Abstract
Abstract: :
Purpose: We have previously associated the induction of inducible nitric oxide synthase (iNOS) with the activation of epidermal growth factor receptor (EGFR) in glaucoma. In addition, astrocytes express iNOS in response to the activation of EGFR in vitro. During postnatal development, activated EGFR appears in the astrocytes of the rat optic nerve in vivo. In the present study, we have demonstrated the expression of iNOS and its association with EGFR in the astrocytes of rat optic nerves during postnatal development. Methods: Expression of iNOS was assayed by immunohistochemistry and immunoblot in the optic nerves from rats of postnatal days 1, 3, 6, 10, 14, 21 and two months old. The effects of EGFR activation on iNOS expression were studied by giving the EGFR tyrosine kinase inhibitor, AG1478, daily by IP injection to newborn pups and detecting iNOS expression at postnatal day 10. The effects of blocking EGFR activation on retinal ganglion cells was determined by Fluorogold and electronmicroscopy. Results: During postnatal development of rat optic nerves, astrocytes proliferate and form cribriform structures within three weeks after birth. During this time, approximately 50% of the axons of the retinal ganglion cells degenerate. Activation of EGFR peaks in the astrocytes of rat optic nerves at postnatal day 6 and is almost undetectable after postnatal day 21. Immunohistochemical labeling for iNOS shows the level of iNOS was detectable in the optic nerves of newborn rats, gradually increases to peak at postnatal day 6, and then decreases and is undetectable after postnatal day 21. Intensive labeling for iNOS is present in the astrocytes surrounding the axons at postnatal day 6. Immunoblot for iNOS in the tissue lysates confirms the presence of iNOS in the optic nerves of rats at postnatal days 1, 6 and 14. The temporal sequence of iNOS expression in the astrocytes corresponds to the activation of EGFR in the astrocytes of the optic nerves during postnatal development. Blocking the activation of EGFR, by giving AG1478 postnatally for 10 days, significantly diminished the expression of iNOS in the astrocytes. The effects of AG1478 on retinal ganglion cells will be reported. Conclusions: iNOS is expressed in response to the activation of EGFR in vivo in rat optic nerves during postnatal development. The expression of iNOS may play an important role in the tissue modeling of the optic nerve in its later stage of development. Perhaps the appearance of activated EGFR and iNOS in the glaucomatous optic nerve is the reinstatement of a developmental process.
Keywords: nitric oxide • astroglia: optic nerve head • growth factors/growth factor receptors