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

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Abstract
1 The giant presynaptic terminal of chick ciliary ganglion was used to examine how protein kinase C (PKC) modulates neurotransmitter release. Cholinergic excitatory postsynaptic currents (EPSCs) were recorded under whole-cell voltage clamp. 2 Although the EPSC was potentiated by phorbol ester (phorbol 12-myristate 13-acetate, PMA; 0.1 μm) in a sustained manner, the nicotine-induced current 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, 4α-PMA, showed no significant effect on EPSCs. The PMA-induced potentiation was antagonized by two PKC inhibitors with different modes of action, sphingosine (20 μm) and bisindolylmaleimide I (10 μm). 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 significantly 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 [Ca2+]i 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, the EPSC was eventually depressed and reached a steady-state level. The recovery process followed 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-state 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 appears to upregulate the Ca2+ sensitivity of the process that controls the exocytotic fusion probability.