Functional probing of neuronal subtypes via intersectional expression of optogenetic actuators reveals non-linear components in a linear circuit

Investigating signal integration in a neural circuit is often challenging when the circuit contains neuronal subtypes that are transcriptomically similar, due to the lack of tools to express optogenetic actuators with high cellular specificity and to deliver light with high spatiotemporal accuracy. Here, we demonstrate the use of a split GAL4-based genetic "AND" gate to express Chrimson in specific touch receptor neuron (TRN) subtypes in the C. elegans touch response circuit. Combining this intersectional strategy for transgene expression with high-throughput optical targeting and behavioral quantification, we optogenetically analyze the role of each TRN subtype in mediating the mechanosensor-induced avoidance response and in integrating signals that trigger the opposite motor output. Surprisingly, we find that although the response of the overall circuit linearly combines the competing anterior and posterior stimuli, this linearity is comprised of antagonistic non-linear contributions from the anterior and posterior sensors, which conspire to generate a linear response.