Protein kinase C potentiates transmitter release from the chick ciliary presynaptic terminal by increasing the exocytotic fusion probability

1. The giant presynaptic terminal of chick ciliary ganglion was used to exa mine how protein kinase C (PKC) modulates neurotransmitter release. Choline rgic excitatory post synaptic currents (EPSCs) were recorded under whole-ce ll voltage clamp. 2. Although the EPSC was potentiated by phorbol ester (phorbol 12-myristate 13-acetate, PMA; 0.1 mu M) in a Sustained manner, the nicotine-induced cur rent was unaffected. PMA increased the quantal content to 2.4 +/- 0.4 (n = 9) of control without changing the quantal size. 3. The inactive isoform of PMA, 4 alpha-PMA, showed no significant effect o n EPSCs. The PMA-induced potentiation was antagonized by two PKC inhibitors with differ ent modes of action, sphingosine (20 mu M) and bisindolylmalei mide I(10 mu M). 4. When stimulated by twin pulses of short interval, the second EPSC was on average larger than the first EPSC (paired-pulse facilitation; PPF). PMA s ignificantly decreased the PPF ratio with a time course similar to that of the potentiation of the first EPSC. 5. PMA did not affect resting [Ca2+](i) or the action potential-induced [Ca 2+], increment in the giant presynaptic terminals. 6. The effect of PMA was less at 10 mM [Ca2+](o) than at 1 mM [Ca2+](o). 7. When a train of action potentials was generated with a short interval, t he EPSC was eventually depressed and reached a steady-state level. The reco very process followed ed a simple exponential relation with a rate constant of 0.132 +/- 0.029 s(-1) PMA did not affect the recovery rate constant of EPSCs from tetanic depression. In addition, PMA did not affect the steady-s tate EPSC which should be proportional to the refilling rate of the readily releasable pool of vesicles. 8. These results conflict with the hypothesis that PKC upregulates the size of the readily releasable pool or the number of release sites. PKC: appear s to upregulate the Ca2+ sensitivity of the process that controls the exocy totic fusion probability.