Author: Webb, Ian Craig
In Syrian hamsters, circadian rhythms can be phase shifted by light at night or by behavioural arousal during the day (usual sleep period). Previous work in this lab has defined arousal procedures that have differential clock resetting effects; arousal stimulated by running in a novel wheel (WC) or by gentle handling (SD) can induce large phase advance shifts, whereas arousal by physical restraint (SLR), by confinement to a platform over water, or by caffeine administration have no phase shifting effects. Pharmacological and immunocytochemical experiments were conducted to identify the neural basis for the differential effects of arousal procedures on circadian rhythms. A preliminary experiment evaluated whether modafinil, a pharmaceutical that produces arousal without stimulating activity or anxiety, can induce or modulate phase shifts. This agent, like caffeine, did not induce phase shifts, and thus represents an additional tool to identify neural correlates of clock resetting. Two extensive mapping experiments were then conducted, using c-fos and double-labeling immunocytochemistry for hypocretin or tryptophan hydroxylase. Double-labelling for c-fos and hypocretin revealed that SD and WC, both of which induce shifts, and SLR, which does not, all were associated with significant c-fos expression in hypocretin cells. Thus, activation of the hypocretin system is not sufficient to induce non-photic shifts. All three procedures also increased c-fos expression in the interge niculate leaflet, revealing no simple relationship between activation of this structure and circadian clock resetting. Examination of the serotonergic raphe nuclei and the noradrenergic locus coeruleus (LC) indicated that both WC and SD failed to increase c-fos expression in the serotonergic and non-serotonergic cells in these areas. Stressful restraint, however, significantly elevated the expression of c-fos in rostral DRN serotonin neurons and in the LC. Therefore, it appears that activation of these areas is neither necessary nor sufficient for non-photic shifts. Instead, the current results suggest that the rostral DRN and the LC may be involved in a stress-induced inhibition of phase shifts to arousal. Further behavioural and pharmacological experiments indicated that, if stress blocks phase shifts to arousal, it likely does so at the input stage and via a mechanism separate from glucocorticoid receptor activation.
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