**Abstract**

**Objectives:** Monoamine oxidase (MAO) A is the key enzyme responsible for the oxidative degradation of several biogenic amines including serotonin in the human brain \[1\]. A previous positron emission tomography (PET) study revealed elevated MAO-A levels in patients with depressive symptoms, potentially leading to lower serotonergic neurotransmission in these subjects \[2\]. Seasonal changes in mood, like blues during the dark time of the year, are common within healthy controls living in areas of high latitude \[3, 4\]. We aimed to demonstrate a light dependent seasonal difference in MAO-A distribution volume (V~T~) in a healthy study population.

**Methods:** 16 healthy subjects (mean age: 37; 14 female) underwent 2 PET scans, one in summer and one in winter, using the radioligand \[^11^C\]harmine. PET images were co-registered to structural magnetic resonance imaging scans and normalized using SPM12. Quantification of MAO-A V~T~ was carried out in PMOD 3.509 using Logan plots for 13 regions of interest \[5\]. Statistical analysis was performed in SPM12 using Pearson´s correlation between regional MAO-A V~T~ and the cumulated amount of individual exposure to global radiation (total light intensity) during the days (1--30) before the PET scans.

**Results:** We found significant negative correlations between cumulated global radiation and MAO-A V~T~ in the amygdala, anterior cingulate cortex and caudate nucleus (r=-0.561, r=-0.550 and r=-0.569; p\<0.05, highest correlation coefficient for the period of 5 to 14 days) in winter PET scans only.

**Conclusions:** These findings suggest an increase in MAO-A during winter associated with light deprivation in regions implicated in previous imaging studies on depression. Although the subjects in our study population showed no signs of depressive symptoms these results shed light on the often experienced \"seasonality\" in healthy people. The lack of a relation between MAO-A and light exposure during the summer months might be explained by a ceiling effect.

References

\[1\] Shih, J.C., Molecular basis of human MAO A and B. Neuropsychopharmacology, 1991. 4(1): p. 1--7.

\[2\] Meyer, J.H., et al., Elevated monoamine oxidase a levels in the brain: an explanation for the monoamine imbalance of major depression. Arch Gen Psychiatry, 2006. 63(11): p. 1209--16.

\[3\] Levitan, R.D., et al., A birth-season/DRD4 gene interaction predicts weight gain and obesity in women with seasonal affective disorder: A seasonal thrifty phenotype hypothesis. Neuropsychopharmacology, 2006. 31(11): p. 2498--503.

\[4\] Praschak-Rieder, N. and M. Willeit, Imaging of seasonal affective disorder and seasonality effects on serotonin and dopamine function in the human brain. Curr Top Behav Neurosci, 2012. 11: p. 149--67.

\[5\] Meyer JH, et al. Brain monoamine oxidase A binding in major depressive disorder: relationship to selective serotonin reuptake inhibitor treatment, recovery, and recurrence. Archives of general psychiatry. 2009;66(12):1304--12.
