PROJECT SUMMARY Identifying how neurons are connected to each other in the brain is an important and necessary step towards understanding how brain activity gives rise to behavior, and how it is perturbed by disease. Unfortunately, currently available methods have limitations that make it challenging to visualize these brain wiring diagrams. In addition, there is an urgent need for a method that will make it possible not only to unveil brain connectivity, but also to genetically modify the functional properties of neurons connected in a circuit. We recently developed a genetic system named TRACT and showed using Drosophila that it possesses both of these features. However, many complex brain functions cannot be examined in Drosophila, and understanding them will require the use of vertebrate animals. In recent years the zebrafish has emerged as a useful vertebrate animal model to study complex brain processes due to its relatively simple yet conserved vertebrate brain, optical transparency during embryonic and larval stages of development, amenability to large-scale behavioral assays, the emergence of complex behaviors after only 5 days of development, and a growing suite of genetic tools that allow observation and manipulation of neuronal circuits in behaving animals. However, the usefulness of zebrafish for neuroscience research is constrained by a lack of methods to identify and perturb synaptically connected neurons. In preliminary studies, we developed a TRACT system that can be used to identify anterograde monosynaptic connections between neurons in the zebrafish brain. Here we propose to further develop TRACT as a tool for transneuronal tracing in zebrafish by developing additional anatomical tracing modalities, and by establishing the use of TRACT to genetically manipulate and functionally characterize synaptically connected neurons. These capabilities will increase the usefulness of zebrafish as a model system to study vertebrate neuronal circuit function, both to reveal general principles of neuronal circuits that underlie specific behaviors, and to model complex brain disorders such as autism, Alzheimer’s disease and schizophrenia.

PUBLIC HEALTH RELEVANCE Many neuropsychiatric disorders are thought to be caused by abnormal connections between neurons in the brain. However, the methods currently available to visualize how neurons are connected to each other have limitations that make it difficult to identify such connections in a comprehensive manner. We propose to develop a new method to visualize how neurons are connected to each other in zebrafish, a vertebrate animal model ideally suited to investigate how genes control the assembly of neuronal connections in the brain, and how this process is affected in disease.