Parameter Space and Potential for Biomarker Development in 25 Years of fMRI Drug Cue Reactivity: A Systematic Review

Importance

In the last 25 years, functional magnetic resonance imaging drug cue reactivity (FDCR) studies have characterized some core aspects in the neurobiology of drug addiction. However, no FDCR-derived biomarkers have been approved for treatment development or clinical adoption. Traversing this translational gap requires a systematic assessment of the FDCR literature evidence, its heterogeneity, and an evaluation of possible clinical uses of FDCR-derived biomarkers.

Objective

To summarize the state of the field of FDCR, assess their potential for biomarker development, and outline a clear process for biomarker qualification to guide future research and validation efforts.

Evidence Review

The PubMed and Medline databases were searched for every original FDCR investigation published from database inception until December 2022. Collected data covered study design, participant characteristics, FDCR task design, and whether each study provided evidence that might potentially help develop susceptibility, diagnostic, response, prognostic, predictive, or severity biomarkers for 1 or more addictive disorders.

Findings

There were 415 FDCR studies published between 1998 and 2022. Most focused on nicotine (122 [29.6%]), alcohol (120 [29.2%]), or cocaine (46 [11.1%]), and most used visual cues (354 [85.3%]). Together, these studies recruited 19 311 participants, including 13 812 individuals with past or current substance use disorders. Most studies could potentially support biomarker development, including diagnostic (143 [32.7%]), treatment response (141 [32.3%]), severity (84 [19.2%]), prognostic (30 [6.9%]), predictive (25 [5.7%]), monitoring (12 [2.7%]), and susceptibility (2 [0.5%]) biomarkers. A total of 155 interventional studies used FDCR, mostly to investigate pharmacological (67 [43.2%]) or cognitive/behavioral (51 [32.9%]) interventions; 141 studies used FDCR as a response measure, of which 125 (88.7%) reported significant interventional FDCR alterations; and 25 studies used FDCR as an intervention outcome predictor, with 24 (96%) finding significant associations between FDCR markers and treatment outcomes.

Conclusions and Relevance

Based on this systematic review and the proposed biomarker development framework, there is a pathway for the development and regulatory qualification of FDCR-based biomarkers of addiction and recovery. Further validation could support the use of FDCR-derived measures, potentially accelerating treatment development and improving diagnostic, prognostic, and predictive clinical judgments.

A methodological checklist for fMRI drug cue reactivity studies: development and expert consensus

Cue reactivity is one of the most frequently used paradigms in functional magnetic resonance imaging (fMRI) studies of substance use disorders (SUDs). Although there have been promising results elucidating the neurocognitive mechanisms of SUDs and SUD treatments, the interpretability and reproducibility of these studies is limited by incomplete reporting of participants' characteristics, task design, craving assessment, scanning preparation and analysis decisions in fMRI drug cue reactivity (FDCR) experiments. This hampers clinical translation, not least because systematic review and meta-analysis of published work are difficult. This consensus paper and Delphi study aims to outline the important methodological aspects of FDCR research, present structured recommendations for more comprehensive methods reporting and review the FDCR literature to assess the reporting of items that are deemed important. Forty-five FDCR scientists from around the world participated in this study. First, an initial checklist of items deemed important in FDCR studies was developed by several members of the Enhanced NeuroImaging Genetics through Meta-Analyses (ENIGMA) Addiction working group on the basis of a systematic review. Using a modified Delphi consensus method, all experts were asked to comment on, revise or add items to the initial checklist, and then to rate the importance of each item in subsequent rounds. The reporting status of the items in the final checklist was investigated in 108 recently published FDCR studies identified through a systematic review. By the final round, 38 items reached the consensus threshold and were classified under seven major categories: 'Participants' Characteristics', 'General fMRI Information', 'General Task Information', 'Cue Information', 'Craving Assessment Inside Scanner', 'Craving Assessment Outside Scanner' and 'Pre- and Post-Scanning Considerations'. The review of the 108 FDCR papers revealed significant gaps in the reporting of the items considered important by the experts. For instance, whereas items in the 'General fMRI Information' category were reported in 90.5% of the reviewed papers, items in the 'Pre- and Post-Scanning Considerations' category were reported by only 44.7% of reviewed FDCR studies. Considering the notable and sometimes unexpected gaps in the reporting of items deemed to be important by experts in any FDCR study, the protocols could benefit from the adoption of reporting standards. This checklist, a living document to be updated as the field and its methods advance, can help improve experimental design, reporting and the widespread understanding of the FDCR protocols. This checklist can also provide a sample for developing consensus statements for protocols in other areas of task-based fMRI.

Intensive longitudinal characterization of multidimensional biobehavioral dynamics in laboratory rats

Rats have been used as animal models for human diseases for more than a century, yet a systematic understanding of basal biobehavioral phenotypes of laboratory rats is still missing. In this study, we utilize wireless tracking technology and videography, collect and analyze more than 130 billion data points to fill this gap, and characterize the evolution of behavior and physiology of group-housed male and female rats (n = 114) of the most commonly used strains (Lister Hooded, Long-Evans, Sprague-Dawley, and Wistar) throughout their development. The resulting intensive longitudinal data suggest the existence of strain and sex differences and bi-stable developmental states. Under standard laboratory 12-h light/12-h dark conditions, our study found the presence of multiple oscillations such as circatidal-like rhythms in locomotor activity. The overall findings further suggest that frequent movement along cage walls or thigmotaxic activity may be a physical feature of motion in constrained spaces, critically affecting the interpretation of basal behavior of rats in cages.

Neurochemical underpinning of hemodynamic response to neuropsychiatric drugs: A meta- and cluster analysis of preclinical studies

Functional magnetic resonance imaging (fMRI) is an extensively used method for the investigation of normal and pathological brain function. In particular, fMRI has been used to characterize spatiotemporal hemodynamic response to pharmacological challenges as a non-invasive readout of neuronal activity. However, the mechanisms underlying regional signal changes are yet unclear. In this study, we use a meta-analytic approach to converge data from microdialysis experiments with relative cerebral blood volume (rCBV) changes following acute administration of neuropsychiatric drugs in adult male rats. At whole-brain level, the functional response patterns show very weak correlation with neurochemical alterations, while for numerous brain areas a strong positive correlation with noradrenaline release exists. At a local scale of individual brain regions, the rCBV response to neurotransmitters is anatomically heterogeneous and, importantly, based on a complex interplay of different neurotransmitters that often exert opposing effects, thus providing a mechanism for regulating and fine tuning hemodynamic responses in specific regions.

Addiction Research Consortium: Losing and regaining control over drug intake (ReCoDe)-From trajectories to mechanisms and interventions

One of the major risk factors for global death and disability is alcohol, tobacco, and illicit drug use. While there is increasing knowledge with respect to individual factors promoting the initiation and maintenance of substance use disorders (SUDs), disease trajectories involved in losing and regaining control over drug intake (ReCoDe) are still not well described. Our newly formed German Collaborative Research Centre (CRC) on ReCoDe has an interdisciplinary approach funded by the German Research Foundation (DFG) with a 12-year perspective. The main goals of our research consortium are (i) to identify triggers and modifying factors that longitudinally modulate the trajectories of losing and regaining control over drug consumption in real life, (ii) to study underlying behavioral, cognitive, and neurobiological mechanisms, and (iii) to implicate mechanism-based interventions. These goals will be achieved by: (i) using mobile health (m-health) tools to longitudinally monitor the effects of triggers (drug cues, stressors, and priming doses) and modify factors (eg, age, gender, physical activity, and cognitive control) on drug consumption patterns in real-life conditions and in animal models of addiction; (ii) the identification and computational modeling of key mechanisms mediating the effects of such triggers and modifying factors on goal-directed, habitual, and compulsive aspects of behavior from human studies and animal models; and (iii) developing and testing interventions that specifically target the underlying mechanisms for regaining control over drug intake.

Early detection and monitoring of cerebral ischemia using calcium-responsive MRI probes

Cerebral ischemia is one of the leading causes of mortality and disability in infants and adults and its timely diagnosis is essential for an efficient treatment. We present a methodology for fast detection and real-time monitoring of fluctuations of calcium ions associated with focal ischemia using a molecular functional MRI approach. We used a dinuclear paramagnetic gadolinium(III) complex chelate that changes MR image contrast through its reversible interaction with extracellular calcium ions, while applying a remote transient middle cerebral artery occlusion as a model for ischemic stroke. Our method sensitively recognizes the onset and follows the dynamics of the ischemic core and penumbra with submillimeter spatial and second-scale temporal resolution, thus paving the way for noninvasive monitoring and development of targeted treatment strategies for cerebral ischemia.

Refined parcellation of the nervous system by algorithmic detection of hidden features within communities

The nervous system can be represented as a multiscale network comprised by single cells or ensembles that are linked by physical or functional connections. Groups of morphologically and physiologically diverse neurons are wired as connectivity patterns with a certain degree of universality across species and individual variability. Thereby, community detection approaches are often used to characterize how neural units cluster into such densely interconnected groups. However, the communities may possess deeper structural features that remain undetected by current algorithms. We present a scheme for refined parcellation of neuronal networks, by identifying local integrator units (LU) that are contained in network communities. An LU is defined as a connected subnetwork in which all neuronal connections are constrained within this unit, and can be formed for instance by a set of interneurons. Our method uses the Louvain algorithm to detect communities and participation coefficients to discriminate local neurons from global hubs. The sensitivity of the algorithm for discovering LUs with respect to the choice of community detection algorithm and network parameters was tested by simulations of different synthetic networks. The appropriateness of the algorithm for real-world scenarios was demonstrated on weighted and binary Caenorhabditis elegans connectomes. The detected LUs are distinctly localized within the worm body and clearly define functional groups. This approach provides a robust, observer-independent parcellation strategy that is useful for functional structure confirmation and potentially contributes to the current efforts in quantitative whole-brain architectonics of different species as well as the analysis of functional connectivity networks.

Quantitative evaluation of cue-induced reinstatement model for evidence-based experimental optimization

Cue-induced reinstatement is a widely used model for investigating relapse of reward-seeking behavior with high face validity in relation to clinical observations. Yet, face validity is not sufficient to evaluate an animal model, and quantitative, evidence-based analysis is required to estimate the ultimate applicability of this paradigm. Furthermore, such analysis would allow an accurate and reproducible design of future experiments. Here, we conducted meta-analysis and cluster analysis to characterize the impact of cue type (visual, auditory, olfactory or combinations thereof), intensity (e.g. light frequency, sound volume and odor concentration), reward type (e.g. different drugs of abuse, sucrose and food pellets) and model parameters (e.g. housing condition, age, gender and strain of animals) on reinstatement levels. We selected 184 publications for meta-analysis based on inclusion criteria with a total number of 3889 rats. Our analysis suggested that the exact level of reinstatement depends on neither cue type, nor intensity nor on the type of reward. While all cues induced reinstatement to reward-seeking behavior, it is the model parameters, in particular, the housing conditions, age and strain, that defined the final reinstatement levels. In particular, single-housed, adolescent, Wistar or Lister Hooded rats showed significantly higher reinstatement than adult, Sparague-Dawley rats housed in groups. Our findings suggest that model parameters (for example, single housing) evoke stress-induced behaviors that affect reinstatement more than cue/reward factors.

Systemic neurotransmitter responses to clinically approved and experimental neuropsychiatric drugs

Neuropsychiatric disorders are the third leading cause of global disease burden. Current pharmacological treatment for these disorders is inadequate, with often insufficient efficacy and undesirable side effects. One reason for this is that the links between molecular drug action and neurobehavioral drug effects are elusive. We use a big data approach from the neurotransmitter response patterns of 258 different neuropsychiatric drugs in rats to address this question. Data from experiments comprising 110,674 rats are presented in the Syphad database [ www.syphad.org ]. Chemoinformatics analyses of the neurotransmitter responses suggest a mismatch between the current classification of neuropsychiatric drugs and spatiotemporal neurostransmitter response patterns at the systems level. In contrast, predicted drug-target interactions reflect more appropriately brain region related neurotransmitter response. In conclusion the neurobiological mechanism of neuropsychiatric drugs are not well reflected by their current classification or their chemical similarity, but can be better captured by molecular drug-target interactions.

Alcohol reduces muscle fatigue through atomistic interactions with nicotinic receptors

Alcohol consumption affects many organs and tissues, including skeletal muscle. However, the molecular mechanism of ethanol action on skeletal muscle remains unclear. Here, using molecular dynamics simulations and single channel recordings, we show that ethanol interacts with a negatively charged amino acid within an extracellular region of the neuromuscular nicotinic acetylcholine receptor (nAChR), thereby altering its global conformation and reducing the single channel current amplitude. Charge reversal of the negatively charged amino acid abolishes the nAChR-ethanol interaction. Moreover, using transgenic animals harboring the charge-reversal mutation, ex vivo measurements of muscle force production show that ethanol counters fatigue in wild type but not homozygous αE83K mutant animals. In accord, in vivo studies of motor coordination following ethanol administration reveal an approximately twofold improvement for wild type compared to homozygous mutant animals. Together, the converging results from molecular to animal studies suggest that ethanol counters muscle fatigue through its interaction with neuromuscular nAChRs.

Adaptive dynamics of the 5-HT systems following chronic administration of selective serotonin reuptake inhibitors: a meta-analysis

Selective serotonin reuptake inhibitors (SSRIs) are the most frequently prescribed antidepressants. However, a major concern is their delayed onset of action, which is hypothesized to be associated with the time required for serotonin (5-HT) autoreceptors to desensitize, which should be reflected by actual neurochemical changes. Numerous in vivo microdialysis studies have been published that report on 5-HT levels in different brain sites following SSRI administration. Here, we performed a meta-analysis on dynamic changes of 5-HT neurotransmission during the course of chronic SSRI treatment. We conducted a meta-analysis on research articles of 5-HT neurotransmission measured by in vivo microdialysis in rat brain after subchronic and chronic SSRI administrations. In total, data from 42 microdialysis studies (798 rats) were analyzed. Within the first week of SSRI treatment, extracellular 5-HT concentrations drop in frontal cortex. Over the next 2 weeks of treatment, a linear increase in extracellular 5-HT levels up to 350% of prior treatment baseline is evident (n = 269). However, in hippocampus, prefrontal cortex, nucleus accumbens, and ventral tegmental area we found increased 5-HT levels within the first 3 days of SSRI administration. The time course of 5-HT dynamics in frontal cortex is in line with the hypothesis that 5-HT autoreceptors desensitize over 2-3 weeks of SSRI treatment and thereby enhanced extracellular 5-HT levels ensue. Yet, in other regions we did not find evidence supporting the traditional autoreceptor-mediated feedback loops hypothesis and thus other neurobiological adaptation mechanisms may also play a role in the delayed onset of SSRI action.

A multiscale cerebral neurochemical connectome of the rat brain

Understanding the rat neurochemical connectome is fundamental for exploring neuronal information processing. By using advanced data mining, supervised machine learning, and network analysis, this study integrates over 5 decades of neuroanatomical investigations into a multiscale, multilayer neurochemical connectome of the rat brain. This neurochemical connectivity database (ChemNetDB) is supported by comprehensive systematically-determined receptor distribution maps. The rat connectome has an onion-type structural organization and shares a number of structural features with mesoscale connectomes of mouse and macaque. Furthermore, we demonstrate that extremal values of graph theoretical measures (e.g., degree and betweenness) are associated with evolutionary-conserved deep brain structures such as amygdala, bed nucleus of the stria terminalis, dorsal raphe, and lateral hypothalamus, which regulate primitive, yet fundamental functions, such as circadian rhythms, reward, aggression, anxiety, and fear. The ChemNetDB is a freely available resource for systems analysis of motor, sensory, emotional, and cognitive information processing.

Towards trans-diagnostic mechanisms in psychiatry: neurobehavioral profile of rats with a loss-of-function point mutation in the dopamine transporter gene

The research domain criteria (RDoC) matrix has been developed to reorient psychiatric research towards measurable behavioral dimensions and underlying mechanisms. Here, we used a new genetic rat model with a loss-of-function point mutation in the dopamine transporter (DAT) gene (Slc6a3_N157K) to systematically study the RDoC matrix. First, we examined the impact of the Slc6a3_N157K mutation on monoaminergic signaling. We then performed behavioral tests representing each of the five RDoC domains: negative and positive valence systems, cognitive, social and arousal/regulatory systems. The use of RDoC may be particularly helpful for drug development. We studied the effects of a novel pharmacological approach metabotropic glutamate receptor mGluR2/3 antagonism, in DAT mutants in a comparative way with standard medications. Loss of DAT functionality in mutant rats not only elevated subcortical extracellular dopamine concentration but also altered the balance of monoaminergic transmission. DAT mutant rats showed deficits in all five RDoC domains. Thus, mutant rats failed to show conditioned fear responses, were anhedonic, were unable to learn stimulus-reward associations, showed impaired cognition and social behavior, and were hyperactive. Hyperactivity in mutant rats was reduced by amphetamine and atomoxetine, which are well-established medications to reduce hyperactivity in humans. The mGluR2/3 antagonist LY341495 also normalized hyperactivity in DAT mutant rats without affecting extracellular dopamine levels. We systematically characterized an altered dopamine system within the context of the RDoC matrix and studied mGluR2/3 antagonism as a new pharmacological strategy to treat mental disorders with underlying subcortical dopaminergic hyperactivity.

Summary: The first systematic RDoc study of a disease mechanism proposes dopamine transporter DAT mutant rats as a model for drug development, targeting a hyperdopaminergic state.

Low μ-Opioid Receptor Status in Alcohol Dependence Identified by Combined Positron Emission Tomography and Post-Mortem Brain Analysis

Blockade of the μ-opioid receptor (MOR) by naltrexone reduces relapse risk in a subpopulation of alcohol-dependent patients. Previous positron-emission-tomography (PET) studies using the MOR ligand [¹¹C]carfentanil have found increased MOR availability in abstinent alcoholics, which may reflect either increased MOR expression or lower endogenous ligand concentration. To differentiate between both effects, we investigated two cohorts of alcoholic subjects using either post-mortem or clinical PET analysis. Post-mortem brain tissue of alcohol-dependent subjects and controls (N=43/group) was quantitatively analyzed for MOR ([³H]DAMGO)-binding sites and OPRM1 mRNA in striatal regions. [¹¹C]carfentanil PET was performed in detoxified, medication free alcohol-dependent patients (N=38), followed by a randomized controlled study of naltrexone versus placebo and follow-up for 1 year (clinical trial number: NCT00317031). Because the functional OPRM1 variant rs1799971:A>G affects the ligand binding, allele carrier status was considered in the analyses. MOR-binding sites were reduced by 23-51% in post-mortem striatal tissue of alcoholics. In the PET study, a significant interaction of OPRM1 genotype, binding potential (BP_ND) for [¹¹C]carfentanil in the ventral striatum, and relapse risk was found. Particularly in G-allele carriers, lower striatal BP_ND was associated with a higher relapse risk. Interestingly, this effect was more pronounced in the naltrexone treatment group. Reduced MOR is interpreted as a neuroadaptation to an alcohol-induced release of endogenous ligands in patients with severe alcoholism. Low MOR availability may explain the ineffectiveness of naltrexone treatment in this subpopulation. Finally, low MOR-binding sites are proposed as a molecular marker for a negative disease course.

Paradoxical augmented relapse in alcohol-dependent rats during deep-brain stimulation in the nucleus accumbens

Case reports indicate that deep-brain stimulation in the nucleus accumbens may be beneficial to alcohol-dependent patients. The lack of clinical trials and our limited knowledge of deep-brain stimulation call for translational experiments to validate these reports. To mimic the human situation, we used a chronic-continuous brain-stimulation paradigm targeting the nucleus accumbens and other brain sites in alcohol-dependent rats. To determine the network effects of deep-brain stimulation in alcohol-dependent rats, we combined electrical stimulation of the nucleus accumbens with functional magnetic resonance imaging (fMRI), and studied neurotransmitter levels in nucleus accumbens-stimulated versus sham-stimulated rats. Surprisingly, we report here that electrical stimulation of the nucleus accumbens led to augmented relapse behavior in alcohol-dependent rats. Our associated fMRI data revealed some activated areas, including the medial prefrontal cortex and caudate putamen. However, when we applied stimulation to these areas, relapse behavior was not affected, confirming that the nucleus accumbens is critical for generating this paradoxical effect. Neurochemical analysis of the major activated brain sites of the network revealed that the effect of stimulation may depend on accumbal dopamine levels. This was supported by the finding that brain-stimulation-treated rats exhibited augmented alcohol-induced dopamine release compared with sham-stimulated animals. Our data suggest that deep-brain stimulation in the nucleus accumbens enhances alcohol-liking probably via augmented dopamine release and can thereby promote relapse.

Activation of Melatonin Receptors Reduces Relapse-Like Alcohol Consumption

Melatonin is an endogenous synchronizer of biological rhythms and a modulator of physiological functions and behaviors of all mammals. Reduced levels of melatonin and a delay of its nocturnal peak concentration have been found in alcohol-dependent patients and rats. Here we investigated whether the melatonergic system is a novel target to treat alcohol addiction. Male Wistar rats were subjected to long-term voluntary alcohol consumption with repeated abstinence phases. Circadian drinking rhythmicity and patterns were registered with high temporal resolution by a drinkometer system and analyzed by Fourier analysis. We examined potential antirelapse effect of the novel antidepressant drug agomelatine. Given that agomelatine is a potent MT1 and MT2 receptor agonist and a 5-HT2C antagonist we also tested the effects of melatonin itself and the 5-HT2C antagonist SB242084. All drugs reduced relapse-like drinking. Agomelatine and melatonin administered at the end of the light phase led to very similar changes on all measures of the post-abstinence drinking behavior, suggesting that effects of agomelatine on relapse-like behavior are mostly driven by its melatonergic activity. Both drugs caused a clear phase advance in the diurnal drinking pattern when compared with the control vehicle-treated group and a reduced frequency of approaches to alcohol bottles. Melatonin given at the onset of the light phase had no effect on the circadian phase and very small effects on alcohol consumption. We conclude that targeting the melatonergic system in alcohol-dependent individuals can induce a circadian phase advance, which may restore normal sleep architecture and reduce relapse behavior.

A structural feature of the non-peptide ligand interactions with mice mu-opioid receptors

By binding to and activating the G-protein coupled μ-, κ- and δ-opioid receptors in the central nervous system, opiates are known to induce analgesic and sedative effects. In particular, non-peptide opioid ligands are often used in clinical applications to induce these therapeutically beneficial effects, due to their superior pharmacokinetics and bioavailability in comparison to endogenous neuropeptides. However, since opioid alkaloids are highly addictive substances, it is necessary to understand the exact mechanisms of their actions, specifically the ligand-binding properties of the target receptors, in order to safely apply opiates for therapeutic purposes. Using an in silico molecular docking approach (AutoDock Vina) combined with two-step cluster analysis, we have computationally obtained the docking scores and the ligand-binding pockets of twelve representative non-peptide nonendogenous agonists and antagonists at the crystallographically identified μ-opioid receptor. Our study predicts the existence of two main binding sites that are congruently present in all opioid receptor types. Interestingly, in terms of the agonist or antagonist properties of the substances on the receptors, the clustering analysis suggests a relationship with the position of the ligand-binding pockets, particularly its depth within the receptor structure. Furthermore, the binding affinity of the substances is directly correlated to the proximity of the binding pockets to the extracellular space. In conclusion, the results provide further insights into the structural features of the functional pharmacology of opioid receptors, suggesting the importance of the binding position of non-peptide agonists and antagonists- specifically the distance and the level of exposure to the extracellular space- to their dissociation kinetics and subsequent potency.

Stress and alcohol interactions: animal studies and clinical significance

Alcohol is frequently consumed for stress relief, but the individual determinants and the temporal course of stress-induced alcohol use are not well understood. Preclinical studies may help shed light on these factors. We synthesize here the findings from numerous rodent studies of stress and alcohol interactions. Stress-induced alcohol consumption is age-dependent, has a high genetic load, and results from an interaction of the stress and reward systems. Specifically, glucocorticoids, acting within the nucleus accumbens (NAc), are important mediators of this stress-induced alcohol intake. In addition, increased activation of the corticotropin-releasing hormone (CRH) system within the extended amygdala appears to mediate stress-induced relapse. Finally, these preclinical studies have helped to identify several attractive targets for novel treatments of alcohol abuse and addiction.

Cluster and meta-analyses on factors influencing stress-induced alcohol drinking and relapse in rodents

Numerous preclinical studies have focused on the identification of biological and environmental factors that modulate stress and alcohol interactions. Although there is a good qualitative description of the determinants of alcohol consumption in rodents, the magnitude of the variables influencing stress-induced ethanol intake and its dynamics are still poorly understood. We therefore carried out a clustered meta-analysis on stress-induced alcohol consumption in 1520 rats. Two-step clustering of the literature-derived dataset suggests a strong dependency of the experimental outcome on the method used to measure alcohol intake. Free-choice home cage drinking versus operant self-administration is the most critical determinant of stress-induced increases in alcohol consumption in rats. Stress does not typically result in enhanced alcohol consumption in operant self-administration paradigms, whereas it leads to increased home cage drinking. Stress-induced alcohol consumption is age dependent, with adults being more sensitive than adolescents. In addition, foot shock and forced swim stress enhance alcohol intake, while restraint stress does not. In contrast, a meta-analysis of 327 rats on stress-induced reinstatement of alcohol-seeking behavior shows less influence of those modulating factors, and usually foot shock or yohimbine leads to a reinstatement of approximately 300 percent of extinction level responding. Via accurate characterization of the significant factors in the interplay of alcohol consumption, relapse and stress, our quantitative description not only improves the understanding of underlying mechanisms, but also provides an appropriate framework for the optimal experimental design of preclinical studies that more accurately translates to the human condition.

A systems medicine research approach for studying alcohol addiction

According to the World Health Organization, about 2 billion people drink alcohol. Excessive alcohol consumption can result in alcohol addiction, which is one of the most prevalent neuropsychiatric diseases afflicting our society today. Prevention and intervention of alcohol binging in adolescents and treatment of alcoholism are major unmet challenges affecting our health-care system and society alike. Our newly formed German SysMedAlcoholism consortium is using a new systems medicine approach and intends (1) to define individual neurobehavioral risk profiles in adolescents that are predictive of alcohol use disorders later in life and (2) to identify new pharmacological targets and molecules for the treatment of alcoholism. To achieve these goals, we will use omics-information from epigenomics, genetics transcriptomics, neurodynamics, global neurochemical connectomes and neuroimaging (IMAGEN; Schumann et al. ) to feed mathematical prediction modules provided by two Bernstein Centers for Computational Neurosciences (Berlin and Heidelberg/Mannheim), the results of which will subsequently be functionally validated in independent clinical samples and appropriate animal models. This approach will lead to new early intervention strategies and identify innovative molecules for relapse prevention that will be tested in experimental human studies. This research program will ultimately help in consolidating addiction research clusters in Germany that can effectively conduct large clinical trials, implement early intervention strategies and impact political and healthcare decision makers.

Global ethanol-induced enhancements of monoaminergic neurotransmission: a meta-analysis study

BACKGROUND

Numerous studies use in vivo microdialysis as a quantification method for studying dynamical alterations of extracellular neurotransmitter concentrations in specific brain regions of various species following acute and chronic administration of ethanol (EtOH). A major focus of these investigations is the EtOH-induced effects on the neurochemistry of forebrain regions, particularly dose-dependent neuroadaptive changes of monoamine systems.

METHODS

Here, we performed a meta-analysis on published data sets of in vivo microdialysis measurements to assess the concentration-dependent effects of EtOH on monoamine levels within 19 distinct brain regions in adult rats, which were identified as major components of a neurocircuitry for modeling drug effects. In total, data sets of 210 research articles (7,407 rats) were analyzed.

RESULTS

The analysis of the basal values of noradrenaline, serotonin, and dopamine in those regions indicated hardly any dependencies on gender, strain, or state of consciousness. However, the acute administrations of EtOH (intraperitoneal 0.25 to 2.5 g/kg) appear to increase the level of monoamines globally and independent of the brain sites up to 270% of the basal concentrations. Moreover, a peak time average of approximately 40 minutes suggests an optimal time interval of maximal 240 minutes length to completely study the effects of different doses of EtOH within the framework of microdialysis experiments. The analysis further revealed a positive correlation between the magnitude of increase (peak % baseline) of local extracellular monoamine concentrations and the applied doses of EtOH, while the temporal occurrence of the EtOH-induced peaks in the concentrations (peak time) was mostly negatively correlated.

CONCLUSIONS

Our results provide a universal database and framework for the optimal design of future in vivo microdialysis and in silico experiments in neurochemistry and pharmacology.

Ethanol-induced alterations of amino acids measured by in vivo microdialysis in rats: a meta-analysis

PURPOSE

In recent years in vivo microdialysis has become an important method in research studies investigating the alterations of neurotransmitters in the extracellular fluid of the brain. Based on the major involvement of glutamate and γ-aminobutyric acid (GABA) in mediating a variety of alcohol effects in the mammalian brain, numerous microdialysis studies have focused on the dynamical behavior of these systems in response to alcohol.

METHODS

Here we performed multiple meta-analyses on published datasets from the rat brain: (i) we studied basal extracellular concentrations of glutamate and GABA in brain regions that belong to a neurocircuitry involved in neuropsychiatric diseases, especially in alcoholism (Noori et al., Addict Biol 17:827-864, 2012); (ii) we examined the effect of acute ethanol administration on glutamate and GABA levels within this network and (iii) we studied alcohol withdrawal-induced alterations in glutamate and GABA levels within this neurocircuitry.

RESULTS

For extraction of basal concentrations of these neurotransmitters, datasets of 6932 rats were analyzed and the absolute basal glutamate and GABA levels were estimated for 18 different brain sites. In response to different doses of acute ethanol administration, datasets of 529 rats were analyzed and a non-linear dose response (glutamate and GABA release) relationship was observed in several brain sites. Specifically, glutamate in the nucleus accumbens shows a decreasing logarithmic dose response curve. Finally, regression analysis of 11 published reports employing brain microdialysis experiments in 104 alcohol-dependent rats reveals very consistent augmented extracellular glutamate and GABA levels in various brain sites that correlate with the intensity of the withdrawal response were identified.

CONCLUSIONS

In summary, our results provide standardized basal values for future experimental and in silico studies on neurotransmitter release in the rat brain and may be helpful to understand the effect of ethanol on neurotransmitter release. Furthermore, this study illustrates the benefit of meta-analyses using the generalization of a wide range of preclinical data.

Postnatal mammalian retinal development: quantitative data and general rules

This article is aimed at providing comparative quantitative data about postnatal mammalian retina development, and at searching for some general rules at both the descriptive and the mechanistic level. In mammals the eye continues to grow, and the retina continues to expand, much after the end of retinal cytogenesis. Thus, although the total number of retinal cells remains constant after cessation of mitotic activity (and the end of 'physiological cell death'), the retinal surface area increases by a factor of two or more. In most mammals, ocular growth exceeds retinal expansion: the neural retina lines 70-80% of the inner ocular surface at the beginning but only about 40-60% in adults. Differential local expansion of the retina (the peripheral area increases more than the central one) can be explained by 'passive stretching' of the retinal tissue by the growing eyeball; it depends on the different biomechanical properties of the peripheral vs. central retinal tissue. The increasing retinal surface area allows for a re-distribution of cells such that the thickness of the (particularly, outer) nuclear layer(s) decreases proportional to the areal expansion. This causes a considerable developmental reduction of the number of cell nuclei 'stacked above each other' by a factor of more than two, and requires a translocation of the somata against their neighbors. We provide a physico-mathematical model of these oblique 'down-sliding' movements of the photoreceptor cell somata along the Müller cell process in the center of their columnar cell unit.

Neurocircuitry for modeling drug effects

The identification and functional understanding of the neurocircuitry that mediates alcohol and drug effects that are relevant for the development of addictive behavior is a fundamental challenge in addiction research. Here we introduce an assumption-free construction of a neurocircuitry that mediates acute and chronic drug effects on neurotransmitter dynamics that is solely based on rodent neuroanatomy. Two types of data were considered for constructing the neurocircuitry: (1) information on the cytoarchitecture and neurochemical connectivity of each brain region of interest obtained from different neuroanatomical techniques; (2) information on the functional relevance of each region of interest with respect to alcohol and drug effects. We used mathematical data mining and hierarchical clustering methods to achieve the highest standards in the preprocessing of these data. Using this approach, a dynamical network of high molecular and spatial resolution containing 19 brain regions and seven neurotransmitter systems was obtained. Further graph theoretical analysis suggests that the neurocircuitry is connected and cannot be separated into further components. Our analysis also reveals the existence of a principal core subcircuit comprised of nine brain regions: the prefrontal cortex, insular cortex, nucleus accumbens, hypothalamus, amygdala, thalamus, substantia nigra, ventral tegmental area and raphe nuclei. Finally, by means of algebraic criteria for synchronizability of the neurocircuitry, the suitability for in silico modeling of acute and chronic drug effects is indicated. Indeed, we introduced as an example a dynamical system for modeling the effects of acute ethanol administration in rats and obtained an increase in dopamine release in the nucleus accumbens-a hallmark of drug reinforcement-to an extent similar to that seen in numerous microdialysis studies. We conclude that the present neurocircuitry provides a structural and dynamical framework for large-scale mathematical models and will help to predict chronic drug effects on brain function.

Inhibition of the casein-kinase-1-ε/δ/ prevents relapse-like alcohol drinking

During the past decade, it has been shown that circadian clock genes have more than a simple circadian time-keeping role. Clock genes also modulate motivational processes and have been implicated in the development of psychiatric disorders such as drug addiction. Recent studies indicate that casein-kinase 1ε/δ (CK1ε/δ)--one of the components of the circadian molecular clockwork-might be involved in the etiology of addictive behavior. The present study was initiated to study the specific role of CK1ε/δ in alcohol relapse-like drinking using the 'Alcohol Deprivation Effect' model. The effect of CK1ε/δ inhibition was tested on alcohol consumption in long-term alcohol-drinking rats upon re-exposure to alcohol after deprivation using a four-bottle free-choice paradigm with water, 5%, 10%, and 20% ethanol solutions, as well as on saccharin preference in alcohol-naive rats. The inhibition of CK1ε/δ with systemic PF-670462 (0, 10, and 30 mg/kg) injections dose-dependently decreased, and at a higher dosage prevented the alcohol deprivation effect, as compared with vehicle-treated rats. The impact of the treatment was further characterized using nonlinear regression analyses on the daily profiles of drinking and locomotor activity. We reveal that CK1ε/δ inhibition blunted the high daytime alcohol intake typically observed upon alcohol re-exposure, and induced a phase shift of locomotor activity toward daytime. Only the highest dose of PF-670462 shifted the saccharin intake daily rhythm toward daytime during treatment, and decreased saccharin preference after treatment. Our data suggest that CK1 inhibitors may be candidates for drug treatment development for alcoholism.

The use of a novel drinkometer system for assessing pharmacological treatment effects on ethanol consumption in rats

BACKGROUND

There are numerous studies in the preclinical alcohol research field showing that pharmacological interventions and many other manipulations can influence ethanol (EtOH) consumption in a free-choice paradigm in rats. Most of these studies are based on 24-hour measurements. These studies provide a measure of the total amount of EtOH consumed per day, but do not provide information on the drinking patterns within this period of measurement. Here, we used a novel drinkometer system in combination with Fourier analysis to provide detailed information on drinking patterns.

METHODS

Our automated drinkometer system measures fluid consumption by means of high-precision sensors attached to the drinking bottles in the home cage of the rat and thereby ameliorates several limitations of a classical lickometer-based drinkometer system. As an example of its application, we used the alcohol deprivation effect (ADE) model for relapse-like drinking and tested as a reference compound lamotrigine, which has a robust effect on the ADE. Fourier analysis was chosen as the main strategy for 24-hour drinking pattern recognition during water/EtOH drinking.

RESULTS

Under baseline conditions, voluntary EtOH consumption in rats can be expressed as characteristic oscillations that follow diurnal activity and differ in their amplitude, depending on the EtOH concentration. This diurnal drinking rhythmicity was altered during a relapse condition. Furthermore, lamotrigine given during the ADE did not significantly affect the drinking frequency or the number of approaches to the EtOH bottles when compared to vehicle-treated animals. However, EtOH intake during a drinking approach was dramatically reduced.

CONCLUSIONS

The use of the drinkometer system and mathematical modeling allows the characterization of treatment effects on relapse-like drinking with a great level of detail. One use of such detailed information may lie in its translational predictability. For instance, owing to lamotrigine treatment's lack of effect on EtOH drinking frequency or the number of approaches to the EtOH bottles, this compound might not be effective in relapse prevention per se but may reduce hedonic EtOH effects and could therefore be used in alcohol-dependent patients if harm reduction is the primary goal of treatment.

The appropriateness of unbiased optical fractionators to assess cell proliferation in the adult hippocampus

Optical fractionators have dominated the field of neural cell counting for two decades. These unbiased stereological techniques are often used for the quantification of hippocampal cell proliferation in neurogenesis experiments. However, the heterogeneous distribution of labeled cells, especially in the form of clusters, confounds the application of these techniques. A critical evaluation of the applicability of the optical fractionator suggests that absolute counting achieves higher efficiency in the quantification of cell proliferation than unbiased estimations.

Substantial changes in synaptic firing frequencies induced by glial ATP hysteresis

Recent experimental studies strongly suggest the influence of glial purinergic transmission in the modulation of synaptic dynamics. By releasing adenosine triphosphate (ATP), which accumulates as adenosine, astrocytes tonically suppressed synaptic transmission. The delayed multi-step feedback of the glial ATP with the neuron suggests the existence of a discrete hysteresis phenomena. By integration of this hysteretic behavior into a delayed leaky integrate-and-fire model for the tripartite synapses, a significant sensitivity of the pre- and postsynaptic firing frequency patterns to the adenosine feedback-delays is observed that might be of importance for adenosine-related neurological deficits, such as sleep disorders.