Interaction of Postsynaptic Receptor Saturation with Presynaptic Mechanisms Produces a Reliable Synapse

Synapses that reliably activate their postsynaptic targets typically release neurotransmitter with high probability, are not very sensitive to changes in calcium entry, and depress. We have determined the mechanisms that give rise to these characteristic features at the climbing fiber to Purkinje cell synapse. We find that saturation of presynaptic calcium entry, of presynaptic release, and of postsynaptic receptors combine to produce a postsynaptic response that is near maximal. Postsynaptic receptor saturation also accelerates recovery from depression, in part by accentuating a rapid calcium-dependent recovery phase. Thus, postsynaptic receptor saturation interacts with presynaptic mechanisms to produce highly reliable synapses that can effectively drive their targets even during sustained activation.