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S. Nirenberg, C. Pandarinath, I. Bomash, J. D. Victor, G. T. Prusky, W. W. Tschetter; How Neural Systems Adjust to Different Environments: An Intriguing Role for Gap Junction Coupling. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3290.
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It is well known that the visual system adjusts to new environments. One of the best-known examples of this is the adjustment in visual integration time that occurs as an animal switches from a day to a night environment. During the day, when photons are abundant, and the signal to noise ratio is high, the visual system is shifted toward short integration times. At night, when photons are limited, and the signal-to-noise ratio drops, the system shifts toward long integration times. How the visual system produces the shift isn’t clear. Here we tracked it from the behavioral level down to the circuitry and identified a mechanism that controls or strongly modulates it. The mechanism is a change in gap junction coupling among horizontal cells.
To assess the role of this coupling, we used transgenic mice that lack the gap junction gene, connexin 571. This gene is expressed exclusively in horizontal cells - nowhere else in the brain1 - and is not a hemichannel2; thus, the knockout provided a clean, specific means for preventing horizontal cell coupling. Effects on visual integration time were measured behaviorally, using an optomotor task, and physiologically, using multi-electrode recording from retinal ganglion cells.
Our results showed that the shift in visual integration time was completely blocked at the behavioral level and almost completely at the physiological level.
Horizontal cell coupling was found to play a critical role in modulating a circuit shift, a shift in visual integration time. The coupling achieves this by producing a shunt, which reduces the activity of the horizontal cells, minimizing their feedback to photoreceptors. With minimal feedback, photoreceptor integration time increases. As shown here, this translated to an increase in visual integration time at the ganglion cell level and the level of behavioral performance. This novel network-shifting mechanism may generalize to other brain areas, as similarly coupled networks are present throughout the brain.1. Hombach et al., (2004). Eur J Neurosci 19, 2633--2640.2. Janssen-Bienhold et. al. (2009) J Comp Neurol 513:363--374.
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