Synaptotagmins 3 and 7 mediate the majority of asynchronous release from synapses in the cerebellum and hippocampus.

Neurotransmitter release consists of rapid synchronous release followed by longer-lasting asynchronous release (AR). Although the presynaptic proteins that trigger synchronous release are well understood, the mechanisms for AR remain unclear. AR is sustained by low concentrations of intracellular Ca and Sr, suggesting the involvement of sensors with high affinities for both ions.

Synaptotagmin-7 Counteracts Short-Term Depression during Phasic Dopamine Release.

Dopamine neurons switch from tonic pacemaker activity to high-frequency bursts in response to salient stimuli. These bursts lead to superlinear increases in dopamine release, and the degree of this increase is highly dependent on firing frequency. The superlinearity and frequency dependence of dopamine release implicate short-term plasticity processes.

Fast resupply of synaptic vesicles requires synaptotagmin-3.

Sustained neuronal activity demands a rapid resupply of synaptic vesicles to maintain reliable synaptic transmission. Such vesicle replenishment is accelerated by submicromolar presynaptic Ca signals by an as-yet unidentified high-affinity Ca sensor. Here we identify synaptotagmin-3 (SYT3) as that presynaptic high-affinity Ca sensor, which drives vesicle replenishment and short-term synaptic plasticity.

Cerebellar Purkinje cell activity modulates aggressive behavior.

Although the cerebellum is traditionally associated with balance and motor function, it also plays wider roles in affective and cognitive behaviors. Evidence suggests that the cerebellar vermis may regulate aggressive behavior, though the cerebellar circuits and patterns of activity that influence aggression remain unclear. We used optogenetic methods to bidirectionally modulate the activity of spatially-delineated cerebellar Purkinje cells to evaluate the impact on aggression in mice.

In Vivo Targeted Expression of Optogenetic Proteins Using Silk/AAV Films.

The quest to understand how neural circuits process information in order to drive behavioral output has been greatly aided by recently-developed optical methods for manipulating and monitoring the activity of neurons in vivo. These types of experiments rely on two main components: 1) implantable devices that provide optical access to the brain, and 2) light-sensitive proteins that change neuronal excitability or provide a readout of neuronal activity. There are a number of ways to express light-sensitive proteins, but stereotaxic injection of viral vectors is currently the most flexible approach because expression can be controlled with genetic, anatomical, and temporal precision.

The high-affinity calcium sensor synaptotagmin-7 serves multiple roles in regulated exocytosis.

Synaptotagmin (Syt) proteins comprise a 17-member family, many of which trigger exocytosis in response to calcium. Historically, most studies have focused on the isoform Syt-1, which serves as the primary calcium sensor in synchronous neurotransmitter release. Recently, Syt-7 has become a topic of broad interest because of its extreme calcium sensitivity and diversity of roles in a wide range of cell types.

Silk Fibroin Films Facilitate Single-Step Targeted Expression of Optogenetic Proteins.

Optical methods of interrogating neural circuits have emerged as powerful tools for understanding how the brain drives behaviors. Optogenetic proteins are widely used to control neuronal activity, while genetically encoded fluorescent reporters are used to monitor activity. These proteins are often expressed by injecting viruses, which frequently leads to inconsistent experiments due to misalignment of expression and optical components.

Synaptotagmin 7 confers frequency invariance onto specialized depressing synapses.

At most synapses in the brain, short-term plasticity dynamically modulates synaptic strength. Rapid frequency-dependent changes in synaptic strength have key roles in sensory adaptation, gain control and many other neural computations. However, some auditory, vestibular and cerebellar synapses maintain constant strength over a wide range of firing frequencies, and as a result efficiently encode firing rates.

The Mechanisms and Functions of Synaptic Facilitation.

The ability of the brain to store and process information relies on changing the strength of connections between neurons. Synaptic facilitation is a form of short-term plasticity that enhances synaptic transmission for less than a second. Facilitation is a ubiquitous phenomenon thought to play critical roles in information transfer and neural processing.

Synaptic Specializations Support Frequency-Independent Purkinje Cell Output from the Cerebellar Cortex.

The output of the cerebellar cortex is conveyed to the deep cerebellar nuclei (DCN) by Purkinje cells (PCs). Here, we characterize the properties of the PC-DCN synapse in juvenile and adult mice and find that prolonged high-frequency stimulation leads to steady-state responses that become increasingly frequency independent within the physiological firing range of PCs in older animals, resulting in a linear relationship between charge transfer and activation frequency. We used a low-affinity antagonist to show that GABA-receptor saturation occurs at this synapse but does not underlie frequency-invariant transmission.

The calcium sensor synaptotagmin 7 is required for synaptic facilitation.

It has been known for more than 70 years that synaptic strength is dynamically regulated in a use-dependent manner. At synapses with a low initial release probability, closely spaced presynaptic action potentials can result in facilitation, a short-term form of enhancement in which each subsequent action potential evokes greater neurotransmitter release. Facilitation can enhance neurotransmitter release considerably and can profoundly influence information transfer across synapses, but the underlying mechanism remains a mystery.

Achieving high-frequency optical control of synaptic transmission.

The optogenetic tool channelrhodopsin-2 (ChR2) is widely used to excite neurons to study neural circuits. Previous optogenetic studies of synapses suggest that light-evoked synaptic responses often exhibit artificial synaptic depression, which has been attributed to either the inability of ChR2 to reliably fire presynaptic axons or to ChR2 elevating the probability of release by depolarizing presynaptic boutons. Here, we compare light-evoked and electrically evoked synaptic responses for high-frequency stimulation at three synapses in the mouse brain.

Regulation of presynaptic strength by controlling Ca2+ channel mobility: effects of cholesterol depletion on release at the cone ribbon synapse.

Synaptic communication requires proper coupling between voltage-gated Ca(2+) (Ca(V)) channels and synaptic vesicles. In photoreceptors, L-type Ca(V) channels are clustered close to synaptic ribbon release sites. Although clustered, Ca(V) channels move continuously within a confined domain slightly larger than the base of the ribbon.

Release from the cone ribbon synapse under bright light conditions can be controlled by the opening of only a few Ca(2+) channels.

Light hyperpolarizes cone photoreceptors, causing synaptic voltage-gated Ca(2+) channels to open infrequently. To understand neurotransmission under these conditions, we determined the number of L-type Ca(2+) channel openings necessary for vesicle fusion at the cone ribbon synapse. Ca(2+) currents (I(Ca)) were activated in voltage-clamped cones, and excitatory postsynaptic currents (EPSCs) were recorded from horizontal cells in the salamander retina slice preparation.

A positive feedback synapse from retinal horizontal cells to cone photoreceptors.

Cone photoreceptors and horizontal cells (HCs) have a reciprocal synapse that underlies lateral inhibition and establishes the antagonistic center-surround organization of the visual system. Cones transmit to HCs through an excitatory synapse and HCs feed back to cones through an inhibitory synapse. Here we report that HCs also transmit to cone terminals a positive feedback signal that elevates intracellular Ca(2+) and accelerates neurotransmitter release.

Role of the synaptic ribbon in transmitting the cone light response.

Cone photoreceptors distinguish small changes in light intensity while operating over a wide dynamic range. The cone synapse encodes intensity by modulating tonic neurotransmitter release, but precise encoding is limited by the quantal nature of synaptic vesicle exocytosis. Cones possess synaptic ribbons, structures that are thought to accelerate the delivery of vesicles for tonic release.