Dopamine transporter function fluctuates across sleep/wake state: potential impact for addiction

Ascorbate insufficiency disrupts glutamatergic signaling and alters electroencephalogram phenotypes in a mouse model of Alzheimer's disease

Clinical studies have reported that increased epileptiform and subclinical epileptiform activity can be detected in many patients with an Alzheimer's disease (AD) diagnosis using electroencephalogram (EEG) and this may correlate with poorer cognition. Ascorbate may have a specific role as a neuromodulator in AD as it is released concomitantly with glutamate reuptake following excitatory neurotransmission. Insufficiency may therefore result in an exacerbated excitatory/inhibitory imbalance in neuronal signaling. Using a mouse model of AD that requires dietary ascorbate (Gulo-/-APPswe/PSEN1dE9), EEG was recorded at baseline and during 4 weeks of ascorbate depletion in young (5-month-old) and aged (20-month-old) animals. Data were scored for changes in quantity of spike trains, individual spikes, sleep-wake rhythms, sleep fragmentation, and brainwave power bands during light periods each week. We found an early increase in neuronal spike discharges with age and following ascorbate depletion in AD model mice and not controls, which did not correlate with brain amyloid load. Our data also show more sleep fragmentation with age and with ascorbate depletion. Additionally, changes in brain wave activity were observed within different vigilance states in both young and aged mice, where Gulo-/-APPswe/PSEN1dE9 mice had shifts towards higher frequency bands (alpha, beta, and gamma) and ascorbate depletion resulted in shifts towards lower frequency bands (delta and theta). Microarray data supported ascorbate insufficiency altering glutamatergic transmission through the decreased expression of glutamate related genes, however no changes in protein expression of glutamate reuptake transporters were observed. These data suggest that maintaining optimal brain ascorbate levels may support normal brain electrical activity and sleep patterns, particularly in AD patient populations where disruptions are observed.

Striatal projection neurons coexpressing dopamine D1 and D2 receptors modulate the motor function of D1- and D2-SPNs

The role of the striatum in motor control is commonly assumed to be mediated by the two striatal efferent pathways characterized by striatal projection neurons (SPNs) expressing dopamine (DA) D1 receptors or D2 receptors (D1-SPNs and D2-SPNs, respectively), without regard to SPNs coexpressing both receptors (D1/D2-SPNs). Here we developed an approach to target these hybrid SPNs in mice and demonstrate that, although these SPNs are less abundant, they have a major role in guiding the motor function of the other two populations. D1/D2-SPNs project exclusively to the external globus pallidus and have specific electrophysiological features with distinctive integration of DA signals. Gain- and loss-of-function experiments indicate that D1/D2-SPNs potentiate the prokinetic and antikinetic functions of D1-SPNs and D2-SPNs, respectively, and restrain the integrated motor response to psychostimulants. Overall, our findings demonstrate the essential role of this population of D1/D2-coexpressing neurons in orchestrating the fine-tuning of DA regulation in thalamo-cortico-striatal loops.

Hidden variables in stress neurobiology research

Among the central goals of stress neurobiology research is to understand the mechanisms by which stressors change neural circuit function to precipitate or exacerbate psychiatric symptoms. Yet despite decades of effort, psychiatric medications that target the biological substrates of the stress response are largely lacking. We propose that the clinical advancement of stress response-based therapeutics for psychiatric disorders may be hindered by 'hidden variables' in stress research, including considerations of behavioral study design (stressors and outcome measures), individual variability, sex differences, and the interaction of the body's stress hormone system with endogenous circadian and ultradian rhythms. We highlight key issues and suggest ways forward in stress neurobiology research that may improve the ability to assess stress mechanisms and translate preclinical findings.

Epworth sleepiness scale is associated with increased striatal dopamine uptake in Parkinson's disease: a cross-sectional study

PURPOSE

A cross-sectional study was designed to investigate the possible association between Epworth sleepiness scale (ESS) scores and striatal dopamine uptake in the early stages of Parkinson's disease (PD).

METHODS

Two groups of PD patients (n = 464) and healthy controls (HC) (n = 162) were enrolled in the current study from the Parkinson's progression markers initiative cohort ( https://www.ppmi-info.org ). All the subjects were evaluated for excessive daytime sleepiness (EDS) using the ESS. They also completed specific measures to be assessed for motor and non-motor symptoms, including cognitive, gait, autonomic, and olfactory dysfunction. Dopamine transporter (DaT) scans were used to identify dopamine transporter impairments. Spatial normalization for DaTscan imaging of participants was performed to reach consistent orientation.

RESULTS

A significant correlation was found between ESS score and right putamen (P < 0.001; correlation coefficient = 0.186) and left putamen (P = 0.003; correlation coefficient = 0.139) dopamine uptake in PD patients. The same results were revealed after adjusted Pearson's correlation for the effects of handedness, age, gender, and education. No significant correlation was found between the ESS score and the amount of caudate nucleus dopamine uptake in PD patients. Moreover, there was a significant association between caudate nucleus dopamine uptake and ESS score in neither PD patients nor HCs.

CONCLUSIONS

The results of our study suggest that increased dopaminergic function of putamen nuclei may be associated with ESS scores in the early stages of PD. Further studies are needed at different PD stages and evaluate PD progression as a possible confounder.

Evaluation and Validation of Commercially Available Dopamine Transporter Antibodies

With a wide variety of dopamine transporter (DAT) antibodies available commercially, it is important to validate which antibodies provide sufficient immunodetection for reproducibility purpose and for accurate analysis of DAT levels and/or location. Commercially available DAT antibodies that are commonly used were tested in western blotting (WB) on wild-type (WT) and DAT-knock-out (DAT-KO) brain tissue and with immunohistology (IH) techniques against coronal slices of unilaterally lesioned 6-OHDA rats, in addition to wild-type and DAT-knock-out mice. DAT-KO mice and unilateral 6-OHDA lesions in rats were used as a negative control for DAT antibody specificity. Antibodies were tested at various concentrations and rated based on signal detection varying from no signal to optimal signal detection. Commonly used antibodies, including AB2231 and PT-22 524-1-AP, did not provide specific DAT signals in WB and IH. Although certain antibodies provided a good DAT signal, such as SC-32258, D6944, and MA5-24796, they also presented nonspecific bands in WB. Many DAT antibodies did not detect the DAT as advertised, and this characterization of DAT antibodies may provide a guide for immunodetection of DAT for molecular studies.

Chronically dysregulated corticosterone impairs dopaminergic transmission in the dorsomedial striatum by sex-divergent mechanisms

Major depressive disorder (MDD) is a leading cause of disability worldwide. Individuals with MDD exhibit decreased motivation and deficits in reward processing. In a subset of MDD patients, chronic dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis occurs, resulting in increased levels of the 'stress hormone' cortisol during the normal rest period (i.e., evening and night). However, the mechanistic relationship between chronically elevated resting cortisol and behavioral deficits in motivation and reward processing remains unclear. Given that women are diagnosed with MDD at twice the rate of men, it is important to understand whether the mechanisms linking cortisol to the symptoms of MDD differ by sex. In this study, we used subcutaneous implants to chronically elevate free plasma corticosterone (the rodent homolog of cortisol; 'CORT') during the rest period in male and female mice and examined changes in behavior and dopamine system function. We found that chronic CORT treatment impaired motivated reward-seeking in both sexes. In female but not male mice, CORT treatment reduced dopamine content in the dorsomedial striatum (DMS). In male but not female mice, CORT treatment impaired the function of the dopamine transporter (DAT) in DMS. From these studies, we conclude that chronic CORT dysregulation impairs motivation by impairing dopaminergic transmission in the DMS, but via different mechanisms in male and female mice. A better understanding of these sex-specific mechanisms could lead to new directions in MDD diagnosis and treatment.

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

Different sensitivity to anesthesia according to ECoG data in dopamine transporter knockout and heterozygous rats

Dopamine in the brain is involved in many important functions, including the regulation of wakefulness. There is also some evidence suggesting that the dopamine function is crucial in anesthetic function. The state of anesthesia is characterized by a change in the level of consciousness and a change in brain electrical activity. Due to impaired mechanisms of dopamine transportation back to the synaptic terminal, dopamine transporter (DAT) knockout and heterozygous rats have increased levels of the extracellular dopamine. In our work, we registered ECoG disturbances in knockout and heterozygous rats, as well as disturbances in tone and activity in acute experiments under the anesthesia Zoletil (tiletamine and zolazepam) from the somatosensory cortex using a NeuroNexus flat multielectrode array to study gamma activity. We also used four low-resistance electrodes to control the slow rhythm. Both low-resistance and high-resistance electrodes showed differences in the ECoG spectrum of heterozygotes and total knockouts from the wild type and from each other. Heterozygous rats for the DAT gene (HET) showed increased rapid beta and gamma activity and decreased slow delta activity, while complete knockouts (KO), on the contrary, showed increased delta activity and decreased beta and gamma activity. Thus, the ECoG spectrum of HET is shifted to the right, while that of KO is shifted to the left. Full knockouts also showed decreased spatial synchronization in the 30-100 Hz gamma range compared to the wild type (WT). It is assumed that sedation of HET and KO is shifted towards opposite directions compared to WT under the same anesthesia conditions.

Changes in striatal dopamine release, sleep, and behavior during spontaneous Δ-9-tetrahydrocannabinol abstinence in male and female mice

Withdrawal symptoms are observed upon cessation of cannabis use in humans. Although animal studies have examined withdrawal symptoms following exposure to delta-9-tetrahydrocannabinol (THC), difficulties in obtaining objective measures of spontaneous withdrawal using paradigms that mimic cessation of use in humans have slowed research. The neuromodulator dopamine (DA) is affected by chronic THC treatment and plays a role in many behaviors related to human THC withdrawal symptoms. These symptoms include sleep disturbances that often drive relapse, and emotional behaviors like irritability and anhedonia. We examined THC withdrawal-induced changes in striatal DA release and the extent to which sleep disruption and behavioral maladaptation manifest during abstinence in a mouse model of chronic THC exposure. Using a THC treatment regimen known to produce tolerance, we measured electrically elicited DA release in acute brain slices from different striatal subregions during early and late THC abstinence. Long-term polysomnographic recordings from mice were used to assess vigilance state and sleep architecture before, during, and after THC treatment. We additionally assessed how behaviors that model human withdrawal symptoms are altered by chronic THC treatment in early and late abstinence. We detected altered striatal DA release, sleep disturbances that mimic clinical observations, and behavioral maladaptation in mice following tolerance to THC. Altered striatal DA release, sleep, and affect-related behaviors associated with spontaneous THC abstinence were more consistently observed in male mice. These findings provide a foundation for preclinical study of directly translatable non-precipitated THC withdrawal symptoms and the neural mechanisms that affect them.

Historical and Modern Evidence for the Role of Reward Circuitry in Emergence

Increasing evidence supports a role for brain reward circuitry in modulating arousal along with emergence from anesthesia. Emergence remains an important frontier for investigation, since no drug exists in clinical practice to initiate rapid and smooth emergence. This review discusses clinical and preclinical evidence indicating a role for two brain regions classically considered integral components of the mesolimbic brain reward circuitry, the ventral tegmental area and the nucleus accumbens, in emergence from propofol and volatile anesthesia. Then there is a description of modern systems neuroscience approaches to neural circuit investigations that will help span the large gap between preclinical and clinical investigation with the shared aim of developing therapies to promote rapid emergence without agitation or delirium. This article proposes that neuroscientists include models of whole-brain network activity in future studies to inform the translational value of preclinical investigations and foster productive dialogues with clinician anesthesiologists.

The Ventral Tegmental Area and Nucleus Accumbens as Circadian Oscillators: Implications for Drug Abuse and Substance Use Disorders

Circadian rhythms convergently evolved to allow for optimal synchronization of individuals’ physiological and behavioral processes with the Earth’s 24-h periodic cycling of environmental light and temperature. Whereas the suprachiasmatic nucleus (SCN) is considered the primary pacemaker of the mammalian circadian system, many extra-SCN oscillatory brain regions have been identified to not only exhibit sustainable rhythms in circadian molecular clock function, but also rhythms in overall region activity/function and mediated behaviors. In this review, we present the most recent evidence for the ventral tegmental area (VTA) and nucleus accumbens (NAc) to serve as extra-SCN oscillators and highlight studies that illustrate the functional significance of the VTA’s and NAc’s inherent circadian properties as they relate to reward-processing, drug abuse, and vulnerability to develop substance use disorders (SUDs).

The Mind After Midnight: Nocturnal Wakefulness, Behavioral Dysregulation, and Psychopathology

Sufficient sleep with minimal interruption during the circadian/biological night supports daytime cognition and emotional regulation. Conversely, disrupted sleep involving significant nocturnal wakefulness leads to cognitive and behavioral dysregulation. Most studies to-date have examined how fragmented or insufficient sleep affects next-day functioning, but recent work highlights changes in cognition and behavior that occur when someone is awake during the night. This review summarizes the evidence for day-night alterations in maladaptive behaviors, including suicide, violent crime, and substance use, and examines how mood, reward processing, and executive function differ during nocturnal wakefulness. Based on this evidence, we propose the Mind after Midnight hypothesis in which attentional biases, negative affect, altered reward processing, and prefrontal disinhibition interact to promote behavioral dysregulation and psychiatric disorders.

Narcolepsy in Parkinson's disease with insulin resistance

Background: Parkinson’s disease (PD) is characterized by its progression of motor-related symptoms such as tremors, rigidity, slowness of movement, and difficulty with walking and balance. Comorbid conditions in PD individuals include insulin resistance (IR) and narcolepsy-like sleep patterns. The intersecting sleep symptoms of both conditions include excessive daytime sleepiness, hallucinations, insomnia, and falling into REM sleep more quickly than an average person. Understanding of the biological basis and relationship of these comorbid disorders with PD may help with early detection and intervention strategies to improve quality of life. Methods: In this study, an integrative genomics and systems biology approach was used to analyze gene expression patterns associated with PD, IR, and narcolepsy in order to identify genes and pathways that may shed light on how these disorders are interrelated. A correlation analysis with known genes associated with these disorders (LRRK2, HLA-DQB1, and HCRT) was used to query microarray data corresponding to brain regions known to be involved in PD and narcolepsy. This includes the hypothalamus, dorsal thalamus, pons, and subcoeruleus nucleus. Risk factor genes for PD, IR, and narcolepsy were also incorporated into the analysis. Results: The PD and narcolepsy signaling networks are connected through insulin and immune system pathways. Important genes and pathways that link PD, narcolepsy, and IR are CACNA1C, CAMK1D, BHLHE41, HMGB1, and AGE-RAGE. Conclusions: We have identified the genetic signatures that link PD with its comorbid disorders, narcolepsy and insulin resistance, from the convergence and intersection of dopaminergic, insulin, and immune system related signaling pathways. These findings may aid in the design of early intervention strategies and treatment regimes for non-motor symptoms in PD patients as well as individuals with diabetes and narcolepsy.

Ultrasensitive electrochemical biosensors for dopamine and cholesterol: recent advances, challenges and strategies

The rapid and accurate determination of the dopamine (neurotransmitter) and cholesterol level in bio-fluids is significant because they are crucial bioanalytes for several lethal diseases, which require early diagnosis. The level of DA in the brain is modulated by the dopamine active transporter (DAT), and is influenced by cholesterol levels in the lipid membrane environment. Accordingly, electrochemical biosensors offer rapid and accurate detection and exhibit unique features such as low detection limits even with reduced volumes of analyte, affordability, simple handling, portability and versatility, making them appropriate to deal with augmented challenges in current clinical and point-of-care diagnostics for the determination of dopamine (DA) and cholesterol. This feature article focuses on the development of ultrasensitive electrochemical biosensors for the detection of cholesterol and DA for real-time and onsite applications that can detect targeted analytes with reduced volumes and sub-picomolar concentrations with quick response times. Furthermore, the development of ultrasensitive biosensors via cost-effective, simple fabrication procedures, displaying high sensitivity, selectivity, reliability and good stability is significant in the impending era of electrochemical biosensing. Herein, we emphasize on recent advanced nanomaterials used for the ultrasensitive detection of DA and cholesterol and discuss in depth their electrochemical activities towards ultrasensitive responses. Key points describing future perspectives and the challenges during detection with their probable solutions are discussed, and the current market is also surveyed. Further, a comprehensive review of the literature indicates that there is room for improvement in the miniaturization of cholesterol and dopamine biosensors for lab-on-chip devices and overcoming the current technical limitations to facilitate full utilization by patients at home.

Individual differences in dopamine uptake in the dorsomedial striatum prior to cocaine exposure predict motivation for cocaine in male rats

A major theme of addiction research has focused on the neural substrates of individual differences in the risk for addiction; however, little is known about how vulnerable populations differ from those that are relatively protected. Here, we prospectively measured dopamine (DA) neurotransmission prior to cocaine exposure to predict the onset and course of cocaine use. Using in vivo voltammetry, we first generated baseline profiles of DA release and uptake in the dorsomedial striatum (DMS) and nucleus accumbens of drug-naïve male rats prior to exposing them to cocaine using conditioned place preference (CPP) or operant self-administration. We found that the innate rate of DA uptake in the DMS strongly predicted motivation for cocaine and drug-primed reinstatement, but not CPP, responding when "price" was low, or extinction. We then assessed the impact of baseline variations in DA uptake on cocaine potency in the DMS using ex vivo voltammetry in naïve rats and in rats with DA transporter (DAT) knockdown. DA uptake in the DMS of naïve rats predicted the neurochemical response to cocaine, such that rats with innately faster rates of DA uptake demonstrated higher cocaine potency at the DAT and rats with DAT knockdown displayed reduced potency compared to controls. Together, these data demonstrate that inherent variability in DA uptake in the DMS predicts the behavioral response to cocaine, potentially by altering the apparent potency of cocaine.

Altered Grooming Syntax and Amphetamine-Induced Dopamine Release in EAAT3 Overexpressing Mice

The excitatory amino acid transporter EAAT3 plays an important role in the neuronal uptake of glutamate regulating the activation of glutamate receptors. Polymorphisms in the gene-encoding EAAT3 have been associated with obsessive-compulsive disorder (OCD), although the mechanisms underlying this relationship are still unknown. We recently reported that mice with increased EAAT3 expression in forebrain neurons (EAAT3 ^{g lo} /CMKII) display behavioral and synaptic features relevant to OCD, including increased grooming, higher anxiety-like behavior and altered cortico-striatal synaptic function. The dopamine neurotransmitter system is implicated in ritualistic behaviors. Indeed, dopaminergic neurons express EAAT3, and mice lacking EAAT3 exhibit decreased dopamine release and decreased expression of the dopamine D1 receptor. Moreover, EAAT3 plays a role on the effect of the psychostimulant amphetamine. As such, we sought to determine if the OCD-like behavior in EAAT3 ^{g lo} /CMKII mice is accompanied by altered nigro-striatal dopaminergic transmission. The aim of this study was to analyze dopamine transmission both in basal conditions and after an acute challenge of amphetamine, using behavioral, neurochemical, molecular, and cellular approaches. We found that in basal conditions, EAAT3 ^{g lo} /CMKII mice performed more grooming events and that they remained in phase 1 of the grooming chain syntax compared with control littermates. Administration of amphetamine increased the number of grooming events in control mice, while EAAT3 ^{g lo} /CMKII mice remain unaffected. Interestingly, the grooming syntax of amphetamine-control mice resembled that of EAAT3 ^{g lo} /CMKII mice in basal conditions. Using in vivo microdialysis, we found decreased basal dopamine levels in EAAT3 ^{g lo} /CMKII compared with control mice. Unexpectedly, we found that after acute amphetamine, EAAT3 ^{g lo} /CMKII mice had a higher release of dopamine compared with that of control mice, suggesting that EAAT3 overexpression leads to increased dopamine releasability. To determine postsynaptic effect of EAAT3 overexpression over dopamine transmission, we performed Western blot analysis of dopaminergic proteins and found that EAAT3 ^{g lo} /CMKII mice have higher expression of D2 receptors, suggesting a higher inhibition of the indirect striatal pathway. Together, the data indicate that EAAT3 overexpression impacts on dopamine transmission, making dopamine neurons more sensitive to the effect of amphetamine and leading to a disbalance between the direct and indirect striatal pathways that favors the performance of repetitive behaviors.

Experimentally imposed circadian misalignment alters the neural response to monetary rewards and response inhibition in healthy adolescents

BACKGROUND

Sleep and circadian timing shifts later during adolescence, conflicting with early school start times, and resulting in circadian misalignment. Although circadian misalignment has been linked to depression, substance use, and altered reward function, a paucity of experimental studies precludes the determination of causality. Here we tested, for the first time, whether experimentally-imposed circadian misalignment alters the neural response to monetary reward and/or response inhibition.

METHODS

Healthy adolescents (n = 25, ages 13-17) completed two in-lab sleep schedules in counterbalanced order: An 'aligned' condition based on typical summer sleep-wake times (0000-0930) and a 'misaligned' condition mimicking earlier school year sleep-wake times (2000-0530). Participants completed morning and afternoon functional magnetic resonance imaging scans during each condition, including monetary reward (morning only) and response inhibition (morning and afternoon) tasks. Total sleep time and circadian phase were assessed via actigraphy and salivary melatonin, respectively.

RESULTS

Bilateral ventral striatal (VS) activation during reward outcome was lower during the Misaligned condition after accounting for the prior night's total sleep time. Bilateral VS activation during reward anticipation was lower during the Misaligned condition, including after accounting for covariates, but did not survive correction for multiple comparisons. Right inferior frontal gyrus activation during response inhibition was lower during the Misaligned condition, before and after accounting for total sleep time and vigilant attention, but only during the morning scan.

CONCLUSIONS

Our findings provide novel experimental evidence that circadian misalignment analogous to that resulting from school schedules may have measurable impacts on healthy adolescents' reward processing and inhibition of prepotent responses.

Neurobiological and Hormonal Mechanisms Regulating Women's Sleep

Sleep is crucial for optimal well-being, and sex differences in sleep quality have significant implications for women's health. We review the current literature on sex differences in sleep, such as differences in objective and subjective sleep measures and their relationship with aging. We then discuss the convincing evidence for the role of ovarian hormones in regulating female sleep, and survey how these hormones act on a multitude of brain regions and neurochemicals to impact sleep. Lastly, we identify several important areas in need of future research to narrow the knowledge gap and improve the health of women and other understudied populations.