Season as a Possible Confounding Factor in Reduced Brain Serotonin Turnover Following Selective Serotonin Reuptake Inhibitor Therapy—Reply

In reply

Dr Papadopoulos raises some interesting and pertinent points. In our study, we did not take into account an individual's history of exposure to sunlight.¹ Whether our depressed cohort was less exposed to sunlight compared with healthy subjects because of limited outdoor activities remains problematic. If so, this might be expected to lower their brain serotonin turnover.²

In the majority of patients (n = 6), the initial investigation was performed in spring with follow-up in summer; in 3 patients, the pattern was autumn/winter; and in the remaining patient, initial testing was in winter with the posttreatment investigation in spring. Sunlight duration pretreatment and posttreatment was not identical (mean [SD], 5.9 [3.5] hours/day vs 7.7 [3.7] hours/day; P = .26). Given this pattern of testing, and that we,² and others,^{3 - 5} have previously demonstrated a positive interaction between brain serotonin, season, and bright light, our finding of diminished brain serotonin turnover following SSRI therapy could not be attributed to the season of testing.

Correspondence: Dr Barton, Baker Heart & Diabetes Institute, St. Kilda Road Central, PO Box 6492, Melbourne, Victoria 8008, Australia (david.barton@bigpond.com).

Financial Disclosure: None reported.