Protein kinase C increases the apparent affinity of the release machinery to Ca2+ by enhancing the release machinery downstream of the Ca2+ sensor

Modulation of the release probability of releasable vesicles in response to Ca2+ influx (Prob(Ca)) is involved in mediating several forms of synaptic plasticity, including short-term depression, short-term augmentation, and p otentiation induced by protein kinases. Given such an important role, howev er, the mechanism underlying modulation of the Prob(Ca) is unclear. We addr essed this question by investigating how the activation of protein kinase C modulates the Prob(Ca) at a calyx-type nerve terminal in rat brainstem. Va rious lengths of step depolarization were applied to the nerve terminal to evoke different amounts of Ca2+ currents and capacitance jumps, the latter of which reflect vesicle release. The relationship between the capacitance jump and the Ca2+ current integral was sigmoidal and was fit well with a Hi ll function. The sigmoidal relationship was shifted significantly to the le ft during the application of the PKC activator 12-myristate 13-acetate (PMA ), suggesting that PMA increases the apparent affinity of the release machi nery to Ca2+. This effect was blocked in large part by the application of t he PKC inhibitor bisindolylmaleimide, suggesting that the effect is mediate d mainly by the activation of PKC. We also found that PMA increased the rat e of miniature EPSCs evoked by the application of hypertonic sucrose soluti on, which triggers release downstream of the Ca2+ influx. Taken together, o ur results suggest that PKC enhances the apparent affinity of the release m achinery to Ca2+ by a mechanism downstream of the binding between Ca2+ and its sensor. These results have provided the first example of the mechanisms underlying modulation of the Prob(Ca).