Cannabinoids Enhance NMDA-Elicited Ca super(2+) Signals in Cerebellar Granule Neurons in Culture

A physiological role for cannabinoids in the CNS is indicated by the presence of endogenous cannabinoids and cannabinoid receptors. However, the cellular mechanisms of cannabinoid actions in the CNS have yet to be fully defined. In the current study, we identified a novel action of cannabinoids to enhance intracellular Ca super(2+) responses in CNS neurons. Acute application of the cannabinoid receptor agonists R(+)-methanandamide, R(+)-WIN, and HU-210 (1-50 nM) dose-dependently enhanced the peak amplitude of the Ca super(2+) response elicited by stimulation of the NMDA subtype of glutamate receptors (NMDARs) in cerebellar granule neurons. The cannabinoid effect was blocked by the cannabinoid receptor antagonist SR141716A and the G sub(i)/G sub(o) protein inhibitor pertussis toxin but was not mimicked by the inactive cannabinoid analog S(-)-WIN, indicating the involvement of cannabinoid receptors. In current-clamp studies neither R(+)-WIN nor R(+)-methanandamide altered the membrane response to NMDA or passive membrane properties of granule neurons, suggesting that NMDARs are not the primary sites of cannabinoid action. Additional Ca super(2+) imaging studies showed that cannabinoid enhancement of the Ca super(2+) signal to NMDA did not involve N-, P-, or L-type Ca super(2+) channels but was dependent on Ca super(2+) release from intracellular stores. Moreover, the phospholipase C inhibitor U-73122 and the inositol 1,4,5-trisphosphate (IP sub(3)) receptor antagonist xestospongin C blocked the cannabinoid effect, suggesting that the cannabinoid enhancement of NMDA-evoked Ca super(2+) signals results from enhanced release from IP sub(3)-sensitive Ca super(2+) stores. These data suggest that the CNS cannabinoid system could serve a critical modulatory role in CNS neurons through the regulation of intracellular Ca super(2+ )signaling.