Journal of Neuroscience, Vol 11, 1040-1048, Copyright © 1991 by Society for Neuroscience
Activity-dependent development of synaptic varicosities at crayfish motor terminals
GA Lnenicka, SJ Hong, M Combatti and S LePage
Department of Biological Sciences, State University of New York, Albany 12222.
Tonically and phasically active crayfish motor terminals have well-
characterized differences in synaptic physiology. During repetitive
activation, the tonic terminals show facilitation and no depression, while
the phasic terminals show dramatic synaptic fatigue. It has been proposed
that this greater capacity for transmitter release from tonic terminals may
be largely due to the presence of large synaptic varicosities along tonic
terminals that contain large mitochondria, synapses, and numerous synaptic
vesicles. In addition, a recent study indicates that in vivo tonic
stimulation of a phasic terminal in young animals increases the fatigue
resistance of the neuromuscular synapses and increases the number of
synaptic varicosities, as well as the size of mitochondria and synapses.
Because these previous morphological studies of crustacean motor terminals
were performed using transmission electron microscopy, only short lengths
of terminal were observed, and changes in synaptic varicosity frequency and
size could not be thoroughly examined. In order to examine the synaptic
varicosities along the terminal, motor terminals were injected with HRP,
and the morphology of contrast-enhanced light microscopic images was
examined. We report here that tonic terminals have much larger and more
numerous synaptic varicosities than phasic terminals. In vivo stimulation
of a phasic motoneuron results in an increase in the frequency of synaptic
varicosities. This represents an increase in the total number of
varicosities because terminal length appears to remain constant. These
synaptic varicosities appear to form along preexisting terminals and
persist for days after the final in vivo stimulation. The results indicate
that the greater transmitter-releasing capabilities of tonic terminals
compared to phasic terminals are due to their larger and more frequent
synaptic varicosities. In addition, the development of these synaptic
varicosities is activity dependent.
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