Subterranean Fishes of the World

3083

2015

AIM 2015

To study the adapting roles of the neural circuits associated with vision in Astyanax mexicanus we have performed two-photon calcium imaging of the retina and the optic tectum in intact, non-anesthetized larvae of both epigean and hypogean forms. We found that both the retina and the optic tectum of cavefish show extensive ongoing spontaneous activities. Yet, activity induced by visual stimuli were never observed. This lack of visually-induced responses comes in sharp contrast to surface fish which show large visually-induced tectal responses. Furthermore, the cavefish optic tectum did not respond to either vibrational or acoustic stimuli which trigger specific behavioral responses. Anatomically, both 5-14 days post fertilization cave and surface larvae displayed similar size optic tecta, with retinal ganglion cells projecting to the optic tectum at comparable extents. Our results also show that while deprived from sensory input, the visual network remains highly active, showing extensive ongoing spontaneous activity. The functional role of the retino-tectal circuit in a developing blind fish is still uncertain. One explanation rests upon the developmental role of the tectum which can be fundamental to patterning the CNS, or maintain the proper connectivity between different brain regions (e.g acting as a relay rather than being a processing brain area). It is worth noticing that cave A. mexicanus could have diverged as early as 10,000 years ago (Avise and Selander, 1972; Chakraborty and Nei, 1974; Mitchell et al., 1977), therefore providing us with only an early snapshot of a continuously adapting brain. Spontaneous activity in the visual pathway of cavefish could therefore be present as a vestigial surface fish code for processing visual information.

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