The interactions between applied direct current electric fields and neurotrophic factors in guiding cultured embryonic Xenopus laevis neurons
I have investigated whether or not neurotrophic factors can alter the effects of an applied electric field on neurite growth. A direct current electric field (150mV/mm) applied for 5 hours affected the growth of dissociated cells from the neutral tube of stage 20 Xenopus laevis. Neurites turned and grew to the cathode, showed a differential rate of growth, with cathodal-facing neurites growing faster than those facing the anode, and exhibited a polarised branching pattern, with the majority of branches emanating from the cathodal-facing side of the neurite. Exposure of neurites to the neurotrophins neurotrophin 3 (NT-3; 50ng/ml and 100ng/ml), brain derived neurotrophic factor (BDNF; 50ng/ml and 100ng/ml) and neurotrophin 4 (NT-4; 100ng/ml) lowered the threshold field strength necessary to elicit a turning response to 100mV/mm. Only BDNF (100ng/ml) enhanced cathodal orientation at 150mV/mm, with neurites turning three times as far as in the field alone. Additionally, in the presence of these neurotrophins the threshold field that evoked a differential rate of growth and polarised branching was also lowered to 100mV/mm. Neurite turning did not occur in a field of 100mV/mm in the presence of 50ng/ml nerve growth factor (NGF), 100ng/ml ciliary neurotrophic factor (CNTF) or 50ng/ml glial-cell-line derived neurotrophic factor (GDNF), or when the Trk receptor tyrosine kinase inhibitors K-252a and K-252b were added concomitantly with 50ng/ml NT-3. Xenopus growth cones release acetylcholine (Ach) spontaneously. This is enhanced by NT-3, BDNF, NT-4. Nicotinic Ach receptor antagonists abolish field-induced cathodal turning, thus Ach release may be crucial for this response. The neurotrophins NT-3, BDNF and NT-4 may therefore modulate field-directed nerve growth because they enhance Ach release. Interactions in vivo, where neurotrophins and electric fields co-exist, would increase the efficacy of endogenous electric fields as modulators of nerve growth.