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ePaper created 2012-11-06, 16:43:12 | version 1.24.0
32 Forschungszentrum Jülich | Annual Report 2011 Molecular Sunglasses – How Visual Cells Work Together in Dim Light Two types of visual cells ensure that we can find our way around both in sunshine and at night by starlight. In dim light, the two work together. How this happens is what Jülich scientists in- vestigated together with an international team of research- ers. In so doing, they discovered an ingenious anti-glare protection in the eye. They published their findings in the high-impact journal Nature Communications. W hen you enter a bright- ly lit room after a stroll at night, you are momentarily blinded. But then after no time at all, your eyes adapt to the room flooded with light. Our eyes are incredi- bly adaptable – from weak moonlight to bright sunshine, they can deal with very different lighting conditions. This is pos- sible thanks to two different types of specialized sensory cells. Cones come into play in bright light; they allow us to see sharply and distinguish between dif- ferent colours. Rods, in contrast, take over when it’s darker; they work in the range of a billionth to a ten-thousandth of the maximum perceptible brightness. However, they only produce black-and- white images – after all “all cats are grey in the dark”. Anti-glare protection prevents chaos The changeover from cones to rods when it gets darker and vice versa when it gets brighter does not happen sud- denly. The two work together during the transition period. “When we can just about distinguish colours in dim light, then both cell types are active,” says neurobiologist Prof. Frank Müller from the Jülich Institute of Complex Systems. In cooperation with researchers from the universities in Tübingen, Oldenburg and Dublin, his group investigated how this collaboration between cones and rods is coordinated. Precise control is essential because when the cones and rods are simultane- ously active, information chaos could re- sult. The highly sensitive rods would send out signals that are so strong that they would block the neural pathways, preventing the weaker signals from the cones getting through – the eye would then be blinded by only a moderate inci- dence of light. Rods and cones work to- gether in two different ways. The signals from both types of cell either converge in certain neurons in the retina when being transmitted, or the cones and rods are directly connected to each oth- er via so-called gap junctions. “In any case, the eye must prevent the rods from registering a feeling of blindness and transmitting this to the neurons downstream,” says Müller. “Only then can the cones transmit their information without any problems.” The researchers were looking for the control mecha- nisms that ensure that the signals re- leased by the cones do not get lost in this network. The key molecular switch in this pro- cess was identified by the researchers as a cellular building block known as HCN1. It creates an opening – an ion channel – in the cell membrane of the rods which opens when the incidence of light increases. This weakens the signal released by the rods. This mechanism works a bit like sunglasses with self-tint- ing lenses – the more intensive the light, the stronger the anti-glare protection. In such a regulated system, the cones also have a chance to function – in this way, the eye makes optimal use of the availa- ble light. The fact that HCN1 is indispensable as the “switch” for this was shown in mice that lacked this molecule. Their eyes did not react like those of normal animals to a rapid sequence of light im-