The A3 adenosine receptor attenuates the calcium rise triggered by NMDA receptors in retinal ganglion cells

Abstract

The A3 adenosine receptor is emerging as an important regulator of neuronal signaling, and in some situations receptor stimulation can limit excitability. As the NMDA receptor frequently contributes to neuronal excitability, this study examined whether A3 receptor activation could alter the calcium rise accompanying NMDA receptor stimulation. Calcium levels were determined from fura-2 imaging of isolated rat retinal ganglion cells as these neurons possess both receptor types. Brief application of glutamate or NMDA led to repeatable and reversible elevations of intracellular calcium. The A3 agonist Cl-IB-MECA reduced the response to both glutamate and NMDA. While adenosine mimicked the effect of Cl-IB-MECA, the A3 receptor antagonist MRS 1191 impeded the block by adenosine, implicating a role for the A3 receptor in the response to the natural agonist. The A1 receptor antagonist DPCPX provided additional inhibition, implying a contribution from both A1 and A3 adenosine receptors. The novel A3 agonist MRS 3558 (1’S,2’R,3’S,4’R,5’S)-4-(2-chloro-6-(3-chlorobenzylamino)-9H-purin-9-yl)-2,3-dihydroxy-N-methylbicyclo[3.1.0] hexane-1-carboxamide and mixed A1/A3 agonist MRS 3630 (1’S,2’R,3’S,4’R,5’S)-4-(2-chloro-6-(cyclopentylamino)-9H-purin-9-yl)-2,3-dihydroxy-N-methylbicyclo [3.1.0]hexane-1-carboxamide also inhibited the calcium rise induced by NMDA. Low levels of MRS 3558 were particularly effective, with an IC50 of 400 pM. In all cases, A3 receptor stimulation inhibited only 30-50% of the calcium rise. In summary, stimulation of the A3 adenosine receptor by either endogenous or synthesized agonists can limit the calcium rise accompanying NMDA receptor activation. It remains to be determined if partial block of the calcium rise by A3 agonists can modify downstream responses to NMDA receptor stimulation.

The A3Adenosine Receptor: History and Perspectives

“…Owing to this very low expression, other authors have reported the impossibility of detecting either the A 3 AR gene or binding site in the CNS from in situ hybridization experiments (Rivkees et al, 2000), and others have described A 3 AR expression in the thalamus and hypothalamus (Yaar et al, 2002). However, electrophysiologic and biochemical evidence suggests the presence of A 3 AR in the rat hippocampus (Dunwiddie et al, 1997;Macek et al, 1998;Lopes et al, 2003) and cortex (Brand et al, 2001); functional studies have also indicated its presence in the brain and retinal ganglion cells von Lubitz et al, 1994;Haskó et al, 2005;Zhang et al, 2006aZhang et al, , 2010. The proposed presence of A 3 AR at motor nerve terminals (Ribeiro and Sebastião 1986) was recently demonstrated through pharmacologic and immunohistochemical studies (Cinalli et al, 2013).…”

Rat, Mouse, and Primate Models of Chronic Glaucoma Show Sustained Elevation of Extracellular ATP and Altered Purinergic Signaling in the Posterior Eye

“…24,52 As these adenosine receptors hyperpolarize ganglion cells, decrease their calcium levels and protect the cells from excitotoxic damage, the rise in adenosine is likely to be beneficial. [52][53][54] Thus, the overall effect of increased extracellular ATP in glaucoma will depend upon whether the extracellular conversion of ATP into adenosine can keep up with the release of ATP, in addition to the relative expression of protective or damaging receptors.…”

Ocular Purine Receptors as Drug Targets in the Eye

“…80 Nucleoside prodrugs of A 3 AR agonists and antagonists have been explored for their action on IOP. 65,66,81,82 Elevated IOP causes degeneration and eventually apoptotic death of retinal ganglion cells (RGCs). ARs are known to be present in the retina 8,83 and their activation provides protection against this apoptosis.…”