![]() Unlike AMPT, evidence suggests that it preferentially affects the release of DA 29 compared with DA receptor antagonists, APTD influences presynaptic release and, consequently, the activation of all DA receptor subtypes. 29, 30 The APTD method has a number of advantages. The purpose of the present study was to investigate the roles of DA and light on mood and motivational states using the acute phenylalanine/tyrosine depletion (APTD) method. 27 Finally, in a preclinical model, rodents kept in constant darkness showed increased monoamine cell body apoptosis - changes that were associated with behavioural alterations indicative of a depressed state. Moreover, in patients with remitted SAD, catecholamine depletion with α-methyl-para-tyrosine (AMPT) reversed the therapeutic efficacy of bright light. 25 Patients with symptomatic SAD also show evidence of altered DA system function, and, compared with healthy controls, 26 striatal DA transporter (DAT) availability is reduced. ![]() 24 Consistent with this, DA D 2/D 3 receptor availability was greater in participants whose measurements were preceded by 30 days with more sunshine than in those tested after less sunshine. For example, acute bright light exposure (7000 lux for 10 min) increased striatal blood flow in healthy volunteers, 23 and in a case series of patients with Parkinson disease, light exposure (1000–1500 lux, 1 hour daily for 2 weeks) improved mood, social activity and motor function and, in some cases, reduced DA replacement therapy dosages by 13%–100%. 19 – 22 However, evidence that light influences DA transmission remains limited and largely indirect. The SAD symptom cluster of lethargy, psychomotor retardation, weight gain and low motivation 18 suggest a hypoactive dopamine (DA) system. Decreasing 5-HT synthesis with acute tryptophan depletion (ATD) temporarily reverses the effects of phototherapy, 15, 16 while in the converse experiment, bright light exposure protects against ATD-induced mood lowering in healthy women with mild seasonal changes to mood and behaviour. A small but more consistent literature implicates serotonin (5-HT). 10 Cortisol is also implicated, but the effects have been inconsistent, as extended bright light exposure has been reported to decrease, 11 increase 12, 13 and to have no effect 14 on plasma cortisol levels in humans. For example, altered diurnal rhythms may be involved, and administration of melatonin at certain times of day can shift rhythms and improve symptoms in patients with SAD. Although the mechanism of action behind the therapeutic effect of bright light remains poorly understood, some potential candidates contributing to this effect have been identified. Bright light can alleviate low mood in patients with seasonal affective disorder (SAD), 1 nonseasonal major depressive disorder, 2, 3 antepartum depression, 4 eating disorders 5, 6 and those with subsyndromal winter mood disturbances 7, 8 (for contrasting results, see Rosenthal and colleagues 9).
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