Abstract

Although the activation of cannabinoid receptor-1 (CB1) receptors by cannabinoids is known to inhibit neuronal hyperexcitability and reduce excitotoxic cell death, the mechanistic links between these two actions remain elusive. We tested the hypothesis that activation of CB1 receptors inhibits N-methyl-d-aspartic acid (NMDA)-mediated calcium influx and cell death via the inositol triphosphate (IP₃) signaling pathway in both primary dorsal root ganglia neurons and a cultured neuronal cell line (F-11 cells). These cells were pretreated with the cannabinoid agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de)-1,4-benzoxazin-6-yl]-1-napthalenylmethanone (R-(+)-WIN 55,212-2; WIN) before exposure to NMDA. Concentrations of cytosolic calcium were measured with the ratiometric calcium indicator, Fura-2, and cell death was determined by a cell viability test. WIN dose-dependently attenuated both the calcium influx and cell death induced by NMDA. These effects were blocked by selective cannabinoid CB1 receptor antagonists N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) or N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251), but not CB2 receptor antagonist N-[(1S)-endo-1,3,3,-trimethylbicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methyl-benzyl)-pyrazole-3-carboxamide (SR144528). It is interesting to note that a transient Ca²⁺ signal was observed after the acute application of WIN. This Ca²⁺ increase was blocked by a CB1 receptor antagonist AM251, IP₃ receptor antagonist 2- aminoethyl diphenylborinate, or by depleting intracellular Ca²⁺ stores with the endoplasmic reticulum Ca²⁺ pump inhibitor thapsigargin. Removal of extracellular Ca²⁺, on the other hand, had no effect on the CB1 receptor-induced Ca²⁺ increase. These data suggest that WIN triggers a cascade of events: it activates the CB1 receptor and the IP₃ signaling pathway, stimulates the release of Ca²⁺ from intracellular stores, raises the cytosolic Ca²⁺ levels, and inhibits the NMDA-mediated Ca²⁺ influx and cell death through a process that remains to be determined.