Substance P effects on IlC-CeA circuit in alcoholism

Antagonism of neurokinin-1 receptors (NK-1), the molecular target for the neuropeptide Substance P (SP), has emerged as a promising treatment option of alcohol-related disorders, which are highly prevalent in the European Union affecting approximately 23 million people. The findings that NK-1 receptor antagonists suppress spontaneous and challenge-induced alcohol craving in detoxified alcoholics and also suppress stress-induced reinstatement of ethanol drinking without affecting baseline ethanol self-administration in animal models provide indirect evidence for altered SP/NK-1 receptor signaling not only during ethanol dependence, but also during ethanol withdrawal. One crucial aspect of SP/NK-1 receptor signaling has not been intensively studied: so far, there is no detailed analysis of the actions of both SP and NK-1 receptor antagonists on synaptic transmission in brain areas critical either in the development of alcohol dependence or alcohol withdrawal. Increased activity of the central nucleus of the amygdala (CeA), the major output of the amygdala complex, is a hallmark of alcohol dependence. I hypothesize that the observed increases of CeA activity in ethanol dependence are in part mediated by alterations in SP/NK-1 receptor signaling in the CeA and the infralimbic cortex (IlC) leading to a dysregulation of cortical inputs (promoting/facilitating excitatory) to the CeA. Specifically, I aim to test the following hypotheses: i.) SP enhances the excitability of IlC-CeA projection neurons in alcohol-dependent rats ii.)Heightened SP release contributes to increased CeA activity in alcohol-withdrawn animals and iii.) Alcohol dependence induces persistent up-regulation of the SP/NK-1 receptor system in the CeA. I will induce alcohol dependence using the chronic intermittent ethanol exposure paradigm and I will identify IlC-CeA projection neurons in rats injecting retrograde tracers into the CeA. Then, SP and NK-1 receptor antagonist effects on both synaptic GABAergic and glutamatergic transmission will be studied using whole-cell electrophysiology. I am confident that the proposed studies will provide important insights into the cellular mechanisms of SP and NK-1 receptor antagonists on neuronal excitability and synaptic transmission in both CeA and IlC, evidence for CeA innervations originating in the IlC and uncover potential neuroadaptions of the SP/NK-1 receptor system elicited by ethanol dependence and withdrawal. The results from these studies are thus of high significance and will significantly advance the field in understanding the translation potential of SP/NK-1 receptor related drugs in the treatment of alcoholism.

Alcohol use disorder and alcohol dependence affect 9.2% and 4.9% of the Austrian population, respectively. Alcoholism represents a chronically relapsing disease characterized by compulsive alcohol seeking and taking, loss of control in limiting intake, and the emergence of dysphoria, anxiety or irritability when drug access is prevented. The pathophysiology of alcoholism is multifaceted involving changes in multiple signaling systems in the brain and despite intensive research, there is no ultimate cure for alcoholism. In the last decade, the neuropeptide substance P and its receptor neurokinin 1 (NK-1) have emerged as a potential promising drug targets based on preclinical and clinical studies. The aim of this project was to gain a detailed understanding of potential neuroadaptations in substance P/NK-1 receptor system/signaling on a cellular level in the infralimbic cortex to central amygdala circuit in models of alcohol dependence and alcohol withdrawal. The central amygdala as part of the extended amygdala is considered a major neural hub for the motivational effects of alcohol (and other drugs of abuse) and thus plays a critical role in the transition to and maintenance of alcohol dependence. A physiological hallmark of alcohol dependence is augmented -aminobutyric acid (GABA the main inhibitory neurotransmitter in the mammalian brain) activity. Here, I could show that substance P like ethanol contributes to increased central amygdala GABAergic activity. This likely enables the subsequent disinhibition of brain areas that are normally tightly controlled by the central amygdala under physiological conditions. Most importantly, both alcohol dependence and alcohol withdrawal lead to a significantly stronger and more sustained activation of the central amygdala by substance P. Interestingly, these enhanced functional effects are accompanied by a significant decrease of NK-1 receptor expression in the central amygdala suggesting potential sensitization of this system in alcoholism. Collectively, my data provide novel and crucial insights into cellular neuroadaptations occurring in the central amygdala during alcohol dependence and withdrawal, and further support an important contribution of substance P and NK1 receptors in the development and maintenance of alcohol dependence.