Axonal outgrowth and neuronal apoptosis in cultured adult mouse dorsal root ganglion preparations: effects of neurotrophins, of inhibition of neurotrophin actions and of prior axotomy.
Dorsal root ganglia (L4 and L5) with attached spinal roots and nerve stumps were isolated from young adult mice and cultured in a layer of extracellular matrix material (matrigel). Within one day, a large number of axons grew out from the cut ends of the nerve and the dorsal root. The average outgrowth length was more than doubled by nerve growth factor, which also strongly increased the number of fibres, showing extensive branching. There was also a significant outgrowth stimulation by neurotrophin-3, but no observable effect by brain-derived neurotrophic factor. In preparations isolated and cultured six days after peripheral nerve transection in vivo, there was an increase in both the outgrowth length (about 1.5- to 2-fold) and in the number of axons. Stimulation of axonal outgrowth, which concerned outgrowth from both the peripheral nerve and the dorsal root, could be further enhanced by the addition of nerve growth factor to the culture. K-252a, a selective inhibitor of neurotrophin receptor-associated tyrosine kinase activity, did not affect either the normal outgrowth or the increased outgrowth in pre-axotomized preparations, at a concentration which abolished the stimulating effects by exogenous nerve growth factor and neurotrophin-3. Under the culturing conditions used, spontaneous apoptosis occurred, but none of the neurotrophins tested, nor K-252a, affected the number of apoptotic neuronal cells analysed by nick-labelling DNA breaks at the end of a 48-h culturing period. Altogether, the present data suggest that for most dorsal root ganglia neurons, signalling through the trk receptors does not influence the apoptosis in vitro and is not required for either the spontaneous axonal outgrowth in matrigel or the increased outgrowth which occurs after prior axotomy in vivo.