The Cellular Effects of Nicotine and Tobacco Particulate Matter on Monoamine Transporters
Cigarette smoking causes nearly 6 million deaths worldwide every year (WHO, 2011). Current smoking cessation therapies available to the public are only marginally effective (Jorenby, 2006; Balfour et al., 2000), partly due to our incomplete understanding of the molecular biology of smoking addiction. The majority of studies examining the molecular biology of smoking addiction have focused on nicotine alone. However, there is a growing body of evidence that non-nicotinic components of cigarette smoke contribute to smoking addiction. Nicotine has previously been shown to modulate the function of the monoamine transporters, but studies in the literature are often contradictory and this effect is not completely understood (see Danielson et al., 2011 for review). Furthermore, very few studies have examined the effects of non-nicotinic components of tobacco smoke on the monoamine transporters. This thesis has examined the effects of nicotine and a tobacco extract (TPM) on the dopamine, serotonin, and norepinephrine transporters (DAT, NET, and SERT). Changes in monoamine transporter function, protein expression, and mRNA expression were measured ex vivo in discrete regions of the rat brain following chronic and acute in vivo nicotine and TPM treatment, and in vitro nicotine and TPM treatment. We found that nicotine and TPM affect monoamine transporter function, in a time- and dose-dependent manner, and that intact whole brain circuitry is required for these effects to be seen. In particular, nicotine (0.35 mg/kg) and TPM (containing 0.35 mg/kg nicotine) significantly decreased DAT function in the NAc at 30 min. This effect did not result in a corresponding decrease in DAT protein expression and was mediated by nicotinic receptors containing β2 subunits. Furthermore, TPM caused some changes in monoamine transporter function and mRNA expression that were not observed with nicotine alone. In functional studies this effect was particularly seen in the striatum of rats treated with nicotine (0.35 mg/kg) or TPM (containing 0.35 mg/kg nicotine). Overall these data demonstrate that nicotine affects monoamine transporter function in a nicotinic receptor-dependent manner, and that nicotine and TPM have different effects on monoamine transporter function and expression. This is the first study to examine the effects of TPM on monoamine transporter function, and supports previous evidence of a contribution of non-nicotinic components of cigarette smoke to neuroadaptations related to smoking. Findings from this study contribute to knowledge on the molecular biology of smoking addiction, which could in future lead to the development of more effective smoking cessation therapies.