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

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.

Acute neural effects of the mood stabiliser lamotrigine on emotional processing in healthy volunteers: a randomised control trial

Lamotrigine is an effective mood stabiliser, largely used for the management and prevention of depression in bipolar disorder. The neuropsychological mechanisms by which lamotrigine acts to relieve symptoms as well as its neural effects on emotional processing remain unclear. The primary objective of this current study was to investigate the impact of an acute dose of lamotrigine on the neural response to a well-characterised fMRI task probing implicit emotional processing relevant to negative bias. 31 healthy participants were administered either a single dose of lamotrigine (300 mg, n = 14) or placebo (n = 17) in a randomized, double-blind design. Inside the 3 T MRI scanner, participants completed a covert emotional faces gender discrimination task. Brain activations showing significant group differences were identified using voxel-wise general linear model (GLM) nonparametric permutation testing, with threshold free cluster enhancement (TFCE) and a family wise error (FWE)-corrected cluster significance threshold of p < 0.05. Participants receiving lamotrigine were more accurate at identifying the gender of fearful (but not happy or angry) faces. A network of regions associated with emotional processing, including amygdala, insula, and the anterior cingulate cortex (ACC), was significantly less activated in the lamotrigine group compared to the placebo group across emotional facial expressions. A single dose of lamotrigine reduced activation in limbic areas in response to faces with both positive and negative expressions, suggesting a valence-independent effect. However, at a behavioural level lamotrigine appeared to reduce the distracting effect of fear on face discrimination. Such effects may be relevant to the mood stabilisation effects of lamotrigine.

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 Augmentation for Treatment-Resistant Unipolar and Bipolar Depression in the Real World: A Systematic Review and Meta-Analysis

BACKGROUND

Pramipexole is a dopamine full agonist approved for the treatment of Parkinson's disease and restless legs syndrome. Its high affinity for the D3 receptor and neuroprotective, antioxidant, and anti-inflammatory activity provides a rationale for the treatment of depression. In this paper, we review studies on the effectiveness and safety of antidepressant pramipexole augmentation in treatment-resistant depression.

METHODS

This comprehensive systematic review and meta-analysis of observational studies on pramipexole-antidepressant augmentation included patients with resistant unipolar and bipolar depression. The primary outcome measure was the treatment response, measured at the study endpoint.

RESULTS

We identified 8 studies including 281 patients overall, 57% women and 39.5% with bipolar disorder and 60.5% with major depressive disorder. The mean follow-up duration was 27.3 weeks (range 8-69). The pooled estimate of treatment response was 62.5%, without significant differences between unipolar and bipolar depression. Safety was good, with nausea and somnolence the most frequent side effects.

CONCLUSIONS

The findings of this systematic review, needing further confirmation, show that off-label use of pramipexole as augmentation of antidepressant treatment could be a useful and safe strategy for unipolar and bipolar treatment-resistant depression.

Catechol-O-methyltransferase activity does not influence emotional processing in men

BACKGROUND

Catechol-O-methyltransferase (COMT) regulates cortical dopaminergic transmission and prefrontal-dependent cognitive function. However, its role in other cognitive processes, including emotional processing, is relatively unexplored. We therefore investigated the separate and interactive influences of COMT inhibition and Val¹⁵⁸Met (rs4680) genotype on performance on an emotional test battery.

METHODS

We recruited 74 healthy men homozygous for the functional COMT Val¹⁵⁸Met polymorphism. Volunteers were administered either a single 200 mg dose of the brain-penetrant COMT inhibitor tolcapone or placebo in a double-blind, randomised manner. Emotional processing was assessed using the emotional test battery, and mood was rated using visual analogue scales and the Profile of Mood States (POMS) questionnaire across the test day.

RESULTS

There were no main or interactive effects of Val¹⁵⁸Met genotype or tolcapone on any of the emotional processing measures or mood ratings.

CONCLUSIONS

Our findings suggest that, at least in healthy adult men, COMT has little or no effect on emotional processing or mood. These findings contrast with several neuroimaging studies that suggest that COMT modulates neural activity during emotional processing. Thus, further studies are required to understand how COMT impacts on the relationship between behavioural output and neural activity during emotional processing. Nevertheless, our data suggest that novel COMT inhibitors under development for treating cognitive dysfunction are unlikely to have acute off target effects on emotional behaviours.

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.

The Effect of the 5-HT4 Agonist, Prucalopride, on a Functional Magnetic Resonance Imaging Faces Task in the Healthy Human Brain

Depression is a common and often recurrent illness with significant negative impact on a global scale. Current antidepressants are ineffective for up to one third of people with depression, many of whom experience persistent symptomatology. 5-HT₄ receptor agonists show promise in both animal models of depression and cognitive deficit. We therefore studied the effect of the 5-HT₄ partial agonist prucalopride (1 mg daily for 6 days) on the neural processing of emotional faces in 43 healthy participants using a randomised placebo-controlled design. Participants receiving prucalopride were more accurate at identifying the gender of emotional faces. In whole brain analyses, prucalopride was also associated with reduced activation in a network of regions corresponding to the default mode network. However, there was no evidence that prucalopride treatment produced a positive bias in the neural processing of emotional faces. Our study provides further support for a pro-cognitive effect of 5-HT₄ receptor agonism in humans. While our current behavioural and neural investigations do not suggest an antidepressant-like profile of prucalopride in humans, it will be important to study a wider dose range in future studies.