A substance that scientists use to screen people for nicotine use can also play a role in making smokers addicted to tobacco. When smoking, nicotine in the body turns into a metabolite called cotinine, which can be detected even in urine, saliva and blood in passive smokers.
According to Brady Phelps of South Dakota State University, Cotinine stays in the body much longer than nicotine. According to Mayo Medical Laboratories, its half-life — the amount of time it takes for half of the substance to leave the human body — is about two hours for nicotine and as much as 15 hours for cotinine.
“Preliminary evidence suggests that nicotine addiction can include more than nicotine: cotinine can enhance or contribute to nicotine addiction,” says Phelps, who was one of the authors of an article preparing for publication in the journal Neuroscience Letters.
In his work, Phelps and his colleagues studied how cotinine affects the behavior of flatworms called planarians (Planariidae): they are usually used as an animal model to study the effects of a drug with a dependency potential. If given a choice, these water worms prefer a dark environment: they exhibit the behavior described as a photosensitive or negative response to phototaxis.
To test the addictive nature of this nicotine metabolite, researchers immersed water worms in solutions containing three levels of cotinine, exposing them to light for 10 minutes. Concentration indicators reflect the levels used to assess the response of worms to nicotine in a previous study. Combining light with exposure to drugs can change the behavior of planarians – a method called conditional preference for a place. Phelps explains that if the phobic behavior of light changes to the opposite and worms gain preference for a medium conjugated with light, this indicates a greater preference for the conditions of this place. A similar method has been used in experiments with many other animal species, including mammals, and is considered one of the methods for evaluating the beneficial or addictive properties of the compound.
The study showed that planarians exposed to cotinine, regardless of level, spend much more time in the light than in the control group. According to Phelps, this is the first demonstration that cotinine influences the establishment of a preferred place for planarians. Depending on nicotine, there may be more factors than nicotine alone, and cotinine is probably one of them. However, scientists warn: their data at this stage are preliminary and must be confirmed by other studies.