade, our model of photo-sensitive membrane has a simplified structure. It contains only first order linear differential equations and static nonlinearities, whereupon complicated responses to different inputs are regulated by feedback loops - believed to be the key mechanisms of optimal adaptation. The combined model is validated by performing intracellular measurements from Drosophila photoreceptors to light stimuli in vivo and by comparing these to the model output for similar inputs. Even in this relatively basic form, our model can predict well the waveforms of voltage responses to simple light inputs. From a practical and systemic point of view, this model can now serve as a preprocessing module for high-order models of the Drosophila visual system that we intend to build due course.
Where applicable, experiments conform with Society ethical requirements