TDP-43 proteinopathies, such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), are marked by the pathological cytoplasmic accumulation of TAR DNA-binding protein 43 (TDP-43), leading to progressive neuronal dysfunction and degeneration. To investigate the early functional consequences of TDP-43 mislocalization, we generated models expressing either wild-type human TDP-43 or a variant with a mutated nuclear localization signal (ΔNLS), specifically in serotonergic neurons. These neurons were chosen because i) serotonin deficits are a feature of ALS/FTD and ii) in , they regulate well-characterized behaviors, providing a straightforward readout of neuronal function. We found that expression of either TDP-43 variant impaired serotonin-dependent behaviors-including pharyngeal pumping, egg-laying, and locomotion slowing upon food encounter-with the cytoplasmic ΔNLS form causing more severe deficits. Serotonergic neurons remained i) morphologically intact, indicating that neuronal dysfunction precedes overt neurodegeneration; and ii) partially responsive to the selective serotonin reuptake inhibitor fluoxetine, suggesting that neurotransmitter release is still partially functional. Altogether, our findings demonstrate that cytoplasmic TDP-43 disrupts neuronal signaling and behavior early in disease progression. This model provides a genetically tractable system to dissect early mechanisms of TDP-43-mediated dysfunction and to identify therapeutic strategies targeting predegenerative stages of ALS/FTD.