FGFR1 expression and FGFR1-FGF-2 colocalisation in rat retina: Sites of FGF-2 action on rat photoreceptors

Aim: To identify sites of FGF-2 action on photoreceptors of the rat retina, by localizing FGFR1 in the intact retina, and to assess the colocalisation of FGF-2 with FGFR1. Methods: Immunohistochemistry and confocal microscopy were used to localise FGF-2 and FGFR1 in cryosections of the rat retina, both normal retina and retina stressed by exposure to bright continuous light (1000 lux, 24 h). Antibodies to synaptophysin (SY), cytochrome oxidase (CO) and opsin were used to relate FGFR1-labelling to synaptic vesicles, mitochondria and the photoreceptor cell membrane. Electron microscopy was used to demonstrate the location of synapses within the outer plexiform layer (OPL). Results: FGFR1 was most prominent in the outer nuclear layer (ONL), as diffuse labelling of photoreceptor cytoplasm and as granules between photoreceptor somas. FGFR1 labelling was also observed in the outer synapse-rich sublayer of the OPL where it colocalised with SY, but not with CO-labelled mitochondria. In stressed retina, both at the edge of normal retina and after light stress, FGFR1 expression was upregulated in both the ONL and the OPL. Colocalisation of FGFR1 with FGF-2 could not be demonstrated in unstressed retina, but was demonstrable in stressed retina, in both the ONL and OPL. Conclusions: FGFR1 is prominent in the cytoplasm of photoreceptors, and in their axon terminals, where it is closely associated with synaptic vesicles. Colocalisation of FGFR1 and FGF-2 could be demonstrated in stressed retina, in the cytoplasm and the axon terminals of photoreceptors. The known protective action of FGF-2 may be exerted at the photoreceptor soma. The action of FGF-2 in inhibiting the ERG b-wave may be exerted at the axon terminal.