Shedding light on the visual system

The retina is exposed to visual intensities varying over several orders of magnitudes. However, the photoreceptors in the retina have smaller dynamic range and display uniform sensitivity to light. The traditional view is that the neuronal circuitry in the retina undergoes adaptation to increase the dynamic range. However, another type of plasticity, facilitation, may be responsible for this subtlety.

The SYNAPSE TRANSMISSION (Synaptic mechanisms underlying neural coding in the retina during visual response) project used multiphoton microscopy together with novel genetically-encoded optical reporters using the retina of zebrafish larvae as a model system. The aim was to determine what happens at the synapses of bipolar and amacrine cells in the retina under different contrast illuminations.

Using such a methodology, initial research results showed that different synaptic terminals from the same bipolar cell can transmit information differently to downstream post-synaptic ganglion cells. Such a "multiplexed" computation broadens the repertoire of synaptic processing and computation and could be applicable to other areas of the brain. The initial research results stimulate further research in this area.

SYNAPSE TRANSMISSION scientists also studied synaptic computation at bipolar and amacrine cell terminals in the presence and absence of Müller cells. Varying luminance intensities and temporal contrast, the results indicated that the retina can still be functional, albeit with a reduced gain, in terms of processing visual information.

Besides this work, efforts were put in to develop a new optical tools that in future will be useful to study synaptic transmission. First, scientists aimed for spatially and temporally controlled activation of synaptic terminals without perturbing the entire neuronal cell – to this end, scientists developed a method to target channelrhodpsin to the presynaptic membrane. Secondly, a collaborative project was started to develop a new spectrally red-shifted optical reporters to monitor glutamate signals in synaptic terminals.

Other sensory systems possess similar challenges. SYNAPSE TRANSMISSION research results may therefore help researchers to better understand neural and synaptic function and how these circuits encode sensory information.