Although there is evidence for a seasonal variation in serotonin neurotransmission,44 and although there seems to be a close relationship between brain serotonin and atypical depressive symptoms,41 serotonergic alterations are not specific for the pathogenesis of SAD or the antidepressant action of light. They rather seem to constitute a pathway common to depressive syndromes and their treatment in general. Inhibitors,research,lifescience,medical Circadian phase shifts More specific for SAD are the theories concerning alterations in circadian and circannual rhythms. Neurons of the hypothalamic suprachiasmatic nucleus (SCN) act as the main “zeitgeber” in the mammalian organism. Having an

intrinsic near to 24-h rhythm, they arc also known as the ‘internal

clock.“ These neurons are reset by environmental light, and they are believed to be the main determinant for the position of the circadian phase. Several body functions, such as hormonal rhythms, including nocturnal melatonin secretion, sleep, or eating behavior, are subjected to a specific Inhibitors,research,lifescience,medical circadian rhythm. Inhibitors,research,lifescience,medical The best studied marker for the position of the circadian phase is the onset of melatonin secretion by the pineal gland.45 In humans, the beginning of melatonin secretion occurs in the evening, usually between 1 and 2 h before falling asleep. I jght can shift the position of the circadian phase, and amount and direction of that shift greatly depend on the time of light exposure: light in the evening leads to a phase delay (eg, melatonin onset occurs later), morning light advances the circadian phase.2,22 Early theories on the pathogenesis of SAD held that a delay in the circadian phase was responsible for the appearance of SAD symptoms.46 Inhibitors,research,lifescience,medical Inhibitors,research,lifescience,medical Although the phasedelay hypothesis on the pathogenesis of SAD did not hold up, as there does not seem a consistent pattern of phase alterations in SAD, recent work has shown the circannual variation in circadian phase to be this website altered in patients with

SAD when compared with healthy control subjects.47 Recent work by Tcrman and colleagues showed a correlation between light-induced changes in the “phase angle” (the relationship between the circadian phase as measured by melatonin onset and, eg, sleep onset) and antidepressant response to light in SAD.5 Practical issues Sufficient and clear instructions to patients are critical for a satisfactory treatment response. Phosphatidylinositol diacylglycerol-lyase Patients should be informed that the beneficial effects of light are not enduring, ie, that a relapse is to be expected after a few days when treatment is discontinued. Although one study48 suggested a transcutaneous effect of light on melatonin secretion, these results have not been replicated.49,50 It is so far safe to state that the antidepressant effect of light is mediated by the eye. The patient therefore needs to make sure that light of sufficient intensity meets the eye.

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