Correlating retinal function with melatonin secretion in subjects with an early or late circadian phase

Evaluate the diurnal variation of retinal function, as measured with the electroretinogram (ERG), in subjects with an early (morning type: M-type) or a late (evening type: E-type) circadian phase.

Subjects (n = 24) were recruited according to their scores on a Morningness-Eveningness Questionnaire assessing preferences in, e.g., bedtime, waketime, and timing of performance. ERG testing was performed twice on each subject, at 22:30 and at 08:00. Luminance-response functions were obtained in scotopic (blue flashes) and in photopic conditions (white, blue, green, and red flashes). Salivary melatonin samples were taken every half-hour from 20:30 to 00:00 and from 06:30 to 09:30.

In scotopic conditions, both groups had lower ERG amplitudes and retinal sensitivity at 08:00. In photopic conditions, the two groups showed an opposite pattern of diurnal variations. The E-types demonstrated a significant reduction in ERG amplitudes at 08:00, whereas the M-types showed an increase in amplitude at the same time. In addition, negative correlations were found between both the cone ERG and mixed rod-cone ERG and the concentration of salivary melatonin, indicating that the ERG amplitude is lowest when melatonin concentration is highest.

The reduction in scotopic ERG responses at 08:00 seen in both groups might be due to the peak of rod disc shedding that takes place, in some mammals, at around light onset. The strong correlation between the cone ERG and salivary melatonin could be attributable to a direct effect of retinal melatonin on the physiology of cones or of the circadian phase of the subjects.