Dopamine Synthesis Predicts Treatment Response in Patients with Psychosis

What's the science?

One type of medication that can help patients with schizophrenia and other forms of psychosis is dopamine antagonists (medications that block the neurotransmitter dopamine), however, not all patients respond well to this medication. Whether or not a patient responds may be related to dopamine synthesis capacity, whereby patients with high levels of dopamine may respond while those with low levels of dopamine do not. This week in Molecular Psychiatry, Jauhar and colleagues studied patients experiencing a first episode of psychosis, to understand whether differences in dopamine synthesis capacity were related to future treatment response.

How did they do it?

Twenty-six patients who had recently experienced a first episode of psychosis and were diagnosed with a psychosis disorder participated, along with 14 healthy controls. Psychosis symptoms were assessed using the Positive and Negative Syndrome Scale (PANSS) before treatment, 4 weeks into treatment, and at 6 months follow-up. Response was defined as a 50% reduction in PANSS score from baseline. Participants underwent a positron emission tomography (PET) scan at baseline after injection of 18F-DOPA, in order to measure dopamine synthesis capacity in the striatum (using the ‘striatal influx constant’).

What did they find?

At baseline, the striatal influx constant in the associative striatum (a region of the striatum involved in cognitive function) was higher in responders compared to non-responders and healthy controls, indicating dopamine synthesis capacity was higher in this group. Dopamine synthesis capacity was positively correlated with percent change in PANSS score, indicating those with higher synthesis capacity were more likely to experience fewer psychosis symptoms after treatment. Higher dopamine synthesis capacity was also found in responders in two specific parts of the associative striatum: in the caudate (compared to healthy controls & non-responders) and in the putamen (compared to non-responders).

                                   Brain,  S  ervier Medical Art,  image by BrainPost,  CC BY-SA 3.0

                                   Brain, Servier Medical Art, image by BrainPost, CC BY-SA 3.0

What's the impact?

This study is the first to find that dopamine synthesis capacity (i.e. dopamine level) in the striatum is higher in individuals who respond well to treatment after a first episode of psychosis. PET imaging to measure dopamine synthesis capacity could be used to help predict who will respond well to treatment for psychosis.

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S. Jauhar et al., Determinants of treatment response in first-episode psychosis: an 18F-DOPA PET study. Molecular Psychiatry (2018). Access the original scientific publication here.

Dyskinesias in Parkinson’s disease are Caused by a Subgroup of Neurons

What's the science?

In Parkinson’s disease, dopamine neurons in the midbrain degenerate resulting in problems with body movement. A dopamine medication called levodopa can be very effective for improving symptoms, however, in some cases it causes involuntary movements called dyskinesias. We know that unwanted neural activity in brain regions such as the striatum, motor cortex and sensorimotor cortex may be involved, but the specific brain region and cells causing dyskinesias are not known. Recently in Neuron, Girasole and colleagues identify a subgroup of neurons responsible for dyskinesias.

How did they do it?

They first used a method called Targeted Recombination in Active Populations (TRAP) in transgenic (genetically modified) mice. TRAP allows certain proteins (acting as labels) to be expressed in active neurons (as opposed to inactive neurons). In mice with levodopa-induced dyskinesias, they identified neurons that were active during the dyskinesias compared to control mice. Second, they then used optogenetics: Controlling neuron activation by shining light on genetically modified neurons of interest. This allowed them to inhibit and activate these specific neurons in the mice to see if they played a causal role in dyskinesias.

What did they find?

Only neurons in the striatum were significantly more active during dyskinesias compared to control mice. When examining these neurons more closely, they found that most of the active neurons were medium spiny neurons (a specific cell type of neuron found in the striatum) that were part of the 'direct pathway', an inhibitory pathway involved in motor function that is defective in Parkinson’s disease. When these neurons were inhibited with optogenetics, the dyskinesias were reduced. Inhibiting the activity of neurons in the motor or sensorimotor cortices did not reduce dyskinesias, demonstrating a causal role for striatal neurons in producing medication-induced dyskinesias.

Images are generated by Life Science Databases(LSDB).,  Striatum , colour by BrainPost,  CC BY-SA 1.0

Images are generated by Life Science Databases(LSDB)., Striatum, colour by BrainPost, CC BY-SA 1.0

What's the impact?

This is the first study to identify the neurons within the striatum that cause dyskinesias in mice. Dyskinesias are a detrimental side effect of levodopa in Parkinson’s disease and can be debilitating to patients who experience them. Understanding which neurons cause dyskinesias brings us one step closer to finding a way to treat them.

Reach out to study author Ally Girasole on Twitter @AllyGirasole

A. E. Girasole et al., A Subpopulation of Striatal Neurons Mediates Levodopa-Induced Dyskinesia. Neuron. 97, 1–9 (2018). Access the original scientific publication here.