Ototoxic effects of mefloquine in cochlear organotypic cultures

Abstract

Mefloquine is a widely used anti–malarial drug. Some clinical reports suggest that mefloquine may be ototoxic and neurotoxic, but there is little scientific evidence from which to draw any firm conclusion. To evaluate the ototoxic and neurotoxic potential of mefloquine, we treated cochlear organotypic cultures and spiral ganglion cultures with various concentrations of mefloquine. Mefloquine caused a dose–dependent loss of cochlear hair cells at doses exceeding 0.01mM. Hair cell loss progressed from base to apex and from outer to inner hair cells with increasing dose. Spiral ganglion neurons and auditory nerve fibers were also rapidly destroyed by mefloquine in a dose–dependent manner. To investigate the mechanisms underlying mefloquine–induced cell death, cochlear cultures were stained with TO–Pro–3 to identify morphological changes in the nucleus, and with carboxyfluorescein FAM–labeled caspase inhibitor 8, 9 or 3 to determine caspase–mediated cell death. TO–Pro–3 –labeled nuclei in hair cells, spiral ganglion neurons and supporting cells were shrunken or fragmented, morphological features characteristic of cells undergoing apoptosis. Both initiator caspase 8 (membrane damage) and caspase 9 (mitochondrial damage), along with executioner caspase 3, were heavily expressed in cochlear hair cells and spiral ganglions after mefloquine treatment. These three caspases were also expressed in support cells, although labeling was less widespread and less intense. These results indicate that mefloquine damages both the sensory and neural elements in the postnatal rat cochlea by initially activating cell death signaling pathways on the cell membrane and in mitochondria.