Activity-Dependent Induction of Functional Secretory Properties at Cultured Neuromuscular Synapses of Helisoma. Poyer, James C. and Mark J. Zoran. Department of Biology, Texas A&M University, College Station, Texas 77843- 3258.
APStracts 3:0137N, 1996.
SUMMARY AND CONCLUSIONS
1. The role of activity-dependent mechanisms in target-mediated induction of secretory properties was investigated at regenerating neuromuscular synapses of the American pond snail, Helisoma trivolvis , in cell culture. 2. Identified motoneurons were isolated into cell culture conditions that promoted neurite outgrowth. Buccal neurons 19 (B19) were cultured alone for 2 d, at which time dissociated muscle fibers were manipulated into contact with newly formed neurites. 3. Immediately prior to the plating of muscle fibers, the sodium channel blocker, tetrodotoxin (TTX), or the acetylcholine receptor antagonist, d-tubocurarine chloride (curare), was added to the culture dish. After 48 h of exposure, the inhibitors were removed by repeated dilution of the culture medium and electrophysiological analyses were performed. 4. Cholinoceptive assay cells were manipulated into contact with the presynaptic neurons to assess secretory properties along neuronal processes. Assay cells were utilized to control for variations in postsynaptic sensitivity that could result from long-term exposure to activity inhibitors. 5. These analyses demonstrated that inhibition of TTX-sensitive presynaptic activity and inhibition of curare-sensitive postsynaptic activation both blocked the induction of excitation-secretion coupling typically induced in these motoneurons by appropriate target contact. Neuron B5, which rapidly acquires functional synaptic properties in vitro was unaffected in its secretory function by 48 hr of activity inhibition. 6. Acquisition of secretory competence was not suppressed due to a reduction in the viability or long-term changes in excitability of the activity-inhibited neurons, as indicated by analyses of electrophysiological properties. 7. Although target-contact and activity both participated in the induction of secretory modifications in neuron B19, target-mediated changes did not involve retrograde effects on presynaptic neuronal excitability. 8. We hypothesize that contact-mediated mechanisms govern the initiation of presynaptic modifications in B19, however, our data indicate that the acquisition of functional excitation-secretion coupling also involves activity-dependent mechanisms. Although the mechanistic role of activity remains undefined, our results suggest that the activation of the target muscle plays a critical role in a retrograde signaling pathway underlying maturation of a functional secretory apparatus in target-contacted neuronal processes. 

Received 5 April 1996; accepted in final form 20 June 1996.
APS Manuscript Number J81-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 4 July 96