INTRODUCTION
Varenicline is a nicotinic acetylcholine receptor (nAChR) agonist successfully used in the treatment of nicotine addiction. Its mechanism of action as a smoking cessation aid is mediated by partial agonism of the α4β2 subtype. Through this mechanism, varenicline stimulates α4β2 nAChR-mediated dopamine release in the ventral tegmental area sufficiently to reduce craving when abstinent from smoking, but inhibits the action of nicotine on α4β2 nAChRs when smoking. Furthermore, this drug exhibits additional effects on other nicotine addiction related nAChR subtypes including α6β2, α3β4, and α7 (Mihalak, Carroll et al. , 2006; Capelli, Castelletti et al. , 2011; Tammimäki, Herderet al. , 2012). However, although varenicline is a very efficacious drug for smoking cessation, some adverse cardiovascular side-effects have been reported in humans (Chelladurai & Sing, 2014; Gershon, Campitelli et al. , 2018; Munarini, Marabelli et al. , 2015; Rigotti, Pipe et al ., 2010; Singh, Loke et al., 2011) and rats (Selçuk, Sungu et al. , 2015). Also, a case report of varenicline-triggered pheochromocytoma crisis in a smoker has been reported (Hukkanen, Hukkola et al. , 2010). The US Food and Drug Administration reviewed a randomized clinical trial of 700 smokers with cardiovascular disease who were treated with varenicline or placebo. In this trial, adverse cardiovascular events were infrequent overall, but certain serious events, including heart attacks, were reported more frequently in patients treated with varenicline than in patients treated with placebo (FDA, 2011).
Because the action of varenicline on the peripheral α3-containing subtypes expressed in human chromaffin cells (Hone, McIntosh et al. , 2015; Hone, McIntosh et al. , 2017) might contribute to the cardiovascular side effects of this drug by triggering the release of catecholamines, we previously investigated whether varenicline might increase the firing of actions potentials in these cells. We reported that varenicline in the presence of nicotine alters the cell’s ability to fire action potentials in response to acetylcholine (ACh) at therapeutically relevant concentrations and, in addition, is capable of evoking actions potentials in the absence of ACh stimulation (Hone, McIntosh et al. , 2017). These results prompted us to investigate whether varenicline together with nicotine could enhance exocytosis of secretory vesicles and catecholamine release in human chromaffin cells of the adrenal gland. The situation in which both drugs are administered together occurs during therapeutic treatment with varenicline in smokers who are in the process of quitting smoking or during the use of varenicline in conjunction with a nicotine replacement therapy (Chang, Chiang et al. , 2015). This study was also conducted using rat chromaffin cells due to the limitations of obtaining and working with human cells. The main nAChR subtypes expressed in rat chromaffin cells are, as in humans, α3β4* (Di Angelantonio, Matteoni et al. , 2003; Hone, Rueda-Ruzafa et al. , 2020) and α7 (Hone, Rueda-Ruzafaet al. , 2020) but an α3β2β4* subtype (not investigated in human chromaffin cells) has also been reported (Hone, Rueda-Ruzafa et al. , 2020).