Seasonal Affective Disorder: Neurobiology, Diagnosis, and Treatment

Dear Colleagues,

Different seasons require unique strategies and clear behavioural, energetic, metabolic and immunological priorities. Recent research suggests a substantial role of seasonality in a variety of human conditions, such as cardiovascular disease, autoimmune disorders, anxiety, obsessive-compulsive disorder, major depression, mania, bipolar disorder and suicide rates.

Studies in seasonal affective disorder (SAD) found that manic episodes tend to peak in spring-summer and that depressive episodes peak at early winter, both as a form of recurrent depression, which usually occurs during winter and spontaneously resolves in spring/summer. Recent classification attempts to classify SAD as a separate disorder, seasonality is a specifier for both depressive and hypo/manic episodes. SAD is usually characterized by atypical symptoms, such as fatigue, hyperphagia, craving for carbohydrate-rich foods, along with weight gain, frequently preceding the functional impairments seen afterwards.

After light reaches photosensitive receptors in the eye, it sends a message through the Retino-Hypothalamic-Tract to the Suprachiasmatic Nucleus (SCN) of the hypothalamus. The SCN activates the Paraventricular nucleus of the hypothalamus (PVN), which liberates CRF and initiates HPA axis response. Serotonin and Norepinephrine neurons have vital connections with the SCN and the PVN, respectively. Serotonin is also a precursor of melatonin, and disturbances in tryptophan cascades may alter melatonin levels, disrupting even further the circadian rhythm, but a specific role for its dysfunction in SAD is still under discussion.

Because of its usual presentation with atypical symptoms, SAD has been described as a hypocortisolemic condition. However, there is no consensus evidence of the relation between SAD and HPA axis function.

SAD provides a unique window of opportunity to study physiological changes in affective disorders. The pharmacological treatment of SAD and the unusual feature that these patients tend to show improvements in mood after light treatment is a key topic in affective disorders.

Dr. Mario F. Juruena
Guest Editor

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