You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Effect of nicotine pretreatment on arsenic-induced oxidative stress in male Wistar rats. Humans are commonly exposed to nicotine, one of the most important lifestyle chemicals. The occurrence of high levels of arsenic in the groundwater of the southeast region of Asia has received much attention in the past decade and has become a global health concern. Predominant occurrence of both these chemicals and ease of their human exposure led us to investigate the effect of nicotine, a major tobacco alkaloid, on arsenic toxicity. Adult male rats were pre-exposed to two different doses of nicotine (0.75 and 3 mg/kg, intraperitoneally) for 7 days followed by 30 days of arsenic exposure (50 ppm sodium arsenite in drinking water). Nicotine pre-exposure resulted in an increased brain arsenic accumulation and a decreased liver arsenic concentration. Arsenic also caused a significant oxidative stress in the blood, brain and liver of the exposed rats. Glutathione-S-transferase, a phase II enzyme, was inhibited by both arsenic and nicotine but no such inhibition was noted in arsenic-treated animals pre-exposed to nicotine. Upon nicotine pre-exposure, brain acetylcholinesterase increased, while monoamine oxidase (MAO) decreased. The toxic effects of MAO significantly attenuated with nicotine pre-exposure. The present study suggests that nicotine may not be the major contributing factor for the previously reported synergistic toxic interaction between tobacco and arsenic. Nicotine pre-exposure in arsenic-exposed animals revealed interesting toxicokinetics and oxidative stress modulating interactions in the brain and liver of rats, which requires further exploration.
{"Chemical": ["Arsenic", "arsenic", "S", "Nicotine", "Glutathione", "nicotine", "sodium arsenite"]}