Dopamine promotes cognitive effort by biasing the benefits versus costs of cognitive work.

Stimulants such as methylphenidate are increasingly used for cognitive enhancement but precise mechanisms are unknown. We found that methylphenidate boosts willingness to expend cognitive effort by altering the benefit-to-cost ratio of cognitive work. Willingness to expend effort was greater for participants with higher striatal dopamine synthesis capacity, whereas methylphenidate and sulpiride, a selective D2 receptor antagonist, increased cognitive motivation more for participants with lower synthesis capacity. A sequential sampling model informed by momentary gaze revealed that decisions to expend effort are related to amplification of benefit-versus-cost information attended early in the decision process, whereas the effect of benefits is strengthened with higher synthesis capacity and by methylphenidate. These findings demonstrate that methylphenidate boosts the perceived benefits versus costs of cognitive effort by modulating striatal dopamine signaling.

Challenges and opportunities for the development of new antipsychotic drugs.

In spite of the significant impact that the serendipitous discovery of drugs with antipsychotic properties had on the care of patients with psychotic disorders, there are significant challenges when aiming at therapeutic goals such as remission, recovery, improved health-related quality of life and functioning. The efficacy and effectiveness of existing antipsychotic drugs fail to address the full spectrum of symptoms and functional deficits that currently prevent patients with psychotic disorders from achieving fulfilling lives. The study of the pharmacological mechanism of action has increased our knowledge on molecular targets and brain circuits related to the antipsychotic properties of this drug class. However, our understanding of how these molecular targets and brain circuits relate to other aspects of disease pathophysiology like cognitive impairment and negative symptoms is incomplete although these are significant clinical unmet needs. Currently, there is still an important knowledge gap between psychopathology and pathophysiology in schizophrenia research. This may have contributed to some recent costly failures of large clinical development programs for drugs targeted at glutamatergic function and nicotinic receptors. The lack of success of these pharmacological approaches to achieve clinical validation raises important questions concerning the underlying hypothesis that guided the choice of molecular targets, and about the predictive validity of translational models that supported the rationale for testing these drugs in clinical studies. From a clinical perspective there is a need to more strongly consider the disease heterogeneity linked to the use of the current diagnostic classification of subjects and to the validity of the psychopathological constructs and assessments that are used to assess clinical outcomes. A paradigm shift in the development of drugs for schizophrenia is needed. This will require among other addressing: the shortcomings of a single diagnostic entity; the needs for in depth clinical phenotyping to leverage the findings of schizophrenia genetics and advance the understanding of the disease biology; the symptom domains that are the major sources of disability in order to improve functional outcomes beyond current treatment options. In spite of the progress achieved during the last century the task ahead is still daunting and will require the efforts of scientists and clinicians through inclusive public-private partnerships and consortia to create the scientific basis for new therapeutic approaches to schizophrenia.

Dopaminergic modulation of the trade-off between probability and time in economic decision-making.

Studies on animals and humans have demonstrated the importance of dopamine in modulating decision-making processes. In this work, we have tested dopaminergic modulation of economic decision-making and its neural correlates by administering either placebo or metoclopramide, a dopamine D2-receptor antagonist, to healthy subjects, during a functional MRI study. The decision-making task combined probability and time delay with a fixed monetary reward. For individual behavioral characterization, we used the Probability Time Trade-off (PTT) economic model, which integrates the traditional trade-offs of reward magnitude-time and reward magnitude-probability into a single measurement, thereby quantifying the subjective value of a delayed and probabilistic outcome. A regression analysis between BOLD signal and the PTT model index permitted to identify the neural substrate encoding the subjective reward-value. Behaviorally, medication reduced the rate of temporal discounting over probability, reflected in medicated subjects being more prone to postpone the reward in order to increase the outcome probability. In addition, medicated subjects showed less activity during the task in the postcentral gyrus as well as frontomedian areas, whereas there were no differences in the ventromedial orbitofrontal cortex (VMOFC) between groups when coding the subjective value. The present study demonstrates by means of behavior and imaging that dopamine modulation alters the probability-time trade-off in human economic decision-making.

Dopamine Regulates Approach-Avoidance in Human Sensation-Seeking.

BACKGROUND: Sensation-seeking is a trait that constitutes an important vulnerability factor for a variety of psychopathologies with high social cost. However, little is understood either about the mechanisms underlying motivation for intense sensory experiences or their neuropharmacological modulation in humans. METHODS: Here, we first evaluate a novel paradigm to investigate sensation-seeking in humans. This test probes the extent to which participants choose either to avoid or self-administer an intense tactile stimulus (mild electric stimulation) orthogonal to performance on a simple economic decision-making task. Next we investigate in a different set of participants whether this behavior is sensitive to manipulation of dopamine D2 receptors using a within-subjects, placebo-controlled, double-blind design. RESULTS: In both samples, individuals with higher self-reported sensation-seeking chose a greater proportion of mild electric stimulation-associated stimuli, even when this involved sacrifice of monetary gain. Computational modelling analysis determined that people who assigned an additional positive economic value to mild electric stimulation-associated stimuli exhibited speeding of responses when choosing these stimuli. In contrast, those who assigned a negative value exhibited slowed responses. These findings are consistent with involvement of low-level, approach-avoidance processes. Furthermore, the D2 antagonist haloperidol selectively decreased the additional economic value assigned to mild electric stimulation-associated stimuli in individuals who showed approach reactions to these stimuli under normal conditions (behavioral high-sensation seekers). CONCLUSIONS: These findings provide the first direct evidence of sensation-seeking behavior being driven by an approach-avoidance-like mechanism, modulated by dopamine, in humans. They provide a framework for investigation of psychopathologies for which extreme sensation-seeking constitutes a vulnerability factor.

Assessment of lactotroph axis functionality in mice: longitudinal monitoring of PRL secretion by ultrasensitive-ELISA.

The pattern of prolactin (PRL) secretion depends on the physiological state. Due to insufficient detection sensitivity of existing assays, the precise description of these patterns in mice is lacking. We described an ultrasensitive ELISA assay that can detect mouse PRL in small fractions of whole blood, allowing longitudinal studies of PRL secretion profiles in freely moving mice. Over a 24-hour period, males displayed no oscillation in PRL levels, whereas virgin and lactating females showed large pulses. Peaks of PRL secretion reached 30-40 ng/mL in lactating female mice and rarely exceeded 10 ng/mL in virgin females. These pulses of PRL in lactating females were associated with suckling. The return of pups after an experimental 12-hour weaning induced a pulse of PRL release, reaching 100 ng/mL. This approach also enabled us to assess the inhibitory tone from hypothalamic dopamine neurons on PRL secretion. We used a dopamine D2 receptor antagonist to relieve pituitary lactotrophs from the tuberoinfundibular dopaminergic inhibitory tone and demonstrate a D2-induced PRL rise that can be used to evaluate both the secretory capacity of lactotrophs and the magnitude of the inhibitory tone on pituitary PRL release. We demonstrate that, although lactotroph function is altered to enhance chronic PRL output, their secretory response to acute stimulus is not modified during lactation and that chronic hyperprolactinemia is linked to a lower inhibitory tone. The combination of a sensitive PRL ELISA and administration of D2 receptor antagonist provide a unique opportunity to investigate the function and plasticity of the lactotroph axis in freely moving mice.

Dopamine D2 receptor occupancy as a predictor of catalepsy in rats: a pharmacokinetic-pharmacodynamic modeling approach.

OBJECTIVES: Dopamine D2 receptor occupancy (D2RO) is the major determinant of efficacy and safety in schizophrenia drug therapy. Excessive D2RO (>80%) is known to cause catalepsy (CAT) in rats and extrapyramidal side effects (EPS) in human. The objective of this study was to use pharmacokinetic and pharmacodynamic modeling tools to relate CAT with D2RO in rats and to compare that with the relationship between D2RO and EPS in humans. METHODS: Severity of CAT was assessed in rats at hourly intervals over a period of 8 h after antipsychotic drug treatment. An indirect response model with and without Markov elements was used to explain the relationship of D2RO and CAT. RESULTS: Both models explained the CAT data well for olanzapine, paliperidone and risperidone. However, only the model with the Markov elements predicted the CAT severity well for clozapine and haloperidol. The relationship between CAT scores in rat and EPS scores in humans was implemented in a quantitative manner. Risk of EPS not exceeding 10% over placebo correlates with less than 86% D2RO and less than 30% probability of CAT events in rats. CONCLUSION: A quantitative relationship between rat CAT and human EPS was elucidated and may be used in drug discovery to predict the risk of EPS in humans from D2RO and CAT scores measured in rats.