UC Berkeley

Synapses are the fundamental units of communication in the nervous system and reliable synaptic transmission is central to key nervous system processes such as learning, memory, and sensory adaptation. Moreover, due to dysfunctions at the synapse, many neurological diseases may develop. While electrophysiological studies of synaptic transmission have been around for a long time, only the recent development of optical quantal analysis (OQA) tools has made possible to correlate morphological and structural elements to transmission properties of individual synapses. Studies using OQA have revealed a much larger diversity in synaptic transmission, even among neighboring synapses, than was previously thought. The advent of higher-resolution OQA, such as the one developed in Chapter 2 of this thesis, called “QuaSOR”, and super-resolution structural imaging methods opens up an exciting frontier of being able to investigate how neural activity shapes (and is shaped by) synaptic diversity, what are the molecular determinants and mechanisms the set this diversity, and how do these molecular determinants shape synaptic diversity. By using OQA, we’ve shown that synaptic diversity, as measured by difference in synaptic strength (i.e., probability of action potential evoked transmission; Pr ) is extremely heterogeneous (Pr: 0.01–0.62) within a single neuron synapsing onto a single target cell. This high degree of heterogeneity leads us to the central hypothesis of my thesis which is that synaptic strength is set by a very precise, local distribution of key proteins. To test this hypothesis I used the model glutamatergic synapse–Drosophila melanogaster larval neuromuscular junction (NMJ)– where I will investigated hundreds of synapses in parallel, in vivo, and addressed synaptic heterogeneity from both functional and structural perspectives at single synapse resolution (50-100nm). In Chapter 2 of this thesis, using a combination of QuaSOR and super-resolution structural imaging methods, we found that essential active zone (AZ) proteins such as Bruchpilot (Brp) and the Ca2+-channel Cacophony (Cac) vary greatly among synapses and can explain ~31% of the diversity that we observe in basal Pr. Moreover, when investigating complexin, which acts as a break on release, we found that it suppresses both spontaneous and evoked release but in different ways. Taking the functional-structural tools developed in Chapter 2, I then wanted to ask how the neuromodulator octopamine, which is released from type II motor neurons (MNs) impacts synaptic release at type I MNs. In Chapter 3, I was able to show that OA affects release at type I MN in an input-specific manner (increasing release at type Ib MNs but having no effect at type Is MNs). Then, combining the QuaSOR method with structural imaging, I was able to show that the effect of modulation by OA on a single-synapse level is at least partially due to the amount of Unc13A at the synapse and interestingly, is dependent on the PLC pathway.