Abstract
Purpose:
Oxytocin (OXT) is a neuropeptide that facilitates paracrine signaling by binding to the oxytocin receptor (OXTR) and activating a cellular phosphatidylinositol-calcium second messenger. Interestingly, multiple functions of the retinal pigment epithelium (RPE) that support the neural retina occur through a similar mechanism. The neural retina solicits RPE support by releasing an assortment of signaling molecules that are yet to be identified. Since OXT has been identified in the retina, we were interested in determining whether OXT may be involved in neural retina-RPE communication.
Methods:
We determined OXTR and OXT protein expression first by indirect fluorescence immunohistochemistry using human and monkey frozen retinal tissue sections. Cultured human fetal RPE (H-RPE from LONZA®, USA) cells were grown to a monolayer and maintained at 37°C and 5% CO2 with a media change every 2 days. These cells were tested for the expression of RPE markers as part of phenotype determination. OXTR protein expression in H-RPE cells was confirmed by standard Western blot techniques. Specific OXT induction of intracellular Ca2+ ([Ca2+]i) mobilization was determined by using live-cell imaging of FURA-2AM ratiometric measurements.
Results:
In both human and monkey frozen sections, OXTR protein was only detected in the RPE. Cellular distribution of OXTR co-localized with specific RPE cell marker RPE65. OXT, on the other hand was concentrated heavily in the cone photoreceptor (PR) outer segments. Cultured H-RPE cells expressed OXTR protein. When a monolayer of these cells was treated with OXT, the cells exhibited a specific, reversible 70-120 nM increase of [Ca2+]i.
Conclusions:
The oxytocin receptor is located in the RPE and oxytocin is found in the cone PR of the eye. Oxytocin’s specific induction of H-RPE [Ca2+]i suggests that oxytocin stimulates RPE signaling through a traditional phosphatidylinositol-Ca2+ second messenger system. Given the adjacent distribution of oxytocin in the PR outer segment with the oxytocin receptor on the RPE cells, we propose that oxytocin is involved in the paracrine regulation of PR-RPE communication via release from the cone photoreceptors to stimulate as yet to be identified RPE functions.
Keywords: 447 cell-cell communication •
701 retinal pigment epithelium •
715 signal transduction: pharmacology/physiology