Functional imaging of circuits commonly associated with vision processing in the Mexican blind cavefish Astyanax mexicanus

DAPHNE SOARES; SEBASTIÁN A. ROMANO; ERNESTO RESTREPO; FILIPPO DEL BENE; GERMÁN SUMBRE

Washington

Congreso; Society for Neuroscience Annual Meeting; 2011

Society for Neuroscience

Troglobitic animals often have evolutionary derived nervous systems due to both constructive and regressive traits. In the Mexican tetra Astyanax mexicanus, we find a particularly good animal model to study the adaptation of neural networks and behavior because of readily available ancestral and derived groups. Astyanax is undergoing allopatric speciation and is extant in two forms: a surface, river dwelling form (epigean) and a blind cave dwelling form (hypogean). Although adult cavefish lack functional eyes, eyes are generated during embryogenesis, but later arrest in development, degenerate, and sink into the orbit. However, a cavefish eye will continue to develop when a surface fish lens is transplanted into a cavefish optic cup, indicating that cavefish derived visual neural substrate may have retained the ancestral ability to process visual information. 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-anaesthetized larvae of both epigean and hypogean forms. We found that both the retina and the optic tectum of cavefish show extensive ongoing spontaneous activity. Yet, activity induced by visual stimuli was never observed. This lack of visually-induced responses comes in sharp contrast to surface fish, which, similar to zebrafish larvae, 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 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.