Evidence that changes in presynaptic calcium currents are not responsible for long-term potentiation in hippocampus

We used two approaches to test the possibility that changes in presynaptic calcium currents might be responsible for the long-term potentiation (LTP) effect induced by high-frequency stimulation in area CA1 of hippocampal slices. In a first series of experiments, we compared the effect of LTP induction on paired-pulse facilitation with that produced by changes in extracellular calcium concentration, a procedure that modifies presynaptic calcium currents during depolarization by changing the ionic gradient for calcium. In hippocampus, as in peripheral synapses, increasing concentrations of extracellular calcium caused a marked reduction in the degree of facilitation obtained with paired-pulse stimulation; LTP, conversely, did not affect the facilitation ratio. The differential effect of changing calcium concentrations versus LTP induction on paired-pulse facilitation was observed with different interpulse intervals as well as in conditions in which the changes in response size produced by the two manipulations were comparable. In the second approach, we measured calcium dependency curves of synaptic responses before and after LTP induction or application of 4-aminopyridine, a blocker of potassium channels that increases presynaptic calcium currents by slowing spike repolarization. Procedures that increase calcium entry into terminals during transmission should shift to the left the sigmoidal function relating extracellular calcium to the slope of the extracellular response. This in turn should result in disproportionate effects of the procedure as a function of the calcium concentration. This prediction was realized with 4-aminopyridine but did not occur following LTP induction: control and potentiated responses were similarly affected by changes in calcium concentration. Although indirectly, these data strongly suggest that LTP is not accompanied by alterations in the presynaptic calcium dynamics associated with transmitter release.