A significant fraction of the total calcium͞calmodulin-dependent protein kinase II (CaMKII) activity in neurons is associated with synaptic connections and is present in nerve terminals, thus suggesting a role for CaMKII in neurotransmitter release. To determine whether CaMKII regulates neurotransmitter release, we generated and analyzed knockout mice in which the dominant ␣-isoform of CaMKII was specifically deleted from the presynaptic side of the CA3-CA1 hippocampal synapse. Conditional CA3 ␣-CaMKII knockout mice exhibited an unchanged basal probability of neurotransmitter release at CA3-CA1 synapses but showed a significant enhancement in the activity-dependent increase in probability of release during repetitive presynaptic stimulation, as was shown with the analysis of unitary synaptic currents. These data indicate that ␣-CaMKII serves as a negative activity-dependent regulator of neurotransmitter release at hippocampal synapses and maintains synapses in an optimal range of release probabilities necessary for normal synaptic operation.
Synapses mediate interneuronal communication and display a variety of properties (1). Presynaptic neurotransmitter release is one of the first steps in the sequence of events underlying synaptic transmission. Discrete packets of neurotransmitter encased in vesicles are released at synaptic connections with a certain probability (probability of release, P r ) in a tightly regulated fashion (1, 2). Calcium͞calmodulin-dependent protein kinase II (CaMKII), a serine͞threonine protein kinase, is well positioned to serve a role in synaptic function regulation, because it is highly expressed in the brain and is known to phosphorylate multiple synaptic proteins (2). Accordingly, CaMKII has previously been implicated in the long-lasting frequency-dependent regulation of synaptic function in mammals (3-9), birds (10), frogs (11), and invertebrates (12).Although a postsynaptic role for CaMKII in synaptic function and plasticity has been well demonstrated (3,4,6,7,13), studies of the presynaptic function of this enzyme are scarce. Early microinjection experiments in the squid giant synapse showed that CaMKII injected presynaptically enhanced neurotransmitter release (14,15). This effect appeared to be mediated by phosphorylation of a synaptic vesicle protein, synapsin I, which in its dephosphorylated form inhibited synaptic transmission. However, mice lacking synapsin I do not demonstrate a major change in the basal P r (16), suggesting that certain differences in the mechanisms of release may exist between vertebrate and invertebrate synapses. It has also been shown that ␣-CaMKII can either potentiate or depress CA3-CA1 synapses in the hippocampus depending on the pattern of presynaptic activation. Pairedpulse facilitation (PPF), an index of presynaptic function, was decreased in the CA1 region of mice heterozygous for a global null mutation of ␣-CaMKII, whereas another form of presynaptic plasticity, which also depends on P r , posttetanic potentiation, was enhanced (17). A direct esti...