AJP - Regulatory, Integrative and Comparative Physiology, Vol 246, Issue 6 949-R954, Copyright © 1984 by American Physiological Society
Dissociation of circadian rhythms in rats with a hypothalamic island
S. Honma, K. Honma and T. Hiroshige
Complete isolation of the medial basal hypothalamus, including the
suprachiasmatic nuclei in the isolated island, from the rest of the central
nervous system was performed in rats. The circadian rhythm of plasma
corticosterone level remained essentially intact in 5 of 16 rats with
complete islands, whereas that of spontaneous locomotor activity was
decomposed into ultradian bursts in 15 rats, resulting in a clear
dissociation of the two rhythms in four rats. One rat, whose circadian
rhythm of both variables persisted after the hypothalamic isolation, showed
a diurnal activity. The 24-h patterns of plasma corticosterone of the other
rats could be characterized as either episodic or continuously low
throughout the day. A prefeeding corticosterone peak was detected under
restricted feeding in rats with episodic fluctuations but not in those with
continuously low hormone levels. It is concluded that the hypothalamic
island includes the fundamental structures necessary for the manifestation
of the entrained circadian rhythm of plasma corticosterone in rats under ad
libitum feeding, whereas it contains only a part of that for spontaneous
locomotor activity. The coupling pathways from the circadian oscillator(s)
to these overt rhythms seem to be separate in the central nervous system,
and the circadian rhythm of plasma corticosterone is not a direct
consequence of that of locomotor activity.
