Seasonal affective disorder\winter type (SAD) is characterized by recurrent depressive episodes during autumn and winter alternating with non-depressive episodes during spring and summer. Light therapy with fullspectrum, bright white light has been shown to be effective for this condition. Several hypotheses have been discussed in the literature about the pathogenesis of SAD. The most prominent includes disturbances in central monoaminergic transmission. Evidence can be inferred from studies showing a seasonal rhythm of central and peripheral serotonergic functioning which may be a predisposing factor for SAD. Some of the symptoms of SAD are believed to represent an attempt to overcome a putative deficit in brain serotonergic transmission. Moreover, 5-HT receptor challenge studies suggest altered activity at or downstream to central 5-HT receptors. Monoamine depletion studies support hypotheses about serotonergic and catecholaminergic dysfunctions in SAD and suggest that light therapy may well compensate for this underlying deficit. Further, albeit indirect, support for the importance of monoaminergic mechanisms in SAD and its involvement in the mechanism of the action of light therapy comes from studies showing antidepressant efficacy of serotonergic and noradrenergic antidepressants in the treatment of SAD. Altogether, disturbances in brain monoaminergic transmission seem to play a key role in the pathogenesis of SAD ; monoaminergic systems may also play an important role in the mechanisms of the action of light therapy.

Animal research suggests a central role of the μ-opioid receptor (MOR) system in regulating affiliative behaviors and in mediating the stress-buffering function of social contact. However, the neurochemistry of stress-related social contact seeking in humans is still poorly understood. In a randomized, double-blind, between-subjects design, healthy female volunteers (N = 80) received either 10 mg of the µ-opioid agonist morphine sulfate, or a placebo. Following a standardized psychosocial stress induction, participants engaged in a social reward task, in which the motivation to obtain skin-to-skin social touch and the hedonic reactions elicited by such touch were assessed. Morphine prevented the increase of salivary cortisol typically observed following acute stress exposure. Notably, this altered HPA axis responsivity was associated with increased negative affect in response to psychosocial stress, and with enhanced subjective wanting of highly rewarding social contact. These findings provide novel evidence on the effect of exogenous opioids administration on the reactions to psychosocial stress and point to a state-dependent regulation of social motivation.

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