Mood disruption in prolonged abstinence from drugs of abuse: molecular mechanisms and gene discovery in the dorsal raphe – ABSTINENCE

In our model, adult mice receive chronic heroin injections. Then, we measure the social interactions, as well as various behavioural responses considered to reflect the animal’s emotional state. These behavioural analyses are conducted following variable abstinence durations of either 1, 4 or 7 weeks.
To further reinforce the validity of this model, we are trying to attenuate or even prevent the emotional deficits in abstinent mice. An antidepressant is administered following chronic heroin treatment throughout the abstinence period. We use fluoxetine, an antidepressant targeting the dorsal raphe nucleus, thereby directly addressing the role of this structure in abstinence.

Our first results indicate that despair-like behavior and social interaction deficits develop during abstinence from chronic heroin treatment, and progressively strengthen during the 7-week period. In a second step, we have shown that these deficits are blocked by chronic antidepressant treatment with fluoxetine during the first 4 weeks of abstinence. Finally, we are currently initiating the study of molecular adaptations induced by heroin in the dorsal raphe nucleus, using sequencing technology.

We have now at our disposal a robust and validated animal model. The next goal of this project is to demonstrate the major role of the dorsal raphe nucleus, and candidate genes, in the emergence of emotional deficits during abstinence. Therefore, we will now study in our abstinence model the behavioural responses of genetically modified mice, most notably for the mu opioid receptor, the direct target of heroin.

The emergence of emotional disorders during protracted abstinence from drugs of abuse perpetuates the addiction vicious cycle by precipitating relapse. Our project aims at better understanding underlying neurobiological mechanisms.

Drug abuse is a chronic relapsing disorder with devastating consequences for individuals and their social environment. A major challenge in recovering from addiction is to maintain a drug-free state, also referred to as abstinent state. Prolonged abstinence is characterized by lowered mood and a negative affective state. These emotional dysfunctions are considered to contribute to relapse, and clinical studies show a marked co-morbidity between addiction and depressive disorders. However, the “abstinence syndrome” has received little attention in preclinical investigations, and the neurobiology of this particular brain state is poorly understood. We have successfully developed a novel mouse model of protracted abstinence to chronic morphine, which reflects some aspects of addiction-depression co-morbidity. In this model, animals previously exposed to a chronic morphine regimen develop despair-like behavior and social interaction deficits. These behavioral modifications are detectable only after prolonged abstinence, and are also observed after heroin or alcohol in pilot experiments. Importantly serotonin (5-HT) levels in the dorsal raphe nucleus (DRN) remain altered after protracted abstinence to morphine, suggesting enduring modifications in 5-HT neurons. Further, Selective Serotonin Uptake Inhibitor (SSRI) treatment during morphine abstinence prevents appearance of the emotional syndrome, indicating a causal implication of the 5-HT system. Serotonergic transmission is a well-established key mediator of emotional homeostasis, but has been neglected in addiction research. The present project will capitalize on this unique mouse model to identify molecular and circuit mechanisms underlying mood disruption in protracted abstinence from drugs of abuse, with a focus on serotonin-associated adaptations.
The proposal has three objectives: Aim 1: we will extend our behavioral model using more sophisticated behavioral testing, as well as neurochemical and gene expression end-points. We will also establish that behavioral deficits generalize to heroin and alcohol, which are most disruptive illicit and licit drugs of abuse respectively. Aim 2: we will characterize molecular and signaling adaptations that occur in the DRN upon protracted abstinence to heroin, at both genome-wide transcriptome and miRNome levels. We will also identify genes and networks associated to SSRI-induced normalization of the emotional syndrome. We will further examine these adaptations in protracted abstinence to alcohol, to test generalization of the model at molecular level. Aim 3: we will test the hypothesis that mu and kappa opioid receptors, known to mediate euphoric and dysphoric states respectively, functionally contribute to emotional disruption via 5HT-mediated mechanisms at the level of DRN circuitry. Hence we will examine both heroin- and alcohol-induced abstinence syndromes in dorsal raphe-conditional knockout mice, both at behavioral and molecular levels. The latter approach will also be applicable to any other gene, signaling network or transmitter system, which will be identified upon genome-wide analyses of the DRN, in order to establish the functional relevance of novel candidate genes.
Altogether this project will address a critical, yet poorly understood aspect of drug abuse that has not been modeled previously in animal research (1). In addition, the proposal will investigate dysfunction of serotonergic transmission in drug abuse, which remains a virtually unexplored field. The genome-wide analysis of molecular plasticity in the DRN (2) and the functional study of mu and kappa opioid receptors (3) provide both exploratory and hypothesis-driven approaches to molecular mechanisms of drug abstinence. Findings from this proposal will help understanding the negative affect characterizing abstinent individuals, and will open novel avenues in both addiction and depression research representing two major fields in molecular psychiatry.