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Summary High-frequency stimulation transiently increases spontaneous synaptic transmission and the amplitude of evoked synaptic transmission (known as post-tetanic potentiation, PTP). Here we examine the roles of the calcium-dependent protein kinase C isoforms PKCα and PKCβ in PTP at the calyx of Held synapse. In PKCα/β double knockouts 80 % of PTP is eliminated, whereas basal synaptic properties are unaffected. PKCα and PKCβ produce PTP by increasing the size of the readily-releasable pool of vesicles evoked by high-frequency stimulation, and by increasing the fraction of this pool released by the first stimulus. PKCα and PKCβ do not facilitate presynaptic calcium currents. The small PTP remaining in double knockouts is mediated partly by an increase in mEPSC amplitude, and partly by a mechanism involving myosin light chain kinase. These experiments establish that PKCα and PKCβ are crucial for PTP, and suggest that long-lasting presynaptic calcium increases produced by tetanic stimulation may activate these isoforms to produce PTP.

Summary Post-tetanic potentiation (PTP) is a widely observed form of short-term plasticity lasting for tens of seconds after high-frequency stimulation. Here we show that although protein kinase C (PKC) mediates PTP at the calyx of Held synapse in the auditory brainstem before and after hearing onset, PTP is produced primarily by an increased probability of release (p) before hearing onset, and by an increased readily-releasable pool of vesicles (RRP) thereafter. We find that these mechanistic differences, which have distinct functional consequences, reflect unexpected differential actions of closely related calcium-dependent PKC isoforms. Prior to hearing onset, when PKCγ and PKCβ are both present, PKCγ mediates PTP by increasing p and partially suppressing PKCβ actions. After hearing onset, PKCγ is absent and PKCβ produces PTP by increasing RRP. In hearing animals, virally expressed PKCγ overrides PKCβ to produce PTP by increasing p. Thus, two similar PKC isoforms mediate PTP in distinctly different ways.

Golgi cells (GoCs) are inhibitory interneurons that influence the cerebellar cortical response to sensory input by regulating the excitability of the granule cell layer. While GoC inhibition is essential for normal motor coordination, little is known about the circuit dynamics that govern the activity of these cells. In particular, while GoC spontaneous spiking influences the extent of inhibition and gain throughout the granule cell layer, it is not known whether this spontaneous activity can be modulated in a long-term manner. Here we describe a form of long-term plasticity that regulates the spontaneous firing rate of GoCs in the rat cerebellar cortex. We find that membrane hyperpolarization, either by mGluR2 activation of potassium channels, or by somatic current injection, induces a long-lasting increase in GoC spontaneous firing. This spike rate plasticity appears to result from a strong reduction in the spike afterhyperpolarization (AHP). Pharmacological manipulations suggest the involvement of calcium-calmodulin dependent kinase II (CaMKII) and calcium-activated potassium channels in mediating these firing rate increases. As a consequence of this plasticity, GoC spontaneous spiking is selectively enhanced, but the gain of evoked spiking is unaffected. Hence this plasticity is well-suited for selectively regulating the tonic output of GoCs rather than their sensory-evoked responses.

In presynaptic boutons, calcium (Ca2+) triggers both neurotransmitter release and short-term synaptic plasticity. Whereas synaptotagmins are known to mediate vesicle fusion through binding of high local Ca2+ to their C2 domains, the proteins that sense smaller global Ca2+ increases to produce short-term plasticity have remained elusive. Here, we identify a Ca2+ sensor for post-tetanic potentiation (PTP), a form of plasticity thought to underlie short-term memory. We find that at the functionally mature calyx of Held synapse the Ca2+-dependent protein kinase C isoforms α and β are necessary for PTP, and the expression of PKCβ in PKCαβ double knockout mice rescues PTP. Disruption of Ca2+ binding to the PKCβ C2 domain specifically prevents PTP without impairing other PKCβ-dependent forms of synaptic enhancement. We conclude that different C2-domain-containing presynaptic proteins are engaged by different Ca2+ signals, and that Ca2+ increases evoked by tetanic stimulation are sensed by PKCβ to produce PTP.DOI: http://dx.doi.org/10.7554/eLife.03011.001

Depolarization of presynaptic terminals that arises from activation of presynaptic ionotropic receptors, or somatic depolarization, can enhance neurotransmitter release; however, the molecular mechanisms mediating this plasticity are not known. Here we investigate the mechanism of this enhancement at the calyx of Held synapse, where presynaptic glycine receptors depolarize presynaptic terminals, elevate resting calcium levels and potentiate release. Using knockout mice of the calcium-sensitive PKC isoforms (PKCCa), we find that enhancement of evoked but not spontaneous synaptic transmission by glycine is mediated primarily by PKCCa. Measurements of calcium at the calyx of Held indicate that deficits in synaptic modulation in PKCCa knockout mice occur downstream of presynaptic calcium increases. Glycine enhances synaptic transmission primarily by increasing the effective size of the pool of readily-releasable vesicles. Our results reveal that PKCCa can enhance evoked neurotransmitter release in response to calcium increases caused by small presynaptic depolarizations.

Our article contained a typo in the last equation of the online supplement. The equation for w 1 should have read w 1 z 1. As this equation was not used to fit the data (for that, we used the full equation given in the main article), the typo had no consequences. The supplement has now been corrected online.

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