PS186. Light exposure and seasonal variation of the serotonin degrading enzyme monoamine oxidase A in the healthy human brain revealed by PET

s | 67 tissue, to identify biophysical alterations, which are represented by a magnetization transfer ratio (MTR). Whole brain voxelbased analysis was used to compare MTR across groups controlling for age and gender, thresholded at p < 0.05 (corrected) for a minimum cluster size of 132 voxels. Moreover, we conducted correlation analyses between the average regional values in affected regions with age, 17-item Hamilton Rating Scale for Depression (HRSD) and illness duration. Results: The patients exhibited significantly reduced MTR in left superior parietal lobule (SPL) and left middle occipital gyrus (MOG) compared to healthy controls (p < 0.05, corrected for multiple comparisons). These abnormalities were not correlated with age, HRSD or illness duration. Conclusions: The first-episode, drug-naïve MDD patients displayed biophysical alterations in the SPL and MOG which were involved in the attentional2 and cognitive dysfunction3. These findings in first-episode, drug-naïve MDD patients may reflect illness-related macromolecular changes close to illness onset, and thus potentially provide important new insight into the early neurobiology of depression.