Striatal astrocytic mechanisms underlying the development of compulsive drug seeking habits

Drug addiction is a chronic, relapsing, brain disorder that afflicts many people in the UK and internationally, placing enormous burdens on health, social and criminal justice systems. With the increase in opiates use and associated deaths in the USA, the UK and the EU, it has never been more urgent to understand the neurobiological mechanisms that confer some individuals the vulnerability to switch from controlled, recreational, heroin use to the persistent, compulsive, drug-seeking behaviour that characterises addiction. Only then we will be in a position to develop new treatments that help break these maladaptive compulsive drug-seeking habits or prevent their development in at-risk populations. Remarkably, despite understanding how addictive substances produce their pleasurable or rewarding effects through specific actions on neurons in the brain, it remains unclear why only a subset of individuals currently using drugs such as heroin subsequently become addicted. Recent discoveries have helped understand how, over the course of a prolonged history of drug exposure, a transition occurs in the brain circuits that control drug-seeking behaviour, from the so-called reward system to the dorsolateral (DLS) dopamine-dependent habit system. These transitions have been considered to depend on the hijacking by drugs of the neurons involved in dopamine-related mechanisms in these circuits. Thus, research has almost exclusively focused on the cellular and molecular mechanisms occurring within the neurons of these circuits in trying to elucidate the biological basis of addiction.
However, exposure to drugs of abuse also triggers cellular and molecular adaptations in astrocytes, the cells hitherto considered only to have a supporting role in the brain, in that they provide neurons with fuel and maintain brain homeostasis. Because astrocytes form a network of their own, or syncytium, that spans broad territories of the brain, they are in a unique position functionally to bridge aberrantly neuronal circuits that are otherwise independent.
Thus, the present research project seeks to understand the contribution of astrocytes and their syncytium to the functional transition from the ventral striatum to the DLS in the control over heroin seeking when it becomes habitual and eventually compulsive.
Capitalizing on our novel behavioural models of compulsive heroin seeking habits in the rat we will first carry a cross-sectional study aiming to determine the spatiotemporal profile of the functional recruitment of the striatal astrocytic syncytium across functional domains of the striatum over the course of the development of compulsive heroin seeking habits and identify molecular correlates of this functional recruitment.
The second objective of this project is to causally establish the role of DLS astrocytes in the expression of DLS-dopamine dependent heroin seeking habits and their maladaptive persistence in the face of punishment. For this, we will combine viral-mediated manipulations of DLS astrocytes with intra DLS infusions of dopamine receptor antagonists in order to determine whether the reliance of well-established heroin seeking on DLS dopamine mechanisms is influenced by Hm4Di-mediated activation or membrane calcium pump-mediated inhibition of astrocyte activity and whether these manipulations influence the tendency to persist in seeking heroin despite punishment.
The third objective of this project is to causally establish the role of astrocytes in the nucleus accumbens in the development of aDLS dopamine-dependent heroin seeking habits and compulsive heroin seeking. For this, we will use a strategy similar to that laid out for objective 2, and we will measure the influence of chronic activation or inhibition of ventral striatal astrocytes throughout the entire history of heroin self-administration on the emergence of DLS-dopamine dependent heroin seeking habits and their maladaptive persistence in the face of punishment.

This project aims (i) to determine the spatiotemporal profile of the functional recruitment of the striatal astrocytic syncytium across functional domains of the striatum over the course of the development of compulsive heroin seeking habits and identify molecular correlates of this functional recruitment (ii) to causally establish the role of the dorsal striatal astrocytes in the expression of habitual, dorsolateral striatum dopamine-dependent heroin seeking and (iii) to causally establish the role of the ventral striatal astrocytes in the functional recruitment of dorsolateral striatum dopamine-dependent mechanisms over the course of the development of compulsive heroin seeking habits.
This original and innovative research project relies on our refined models of persistent cue-controlled heroin seeking deployed that we will deploy in
(i) cross-sectional studies in combination with spatiotemporal mapping of the functional recruitment of striatal astrocyte syncytium using cell-specific RNAscope multiplex in situ transcriptomics against GFAP or neuronal markers, and markers of cellular activity/plasticity (e.g., c-fos/zif268/arc) and/or candidate genes such as the glutamate and dopamine transporter, adenosine and dopamine receptors.
(ii) longitudinal studies, in combination with our well established causal interrogations of the reliance of cue-controlled drug-seeking on aDLS dopamine-dependent mechanisms together with direct activation or inhibition or dorsal vs ventral striatal astrocyte function, mediated by the GFAP-promoter dependent expression of commercially available hM4Di DREAADs, calcium extruders or empty controls delivered by AAVS infused into different striatal domains.
While the combination of these approaches is novel, they are all routinely used in the laboratory, thereby greatly minimizing the overall technical risk of the project.