Blockade of the long-term synaptic effect of NT-3 by C.mycin -induced PKR activation. (A) Sample recordings showing long-term synaptic potentiation by NT-3. The Xenopus nerve-muscle co-cultures were grown in the presence or absence of NT-3 (5 ng/ml) for 2 days. Expression of GyrB-PKR alone did not prevent NT-3-mediated long-term synaptic potentiation. However, application of C.mycin to synapses expressing GyrB-PKR presynaptically completely prevented long-term synaptic potentiation by NT-3. (B) Summary of long-term effect on NT-3 on synapses without GyrB-PKR expression, and synapses with pre- or post-syanptic expression of GyrB-PKR. Each data point represents SSCs frequency (averaged from 30 min of recording) form a single synapse with or without coumermycin treatment. Note that application of C.mycin alone could not attenuate the increase in SSC frequency by NT-3 chronic treatment. Application of C.mycin to synapses expression GyrB-PKR presynaptically, but not postsynaptically, completely prevented long-term synaptic effect of NT-3. The number associated with each column represents the number of cells analyzed. Data are presented as the mean ± the SEM.