Background: The dysfunctional activity of the medial prefrontal cortex has been associated with the appearance of the somatic symptom disorder, a key feature of the Parkinson's disease (PD) psychosis complex. Objectives: The objectives of this study were to investigate whether the basal contents of inhibitory γ-aminobutyric acid and excitatory glutamate plus glutamine neurotransmitter levels are changed in the medial prefrontal cortex of patients with PD with somatic symptom disorder and whether this alteration represents a marker of susceptibility of PD to somatic symptom disorder, thus representing a signature of psychosis complex of PD. Methods: Levels of the γ-aminobutyric acid and glutamate plus glutamine were investigated, at rest, with proton magnetic resonance spectroscopy. Total creatine was used as an internal reference. The study cohort included 23 patients with somatic symptom disorder plus PD, 19 patients with PD without somatic symptom disorder, 19 healthy control subjects, and 14 individuals with somatic symptom disorder who did not show other psychiatric or neurological disorders. Results: We found that, compared with patients with PD without somatic symptom disorder or healthy control individuals, patients with somatic symptom disorder, with or without PD, show increased γ-aminobutyric acid/total creatine levels in the medial prefrontal cortex. The medial prefrontal cortex contents of glutamate plus glutamine/ total creatine levels or γ-aminobutyric acid/glutamate plus glutamine were not different among groups. Conclusions: Our findings highlight a crucial pathophysiologic role played by high γ-aminobutyric acid within the medial prefrontal cortex in the production of somatic symptom disorder. This phenomenon represents a signature of psychosis complex in patients with PD.

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