Vision and visual cortical maps in mice with a photoreceptor synaptopathy: reduced but robust visual capabilities in the absence of synaptic ribbons.

How little neurotransmission in the visual system is sufficient to promote decent visual capabilities? This question is of key importance for therapeutic approaches to restore vision in patients who suffer from degenerative retinal diseases. In the retinae of mice, mutant for the presynaptic scaffolding protein Bassoon (Bsn), signal transfer at photoreceptor ribbon synapses is severely disturbed due to impaired ribbon attachment to the active zone. We have used two different behavioural tasks and optical imaging of intrinsic signals to probe vision in young and adult Bsn-/- mice and their wild-type littermates. Here we show that while visual acuity was significantly reduced in mutants compared to controls, vision guided behavioural decisions and optical imaging revealed essentially unperturbed cortical signals and retinotopy in spite of the photoreceptor synaptopathy. In addition, both vision and visual cortical maps were adult-like at 4 weeks of age. These results show that (i) while Bassoon-dependent fast exocytosis is essential for normal vision surprisingly good visual performance can be achieved in the absence of synaptic ribbons, (ii) both the development and maintenance of visual cortical maps is independent of synaptic ribbons and (iii) visual development in the mutants is completed at 4 weeks of age indicating that later developing ectopic synapses do not affect vision. Thus, the central visual system can make use of slow and weak retinal signals to subserve surprisingly robust vision.