Modelling of light responses of Drosophila Photoreceptor
It remains unclear how a network of photoreceptors and interneurons, whose responses are shaped together through feedforward and feedback synapses, co-process visual information. We began examining this question in a relatively simple eye of Drosophila by constructing a mathematical model, which describes voltage responses of R1-R6 photoreceptors to light stimuli. This model is proposed as an early input stage for later network processing in this supposedly visual ''motion''-pathway.
Photoreceptors transform images projected on the eyes into voltage responses that are transmitted toward the brain for further processing and to update the neural representation of the visual world. Drosophila photoreceptors also receive feedbacks from higher order neurons that shape their responses further. However, relatively little is known how these dynamic interactions function. By constructing a model of a Drosophila photoreceptor, based on experimentally measured parameters, we aim to learn more about feedback interactions within photoreceptors and between photoreceptors and higher-order neurons.
A fly photoreceptor consists of a photo-sensitive part (rhabdomere) and a photo-insensitive part (body). It is believed that photo-transduction cascade, which translates light-quanta into light current, happens in the photo-sensitive part of photoreceptors, whereas the photo-insensitive membrane converts light current into a voltage response. Accordingly, there are two parts in our model. The first part of the model simulates the photo-transduction cascade based on known biochemical interactions of major proteins (rhodopsin, metarhodopsin, G-protein, PLC, PIP2, DAG, Na+/Ca2+,exchanger, CAM), several of which are feedback targets for Ca2+ fluxing in via light-gated channels [1]. These trans-membrane currents then drive the second part of the model: the photo-insensitive body, which uses Hodgkin-Huxley-formalism to approximate the dynamics of the known voltage-gated ion-channels [3].
The rhabdomere part of the model is simulated for bright and dim inputs separately to produce macroscopic light-induced currents; integrated by run down currents from ~30,000 microvilli. For dim impulses, assuming that photons are absorbed by different microvilli and integrated in a linear way, then macroscopic light induced current is obtained by convolution of the latency distribution (Gillespie algorithm [5]) with a quantum bump (single photon response, provided by a quantum bump model [2,4]). For bright impulses, light adaptation is involved, thus microvilli are divided into several adaptation categories, and macroscopic light induced current is then integrated from responses from each category of microvilli.
The model is validated by performing intracellular measurements from Drosophila photoreceptors to light impulses in vivo and by comparing these to the model output for the same light inputs. Even in this relatively basic form, our model can predict well the waveforms of voltage responses. From a practical and systemic point of view, this model can serve as a foundation to a preprocessing module for higher order models of the Drosophila visual system that we intend to build in due course.
References
1. Hardie, R.C., Raghu, P, Nature 413, 186-193 (2001)
2. Luo C.H., Rudy Y., Cir. Res. 74, 1071- 1096 (1994)
3. Vähäsöyrinki, M. Thesis, University of Oulu(2004)
4. Pumir, A. et al. PNAS 10354-10359 (2008)
5. Gillespie D.T., J. Comp. Phys. A. 22, 403-434 (1976)
Conference:
Computational and systems
neuroscience 2009, Salt Lake City, UT, United States, 26 Feb - 3 Mar, 2009.
Presentation Type:
Poster Presentation
Topic:
Poster Presentations
Citation:
(2009). Modelling of light responses of Drosophila Photoreceptor.
Front. Syst. Neurosci.
Conference Abstract:
Computational and systems
neuroscience 2009.
doi: 10.3389/conf.neuro.06.2009.03.030
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Received:
30 Jan 2009;
Published Online:
30 Jan 2009.