It is now well established that growth hormone (GH) gene expression is not confined to the pituitary gland and occurs in many extrapituitary tissues, including the central and peripheral nervous systems. GH is present in neuronal and glial cells and in cerebrospinal fluid, and is present in the brain prior to the ontogenic differentiation of the pituitary gland. The colocalization of GH-receptors and GH-binding proteins in these sites suggests they are also sites of GH action. This possibility is supported by the presence of GH-responsive genes in neural tissues and by demonstrated actions of GH in neural development, behaviour and neurotransmission. GH may also have synaptic or extrasynaptic actions in the neural retina during the development of the eye.

GH immunoreactivity is abundantly present in cells of the neural retina in embryonic chicks, although this immunoreactivity is lost following hatch. This immunoreactivity is primarily associated with two proteins that are of smaller molecular size (15 and 17 kDa) than ‘monomer’ pituitary GH (24 kDa). The 15 kDa moiety (but not the 17 kDa variant) is present in the media of cultured retinal explants and is richly abundant in vitreous humour, suggesting that it is synthesized and secreted by the neural retina. The presence of GH mRNA in retinal tissue and the likely exclusion of circulating GH by the blood-retinal barrier supports this view. The presence of GH receptors in the retina also suggests autocrine/paracrine actions of this GH moiety in retinal neural tissue. However, as GH receptors are also present in the lens and cornea, there may also be non-neural sites of retinal GH action and the vitreous humour may provide a novel conduit for this retinal secretory product. GH may thus have synaptic or extrasynaptic actions within the eye.