Pavlovian fear conditioning and extinction represent learning mechanisms underlying exposure-based interventions. While increasing evidence indicates a pivotal role of disgust in the development of contamination-based obsessive-compulsive disorder (C-OCD), dysregulations in conditioned disgust acquisition and maintenance, in particular driven by higher-order conceptual processes, have not been examined. Here, we address this gap by exposing individuals with high (HCC, n = 41) or low (LCC, n = 41) contamination concern to a conceptual-level disgust conditioning and extinction paradigm. Conditioned stimuli (CS+) were images from one conceptual category partially reinforced by unconditioned disgust-eliciting stimuli (US), while images from another category served as non-reinforced conditioned stimuli (CS-). Skin conductance responses (SCRs), US expectancy and CS valence ratings served as primary outcomes to quantify conditioned disgust responses. Relative to LCC, HCC individuals exhibited increased US expectancy and CS+ disgust experience, but comparable SCR levels following disgust acquisition. Despite a decrease in conditioned responses from the acquisition phase to the extinction phase, both groups did not fully extinguish the learned disgust. Importantly, the extinction resilience of acquired disgust was more pronounced in HCC individuals. Together, our findings suggest that individuals with high self-reported contamination concern exhibit increased disgust acquisition and resistance to extinction. The findings provide preliminary evidence on how dysregulated disgust learning mechanism across semantically related concepts may contribute to C-OCD.

Associative learning allows animals to establish links between stimuli based on their concomitance. In the case of Pavlovian conditioning, a single stimulus A (the conditional stimulus, CS) is reinforced unambiguously with an unconditional stimulus (US) eliciting an innate response. This conditioning constitutes an \'elemental\' association to elicit a learnt response from A+ without US presentation after learning. However, associative learning may involve a \'complex\' CS composed of several components. In that case, the compound may predict a different outcome than the components taken separately, leading to ambiguity and requiring the animal to perform so-called non-elemental discrimination. Here, we focus on such a non-elemental task, the negative patterning (NP) problem, and provide the first evidence of NP solving in Drosophila. We show that Drosophila learn to discriminate a simple component (A or B) associated with electric shocks (+) from an odour mixture composed either partly (called \'feature-negative discrimination\' A+ versus AB-) or entirely (called \'NP\' A+B+ versus AB-) of the shock-associated components. Furthermore, we show that conditioning repetition results in a transition from an elemental to a configural representation of the mixture required to solve the NP task, highlighting the cognitive flexibility of Drosophila.

Standard fear extinction relies on the ventromedial prefrontal cortex (vmPFC) to form a new memory given the omission of threat. Using fMRI in humans, we investigated whether replacing threat with novel neutral outcomes (instead of just omitting threat) facilitates extinction by engaging the vmPFC more effectively than standard extinction. Computational modeling of associability (indexing surprise strength and dynamically modulating learning rates) characterized skin conductance responses and vmPFC activity during novelty-facilitated but not standard extinction. Subjects who showed faster within-session updating of associability during novelty-facilitated extinction also expressed better extinction retention the next day, as expressed through skin conductance responses. Finally, separable patterns of connectivity between the amygdala and ventral versus dorsal mPFC characterized retrieval of novelty-facilitated versus standard extinction memories, respectively. These results indicate that replacing threat with novel outcomes stimulates vmPFC involvement on extinction trials, leading to a more durable long-term extinction memory.SIGNIFICANCE STATEMENT Psychiatric disorders characterized be excessive fear are a major public health concern. Popular clinical treatments, such as exposure therapy, are informed by principles of Pavlovian extinction. Thus, there is motivation to optimize extinction strategies in the laboratory so as to ultimately develop more effective clinical treatments. Here, we used functional neuroimaging in humans and found that replacing (rather than just omitting) expected aversive events with novel and neutral outcomes engages the ventromedial prefrontal cortex during extinction learning. Enhanced extinction also diminished activity in threat-related networks (e.g., the insula, thalamus) during immediate extinction and a 24 h extinction retention test. This is new evidence for how behavioral protocols designed to enhance extinction affects neurocircuitry underlying the learning and retention of extinction memories.

Placebo analgesia is the beneficial effect that follows despite a pharmacologically inert treatment. Modern neuroimaging studies in humans have delineated the hierarchical brain regions involved in placebo analgesia. However, because of the lack of proper approaches to perform molecular and cellular manipulations, the detailed molecular processes behind it have not been clarified. To address this issue, we developed an animal model of placebo analgesia in rats and analyzed the placebo analgesia related brain activity using small-animal neuroimaging method. We show here that gabapentin-based Pavlovian conditioning successfully induced placebo analgesia in neuropathic pain model rats and hierarchical brain regions are involved in placebo analgesia in rats, including the prelimbic cortex (PrL) of the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), ventrolateral periaqueductal gray matter (vlPAG), etc. The functional couplings in placebo responders between the mPFC and vlPAG was interrupted by naloxone, an antagonist of μ opioid receptor. Moreover, both local chemical lesion and microinfusion of naloxone in the mPFC suppressed the placebo analgesia. These results suggest that the intrinsic μ opioid system in the mPFC causally contribute to placebo analgesia in rats, and the small-animal neuroimaging approach could provide important insights toward understanding the placebo effect in great detail.

Poor conditioning to punishment, such as loud tones or electric shock, has been proposed as an important factor involved in the etiology of aggressive and psychopathic behavior. However, it is not known whether the association holds when monetary or social stimulus is used as the unconditioned stimulus, and if aggressive individuals also have impaired conditioning to rewards. In this study, skin conductance responses in a conditioning task involving both monetary/social reward and punishment as unconditioned stimuli were assessed in 340 male and female 8- to 9-year-old children from the community. Children reported their reactive and proactive aggression using the Reactive and Proactive Aggression Questionnaire (RPQ; Raine et al., 2006). Results showed that monetary/social reward and punishment were effective in eliciting physiological classical conditioning in children, and that reduced reward conditioning was associated with high levels of proactive aggression in particular. Findings highlight the importance of distinguishing between reactive and proactive aggression when examining antisocial behavior in children, and suggest that reward-oriented treatment programs may not be effective for children with more proactive, instrumental aggressive behavior.

The ability to predict reward promotes animal survival. Both dopamine neurons in the ventral tegmental area and serotonin neurons in the dorsal raphe nucleus (DRN) participate in reward processing. Although the learning effects on dopamine neurons have been extensively characterized, it remains largely unknown how the response of serotonin neurons evolves during learning. Moreover, although stress is known to strongly influence reward-related behavior, we know very little about how stress modulates neuronal reward responses. By monitoring Ca2+ signals during the entire process of Pavlovian conditioning, we here show that learning differentially shapes the response patterns of serotonin neurons and dopamine neurons in mice of either sex. Serotonin neurons gradually develop a slow ramp-up response to the reward-predicting cue, and ultimately remain responsive to the reward, whereas dopamine neurons increase their response to the cue but reduce their response to the reward. For both neuron types, the responses to the cue and the reward depend on reward value, are reversible when the reward is omitted, and are rapidly reinstated by restoring the reward. We also found that stressors including head restraint and fearful context substantially reduce the response strength of both neuron types, to both the cue and the reward. These results reveal the dynamic nature of the reward responses, support the hypothesis that DRN serotonin neurons signal the current likelihood of receiving a net benefit, and suggest that the inhibitory effect of stress on the reward responses of serotonin neurons and dopamine neurons may contribute to stress-induced anhedonia.SIGNIFICANCE STATEMENT Both serotonin neurons in the dorsal raphe and dopamine neurons in the ventral tegmental area are intimately involved in reward processing. Using long-term fiber photometry of Ca2+ signals from freely behaving mice, we here show that learning produces a ramp-up activation pattern in serotonin neurons that differs from that in dopamine neurons, indicating complementary roles for these two neuron types in reward processing. Moreover, stress treatment substantially reduces the reward responses of both serotonin neurons and dopamine neurons, suggesting a possible physiological basis for stress-induced anhedonia.

The lateral habenula (LHb) is believed to encode negative motivational values. It remains unknown how LHb neurons respond to various stressors and how learning shapes their responses. Here, we used fiber-photometry and electrophysiology to track LHb neuronal activity in freely-behaving mice. Bitterness, pain, and social attack by aggressors intensively excite LHb neurons. Aversive Pavlovian conditioning induced activation by the aversion-predicting cue in a few trials. The experience of social defeat also conditioned excitatory responses to previously neutral social stimuli. In contrast, fiber photometry and single-unit recordings revealed that sucrose reward inhibited LHb neurons and often produced excitatory rebound. It required prolonged conditioning and high reward probability to induce inhibition by reward-predicting cues. Therefore, LHb neurons can bidirectionally process a diverse array of aversive and reward signals. Importantly, their responses are dynamically shaped by learning, suggesting that the LHb participates in experience-dependent selection of behavioral responses to stressors and rewards.

The orbitofrontal cortex (OFC) is important for the cognitive processes of learning and decision making. Previous recordings have revealed that OFC neurons encode predictions of reward outcomes. The OFC is interconnected with the dorsal raphe nucleus (DRN), which is a major serotonin (5-HT) center of the brain. Recent studies have provided increasing evidence that the DRN encodes reward signals. However, it remains unclear how the activity of DRN neurons affects the prospective reward coding of OFC neurons. By combining single-unit recordings from the OFC and optogenetic activation of the DRN in behaving mice, we found that DRN stimulation is sufficient to organize and modulate the anticipatory responses of OFC neurons. During pavlovian conditioning tasks for mice, odorant cues were associated with the delayed delivery of natural rewards of sucrose solution or DRN stimulation. After training, OFC neurons exhibited prospective responses to the sucrose solution. More importantly, the coupling of an odorant with delayed DRN stimulation resulted in tonic excitation or inhibition of OFC neurons during the delay period. The intensity of the prospective responses was affected by the frequency and duration of DRN stimulation. Additionally, DRN stimulation bidirectionally modulated the prospective responses to natural rewards. These experiments indicate that signals from the DRN are incorporated into the brain reward system to shape the cortical prospective coding of rewards.

Iptakalim is an ATP-sensitive potassium channel opener, as well as an α4β2-containing nicotinic acetylcholine receptor (nAChR) antagonist. Pretreatment with iptakalim diminishes nicotine-induced dopamine (DA) and glutamate release in the nucleus accumbens. This neuropharmacological profile suggests that iptakalim may be useful for treatment of nicotine dependence. Thus, we examined the effects of iptakalim in two preclinical models. First, the impact of iptakalim on the interoceptive stimulus effect of nicotine was evaluated by training rats in a discriminated goal-tracking task that included intermixed nicotine (0.4 mg/kg, SC) and saline sessions. Sucrose was intermittently presented in a response-independent manner only on nicotine sessions. On intervening test days, rats were pretreated with iptakalim (10, 30, 60 mg/kg, IP). Results revealed that iptakalim attenuated nicotine-evoked responding controlled by the nicotine stimulus in a dose-dependent manner. In a separate study, the impact of iptakalim on the reinforcing effects of nicotine was investigated by training rats to lever-press to self-administer nicotine (0.01 mg/kg/infusion) [Dosage error corrected]. Results revealed that pretreatment with iptakalim (1, 3, 6 mg/kg, IV) decreased nicotine intake (i.e., less active lever responding). Neither behavioral effect was due to a non-specific motor effect of iptakalim, nor to an ability of iptakalim to inhibit DA transporter (DAT) or serotonin transporter (SERT) function. Together, these finding support the notion that iptakalim may be an effective pharmacotherapy for increasing smoking cessation and a better understanding of its action could contribute to medication development.