Time-dependence and cell-type specificity of synergistic neurotrophin actions on spiral ganglion neurons.

The neurotrophins brain-derived neurotrophin (BDNF) and neurotrophin-3 (NT-3) synergistically enhance survival of spiral ganglion neurons such that simultaneous exposure to both compounds produces a larger response than would be expected from their individual effects. To elucidate the functional role of this neurotrophin interaction, we examined its temporal and cell-type specificity in vitro for both mouse and gerbil spiral ganglion neurons. Synergistic effects were transient; they were maximal within the first two postnatal days and declined during the first postnatal week. Both neurotrophins were, however, still efficacious at increasing cell survival. After postnatal day 10, the effects of coexposure to BDNF and NT-3 were additive rather than synergistic. Synergism declined more rapidly in mouse than gerbil neurons, reflecting the difference in cochlear development for each species. Only neurons without peripherin epitopes, putative type I neurons, showed synergistic survival effects; survival of peripherin-expressing neurons was purely additive. Therefore, during a restricted time period, identical neurotrophin stimuli are capable of preferentially enhancing survival of one class of neurons that compose approximately 95% of the adult spiral ganglion.