CAPSAICIN-INDUCED NEUROTOXICITY IN CULTURED DORSAL-ROOT GANGLION NEURONS - INVOLVEMENT OF CALCIUM-ACTIVATED PROTEASES

We examined the mechanism by which capsaicin produces its toxic effect s on cultures of rat sensory neurons. Capsaicin caused a robust increa se in [Ca2+](i) in a subpopulation of cultured rat dorsal root ganglio n neurons. Similarly, a brief exposure to capsaicin resulted in delaye d degeneration of a subpopulation of the cells. This subpopulation (ab out 35% of the cells present) was characterized by a capsaicin-induced uptake of Co2+, which could be detected cytochemically. Both capsaici n-induced Co2+ uptake and capsaicin-induced cell death were blocked by the capsaicin antagonist Ruthenium Red. Cell death was also prevented by removal of external calcium or by inhibiting calcium-activated pro teases such as calpain. Evidence that calpain activity was increased w as provided by examining the amount of degradation of the preferred ca lpain substrate alpha-spectrin. Capsaicin treatment produced a signifi cant increase in the levels of the 150,000 molecular weight spectrin b reakdown product. Furthermore, applying the protease inhibitors E64 or MDL 28,170 reduced capsaicin-mediated cell death. It is concluded tha t capsaicin kills a subpopulation of sensory neurons by activating a r eceptor-operated channel. The consequent Ca2+ ion influx causes large increases in [Ca2+](i) and subsequent activation of Ca2+-sensitive pro teases. This model provides support for the role of [Ca2+](i) as the o rchestrator of delayed neuronal degeneration.