Cannabinoid receptor-mediated inhibition of calcium signaling in rat retinal ganglion cells

Purpose: The physiological actions of CB₁, cannabinoid receptors (CB₁Rs) in mammalian retina have yet to be fully described in all cell types. Here we investigate the actions of CB₁R activation on high-voltage-activated (HVA) Ca²⁺ channtel currents in purified cultures of rat retina] ganglion cells (RGCs). Methods: Reverse transcriptase polymerase chain reaction (RT-PCR) and immunocytochemistry were used to determine the presence of CB₁R mRNA and protein in a purified RGC culture generated from neonatal rats using a two-step panning procedure. Ruptured-patch whole-cell voltage clamp was used to test the effect of CB₁R agonists (WIN 55,212-2) and antagonists (SR 141716A, AM281) on HVA Ca²⁺ channel currents. Results: RT-PCR analysis contirmed CB₁B mKNA in cultured RGCS and immunocytoctiermstry tor CB₁R protein revealed labeling in both the cell body and neurites of isolated RGCs. Patch-clamp recording from cultured rat RGCs showed that the CB₁R agonist WIN 55,212-2 inhibited HVA Ca²⁺ channel currents up to 50% in a concentration-dependent manner (0.5, 1, and 5 μM). The Ca²⁺ channel current inhibition by WIN 55,212-2 was blocked by CB₁R antagonists AM281 and SR141716. Conclusions: Activation of CB₁Rs in cultured RGCs inhibits HVA Ca²⁺ channel currents. These data show that cannabinoids can modify the excitability of RGCs and could affect retinal output. This finding has implications for retinal signal processing as'it suggests that endogenous cannabinoids have inhibitory effects on RGCs and that exogenous cannabinoids could modulate retinal function by this pathway as well.