Background
In chickens, retinal glucagon amacrine cells play an important role in emmetropization, since they express the transcription
factor ZENK (also known as NGFI-A, zif268, tis8, cef5, Krox24) in correlation with the sign of imposed image defocus. Pharmacological
studies have shown that glucagon can act as a stop signal for axial eye growth, making it a promising target for pharmacological
intervention of myopia. Unfortunately, in mammalian retina, glucagon itself has not yet been detected by immunohistochemical
staining. To learn more about its possible role in emmetropization in mammals, we studied the expression of different members
of the glucagon hormone family in mouse retina, and whether their abundance is regulated by visual experience.
Methods
Black wildtype C57BL/6 mice, raised under a 12/12 h light/dark cycle, were studied at postnatal ages between P29 and P40.
Frosted hemispherical thin plastic shells (diffusers) were placed in front of the right eyes to impose visual conditions that
are known to induce myopia. The left eyes remained uncovered and served as controls. Transversal retinal cryostat sections
were single- or double-labeled by indirect immunofluorescence for early growth response protein 1 (Egr-1, the mammalian ortholog
of ZENK), glucagon, glucagon-like peptide-2 (GLP-2), glucose-dependent insulinotropic polypeptide (GIP), peptide histidine
isoleucine (PHI), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase-activating polypeptide (PACAP), secretin,
and vasoactive intestinal polypeptide (VIP). In total, retinas of 45 mice were studied, 28 treated with diffusers, and 17
serving as controls.
Results
Glucagon itself was not detected in mouse retina. VIP, PHI, PACAP and GIP were localized. VIP was co-localized with PHI and
Egr-1, which itself was strongly regulated by retinal illumination. Diffusers, applied for various durations (1, 2, 6, and
24 h) had no effect on the expression of VIP, PHI, PACAP, and GIP, at least at the protein level. Similarly, even if the analysis
was confined to cells that also expressed Egr-1, no difference was found between VIP expression in eyes with diffusers and
in eyes with normal vision.
Conclusions
Several members of the glucagon super family are expressed in mouse retina (although not glucagon itself), but their expression
pattern does not seem to be regulated by visual experience.