Journal of Neuroscience, Vol 12, 338-344, Copyright © 1992 by Society for Neuroscience
Modulation of la EPSP amplitude: the effects of chronic synaptic inactivity
CB Webb and TC Cope
Physiology Department, Hahnemann University, Philadelphia, Pennsylvania 19107.
In this study, we test the hypothesis that monosynaptic connections between
la afferents and spinal motoneurons are strengthened by chronic disuse.
Impulse activity along the medial gastrocnemius (MG) nerve was blocked for
2 weeks using TTX delivered by an osmotic minipump to a Silastic cuff
placed around the nerve. The duration and specificity of this block were
confirmed by chronic EMG recordings from several triceps surae muscles. The
effect of TTX-induced inactivity of presynaptic elements on EPSP amplitude
was distinguished from the effect of treating the postsynaptic target by
comparing the results from heteronymous synaptic connections, where only
one or the other element was treated. After 2 weeks of synaptic inactivity,
the heteronymous EPSPs generated by MG la afferents in lateral
gastrocnemius/soleus (LG-Sol) motoneurons were significantly (p less than
0.005) larger than control values (48%). Sample differences in rheobase
current and half-afterhyperpolarization, both of which may covary with EPSP
amplitude, did not account for the differences between groups. Segregation
of the two samples of motoneurons by rheobase current identified the
increase as being confined to those LG-Sol cells whose rheobase fell below
10 nA. In addition, EPSPs generated by untreated LG-Sol la afferents in
treated MG motoneurons were significantly enhanced (39%, p less than 0.05).
Thus, TTX treatment of either presynaptic or postsynaptic elements
increases synaptic strength. This increase in monosynaptic EPSP amplitude
following TTX- induced inactivity may reflect an alteration intrinsic to
the la afferent to motoneuron synapse, but influences from extrinsic
sources cannot be discounted.
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