Credit: Simon Bradbrook/NPG

Closely spaced action potentials can cause an increase in neurotransmitter release known as facilitation. The underlying mechanism for this has remained elusive, but now Jackman et al. show that the calcium sensor synaptotagmin 7 (SYT7) has a crucial role in mediating this response.

SYT7 presynaptic action potentials produce a large, localized increase in presynaptic intracellular calcium (Ca2+) that triggers neurotransmitter release through binding of the Ca2+ to fast synaptotagmin isoforms such as SYT1 and SYT2. This is followed by a smaller residual Ca2+ signal (Ca2+res) that lasts for several tens of miliseconds and has been implicated in synaptic facilitation. SYT7 is present presynaptically and binds Ca2+ with high affinity and slow kinetics, making it a promising candidate sensor for Ca2+res.

SYT7 is likely to be the calcium sensor that mediates the increase in p [synapse initial release probability] during facilitation

The authors found that Syt7-knockout mice do not exhibit facilitation, and used a combination of electrophysiology, pharmacology and optogenetics to determine why. They focused their attention on the hippocampal CA1–CA3 Schaffer collateral synapse, which exhibits facilitation following a paired-pulse facilitation (PPF) protocol (that is, the application of closely spaced stimuli). They found that, following synaptic stimulation, the amplitude and time course of Ca2+ influx and Ca2+res were similar in wild-type mice and Syt7-knockout mice. This suggests that the loss of facilitation in Syt7-knockout mice is not due to a change in the Ca2+ signal.

The authors then turned their attention to measuring the initial release probability (p), as an increase in p could produce synaptic depression that would obscure facilitation. They compared p in Syt7-knockout and wild-type mice using the use-dependent blocker of NMDA receptors (NMDARs), MK-801. In the presence of this agent, higher p results in more glutamate release and faster blockade of NMDARs; thus, the rate of attenuation of NMDAR-mediated synaptic responses can be used to assess changes in p. Following single stimuli, the rate of NMDAR blockade in the presence of MK-801 was similar between Syt7-knockout and wild-type mice, indicating a similar initial p. However, NMDAR-mediated responses evoked by brief trains of stimuli decayed more quickly (indicative of greater glutamate release) in wild-type than in Syt7-knockout mice, suggesting that the mutant mice lacked the ability to increase release probability and thus produce facilitation.

Finally, the authors co-expressed SYT7 and channelrhodopsin-2 (ChR2) in CA3 terminals enabling optogenetic activation of SYT7-expressing neurons. SYT7 expression in hippocampal CA3 terminals of Syt7-knockout mice restored facilitation in a cell-autonomous manner. In wild-type mice, expression of a mutant SYT7 that lacked the calcium-sensing C2A domain substantially attenuated facilitation. These data suggest that SYT7 is likely to be the calcium sensor that mediates the increase in p during facilitation.