1. SCN lesion abolished ultradian rhythms at < 24 h
Lesioning the suprachiasmatic nucleus abolishes ultradian rhythms of locomotor activity in rats
Steven A. Shea1, Kun Hu1, Ruud M. Buijs2,3, Frank A.J.L. Scheer1,2
1 Division of Sleep Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02215, USA
2 Department of Hypothalamic Integration Mechanisms, Netherlands Institute for Brain Research, Amsterdam, The Netherlands
3 Department Physiology, Instituto Biomedicas, UNAM Mexico
Power spectrum
Power spectrum for each rat (3600 points, 240 hours) was obtained with a spectral resolution (15 hour-1).
Group averages for control and SCNx groups were obtained from individual spectral densities that were first normalized.
Artifacts in data caused by sporadic handling and cage management, were and replaced by the individual’s mean data (total 5 hours over the total 240 hours recording).
Introduction
In control rats:
‘Apparent’ ultradian rhythms at precise harmonic frequencies of 24h (i.e., 24/2=12h, 24/3=8h, 24/4=6h, 24/5=4.8h, 24/6=4h and 24/7=3.4h).
Negligible spectral power between these harmonics.
In SCNx rats:
All rhythms in the circadian and ultradian ranges were abolished.
Locomotor activity in rats exhibits a circadian rhythm at ~24h and ultradian rhythms at shorter time scales [1-4]. Circadian rhythms in activity are generated by the suprachiasmatic nucleus (SCN) [4-9], but the neural sites responsible for ultradian rhythms are unknown.
Goal of study
We tested whether the SCN contributes to the generation of ultradian rhythms in locomotor activity.
Results
Conclusions
The SCN is critically involved in regulating locomotor activity not only at a frequency of ~24h but also in the ultradian range (<24h) at precise harmonic frequencies of the underlying circadian rhythm.
Data and Methods
Absence of circadian rhythms in activity confirmed the complete SCN lesion
Data Acquisition
Activity was recorded for 10 days in:
Two possible control mechanisms for ultradian rhythms
7 control rats living under 24 h light/dark cycles, and again under constant darkness; and
7 rats with SCN lesions living under 24 h light/dark cycles, and again under constant darkness.
Mechanism I
Mechanism II
Movements cross the space in a cage 39x38x38 cm
Two infrared sensors
Sampling frequency: 0.8 Hz
Epoch length: 4min
SCN lesion did not alter the 24 h mean level of activity:
LD: Control = 10129; SCNx = 9314;
DD: Control = 10327; SCNx = 9620; P >0.5; ANOVA
SCN lesion altered the 24 h waveform of activity.
Intact SCN
SCN lesion
0.2mm
Ultradian rhythms in activity
i) may emanate from the SCN itself as it regulates a circadian rhythm in activity with a 24 hour shape that is not purely sinusoidal [10]. OR
ii) are generated from locations outside the SCN, with such ultradian oscillator interacting with or coupled to the SCN pacemaker [11].
SCN lesion
Rats were first anesthetized with Hypnorm and mounted in a stereotactic instrument.
Two electrode tips were then placed bilaterally in the SCN (tooth bar at +5.0 mm; angle of -6°; coordinates: -2 mm rostral to bregma; +2 mm to midline; 7.4 mm below brain surface) and heated at 80 °C for 1 min6.
SCN lesion abolished ultradian rhythms at < 24 h
Acknowledgement
The research is supported by grants from NIH/NHLBI K24 HL to S.A.S., Pickwick fellowship to F.A.J.L.S..
To avoid the inclusion of rats with incomplete lesions, the day/night rhythms of in drinking water were investigated at least one month after the surgery, and vasoactive intestinal polypeptide (VIP) and vasopressin (VP) within the SCN in perfusion fixed brains (4% paraformaldehyde) were checked.
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Average circadian waveform in a circadian period
In the DD, circadian period was estimated from rhythmicity in core body temperature for control rats; and was assigned as the average circadian period (i.e., 24.1 h) of all control rats for SCNx rats.
In the LD, circiadian period was 12 h.