Rate-limiting step for transmission at excitatory synapses in hippocampus

To examine mechanisms that might be responsible for limiting transmission at excitatory synapses in hippocampus, we analyzed the relationship between extracellular calcium concentrations (1-6 mM) and postsynaptic responses in field CA1 of hippocampal slices using low stimulation intensities and a paired-pulse paradigm. Three effects were observed: One, the relationship between calcium levels and the slope (or amplitude) of the postsynaptic response was described by a sigmoidal function with an asymptote at about 4 mM. Double reciprocal pilots relating calcium concentration to the initial slope of EPSPs provided evidence for the cooperativity expected between calcium ions and transmitter release. Two, both the rise time and half-decay time of the postsynaptic responses were reduced with increasing calcium concentrations. These effects of calcium were more pronounced on the first response elicited by paired-pulse stimulation and were considerably attenuated by 2 microM bicuculline, indicating that feed-forward inhibition was positively related to calcium concentration and differentially activated by repetitive stimulation. However, inhibition was not responsible for the asymptotic relationship observed between calcium and response size. Three, while increasing the calcium concentration beyond 4 mM did not further affect the initial slope of excitatory postsynaptic potentials (EPSPs), paired-pulse facilitation and 4-aminopyridine were still effective in increasing response size. These results suggest 1) that neither the number of postsynaptic receptors nor the number of transmitter quanta available for release were limiting transmission as a function of the calcium concentration; and 2) that calcium entry into presynaptic terminals was likely to represent the limiting step under the conditions used.