To develop an animal behavioral model capable of elucidating the role of genetics in the vulnerability to smoke cigarettes during adolescence, we focused on nicotine, the principal psychoactive agent in tobacco smoke. We found that nicotine-reinforced operant behavior varies significantly across a panel of 12 isogenic strains of rats in mid-to-late adolescence. These included 6 unique F1 hybrids. These 12 strains displayed a full spectrum of motivated nicotine intake, ranging from an average of 1.4 to 13.0 infusions per 2 h session. This variation in behavior across isogenic strains was largely due to inheritance (0.64). We also found no correlation between motivated nicotine SA and food reward, indicating that genetic control of the motivation to obtain nicotine is distinct from natural rewards.
Genetics plays a major role in the susceptibility to substance abuse and addictive disorders 
. Human studies 
have reported a heritability of about 0.5 for various cigarette-smoking phenotypes (e.g., cigarettes per day). This is similar to the estimated heritability of nicotine intake found in our study (0.64), indicating that genetic mechanisms influence the vulnerability to abuse nicotine to a similar degree in both rodents and humans. Despite the large effort involved in several recent genome-wide association studies, few specific risk genes have been identified. One of them is the CHRNA5-CHRNA3-CHRNB4 gene cluster located on chromosome 15q24-25 
. The contribution of several other candidate genes to the vulnerability to cigarette-smoking has been reported, such as catechol O-methyltransferase 
, dopamine receptor 2 
, opioid receptor 
as well as genes related to nicotine metabolism, such as cytochrome P450 CYP 2A6 
. The effects of these genes are often dependent on the ethnicity of the population 
. In general, the contribution of each gene is quite small, consistent with the notion that smoking is a complex trait determined by many genetic loci.
The inability to control for myriad environmental variables is one of the foremost limitations in detecting the genetic loci that determine the human vulnerability to smoke cigarettes. Rodent models circumvent these limitations, especially when a dedicated team breeds and evaluates all strains in the same facility to avoid inadvertent stressors, such as those due to shipping. Although mice have traditionally been the principal model for genetic studies, establishing nicotine SA in mice is fraught with difficulties. These include not only technical difficulties (e.g., implanting and maintaining a chronically, patent indwelling catheter), but more critically, the challenge of unambiguously attributing the observed behavior directly to i.v. nicotine 
. For example, the number of self-administered nicotine infusions appear to be similar 
to the number of saline infusions obtained by control mice (but see 
). In contrast, established rat models of nicotine SA consistently surmount these limitations 
The recent availability of rat genomic resources enables genetic studies of rat behavior. For example, the genome sequence of both BN 
and SHR 
strains have been published, and the sequences of many other rat strains (currently 17) are also available (http://rgd.mcw.edu
). Additionally, single nucleotide polymorphisms, comprising ~20,000 locations, are available for 167 strains of rat 
. These resources provide the foundation for behavioral genetic studies of inbred and F1 rats.
This study identified a large difference in stable nicotine intake among isogenic strains, with F344 and LEW strains at the extreme ends of the spectrum, respectively. Although the current study used a 2 h limited access model of nicotine SA, the contrasting behavior of the LEW and F344 strains is in agreement with our previous findings obtained from a 23 h model of virtually unlimited access to nicotine SA 
. In fact, these two strains are the most frequently studied inbred strains in models of drug addiction. In general, LEW rats self-administer more morphine than F344 rats 
but F344 rats self-administer more cocaine than LEW rats 
. However, the precise hierarchy of strain-dependent nicotine SA reported herein may be affected by unknown differences in the dose-response profiles between strains, such that nicotine 30 µg/kg might be on the descending limb in some strains. Our finding that food maintained behavior was not significantly different between these two strains () is consistent with our previous report . Taken together, these results suggest that not only are the genetic mechanisms underlying natural vs. drug-reinforced behavior different, but the genetic control of drug abuse is substance-specific.
In the present studies, F1 hybrids were used to identify additional complexity in the genetic control of nicotine-reinforced operant behavior. Indeed, in four of the F1 crosses, the amount of nicotine intake was different from the expected additive genetic effects of the parental strains (). This is consistent with the impact of specific gene-gene interactions on a complex trait like nicotine SA.
In summary, we developed a unique panel of isogenic rat strains that effectively model the overall impact of genetics on the vulnerability to acquire nicotine-reinforced behavior during adolescence. The influence of heredity on this process (h2
0.64) is similar to that reported for humans. Moreover, in this model, the genetic control of the motivation to obtain nicotine is distinctly different from food reward, indicating the specificity of the underlying genetic mechanisms. Significant gene-gene interactions were found to determine the susceptibility to abuse nicotine in any particular rat strain, as shown by the failure to accurately predict F1 behavior based simply on the inheritance of additive genetic factors from the parental strains. Taken together, these characteristics of the model indicate its strong potential to identify specific genes mediating the human vulnerability to smoke cigarettes, a problem that is exceedingly difficult to resolve by human studies alone.