Rapid structural alterations of the active zone lead to sustained changes in neurotransmitter release

Abstract

The likelihood with which an action potential elicits neurotransmitter release, the release probability (p r ), is an important component of synaptic strength. Regulatory mechanisms controlling several steps of synaptic vesicle (SV) exocytosis may affect p r , yet their relative importance in determining p r and eliciting temporal changes in neurotransmitter release at individual synapses is largely unknown. We have investigated whether the size of the active zone cytomatrix is a major determinant of p r and whether changes in its size lead to corresponding alterations in neurotransmitter release. We have used a fluorescent sensor of SV exocytosis, synaptophysin-pHluorin, to measure p r at individual synapses with high accuracy and employed a fluorescently labeled cytomatrix protein, Bassoon, to quantify the amount of active zone cytomatrix present at these synapses. We find that, for synapses made by a visually identified presynaptic neuron, p r is indeed strongly correlated with the amount of active zone cytomatrix present at the presynaptic specialization. Intriguingly, active zone cytomatrices are frequently subject to synapse-specific changes in size on a time scale of minutes. These spontaneous alterations in active zone size are associated with corresponding changes in neurotransmitter release. Our results suggest that the size of the active zone cytomatrix has a large influence on the reliability of synaptic transmission. Furthermore, they implicate mechanisms leading to rapid structural alterations at active zones in synapse-specific forms of plasticity. release probability | synaptic vesicle docking | active zone cytomatrix | plasticity | pHluorin S ynaptic transmission between most neurons is unreliable as a presynaptic action potential elicits neurotransmitter (NT) release at any individual synapse with a release probability (p r ) of less than one. This probability can vary considerably among synapses of the same axon, often in a target-specific manner (1-3). Similarly, synapses onto a single neuron often display widely varying release probabilities (4-7). Moreover, p r can undergo sustained activity-dependent changes over time (8)(9)(10)(11). These observations suggest that p r is an important determinant of synaptic strength that is tightly controlled by the presynaptic neuron and/or, indirectly, by its postsynaptic target.Although numerous studies have highlighted specific regulatory mechanisms affecting p r of synapses at rest, a concise picture of how p r at individual synapses is determined is only beginning to emerge. Synaptic vesicle (SV) exocytosis is a multistep process during which SVs have to dock at the active zone cytomatrix and undergo a priming step before they are "readily releasable," i.e., immediately available for fusion with the plasma membrane in response to calcium influx through voltage-gated calcium channels (12). Evidence that the size of a synapse's readily releasable pool (RRP) of SVs is closely correlated with its p r (7, 13) suggests that p r is mainly determined by pro...

Rapid Active Zone Remodeling during Synaptic Plasticity

“…It also corresponds well to the finding that prolonged depolarization leads to a reduction of both N and p vr (Moulder et al, 2004). Moreover, in a recent study, Matz et al (2010) used genetically expressed pHluorin to examine p syn in hippocampal cell cultures. They find that p syn changes spontaneously on a timescale of minutes.…”

“…However, rapid structural remodeling of synapses has been demonstrated previously. Another recent study at cultured hippocampal neurons found that the amount of fluorescently labeled Bassoon molecules at AZs changes on a timescale of minutes (Matz et al, 2010), suggesting a very dynamic structure of the cytomatrix. Furthermore, at postsynaptic densities, structural remodeling on the timescale of even milliseconds has been shown to influence paired-pulse ratios by diffusional exchange of desensitized and un-desensitized glutamate receptors (Heine et al, 2008).…”

Increased axonal bouton dynamics in the aging mouse cortex

“…Correlated bouton size changes might arise because boutons form contacts with postsynaptic cells displaying similar tuning properties (e.g., in the same cortical column), whereas spines along a dendrite likely make synapses with axons exhibiting different firing patterns. At the same time, fluctuations in EPB size could decrease the reliability of synaptic transmission (20), degrading the saliency of output signals. Long-term rather than short-term changes in synaptic weight may be at the basis of memory encoding (41).…”

“…Furthermore, because the size of the active zone correlates with the reliability of synaptic transmission (20), and synaptic vesicles are exchanged at high rates between neighboring EPBs along the axon in an activity-dependent manner (50), it is plausible that a combination of the aforementioned mechanisms could contribute to the increased rate of fluctuations in the size of Ag cortical EPBs (Fig. 5).…”

“…15) and behavioral training (13,16). Because the size of a synapse is directly related to its strength (17)(18)(19)(20), changes in the size of persistent synapses could play a significant role in mediating optimal cognitive function and experience-dependent plasticity alongside synapse formation, elimination, and stabilization (13,14,16,(21)(22)(23). However, little is known about structural changes of persistent synapses in the living brain.…”