UC Santa Barbara

In my dissertation studies, I have focused on uncovering the neural circuitry driving sensorimotor behaviors in a remarkable animal - Ciona. Understanding the signal transmission properties neurons use to elicit behavior is essential for determining functionality. An advantage to using the Ciona larval tadpole is its relatively simple nervous system. Another, and powerful, advantage is the availability of the completely described connectome for the Ciona larval-stage central nervous system (CNS). Furthermore, Ciona are tunicates, a subphylum of chordates, and are the closest living relatives of vertebrates, making their connectome the sole representative in the chordate phylum. The Ciona tadpole larva shows several vertebrate-like features, including a CNS that shows strong conservation with vertebrate CNSs. Despite having only ~180 neurons, Ciona larvae have a surprisingly complex set of behaviors. Among these behaviors are negative phototaxis and a response to rapid light dimming called the dim response (also known as the looming shadow behavior). Previous work in the Smith lab investigated these two behaviors and showed that they are mediated by distinct groups of photoreceptors. However, the details of the circuits, such as neurotransmitters used, were still unknown. To explore these circuits, I used a range of approaches, including gene expression analysis to discern the distribution of neurotransmitters and their receptors in individual neurons, and behavioral studies using pharmacological agents and behavioral mutants. Together, these approaches allowed me to fill in many of the details of the circuits predicted by the connectome, and to construct models that link circuits to behavior. In my first published study, I found that the two photoreceptor groups have distinct, but overlapping, circuits. The first circuit is excitatory and responds to the direction of light, driving phototaxis. The second circuit is disinhibitory and responds to rapid changes in light, driving the dim response. In my second published study, I found that both circuits detect fold-change differences. In fold-change detection (FCD), behavioral (i.e., swimming) responses scales with relative change in sensory input, and not to the overall magnitude of the stimulus. Furthermore, the two visuomotor behaviors have different input/output relationships, indicating different FCD strategies. Pharmacological manipulation of specific relay neurons in the posterior Brain Vesicle (pBV) led to an extinction of FCD without eliminating the visuomotor behavior, suggesting the FCD circuits lie at the neuronal level outside of the visual organ, as opposed to a mechanism of the photoreceptors. The role of pBV in sensory processing, along with it receiving converging inputs from other sensory systems, has lent further support of the pBV being a vertebrate midbrain homolog. By examining neurotransmitter receptor expression with in situ hybridization, I found broad expression of the glutamate receptor, NMDA-R, in the Ciona CNS, except in the photoreceptors. When NMDA-R is pharmacologically inhibited, the larvae lose their ability to respond to sensory input, suggesting an important role of the receptor in sensory processing. Further work is necessary to determine the specific components involved in visual processing and FCD, as well as the role of NMDA-R across the sensory systems. The work described here established a model to study sensory neural circuits for behavior in a new chordate model system.