Researchers explore role played by nicotine in grouping of nicotinic receptors within the brain

Nicotine -- the primary compound found within tobacco smoke -- is known to change the grouping of some subtypes of nicotinic receptors, but the mechanisms for nicotine addiction remain unclear.

This inspired a group of University of Kentucky researchers to explore the role nicotine plays in the assembly of nicotinic receptors within the brain.

During the Biophysical Society's 61st Annual Meeting, being held Feb. 11-15, 2017, in New Orleans, Louisiana, Faruk Moonschi, a graduate student in the Department of Chemistry at the University of Kentucky, will present the group's work, which centers on a fluorescence-based "single molecule" technique they developed.

"Like many people in my country, Bangladesh, my father is a heavy smoker," Moonschi said, explaining his interest in studying nicotine addiction. "Despite the fact that my mother and siblings have encouraged him to quit, he has never been able to stop smoking. I've always been interested in what led to his addiction, so I joined a research group working within the area of addiction to study the underlying mechanisms of nicotine dependency."

The group explored whether nicotine exposure increases the total number of nicotinic receptors on cell surfaces and if it changes the way the receptor is grouped. "To do this, we use custom-built microscopes to expose our samples to laser excitation while we detect the fluorescence signal given off from the labeled proteins," Moonschi said.

Generally, a ligand -- which can be a small molecule or peptide -- binds with the corresponding receptor protein on its cell surface and causes an effect. "Contrary to this general process, some researchers hypothesized that nicotine actually gets inside the cells of smokers' brains and changes the assembly of nicotinic receptors by altering the ratio of nicotinic receptor subunits and potentially altering the trafficking of some subtypes of nicotinic receptors to the cell surface," Moonschi pointed out.

So, Moonschi and colleagues developed a single molecule technique that allows them to separate freshly assembled nicotinic receptors -- from the endoplasmic reticulum of cells -- from those already assembled and transferred to the cell surface. "By doing this, we were able to show that nicotine changes the subunit ratio of nicotinic receptors in the endoplasmic reticulum. Further, we showed that one assembly is preferentially transported from the endoplasmic reticulum to the plasma membrane," he added.

These findings are an important contribution to understanding the mechanism of nicotine addiction. "Understanding how nicotine alters the assembly of receptors in the context of addiction should provide insight into therapeutic targets for smoking cessation compounds," Moonschi noted.

Next, Moonschi and colleagues will "extend their research to in vivo studies, using genetically modified mice to determine whether similar changes in stoichiometry are seen in live animals due to nicotine exposure," he said. "We'll also study how smoking cessation compounds affect the stoichiometry of nicotinic receptors."