Abstract
An expanding body of evidence indicates that there are substantial seasonal fluctuations in various parameters of serotonin (5-HT) function in humans. These parameters include hypothalamic 5-HT content, CSF 5-HIAA, platelet 5-HT uptake, platelet 3H-imipramine binding, plasma L-TRP levels, and plasma melatonin levels, all of which are reviewed and compared in terms of their functional interrelationships across time. New data demonstrating seasonal fluctuations of prolactin responses to m-CPP and L-TRP challenges are presented, which reveal significant differences primarily between summer and winter. These findings strongly suggest that all future research studies of the 5-HT system can be controlled for seasonality. In addition, these data may be relevant to the understanding of seasonal variations in clinical phenomena thought to be related to 5-HT dysfunction, particularly affective illness and suicide, but also perhaps violent behavior, bulimia nervosa, obesity, and alcoholism.