Chronic exposure to kappa-opioids enhances the susceptibility of immortalized neurons (F-11kappa 7) to apoptosis-inducing drugs by a mechanism that may involve ceramide.

Chronic exposure of embryonic brain to opioids leads to microcephaly and developmental abnormalities. An immortalized mouse neuroblastoma x dorsal root ganglion hybrid cell line stably transfected to overexpress kappa-opioid receptors (F-11kappa7) showed complete loss of kappa-receptor binding to [3H]U69,593 after exposure to the kappa-agonist U69,593 for 24 h. U69,593 had no measurable effect on cell viability as determined by either cell viability or DNA fragmentation assays. However, when cell death (apoptosis) was induced by either staurosporine or the phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002, cells pretreated with U69,593 for 24 h showed increased apoptosis compared with untreated cells. Thus, staurosporine (50 nM), wortmannin (4 microM), and LY294002 (30 microM) treatment for 24 h induced a 50% loss of cell viability and DNA fragmentation in 24 h. U69,593 pretreatment produced the same killing at lower concentrations, namely, 20 nM staurosporine, 2 microM wortmannin, and 14 microM LY294002, respectively. The effects of U69,593 were time-, dose-, and naloxone-reversible, suggesting that they are receptor-mediated. However, coaddition of U69,593 at the same time as staurosporine, wortmannin, or LY294002 did not enhance apoptosis. All three drugs that induced apoptosis were found to increase the level of ceramide, and pretreatment with U69,593 further increased the rate of formation of ceramide, a lipid that induces apoptosis in cells. We propose that chronic exposure to kappa-receptor agonists promotes increased vulnerability of neurons to apoptosis.