Flavor compounds present in E-cigarettes Modulate oral Nicotine Consumption and Neuronal Activity in Ventral Tegmental Area and Nucleus Tactus Solitarius in mice.

Abstract

Background - The use of electronic cigarettes has increased greatly over the past decade. They are available in a variety of flavors which increases their attractiveness and use especially among young and new smokers. The impact of these individual flavors on nicotine consumption is not well understood. It has been previously shown that certain E-liquid flavors increase nicotine consumption. The objective of this study was to compare the effect of flavor compounds found either in fruit-flavored e-liquid (trans-2-hexanal (T2H) & isoamyl acetate (IAA)) or tobacco-flavored e-liquid (beta-damascone (BD)) in a mouse model of nicotine consumption. We hypothesized that T2H & IAA would increase nicotine consumption & preference, whereas BD would have no effect on nicotine consumption and preference. We also hypothesized that the different compounds show different neuronal activity in the brain region, Ventral Tegmental Area (VTA), and Nucleus Tactus Solitarius (NTS).Methods - Adult male mice C57BL/6J voluntarily consumed 75μg/mL nicotine alone, nicotine with a flavor compound, or a flavor compound alone in a series of voluntary, chronic two-bottle choice tests. The concentration of the flavor compound was increased weekly over five weeks while the nicotine concentration was kept constant. We measured the average nicotine consumption (mg/kg/day) and percent preference for the nicotine or flavor, and data were analyzed using repeated measures 2-way ANOVA with multiple comparisons tests. An immunohistochemistry (IHC) assay was performed to analyze the neuronal activation in VTA and NTS after involuntary oral administration of the test compounds. Results - Mice showed higher consumption and preference for BD plus nicotine compared with nicotine alone. IAA plus nicotine increased the consumption and preference at lower concentration (1μg/ml) and decreased at higher concentration (100 μg/ml) whereas T2H did not alter the nicotine consumption at any concentration. This change in consumption was not due to the flavor as the preference for all three flavor compounds was similar. A significant difference was observed in the number of activated cells by BD, IAA, nicotine, saccharine, quinine, and water in VTA. Saccharine showed the highest cFos activation compared to other groups. There was also a significant difference between IAA and nicotine, as well as between BD and IAA. In contrast, no significant differences in cFos activation were observed in the NTS region. Conclusion - Our data suggest that the flavor compounds can differentially modulate voluntary nicotine consumption and preference, and these compounds have their own neuronal activity in the brain, which can have important implications for the inclusion of flavor compounds in e-cigarette formulations.