Re-evaluation of phorbol ester-induced potentiation of transmitter release from mossy fibre terminals of the mouse hippocampus

To investigate the mechanisms by which phorbol esters potentiate transmitter release from mossy fibre terminals we used fura dextran to measure the intraterminal Ca2+ concentration in mouse hippocampal slices. A phorbol ester, phorbol 12,13-diacetate (PDAc), potentiated the field excitatory postsynaptic potential (fEPSP) slope. PDAc also enhanced the stimulation-dependent increase of [Ca2+]i in the mossy fibre terminal (Delta[Ca2+]pre). The magnitude of the PDAc-induced fEPSP potentiation (463+/-57% at 10 microM) was larger than that expected from the enhancement of Delta[Ca2+]pre (153+/-5%). The Delta[Ca2+]pre was suppressed by omega-agatoxin IVA (omega-AgTxIVA, 200 nM), a P/Q-type Ca2+ channel-specific blocker, by 31%. The effect of PDAc did not select between omega-AgTxIVA-sensitive and -resistant components. The PDAc-induced potentiation of the fEPSP slope was partially antagonized by the protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIS-I, 10 microM), whereas the Delta[Ca2+]pre was completely blocked by BIS-I. Although the BIS-I-sensitive fEPSP potentiation was accompanied by a reduction of the paired-pulse ratio (PPR), the BIS-I-resistant component was not. Whole-cell patch clamp recording from a CA3 pyramidal neuron in a BIS-I-treated slice demonstrated that PDAc (10 microM) increased the frequency of miniature excitatory postsynaptic currents (mEPSCs, 259+/-33% of control) without a noticeable change in their amplitude (102+/-5% of control). These results suggest that PKC potentiates transmitter release by at least two distinct mechanisms, one Delta[Ca2+]pre dependent and the other Delta[Ca2+]pre independent. In addition, some phorbol ester-mediated potentiation of synaptic transmission appears to occur without activating PKC.