Expression of receptors for glial cell line-derived neurotrophic factor (GDNF) and neurturin in the inner blood-retinal barrier of rats

The retina is protected from somatic circulation by the blood-retinal barrrier (BRB) composed of tight junctions between retinal vascular endothelial cells (the inner BRB) and those between retinal pigment epithelial cells (the outer BRB). Our recent studies showed that glial cell line-derived neurotrophic factor (GDNF) secreted from astrocytes regulates the permeability of the BBB. In the present study, we immunohistochemically examined the expression of GDNF, neurturin (NTN) and their receptors, GFR alpha1 for GDNF and GFR alpha2 for NTN, because the capillaries of the inner BRB show specialization very similar to the blood-brain barrier (BBB), GDNF and NTN were detected in glial fibrillary acidic protein (GFAP)-positive cells, including Muller cells. GFR alpha1 and GFR alpha2 were localized in von Willebrand factor-positive cells, GDNF and NTN enhanced the barrier function of endothelial cells derived from porcine brain cortex. These results strongly suggest that the barrier function of the BRB is regulated by GDNF and NTN secreted from glial cells, like the BBB.