products is 18. However, there is no minimum age requirement to use tobacco or nicotine products. In media In some anti-smoking literature, the harm that tobacco smoking and nicotine addiction does is personified as Nick O'Teen, represented as a humanoid with some aspect of a cigarette or cigarette butt about him or his clothes and hat. Nick O'Teen was a villain that was created for the Health Education Council. Nicotine was often compared to caffeine in advertisements in the 1980s by the tobacco industry, and later in the 2010s by the electronic cigarettes industry, in an effort to reduce the stigmatization and the public perception of the risks associated with nicotine use. Research Central nervous system While acute/initial nicotine intake causes activation of neuronal nicotine receptors, chronic low doses of nicotine use leads to desensitization of those receptors (due to the development of tolerance) and results in an antidepressant effect, with early research showing low dose nicotine patches could be an effective treatment of major depressive disorder in non-smokers. Though tobacco smoking is associated with an increased risk of Alzheimer's disease, there is evidence that nicotine itself has the potential to prevent and treat Alzheimer's disease. Smoking is associated with a decreased risk of Parkinson's Disease; however, it is unknown whether this is due to people with healthier brain dopaminergic reward centers (the area of the brain affected by Parkinson's) being more likely to enjoy smoking and thus pick up the habit, nicotine directly acting as a neuroprotective agent, or other compounds in cigarette smoke acting as neuroprotective agents. Immune system Immune cells of both the Innate immune system and adaptive immune systems frequently express the α2, α5, α6, α7, α9, and α10 subunits of nicotinic acetylcholine receptors. Evidence suggests that nicotinic receptors which contain these subunits are involved in the regulation of immune function. Optopharmacology A photoactivatable form of nicotine, which releases nicotine when exposed to ultraviolet light with certain conditions, has been developed for studying nicotinic acetylcholine receptors in brain tissue. Oral health Several in vitro studies have investigated the potential effects of nicotine on a range of oral cells. A recent systematic review concluded that nicotine was unlikely to be cytotoxic to oral cells in vitro'' in most physiological conditions but further research is needed. Understanding the potential role of nicotine in oral health has become increasingly important given the recent introduction of novel nicotine products and