Parkinson's disease duration determines effect of dopaminergic therapy on ventral striatum function.

We investigated the hypothesis that variation in endogenous dopamine (DA) across brain regions explains dissimilar effects of dopaminergic therapy on aspects of cognition in early Parkinson's disease (PD). Extensive degeneration of DA-producing cells in the substantia nigra cause dorsal striatum (DS) DA deficiency and movement abnormalities. Particularly in early PD, this contrasts with relative sparing of the dopaminergic cells of the ventral tegmental area (VTA). The hypothesis predicts that DS-mediated cognitive functions are deficient at baseline and improved by DA replacement, whereas functions depending upon VTA-innervated brain regions are normal off medication and worsen with treatment. The latter pattern presumably owes to overdose of relatively DA-replete VTA-supplied brain regions with medication levels titrated to DS-mediated motor symptoms. As PD progresses, however, VTA degeneration increases. Impairment in cognitive operations performed by VTA-innervated brain regions, such as the ventral striatum (VS), is expected. We compared the performance of early and late PD patients, on and off dopaminergic medication, relative to age-matched controls, on reward learning, previously shown to implicate the VS. As expected, in early PD, stimulus-reward learning was normal off medication, but worsened with DA replacement. At more advanced disease stages, PD patients learned stimulus-reward contingencies more poorly than controls and early PD patients off medication. Furthermore, dopaminergic medication did not worsen reward learning in late PD patients, in line with the dopamine overdose hypothesis. Unlike its effect on DS-mediated functions, however, DA-replacement therapy did not improve reward learning in late PD patients.

The effect of dopamine therapy on ventral and dorsal striatum-mediated cognition in Parkinson's disease: support from functional MRI.

The central aim of our study was to elucidate functions mediated by the ventral and dorsal striatum, respectively, to better understand the cognitive effects of dopamine replacement in Parkinson's disease. We proposed that the ventral striatum underlies general learning of stimulus associations, whereas the dorsal striatum promotes integration of various influences on selecting. In Parkinson's disease, dopamine depletion is substantially less notable in the ventral relative to the dorsal striatum, and therefore greater improvements are expected for dorsal striatum-mediated functions with dopamine replacement. Using a simple selection task, we found that dopamine replacement impaired encoding and facilitation of consistent stimulus-stimulus relations across trials. This finding was in line with our contention that ventral striatum mediates learning stimulus associations, even when explicit feedback or reward is not provided. In contrast, dopamine replacement enhanced interference related to assimilating conflicting influences on selection across trials, consistent with our hypothesis that the dorsal striatum supports deciding in ambiguous contexts. We further confirmed these separable roles for the ventral and dorsal striatum in our selection task with healthy young volunteers using functional magnetic resonance imaging. In summary, we present a within-subject, double dissociation of the effects of dopamine replacement in patients with Parkinson's disease for ventral striatum-mediated facilitation and dorsal striatum-mediated interference, confirmed in a separate functional magnetic resonance imaging experiment. Defining the distinct functions of the ventral and dorsal striatum will have direct clinical implications. Titration of therapy in Parkinson's disease is generally geared towards optimizing dorsal striatum-mediated motor symptoms, possibly at the expense of ventral striatum operations, a consequence that is only beginning to be recognized. Enhanced awareness of these different processes will translate into medication strategies that take into account those symptoms that dopamine replacement might hinder, as well as improve. Here, we show impairments in learning new stimulus associations compared with improvements in integrating varied influences related to selection. Ultimately, this knowledge will lead clinicians to survey a broader range of symptoms in determining optimal therapy based on individual patient priorities.

Predictors of in-laboratory polysomnography attendance in a cohort of patients with stroke or TIA.

STUDY OBJECTIVES: After cerebrovascular events, obstructive sleep apnea (OSA) is associated with poor functional outcomes, an increased risk of recurrent stroke, and mortality. Although the significant under-diagnosis of OSA is likely multifactorial in nature, factors associated with attendance at in-laboratory polysomnography (iPSG) are poorly understood. We evaluated demographic, lifestyle, and clinical variables that predicted attendance at iPSG in a cohort of patients with stroke and transient ischemic attack (TIA) enrolled in a clinical research study. METHODS: Demographic, lifestyle, and clinical variables were summarized for the patients who did and did not attend iPSG. Multivariable binary logistic regression analyses were performed to examine four models that thematically aggregated predictors expected to relate to attendance. Further, t-tests for continuous variables, and chi-squared tests for categorical variables, were performed to uncover differences between the two groups. RESULTS: In the 122 participants included in our analyses, mean age was 66.6 ± 15.7 (64.8% males) with only 59.0% of participants attending iPSG. Only Model 1, which investigated the impact of demographic, lifestyle, and cognitive status, was significant (χ2 = 35.31, p < 0.001). Among our variables, (1) younger age, (2) better cognition, (3) higher level of education, and (4) never-smoking status significantly predicted attendance at iPSG. CONCLUSION: By understanding the relationship between variables that significantly predict attendance, we hope our results will translate into practices that promote iPSG attendance, which may help improve outcomes in the stroke/TIA population. CLINICAL TRIALS: SLEep APnea Screening Using Mobile Ambulatory Recorders After TIA/Stroke (SLEAP SMART) (NCT02454023); https://clinicaltrials.gov/ct2/show/NCT02454023.

SLC6A3 Polymorphism Predisposes to Dopamine Overdose in Parkinson's Disease.

In Parkinson's disease (PD), cognitive functions mediated by brain regions innervated by ventral tegmental area (VTA) worsen with dopamine replacement therapy, whereas processes relying on regions innervated by the substantia nigra pars compacta (SNc) improve. The SLC6A3 gene encodes the dopamine transporter (DAT). The common 9R polymorphism produces higher DAT concentrations and consequently lower baseline dopamine than SLC6A3 wildtype. Whether SLC6A3 genotype modulates the effect of dopaminergic therapy on cognition in PD is not known. We investigated the effect of dopaminergic therapy and SLC6A3 genotype on encoding and recall of abstract images using the Aggie Figures Learning Test in PD patients. Encoding depends upon brain regions innervated by the VTA, whereas recall is mediated by widespread brain regions, a number innervated by the SNc. We found that dopaminergic therapy worsened encoding of abstract images in 9R carriers only. In contrast, dopaminergic therapy improved recall of abstract images in all PD patients, irrespective of SLC6A3 genotype. Our findings suggest that 9R-carrier PD patients are more predisposed to dopamine overdose and medication-induced impairment of cognitive functions mediated by VTA-innervated brain regions. Interestingly, PD patients without the 9R polymorphism did not show such an impairment. SLC6A3 genotype does not modulate the dopaminergic therapy-induced improvement of functions mediated by SNc-innervated regions in PD patients.

Anesthesia and neuroimaging: investigating the neural correlates of unconsciousness.

In the past 15 years, rapid technological development in the field of neuroimaging has led to a resurgence of interest in the study of consciousness. However, the neural bases of consciousness and the boundaries of unconscious processing remain poorly understood. Anesthesia combined with functional neuroimaging presents a unique approach for studying neural responses as a function of consciousness. In this review we summarize findings from functional neuroimaging studies that have used anesthetic drugs to study cognition at different levels of conscious awareness. We relate the results to those of psychophysical studies of cognition and explore their potential usefulness in interpreting clinical findings from studies of non-responsive patients.

Examining dorsal striatum in cognitive effort using Parkinson's disease and fMRI.

OBJECTIVE: Understanding cognition mediated by the striatum can clarify cognitive deficits in Parkinson's disease (PD). Previously, we claimed that dorsal striatum (DS) mediates cognitive flexibility. To refute the possibility that variation in cognitive effort confounded our observations, we reexamined our data to dissociate cognitive flexibility from effort. PD provides a model for exploring DS-mediated functions. In PD, dopamine-producing cells supplying DS are significantly degenerated. DS-mediated functions are impaired off and improved on dopamine replacement medication. Functional magnetic resonance imaging (fMRI) can confirm striatum-mediated functions. METHODS: Twenty-two PD patients, off-on dopaminergic medication, and 22 healthy age-matched controls performed a number selection task. Numerical distance between number pairs varied systematically. Selecting between two numbers that are closer versus distant in magnitude is more effortful: the symbolic distance effect. However, selecting between closer versus distant number pairs is equivalent in the need to alter attention or response strategies (i.e., cognitive flexibility). In Experiment 2, 28 healthy participants performed the same task with simultaneous measurement of brain activity with fMRI. RESULTS: The symbolic distance effect was equivalent for PD versus control participants and across medication sessions. Furthermore, symbolic distance did not correlate with DS activation using fMRI. In this dataset, we showed previously that integrating conflicting influences on decision making is (1) impaired in PD and improved by dopaminergic therapy and (2) associated with preferential DS activation using fMRI. INTERPRETATION: These findings support the notion that DS mediates cognitive flexibility specifically, not merely cognitive effort, accounting for some cognitive deficits in PD and informing treatment.

Differential effects of Parkinson's disease and dopamine replacement on memory encoding and retrieval.

Increasingly memory deficits are recognized in Parkinson's disease (PD). In PD, the dopamine-producing cells of the substantia nigra (SN) are significantly degenerated whereas those in the ventral tegmental area (VTA) are relatively spared. Dopamine-replacement medication improves cognitive processes that implicate the SN-innervated dorsal striatum but is thought to impair those that depend upon the VTA-supplied ventral striatum, limbic and prefrontal cortices. Our aim was to examine memory encoding and retrieval in PD and how they are affected by dopamine replacement. Twenty-nine PD patients performed the Rey Auditory Verbal Learning Test (RAVLT) and a non-verbal analogue, the Aggie Figures Learning Test (AFLT), both on and off dopaminergic medications. Twenty-seven, age-matched controls also performed these memory tests twice and their data were analyzed to correspond to the ON-OFF order of the PD patients to whom they were matched. We contrasted measures that emphasized with those that accentuated retrieval and investigated the effect of PD and dopamine-replacement on these processes separately. For PD patients relative to controls, encoding performance was normal in the off state and was impaired on dopaminergic medication. Retrieval was impaired off medication and improved by dopamine repletion. This pattern of findings suggests that VTA-innervated brain regions such as ventral striatum, limbic and prefrontal cortices are implicated in encoding, whereas the SN-supplied dorsal striatum mediates retrieval. Understanding this pattern of spared functions and deficits in PD, and the effect of dopamine replacement on these distinct memory processes, should prompt closer scrutiny of patients' cognitive complaints to inform titration of dopamine replacement dosages along with motor symptoms.