Simultaneous Monitoring of Presynaptic Transmitter Release and Postsynaptic Receptor Trafficking Reveals an Enhancement of Presynaptic Activity in Metabotropic Glutamate Receptor-Mediated Long-Term Depression

Abstract

Although the contribution of postsynaptic mechanisms to long-term synaptic plasticity has been studied extensively, understanding the contribution of presynaptic modifications to this process lags behind, primarily because of a lack of techniques with which to directly and quantifiably measure neurotransmitter release from synaptic terminals. Here, we developed a method to measure presynaptic activity through the biotinylation of vesicular transporters in vesicles fused with presynaptic membranes during neurotransmitter release. This method allowed us for the first time to selectively quantify the spontaneous or evoked release of glutamate or GABA at their respective synapses. Using this method to investigate presynaptic changes during the expression of group I metabotropic glutamate receptor (mGluR1/5)-mediated long-term depression (LTD) in cultured rat hippocampal neurons, we discovered that this form of LTD was associated with increased presynaptic release of glutamate, despite reduced miniature EPSCs measured with whole-cell recording. Moreover, we found that specific blockade of AMPA receptor (AMPAR) endocytosis with a membrane-permeable GluR2-derived peptide not only prevented the expression of LTD but also eliminated LTD-associated increase in presynaptic release. Thus, our work not only demonstrates that mGluR1/5-mediated LTD is associated with increased endocytosis of postsynaptic AMPARs but also reveals an unexpected homeostatic/compensatory increase in presynaptic release. In addition, this study indicates that biotinylation of vesicular transporters in live cultured neurons is a valuable tool for studying presynaptic function.

Illuminating Relationships Between the Pre- and Post-synapse

“…Indeed, there are other points of similarity between these forms of plasticity. For example, in some conditions, glutamate release is enhanced in mGluR-LTD (Xu et al, 2013), the AMPAR trafficking in synaptic scalingdown occludes that induced by DHPG (Hu et al, 2010) and some molecular mechanisms are utilized in both forms of plasticity, notably tyrosine dephosphorylation (Moult et al, 2006;Hu et al, 2010). However, it is worth noting that not all molecular mechanisms are necessarily shared.…”

The interplay between Hebbian and homeostatic synaptic plasticity

“…This stipulates that durable forms of synaptic plasticity create a mismatch of pre-and postsynaptic function. A recent study in dissociated hippocampal culture has monitored the change in presynaptic vesicle turnover by surface biotinylation of synaptic vesicle components, while concurrently monitoring quantal amplitudes that reflect the number of AM-PARs (Xu et al, 2013). Induction of mGluR-dependent LTD accompanies a decrease in quantal amplitude by facilitated endocytosis of AMPARs, and unexpectedly, presynaptic release (Bozdagi et al, 2000(Bozdagi et al, , 2010.…”

Presynaptic long-term plasticity

“…In principle these tools could be used in brain slice preparations to study mechanisms of long-term synaptic plasticity. More recent advances, such as a molecular glutamate sensor which has been used in tissue preparations and has excellent signal-to-noise properties (Marvin et al, 2013) or biotinylated vesicular neurotransmitter reporters (Xu et al, 2013), also show great promise for directly monitoring presynaptic function. Finally, optogenetic techniques (Boyden et al, 2005) can also be employed to achieve activation of single presynaptic boutons to facilitate the study of presynaptic plasticity mechanisms.…”