**6. Conclusion**

in the extinction and reinstatement of ethanol CPP, as they are important for the extinction and reinstatement of CPP of other addictive substances. Investigating the involvement of these regions in extinction and reinstatement of ethanol CPP will be an important step for under‐

In summary, these studies show the importance of several brain regions heavily implicated in drug reward to ethanol seeking as well. Most of these studies evaluated the structures situated downstream the VTA that have been implicated in drug seeking based on their efferent dopaminergic input. In addition to these studies demonstrating that downstream sites are involved, findings from Bechtholt and Cunningham [115] further illustrate the importance of the VTA in ethanol-seeking behavior. As opioid receptors are situated presynaptically on local GABAergic inputs to DA cells, the authors hypothesized that methylnaloxonium disinhibited VTA GABAergic interneurons. This likely inhibited VTA DA activity, thereby reducing ethanol place preference expression. Conversely, baclofen presumably reduced CPP by acting directly on VTA DA cells, as they express GABAB receptors. Thus, the VTA is a critical structure for drug and, specifically, ethanol cue associations. Although these findings and proposed mechanisms underscore the importance of VTA DA in ethanol CPP expression, they provide little information on the inputs to the VTA that modulate DA cell activity. Thus, the origins and neurochemical sources of VTA DA cell innervation involved in ethanol-seeking behavior remain unknown. Accordingly, the next section discusses the involvement of VTA input in reward and identifies several inputs that may be of importance and deserve further study for

Excitatory (glutamatergic) afferents of the VTA arise from virtually all structures to which this region projects, with the exception of the NAc and lateral septum (LS), which provide strong GABAergic inputs [124]. This suggests that there is a broad network of excitatory reciprocal projections to and from the VTA, with much of the glutamatergic input to VTA neurons arising from subcortical regions that include but are not limited to amygdala, mesopontine nuclei, lateral habenula, and hypothalamus [125]. Although this reciprocal flow of neurotransmission is also found between the VTA and cortex, the PFC serves as the only cortical source of glutamate to the VTA [126]. Importantly, these glutamatergic afferents play a critical role in regulating VTA neuron firing. Specifically, glutamatergic input to the VTA appears to be critical for behaviorally relevant burst firing of VTA DA neurons [127]. The resulting phasic release of DA from the VTA is intimately associated with goal-directed behaviors and drug

In addition to the contemporary methods described earlier, recent advances in immunohisto‐ logical techniques have facilitated a more precise mapping of afferent and efferent projections of the VTA. As a result, an updated view on the role of broad neural circuit activity (including that of the VTA) in relation to behavior has been formed. In this current view, the net result of cell-specific projections onto cell-specific targets is accounted for and the subsequent plotting

standing the circuitry of relapse.

66 Recent Advances in Drug Addiction Research and Clinical Applications

their role in ethanol CPP.

**5.4. Inputs to the VTA**

reward [128].

**5.3. Summary**

Alcohol is a widely used legal intoxicant that produces a huge financial toll (\$223.5 billion in 2006 alone) on the United States (Centers for Disease Control and Prevention, 2014) due to the workplace, healthcare, legal, and criminal consequences of excessive consumption of alcohol. Despite the prevalent societal knowledge of the detriment of excessive alcohol use, AUDs continue to be a common disorder that is difficult to treat. The primary challenge of providing lasting treatments for AUDs is the high propensity for those with AUDs to relapse into alcohol use. Relapse is often caused when a cue associated with alcohol (i.e., a bar, alcohol advertise‐ ment) or alcohol itself is presented to a person with an AUD. This cue recalls the associative effects of ethanol and induces a craving and an internal drive to seek and consume alcohol. Thus, it is critical to understand how alcohol cue associations are formed and maintained despite receiving treatment for alcohol abuse. This chapter has outlined the animal models that are being used by preclinical researchers to better understand the formation, expression, extinction, and reinstatement (relapse) of alcohol cue associations that promote ethanol seeking and has summarized each of their advantages and disadvantages. Of particular use is the CPP paradigm. CPP allows the experimenter to separate distinct phases of acquiring, expressing, extinguishing, and reexpressing conditioned ethanol seeking and thus can easily study the neural mechanisms involved in each. This chapter also presented the classical and contemporary tools that can be used separately and in conjunction to probe the exact neural structures and circuitry involved in alcohol cues and seeking. Although classical tools have given us the greatest insight into the neurobiology of ethanol seeking thus far, contemporary tools have been and will allow for a much clearer and specific understanding of the structures involved in animal models of alcohol seeking. Finally, this chapter presented evidence from ethanol self-administration and ethanol CPP studies of the modulation of ethanol seeking by the mesolimbic structures (VTA, NAc), the limbic system (amygdala, BNST, ACC), and cortical structures (mPFC). Of particular importance is the VTA that sends vast dopaminergic input to many of these structures. The challenge of future research is to identify more structures critical for the acquisition and expression, and especially the extinction and reinstatement, of ethanol CPP and the inputs of the VTA that modulate dopaminergic tone and thus ethanolseeking behavior (such as the BNST-VTA projection). A better understanding of the whole circuit driving every aspect of ethanol seeking will improve our knowledge of AUDs and treatment options.
