Effects of age on nicotine consumption and enzyme activity following methoxsalen administration in C57BL/6J male mice

Open Access
Author:
Kapelewski, Christine Halina
Graduate Program:
Neuroscience
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
December 03, 2010
Committee Members:
  • Laura Klein, Dissertation Advisor
  • Laura Klein, Committee Chair
  • Steven A Branstetter, Committee Member
  • David John Vandenbergh, Committee Member
  • John Patrick Vanden Heuvel, Committee Member
Keywords:
  • methoxsalen
  • mouse
  • smoking cessation
  • nicotine
  • adolescent
  • CYP2a5
Abstract:
With tobacco use and nicotine addiction reaching epidemic proportions worldwide, the race to find effective smoking cessation aids and methods for all populations of individuals is in full swing. There are several drugs on the market today specific for this purpose; however, many of these pharmacotherapies are not universally helpful, particularly for adolescents. Age, along with interindividual genetic polymorphisms in the enzyme responsible for nicotine metabolism (CYP2A6), may contribute to these disparities. Mimicking the genetic variants of this enzyme with known CYP2A6 inhibitors (e.g., methoxsalen) in adolescents and adults could provide crucial evidence for the role of these biological and genetic influences on cigarette consumption and nicotine dependence. This dissertation is aimed to understand the effects of age on nicotine consumption and metabolism following acute and chronic methoxsalen administration in adolescent and adult male C57BL/6J mice. Experiment I was a 1-day study in which adolescent (n=60) and adult (n=60) male C57BL/6J mice received either no injection or a single subcutaneous injection of either physiological saline, vehicle (Emulphor, distilled water and ethyl alcohol) or one of two doses of methoxsalen (5 or 10 mg/kg), followed by access to a 3-bottle oral nicotine paradigm [tap water, 50 (LOW NIC) and 200 (HIGH NIC) ug/ml (-)-freebase nicotine dissolved in tap water] for 12 hours. To further investigate the age differences in nicotine consumption and drug effects on serum cotinine (ng/ml/g), an 8-day two-phase study with adolescent (n=32) and adult (n=32) male C57BL/6J mice (Experiment II) was conducted. For the first four days (Phase I), animals received 24-hour unlimited access to the 3-bottle oral nicotine iii paradigm (tap water, LOW NIC and HIGH NIC). During the four days of Phase II, animals continued to have access to the water and nicotine bottles, but were also given daily subcutaneous injections of either vehicle or 10 mg/kg 8MOP at the end of the light cycle. For Experiment I and II, nicotine exposure continued until sacrifice when livers were removed and trunk blood was collected to assess hepatic CYP2a5 and CYP2e1 gene expression and serum cotinine levels, respectively. In Experiment I, adults consumed more nicotine (mL, mg/kg, % total fluid intake) than did adolescents, regardless of drug treatment. Results for Experiment II regarding nicotine consumption were not conclusive. For both studies, however, methoxsalen was found to be an age-dependent CYP2a5 inhibitor, such that serum cotinine levels (ng/ml/g) were reduced with increasing dosages of methoxsalen in adults, but not adolescents. Drug treatment and age did not alter relative gene expression levels for CYP2a5 or CYP2e1. Results of these two experiments provide significant contributions to the small pool of scientific literature regarding the effects of methoxsalen on nicotine consumption and pharmacokinetics in mouse models. Alternative methods for future studies that are indicated below are necessary to test explanations for the effects observed in this dissertation as well as to begin to understand what effect other biological determinants (e.g., sex) have on nicotine consumption and metabolism. Preclinical studies, such as Experiment I and II, are important stepping stones to answer the overarching question of whether or not smoking cessation aids can be tailored to the individual.