Alterations in ZENK and glucagon RNA transcript expression during increased ocular growth in chickens

To examine in detail the time-course of changes in Zif268, Egr-1, NGFI-A, and Krox-24 (ZENK) and pre-proglucagon (PPG) RNA transcript levels in the chick retina during periods of increased ocular growth induced by form-deprivation and negative-lens wear. To further elucidate the role of ZENK in the modulation of ocular growth, we investigated the effect of intravitreal injections of the muscarinic antagonist atropine and the dopamine agonist 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (ADTN), both of which block the development of experimental myopia, on the expression of ZENK in eyes fitted with negative-lenses.

Myopia was induced by fitting translucent diffusers or -10D polymethyl methacrylate (PMMA) lenses over one eye of the chicken. At times from 1 h to 10 days after fitting of the diffusers or negative lenses, retinal RNA transcript levels of the selected genes were determined by semi-quantitative real-time reverse transcriptase polymerase chain reaction (RT-PCR). For the pharmacology experiments, -10D lenses were fitted over the left eye of chicks for a period of 1h. Intravitreal injections of atropine (10 mul-25 mM), ADTN (10 mul-10 mM), or a vehicle solution were made immediately before fitting of the lenses.

ZENK RNA transcript levels were rapidly and persistently down-regulated following the attachment of the optical devices over the eye. With a delay relative to ZENK, PPG transcript levels were also down-regulated. Induced changes in gene expression were similar for both form-deprivation and negative-lens wear. When atropine or ADTN were administered immediately before lens attachment, the rapid down-regulation in ZENK RNA transcript levels normally seen following 1 h of negative-lens wear was not seen, and ZENK transcript levels rose above those values seen in control eyes. However, injection of atropine or ADTN into untreated eyes had no effect on ZENK transcript levels.

Both form-deprivation and negative-lens wear modulated the retinal expression of ZENK and PPG RNA transcripts, with a similar time-course and strength of response. The ability of the tested drugs to prevent the down-regulation of ZENK in both lens-induced myopia (LIM) and form-deprivation myopia (FDM) suggests that atropine and ADTN act directly and rapidly on retinal circuits to enhance sensitivity early in the signaling process. These findings suggest that very similar molecular pathways are involved in the changes in eye growth in response to form-deprivation and negative lenses at 1 h after the fitting of optical devices.