CRH signaling and the dark side of addiction: Long-lasting hyper-reactivity to stress in animals with a history of ethanol dependence

It is increasingly recognized that anti-reward systems (i.e. negative reinforcement through stress and fear systems) are progressively recruited during the development of alcoholism, and may constitute a key component in maintaining the dependent phenotype. Extra-hypothalamic CRH (corticotrophin releasing hormone) signaling is thought to be of crucial importance in this process. However, understanding the impact of the CRH mediated neurotransmission in ethanol dependence and its potential value as a candidate target for treatment of ethanol use disorders requires modeling the neuroadaptations which occur with prolonged exposure of the brain to ethanol. To this end we have recently developed an animal model, which based on repeated cycles of intoxication and withdrawal, mimics the natural history of alcoholism and triggers long lasting plasticity. Methods: Male Wistar rats were exposed to either 4 or 7 weeks of daily cycles of ethanol vapor intoxication and withdrawal followed by 3 weeks of abstinence. Animals were then assigned to 3 separate experiments to investigate: 1) voluntary home cage ethanol consumption in a 2-bottle, free choice drinking paradigm; 2) fear-induced suppression of behavior in a punished drinking paradigm with or without pretreatment with a CRH receptor 1 antagonist; and 3) sacrificed for the study CRH and its receptors in the medial prefrontal cortex and the extended amygdala by in situ hybridization and receptor autoradiography. Results: Only 7-weeks exposed animals show signs of withdrawal at the end of the exposure cycle and consume significantly more ethanol after the 3 weeks resting period (p < 0.05, 7-weeks exposed vs. control). Associated with the drinking phenotype was an increased fear response in the conflict test (p < 0.01), increased expression of CRH in the central amygdala (p < 0.05) and its receptor CRH-R1 in central (p < 0.05), medial (p < 0.01) as well as basolateral amygdala (p < 0.001) in 7-weeks exposed animals compared to non-exposed controls. There was a trend to significant increased fear- suppression also in animals with a history of only 4 weeks cyclic intoxication and withdrawal, but no alterations in CRH signaling. Furthermore, heightened fear response was still visible fourteen weeks after the induction of the high-drinking phenotype and was completely abolished by a specific, highly brain-penetrant CRH-R1 antagonist. Discussion: Enhanced fear suppression seems to be an early onset and long-lasting phenotype in the development of ethanol dependence and is accompanied by long-term upregulation of CRH circuits in the amygdala. Together, these data support the hypothesis of an early recruitment of anti-reward systems in the descent into alcoholism and emphasize the potential of the CRH-R1 receptor as a treatment target for this disorder.