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.