Interaction between trophic action and electrical activity in spinal cord cultures.

The effect of conditioned medium (CM) on tetrodotoxin (TTX)-mediated neuronal cell death was investigated in dissociated spinal cord-dorsal root ganglion (SC-DRG) cultures. Nutrient medium was collected from donor SC-DRG cultures at 3--4-day intervals throughout development. The presence of survival-promoting neurotrophic material (NTM) in the conditioned medium was tested on TTX-treated cultures which were within the critical period of vulnerability to electrical blockade (days 7-21 in culture). Cultures were assayed by neuronal cell counts, choline acetyltransferase (ChAT) activity or fixation of [125I]tetanus toxin, a neuronal surface marker. Evidence of NTM was found in CM collected prior to day 8 and after day 21 in culture. Increasing the percentage of CM in fresh nutrient medium resulted in a dose-dependent increase in [125I]tetanus toxin fixation and ChAT activity in TTX-treated cultures. In addition, CM plus TTX treatment produced a 25% increase in neuronal cell counts as compared to controls. TTX treatment alone resulted in 20-25% decrease in neuronal number from that of control cultures. Cultures treated with CM alone had neuronal counts that were similar to controls. When electrical activity was blocked with TTX, NTM was not detectable in CM collected from donor cells during days 1-5. CM obtained from control cultures during the same interval had NTM activity. The existence of the critical period for electrical blockade-associated neuron death is associated with a decrease in the availability of NTM. Furthermore, the release of NTM from donor cells and the survival response of target neurons to NTM are apparently dependent on ongoing electrical activity.