Sandeep Kumar

Co-transmission expands the chemical repertoire of neurons, but whether coexisting neurotransmitters can be independently organized and remodeled by experience to control distinct aspects of behavior remains unclear. Here, we address this question in the AFD thermosensory neuron of Caenorhabditis elegans , which directs navigation toward a learned temperature preference. We find that glutamatergic and cholinergic vesicular transporters coexist within individual presynaptic boutons but differ in axonal trafficking, subsynaptic organization, and dependence on presynaptic machinery. Sensory experience differentially regulates the two systems: animals trained to prefer higher temperatures exhibit increased VGLUT/EAT-4 abundance, whereas those trained to prefer lower temperatures exhibit increased VAChT/UNC-17 abundance. The two transmitters also make distinct, context-dependent contributions to goal-directed navigation. Glutamatergic transmission promotes movement toward the learned thermal preference, whereas cholinergic transmission restrains continued migration beyond it. Analysis of locomotor trajectories further reveals that the two transmitters differentially regulate reorientation decisions across regions of the thermal gradient. Together, these findings identify adaptive co-transmission , a form of synaptic plasticity in which coexisting neurotransmitter systems are differentially organized and regulated by experience, and make distinct, context-dependent contributions to behavior.

Just as genomes revolutionized molecular genetics, connectomes (maps of neurons and synapses) are transforming neuroscience. To date, the only organisms with complete connectomes are worms ^1–3 , sea squirts ⁴ and comb jellies ⁵ (10 ³ –10 ⁴ synapses). By contrast, the fruit fly is more complex (10 ⁸ synaptic connections), with a brain that supports learning and spatial memory ^6,7 and an intricate ventral nerve cord analogous to the vertebrate spinal cord ^8–12 . Here we report a densely reconstructed adult fly connectome that unites the brain and ventral nerve cord, and we leverage this resource to investigate principles of neural control. We show that effector neurons (motor neurons, endocrine cells and efferent neurons targeting the viscera) are primarily influenced by sensory neurons in the same body part, forming local feedback loops. These local loops are linked by long-range circuits that involve ascending and descending neurons organized into behaviour-centric modules. Single ascending and descending neurons are often positioned to influence the voluntary movements of multiple body parts, together with the endocrine cells or visceral organs that support those movements. Brain regions involved in learning and navigation supervise these circuits. These results reveal an architecture that is distributed, parallelized and embodied, reminiscent of distributed control architectures in engineered systems ^13,14 .

As a basic unit of the nervous system, the sensory-motor reflex circuit is fast and robust. However, it is not entirely clear how this robustness is achieved, given that various genetic perturbations can alter the function of the sensory neurons. By mapping the molecular basis of neuronal connections in the touch response circuit of Caenorhabditis elegans , we found prevalent genetic redundancy at neural pathway, synaptic, and molecular levels, which ensures that sensory signals can be relayed to command interneurons that control motor output. We also found developmental remodeling of the anterior circuit, which leads to the pruning of larval synapses, establishment of a second pathway that activates additional interneurons, and lateralization of the circuit. Finally, we found that the synapses that appeared to be functionally redundant in a simple touch assay contribute to the extent of reversal response in an additive manner, which may help the organism escape from predators.

Summary: Sensory neurons must extract behaviorally relevant features from dynamic environments while maintaining sensitivity across wide stimulus ranges. To understand how sensory encoding adapts to experience during behavior, we combine long-duration calcium imaging in freely moving C. elegans with a temperature-trajectory playback paradigm to determine how the thermosensory neuron AFD extracts behaviorally relevant sensory features during navigation. We observe that AFD functions as a leaky integrator of recently experienced temperature changes, accumulating thermal inputs over a rolling window of tens of seconds, resulting in calcium levels that represent recent temperature dynamics during runs. Importantly, we determine that AFD selectively amplifies responses to temperature changes near its learned preferred temperature. This experience-dependent gain control aligns encoding with the navigational goal, providing a mechanism for representing temperature preference within a derivative-based sensory system. A minimal mathematical model incorporating derivative detection, leaky integration, and temperature-dependent gain captures the calcium dynamics over a range of stimuli, and a simulation based on the mathematical model predicts goal-oriented locomotor strategies across stimulus regimes. Together, these findings show how gain control allows a derivative-based sensory code to represent an absolute goal and guide locomotory strategies during navigation. Highlights: AFD encodes recent temperature changes over behaviorally relevant timescales. AFD amplifies temperature-change responses within a learned temperature-centered gain window. Gain control provides a mechanism for representing distance to temperature preference within the derivative-based sensory system. A mathematical model and simulation of derivative sensing, leaky integration, and gain control predicts AFD calcium responses and goal-oriented locomotory strategies across stimulus regimes.

SUMMARY: As a basic unit of the nervous system, the sensory-motor reflex circuit is fast and robust. However, it is not entirely clear how this robustness is achieved, given that various genetic perturbations can disrupt the function of the sensory neurons. By mapping the molecular basis of neuronal connections in the touch response circuit of Caenorhabditis elegans , we found prevalent genetic redundancy at neural pathway, synaptic, and molecular levels, which ensures that sensory signals can be relayed to command interneurons that control motor output. We also discovered developmental remodeling of the anterior circuit, which leads to the pruning of larval synapses, establishment of a second pathway that activates additional interneurons, and lateralization of the circuit. Finally, we found that the synapses that appeared to be functionally redundant in a simple touch assay contribute to the extent of reversal response in an additive manner, which may help the organism escape from predators.

Just as genomes revolutionized molecular genetics, connectomes (maps of neurons and synapses) are transforming neuroscience. To date, the only species with complete connectomes are worms ^1-3 and sea squirts4 (10 ³ -10 ⁴ synapses). By contrast, the fruit fly is more complex (10 ⁸ synaptic connections), with a brain that supports learning and spatial memory ^5,6 and an intricate ventral nerve cord analogous to the vertebrate spinal cord ^7-11 . Here we report the first densely reconstructed adult fly connectome that unites the brain and ventral nerve cord, and we leverage this resource to investigate principles of neural control. We show that effector neurons (motor neurons, endocrine cells and efferent neurons targeting the viscera) are primarily influenced by sensory neurons in the same body part, forming local feedback loops. These local loops are linked by long-range circuits involving ascending and descending neurons organized into behavior-centric modules. Single ascending and descending neurons are often positioned to influence the voluntary movements of multiple body parts, together with the endocrine cells or visceral organs that support those movements. Brain regions involved in learning and navigation supervise these circuits. These results reveal an architecture that is distributed, parallelized and embodied, reminiscent of distributed control architectures in engineered systems ^12,13 .

Sex-specific behaviors are often attributed to differences in neuronal wiring and molecular composition, yet how genetic sex shapes the molecular architecture of the nervous system at the individual neuron level remains unclear. Here, we use single-cell RNA sequencing to profile the transcriptome of sex-shared neurons in adult Caenorhabditis elegans males and hermaphrodites. We uncover widespread molecular dimorphism across the nervous system, including in previously unrecognized neuron-types such as the touch receptors. Neuropeptides and signaling-related genes exhibit strong sex-biased expression, particularly in males, reinforcing the notion that neuropeptides are crucial for diversifying connectome outputs. Despite these differences, neurotransmitter identities remain largely conserved, indicating that functional dimorphism arises through modulatory, not identity-defining, changes. We show that sex-biased expression of neurotransmitter-related genes correlates with bias in outgoing synaptic connectivity and identify regulatory candidates for synaptic wiring, including both shared and sex-specific genes. This dataset provides a molecular framework for understanding how subtle regulatory differences tune conserved circuits to drive sex-specific behaviors.

Summary: Investigating signal integration in a neural circuit is oftentimes 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 interrogated the role of each TRN subtype in mediating the mechanosensor-induced escape response and in integrating signals that trigger the opposite motor output. Surprisingly, we found 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.

Sex-specific behaviors are often attributed to differences in neuronal wiring and molecular composition, yet how genetic sex shapes the molecular architecture of the nervous system at the individual neuron level remains unclear. Here, we use single-cell RNA sequencing to profile the transcriptome of sex-shared neurons in adult Caenorhabditis elegans males and hermaphrodites. We uncover widespread molecular dimorphism across the nervous system, including in previously unrecognized neuron-types such as the touch receptors. Neuropeptides and signaling-related genes exhibit strong sex-biased expression, particularly in males, reinforcing the notion that neuropeptides are crucial for diversifying connectome outputs. Despite these differences, neurotransmitter identities remain largely conserved, indicating that functional dimorphism arises through modulatory, not identity-defining, changes. We show that sex-biased expression of neurotransmitter-related genes correlates with bias in outgoing synaptic connectivity and identify regulatory candidates for synaptic wiring, including both shared and sex-specific genes. This dataset provides a molecular framework for understanding how subtle regulatory differences tune conserved circuits to drive sex-specific behaviors.