Probabilistic Synaptic Transmission in Hippocampal Neurons: Role of Presynaptic Calcium
lie factors governing the apparently stochastic release of neurotransmitter at central synapses are largely unknown. One potential factor is fluctuation in calcium entry into release sites - for instance, failure of transmitter release could be due to failure of calcium entry into preynaptic terminals. We tested this hpthesis by recording action potential-evoked calcium transients at single presynaptic sites of cultured hippocampal neurons. Calcium concentration rises were observed reliably at putative release sites, with no apparent calcium entry failures. However, in some release sites, there was evidenceforf7uctuations in amplitude of calcium transients that exceeded variation estimated @om baseline noise. This trial-to-trial fluctuation in calcium entry could potentially play a role in determining transmitter release probability.