Rogel-Hernandez, L. E. | Search bioRxiv

bioRxiv2025

The anticonvulsant and mood-stabilizing drug valproic acid attracts C. elegans and activates chemosensory neurons via a cGMP signaling pathway

Valproic acid (VPA) is a drug with both anticonvulsant and antimanic properties. It has been widely prescribed to treat epilepsy, bipolar disorder, and other neuropsychiatric conditions for decades, but prenatal exposure is linked to developmental anomalies, cognitive deficits, and an increased risk of autism. Remarkably, the molecular basis of VPA action in the brain is not fully understood. To determine VPAs effect on the nervous system without requiring systemic drug application that induce toxic effects, we exploited the well-characterized chemotaxis behavior of the nematode Caenorhabditis elegans. We found that C. elegans are attracted to VPA, and this behavior is missing in animals lacking the tax-4 ion channel and the tax-4-expressing AWC chemosensory neurons. To test the idea that VPA directly activates the AWC neurons, we performed calcium imaging studies in a line expressing GCaMP6s in all amphid chemosensory neurons in the head. We found that VPA evoked calcium transients consistently in the AWC neurons and variably in AWB and ASH. As cyclic nucleotide-gated channels, the active state of tax-4 increases upon the binding of cGMP. Within the worms chemosensory nervous system, receptor guanylate cyclases (rGCs) facilitate the synthesis of cGMP and serve as chemoreceptors for various chemical cues. By performing chemotaxis assays and calcium imaging experiments with rGC mutants against VPA, we found that odr-1 and gcy-28 are essential for mediating VPA attraction, as their absence disrupts both behavior and control AWC calcium transients. However, given their broad use in chemosensation, odr-1 and gcy-28 most likely act as downstream effectors rather than as direct targets. These findings compelled us to investigate whether chemoreceptors belonging to the G protein-coupled receptor (GPCR) family contribute to VPA sensing. We approached this hypothesis by conducting chemotaxis assays with G mutants and identified several (odr-3, egl-30, gpa-2;gpa-3) whose absence leads to a loss of VPA attraction. Thus, future studies should focus on elucidating the mechanisms by which GPCRs contribute to VPA sensing and cGMP signaling.

neuroscience↗

bioRxiv2023

An efficient behavioral screening platform classifies natural products and other chemical cues according to their chemosensory valence in C. elegans

Throughout history, humans have relied on plants as a source of medication, flavoring, and food. Plants synthesize large chemical libraries and release many of these compounds into the rhizosphere and atmosphere where they affect animal and microbe behavior. To survive, nematodes must have evolved the sensory capacity to distinguish plant-made small molecules (SMs) that are harmful and must be avoided from those that are beneficial and should be sought. This ability to classify chemical cues as a function of their value is fundamental to olfaction, and represents a capacity shared by many animals, including humans. Here, we present an efficient platform based on multi-well plates, liquid handling instrumentation, inexpensive optical scanners, and bespoke software that can efficiently determine the valence (attraction or repulsion) of single SMs in the model nematode, Caenorhabditis elegans. Using this integrated hardware-wetware-software platform, we screened 90 plant SMs and identified 37 that attracted or repelled wild-type animals, but had no effect on mutants defective in chemosensory transduction. Genetic dissection indicates that for at least 10 of these SMs, response valence emerges from the integration of opposing signals, arguing that olfactory valence is often determined by integrating chemosensory signals over multiple lines of information. This study establishes that C. elegans is an effective discovery engine for determining chemotaxis valence and for identifying natural products detected by the chemosensory nervous system.

neuroscience↗