Activation of Ca²⁺-dependent currents in cultured rat dorsal root ganglion neurones by a sperm factor and cyclic ADP-ribose

The effects of intracellular application of two novel Ca²⁺ releasing agents have been studied in cultured rat dorsal root ganglion (DRG) neurones by monitoring Ca²⁺-dependent currents as a physiological index of raised free cytosolic Ca²⁺ ([Ca²⁺]_i). A protein based sperm factor (SF) extracted from mammalian sperm, has been found to trigger Ca²⁺ oscillations and to sensitize unfertilized mammalian eggs to calcium induced calcium release (CICR). In this study intracellular application of SF activated Ca²⁺-dependent currents in approximately two-thirds of DRG neurones. The SF induced activity was abolished by heat treatment, attenuated by increasing the intracellular Ca²⁺ buffering capacity of the cells and persisted when extracellular Ca²⁺ was replaced by Ba²⁺. In addition, activity could be triggered or potentiated by loading the cells with Ca²⁺ by activating a series of voltage-gated Ca²⁺ currents. Ca²⁺-activated inward current activity was also generated by intracellular application of cyclic ADP-ribose (cADPR), a metabolite of NAD⁺, which causes Ca²⁺ release in sea urchin eggs. This activity could also be enhanced by loading the cells with Ca²⁺. The cADPR induced activity, but not the SF induced activity, was abolished by depleting the caffeine sensitive Ca²⁺ store. Ruthenium red markedly attenuated SF induced activity but had little action on cADPR induced activity or caffeine induced activity. Our results indicate that both SF and cADPR release intracellular Ca²⁺ pools in DRG neurones and that they appear to act on subtly distinct stores or distinct intracellular Ca²⁺ release mechanisms, possibly by modulating CICR.