Dopamine-modulated behaviors, including information processing and reward, are subject to behavioral plasticity. Disruption of these behaviors is thought to support drug addictions and psychoses. The plasticity of dopaminemediated behaviors, for example, habituation and sensitization, are not well understood at the molecular level. We show that in the nematode Caenorhabditis elegans, a D1-like dopamine receptor gene (dop-1) modulates the plasticity of mechanosensory behaviors in which dopamine had not been implicated previously. A mutant of dop-1 displayed faster habituation to nonlocalized mechanical stimulation. This phenotype was rescued by the introduction of a wild-type copy of the gene. The dop-1 gene is expressed in mechanosensory neurons, particularly the ALM and PLM neurons. Selective expression of the dop-1 gene in mechanosensory neurons using the mec-7 promoter rescues the mechanosensory deficit in dop-1 mutant animals. The tyrosine hydroxylase-deficient C. elegans mutant (cat-2) also displays these specific behavioral deficits. These observations provide genetic evidence that dopamine signaling modulates behavioral plasticity in C. elegans.

The peroxisomal targeting signal 1 (PTS1), consisting of a C‐terminal tripeptide (SKL and variants), directs polypeptides to the peroxisome matrix in evolutionarily diverse organisms. Previous studies in the methylotrophic yeast Pichia pastoris identified a 68 kDa protein, PAS8p, as a potential component of the PTS1 import machinery. We now report several new properties of this molecule which, taken together, show that it is the peroxisomal PTS1 receptor. (i) PAS8p is localized to and tightly associated with the cytoplasmic side of the peroxisomal membrane, (ii) peroxisomes of wild‐type, but not of pas8 delta (null) mutant, P.pastoris cells bind a PTS1‐containing peptide (CRYHLKPLQSKL), (iii) CRYHLKPLQSKL can be cross‐linked to PAS8p after binding at the peroxisome membrane and (iv) purified PAS8p binds CRYHLKPLQSKL with high affinity (nanomolar dissociation constant). In addition, the tetratricopeptide repeat (TPR) domain of PAS8p is identified as the PTS1 binding region.

We demonstrate that Caenorhabditis elegans is able to form an association between the presence of the odorant benzaldehyde and the food content of its environment. When exposed to 100% benzaldehyde for 1 h in the absence of food the naive attractive response is reduced, and we have found that this olfactory adaptation is attenuated by the presence of food. Contrary to nonassociative (single stimulus) learning theory, this response is not a function of the total time of exposure to benzaldehyde but rather an associative function of the ability of benzaldehyde to predict a nutrient-deficient environment. Genetic and pharmacological evidence revealed that the effects of food in this learning paradigm are mediated by serotonergic signaling. Food is a powerful motivator for animals, and many behaviors depend on food availability and recent nutritional history. For the nematode Caenorhabditis elegans, the presence of Escherichia coli, the standard laboratory food source, modulates multiple behaviors including locomotion, egg laying, and pharyngeal pumping (1, 2). In addition to these acute modulatory effects, food deprivation has been shown to alter more complex behavioral phenomena such as thermotaxis (3) and chemotaxis (CTX; ref. 4), and prolonged starvation can lead to the expression of an alternative developmental pathway leading to a period of relative metabolic stasis, the dauer larva stage (5). Food also has been used as an unconditioned stimulus (US) in associativelearning paradigms. Differential pairing of food with watersoluble chemoattractants (e.g., Na ϩ or Cl Ϫ ) leads to associative conditioning such that ions recently paired with food (CS ϩ ion) are preferred to ions that have not been paired recently with food (CS Ϫ ion; ref. 6).Food is clearly a salient environmental cue for C. elegans, and it is likely that nematodes use odor cues to locate food and avoid detrimental environments. C. elegans utilizes six primary sensory neurons to respond to more than 40 different volatile attractants and repellents (7). Prolonged exposure to some odorants leads to a decrease in the level of attraction to that specific odorant because of a phenomenon called olfactory adaptation (8). For example, exposure to benzaldehyde for 1 h causes a significant decrease in subsequent chemotactic scores. Adaptation in this context is used to refer to a behavioral response rather than its mechanisms. Previous investigations into this behavioral response have demonstrated that the reduction of CTX is susceptible to dishabituation, confirming that it is not the result of fatigue in sensory or motor systems but rather the result of a learning process (9). Indeed, prolonged exposure to a high concentration of benzaldehyde leads to the development of an aversive response to the odorant such that the animals will actively avoid a point source of benzaldehyde. This behavioral switch is produced by the dynamic interaction of two separate (attractive and aversive) benzaldehyde-responsive motivational systems (9).We investigated the relation...

Abstract. Two peroxisomal targeting signals, PTS1 and

We describe the isolation and characterization of peroxisomal assembly mutants in the genetically manipulable yeast Yarrowia lipolytica (pay mutants). These mutants were initially identified as oleic acid‐non‐utilizers by their inability to grow on oleic acid, the utilization of which requires peroxisomal β‐oxidation enzymes. Identification of a subset of oleic acid‐non‐utilizers as pay mutants was obtained by a rapid immunofluorescence procedure using antibodies to the peroxisomal targeting signal Ser‐Lys‐Leu‐CO2H. Punctate structures characteristic of peroxisomes were not detected in pay mutants using this technique. This rapid identification by immunofluorescence should be generally applicable to the selection of peroxisomal assembly mutants in other yeasts. To take advantage of the pay mutant system, we constructed a genomic library in the autonomously replicating vector pINA445 and developed an efficient and rapid electroporation procedure for the functional complementation of these mutants. We have been successful in functionally complementing two independent pay mutants. Molecular analysis of these and other complementing genes will allow for characterization of some of the cellular elements involved in peroxisomal assembly.