Dopaminergic systems create reward seeking despite adverse consequences

Uncontrolled reward-seeking behavior is a hallmark of several disorders, particularly substance use disorders. While dysfunction in the brain's dopaminergic valuation system is implicated, the underlying mechanisms remain unclear. This study leverages the relative simplicity of the Drosophila melanogaster dopaminergic system to investigate these mechanisms at a cellular resolution. The Drosophila system offers advantages over mammalian models due to its lower complexity, facilitating the study of reward memory and seeking with greater precision. Previous research has demonstrated that activating specific dopaminergic neurons (DANs) in Drosophila can reinforce olfactory memories and assign positive valence to associated sensory stimuli. Conversely, other DAN populations mediate aversive reinforcement. This study hypothesizes that simultaneous engagement of multiple reward-specific signals might create a 'compound reward' memory, leading to reward seeking even in the face of negative consequences. Understanding these mechanisms in Drosophila could provide valuable insights into similar maladaptive behaviors in mammals, such as compulsive drug seeking.

Existing literature establishes a link between dopaminergic dysfunction and uncontrolled reward seeking. Studies in rodents, using electrical or optogenetic self-stimulation of DANs, have shown persistent self-administration even with punishment, mirroring behavior after cocaine administration. However, the heterogeneity of DANs in mammalian brains and difficulties in targeting specific subpopulations hinder a precise understanding of the underlying mechanisms. In contrast, the Drosophila model offers a simplified system with distinct DAN populations mediating reward and aversion. Prior work has shown that specific DAN populations in Drosophila are involved in reinforcing olfactory and place memories and in assigning positive valence to stimuli. The heterogeneity within reward-encoding DANs, allowing parallel coding of various rewarding stimuli, has also been established. This existing research provides the foundation for the current study, which aims to identify specific DAN populations responsible for unconstrained reward seeking and to elucidate the underlying neural mechanisms.

The study employed several methodologies. First, associative olfactory learning paradigms were used. Flies were trained to associate an odor (conditioned stimulus, CS+) with either sucrose (natural reward) or optogenetic activation of specific DANs (artificial reward). A control odor (CS-) was presented without reward. Flies were then tested in a T-maze for their preference between the CS+ and CS- odors, with the CS+ odor sometimes paired with electric shocks. The performance index measured the preference for the rewarded odor, with positive values indicating preference for CS+ and negative values for CS-. Optogenetic manipulation involved the use of CsChrimson (CsChr), a red-light-sensitive cation channel, to artificially activate specific DANs. Single-cell RNA sequencing (scRNA-seq) was utilized to characterize the cell types labeled by the 0273-GAL4 driver line, which labels a group of DANs. Cell-type-specific GAL80 was used to suppress GAL4 expression in specific neuron types. Two-photon in vivo calcium imaging was used to monitor the activity of specific DANs in response to odor presentation and sucrose feeding under different physiological states (starved, dehydrated, satiated). Connectome analysis of electron microscopy data was employed to characterize the input structure of the reward-encoding DANs. This comprehensive approach combines behavioral assays, genetic manipulation, molecular characterization and connectomics to dissect the complex neural circuits involved in reward seeking.

The study revealed that optogenetic activation of 0273 neurons, a subset of reward-encoding DANs in the PAM cluster, produced a strong preference for the associated odor cue even when it was paired with electric shocks. This shock-resistant reward seeking was not observed with sucrose reward or activation of other DAN populations. scRNA-seq data indicated that 0273-GAL4 labels a mixed population of neurons, including DANs, cholinergic, GABAergic, and glutamatergic neurons. Further experiments showed that DANs are crucial for the shock-resistant reward seeking, with specific PAM DAN populations (β'2 & γ4 DANs) being particularly important. These β'2 & γ4 DANs, when activated, created a shock-resistant reward memory that was state-independent (i.e., present regardless of hunger). Optogenetic silencing of 0273 neurons or β'2&γ4 DANs implanted aversive memories, and this aversion was diminished when aversive DANs were co-inhibited, indicating an antagonistic relationship between reward and aversion DANs. Activation of reward DANs impaired subsequent aversive learning and naive shock avoidance, suggesting that activation of reward DANs interferes with the processing of aversive stimuli. Importantly, flies with artificially induced reward-seeking behavior neglected available food to seek the artificially rewarded odor, demonstrating that this behavior overrides physiological needs. Two-photon calcium imaging showed that β'2 and γ4 DANs respond strongly to odors, while γ5n DANs respond strongly to sucrose. The γ5n DAN responses showed adaptation with repeated sucrose presentations, suggesting a satiety signal. Finally, connectome analysis showed highly heterogeneous and parallel input to the reward-encoding DANs, consistent with the parallel representation of diverse rewards.

This study demonstrates that specific dopaminergic neuron populations in Drosophila can drive reward-seeking behavior that persists even in the face of adverse consequences such as electric shocks and food deprivation. The findings highlight the importance of specific DAN subsets (β'2 and γ4) in mediating this behavior. The antagonistic interaction between reward and aversion DANs, demonstrated through optogenetic manipulation, sheds light on how reward seeking can override aversion processing. The observation that artificial reward seeking bypasses the usual state-dependent regulation of reward processing, suggests a mechanism for uncontrolled reward-seeking behaviors. The highly heterogeneous input structure of the reward-encoding DANs further emphasizes the complexity of reward processing and its potential susceptibility to disruption. The Drosophila model offers valuable insights into similar maladaptive behaviors seen in mammals, suggesting potential conservation of these mechanisms across species. The observed impairment of subsequent aversive learning and naive shock avoidance following reward DAN activation suggests that dysfunction in these pathways may contribute to the development of maladaptive reward-seeking behaviors.

This study reveals specific dopaminergic mechanisms underlying compulsive-like reward seeking in Drosophila, characterized by shock resistance and need neglect. The study identified key DAN subpopulations (β'2 & γ4) crucial for this behavior, highlighting the interplay between reward and aversion processing. The results suggest that dysfunction in dopaminergic pathways, disrupting the balance between reward and aversion and overriding state-dependent regulation, may contribute to maladaptive reward seeking. The Drosophila model provides a valuable system for further investigation into the complex neural circuits involved in compulsive behaviors and their potential therapeutic targets.

The study primarily focuses on artificially induced reward seeking through optogenetic manipulation. While this approach allows for precise control of DAN activity, it might not fully capture the complexity of naturally occurring reward seeking. The Drosophila model, while advantageous due to its simplicity, may not fully replicate all aspects of mammalian reward systems. The study does not investigate the long-term consequences or the impact of repeated exposure to these reward-seeking paradigms. Future studies could address these limitations by examining natural reward-seeking behaviors, focusing on specific pharmacological or environmental manipulations, and exploring long-term behavioral changes in the flies.