Introduction
For people dealing with addictions to alcohol or other drugs, relapse is one of the greatest obstacles to recovery, as alcohol-related cues can trigger people to seek alcohol even after long periods of abstinence. Extinction-based interventions, which are methods that aim to stop alcohol addiction by removing the rewards associated with it, can reduce potential relapse, but they do not erase the original connections, meaning that they can reappear under conditions where alcohol-related cues are involved.
In the dorsomedial striatum, or DMS, sparse ensembles of activity-defined direct-pathway medium spiny neurons (dMSNs) encode drug-related actions and regulate drug seeking. Recent evidence indicates that groups of dMSNs within the DMS that were activated during voluntary alcohol use create ensembles that encode drug-associated memories. When these dMSN ensembles are reactivated during exposure to drug-related cues, relapse-like behavior is triggered, revealing a connection between striatal activity and relapse of alcohol seeking.
Studies showing how extinction-responsive neuronal ensembles oppose reward-seeking ensembles suggest that extinction-based interventions may suppress alcohol seeking by reducing the reactivation of dMSN ensembles associated with drug-related memories. However, it is unclear if extinction affects late ensemble activation or if sustained ensemble activity alters extinction-related behaviors and relapse.
Methodology
Drd1a-tdTomato, Snap25-LSL-F2A-GFP, FosTRAP, ArcTRAP, and MOR-Cre mice were used in the study. FosTRAP, Snap25-GFP, and D1-tdTomato mice were crossed to create triple transgenic FosTRAP; Snap25-GFP; D1-tdTomato mice. Sex distribution was unbalanced and not considered, but all mice were in the 3-8 months old age group. Researchers performed a total of four experiments, each with different conditions.
For the first experiment, wild-type mice were trained in operant alcohol self-administration before being put into a control group (no-extinction) or an experimental group (extinction). Extinction mice had nine lever-extinction sessions every day, in which the mice were placed in a chamber where pushing the lever did not dispense alcohol or activate a cue light. The no-extinction mice were exposed to the operant chamber every day, in which the lever did dispense alcohol and activate the light. Afterwards, the two groups underwent cue-induced testing.
For the second experiment, triple transgenic FosTRAP; Snap25-GFP; D1-tdTomato mice were used to label neurons acquired during alcohol exposure and identify dMSNs. They were then assigned to no-extinction or extinction groups, which went through similar sessions as the mice in the first experiment. They then underwent cue-induced testing and had tissue collected in order to detect the c-Fos protein.
In the third experiment, ArcTrap mice received an infusion of AAVretro-DIO-FLPo into the substantia nigra pars reticulata (SNr) and either AAV-fDIO-hM3Dq-mCherry or AAV-FDIO-mCherry into the DMS. Their acquisition-recruited neurons were tagged immediately after the acquisition sessions. After recombination of genes and extinction training, the mice underwent cue-induced testing.
In the fourth experiment, MOR-Cre mice were also infused with AAVretro-DIO-FLPo into the SNr and either AAV-fDIO-hM3Dq-mCherry or AAV-FDIO-mCherry into the DMS. They went through the same process as the mice in the third experiment and had their locomotor activity evaluated using an open-field procedure.
Results
The first experiment supported the hypothesis that extinction training suppresses cue-induced relapse to alcohol seeking, as the experimental group of mice that underwent extinction showed alcohol seeking behaviors at a much lower rate compared to the control group.
Mapping the neurons of the second group showed that mice that underwent an extinction process had a much lower density of reactivation of dMSNs acquired during alcohol exposure.
The third experiment showed that maintaining activity in acquisition-recruited dMSNs during extinction hindered extinction-related responding and showed greater relapse.
Finally, the fourth experiment showed that increasing activity in MOR-expressing dMSNs during extinction accelerated the reduction in alcohol seeking.
Conclusion
The four experiments on these mice suggest various possibilities regarding extinction. Notably, extinction suppresses reactivation of dMSN ensembles recruited during alcohol exposure. Increasing activity in MOR-expressing dMSNs and decreasing activity in acquisition-related dMSNs provides the best framework for accelerating recovery from alcohol seeking and preventing relapse.
However, the study has not been peer-reviewed by a journal and only investigates mice, meaning the same effects may not be present in humans. Further research on this topic may help us validate results and better understand how we can help those who are dealing with alcohol addiction.
