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One pacemaker in seasonal affective disorder
Medical Hypotheses (1998) 51,297-298 © Harcourt Brace & Co. Ltd 1998
One pacemaker in seasonal affective disorder T. PARTONEN Department of Psychiatry, University of Helsinki, Tukholmankatu 8 C, FIN-O0290 Helsinki, Finland (Phone: +358 9 471 3506; Fax: +356 9 471 5478)
Abstract - - Patients with winter seasonal affective disorder (SAD) often have abnormalities in the resetting of their circadian clock. The circadian cycle appears to be variable across days among these patients, deviating from the exact 24 hours, and peaking at unstable times. The findings suggest that the mean free-running circadian period is longer in winter SAD patients compared to healthy subjects. It is hypothesized that there would be a singular circadian pacemaker in the patients against the two pacemakers in healthy subjects.
Introduction
Discussion
Winter seasonal affective disorder (SAD) is frequently characterized by abnormalities in the synchronization of circadian variation. The phase is delayed and circadian oscillation may also be suppressed in some patients with winter SAD. According to the current chronobiological hypotheses, bright light treatment is suggested to work by restoring either the appropriate timing or amplitude of circadian rhythms. The circadian cycle appears to be more variable across days among these patients, deviating more from 24 hours, and peaking at more unstable times than in healthy subjects (1). The weakened entrainment is thought to be a consequence of less consistent resetting of the circadian clock by light during the decreasing photoperiod. In winter SAD patients, the perceived photoperiod is in fact 4-5 hours longer in summer than in winter, with a later perceived dusk contributing more to the lengthening of perceived photoperiod than an earlier perceived dawn (2).
The basic period of the overt circadian rhythm is determined by cells of the suprachiasmatic region (3). After transplantation of the suprachiasmatic nucleus (SCN), the restored rhythms exhibit the period of the donor genotype, regardless of the direction of the transplant or the genotype of the host. There is also evidence that each of the right and left SCN alone is a competent circadian pacemaker in hamsters (4). However, the hamsters that receive both of the SCNs have a longer free-running period of restored rhythms than those that receive one of the two. I hypothesize that, since the circadian cycle deviates more from 24 hours, the mean free-running circadian period is longer in winter SAD patients compared to healthy subjects. Further, I hypothesize that there is a single circadian pacemaker in winter SAD patients against the two integrated pacemakers present in healthy subjects. Because of this unicellularity, with or without anatomical asymmetry, more effort is needed to be able to keep pace with the
Received 17 April 1997 Accepted 13 May 1997
297
298
environmental time-givers, pressuring precise resynchronization of the circadian pacemaker. Due to the weakened capability of resetting, the circadian cycle tends to peak at unstable times among winter SAD patients. References 1. Teicher M H, Glod C A, Magnus E et al. Circadian rest-activity
MEDICAL HYPOTHESES
disturbances in seasonal affective disorder. Arch Gen Psychiatr 1997; 54: 124-130. 2. Eastman C I. Natural summer and winter sunlight exposure patterns in seasonal affective disorder. Physiol Behav 1990; 48: 611-616. 3. Ralph M R, Foster R G, Davis F C, Menaker M. Transplanted suprachiasmatic nucleus determines circadian period. Science 1990; 247: 975-978. 4. Davis F C, Viswanathan N. The effect of transplanting one or two suprachiasmatic nuclei on the period of the restored rhythm. J Biol Rhythms 1996; 11: 291-301.
One pacemaker in seasonal affective disorder T. PARTONEN Department of Psychiatry, University of Helsinki, Tukholmankatu 8 C, FIN-O0290 Helsinki, Finland (Phone: +358 9 471 3506; Fax: +356 9 471 5478)
Abstract - - Patients with winter seasonal affective disorder (SAD) often have abnormalities in the resetting of their circadian clock. The circadian cycle appears to be variable across days among these patients, deviating from the exact 24 hours, and peaking at unstable times. The findings suggest that the mean free-running circadian period is longer in winter SAD patients compared to healthy subjects. It is hypothesized that there would be a singular circadian pacemaker in the patients against the two pacemakers in healthy subjects.
Introduction
Discussion
Winter seasonal affective disorder (SAD) is frequently characterized by abnormalities in the synchronization of circadian variation. The phase is delayed and circadian oscillation may also be suppressed in some patients with winter SAD. According to the current chronobiological hypotheses, bright light treatment is suggested to work by restoring either the appropriate timing or amplitude of circadian rhythms. The circadian cycle appears to be more variable across days among these patients, deviating more from 24 hours, and peaking at more unstable times than in healthy subjects (1). The weakened entrainment is thought to be a consequence of less consistent resetting of the circadian clock by light during the decreasing photoperiod. In winter SAD patients, the perceived photoperiod is in fact 4-5 hours longer in summer than in winter, with a later perceived dusk contributing more to the lengthening of perceived photoperiod than an earlier perceived dawn (2).
The basic period of the overt circadian rhythm is determined by cells of the suprachiasmatic region (3). After transplantation of the suprachiasmatic nucleus (SCN), the restored rhythms exhibit the period of the donor genotype, regardless of the direction of the transplant or the genotype of the host. There is also evidence that each of the right and left SCN alone is a competent circadian pacemaker in hamsters (4). However, the hamsters that receive both of the SCNs have a longer free-running period of restored rhythms than those that receive one of the two. I hypothesize that, since the circadian cycle deviates more from 24 hours, the mean free-running circadian period is longer in winter SAD patients compared to healthy subjects. Further, I hypothesize that there is a single circadian pacemaker in winter SAD patients against the two integrated pacemakers present in healthy subjects. Because of this unicellularity, with or without anatomical asymmetry, more effort is needed to be able to keep pace with the
Received 17 April 1997 Accepted 13 May 1997
297
298
environmental time-givers, pressuring precise resynchronization of the circadian pacemaker. Due to the weakened capability of resetting, the circadian cycle tends to peak at unstable times among winter SAD patients. References 1. Teicher M H, Glod C A, Magnus E et al. Circadian rest-activity
MEDICAL HYPOTHESES
disturbances in seasonal affective disorder. Arch Gen Psychiatr 1997; 54: 124-130. 2. Eastman C I. Natural summer and winter sunlight exposure patterns in seasonal affective disorder. Physiol Behav 1990; 48: 611-616. 3. Ralph M R, Foster R G, Davis F C, Menaker M. Transplanted suprachiasmatic nucleus determines circadian period. Science 1990; 247: 975-978. 4. Davis F C, Viswanathan N. The effect of transplanting one or two suprachiasmatic nuclei on the period of the restored rhythm. J Biol Rhythms 1996; 11: 291-301.