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Abstract

The concept of “stress” is often associated with unpleasant feelings, even though it is the major trigger of the evolutionary processes. The refined system responsible for the regulation of stress in animals as well as humans controls the release of Corticotropin Releasing Hormone (CRH) in the brain. In the presence of threats, physiological reactions are activated as fast as deactivated once the stressful stimulus has vanished. However, prolonged stimulation of the CRH system leads to the development of anxiety-associated disorders. Increasing evidence indicates that the dopamine (DA) system plays a role in the modulation of emotional responses. The vulnerability to stress is not only a crucial characteristic of anxiety-related diseases, but it is also a major component underlying different stages in the development of addiction. Indeed, alcohol dependence is a chronic-relapsing disease which induces long-term neuroadaptations that recruit a negative emotional state leading to excessive alcohol ingestion motivated by relief of negative emotionality. Thus, this PhD thesis aims to establish the cell-specific involvement of the CRH receptor subtype 1 (CRHR1) in the regulation of emotional responses to stress under healthy conditions and in alcohol dependence.

Specifically, Study 1 demonstrates that the amygdala-specific increase in D1 binding sites upon pharmacological activation of CRHR1 is accompanied by an augmented anxiety-like behavior, which is abolished by pretreatment with a D1 antagonist. Further experiments using mice lacking CRHR1 expression in D1-containing neurons revealed that a D1-CRHR1 interaction is due to co-localized receptors, and bioluminescence resonance energy transfer (BRET) assay suggests that D1 and CRHR1 forms heterocomplexes. Together these results demonstrate that amygdala CRHR1 acts via D1 to mediate anxiety-like responses. In alcohol dependence, a maladaptive interaction between the two receptors seems to underlie the increased vulnerability to stress, a hallmark of the post-dependent (PD) phenotype that subsequently triggers stress-induced relapse to alcohol drinking (Study 2). Study 3 reveals that CRHR1 is differentially involved in stress-induced alcohol drinking whether it is expressed in DAT- or D1- neurons. These findings are supported by the anatomical separation within the VTA of D1-containing neurons, expressed in the dorsal portion, and the dopaminergic neuronal population distributed in the ventral part, and both co-localized with CRHR1. The cell-specific impact of CRHR1 in stress-induced alcohol consumption is further investigated in Study 4. Virus-induced αCaMKII-dependent over-expression of the receptor in the central nucleus of the amygdala (CeA) resulted in an increased alcohol-seeking behavior during stress-, but not cue- induced reinstatement in transgenic rats. Finally, an opposite functional impact of D1-CRHR1 and dopaminergic CRHR1 has been established for cue-induced reinstatement of cocaine seeking behavior using two double transgenic mouse lines lacking CRHR1 in D1- and DAT-expressing neurons, demonstrating that the modulation of relapse-like behavior for cocaine is related to the neuron-specific expression of CRHR1 (Study 5).

In summary, the results presented in this thesis demonstrate that an interaction of D1 and CRHR1 receptors within the amygdala functions as a novel mechanism involved in the regulation of the emotional responses to stress under healthy condition. However, during alcohol dependence, the maladaptive functionality of the D1-CRHR1 interaction results in a hyper-responsivity to stress, leading to an increase in alcohol consumption as a relief to the negative affective state. Furthermore, our findings reveal the functional impact of dopaminergic and dopaminoceptive CRHR1 on stress-induced alcohol-seeking and cocaine-mediated behavior. In addition, this thesis gives support for the functional relevance of receptor-receptor interaction in stress and addiction-related processes, and thus the promise of the development of pharmacological compounds acting on these receptor-heterocomplexes as medication therapies.