D1 and D2 medium spiny neurons in the nucleus accumbens core have distinct and valence-independent roles in learning.

At the core of value-based learning is the nucleus accumbens (NAc). D1- and D2-receptor-containing medium spiny neurons (MSNs) in the NAc core are hypothesized to have opposing valence-based roles in behavior. Using optical imaging and manipulation approaches in mice, we show that neither D1 nor D2 MSNs signal valence. D1 MSN responses were evoked by stimuli regardless of valence or contingency. D2 MSNs were evoked by both cues and outcomes, were dynamically changed with learning, and tracked valence-free prediction error at the population and individual neuron level. Finally, D2 MSN responses to cues were necessary for associative learning. Thus, D1 and D2 MSNs work in tandem, rather than in opposition, by signaling specific properties of stimuli to control learning.

Nucleus accumbens core single cell ensembles bidirectionally respond to experienced versus observed aversive events.

Fear learning is a critical feature of survival skills among mammals. In rodents, fear learning manifests itself through direct experience of the aversive event or social transmission of aversive stimuli such as observing and acting on conspecifics' distress. The neuronal network underlying the social transmission of information largely overlaps with the brain regions that mediate behavioral responses to aversive and rewarding stimuli. In this study, we recorded single cell activity patterns of nucleus accumbens (NAc) core neurons using in vivo optical imaging of calcium transients via miniature scopes. This cutting-edge imaging methodology not only allows us to record activity patterns of individual neurons but also lets us longitudinally follow these individual neurons across time and different behavioral states. Using this approach, we identified NAc core single cell ensembles that respond to experienced and/or observed aversive stimuli. Our results showed that experienced and observed aversive stimuli evoke NAc core ensemble activity that is largely positive, with a smaller subset of negative responses. The size of the NAc single cell ensemble response was greater for experienced aversive stimuli compared to observed aversive events. Our results also revealed sex differences in the NAc core single cell ensembles responses to experience aversive stimuli, where females showed a greater accumbal response. Importantly, we found a subpopulation within the NAc core single cell ensembles that show a bidirectional response to experienced aversive stimuli versus observed aversive stimuli (i.e., negative response to experienced and positive response to observed). Our results suggest that the NAc plays a role in differentiating somatosensory experience from social observation of aversion at a single cell level. These results have important implications for psychopathologies where social information processing is maladaptive, such as autism spectrum disorders.

Dopamine release at the time of a predicted aversive outcome causally controls the trajectory and expression of conditioned behavior.

Dopamine release in the nucleus accumbens (NAc) is causally linked to adaptive aversive learning, and its dysregulation is a core phenotype in anxiety and stress disorders. Here, we record NAc core dopamine during a task where mice learn to discriminate between cues signaling two types of outcomes: (1) footshock presentation and (2) footshock omission. We show that dopamine release is evoked by footshock omission. This dopamine response is largest when the omission is unexpected and decreases over learning, and artificially increasing this signal disrupts discrimination learning. Conversely, optogenetic inhibition of dopamine responses to the footshock itself impairs learning. Finally, theory-driven computational modeling suggests that these effects can be explained by dopamine signaling the perceived saliency of predicted aversive events. Together, we elucidate the role of NAc dopamine in aversive learning and offer potential avenues for understanding the neural mechanisms involved in anxiety and stress disorders.

Nucleus accumbens core single cell ensembles bidirectionally respond to experienced versus observed aversive events.

Empathy is the ability to adopt others' sensory and emotional states and is an evolutionarily conserved trait among mammals. In rodents, empathy manifests itself as social modulation of aversive stimuli such as acknowledging and acting on conspecifics' distress. The neuronal network underlying social transmission of information is known to overlap with the brain regions that mediate behavioral responses to aversive and rewarding stimuli. In this study, we recorded single cell activity patterns of nucleus accumbens (NAc) core neurons using in vivo optical imaging of calcium transients via miniature scopes. This cutting-edge imaging methodology not only allows us to record activity patterns of individual neurons but also lets us longitudinally follow these individual neurons across time and different behavioral states. Using this approach, we identified NAc core single cell ensembles that respond to experienced and/or observed aversive stimuli. Our results showed that experienced and observed aversive stimuli evoke NAc core ensemble activity that is largely positive, with a smaller subset of negative responses. The size of the NAc single cell ensemble response was greater for experienced aversive stimuli compared to observed aversive events. Our results also revealed a subpopulation within the NAc core single cell ensembles that show a bidirectional response to experienced aversive stimuli versus observed aversive stimuli (i.e., negative response to experienced and positive response to observed). These results suggest that the NAc plays a role in differentiating somatosensory experience from social observation of aversion at a single cell level. This has important implications for psychopathologies where social information processing is maladaptive, such as autism spectrum disorders.

Dopamine signaling in the nucleus accumbens core mediates latent inhibition.

Studies investigating the neural mechanisms by which associations between cues and predicted outcomes control behavior often use associative learning frameworks to understand the neural control of behavior. These frameworks do not always account for the full range of effects that novelty can have on behavior and future associative learning. Here, in mice, we show that dopamine in the nucleus accumbens core is evoked by novel, neutral stimuli, and the trajectory of this response over time tracked habituation to these stimuli. Habituation to novel cues before associative learning reduced future associative learning, a phenomenon known as latent inhibition. Crucially, trial-by-trial dopamine response patterns tracked this phenomenon. Optogenetic manipulation of dopamine responses to the cue during the habituation period bidirectionally influenced future associative learning. Thus, dopamine signaling in the nucleus accumbens core has a causal role in novelty-based learning in a way that cannot be predicted based on purely associative factors.

Dopamine release in the nucleus accumbens core signals perceived saliency.

A large body of work has aimed to define the precise information encoded by dopaminergic projections innervating the nucleus accumbens (NAc). Prevailing models are based on reward prediction error (RPE) theory, in which dopamine updates associations between rewards and predictive cues by encoding perceived errors between predictions and outcomes. However, RPE cannot describe multiple phenomena to which dopamine is inextricably linked, such as behavior driven by aversive and neutral stimuli. We combined a series of behavioral tasks with direct, subsecond dopamine monitoring in the NAc of mice, machine learning, computational modeling, and optogenetic manipulations to describe behavior and related dopamine release patterns across multiple contingencies reinforced by differentially valenced outcomes. We show that dopamine release only conforms to RPE predictions in a subset of learning scenarios but fits valence-independent perceived saliency encoding across conditions. Here, we provide an extended, comprehensive framework for accumbal dopamine release in behavioral control.

An optimized procedure for robust volitional cocaine intake in mice.

Substance use disorder (SUD) is a behavioral disorder characterized by volitional drug consumption. Mouse models of SUD allow for the use of molecular, genetic, and circuit-level tools, providing enormous potential for defining the underlying mechanisms of this disorder. However, the relevance of results depends on the validity of the mouse models used. Self-administration models have long been the preferred preclinical model for SUD as they allow for volitional drug consumption, thus providing strong face validity. While previous work has defined the parameters that influence intravenous cocaine self-administration in other species-such as rats and primates-many of these parameters have not been explicitly assessed in mice. In a series of experiments, we showed that commonly used mouse models of self-administration, where behavior is maintained on a fixed-ratio schedule of reinforcement, show similar levels of responding in the presence and absence of drug delivery-demonstrating that it is impossible to determine when drug consumption is and is not volitional. To address these issues, we have developed a novel mouse self-administration procedure where animals do not need to be pretrained on sucrose and behavior is maintained on a variable-ratio schedule of reinforcement. This procedure increases rates of reinforcement behavior, increases levels of drug intake, and results in clearer delineation between drug-reinforced and saline conditions. Together, these data highlight a major issue with fixed-ratio models in mice that complicates subsequent analysis and provide a simple approach to minimize these confounds with variable-ratio schedules of reinforcement. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Sex differences in dopamine release regulation in the striatum.

The mesolimbic dopamine system-which originates in the ventral tegmental area and projects to the striatum-has been shown to be involved in the expression of sex-specific behavior and is thought to be a critical mediator of many psychiatric diseases. While substantial work has focused on sex differences in the anatomy of dopamine neurons and relative dopamine levels between males and females, an important characteristic of dopamine release from axon terminals in the striatum is that it is rapidly modulated by local regulatory mechanisms independent of somatic activity. These processes can occur via homosynaptic mechanisms-such as presynaptic dopamine autoreceptors and dopamine transporters-as well as heterosynaptic mechanisms, such as retrograde signaling from postsynaptic cholinergic and GABAergic systems, among others. These regulators serve as potential targets for the expression of sex differences in dopamine regulation in both ovarian hormone-dependent and independent fashions. This review describes how sex differences in microcircuit regulatory mechanisms can alter dopamine dynamics between males and females. We then describe what is known about the hormonal mechanisms controlling/regulating these processes. Finally, we highlight the missing gaps in our knowledge of these systems in females. Together, a more comprehensive and mechanistic understanding of how sex differences in dopamine function manifest will be particularly important in developing evidence-based therapeutics that target this system and show efficacy in both sexes.

Direct dopamine terminal regulation by local striatal microcircuitry.

Regulation of axonal dopamine release by local microcircuitry is at the hub of several biological processes that govern the timing and magnitude of signaling events in reward-related brain regions. An important characteristic of dopamine release from axon terminals in the striatum is that it is rapidly modulated by local regulatory mechanisms. These processes can occur via homosynaptic mechanisms-such as presynaptic dopamine autoreceptors and dopamine transporters - as well heterosynaptic mechanisms such as retrograde signaling from postsynaptic cholinergic and dynorphin systems, among others. Additionally, modulation of dopamine release via diffusible messengers, such as nitric oxide and hydrogen peroxide, allows for various metabolic factors to quickly and efficiently regulate dopamine release and subsequent signaling. Here we review how these mechanisms work in concert to influence the timing and magnitude of striatal dopamine signaling, independent of action potential activity at the level of dopaminergic cell bodies in the midbrain, thereby providing a parallel pathway by which dopamine can be modulated. Understanding the complexities of local regulation of dopamine signaling is required for building comprehensive frameworks of how activity throughout the dopamine system is integrated to drive signaling and control behavior.

A novel multidimensional reinforcement task in mice elucidates sex-specific behavioral strategies.

A large body of work has focused on understanding stimulus-driven behavior, sex differences in these processes, and the neural circuits underlying them. Many preclinical mouse models present rewarding or aversive stimuli in isolation, ignoring that ethologically, reward seeking requires the consideration of potential aversive outcomes. In addition, the context (or reinforcement schedule under) in which stimuli are encountered can engender different behavioral responses to the same stimulus. Thus, delineating neural control of behavior requires a dissociation between stimulus valence and stimulus-driven behavior. We developed the Multidimensional Cue Outcome Action Task (MCOAT) to dissociate motivated action from cue learning and valence in mice. First, mice acquire positive and negative reinforcement in the presence of discrete discriminative stimuli. Next, discriminative stimuli are presented concurrently allowing for parsing innate behavioral strategies based on reward seeking and avoidance. Lastly, responding in the face of punishment is assessed, thus examining  how positive and negative outcomes are relatively valued. First, we identified sex-specific behavioral strategies, showing that females prioritize avoidance of negative outcomes over seeking positive, while males have the opposite strategy. Next, we show that chemogenetically inhibiting D1 medium spiny neurons (MSNs) in the nucleus accumbens-a population that has been linked to reward-driven behavior-reduces positive and increases negative reinforcement learning rates. Thus, D1 MSNs modulate stimulus processing, rather than motivated responses or the reinforcement process itself. Together, the MCOAT has broad utility for understanding complex behaviors as well as the definition of the discrete information encoded within cellular populations.

Feminization of Male Brown Treesnake Methyl Ketone Expression via Steroid Hormone Manipulation.

Pheromones are useful tools for the management of invasive invertebrates, but have proven less successful in field applications for invasive vertebrates. The brown treesnake, Boiga irregularis, is an invasive predator that has fundamentally altered the ecology of Guam. The development of control tools to manage Boiga remains ongoing. Skin-based, lipophilic pheromone components facilitate mating in brown treesnakes, with females producing the same long-chain, saturated and monounsaturated (ketomonoene) methyl ketones known to function as pheromones in garter snakes, Thamnophis sirtalis. Boiga also express novel, diunsaturated methyl ketones (ketodienes) with a purported function as a sex pheromone. In our study, we implanted 17 ß-estradiol in adult male brown treesnakes in order to manipulate methyl ketone expression as sex attractants, an effect that would mirror findings with garter snakes. Specifically, estrogen promoted production of two ketomonoenes, pentatriaconten-2-one and hexatriaconten-2-one, and suppressed production of one ketodiene, heptatriacontadien-2-one. In bioassays, estrogen-implanted males elicited tongue-flicking and chin rubbing behavior from unmanipulated males, though the responses were weaker than those elicited by females. On Guam, wild males exhibited greatest responses to whole female skin lipid extracts and only weak responses to the methyl ketone fractions from females and implanted males. Our results suggest that sex identity in brown treesnakes may be conferred by the ratio of ketomonoenes (female) to ketodienes (male) from skin lipids and may be augmented by a sex-specific endocrine signal (estradiol). However, a blend of long-chain methyl ketones alone is not sufficient to elicit maximal reproductive behaviors in male Boiga.

High-Fat, High-Sugar Diet Disrupts the Preovulatory Hormone Surge and Induces Cystic Ovaries in Cycling Female Rats.

Diet-induced obesity has been associated with various metabolic and reproductive disorders, including polycystic ovary syndrome. However, the mechanisms by which obesity influences the reproductive system are still not fully known. Studies have suggested that impairments in hormone signaling are associated with the development of symptoms such as acyclicity and ovarian cysts. However, these studies have often failed to address how these hormonal changes arise and how they might contribute to the progression of reproductive diseases. In the present study, we used a high-fat, high-sugar (HFHS) diet to induce obesity in a female rodent model to determine the changes in critical reproductive hormones that might contribute to the development of irregular estrous cycling and reproductive cycle termination. The HFHS animals exhibited impaired estradiol, progesterone (P4), and luteinizing hormone (LH) surges before ovulation. The HFHS diet also resulted in altered basal levels of testosterone (T) and LH. Furthermore, alterations in the basal P4/T ratio correlated strongly with ovarian cyst formation in HFHS rats. Thus, this model provides a method to assess the underlying etiology of obesity-related reproductive dysfunction and to examine an acyclic reproductive phenotype as it develops.