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-
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
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