A novel phosphodiesterase 1 inhibitor reverses L-dopa-induced dyskinesia, but not motivation deficits, in monkeys

The enzyme phosphodiesterase 1 (PDE1) is highly expressed in the striatum and cortex. However, its role in corticostriatal function has not been fully investigated. The present study was aimed at evaluating the therapeutic potential of PDE1 inhibitors in treating motivation deficits and 3,4-dihydroxy-L-phenylalanine (L-dopa)-induced dyskinesia, which are pathological conditions of the corticostriatal system. We used a novel PDE1 inhibitor 3-ethyl-2-{[trans-4-(methoxymethyl)cyclohexyl]oxy}-7-(tetrahydro-2H-pyran-4-yl)-imidazo[5,1-f][1,2,4]triazin-4(3H)-one (DSR-143136), which was identified in our drug discovery program. Motivation in monkeys was measured using a progressive ratio task. L-Dopa-induced dyskinesia and disability scores were measured in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys. DSR-143136 had a high selectivity for PDE1 over other PDE families and 67 other biologic targets. A dopamine D₁ receptor antagonist SCH-39166 at 0.01, 0.03 and 0.1 mg/kg potently decreased motivation in monkeys. However, DSR-143136 at 0.3 and 3 mg/kg did not affect motivation deficits induced by low-dose SCH-39166 (0.01 mg/kg). On the other hand, DSR-143136 at 3 mg/kg potently decreased L-dopa-induced dyskinesia in the Parkinsonian monkey model. Importantly, this antidyskinesic efficacy was NOT accompanied by detrimental effects on motor function. Further, this compound decreased on-time with marked or severe dyskinesia, without affecting on-time itself. These findings suggest that PDE1 inhibitor could be a therapeutic candidate for treating L-dopa-induced dyskinesia in Parkinson's disease, but not for motivation deficits.

Motivational deficits in schizophrenia relate to abnormalities in cortical learning rate signals

Individuals from across the psychosis spectrum display impairments in reinforcement learning. In some individuals, these deficits may result from aberrations in reward prediction error (RPE) signaling, conveyed by dopaminergic projections to the ventral striatum (VS). However, there is mounting evidence that VS RPE signals are relatively intact in medicated people with schizophrenia (PSZ). We hypothesized that, in PSZ, reinforcement learning deficits often are not related to RPE signaling per se but rather their impact on learning and behavior (i.e., learning rate modulation), due to dysfunction in anterior cingulate and dorsomedial prefrontal cortex (dmPFC). Twenty-six PSZ and 23 healthy volunteers completed a probabilistic reinforcement learning paradigm with occasional, sudden, shifts in contingencies. Using computational modeling, we found evidence of an impairment in trial-wise learning rate modulation (α) in PSZ before and after a reinforcement contingency shift, expressed most in PSZ with more severe motivational deficits. In a subsample of 22 PSZ and 22 healthy volunteers, we found little evidence for between-group differences in VS RPE and dmPFC learning rate signals, as measured with fMRI. However, a follow-up psychophysiological interaction analysis revealed decreased dmPFC-VS connectivity concurrent with learning rate modulation, most prominently in individuals with the most severe motivational deficits. These findings point to an impairment in learning rate modulation in PSZ, leading to a reduced ability to adjust task behavior in response to unexpected outcomes. At the level of the brain, learning rate modulation deficits may be associated with decreased involvement of the dmPFC within a greater RL network.

Effort-based decision-making impairment in patients with clinically-stabilized first-episode psychosis and its relationship with amotivation and psychosocial functioning

Effort-based decision-making has recently been proposed as a potential mechanism contributing to motivational deficits (amotivation) in psychotic disorder. Previous research has identified altered effort allocation in chronic schizophrenia, but produced mixed results regarding its relationship with amotivation. No study has investigated effort allocation in first-episode psychosis (FEP). We examined effort-based decision-making in 45 clinically-stabilized FEP patients and 45 demographically-matched controls, using Effort-Expenditure for Reward Task (EEfRT) which measures allocation of physical effort for monetary reward at varying magnitude and probability levels. Our results showed that FEP patients did not demonstrate overall reduction in effort expenditure but displayed reduced willingness to expend effort for high-value/high-probability reward as compared to controls. In particular, such selective effort-related impairment was most pronounced in patients with high levels of amotivation. Furthermore, reduced allocation of greater effort for higher probability reward was related to poorer psychosocial functioning. Decreased effort exertion was generally unrelated to other symptom dimensions, self-report anhedonia, antipsychotic dose and cognitive deficits. This study thus provides the first evidence of effort-based decision-making impairment in FEP, and indicates that first-episode patients were not generally effort-averse but exhibited inefficient effort allocation by failing to make high-effort choices to maximize reward receipt. Our findings affirm the critical role of amotivation on aberrant effort allocation, and support the link between laboratory-based effort-cost measures and real-world psychosocial functioning in medicated FEP. Further longitudinal research is required to clarify trajectory of suboptimal effort allocation and its potential utility in predicting amotivation and functional outcomes in the early course of illness.

Impaired Expected Value Computations Coupled With Overreliance on Stimulus-Response Learning in Schizophrenia

BACKGROUND

While many have emphasized impaired reward prediction error signaling in schizophrenia, multiple studies suggest that some decision-making deficits may arise from overreliance on stimulus-response systems together with a compromised ability to represent expected value. Guided by computational frameworks, we formulated and tested two scenarios in which maladaptive representations of expected value should be most evident, thereby delineating conditions that may evoke decision-making impairments in schizophrenia.

METHODS

In a modified reinforcement learning paradigm, 42 medicated people with schizophrenia and 36 healthy volunteers learned to select the most frequently rewarded option in a 75-25 pair: once when presented with a more deterministic (90-10) pair and once when presented with a more probabilistic (60-40) pair. Novel and old combinations of choice options were presented in a subsequent transfer phase. Computational modeling was employed to elucidate contributions from stimulus-response systems (actor-critic) and expected value (Q-learning).

RESULTS

People with schizophrenia showed robust performance impairments with increasing value difference between two competing options, which strongly correlated with decreased contributions from expected value-based learning (Q-learning). Moreover, a subtle yet consistent contextual choice bias for the probabilistic 75 option was present in people with schizophrenia, which could be accounted for by a context-dependent reward prediction error in the actor-critic.

CONCLUSIONS

We provide evidence that decision-making impairments in schizophrenia increase monotonically with demands placed on expected value computations. A contextual choice bias is consistent with overreliance on stimulus-response learning, which may signify a deficit secondary to the maladaptive representation of expected value. These results shed new light on conditions under which decision-making impairments may arise.

Distress related to subclinical negative symptoms in a non-clinical sample: Role of dysfunctional attitudes

Negative symptoms are a prominent feature of schizophrenia that are intimately linked to poor outcomes characterizing the illness. One mechanistic model suggests that these symptoms are produced and maintained, at least in part, through maladaptive attitudes. Beyond mechanisms, it remains phenomenologically unclear if these symptoms are particularly distressing. In the present study we examined whether subclinical negative symptoms evaluated in a non-clinical sample of young adults (N=370) were distressful or bothersome to participants and, further, whether these symptoms were associated with dysfunctional attitudes. We found that greater severity of subclinical negative symptoms such as amotivation and anhedonia were associated with higher ratings of distress specifically attributable to these symptoms. This relationship held even after controlling for severity of depressive symptoms. Moreover, greater negative symptom burden was associated with greater endorsement of defeatist performance beliefs. Negative symptoms expressed in the general population were found to be particularly distressing. Maladaptive cognitive schemas are implicated in the expression of these symptoms, as well as the amount of distress these symptoms instil. A greater understanding of the mechanisms underlying negative symptoms, including both neurobiological and cognitive, is needed in order to effectively develop treatment strategies for these disabling symptoms.

Dissecting negative symptoms in schizophrenia: opportunities for translation into new treatments

Among the constellation of symptoms that characterize schizophrenia, negative symptoms have emerged as a critical feature linked to the functional impairment experienced by affected individuals. Despite advances in our understanding of the role of negative symptoms in the illness, effective treatments for these debilitating symptoms have remained elusive. In this review we explore the contemporary conceptualization of negative symptoms in schizophrenia, including the identification of two key subdomains of diminished expression and amotivation, and clarifications around hedonic capacity. We then explore strategies for clinical assessments of negative symptoms, followed by findings using objective paradigms for evaluating discrete aspects of these negative symptoms in clinical populations and animal models, both for symptoms of diminished expression and within the multifaceted motivation system. We conclude with a consideration of current strategies for drug development for these negative symptoms, the role of heterogeneity in the clinical presentation of symptoms in schizophrenia and opportunities for personalized assessment and treatment approaches, as well as a commentary on current clinical drug trial design and the role of environmental opportunities for novel treatments to effect change and improve outcomes for affected individuals.

Methodological approaches and magnitude of the clinical unmet need associated with amotivation in mood disorders

BACKGROUND

There is growing research interest in studying motivational deficits in different neuropsychiatric disorders because these symptoms appear to be more common than originally reported and negatively impact long-term functional outcomes. However, there is considerable ambiguity in the terminology used to describe motivational deficits in the scientific literature. For the purposes of this manuscript, the term "amotivation" will be utilised in the context of mood disorders, since this is considered a more inclusive/appropriate term for this patient population. Other challenges impacting the study of amotivation in mood disorders, include: appropriate patient population selection; managing or controlling for potential confounding factors; the lack of gold-standard diagnostic criteria and assessment scales; and determination of the most appropriate study duration.

METHODS

This paper summarises the search for a consensus by a group of experts in the optimal approach to studying amotivation in mood disorders.

RESULTS

The consensus of this group is that amotivation in mood disorders is a legitimate therapeutic target, given the magnitude of the associated unmet needs, and that proof-of-concept studies should be conducted in order to facilitate subsequent larger investigations. The focus of this manuscript is to consider the study of amotivation, as a residual symptom of major depressive disorder (MDD) or bipolar depression (BD), following adequate treatment with a typical antidepressant or mood stabiliser/antipsychotic, respectively.

DISCUSSION

There is a paucity of data studying amotivation in mood disorders. This manuscript provides general guidance on the most appropriate study design(s) and methodology to assess potential therapeutic options for the management of residual amotivation in mood disorders.