# Imaging Synaptic Transmission of Individual Active Zones

> **NIH NIH R01** · MASSACHUSETTS INSTITUTE OF TECHNOLOGY · 2021 · $384,994

## Abstract

We propose to use Drosophila as a model system for determining how neurotransmitter release
and plasticity are regulated at individual active zones (AZs). Synaptic vesicle fusion occurs
through a highly probabilistic process, often with only a small percent of action potentials
triggering release from individual AZs. Although AZs largely share the same complement of
proteins, release probability (Pr) is highly variable across different neurons and between AZs of
the same neuron. Indeed, some AZ-specific proteins are non-uniformly distributed, and the
molecular composition of AZs can undergo rapid changes. To date, Ca2+ channel abundance
and Ca2+ influx have been most strongly linked to Pr heterogeneity, though other factors are
likely to contribute as well. The Drosophila neuromuscular junction (NMJ) has emerged as a
robust model system to characterize determinants of Pr. By transgenically expressing GCaMP
Ca2+ sensors targeted to the postsynaptic membrane, single synaptic vesicle fusion events at
individual AZs can be imaged by following spatially localized Ca2+ influx induced upon glutamate
receptor opening. This enabled us to generate Pr maps for evoked and spontaneous fusion for
all AZs, leading to the surprising observation that AZs formed by a single motor neuron have a
heterogeneous distribution of Pr, with neighboring AZs often showing ~50-fold differences in
strength. In addition, 10% of the AZ population supports only spontaneous release, while
another 15% are functionally silent for both evoked and spontaneous fusion. In this proposal, we
will determine how Pr is uniquely set for individual AZs and what molecular, structural, and
developmental variables govern Pr heterogeneity. We will also examine how presynaptic Ca2+
channels traffic to and between AZs, and how plasticity alters these processes. These
approaches should provide new insights into the complement of AZ proteins that functionally
regulate Pr, spontaneous release, and silent synapses, and how they cooperate with
presynaptic Ca2+ channels to set Pr across a functionally diverse set of AZs. Disruptions of
synapse formation and function have been linked to a host of neurological and psychiatric
diseases, reflecting the importance of these processes. The experiments described in this
proposal will generate new insights into important elements that define the strength and release
mode of individual AZs at an unprecedented resolution.

## Key facts

- **NIH application ID:** 10071214
- **Project number:** 5R01MH104536-08
- **Recipient organization:** MASSACHUSETTS INSTITUTE OF TECHNOLOGY
- **Principal Investigator:** J. TROY LITTLETON
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2021
- **Award amount:** $384,994
- **Award type:** 5
- **Project period:** 2014-06-01 → 2023-12-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10071214

## Citation

> US National Institutes of Health, RePORTER application 10071214, Imaging Synaptic Transmission of Individual Active Zones (5R01MH104536-08). Retrieved via AI Analytics 2026-05-22 from https://api.ai-analytics.org/grant/nih/10071214. Licensed CC0.

---

*[NIH grants dataset](/datasets/nih-grants) · CC0 1.0*