A Noradrenergic Mechanism Influences the Circadian Timing System in Rats and Hamsters

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Abstract
Brainstem monoaminergic projections to the suprachiasmatic nucleus (SCN), and to the intergeniculate leaflet (IGL), appear to modulate both photic and non-photic effects on the circadian system. Recent work in this laboratory has concentrated on the role of noradrenaline in the regulation of circadian period and phase. Previously, this lab has shown that chronic administration of the alpha2 adrenergic agonist, clonidine, to rats maintained in constant light (LL) shortens free-running circadian period and promotes dissociation of rhythmicity, while acute clonidine administration to hamsters produces phase shifts similar to those observed with photic stimuli. These results suggest an interaction between clonidine and photic input on circadian rhythmicity, and so the present study was designed to examine systematically the relationship between chronic clonidine administration and photic input in both rats and hamsters. In DD and low intensity LL, clonidine did not alter free-running circadian wheel-running rhythms of rats, but under moderate to high intensity LL, clonidine significantly reduced the period-lengthening effects of LL. Chronic clonidine administration also altered several aspects of circadian phase in hamsters; phase shifts in response to light pulses of varying intensity at CT 19 were reduced; steady-state entrainment phase under a 24-h light-dark cycle (LD 14:10)was delayed; and synchronization to a 23-h light-dark cycle (LD 13:10) was impaired. Clonidine appeared to have little effect on free-running period of hamsters, but a trend towards dissociation of rhythmicity under LL was observed. These effects may reflect an action of clonidine at the photic input pathways to the circadian system, or directly at the circadian pacemaker, since alpha 2 adrenoceptors have been localized both in the suprachiasmatic nucleus (SCN) and in several of its projection areas. As both clinical and experimental studies suggest that clonidine may have depressogenic properties, chronic administration of clonidine to rodents may provide an animal model of the alterations in circadian rhythmicity seen in human depression.

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