Drugs that prevent mouse sleep also block light-induced locomotor suppression, circadian rhythm phase shifts and the drop in core temperature.

Exposure of mice to a brief light stimulus during their nocturnal active phase induces several simultaneous behavioral or physiological responses, including circadian rhythm phase shifts, a drop in core body temperature (Tc), suppression of locomotor activity and sleep. Each response is triggered by light, endures for a relatively fixed interval and does not require additional light for expression. The present studies address the ability of the psychostimulant drugs, methamphetamine (MA), modafinil (MOD) or caffeine (CAF), to modify the light-induced responses. Drug or vehicle (VEH) was injected at CT11 into constant dark-housed mice then exposed to 5-min 100µW/cm(2) light or no light at CT13. Controls (VEH/Light) showed approximately 60-min phase delays. In contrast, response was substantially attenuated by each drug (only 12-15min delays). Under a 12-h light:12-h dark (LD12:12) photoperiod, VEH/light-treated mice experienced a Tc drop of about 1.3°C coincident with locomotor suppression and both effects were abolished by drug pre-treatment. Each drug elevated activity during the post-injection interval, but there was also evidence for CAF-induced hypoactivity in the dark prior to the photic test stimulus. CAF acutely elevated Tc; MA acutely lowered it, but both drugs reduced Tc during the early dark (ZT12.5-ZT13). The ability of the psychostimulant drugs to block the several effects of light exposure is not the result of drug-induced hyperactivity. The results raise questions concerning the manner in which drugs, activity, sleep and Tc influence behavioral and physiological responses to light.

Looking for the keys to diurnality downstream from the circadian clock: role of melatonin in a dual-phasing rodent, Octodon degus.

Melatonin is an essential component for circadian system function, whose daily plasma secretory rhythm is driven by the suprachiasmatic nucleus (SCN), contributing to the communication of temporal messages from the central circadian clock to all cells. Melatonin secretion peaks in the dark, regardless of whether animals are diurnal or nocturnal. To date, the precise mechanisms that explain how the circadian system is configured as nocturnal or diurnal remain unknown. The present study examines mid-day and midnight melatonin plasma levels and the influence of exogenous melatonin on the circadian system phasing of Octodon degus, a diurnal rodent, which exhibits nocturnal and diurnal chronotypes when free access to a wheel is provided. Plasma levels of melatonin were determined by RIA in blood samples taken from the jugular vein at mid-light (ML) and mid-dark (MD). Melatonin (0.5 mg/kg b.wt.) was orally administered in their drinking water for 30 days, 2 hr before the onset of darkness. The results showed that plasma melatonin levels and their qualitative effects, hypothermia and improved synchronization with no modification in the 24-hr wheel running activity (WR), were similar in both nocturnal and diurnal degus. Furthermore, melatonin can be used to improve the impaired circadian rhythmicity observed in aged animals, with no rebound effect after ceasing the treatment. It is concluded that plasma melatonin levels and the differential responses to melatonin do not seem to be responsible for nocturnal and diurnal chronotypes, and thus other mechanisms upstream, within, or downstream from the SCN should be investigated.