(34) INHIBITION OF GLUTAMATE-INDUCED MITOCHONDRIAL DEPOLARIZATION BY TAMOXIFEN IN INTACT CULTURED NEURONS

Kari R. Hoyt
Department of Neurology, The Ohio State University College of Medicine, Columbus, OH 43210

The mitochondrial permeability transition (MPT) has been identified as a possible common effector of the cell death of numerous cell types in response to a variety of toxic agents. In central neurons, glutamate receptor activation causes massive calcium influx and induces a mitochondrial depolarization which is blocked by the MPT inhibitor cyclosporine A. This glutamate-induced neurotoxicity is involved in the cell loss caused by stroke and trauma, as well as potentially chronic neurodegenerative diseases.

It has been recently reported that tamoxifen (an anti-estrogen chemotherapeutic agent) blocks the MPT in isolated liver mitochondria, like cyclosporine A (Custodio, Moreno, and Wallace. Tox. Appl. Pharm., In Press ). Since MPT may be central to the cell death caused by glutamate receptor activation, we tested whether tamoxifen inhibits the mitochondrial depolarization induced by glutamate receptor activation in intact cultured neurons. Neurons were loaded with the fluorescent dye, JC-1, which reports disruptions in mitochondrial membrane potential, which can be caused by MPT activation. We found that glutamate (100 uM for 10 min) causes a robust mitochondrial depolarization which is partially inhibited by tamoxifen. The maximum inhibitory concentration of tamoxifen was 300 nM, with concentrations higher and lower than 300 nM being less effective. Tamoxifen (300 nM) did not inhibit glutamate receptor activated increases in intracellular calcium suggesting that it does not directly inhibit receptor activation. However, while tamoxifen (300 nM) blocked glutamate-induced mitochondrial depolarization, preliminary data suggest that it does not block glutamate-induced neuronal death, unlike cyclosporine A. A relatively high concentration of tamoxifen (100 uM) caused mitochondrial depolarization itself, and was neurotoxic.

These data suggest that tamoxifen is a potent inhibitor of MPT in intact neurons, in agreement with previous studies in isolated liver mitochondria, adding to the relatively short list of drugs with such action. Given that tamoxifen is already a widely-used drug clinically for chemotherapy and chemoprevention, tamoxifen may ultimately be of some benefit in conditions which may arise from activation of MPT such as ischemic damage.