A behavioral investigation of L-type Ca²⁺ channel subtypes Caᵥ1.2 and Caᵥ1.3 in Nicotine addiction

Harding, Meghan L. (Meghan Lucy) (2010) A behavioral investigation of L-type Ca²⁺ channel subtypes Caᵥ1.2 and Caᵥ1.3 in Nicotine addiction. Masters thesis, Memorial University of Newfoundland.

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Abstract

Nicotine has become one of the most addictive and devastating drugs in recent history. Not only has chronic nicotine use, by way of smoking, been linked to cardiovascular problems, cancer and lung diseases from bronchitis to lung cancer, but it kills 40,000 Canadians every year. The mesolimbic dopaminergic pathway has been primarily associated with addiction: cells in the ventral tegmental area release dopamine in the nucleus accumbens, which leads to memory and addiction formation. Previously, our lab investigated what initiates burst firing of ventral tegmental area dopamine neurons when cholinergic nicotinic receptors are activated, as burst firing leads to a higher release probability than spike firing. L-type calcium channels have been shown to be crucial in ventral tegmental area dopamine cell burst firing induced by cholinergic activation. While in vitro work in our lab undoubtedly indicates L-type calcium channels mediate burst firing of dopamine cells, the question of whether they also mediate nicotinic activation of the central dopamine system in behaving animals remains open. -- Nicotine is known to decrease anxiety in chronic smokers and is generally followed by anxiogenic side effects upon sudden nicotine cessation, as seen most often with smokers quitting. In order to test what role L-type calcium channels have in nicotine addiction, two transgenic mice models were tested, one lacking the L-type calcium channel subtype Caᵥ1.3 (Caᵥ1.3-/-) and the other having a mutated DHP site in the Caᵥ1.2 alpha subunit (Caᵥ1.2DHP-/-). We tested Caᵥ1.2DHP-/- mice in the elevated plus maze to examine how L-type calcium channels affect anxiety levels following nicotine treatment and blockade of the Caᵥ1.3 subtype. The conditional place preference test allowed us to investigate the addictive properties of nicotine in the two transgenic mouse models and how L-type calcium channel antagonists (nifedipine) would affect them. -- The elevated plus maze revealed that Caᵥ1.2DHP-/- had different baseline anxiety levels compared to wildtype mice. Nicotine was anxiolytic acutely (following one day of treatment) in Caᵥ1.2DHP-/- mice, while in wildtype mice repeated nicotine was seen to induce anxiolytic effects. Upon the application of nifedipine, an L-type calcium channel DHP antagonist (which mechanism of action only is able to block Caᵥ1.3 due to Caᵥ1.2 mice being DHP insensitive), wildtype mice showed a further decrease in anxiety, while Caᵥ1.2DHP-/- showed no change, indicating that Caᵥ1.3 subtype may play a lesser role in anxiety than Caᵥ1.2. -- Wildtype and Caᵥ1.2DHP-/- mice showed a strong nicotine place preference (time spent in the drug conditioned arm over the neutral arm) of approximately 100 seconds. Nifedipine pretreatment in Caᵥ1.2DHP-/- and wildtype (which blocked both Caᵥ1.2 and Caᵥ1.3) mice abolished the conditional preference for nicotine, suggesting Caᵥ1.3 mediated nicotinic preference. However, when Caᵥ1.3-/- mice were tested for nicotine preference, their conditioned place preference was similar to that in wildtype and Caᵥ1.2DHP-/- mice. Further work is needed to understand the differential involvement of these subtypes in nicotine-induced place conditioning. -- In conclusion our results indicate that the L-type calcium channel subtype Caᵥ1.2 may be more actively involved in anxiety, while Caᵥ1.3, when present, appears to be more critical in inducing nicotine preference. Failure to confirm this finding in Caᵥ1.3-/- mice may be due to compensatory changes in other L-type subtypes or redundant reward circuitry.

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