To the Identification of Molecular Mediators of Stress – TIMMS

Recent clinical and preclinical research strongly suggests that the stress response is a key determinant in the appearance of various disorders of the nervous system such as depression, anxiety, panic disorders and associated cognitive impairments, and addiction. In response to stress, the adrenal glands rapidly release glucocorticoid hormones (GCs), which activate the glucocorticoid receptor (GR), a transcription factor involved in orchestrating adaptive responses.

Stress-induced alterations of the dopamine system may be associated with addictive behaviors, anhedonia and other cardinal symptoms of stress-related disorders. Thus, our proposal focuses on GR gene function in the mesolimbic dopamine pathway. GR is ubiquitously expressed. The first major challenge is therefore to identify the cells in which GR activation may modulate behavior and lead to pathology. We developed precisely targeted mouse models using two complementary strategies based on cell-type specific GR gene inactivation or controlled over-expression. We will examine three main neuronal populations, the dopamine neurons and the two major dopaminoceptive neuronal populations expressing either the dopamine receptors D1a or D2. We will study behavior (responses to drugs of abuse, transition to addiction, anxiety- and depression-like behaviors) under basal conditions and after challenge (GCs treatment, chronic stress), in the different mutants and respective controls.

We recently demonstrated that GR gene inactivation in dopamine neurons only (GRDATCre mice) does not affect intravenous self-administration of cocaine. By contrast, inactivation of the GR gene in dopaminoceptive neurons (expressing dopamine receptors 1a, GRD1aCre mice) substantially reduces their responses to cocaine, including their motivation to self-administer cocaine (Ambroggi et al., Nat. Neuro 2009). Surprisingly, responses to morphine, although affected by GCs, are not modified (unpublished). Preliminary data suggest that it exists a clear neuron-dependent and state-dependent dichotomy for the role of GR in the responses to cocaine and morphine since stress-induced potentiation of morphine’s locomotor responses is blocked in GRDATCre mice but not in GRD1aCre mice. This result will have to be confirmed using the other proposed tests for responses to morphine.

In the relevant models, dopamine neuronal activity (firing, dopamine release), dopaminoceptive neuronal activity (in vivo electrophysiology), synaptic plasticity (patch-clamp, dendritic arborization) will be thoroughly investigated.

Once a relevant cell type, for a given behavior, is identified, a major challenge will be to discover downstream-regulated genes responsible for long-lasting behavioral effects, and to understand how GR control their expression. We will perform comparative transcriptome analyses with functional validation in case of particular interest. Chromatin immunoprecipitation (ChIP) and existing conditional mutants of GR partners in transcriptional control will allow elucidating the molecular mechanisms underlying the GR-mediated control of these genes.