“Summertime sadness”: Striatal dopamine binding decreases during warmer seasons in patients with severe depression

• No overall seasonal difference in striatal DAT between fall–winter and spring–summer • Severe depression (HAMD > 25) shows lower left putamen DAT, accentuated in spring–summer • Psychomotor retardation (DRRS > 18) links to reduced left putamen DAT, extending pan-striatal in spring–summer • Suicidal ideation associates with decreased bilateral putamen DAT • Anhedonia severity is higher in fall–winter : Seasonal changes, particularly increased daylight exposure, are known to influence dopamine transporter (DAT) availability, potentially affecting mood disorders such as major depressive disorder (MDD). This study aimed to evaluate seasonal variations in the striatum using ¹²³I-FP-CIT SPECT in patients with MDD, and examine associations with specific psychopathological symptoms. : In this retrospective study, DAT SPECT scans from 85 patients with MDD were analyzed according to the season of imaging—fall-winter (FW) or spring-summer (SS). Psychometric assessments included the Hamilton Depression Rating Scale (HAMD), Hamilton Anxiety Rating Scale (HAMA), Snaith-Hamilton Pleasure Scale (SHAPS), and Depression Retardation Rating Scale (DRRS). : No overall differences in DAT availability were observed between FW and SS. However, anhedonia levels were higher in FW (p = 0.050). Patients with severe depression (HAMD ≥ 25) showed lower DAT availability in the left putamen, especially during SS (p = 0.014). Patients with marked psychomotor retardation (DRRS ≥ 18) exhibited reduced DAT availability in the left putamen (p = 0.002), with further reductions across all striatal regions during SS. Patients with suicidal ideation showed decreased DAT in the right (p = 0.029) and left putamen (p = 0.015). A negative correlation was found between DRRS scores and left putamen DAT availability (p = 0.034). : Reduced DAT availability is associated with key depressive symptoms, notably psychomotor retardation and suicidal ideation. Seasonal effects, especially in the SS period, may exacerbate dopaminergic dysregulation. These findings support integrating seasonal and neurobiological factors in the assessment and management of severe MDD.