The catecholamine dopamine (DA) functions as a modulatory neurotransmitter in vertebrate and invertebrate nervous systems. In vertebrates, DA mediates behavioral processes such as cognition, reward, and locomotion, and is implicated in the pathophysiology of several neuropsychiatric diseases. In C. elegans, DA influences behaviors including egg-laying, defecation, response to food, habituation to touch, and basal locomotor activity. Though a modulatory role for DA in these behaviors is well-established, much remains unclear about the regulation of DA signaling itself. One important regulator of DA neurotransmission is the presynaptic DA transporter (DAT), which is a 12 transmembrane domain protein that mediates the re-uptake of DA into nerve terminals and is the primary mechanism by which DA signaling is terminated. DAT proteins are functionally and structurally conserved across phylogeny, with orthologs in both vertebrates and invertebrates. Mice lacking the DAT are behaviorally hyperactive, display both cognitive and sensorimotor gating defects, and exhibit altered responsiveness to psychostimulants. The C. elegans DAT (DAT-1) is 43% identical to mammalian DA transporters, preferentially transports DA, and is expressed in all known DA neurons in the nematode. Unlike DAT-KO mice, worms lacking a functional
dat-1 gene (
dat-1 (
ok157)) display no overt locomotor phenotype under normal culture conditions. To further investigate potential behavioral defects in worms unable to clear extracellular DA via DAT-1, we examined thrashing behavior in wild-type (N2) and
dat-1 (
ok157) worms. Whereas well-fed, young adult N2 worms substantially increase their locomotor rates when placed in a liquid environment and continue to thrash at this rate for at least 2 hours,
dat-1 (
ok157) worms exhibit paralysis within 5-10 minutes. Tyrosine hydroxylase-deficient,
cat-2 (
e1112) worms, which lack detectable DA, do not display this paralysis phenotype, consistent with a reliance on released DA. These findings suggest that inability to clear endogenous DA results in overstimulation of C. elegans DA receptors, triggering paralysis in
dat-1 (
ok157) animals. Loss and gain-of function forward genetic approaches targeting DAT-1 dependent, endogenous DA clearance may allow for the identification of genes supporting DAT-1 expression, localization and function.