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A pharmacological characterization has been performed of the opioid receptor involved in modulation of phagocytosis in the protozoan ciliate Tetrahymena. Studies on inhibition of phagocytosis by mammalian prototypic opioid agonists revealed that morphine and beta-endorphin have the highest intrinsic activity, whereas all the other opioids tested can only be considered partial agonists. However, morphine (a mu-receptor agonist) is twice as potent as beta-endorphin (a delta-receptor agonist). Furthermore, the sensitivity for the opioid antagonist naloxone, determined in the presence of morphine and beta-endorphin, is very similar to the sensitivity exhibited by mammalian tissues rich in mu-opioid receptors. We suggest that the opioid receptor coupled to phagocytosis in Tetrahymena is mu-like in some of its pharmacological characteristics and may serve as a model system for studies on opioid receptor function and evolution.

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Nature has a great diversity of organisms whose bioactive compounds may potentially be studied. When it comes to aquatic life we find that algae are organisms that are well suited for screening and identification of bioactive compounds due to their widespread distribution in both salt and freshwater. Our hypothesis is that a crude organic extract of the brown algae Stypopodium zonale can decrease anxiety-related behaviors in Drosophila melanogaster. Stypopodium zonale was collected in the south coast of Puerto Rico and the potential anxiolytic-like effects of the extract were studied in an anxiogenic-like behavioral paradigm in Drosophila melanogaster. This behavior is called centrophobia and is measured using an Open Field Arena (OFA). Validation of the paradigm gave the expected results as reported in the literature, in which Drosophila exhibits a phobia (avoidance) of remaining in the center of the OFA, which corresponds to a behavior with anxiety components. The organic extract was dissolved with dimethyl sulfoxide (DMSO). Toxicity tests were performed both for DMSO and the crude organic extract, and neither showed positive results. To perform the behavioral trials, 1 mL of the crude extract and 4 mL of water were mixed with 1.8 g of Drosophila food. The final concentration of the crude extract in the food was 5.4 mg/mL. The adult flies were grown in a tube with the extract until a considerable quantity of larvae was observed, and then the adults were removed. These new larvae, once turned into adult flies, were used for the behavioral trials. The behavior of control flies (food without extract) and experimental flies (extract containing food) was recorded with a video camera and the results of the centrophobic behavior were analyzed and compared using quantitative criteria. Both the control and experimental trials were performed in triplicate. The results show that flies grown in food containing the crude extract present a significant reduction in centrophobia compared with control flies. In conclusion, our results suggest that the organic crude extract from Stypopodium zonale has anxiolytic-like effects in a Drosophila melanogaster model with anxiety components. We are currently performing Nuclear Magnetic Resonance (NMR) studies on the crude extracts to identify the most abundant secondary metabolites. Future experiments should include the administration of the crude extracts (or fractions of the most abundant secondary metabolites) to a vertebrate model in to test the effect in a behavior with anxiety components. We are also in the process of developing a preliminary model of possible mechanisms of action of the crude organic extract in the reduction of centrophobia. KEYWORDS: Anxiety; Algae; Drosophila melanogaster; Open Field Arena; Organic extracts; Centrophobia; Stypopodium zonale