Caffeine exposure alters adenosine system and neurochemical markers during retinal development

Evidence points to beneficial properties of caffeine in the adult central nervous system, but teratogenic effects have also been reported. Caffeine exerts most of its effects by antagonizing adenosine receptors, especially A(1) and A(2A) subtypes. In this study, we evaluated the role of caffeine on the expression of components of the adenosinergic system in the developing avian retina and the impact of caffeine exposure upon specific markers for classical neurotransmitter systems. Caffeine exposure (5-30mg/kg by in ovo injection) to 14-day-old chick embryos increased the expression of A(1) receptors and concomitantly decreased A(2A) adenosine receptors expression after 48h. Accordingly, caffeine (30mg/kg) increased [H-3]-8-cyclopentyl-1,3-dipropylxanthine (A(1) antagonist) binding and reduced [H-3]-ZM241385 (A(2A) antagonist) binding. The caffeine time-response curve demonstrated a reduction in A(1) receptors 6h after injection, but an increase after 18 and 24h. In contrast, caffeine exposure increased the expression of A(2A) receptors from 18 and 24h. Kinetic assays of [H-3]-S-(4-nitrobenzyl)-6-thioinosine binding to the equilibrative adenosine transporter ENT1 revealed an increase in B-max with no changes in K-d, an effect accompanied by an increase in adenosine uptake. Immunohistochemical analysis showed a decrease in retinal content of tyrosine hydroxylase, calbindin and choline acetyltransferase, but not Brn3a, after 48h of caffeine injection. Furthermore, retinas exposed to caffeine had increased levels of phosphorylated extracellular signal-regulated kinase and cAMP-response element binding protein. Overall, we show an invivo regulation of the adenosine system, extracellular signal-regulated kinase and cAMP-response element binding protein function and protein expression of specific neurotransmitter systems by caffeine in the developing retina.