Acute Stress Enhances Associative Learning via Dopamine Signaling in the Ventral Lateral Striatum

A Wearable Electrochemical Sensor Based on a Molecularly Imprinted Polymer Integrated with a Copper Benzene-1,3,5-Tricarboxylate Metal-Organic Framework for the On-Body Monitoring of Cortisol in Sweat

Owing to their potential to transform traditional medical diagnostics and health monitoring, wearable biosensors have become an alternative evolutionary technology in the field of medical care. However, it is still necessary to overcome some key technique challenges, such as the selectivity, sensitivity, and stability of biometric identification. Herein, a novel, wearable electrochemical sensor based on a molecularly imprinted polymer (MIP) integrated with a copper benzene-1,3,5-tricarboxylate metal-organic framework (MOF) was designed for the detection of stress through the on-body monitoring of cortisol in sweat. The MOF was used as the substrate for MIP deposition to enhance the stability and sensitivity of the sensor. The sensor consisted of two layers, with a microfluidic layer as the top layer for spontaneous sweating and a modified electrode as the bottom layer for sensing. The sensor measured cortisol levels by detecting the current change that occurred when the target molecules bound to the imprinted cavities, using Prussian blue nanoparticles embedded in the MIP framework as the REDOX probe. The proposed sensor exhibited a linear detection range of 0.01-1000 nM with a detection limit of 0.0027 nM, and favorable specificity over other analogies. This facile anti-body free sensor showed excellent stability, and can be successfully applied for in situ cortisol monitoring.

Associations of schizophrenia with the activities of the HPA and HPG axes and their interactions characterized by hair-based biomarkers

BACKGROUND

Past studies on schizophrenia (SCZ) and the stress-sensitive neuroendocrine systems have mostly focused on a single system and traditionally utilized acute biomarkers (e.g., biomarkers from blood, urine and saliva) that poorly match the chronic course of schizophrenia in time span. Using eight biomarkers in hair, this study aimed to explore the functional characteristics of SCZ patients in the hypothalamic-pituitary-adrenocortical (HPA) and hypothalamic-pituitary-gonadal (HPG) axes and the interaction between the two axes.

METHODS

Hair samples were taken from 137 SCZ patients and 73 controls. The SCZ patients were diagnosed by their attending physician according to the Diagnostic and Statistical Manual of Mental Disorders IV and were clinically stable after treatment. Gender, age, BMI, frequency of hair washing, marital status, education level, family history of mental illness and clozapine dosage were concurrently collected as covariates. The 10-item perceived stress scale (PSS-10) and the social readjustment rating scale were used to assess chronic stress status in SCZ patients. Eight hair biomarkers, cortisol, cortisone, dehydroepiandrosterone (DHEA), testosterone, progesterone, cortisol/cortisone, cortisol/DHEA and cortisol/testosterone, were measured by high performance liquid chromatography tandem mass spectrometer. Among them, cortisol, cortisone, DHEA and cortisol/DHEA reflected the functional activity of the HPA axis, and testosterone and progesterone reflected the functional activity of the HPG axis, and cortisol/cortisone reflected the activity of 11β-hydroxysteroid dehydrogenase types 2 (11β-HSD 2), and cortisol/testosterone reflected the HPA-HPG interaction.

RESULTS

SCZ patients showed significantly higher cortisone and cortisol/testosterone than controls (p<0.001, η²p=0.180 and p=0.015, η²p=0.031), lower testosterone (p=0.009, η²p=0.034), progesterone (p<0.001, η²p=0.069) and cortisol/cortisone (p=0.001, η²p=0.054). There were significant intergroup differences in male and female progesterone (p=0.003, η²p=0.088 and p=0.030, η²p=0.049) and female testosterone (p=0.028, η²p=0.051). In SCZ patients, cortisol, cortisol/cortisone, cortisol/DHEA and cortisol/testosterone were positively associated with PSS-10 score (ps<0.05, 0.212

CONCLUSION

The function of the HPA and HPG axes, the activity of 11β-HSD 2 and the HPA-HPG interaction were abnormal in SCZ patients. The abnormality of neuroendocrine systems was associated with chronic stress status in SCZ patients. This study provided evidence for abnormalities in the neuroendocrine systems in SCZ patients.

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Key Words

• 11β-HSD 2
• HPA axis
• HPG axis
• Hair biomarkers
• Schizophrenia

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MESH Headings

• Humans
• Female
• Male
• Hypothalamo-Hypophyseal System/metabolism
• Schizophrenia/metabolism
• Pituitary-Adrenal System/metabolism
• Pituitary-Adrenal System/physiopathology
• Hair/chemistry
• Hair/metabolism
• Biomarkers/metabolism
• Adult
• Hydrocortisone/metabolism
• Hydrocortisone/analysis
• Cortisone/metabolism
• Cortisone/analysis
• Testosterone/metabolism
• Testosterone/analysis
• Dehydroepiandrosterone/metabolism
• Dehydroepiandrosterone/analysis
• Stress, Psychological/metabolism
• Middle Aged
• Progesterone/metabolism
• Progesterone/analysis
• Case-Control Studies

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Affiliation(s)

Deyi Qi Department of Brain and Learning Science, School of Biological Science & Medical Engineering, Southeast University, Nanjing 211189, China; Key Laboratory of Child Development and Learning Science (Southeast University), Ministry of Education, Nanjing 211189, China; Institute of Child Development and Education, Southeast University, Nanjing 211189, China
Weiliang Wang School of Nursing, Harbin Medical University, Harbin 163319, China.
Liuxi Chu Department of Brain and Learning Science, School of Biological Science & Medical Engineering, Southeast University, Nanjing 211189, China; Key Laboratory of Child Development and Learning Science (Southeast University), Ministry of Education, Nanjing 211189, China; Institute of Child Development and Education, Southeast University, Nanjing 211189, China
Yan Wu Department of Brain and Learning Science, School of Biological Science & Medical Engineering, Southeast University, Nanjing 211189, China; Key Laboratory of Child Development and Learning Science (Southeast University), Ministry of Education, Nanjing 211189, China; Institute of Child Development and Education, Southeast University, Nanjing 211189, China
Wei Wang Department of Brain and Learning Science, School of Biological Science & Medical Engineering, Southeast University, Nanjing 211189, China; Key Laboratory of Child Development and Learning Science (Southeast University), Ministry of Education, Nanjing 211189, China; Institute of Child Development and Education, Southeast University, Nanjing 211189, China
Minhui Zhu Department of Brain and Learning Science, School of Biological Science & Medical Engineering, Southeast University, Nanjing 211189, China; Key Laboratory of Child Development and Learning Science (Southeast University), Ministry of Education, Nanjing 211189, China; Institute of Child Development and Education, Southeast University, Nanjing 211189, China
Lin Yuan Department of Brain and Learning Science, School of Biological Science & Medical Engineering, Southeast University, Nanjing 211189, China; Key Laboratory of Child Development and Learning Science (Southeast University), Ministry of Education, Nanjing 211189, China; Institute of Child Development and Education, Southeast University, Nanjing 211189, China
Wei Gao Institute of Child Development and Education, Southeast University, Nanjing 211189, China; School of Psychology, Nanjing Normal University, Nanjing 210024, China
Huihua Deng Department of Brain and Learning Science, School of Biological Science & Medical Engineering, Southeast University, Nanjing 211189, China; Key Laboratory of Child Development and Learning Science (Southeast University), Ministry of Education, Nanjing 211189, China; Institute of Child Development and Education, Southeast University, Nanjing 211189, China.

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Effects of GABA, Sex, and Stress on Reward Learning in Current and Remitted Major Depression

BACKGROUND

Neurocognitive factors including aberrant reward learning, blunted GABA (gamma-aminobutyric acid), and potentiated stress sensitivity have been linked to anhedonia, a hallmark depressive symptom, possibly in a sex-dependent manner. However, previous research has not investigated the putative associations among these factors or the extent to which they represent trait- or state-based vulnerabilities for depression.

METHODS

Young adults with current major depressive disorder (MDD) (n = 44), remitted MDD (n = 42), and healthy control participants (HCs) (n = 44), stratified by sex assigned at birth, underwent magnetic resonance spectroscopy to assess macromolecular contaminated GABA (GABA+) and then a reward learning task before and after acute stress. We assessed changes in reward learning after stress and associations with GABA+.

RESULTS

Results revealed blunted baseline reward learning in participants with remitted MDD versus participants with current MDD and HCs but, surprisingly, no differences between participants with current MDD and HCs. Reward learning was reduced following acute stress regardless of depressive history. GABA+ in the rostral anterior cingulate cortex, but not the dorsolateral prefrontal cortex, was associated with reduced baseline reward learning only in female participants. GABA+ did not predict stress-related changes in reward learning.

CONCLUSIONS

To our knowledge, this is the first study to investigate associations among GABA, reward learning, and stress reactivity in current versus past depression. Hypothesized depression-related differences in reward learning did not emerge, precluding claims about state versus trait vulnerabilities. However, our finding that blunted GABA was associated with greater reward learning in female participants provides novel insights into sex-selective associations between the frontal GABAergic inhibitory system and reward processing.

The Granular Retrosplenial Cortex Is Necessary in Male Rats for Object-Location Associative Learning and Memory, But Not Spatial Working Memory or Visual Discrimination and Reversal, in the Touchscreen Operant Chamber

The retrosplenial cortex (RSC) is a hub of diverse afferent and efferent projections thought to be involved in associative learning. RSC shows early pathology in mild cognitive impairment and Alzheimer's disease (AD), which impairs associative learning. To understand and develop therapies for diseases such as AD, animal models are essential. Given the importance of human RSC in object-location associative learning and the success of object-location associative paradigms in human studies and in the clinic, it would be of considerable value to establish a translational model of object-location learning for the rodent. For this reason, we sought to test the role of RSC in object-location learning in male rats using the object-location paired-associates learning (PAL) touchscreen task. First, increased cFos immunoreactivity was observed in granular RSC following PAL training when compared with extended pretraining controls. Following this, RSC lesions following PAL acquisition were used to explore the necessity of the RSC in object-location associative learning and memory and two tasks involving only one modality: trial-unique nonmatching-to-location for spatial working memory and pairwise visual discrimination/reversal. RSC lesions impaired both memory for learned paired-associates and learning of new object-location associations but did not affect performance in either the spatial or visual single-modality tasks. These findings provide evidence that RSC is necessary for object-location learning and less so for learning and memory involving the individual modalities therein.

Neural sensitivity following stress predicts anhedonia symptoms: a 2-year multi-wave, longitudinal study

Animal models of depression show that acute stress negatively impacts functioning in neural regions sensitive to reward and punishment, often manifesting as anhedonic behaviors. However, few human studies have probed stress-induced neural activation changes in relation to anhedonia, which is critical for clarifying risk for affective disorders. Participants (N = 85, 12-14 years-old, 53 female), oversampled for risk of depression, were administered clinical assessments and completed an fMRI guessing task during a baseline (no-stress) period to probe neural response to receipt of rewards and losses. After the initial task run of the fMRI guessing task, participants received an acute stressor and then, were re-administered the guessing task. Including baseline, participants provided up to 10 self-report assessments of life stress and symptoms over a 2 year period. Linear mixed-effects models estimated whether change in neural activation (post- vs. pre-acute stressor) moderated the longitudinal associations between life stress and symptoms. Primary analyses indicated that adolescents with stress-related reductions in right ventral striatum response to rewards exhibited stronger longitudinal associations between life stress and anhedonia severity (β = -0.06, 95%CI[-0.11, -0.02], p = 0.008, pFDR = 0.048). Secondary analyses showed that longitudinal positive associations between life stress and depression severity were moderated by stress-related increases in dorsal striatum response to rewards (left caudate β = 0.11, 95%CI[0.07,0.17], p < 0.001, pFDR = 0.002; right caudate β = 0.07, 95%CI[0.02,0.12], p = 0.002, pFDR = 0.003; left putamen β = 0.09, 95%CI[0.04, 0.14], p < 0.001, pFDR = 0.002; right putamen β = 0.08, 95%CI[0.03, 0.12], p < 0.001, pFDR = 0.002). Additionally, longitudinal positive associations among life stress and anxiety severity were moderated by stress-related reductions in dorsal anterior cingulate cortex (β = -0.07, 95%CI[-0.12,.02], p = 0.008, pFDR = 0.012) and right anterior insula (β = -0.07, 95%CI[-0.12,-0.02], p = 0.002, pFDR = 0.006) response to loss. All results held when adjusting for comorbid symptoms. Results show convergence with animal models, highlighting mechanisms that may facilitate stress-induced anhedonia as well as a separable pathway for the emergence of depressive and anxiety symptoms.

Stress to inflammation and anhedonia: Mechanistic insights from preclinical and clinical models

Anhedonia, as evidenced by impaired pleasurable response to reward, reduced reward motivation, and/or deficits in reward-related learning, is a common feature of depression. Such deficits in reward processing are also an important clinical target as a risk factor for depression onset. Unfortunately, reward-related deficits remain difficult to treat. To address this gap and inform the development of effective prevention and treatment strategies, it is critical to understand the mechanisms that drive impairments in reward function. Stress-induced inflammation is a plausible mechanism of reward deficits. The purpose of this paper is to review evidence for two components of this psychobiological pathway: 1) the effects of stress on reward function; and 2) the effects of inflammation on reward function. Within these two areas, we draw upon preclinical and clinical models, distinguish between acute and chronic effects of stress and inflammation, and address specific domains of reward dysregulation. By addressing these contextual factors, the review reveals a nuanced literature which might be targeted for additional scientific inquiry to inform the development of precise interventions.

Neural Sensitivity following Stress Predicts Anhedonia Symptoms: A 2-Year Multi-wave, Longitudinal Study

Animal models of depression show that acute stress negatively impacts functioning in neural regions sensitive to reward and punishment, often manifesting as anhedonic behaviors. However, few human studies have probed stress-induced neural activation changes in relation to anhedonia, which is critical for clarifying risk for affective disorders. Participants (N = 85 , 12-14-years-old, 53 female), oversampled for risk of depression, were administered clinical assessments and completed an fMRI guessing task to probe neural response to receipt of rewards and losses. After the initial task run, participants received an acute stressor and then, were re-administered the guessing task. Including baseline, participants provided up to 10 self-report assessments of life stress and symptoms over a 2-year period. Linear mixed-effects models estimated whether change in neural activation (post- vs. pre-acute stressor) moderated the longitudinal associations between life stress and symptoms over time. Primary analyses indicated that adolescents with stress-related reductions in right ventral striatum response to rewards exhibited stronger longitudinal associations between life stress and anhedonia severity p F D R = . 048 . Secondary analyses showed that longitudinal associations among life stress and depression severity were moderated by stress-related increases in dorsal striatum response to rewards p F D R < . 002 . Additionally, longitudinal associations among life stress and anxiety severity were moderated by stress-related reductions in dorsal anterior cingulate cortex and right anterior insula response to loss p F D R ≤ . 012 . All results held when adjusting for comorbid symptoms. Results show convergence with animal models, highlighting mechanisms that may facilitate stress-induced anhedonia as well as a separable pathway for the emergence of depressive and anxiety symptoms.

Acute stress alters probabilistic reversal learning in healthy male adults

Behavioural adaptation is a fundamental cognitive ability, ensuring survival by allowing for flexible adjustment to changing environments. In laboratory settings, behavioural adaptation can be measured with reversal learning paradigms requiring agents to adjust reward learning to stimulus-action-outcome contingency changes. Stress is found to alter flexibility of reward learning, but effect directionality is mixed across studies. Here, we used model-based functional MRI (fMRI) in a within-subjects design to investigate the effect of acute psychosocial stress on flexible behavioural adaptation. Healthy male volunteers (n = 28) did a reversal learning task during fMRI in two sessions, once after the Trier Social Stress Test (TSST), a validated psychosocial stress induction method, and once after a control condition. Stress effects on choice behaviour were investigated using multilevel generalized linear models and computational models describing different learning processes that potentially generated the data. Computational models were fitted using a hierarchical Bayesian approach, and model-derived reward prediction errors (RPE) were used as fMRI regressors. We found that acute psychosocial stress slightly increased correct response rates. Model comparison revealed that double-update learning with altered choice temperature under stress best explained the observed behaviour. In the brain, model-derived RPEs were correlated with BOLD signals in striatum and ventromedial prefrontal cortex (vmPFC). Striatal RPE signals for win trials were stronger during stress compared with the control condition. Our study suggests that acute psychosocial stress could enhance reversal learning and RPE brain responses in healthy male participants and provides a starting point to explore these effects further in a more diverse population.

Mesolimbic dopamine release conveys causal associations

Learning to predict rewards based on environmental cues is essential for survival. It is believed that animals learn to predict rewards by updating predictions whenever the outcome deviates from expectations, and that such reward prediction errors (RPEs) are signaled by the mesolimbic dopamine system-a key controller of learning. However, instead of learning prospective predictions from RPEs, animals can infer predictions by learning the retrospective cause of rewards. Hence, whether mesolimbic dopamine instead conveys a causal associative signal that sometimes resembles RPE remains unknown. We developed an algorithm for retrospective causal learning and found that mesolimbic dopamine release conveys causal associations but not RPE, thereby challenging the dominant theory of reward learning. Our results reshape the conceptual and biological framework for associative learning.

Stressed and wired: The effects of stress on the VTA circuits underlying motivated behavior

Stress affects many brain regions, including the ventral tegmental area (VTA), which is critically involved in reward processing. Excessive stress can reduce reward-seeking behaviors but also exacerbate substance use disorders, two seemingly contradictory outcomes. Recent research has revealed that the VTA is a heterogenous structure with diverse populations of efferents and afferents serving different functions. Stress has correspondingly diverse effects on VTA neuron activity, tending to decrease lateral VTA dopamine (DA) neuron activity, while increasing medial VTA DA and GABA neuron activity. Here we review the differential effects of stress on the activity of these distinct VTA neuron populations and how they contribute to decreases in reward-seeking behavior or increases in drug self-administration.

Critical periods when dopamine controls behavioral responding during Pavlovian learning

RATIONALE

Learning the association between rewards and predictive cues is critical for appetitive behavioral responding. The mesolimbic dopamine system is thought to play an integral role in establishing these cue-reward associations. The dopamine response to cues can signal differences in reward value, though this emerges only after significant training. This suggests that the dopamine system may differentially regulate behavioral responding depending on the phase of training.

OBJECTIVES

The purpose of this study was to determine whether antagonizing dopamine receptors elicited different effects on behavior depending on the phase of training or the type of Pavlovian task.

METHODS

Separate groups of male rats were trained on Pavlovian tasks in which distinct audio cues signaled either differences in reward size or differences in reward rate. The dopamine receptor antagonist flupenthixol was systemically administered prior to either the first ten sessions of training (acquisition phase) or the second ten sessions of training (expression phase), and we monitored the effect of these manipulations for an additional ten training sessions.

RESULTS

We identified acute effects of dopamine receptor antagonism on conditioned responding, the latency to respond, and post-reward head entries in both Pavlovian tasks. Interestingly, dopamine receptor antagonism during the expression phase produced persistent deficits in behavioral responding only in rats trained on the reward size Pavlovian task.

CONCLUSIONS

Together, our results illustrate that dopamine's control over behavior in Pavlovian tasks depends upon one's prior training experience and the information signaled by the cues.

Sex differences in cognitive flexibility are driven by the estrous cycle and stress-dependent

Stress is associated with psychiatric disorders such as post-traumatic stress disorder, major depressive disorder, anxiety disorders, and panic disorders. Women are more likely to be diagnosed with these stress-related psychiatric disorders than men. A key phenotype in stress-related psychiatric disorders is impairment in cognitive flexibility, which is the ability to develop new strategies to respond to different patterns in the environment. Because gonadal hormones can contribute to sex differences in response to stress, it is important to consider where females are in their cycle when exposed to stress and cognitive flexibility testing. Moreover, identifying neural correlates involved in cognitive flexibility could not only build our understanding of the biological mechanisms behind this crucial skill but also leads to more targeted treatments for psychiatric disorders. Although previous studies have separately examined sex differences in cognitive flexibility, stress effects on cognitive flexibility, and the effect of gonadal hormones on cognitive flexibility, many of the findings were inconsistent, and the role of the estrous cycle in stress-induced impacts on cognitive flexibility is still unknown. This study explored potential sex differences in cognitive flexibility using an operant strategy shifting-paradigm after either control conditions or restraint stress in freely cycling female and male rats (with estrous cycle tracking in the female rats). In addition, we examined potential neural correlates for any sex differences observed. In short, we found that stress impaired certain aspects of cognitive flexibility and that there were sex differences in cognitive flexibility that were driven by the estrous cycle. Specifically, stress increased latency to first press and trials to criterion in particular tasks. The female rats demonstrated more omissions and perseverative errors than the male rats; the sex differences were mostly driven by proestrus female rats. Interestingly, the number of orexinergic neurons was higher in proestrus female rats than in the male rats under control conditions. Moreover, orexin neural count was positively correlated with number of perseverative errors made in cognitive flexibility testing. In sum, there are sex differences in cognitive flexibility that are driven by the estrous cycle and are stress-dependent, and orexin neurons may underlie some of the sex differences observed.

Plasma concentration of dopamine varies depending on breed, sex, and the genotype of DRD4 in horses

Dopamine (DA) is known to be a key modulator of animal behaviors. Thus, the plasma concentration of DA might be used as a biomarker for the behavioral characteristics of horses. The behavioral characteristics of horses vary depending on the breed, age, and sex. Moreover, the DA receptor genotypes are also related to horse behaviors. Thus, the aim of this study was to investigate the DA concentration variations of horse plasma by breed, age, sex, or genotype of its receptor. The horses were divided by breed into Thoroughbred (n = 13), Pony (n = 9), Warmblood (n = 4), and Haflinger (n = 5). The age variable was divided into three different groups: post-pubertal (2–5 years, n = 6), adult (6–13 years, n = 19), and aged horses (15–24 years, n = 6). The sex variable was divided into geldings (n = 8) and mares (n = 23). Approximately 10 mL of blood was collected, and an ELISA kit was used to measure the plasma concentration of DA. Polymerase chain reaction analysis was performed to identify the genetic variation in the DA D4 receptor gene (DRD4). SPSS statistical software was used for statistical analysis. The DA concentrations in geldings were significantly lower than those in mares. There was no significant difference in DA concentrations among breed and age groups. Horses with the GG and GA genotypes had significantly higher plasma concentrations of DA compared to horses with the AA genotype for the G292A gene. Briefly, the plasma concentration of DA varied depending on the sex and genotype of G292A. These factors should be considered when the concentration of DA is used as a biomarker for the behavioral characteristics of horses. In conclusion, the DA concentration or DRD4 genotype of horse plasma has the potential to be used as a biomarker that can predict the behavioral characteristics of horses.

Asymmetric effects of acute stress on cost and benefit learning

BACKGROUND

Humans are continuously exposed to stressful challenges in everyday life. Such stressful events trigger a complex physiological reaction - the fight-or-flight response - that can hamper flexible decision-making and learning. Inspired by key neural and peripheral characteristics of the fight-or-flight response, here, we ask whether acute stress changes how humans learn about costs and benefits.

METHODS

Healthy adults were randomly exposed to an acute stress (age mean=23.48, 21/40 female) or no-stress control (age mean=23.80, 22/40 female) condition, after which they completed a reinforcement learning task in which they minimize cost (physical effort) and maximize benefits (monetary rewards). During the task pupillometry data were collected. A computational model of cost-benefit reinforcement learning was employed to investigate the effect of acute stress on cost and benefit learning and decision-making.

RESULTS

Acute stress improved learning to maximize rewards relative to minimizing physical effort (Condition-by-Trial Type interaction: F(1,78)= 6.53, p = 0.01, n²_G= 0.04; reward > effort in stress condition: t(39) = 5.40, p < 0.01). Computational modelling revealed that asymmetric learning could be explained by changes in the learning rates of reward value and action cost [condition-by-learning rate (αR, αE) interaction: F(1,78)= 6.42, p = 0.01, n²_G= 0.03; αE> αR in control condition: t(39) = -4.75, p < 0.001]. This process was associated with distinct alterations in pupil size fluctuations. Data and scripts are available (https://osf.io/ydv2q/).

CONCLUSIONS

Here we demonstrate that acute stress is associated with asymmetric learning about reward value versus action cost, thereby providing new insights into learning strategies under acute stress, which, depending on the context, may be maladaptive or beneficial. Our pupillometry and physiological results tentatively link asymmetric cost and benefit learning to stress-related changes in catecholamine activity.

Sex Differences in Behavioral Responding and Dopamine Release during Pavlovian Learning

Learning associations between cues and rewards require the mesolimbic dopamine system. The dopamine response to cues signals differences in reward value in well trained animals. However, these value-related dopamine responses are absent during early training sessions when cues signal differences in the reward rate. These findings suggest cue-evoked dopamine release conveys differences between outcomes only after extensive training, though it is unclear whether this is unique to when cues signal differences in reward rate, or whether this is also evident when cues signal differences in other value-related parameters such as reward size. To address this, we used a Pavlovian conditioning task in which one audio cue was associated with a small reward (one pellet) and another audio cue was associated with a large reward (three pellets). We performed fast-scan cyclic voltammetry to record changes in dopamine release in the nucleus accumbens of male and female rats throughout learning. While female rats exhibited higher levels of conditioned responding, a faster latency to respond, and elevated post-reward head entries relative to male rats, there were no sex differences in the dopamine response to cues. Multiple training sessions were required before cue-evoked dopamine release signaled differences in reward size. Reward-evoked dopamine release scaled with reward size, though females displayed lower reward-evoked dopamine responses relative to males. Conditioned responding related to the decrease in the peak reward-evoked dopamine response and not to cue-evoked dopamine release. Collectively, these data illustrate sex differences in behavioral responding as well as in reward-evoked dopamine release during Pavlovian learning.

Optogenetic stimulation of dynorphinergic neurons within the dorsal raphe activate kappa opioid receptors in the ventral tegmental area and ablation of dorsal raphe prodynorphin or kappa receptors in dopamine neurons blocks stress potentiation of cocaine reward

Behavioral stress exposure increases the risk of drug-taking in individuals with substance use disorders by mechanisms involving the dynorphins, which are the endogenous neuropeptides for the kappa opioid receptor (KOR). KOR agonists have been shown to encode dysphoria, aversion, and changes in reward valuation, and kappa opioid antagonists are in clinical development for treating substance use disorders. In this study, we confirmed that KORs were expressed in dopaminergic neurons in the ventral tegmental area (VTA) of male C57BL6/J mice. Genetic ablation of KORs from dopamine neurons blocked the potentiating effects of repeated forced swim stress on cocaine conditioned place preference (CPP). KOR activation inhibited dopamine neuron GCaMP6m calcium activity in VTA during swim stress and caused a rebound enhancement during the period after stress exposure. Transient optogenetic inhibition of VTA dopamine neurons with AAV5-DIO-SwiChR was acutely aversive in a real time place preference assay and blunted cocaine CPP when inhibition was administered concurrently with cocaine conditioning. However, when inhibition preceded cocaine conditioning by 30 min, cocaine CPP was enhanced. Retrograde tracing with CAV2-DIO-ZsGreen identified a population of prodynorphin^Cre neurons in the dorsal raphe nucleus (DRN) projecting to the VTA. Optogenetic stimulation of dynorphinergic neurons within the DRN by Channelrhodopsin2 activated KOR in VTA and ablation of prodynorphin blocked stress potentiation of cocaine CPP. Together, these studies demonstrate the presence of a dynorphin/KOR midbrain circuit that projects from the DRN to VTA and is involved in altering the dynamic response of dopamine neuron activity to enhance drug reward learning.

TIP60 buffers acute stress response and depressive behaviour by controlling PPARγ-mediated transcription

Tat-interacting protein 60 (TIP60) as nuclear receptors (NRs) coregulator, acts as a tumor suppressor and also has promising therapeutic potential to target Alzheimer's disease. Stress has been implicated in many psychiatric disorders, and these disorders are characterized by impairments in cognitive function. Until now, there are no experimental data available on the regulatory effect of TIP60 in acute stress and depression. There is also no definitive explanation on which specific modulation of target gene expression is achieved by TIP60. Here, we identify TIP60 as a novel positive regulator in response to acute restraint stress (ARS) and a potentially effective target of antidepressants. Firstly, we discovered increased hippocampal TIP60 expressions in the ARS model. Furthermore, using the TIP60 inhibitor, MG149, we proved that TIP60 function correlates with behavioral and synaptic activation in the two-hour ARS. Secondly, the lentivirus vector (LV)-TIP60^{overexpression (OE)} was injected into the hippocampus prior to the chronic restraint stress (CRS) experiments and it was found that over-expressed TIP60 compensates for TIP60 decrease and improves depression index in CRS. Thirdly, through the intervention of TIP60 expression in vitro, we established the genetic regulation of TIP60 on synaptic proteins, confirmed the TIP60 function as a specific coactivator for PPARγ and found that the PPARγ-mediated TIP60 function modulates transcriptional activation of synaptic proteins. Finally, the LV-TIP60^OE and PPARγ antagonist, GW9662, were both administered in the CRS model and the data indicated that blocking PPARγ significantly weakened the protective effect of TIP60 against the CRS-induced depression. Conclusively, these findings together support TIP60 as a novel positive factor in response to acute stress and interacts with PPARγ to modulate the pathological mechanism of CRS-induced depression.

Two-hour acute restraint stress facilitates escape behavior and learning outcomes through the activation of the Cdk5/GR P S211 pathway in male mice

Acute stress exerts pleiotropic actions on learning behaviors. The induced negative effects are sometimes adopted to measure the efficacy of particular drugs. Until now, there are no detailed experimental data on the time-gradient effects of acute stress. Here, we developed the time gradient acute restraint stress (ARS) model to precisely assess the roles of different restrain times on inducing acute stress. Time gradient ARS facilitates escape behaviors and learning outcomes, peaking at 2 h-ARS and then declining to baseline at 3.5 h-ARS as confirmed by time gradient post-stress data. Furthermore, time gradient ARS activates glucocorticoid receptor (GR) phosphorylation site at Serine211 (P S221) as an inverted V-shaped pattern peaking at 2 h-ARS, whereas that of the GR phosphorylation site at Serine226 (P S226) from 2 h-ARS to 3.5 h-ARS. The 2 h-ARS but not 3.5 h-ARS enhances synaptic plasticity and genes transcription associated with learning and memory in the hippocampus of male mice. The Cdk5 inhibitor, roscovitine, blocks this facilitation effect by intervening in GR phosphorylation at Serine211 in the 2 h-ARS mice. Altogether, these findings show that the time gradient ARS selectively activates GR phospho-isoforms and differentially influences the behaviors along with maintaining a relationship between 2 h-ARS and Cdk5/GR P S211-mediated transcriptional activity.

Dopamine release and its control over early Pavlovian learning differs between the NAc core and medial NAc shell

Dopamine neurons respond to cues to reflect the value of associated outcomes. These cue-evoked dopamine responses can encode the relative rate of reward in rats with extensive Pavlovian training. Specifically, a cue that always follows the previous reward by a short delay (high reward rate) evokes a larger dopamine response in the nucleus accumbens (NAc) core relative to a distinct cue that always follows the prior reward by a long delay (low reward rate). However, it was unclear if these reward rate dopamine signals are evident during early Pavlovian training sessions and across NAc subregions. To address this, we performed fast-scan cyclic voltammetry recordings of dopamine levels to track the pattern of cue- and reward-evoked dopamine signals in the NAc core and medial NAc shell. We identified regional differences in the progression of cue-evoked dopamine signals across training. However, the dopamine response to cues did not reflect the reward rate in either the NAc core or the medial NAc shell during early training sessions. Pharmacological experiments found that dopamine-sensitive conditioned responding emerged in the NAc core before the medial NAc shell. Together, these findings illustrate regional differences in NAc dopamine release and its control over behavior during early Pavlovian learning.

Stress and the dopaminergic reward system

Dopamine regulates reward-related behavior through the mesolimbic dopaminergic pathway. Stress affects dopamine levels and dopaminergic neuronal activity in the mesolimbic dopamine system. Changes in mesolimbic dopaminergic neurotransmission are important for coping with stress, as they allow adaption to behavioral responses to various environmental stimuli. Upon stress exposure, modulation of the dopaminergic reward system is necessary for monitoring and selecting the optimal process for coping with stressful situations. Aversive stressful events may negatively regulate the dopaminergic reward system, perturbing reward sensitivity, which is closely associated with chronic stress-induced depression. The mesolimbic dopamine system is excited not only by reward but also by aversive stressful stimuli, which adds further intriguing complexity to the relationship between stress and the reward system. This review focuses on lines of evidence related to how stress, especially chronic stress, affects the mesolimbic dopamine system, and discusses the role of the dopaminergic reward system in chronic stress-induced depression.