Caenorhabditis elegans modulates its locomotory rate in response to its food, bacteria, in two ways. First, well-fed wild-type animals move more slowly in the presence of bacteria than in the absence of bacteria. This basal slowing response is mediated by a dopamine-containing neural circuit that senses a mechanical attribute of bacteria and may be an adaptive mechanism that increases the amount of time animals spend in the presence of food. Second, food-deprived wild-type animals, when transferred to bacteria, display a dramatically enhanced slowing response that ensures that the animals do not leave their newly encountered source of food. This experience-dependent response is mediated by serotonergic neurotransmission and is potentiated by fluoxetine (Prozac). The basal and enhanced slowing responses are distinct and separable neuromodulatory components of a genetically tractable paradigm of behavioral plasticity.

The Caenorhabditis elegans gene eat-4 affects multiple glutamatergic neurotransmission pathways. We find that eat-4 encodes a protein similar in sequence to a mammalian brainspecific sodium-dependent inorganic phosphate cotransporter I (BNPI). Like BNPI in the rat CNS, eat-4 is expressed predominantly in a specific subset of neurons, including several proposed to be glutamatergic. Loss-of-function mutations in eat-4 cause defective glutamatergic chemical transmission but appear to have little effect on other functions of neurons. Our data suggest that phosphate ions imported into glutamatergic neurons through transporters such as EAT-4 and BNPI are required specifically for glutamatergic neurotransmission.

We isolated two mutants defective in the uptake of exogenous serotonin (5-HT) into the neurosecretory motor neurons of Caenorhabditis elegans. These mutants were hypersensitive to exogenous 5-HT and hyper-responsive in the experiencedependent enhanced slowing response to food modulated by 5-HT. The two allelic mutations defined the gene mod-5 (modulation of locomotion defective), which encodes the only serotonin reuptake transporter (SERT) in C. elegans. The selective serotonin reuptake inhibitor fluoxetine (Prozac) potentiated the enhanced slowing response, and this potentiation required mod-5 function, establishing a 5-HT-and SERT-dependent behavioral effect of fluoxetine in C. elegans. By contrast, other responses of C. elegans to fluoxetine were independent of MOD-5 SERT and 5-HT. Further analysis of the MOD-5-independent behavioral effects of fluoxetine could lead to the identification of novel targets of fluoxetine and could facilitate the development of more specific human pharmaceuticals.Key words: C. elegans; SERT; fluoxetine; serotonin; reuptake; modulation of behavior; SSRI The activity of serotonin (5-HT), a key neuromodulator, is mediated postsynaptically through metabotropic (Martin et al., 1998) and ionotropic (Maricq et al., 1991;Ranganathan et al., 2000) 5-HT receptors and their downstream signaling components (Hille, 1992). 5-HT modulates several behaviors of the nematode Caenorhabditis elegans, including egg laying and locomotion (Horvitz et al., 1982;Trent et al., 1983;Avery and Horvitz, 1990;Schafer and Kenyon, 1995). 5-HT also mediates the enhanced slowing response exhibited by food-deprived nematodes after they encounter bacteria (Sawin et al., 2000). 5-HT neurotransmission can be regulated by the removal of 5-HT from the synaptic cleft by a serotonin reuptake transporter (SERT;Cooper et al., 1996). Na ϩ /Cl Ϫ -dependent SERTs were cloned first from rats (Blakely et al., 1991;Hoffman et al., 1991) and subsequently from other species (Mortensen et al., 1999, and references therein), including humans and Drosophila melanogaster. SERT antagonists, such as the selective serotonin reuptake inhibitors (SSRIs) fluoxetine (Prozac), paroxetine (Paxil), and sertraline (Zoloft), are broadly used in the treatment of psychiatric disorders (Schloss and Williams, 1998). Therefore, in addition to the basic question of SERT function and regulation, it is of particular clinical importance to understand SERT function in vivo. SERT-deficient mice do not show gross developmental defects but have reduced 5-HT levels in the brain (Bengel et al., 1998), are insensitive to 3,4-methylenedioxymethamphetamine (ecstasy)-induced hyperactivity (Bengel et al., 1998), and show a brain region-and gender-specific reduction in the density and expression of 5-HT 1a receptors (Li et al., 2000). However, no obvious behavioral defects or abnormal responses to SSRIs in these SERT-deficient mice have been reported.In this article, we report the isolation of two C. elegans SERTdeficient mutants and describe studies of these mutants...