Ethanol drinking in rodents: is free-choice drinking related to the reinforcing effects of ethanol?

Abstract

Many studies have used voluntary ethanol consumption by animals to assess the influence of genetic and environmental manipulations on ethanol drinking. However, the relationship between home cage ethanol consumption and more formal assessments of ethanol-reinforced behavior using operant and instrumental conditioning procedures is not always clear. The present review attempted to evaluate whether there are consistent correlations between mouse and rat home cage ethanol drinking on the one hand, and either operant oral self-administration (OSA), conditioned taste aversion (CTA), or conditioned place preference (CPP) with ethanol on the other. We also review literature on intravenous ethanol self-administration (IVSA). To collect data, we evaluated a range of genetic manipulations that can change both genes and ethanol drinking behavior including selective breeding, transgenic and knockout models, and inbred and recombinant inbred strain panels. For a genetic model to be included in the analysis, there had to be published data resulting in differences on home cage drinking and data for at least one of the other behavioral measures. A consistent, positive correlation was observed between ethanol drinking and OSA, suggesting that instrumental behavior is closely genetically related to consummatory and ingestive behavior directed at ethanol. A negative correlation was observed between CTA and drinking, suggesting that ethanol's aversive actions may limit oral consumption of ethanol. A more modest, positive relationship was observed between drinking and CPP, and there were not enough studies available to determine a relationship with IVSA. That some consistent outcomes were observed between widely disparate behavioral procedures and genetic populations may increase confidence in the validity of findings from these assays. These findings may also have important implications when researchers decide which phenotypes to use in measuring alcohol-reward relevant behaviors in novel animal models.

Introduction

Ethanol is thought to be one of the most widely used drugs in the world today. While most ethanol users can be characterized as casual drinkers, the abuse rate is substantial. Alcoholism is a complex psychiatric disorder with an estimated heritability of 50–60%. It has a fairly common prevalence worldwide, with the United States showing a prevalence of ethanol dependence as high as 20% in men and 8% in women (Enoch, 2003).

Genetic vulnerability to alcoholism is theorized to be due to multiple interacting genetic loci, each with a small to modest effect combining under certain environmental influences to contribute to vulnerability to ethanol dependence. Animal models such as selectively bred rodent lines as well as inbred and recombinant inbred strains can be used to address this hypothesis of genetic vulnerability. Other animal model literature uses knockout (KO) and transgenic models to focus on single gene alterations to address the specific pharmacogenetics of ethanol use and vulnerability to alcoholism. All of these animal models have been used to study various ethanol-related behaviors, including ethanol drinking. These animal models may lack many aspects of human alcoholism, but experimenters are able to control their genetic and environmental history to research scientific theories difficult to address in human studies due to logistical and ethical considerations. Furthermore, these genetically based animal models may reveal behavioral, genetic, and physiological characteristics that demonstrate genetic links to behaviors such as ethanol drinking.

Differences in free-choice ethanol consumption have frequently been used to study the genetic and neurobiological mechanisms underlying high-ethanol drinking behavior, whether the animal model was created using selective breeding, inbreeding, or targeted gene alteration (Crabbe et al., 1992, Li et al., 1993). One question often arising in the interpretation of these studies is whether high-drinking lines show greater ethanol-reinforced behavior than low-drinking lines. In other words, high drinking is implicitly assumed by many to be evidence of high-ethanol reinforcement per se. However, other intervening variables such as anxiety (Pohorecky, 1991) or novelty-seeking (Cloninger et al., 1988) have also been speculated to be related to excessive drinking. Furthermore, other variables, such as avoidance of ethanol taste, may interfere with oral measures of ethanol reinforcement. For example, although DBA/2J mice drink so little alcohol that they likely never encounter its pharmacological effects (Belknap et al., 1978), they freely self-administer ethanol when it is delivered intravenously (Grahame and Cunningham, 1997). Therefore, it is an open question as to whether animal studies support the idea that genetic differences in free-choice drinking are correlated with genetic differences in more formal assessments of ethanol-reinforced behavior, including those using operant and classical conditioning. The purpose of the present review is to address whether there is, in fact, a consistent relationship between ethanol drinking and behaviors seen when using other methodologies for assessing the reinforcing properties of ethanol.

No matter what method is used to manipulate genes—breeding strategies such as phenotypic selection and inbreeding, or targeted gene alterations using transgenic or KO techniques—genetic correlations between drinking and other behaviors can be determined. For example, when a pair of selected lines is found to differ significantly on some trait other than the one for which they were selected, one may say that a genetic correlation between the traits exists. Taking this one step further, there may be a common set of genes or gene for the two responses (Crabbe et al., 1990). Such assessment can be useful for eventually understanding underlying mechanisms of action in complex behaviors such as ethanol abuse.

In this review, we will cast as wide a net as possible over different methods for inducing genetic differences to be able to detect consistent relationships among differing behavioral phenotypes relevant to ethanol reward and reinforcement. With the nontraditional addition of transgenic and KO models, one has better power to detect genetic relationships among behaviors because the ability to detect genetic correlations is directly related to the number of genetically varying populations studied (Crabbe et al., 1990).

Oral ethanol self-administration has long been regarded as an index of the reinforcing efficacy of ethanol (e.g., Myers et al., 1972). Nonetheless, there are other experimental tasks available to address the reinforcing and aversive properties of ethanol. There are two main approaches: Pavlovian and operant conditioning. Pavlovian conditioning includes conditioned place preference (CPP) and conditioned taste aversion (CTA). In these procedures, an association is established between an environmental stimulus, referred to as the conditioned stimulus (CS), and a drug unconditioned stimulus (US). In CTA, the CS is a flavor solution that the subject may consume before administration of the drug US, while in CPP, the CS is an environmental cue paired with the effects of the drug US. CPP is frequently used to index the reinforcing properties of self-administered drugs (Tzschentke, 1998). CPP involves pairing a distinct environment with the pharmacological effect of a drug, and a different environment with the absence of the drug. Later, the animal is tested in the absence of the drug and freely allowed to occupy or exit the drug-paired environment. Unlike operant oral self-administration (OSA) procedures, these tasks allow the ethanol exposure and dose to be controlled by the experimenter. This is a distinct advantage when assessing the rewarding actions of ethanol in genotypes (such as an alcohol nonpreferring [NP] rat or DBA/2J mouse) that would not encounter pharmacologically relevant doses of ethanol in a self-administration procedure. Typically, drugs abused by humans cause animals to prefer the drug-paired environment. An important exception is ethanol in rats, in which conditioned place aversion is the typical result. On the other hand, most self-administered drugs produce a CTA, though this behavior has been hypothesized to be positively correlated with sensitivity to drug reinforcement (Hunt and Amit, 1987). These tasks can also be less time-consuming than free-choice drinking experiments (see Chester and Cunningham, 2002 for review).

The other common method of studying reinforcement is to use operant behavioral procedures. Here, a learned instrumental action is required to produce a certain outcome, and the dependent variable is typically the amount consumed, or frequency of behavior directed at the desired outcome. Operant procedures can be used to assess reinforcing properties of ethanol. These procedures include the previously mentioned OSA, intravenous self-administration (IVSA), as well as intragastric and intracranial administration of ethanol. Because intragastric and intracranial routes of administration have been assessed in very limited amounts and models, these methodologies were not included here. Intravenous self-administration has also received somewhat limited publication but is included here, despite the failure of this approach in rat models (Hyytia et al., 1996), to address the lack of data on this behavioral model in the literature and bring attention to the positive aspects of this model for addressing the reinforcing properties of ethanol.

The main purpose of this critical synthetic review, however, is to use the technique of genetic correlation to assess the relationship between free-choice consumption of ethanol and methods using operant and classical conditioning to assess ethanol's reinforcing actions. By combining the broadest possible literature on genetic variation, we sought to detect consistent relationships among drinking and other behavioral models of alcohol's rewarding and aversive effects such as OSA, CPP, and CTA.