Melatonin suppression by light in euthymic bipolar and unipolar patients

Background: Previous studies have suggested that bipolar patients are super sensitive to light suppression of melatonin and that this may be a trait ma rker for genetic vulnerability. The present study was an attempt to replica te and extend this observation. Propranolol hydrochloride effects were comp ared with light effects because of the documented influence of beta-adrener gic receptors on melatonin production. Nighttime levels of corticotropin an d cortisol were also examined as potential trait vulnerability markers. Methods: Melatonin levels in euthymic bipolar patients (n=29) were tested b efore and after 500-lux light was administered between 2 and 4 AM and on a separate night in the dark. Results were compared with those of a group of patients with unipolar depression (n=24) and with those of a group of non-p sychiatrically ill control subjects (n=50). Lithium effects and propranolol effects were tested in subgroups. Results: No group differences were seen in light suppression among bipolar patients, unipolar patients, and controls; an analysis of the whole group d id not reveal differences in propranolol effect, differences in corticotrop in or cortisol levels, or evidence for a lithium effect. Holo ever, patient s with bipolar I affective disorder showed the following: (1) significantly lower melatonin levels on the light night, at baseline and following light exposure; and (2) a later peak time for melatonin on the dark night. Conclusions: The general hypothesis of increased light sensitivity in bipol ar patients was not supported. However, melatonin secretion abnormalities w ere confirmed in the subgroup with bipolar I disorder. Further assessments of circadian rhythm disruption as a vulnerability marker in bipolar illness are indicated.