Genetic tools are also providing evidence for the numerous neurotransmitters and receptors (and postreceptor signaling pathways) beyond the initial drug target that can modify responses to acute and chronic drug exposure. In some cases, the information obtained has confirmed earlier evidence from pharmacological approaches with receptor agonists and antagonists. In other cases, genetic studies have yielded fundamentally new insight into mechanisms of drug action.
As just one example, mice lacking the serotonin 5-HT1B receptor show enhanced responsiveness to cocaine and alcohol(Drug information on alcohol) in several behavioral paradigms. Most importantly, the mice self-administer the drugs at higher levels, compared to wildtype controls (Crabbe et al., 1996; Rocha et al., 1998). The mice also show higher levels of 
FosB, a Fos-like transcription factor implicated in addiction (see below), under basal conditions. These data suggest particular mechanisms through which serotonergic systems may be involved in addiction.
Nonmammalian model organisms are also proving useful in identifying novel biochemical pathways related to the actions of drugs of abuse. In one recent study, Drosophila melanogaster lacking the clock gene (the master regulator of circadian rhythms) showed reduced locomotor sensitization to cocaine (Andretic et al., 1999). This evidence raises the possibility that circadian genes in mammals may contribute to the mechanisms by which the brain responds to cocaine exposure.
At first it may seem implausible that something as complex as addiction can be modeled in flies; however, it is important to clarify two points. First, the locomotor responses of flies to cocaine (acting via dopamine(Drug information on dopamine) pathways) are remarkably similar to that seen in mammals, which means that the use of dopaminergic neurons in motor circuits was obligated over 1 billion years ago in evolution. Second, even if flies may not develop the more complex (e.g., cognitive, emotional) aspects of addiction seen in mammals, they certainly can be used to identify the types of genetic and biochemical pathways that are perturbed as nerve cells adapt to a drug of abuse over long periods of time.
Identifying Transcriptional MechanismsThe stability of the behavioral abnormalities that characterize addiction suggests that drug-induced changes in gene expression may be one important mechanism involved. Since classic pharmacological agonists and antagonists are not yet available for most proteins involved in gene regulation, genetic tools have provided the best approach to explore such processes in addiction.
One such mechanism is related to the Fos-like transcription factor FosB (Nestler et al., 2001). It accumulates in the nucleus accumbens (a target of the mesolimbic dopamine system) after chronic, but not acute, exposure to any of several drugs of abuse, including opiates, cocaine, amphetamine, alcohol, nicotine(Drug information on nicotine) and phencyclidine (PCP). This is in contrast to all other Fos-like proteins, which are induced only transiently after acute drug administration. The FosB protein accumulates because it is highly stable, unlike other Fos-like proteins. Consequently, FosB persists in the nucleus accumbens long after drug-taking ceases and thereby provides one mechanism by which changes in gene expression (and resulting changes in neural function and behavior) can be relatively stable.
Such a role for FosB has been confirmed recently in transgenic mice, in which FosB can be induced selectively within the same subset of nucleus accumbens neuron where it is normally induced by drug administration. Such mice show enhanced responsiveness to cocaine in several animal models, including locomotor activity, place conditioning, self-administration and relapse assays, which suggests that FosB may function as a relatively sustained molecular switch that contributes to a state of addiction.
One challenge of current research is to identify target genes through which FosB, as well as other transcription factors, produce their behavioral effects. Two general approaches have been used. One approach considers particular candidate genes that contain putative response elements for the transcription factor in question or whose products are implicated in drug mechanisms within the brain region of interest. The other approach is more open-ended and involves analysis of differential gene expression in certain brain regions (e.g., nucleus accumbens) under control and drug-treated conditions. For example, there is currently a great deal of excitement in the use of DNA array technology to identify genes involved in addiction. In encouraging news, the use of various arrays (filter-, glass- and chip-based) in preliminary studies has led to the identification of thousands of potential drug-regulated genes. The daunting news is that the field needs to learn how to better evaluate this vast amount of new information (beyond evaluating single genes with traditional approaches) to identify which genes truly are drug-regulated and contribute to addiction.
In contrast to the difficulty in identifying genes that underlie individual differences in vulnerability to addiction, genetic tools have been invaluable in increasing our understanding of neurobiological mechanisms involved in the addiction process.
One weakness of the field is that, in some cases, a genetic mutation is shown to result in altered behavioral responses to a drug of abuse, but there is no plausible scheme explaining how the mutation actually causes the abnormal behavior. However, the increasing sophistication of genetic tools and the increasing predictive value of animal models of addiction make it increasingly feasible to fill in the missing pieces and to understand the cellular mechanisms and neural circuitry that ultimately connect molecular events with complex behavior.
AcknowledgementThis essay is based on a recent review (Nestler, 2000).
