How the brain’s dopaminergic system drives compulsive behaviours
‘Directed behaviour’ is the pursuit of rewards, most easily understood (and studied) when driven by need, such as seeking water when thirsty. “But this can spiral out of control, into compulsive or addiction behaviours,” notes Scott Waddell, coordinator of the SCCMMI project, which was funded by the European Research Council(opens in new window). Critical to such motivated behaviours, SCCMMI wanted to better understand the role of neuromodulation(opens in new window), which can switch the brain into different ‘states’ to best serve the most pressing need. Single-cell sequencing (scSeq) of fly brains identified transcriptional (copying of DNA sequences) changes dependent on brain-states occurring throughout the brain, which SCCMMI could assign to specific cell-types.
Single-cell sequencing fly brains
“Memories of sensory cues such as colours or smells, are represented as changes in the brain’s synaptic connections, directed by different neurons releasing the neurotransmitter dopamine. This suggests these neural networks are ideal for investigating brain changes during need or compulsive states,” explains Waddell. Flies were studied that were: thirsty; hungry and trained to seek sugar; fed a memory-enhancing diet; or exhibited compulsive behaviour. The team used 10X Genomics(opens in new window) technology which captures single brain cells in individual liquid droplets, accompanied by barcoded primers. The DNA collected from all the droplets was sequenced, with the barcodes allowing sequences belonging to the same cell to be regrouped, creating single-cell transcriptome profiles. Tens of thousands of these profiles could then be compared and clustered into groups of similar cells, each representing a specific cell-type. Comparing profiles from flies under different conditions, such as thirsty or not, the team identified genes whose expression varied in the same cell-type, dependent on the condition. Crucially, cell-specific genetic manipulation, straightforward in flies, enabled testing of the behavioural relevance of each differentially expressed gene. For example, to ascertain whether a gene is needed for increased drinking when thirsty or to remember a reward-paired smell.
Change in gene expression in specific cells
Looking at the profiles of thirsty flies, the team unexpectedly identified a change in the gene expression of astrocyte(opens in new window) glial cells, including increased expression of a key enzyme producing the neuromodulator D-serine. “Without this enzyme, flies don’t exhibit clear thirst-directed behaviours. We also mimicked thirst by loading flies with D-serine, which both restored thirst-directed behaviour in flies that could not produce it and induced thirst in those already water-sated,” adds Waddell. Key populations of dopaminergic neurons were also identified which, if activated during learning, induced memories that guide compulsive reward seeking(opens in new window). These flies sought illusory rewards at cost, such as enduring electric shocks, and even foregoing food when hungry to seek an alternative compulsion-driven reward. “Our prior work suggested different subsets of dopaminergic neurons represent different types of reward. Here, we discovered that simultaneously activating a collection of dopaminergic neurons, which we predict represents many specific rewards, effectively a ‘super reward’, triggers compulsive reward-seeking behaviour. Additionally, engagement of these reward pathways inhibits processing of aversion, possibly explaining inappropriate risk-taking behaviour,” notes Waddell.
Potential treatments for memory loss
The team are now investigating how strong reward-seeking overrides aversion processing, to explore how dopaminergic processes change gene expression and the physiological state of target neurons. Genes differently expressed in specific neurons after memory formation have also been identified with their functional implications currently being analysed. Indeed, some signalling pathway manipulations in specific neurons have been found to enhance memory. “We are working with genes found to change expression in particular neurons. We expect these to reveal how states and memories are represented as physiological changes in different types of neurons, with some likely critical for compulsions. We are also testing the memory effects of small molecule drugs known to alter the function of some of these genes,” says Waddell.
