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

1. 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. 2. A phorbol ester, phorbol 12,13-diacetate (PDAc), potentiated the held 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 muM) was larger than that expected from the enhancement of Delta [Ca2+](pre) (153 +/- 5%). 3. The Delta [Ca2+](pre) was suppressed by omega -agatoxin IVA (omega -AgTx(IVA), 200 nM), a P/Q-type Ca2+ channel-specific blocker, by 31%. The effect of PDAc did not select between omega -AgTx(IVA)- sensitive and -resistant; components. 4. The PDAc-induced potentiation of the fEPSP slope was partially antagonized by the protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIS-I, 10 muM), 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. 5. Whole-cell patch clamp recording from a CA3 pyramidal neuron in a BI-I-treated slice demonstrated that PDAc (10 muM) increased the frequency of miniature excitatory postsynaptic currents (mEPSCs, 259 +/- 33 % of control) without a noticeable change in their amplitude (102 +/- 5 % of control). 6. 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.