In order to respond to changing environments and fluctuations in internal states, animals adjust their behavior through diverse neuromodulatory mechanisms. In this study we show that electrical synapses between the ASH primary quinine-detecting sensory neurons and the neighboring ASK neurons are required for modulating the aversive response to the bitter tastant quinine in C. elegans. Mutant worms that lack the electrical synapse proteins INX-18 and INX-19 become hypersensitive to dilute quinine. Cell-specific rescue experiments indicate that inx-18 operates in ASK while inx-19 is required in both ASK and ASH for proper quinine sensitivity. Imaging analyses find that INX-19 in ASK and ASH localizes to the same regions in the nerve ring, suggesting that both sides of ASK-ASH electrical synapses contain INX-19. While inx-18 and inx-19 mutant animals have a similar behavioral phenotype, several lines of evidence suggest the proteins encoded by these genes play different roles in modulating the aversive quinine response. First, INX-18 and INX-19 localize to different regions of the nerve ring, indicating that they are not present in the same synapses. Second, removing inx-18 disrupts the distribution of INX-19, while removing inx-19 does not alter INX-18 localization. Finally, by using a fluorescent cGMP reporter, we find that INX-18 and INX-19 have distinct roles in establishing cGMP levels in ASK and ASH. Together, these results demonstrate that electrical synapses containing INX-18 and INX-19 facilitate modulation of ASH nociceptive signaling. Our findings support the idea that a network of electrical synapses mediates cGMP exchange between neurons, enabling modulation of sensory responses and behavior.

Highlights d Neuron loss in a C. elegans chemosensory circuit disrupts chemosensation d A genetically inserted electrical synapse circumvents the damage d Alternative pathways for information flow are established d Weakened signaling is enhanced due to new lateral left-right electrical coupling

45Animals are constantly adjusting their behavior to respond to changes in the environment or to 46 their internal state. This behavior modulation is achieved by altering the activity of neurons and 47 circuits through a variety of neuroplasticity mechanisms. Chemical synapses are known to impact 48 neuroplasticity in several different ways, but the diversity of mechanisms by which electrical 49 synapses contribute is still being investigated. Electrical synapses are specialized sites of 50 connection between neurons where ions and small signaling molecules can pass directly from one 51 cell to the next. By passing small molecules through electrical synapses, neurons may be able to 52 modify the activity of their neighbors. In this study we identify two genes that contribute to 53 electrical synapses between two sensory neurons in C. elegans. We show that these electrical 54 synapses are crucial for proper modulation of sensory responses, as without them animals are 55 overly responsive to an aversive stimulus. In addition to pinpointing their sites of action, we 56 present evidence that they may be contributing to neuromodulation by facilitating passage of the 57 small molecule cGMP between neurons. Our work provides evidence for a role of electrical 58 synapses in regulating animal behavior. 59Electrical synapses (also known as gap junctions) are composed of membrane channels that 69 join the cytoplasm of two cells [6]. They are found throughout vertebrate and invertebrate nervous 70 systems [6][7][8][9] where they pass both electrical and chemical signals between connected cells [10]. 71Electrical synapses have been primarily studied for their ability to synchronize electrical activity 72 between pairs or groups of neurons [11][12][13], but can also pass small molecules such as calcium [14, 73 15], cAMP [16][17][18][19], cGMP [17, 20], IP3 [15, 21], and even small miRNA [22, 23]. Interestingly, while 74 electrical synapses share similar function and protein topology in vertebrates and 75 invertebrates[24], genes encoding electrical synapse components are evolutionarily unrelated[6, 76 10]. As a result, electrical synapses in vertebrates are composed of connexins, while those in 77 invertebrates are composed of innexins (INXs). The separate evolution of electrical synapses 78suggests the functional necessity of these channels, although their role in neural plasticity and brain 79 function is not fully understood. 80Recently, it was discovered that innexin networks play a crucial role in cGMP-dependent 81 sensory modulation in Caenorhabditis elegans [25]. Krzyzanowski and colleagues found that cGMP 82 functions within the sensory neuron ASH to dampen nociceptive sensitivity but is produced in 83 neighboring neurons [26]. They further showed that cGMP-mediated dampening of ASH nociceptive 84 sensitivity requires an innexin-based network [25]. These findings uncover a new strategy of 85 network regulation that may contribute to the modulation of neural activity. ASH is the primary 86 nociceptive neuron pair in ...

Neuronal loss can considerably diminish neural circuit function, impairing normal behavior by disrupting information flow in the circuit. We reasoned that by rerouting the flow of information in the damaged circuit it may be possible to offset these negative outcomes. We examined this possibility using the well-characterized chemosensory circuit of the nematode worm C. elegans. In this circuit, a main sensory neuron class sends parallel outputs to several interneuron classes. We found that the removal of one of these interneuron classes impairs chemotaxis to attractive odors, revealing a prominent path for information flow in the circuit.To alleviate these deficiencies, we sought to reinforce a remaining neural pathway. We used genetically engineered electrical synapses for this purpose, and observed the successful recovery of chemotaxis performance. However, we were surprised to find that the recovery was largely mediated by inadvertently formed left-right lateral electrical connections within individual neuron classes. Our analysis suggests that these additional electrical synapses help restore circuit function by amplifying weakened neuronal signals in the damaged circuit.These results demonstrate the power of genetically engineered synapses to regulate information flow and signal intensity in damaged neural circuits.