Subterranean Fishes of the World

3076

2015

AIM 2015

Adaptation to the cave environment involves many modifications at morphological and behavioural levels. In the vertebrate forebrain, the preoptic region and the hypothalamus contain neuro-endocrine cell types that control complex motivated behaviours as well as body energy homeostasis. Previous work in the group showed that the hypothalamus of the cavefish is larger than that of the surface fish, that both the hypothalamus and preoptic area of the cavefish proliferate more than those of the surface fish during embryonic and larval development (Menuet et al., 2007), and that specific neuronal groups such as the serotonin paraventricular nucleus is enlarged in cavefish (Elipot et al., 2013). Moreover, we proposed a potential developmental trade-off between the neural plate regions fated to become the ventral part of the retina –which is lacking or reduced in the cavefish- and those fated to give rise to the hypothalamus/preoptic area, through a modification in Fgf8/Shh signalling (Pottin et al., 2011). Here, we have therefore examined in details the development, morphogenesis and neural fates in the hypothalamus and preoptic region in a comparative manner in cavefish and surface fish. We generated a map of neuropeptide-expressing neurons and more particularly appetiteregulating peptidergic neurons (including orexigenic NPY and Hypocretin, and anorexigenic POMCa and POMCb). We found an increase in the numbers of orexigenic cells and a decrease in the numbers of anorexigenic cells in specific cell groups in cavefish. Further asking the origin of these modifications, we examined expression patterns of neuronal specifying transcription factors of the LIM-homeodomain family (including Lhx9 and Lhx7) and found some local modifications of their expression, generally just prior the first appearance of neuropeptide expression. Finally, we manipulated Fgf8 signalling through pharmacological treatments at neural plate stage, and found correlated changes in the morphology of the preoptic region and the size of neuropeptidergic groups. In summary, we will show that the increase in size of the hypothalamus and preoptic area is not the result of a global increase in all neuronal types, but rather of a complex modification of cell fates.

1