Regulation of δ opioid receptors by Δ9-tetrahydrocannabinol in NG108-15 hybrid cells

Abstract

In this study we employed the neuroblastoma x glioma NG 108-15 cell line as a model for investigating the effects of long-term activation of cannabinoid receptors on δ opioid receptor desensitization, down-regulation and gene expression. Exposure of NG 108-15 cells to (−)- Δ 9-tetrahydrocannabinol ( Δ 9-THC) reduced opioid receptor binding, evaluated in intact cells, by ≈ 40 – 45% in cells exposed for 24 h to 50 and 100 nM Δ 9-THC and by ≈ 25 % in cells exposed to 10 nM Δ 9-THC. Lower doses of Δ 9-THC (0.1 and 1 nM) or a shorter exposure time to the cannabinoid (6 h) were not effective. Down-regulation of δ opioid receptors was not observed in cells exposed for 24 h to pertussis toxin (PTX) and then treated for 24 h with 100 nM Δ 9-THC. In cells that were exposed for 24 h to the cannabinoid, the ability of Δ 9-THC and of the δ opioid receptor agonist [D-Ser 2, Leu 5, Thr 6]enkephalin to inhibit forskolin-stimulated cAMP accumulation was significantly attenuated. Prolonged exposure of NG 108-15 cells to 100 nM Δ 9-THC produced a significant elevation of steady-state levels of δ opioid receptor mRNA. This effect was not observed in cells pretreated with PTX. The selective cannabinoid receptor antagonist SR 141716A blocked the effects elicited by Δ 9-THC on δ opioid receptor desensitization, down-regulation and gene expression; thus indicating that these are mediated via activation of cannabinoid receptors. These data demonstrate the existence, in NG 108-15 cells, of a complex cross-talk between the cannabinoid and opioid receptors on prolonged exposure to Δ 9-THC triggered by changes in signaling through G i and/or G 0-coupled receptors.