Tooth pulp tissue promotes neurite outgrowth from rat trigeminal ganglia in vitro

The mammalian tooth pulp becomes innervated by nociceptive and sympathetic axons relatively late during development, when part of the root has formed. In the adult, regenerating axons from an injured tooth nerve or sprouting axons from uninjured nerves in the vicinity rapidly reinnervate denervated tooth pulps. These observations indicate that tooth pulp tissue can use mol ecular factors to attract pulpal axons from local nerve trunks. The present study examines the hypothesis that these factors include nerve growth fact or (NGF), brain derived neurotrophic factor (BDNF) and glial cell line deri ved neurotrophic factor (GDNF). Explants of trigeminal ganglia from neonata l rat pups showed a distinct neurite outgrowth when co-cultured with pulpal explants collected from molar teeth of 12-day old pups, or after applicati on of a pulpal extract. Control cultures, containing single ganglionic expl ants, or explants co-cultured with heat-treated pulpal tissue, exhibited a sparse neurite outgrowth. Exogenous NGF and/or GDNF, but not exogenous BDNF , stimulated neurite outgrowth from ganglionic explants. Unexpectedly, appl ication of antibodies against NGF, BDNF and/or GDNF to co-cultures of gangl ionic and pulpal explants did not inhibit neuritogenesis. Control experimen ts showed that IgG molecules readily penetrate the gel used for culture and that even very high concentrations of NGF and GDNF antibodies in combinati on failed to block neurite growth. On the basis of these data we suggest th at other as yet unknown neurite-promoting factors might be present and acti ve in TG/pulpal co-cultures.