Growth Hormone and Synaptogenesis in the Chick Retina

Abstract

In the chicken embryo, GH gene expression occurs in the neural retina, where it promotes cell survival and induces axonal growth of retinal ganglion cells (RGCs). Neuroretinal GH is therefore of functional importance before the appearance of somatotrophs and the onset of pituitary GH secretion to the peripheral plasma which occurs between embryonic days (ED) 15-17. In this present study we provide evidence that GH is able to promote synaptogenesis in the embryonic chick neuroretina by increasing the expression of SNAP25, PSD95, and GAP43. We have also investigated its effects in the neonatal chick through the use of a kainate treatment model to study GHs effects on damaged retinal synapses. In response to damage, an increase in fluorescently labelled Cy3-GH internalization into RGCs was observed which was correlated with an increase in BDNF and PSD95 expression, suggesting a neuroprotective effect on the dendritic trees of RGCs the inner plexiform layer (IPL). In addition, we observed the presence of PSD95 positive Muller glia, which may suggest GH is having a neuroregenerative effect in damaged retinas. This work puts forth further evidence that GH acts as a synaptogenic modulator in the chick retina and opens a new possibility for the use of GH in retinal regeneration research.