The Effects of Visual Stimulation on Glucagon and Glucagon Receptor Expression

Abstract

Abstract: : Purpose: Previously, it was shown that glucagonergic ZENK-immunoreactive amacrine cells respond to defocus in the retinal image and even to its sign. Moreover, it was found that the amount of retinal glucagon mRNA increased during treatment with positive lenses and pharmacological studies supported the idea that glucagon may act as a stop signal for eye growth. Here we aimed to demonstrate the temporal sequence of changes in glucagon and glucagon receptor expression after imposed defocus and to investigate whether changes of gene expression occur first in the retina or in the choroid. Methods: Ten to fourteen day old male White Leghorn chickens were treated with positive and negative lenses (– / +7D) for different periods of time (2, 4, 6 and 24 h). In addition chicks were intravitreally injected with 25 nmol of the glucagon agonist Lys^17,18,Glu²¹-glucagon three times over a period of five days. This concentration inhibits myopia development completely. Following the extraction of the retinal and choroidal RNA the relative expression of glucagon and glucagon receptor was determined by semiquantitative real time RT-PCR (n = 4–8). 18S-rRNA served as an internal standard. Results: (1) Glucagon mRNA expression was significantly increased after 2 hours of +7D lens treatment (p<0.05) in the choroid and after 4 hours in the retina (p<0.05). (2) A decrease of glucagon mRNA in the retina after glucagon agonist Lys^17,18,Glu²¹-glucagon injection could be shown (p<0.05). (3) Glucagon receptor expression in the RPE was significantly higher than in the retina. (4) At least a trend towards a down-regulation of glucagon mRNA was seen after 2 hours of –7D lens treatment in the choroid. Conclusions: Up to now, none of the investigated treatments influenced glucagon receptor expression significantly. In contrary, glucagon expression was significantly up-regulated as a result of plus lens treatment. A 70% increase in glucagon expression could first be detected in the choroid followed by an increased glucagon expression (40%) in the retina, which is in line with the idea that glucagon acts as a stop signal to eye growth. Further studies are underway to confirm the finding that glucagon mRNA levels increase in the choroid prior to retinal expression changes.

Keywords: myopia • gene/expression • retina