Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 5 1 2013 03 15 91 98 EN Manizheh Karami Department of Biology, Faculty of Basic Sciences, Shahed University, Tehran, Iran. Seyed Sajad Shahrokhi Department of Biology, Faculty of Basic Sciences, Shahed University, Tehran, Iran. Bahram Kazemi Biotechnology & Molecular Biology Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran. Seyedeh Samaneh Moezzi Department of Biology, Faculty of Basic Sciences, Shahed University, Tehran, Iran. 2015 10 16 Background and Objective: Nitric oxide (NO) plays a role in thermoregulation and growth of protozoa. This work aimed to add the molecule NO in physiology of protozoa in contact with abused narcotic substances. Materials and Methods: A sedative drug, morphine, was infused into a cell chamber containing Paramecia. The cell response to the drug was recorded promptly after drug infusion using a potency protocol provided for the first time at this laboratory. A precursor of NO, L-arginine, was treated jointly with drug to involve the NO system in protozoan performance to drug exposure. Marking of NADPH-diaphorase (NADPH-d) was followed to provide data to explain the mechanisms. Results: Morphine, particularly 0.5 to 60 μg/μl, aggregated the Paramecia. The infusion of L-arginine (1 to 8 µg/µl) together with morphine potentiated this effect, though, pre-usage of L-NAME (1 to 8 µg/µl), a blocker of NO production, reversed the response. Notably the activation of NADPH-d in solely morphine or L-arginine plus morphine samples was revealed. However, the expression of marker was attenuated upon pre-infusion with L-NAME. Conclusion: This study introduces a new approach to involve NO in physiology of aggregation of Paramecia following exposure to the misused sedative drug, morphine. https://ijm.tums.ac.ir/index.php/ijm/article/view/643 https://ijm.tums.ac.ir/index.php/ijm/article/download/643/415