PET-measured human dopamine synthesis capacity and receptor availability predict trading rewards and time-costs during foraging.

Foraging behavior requires weighing costs of time to decide when to leave one reward patch to search for another. Computational and animal studies suggest that striatal dopamine is key to this process; however, the specific role of dopamine in foraging behavior in humans is not well characterized. We use positron emission tomography (PET) imaging to directly measure dopamine synthesis capacity and D1 and D2/3 receptor availability in 57 healthy adults who complete a computerized foraging task. Using voxelwise data and principal component analysis to identify patterns of variation across PET measures, we show that striatal D1 and D2/3 receptor availability and a pattern of mesolimbic and anterior cingulate cortex dopamine function are important for adjusting the threshold for leaving a patch to explore, with specific sensitivity to changes in travel time. These findings suggest a key role for dopamine in trading reward benefits against temporal costs to modulate behavioral adaptions to changes in the reward environment critical for foraging.

Polymorphism in the ZNF804A Gene and Variation in D1 and D2/D3 Dopamine Receptor Availability in the Healthy Human Brain: A Dual Positron Emission Tomography Study.

BACKGROUND: The rs1344706 single nucleotide polymorphism in the ZNF804A gene has been associated with risk for psychosis in multiple genome-wide association studies, yet mechanisms underlying this association are not known. Given preclinical work suggesting an impact of ZNF804A on dopamine receptor gene transcription and clinical studies establishing dopaminergic dysfunction in patients with schizophrenia, we hypothesized that the ZNF804A risk single nucleotide polymorphism would be associated with variation in dopamine receptor availability in the human brain. METHODS: In this study, 72 healthy individuals genotyped for rs1344706 completed both [18F]fallypride and [11C]NNC-112 positron emission tomography scans to measure D2/D3 and D1 receptor availability, respectively. Genetic effects on estimates of binding potential for each ligand were tested first with canonical subject-specific striatal regions of interest analyses, followed by exploratory whole-brain voxelwise analyses to test for more localized striatal signals and for extrastriatal effects. RESULTS: Region of interest analyses revealed significantly less D2/D3 receptor availability in risk-allele homozygotes (TT) compared with non-risk allele carriers (G-allele carrier group: TG and GG) in the associative striatum and sensorimotor striatum, but no significant differences in striatal D1 receptor availability. CONCLUSIONS: These data suggest that ZNF804A genotype may be meaningfully linked to dopaminergic function in the human brain. The results also may provide information to guide future studies of ZNF804A-related mechanisms of schizophrenia risk.

Longitudinal Positron Emission Tomography of Dopamine Synthesis in Subjects with GBA1 Mutations.

Mutations in GBA1, the gene mutated in Gaucher disease, are a common genetic risk factor for Parkinson disease, although the penetrance is low. We performed [18 F]-fluorodopa positron emission tomography studies of 57 homozygous and heterozygous GBA1 mutation carriers (15 with parkinsonism) and 98 controls looking for early indications of dopamine loss using voxelwise analyses to identify group differences in striatal [18 F]-fluorodopa uptake (Ki ). Forty-eight subjects were followed longitudinally. Cross-sectional and longitudinal comparisons of Ki and Ki change found significant effects of Parkinson disease. However, at baseline and over time, striatal [18 F]-fluorodopa uptake in mutation carriers without parkinsonism did not significantly differ from controls. ANN NEUROL 2020;87:652-657.

Presynaptic Dopamine Synthesis Capacity in Schizophrenia and Striatal Blood Flow Change During Antipsychotic Treatment and Medication-Free Conditions.

Standard-of-care biological treatment of schizophrenia remains dependent upon antipsychotic medications, which demonstrate D2 receptor affinity and elicit variable, partial clinical responses via neural mechanisms that are not entirely understood. In the striatum, where D2 receptors are abundant, antipsychotic medications may affect neural function in studies of animals, healthy volunteers, and patients, yet the relevance of this to pharmacotherapeutic actions remains unresolved. In this same brain region, some individuals with schizophrenia may demonstrate phenotypes consistent with exaggerated dopaminergic signaling, including alterations in dopamine synthesis capacity; however, the hypothesis that dopamine system characteristics underlie variance in medication-induced regional blood flow changes has not been directly tested. We therefore studied a cohort of 30 individuals with schizophrenia using longitudinal, multi-session [15O]-water and [18F]-FDOPA positron emission tomography to determine striatal blood flow during active atypical antipsychotic medication treatment and after at least 3 weeks of placebo treatment, along with presynaptic dopamine synthesis capacity (ie, DOPA decarboxylase activity). Regional striatal blood flow was significantly higher during active treatment than during the placebo condition. Furthermore, medication-related increases in ventral striatal blood flow were associated with more robust amelioration of excited factor symptoms during active medication and with higher dopamine synthesis capacity. These data indicate that atypical medications enact measureable physiological alterations in limbic striatal circuitry that vary as a function of dopaminergic tone and may have relevance to aspects of therapeutic responses.

Reward-Guided Learning with and without Causal Attribution.

When an organism receives a reward, it is crucial to know which of many candidate actions caused this reward. However, recent work suggests that learning is possible even when this most fundamental assumption is not met. We used novel reward-guided learning paradigms in two fMRI studies to show that humans deploy separable learning mechanisms that operate in parallel. While behavior was dominated by precise contingent learning, it also revealed hallmarks of noncontingent learning strategies. These learning mechanisms were separable behaviorally and neurally. Lateral orbitofrontal cortex supported contingent learning and reflected contingencies between outcomes and their causal choices. Amygdala responses around reward times related to statistical patterns of learning. Time-based heuristic mechanisms were related to activity in sensorimotor corticostriatal circuitry. Our data point to the existence of several learning mechanisms in the human brain, of which only one relies on applying known rules about the causal structure of the task.

Common Variation in the DOPA Decarboxylase (DDC) Gene and Human Striatal DDC Activity In Vivo.

The synthesis of multiple amine neurotransmitters, such as dopamine, norepinephrine, serotonin, and trace amines, relies in part on DOPA decarboxylase (DDC, AADC), an enzyme that is required for normative neural operations. Because rare, loss-of-function mutations in the DDC gene result in severe enzymatic deficiency and devastating autonomic, motor, and cognitive impairment, DDC common genetic polymorphisms have been proposed as a source of more moderate, but clinically important, alterations in DDC function that may contribute to risk, course, or treatment response in complex, heritable neuropsychiatric illnesses. However, a direct link between common genetic variation in DDC and DDC activity in the living human brain has never been established. We therefore tested for this association by conducting extensive genotyping across the DDC gene in a large cohort of 120 healthy individuals, for whom DDC activity was then quantified with [(18)F]-FDOPA positron emission tomography (PET). The specific uptake constant, Ki, a measure of DDC activity, was estimated for striatal regions of interest and found to be predicted by one of five tested haplotypes, particularly in the ventral striatum. These data provide evidence for cis-acting, functional common polymorphisms in the DDC gene and support future work to determine whether such variation might meaningfully contribute to DDC-mediated neural processes relevant to neuropsychiatric illness and treatment.