Every year, more than 5 million people die primarily from developing smoking related lung cancer. Tobacco contains nicotine, the major compound responsible for driving the strong addiction to smoking. Nicotine is addictive because of its effects on the central nervous system mediated by nicotinic acetylcholine receptors (nAChRs). The ability to sense the bitter taste of phenylthiocarbamide due to polymorphisms in a taste receptor gene (T2R38) protects a subject from developing a cigarette smoking addiction and reduces the positive reinforcement from smoking. This suggests that changes in taste play a significant role in the context of cigarette smoking and provide important cues involved in nicotine addiction. Although nicotine is described as bitter tasting, the molecular and cellular mechanisms that encode the taste of nicotine remain unclear. Our studies show that nicotine activates the taste system via two parallel receptor-mediated pathways. One pathway involves a G-protein coupled receptor (T2R) and down stream effectors (a G-protein gustducin, an enzyme PLC?2 and a cation channel, TRPM5) that are common to sweet, bitter and umami bitter sensing. The second pathway utilizes nAChRs and responds to nicotine, acetylcholine, ethanol and perhaps other bitter tastants and is essential for their complete sensory representation. The overall aim of this study is to increase our understanding of the contribution of different nAChR subunits to the bitter taste of nicotine, acetylcholine, ethanol and other bitter compounds. Studies are designed to investigate the presence of different nAChR subunits in taste bud cells using molecular techniques (RT-PCR, quantitative-RT-PCT, immunohistochemical and in situ hybridization). To investigate the role of nAChRs in peripheral taste transduction we will monitor the effects of pharmacological agonists and antagonists of nAChRs on single isolated taste cells using whole cell patch clamp technique, chorda tympani taste nerve responses and on lick rates as a measure of altered behavioral responses to nicotine, acetylcholine, ethanol and other bitter tastants in several animal models. Specifically, we will use genetically modified mice in which specific nAChR subunit genes or genes for TRPM5 cation channel have been deleted. The information obtained in this study on the nAChR-dependent bitter- sensing pathway may provide us with information regarding taste-related events that may be either involved in or predictors of nicotine and alcohol addiction. The effects of cigarette smoking on food consumption and taste preferences may occur from peripheral taste modulation due to chronic exposure to nicotine acting on nAChRs in taste buds among other factors.
Alterations in taste play a significant role in the context of cigarette smoking and alcohol abuse and provide important cues involved in their addiction. Therefore, it is important to understand the molecular and cellular mechanisms that encode the bitter taste of nicotine and ethanol. Our studies indicate that nicotinic acetylcholine receptors provide another bitter sensing mechanism that works in parallel with the TRPM5- dependent transduction pathway and is necessary for the complete representation of the bitter taste of nicotine, ethanol and perhaps other bitter tastants.
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