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

PURPOSE: The physiological actions of CB(1) cannabinoid receptors (CB(1)Rs) in mammalian retina have yet to be fully described in all cell types. Here we investigate the actions of CB(1)R activation on high-voltage-activated (HVA) Ca(2+) channel currents in purified cultures of rat retinal ganglion cells (RGCs).
METHODS: Reverse transcriptase polymerase chain reaction (RT-PCR) and immunocytochemistry were used to determine the presence of CB(1)R 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 CB(1)R agonists (WIN 55,212-2) and antagonists (SR141716A, AM281) on HVA Ca(2+) channel currents.
RESULTS: RT-PCR analysis confirmed CB(1)R mRNA in cultured RGCs and immunocytochemistry for CB(1)R protein revealed labeling in both the cell body and neurites of isolated RGCs. Patch-clamp recording from cultured rat RGCs showed that the CB(1)R agonist WIN 55,212-2 inhibited HVA Ca(2+) channel currents up to 50% in a concentration-dependent manner (0.5, 1, and 5 muM). The Ca(2+) channel current inhibition by WIN 55,212-2 was blocked by CB(1)R antagonists AM281 and SR141716.
CONCLUSIONS: Activation of CB(1)Rs in cultured RGCs inhibits HVA Ca(2+) 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.