α2-Adrenergic receptors are expressed on chicken Müller glia cells and stimulate the MAPkinase pathway in-vivo and in-vitro

Muller cells are the principle glia cells of the retina that maintain the integrity and function of the retina and are thought to support and protect neurons during various insults. α2-adrenergic receptor agonists have been shown to be neuroprotective and Mu[[unable to display character: ̈]]ller cells are suggested to take part in these effects. The purpose of this project was to study the expression of α2-adrenergic receptors and the effects of α2-adrenergic agonists on Mu[[unable to display character: ̈]]ller cell-activation and to investigate the signaling pathway that mediates this activation. Retina from embryonic day (E) 18 White-Leghorn chicken and primary Mu[[unable to display character: ̈]]ller cell cultures, established from E14 chick retinas were used for expression analysis. We studied immunoreactivity (IR) and mRNA levels of α2A, 2B and 2C adrenergic receptors in Müller cells. Retinas and Müller cell culture were analyzed by using immunohistochemistry, qRT-PCR and western blot techniques. The results showed that cell soma positive for Müller cell markers, 2M6 and Sox2 in the inner nuclear layer and processes at the outer limiting membrane were positive for α2A receptors. Analysis of primary cultures for IR and mRNA expression confirmed that Müller cells were positive for the α2A receptor. No or very low α2B or α2C expression was detected. Next we analyzed the effect of an α2-adrenergic agonist: Brimonidine (BMD). BMD was tested by injection into E18 chicken eyes or by treatment of the Mu[[unable to display character: ̈]]ller cells in culture. We observed activation of Müller cells after injection of BMD as demonstrated by phospho-ERK1/2 IR in Müller cells with a distinct increase of IR within 2 hours and a gradual decrease to background levels by 24 hours. In-vitro, phospho-ERK1/2 IR was seen already within 5 minutes after BMD addition to the Müller cell culture with peak-levels at 15 minutes and a gradual decrease to pre-exposure levels by 60 minutes. In conclusion, our results show that chick Müller cells mainly expresses α2A adrenergic receptors and that stimulation of the α2-adrenergic receptors triggers the MAPK signaling pathway both in-vivo and in cultured Müller cells in-vitro.