Genetic factors control nicotine self-administration in isogenic adolescent rat strains.

Adult cigarette smokers usually become dependent on cigarettes during adolescence. Despite recent advances in addiction genetics, little data delineates the genetic factors that account for the vulnerability of humans to smoke tobacco. We studied the operant nicotine self-administration (SA) behavior of six inbred strains of adolescent male rats (Fisher 344, Brown Norway, Dark Agouti, Spontaneous Hypertensive Rat, Wistar Kyoto and Lewis) and six selected F1 hybrids. All rats were trained to press a lever to obtain food starting on postnatal day (PN) 32, and then nicotine (0.03 mg/kg/infusion, i.v.) reinforcement was made available on PN41-42 (10 consecutive daily 2 h sessions). Of the 12 isogenic strains, Fisher rats self-administered the fewest nicotine infusions (1.45 ± 0.36/d) during the last 3 d, while Lewis rats took the most nicotine (13.0 ± 1.4/d). These strains sorted into high, intermediate and low self-administration groups in 2, 2, and 8 strains, respectively. The influence of heredity on nicotine SA (0.64) is similar to that reported for humans. Therefore, this panel of isogenic rat strains effectively models the overall impact of genetics on the vulnerability to acquire nicotine-reinforced behavior during adolescence. Separate groups of rats responded for food starting on PN41. The correlation between nicotine and food reward was not significant. Hence, the genetic control of the motivation to obtain nicotine is distinctly different from food reward, indicating the specificity of the underlying genetic mechanisms. Lastly, the behavior of F1 hybrids was not predicted from the additive behavior of the parental strains, indicating the impact of significant gene-gene interactions on the susceptibility to nicotine reward. Taken together, the behavioral characteristics of this model indicate its strong potential to identify specific genes mediating the human vulnerability to smoke cigarettes

Nicotine infusion during the last 3 d of SA (Tukey HSD adjusted pair-wise p values).

<p>The mean number of nicotine infusions taken by each strain during the last 3 d of SA was compared using a post-hoc Tukey HSD procedure. The p values are shown for all the pair-wise comparisons. Comparisons that achieved statistical significance (p<0.05) are in bold and italics.</p

<b>Table 3.</b> Strain differences in food rewards earned during the last 3 d of SA.

<p>The mean number of food pellets earned by each strain during the last 3 d of SA was compared using a post-hoc Tukey HSD procedure. The p values are shown for all the pair-wise comparisons. Comparisons that achieved statistical significance (p<0.05) are in bold and italics.</p

Strain differences in food rewarded behavior.

<p>The means for the number of active and inactive lever presses, as well as food reward earned, are shown for the last 3 d of SA. Strains are ordered by the number of food rewards earned. Results for pair-wise comparisons are shown in Table 3.</p

Nicotine self-administration by isogenic male adolescent rats.

<p>Six inbred and six isogenic F1 hybrid strains were trained to press a lever for food on postnatal day 33. Nicotine SA started on postnatal day 41 or 42. Nicotine (30 µg/kg/infusion, i.v.) was delivered using a fixed-ratio 1 schedule. Each session lasted 2 h. A total of 10 daily sessions were conducted without interruption. Statistical analyses of the number of lever presses and nicotine infusions for the last 3 d were shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0044234#pone-0044234-t001" target="_blank">Table 1</a>. BN: Brown Norway; DA: Dark Agouti; F344: Fisher 344; LEW: Lewis; SHR: Spontaneous hypertensive rat; WKY: Wistar Kyoto. F1 hybrids were identified by the initials of the parental strains.</p

Food rewarded behavior by isogenic male adolescent rats.

<p>Six inbred and six isogenic F1 hybrid strains were trained to press a lever for food on postnatal day 33–36. Food reward resumed on postnatal day 41 or 42. Food pellets were delivered using a fixed-ratio 1 schedule. Each session lasted 2 h. A total of 10 daily sessions were conducted without interruption. Statistical analyses of the number of lever presses and food rewards earned are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0044234#pone-0044234-t001" target="_blank">Table 1</a>. (Y-axis is logarithmic).</p

Difference between F1 and the mean expected from the parental strains.

<p>The amount of nicotine intake or food reward earned by each F1 hybrid strain during the last 3 d was compared to the mean expected from the two parental strains. This value for the mean of the two parental strains predicts F1 behaviors if the genetic determinants were additive. Comparisons that achieved statistical significance (p<0.05) are in bold and italics.</p