ORIGIN OF SYNAPTIC DEPRESSION

It has been shown that repetitive stimulation leads to depression of p ostsynaptic potentials. The prevalent interpretation of this phenomeno n is the reduction in the number of "quanta" of the transmitter releas ed from the cell. The synaptic depression occurs only in synapses with a high level of release and the extent of depression depends on the q uantal content. Therefore, it was suggested that the depression result ed from depletion of the immediately available store of transmitter. I n many, although not all, cases the statistical studies support this v iew and show that depression is associated with a decrease in the bino mial parameter n. Usually this parameter is interpreted as an indicati on of the number of transmitter vesicles in the immediately available store. However, studies with prolonged stimulation at central synapses in fishes and at neuromuscular junctions in frogs show that a reducti on in quantal size may also significantly contribute to depression. In complete filling of vesicles would not be surprising if, as many studi es suggest, the newly formed transmitter is preferentially incorporate d into small but metabolically active store of quanta. Furthermore, th e synaptic depression can be affected greatly by the stimulation induc ed increase in the capacity of the immediately available transmitter s tore or by desensitization of postsynaptic transmitter receptors.