Characterization of the turning response of dorsal root neurites toward nerve growth factor.

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Abstract
Chick dorsal root ganglion neurites undergo a rapid (20 min) reorientation of their direction of growth in response to nerve growth factor (NGF) concentration gradients in vitro. Dorsal root ganglia from chick embryos were explanted onto a collagen-poly-L-lysine substrate. After 24-48 h in culture, NGF gradients were applied to individual growth cones via a micropipette containing 50 biological units NGF/ml. The growth cones turned and grew toward these NGF sources. This turning response was not caused by the trophic effects of NGF on neurite initiation, survival or growth rate. Dorsal root neurites also grew toward sources of mono- and dibutyryl [dB] cAMP (cGMP), and elevated Cal in the presence of the Cal ionophore A23187. These results are consistent with the hypothesis that intracellular levels of cAMP and/or cGMP and Cal may play a role in the turning response of dorsal root neurites toward NGF, but do not establish a causal relationship between the mechanisms of action of NGF, cyclic neucleotides and Cal. Total growth cone adherence to the substrate was measured using a timed microjet of perfusion medium. NGF increased the adherence of growth cones to the substrate, but caffeine and dB cAMP which also elicit the positive turning response decreased growth cone adherence. Cal, which did not elicit the positive turning response, produced a greater growth cone adherence to the substrate than that observed with NGF. Although these results do not rule out a role of adhesion changes in axonal turning to NGF, they show that a general increase in adherenc does not correlate well with the rapid turning response observed.