Gamma oscillations (30 -120 Hz) are thought to be important for various cognitive functions, including perception and working memory, and disruption of these oscillations has been implicated in brain disorders, such as schizophrenia and Alzheimer's disease. The cornu ammonis area 1 (CA1) of the hippocampus receives gamma frequency inputs from upstream regions (cornu ammonis area 3 and medial entorhinal cortex) and generates itself a faster gamma oscillation. The exact nature and origin of the intrinsic CA1 gamma oscillation is still under debate. Here, we expressed channelrhodopsin-2 under the CaMKII␣ promoter in mice and prepared hippocampal slices to produce a model of intrinsic CA1 gamma oscillations. Sinusoidal optical stimulation of CA1 at theta frequency was found to induce robust theta-nested gamma oscillations with a temporal and spatial profile similar to CA1 gamma in vivo. The results suggest the presence of a single gamma rhythm generator with a frequency range of 65-75 Hz at 32°C. Pharmacological analysis found that the oscillations depended on both AMPA and GABA A receptors. Cell-attached and whole-cell recordings revealed that excitatory neuron firing slightly preceded interneuron firing within each gamma cycle, suggesting that this intrinsic CA1 gamma oscillation is generated with a pyramidal-interneuron circuit mechanism.

Synaptotagmin 1 (Syt1) synchronizes neurotransmitter release to action potentials (APs) acting as the fast Ca 2+ release sensor and as the inhibitor (clamp) of spontaneous and delayed asynchronous release. While the Syt1 Ca 2+ activation mechanism has been wellcharacterized, how Syt1 clamps transmitter release remains enigmatic. Here we show that C2B domain-dependent oligomerization provides the molecular basis for the Syt1 clamping function. This follows from the investigation of a designed mutation (F349A), which selectively destabilizes Syt1 oligomerization. Using a combination of fluorescence imaging and electrophysiology in neocortical synapses, we show that Syt1 F349A is more efficient than wild-type Syt1 (Syt1 WT ) in triggering synchronous transmitter release but fails to clamp spontaneous and synaptotagmin 7 (Syt7)-mediated asynchronous release components both in rescue (Syt1 −/− knockout background) and dominant-interference (Syt1 +/+ background) conditions. Thus, we conclude that Ca 2+ -sensitive Syt1 oligomers, acting as an exocytosis clamp, are critical for maintaining the balance among the different modes of neurotransmitter release. synaptic transmission | synaptotagmin | C2B domain | fusion clamp T ightly regulated synaptic release of neurotransmitters forms the basis of neuronal communication in the brain. Synaptotagmin 1 (Syt1) plays a key role in this process, both as the major Ca 2+ sensor for fast synchronous action potential (AP)-evoked transmitter release and as an inhibitor of spontaneous and delayed evoked asynchronous release. Syt1 is an integral membrane protein of synaptic vesicles containing a large cytosolic part composed of 2 tandemly arranged Ca 2+ -binding C2 domains (C2A and C2B). The C2B domain also binds SNARE (soluble Nethylmaleimide-sensitive factor attachment protein receptor) complexes and acidic lipids on the presynaptic membrane, and thus supports the generation and maintenance of the readily releasable pool (RRP) of vesicles docked at the synaptic active zone (1-3). Action potential-evoked depolarization triggers opening of presynaptic voltage-gated Ca 2+ channels, resulting in a transient and spatially restricted increase of [Ca 2+ ] at the vesicular release sites. Upon Ca 2+ binding, the adjacent aliphatic loops on Syt1 C2 domains insert into the presynaptic membrane and this triggers rapid (submillisecond timescale) fusion of RRP vesicles (4, 5).In physiological conditions, Syt1 also acts as a suppressor (or "clamp") of spontaneous transmitter release (that occurs in the absence of neuronal spiking) and of the delayed lasting increase in vesicular exocytosis that follows APs (asynchronous release). Indeed, at many synapses, genetic deletion of Syt1 not only abolishes the fast synchronous release component but also leads to a severalfold enhancement of spontaneous and asynchronous release (6-8). Similarly, deletion of the SNARE-binding presynaptic protein complexin partially abrogates the clamping phenotype, suggesting that a synergistic action of both Syt1 and complex...

Synaptotagmin1 (Syt1) synchronises neurotransmitter release to action potentials acting as the fast Ca 2+ release sensor and as the inhibitor (clamp) of spontaneous and delayed asynchronous release. Whilst the Syt1 Ca 2+ activation mechanism has been well characterised, how Syt1 clamps transmitter release remains enigmatic. Here we show that C2B domain-dependent oligomerisation provides the molecular basis for the Syt1 clamping function. This follows from the investigation of a designed mutation (F349A), which selectively destabilises Syt1 oligomerisation. Using combination of fluorescence imaging and electrophysiology in neocortical synapses we show that Syt1 F349A is more efficient than wild type Syt1 (Syt1 WT ) in triggering synchronous transmitter release but fails to clamp spontaneous and Synaptotagmin7 (Syt7)-mediated asynchronous release components both in rescue (Syt1 -/knock-out background) and dominant-interference (Syt1 +/+ background) conditions. Thus we conclude that Ca 2+ -sensitive Syt1 oligomers, acting as an exocytosis clamp, are critical for maintaining the balance among the different modes of neurotransmitter release.