The effects of pramipexole on motivational vigour during a saccade task: a placebo-controlled study in healthy adults.

Motivation allows us to energise actions when we expect reward and is reduced in depression. This effect, termed motivational vigour, has been proposed to rely on central dopamine, with dopaminergic agents showing promise in the treatment of depression. This suggests that dopaminergic agents might act to reduce depression by increasing the effects of reward or by helping energise actions. The aim of the current study was to investigate whether the dopamine agonist pramipexole enhanced motivational vigour during a rewarded saccade task. In addition, we asked whether the effects of pramipexole on vigour differ between reward contingent on performance and guaranteed reward. Healthy adult participants were randomised to receive either pramipexole (n = 19) or placebo (controls n = 18) for 18 days. The vigour of saccades was measured twice, once before the administration of study medication (Time 1) and after taking it for 12-15 days (Time 2). To separate motivation by contingency vs. reward, saccadic vigour was separately measured when (1) rewards were contingent on performance (2) delivered randomly with matched frequency, (3) when reward was guaranteed, (4) when reward was not present at all. Motivation increased response vigour, as expected. Relative to placebo, pramipexole also increased response vigour. However, there was no interaction, meaning that the effects of reward were not modulated by drug, and there was no differential drug effect on contingent vs. guaranteed rewards. The effect of pramipexole on vigour could not be explained by a speed/accuracy trade-off, nor by autonomic arousal as indexed by pupillary dilation. Chronic D2 stimulation increases general vigour, energising movements in healthy adults irrespective of extrinsic reward.

Pramipexole Enhances Reward Learning by Preserving Value Estimates.

BACKGROUND: Dopamine D2-like agonists show promise as treatments for depression. They are thought to act by enhancing reward learning; however, the mechanisms by which they achieve this are not clear. Reinforcement learning accounts describe 3 distinct candidate mechanisms: increased reward sensitivity, increased inverse decision-temperature, and decreased value decay. As these mechanisms produce equivalent effects on behavior, arbitrating between them requires measurement of how expectations and prediction errors are altered. We characterized the effects of 2 weeks of the D2-like agonist pramipexole on reward learning and used functional magnetic resonance imaging measures of expectation and prediction error to assess which of these 3 mechanistic processes were responsible for the behavioral effects. METHODS: Forty healthy volunteers (50% female) were randomized to 2 weeks of pramipexole (titrated to 1 mg/day) or placebo in a double-blind, between-subject design. Participants completed a probabilistic instrumental learning task before and after the pharmacological intervention, with functional magnetic resonance imaging data collected at the second visit. Asymptotic choice accuracy and a reinforcement learning model were used to assess reward learning. RESULTS: Pramipexole increased choice accuracy in the reward condition with no effect on losses. Participants who received pramipexole had increased blood oxygen level-dependent response in the orbital frontal cortex during the expectation of win trials but decreased blood oxygen level-dependent response to reward prediction errors in the ventromedial prefrontal cortex. This pattern of results indicates that pramipexole enhances choice accuracy by reducing the decay of estimated values during reward learning. CONCLUSIONS: The D2-like receptor agonist pramipexole enhances reward learning by preserving learned values. This is a plausible mechanism for pramipexole's antidepressant effect.

Pramipexole for the Treatment of Depression: Efficacy and Mechanisms.

Dopaminergic mechanisms are a plausible treatment target for patients with clinical depression but are relatively underexplored in conventional antidepressant medications. There is continuing interest in the potential antidepressant effects of the dopamine receptor agonist, pramipexole, with data from both case series and controlled trials indicating that this agent may produce benefit for patients with difficult-to-treat depression. Pramipexole's therapeutic utility in depression is likely to be expressed through alterations in reward mechanisms which are strongly influenced by dopamine pathways and are known to function abnormally in depressed patients. Our work in healthy participants using brain imaging in conjunction with computational modelling suggests that repeated pramipexole facilitates reward learning by inhibiting value decay. This mechanism needs to be confirmed in larger clinical trials in depressed patients. Such studies will also allow assessment of whether baseline performance in reward learning in depression predicts therapeutic response to pramipexole treatment.

Enhanced Taste Recognition Following Subacute Treatment With The Dopamine D2/D3 Receptor Agonist Pramipexole in Healthy Volunteers.

BACKGROUND: Patients with Parkinson's disease (PD) show impaired performance in taste recognition tests, which suggests a possible dopaminergic influence on gustatory functioning. To experimentally test this hypothesis, we assessed whether pharmacological manipulation of dopaminergic signaling in healthy volunteers can affect performance in a standardized taste recognition test. METHODS: Physically and mentally healthy volunteers (n = 40, age 18-43 years) were randomly allocated to treatment with either pramipexole or placebo using a double-blind, parallel-group design. After 12 to 15 days of treatment (dose titrated up from 0.25 mg/d of pramipexole salt to 1.0 mg/d), taste recognition performance was assessed using a standardized and validated assay (taste strip test). Additionally, visual analogue scale ratings of subjective pleasantness and disgustingness of taste samples were obtained. RESULTS: Compared with the placebo group, participants receiving pramipexole showed significantly higher total recognition accuracy (medianpramipexole = 14.0, medianplacebo = 13.0, U = 264.5, P = .04). This was driven by a higher sensitivity for taste in the pramipexole group. Exploratory analysis of pleasantness and disgustingness ratings of appetitive (sweet) vs aversive (bitter) stimuli suggested that pramipexole treatment was associated with overall blunted hedonic responses, but this effect did not survive the inclusion of nausea (a side effect of treatment) as a covariate in the analysis. CONCLUSIONS: Healthy volunteers who received subacute pramipexole treatment exhibited higher taste recognition performance compared with the placebo group. This finding is consistent with a proposed role of the dopaminergic system in gustatory functioning and could have important theoretical and clinical implications.

An Experimental Medicine Investigation of the Effects of Subacute Pramipexole Treatment on Emotional Information Processing in Healthy Volunteers.

Treatment with the dopamine D2/D3 receptor agonist pramipexole has demonstrated promising clinical effects in patients with depression. However, the mechanisms through which pramipexole might alleviate depressive symptoms are currently not well understood. Conventional antidepressant drugs are thought to work by biasing the processing of emotional information in favour of positive relative to negative appraisal. In this study, we used an established experimental medicine assay to explore whether pramipexole treatment might have a similar effect. Employing a double-blind, parallel-group design, 40 healthy volunteers (aged 18 to 43 years, 50% female) were randomly allocated to 12 to 15 days of treatment with either pramipexole (at a peak daily dose of 1.0 mg pramipexole salt) or placebo. After treatment was established, emotional information processing was assessed on the neural level by measuring amygdala activity in response to positive and negative facial emotional expressions, using functional magnetic resonance imaging (MRI). In addition, behavioural measures of emotional information processing were collected at baseline and on drug, using an established computerized task battery, tapping into different cognitive domains. As predicted, pramipexole-treated participants, compared to those receiving placebo, showed decreased neural activity in response to negative (fearful) vs. positive (happy) facial expressions in bilateral amygdala. Contrary to our predictions, however, pramipexole treatment had no significant antidepressant-like effect on behavioural measures of emotional processing. This study provides the first experimental evidence that subacute pramipexole treatment in healthy volunteers modifies neural responses to emotional information in a manner that resembles the effects of conventional antidepressant drugs.