REGULATION OF QUANTAL SECRETION BY NEUROTROPHIC FACTORS AT DEVELOPINGMOTONEURONS IN XENOPUS CELL-CULTURES

1. The ability of different neurotrophic factors to maintain and regul ate synaptic function at the developing motoneuron was studied in Xeno pus nerve-muscle co-cultures. Spontaneous synaptic currents (SSCs) wer e measured by using whole-cell voltage-clamped myocytes. 2. Compared w ith natural synapses, motoneurons without contact on a myocyte (naive neurons) released ACh in smaller quantal packets, the amplitude being inversely proportional to the days in culture. The mean SSC amplitudes of naive neurons, which were measured by manipulating a myoball, into contact with the myocyte-free nerve terminals to form a manipulated s ynapse, were 99.5 +/- 6.7 and 48.2 +/- 1.9 pA for day-1 and day-3 cult ures, respectively. 3. Chronic treatment of day-1 cultures with brain- derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophi n-l (NT-4), ciliary neurotrophic factor (CNTF) or glial cell line-deri ved neurotrophic factor (GDNF) for 2 days, increased the ACh quantal s ize of naive motoneurons in a concentration-dependent manner, whereas insulin-like growth factor-1 (IGF-1) and basic fibroblast growth facto r (bFGF) had no effect., even at high concentrations. 4. The interacti on of various neurotrophic factors was examined, using concentrations that gave maximal effects. Combination of CNTF plus BDNF or CNTF plus NT-3 had synergistic effects in potentiating SSC amplitude of the mani pulated synapse of naive neurons, whereas NT-3 plus BDNF, NT-3 plus GD NF, BDNF plus GDNF or CNTF plus GDNF had no synergistic action. 5. Chr onic treatment with d-tubocurarine for 2 days resulted in a reduction of the quantal size of natural synapses. Concomitant treatment with BD NF, NT-3, GDNF, CNTF but not bFGF or IGF-1, reconstituted the SSC ampl itude. 6. Taken together, these findings suggest that BDNF, NT-3, NT-4 , CNTF and GDNF may regulate and maintain the synaptic function of dev eloping motoneurons, and different neurotrophic factors utilizing dist inct signalling mechanisms may have synergistic actions.