Presynaptic control of quantal size: kinetic mechanisms and implications for synaptic transmission and plasticity

Although the strength of quantal synaptic transmission is jointly controlled by pre- and post-synaptic mechanisms, the presynaptic mechanisms remain substantially less well characterized. Recent studies reveal that a single package of neurotransmitter is generally insufficient to activate all available postsynaptic receptors, whereas the sum of transmitter from multiple vesicles can result in receptor saturation. Thus, depending upon the number of vesicles released, a given synaptic pathway might be either 'reliable' or 'unreliable'. A lack of receptor saturation in turn makes it possible to modify quantal size by altering the flux of transmitter through the synaptic cleft. Studies are now illuminating several new mechanisms behind the regulation of this transmitter flux--characteristics that control how transmitter is loaded into vesicles, how it is released and the manner by which it interacts with postsynaptic receptors.