Looking for the keys to diurnality downstream from the circadian clock: role of melatonin in a dual-phasing rodent, Octodon degus Introduction Throughout their lifetimes, the daily light–dark transition has served for almost all species as a Zeitgeber to entrain their internal clocks, and thus accommodate their behavior to a 24-hr period to optimize the survival. Although most animals display stable nocturnal or diurnal behaviors conditioned by physiological adaptation to light or dark- ness, some rare species, including fish (European sea bass, Dicentrarchus labrax; goldfish, Carassius auratus) and mammals (mole rat, Spalax ehrenbergi; Nile grass rat, Arvicantis niloticus) [1–4] can be considered dual species because of their ability to switch from a nocturnal to a diurnal behavior and vice versa in just a few days. Octodon degus is also able to phase shift its daily activity cycle from diurnal to nocturnal [5], with predator avoidance, food availability and thermoregulatory constraints being some of the causes proposed to explain this locomotor rhythm flexibility in natural environments [6, 7]. Laboratory experiments suggest the presence in these rodents of a switch to control their phase preferences for diurnal or nocturnal activity; however, the molecular and neural mechanisms underlying diurnality remain unclear. Several lines of evidence indicate that the pacemaker in the suprachiasmatic nucleus (SCN) operates quite similarly in both nocturnal and diurnal species [8]. Therefore, it has been suggested that the mechanism determining the activity phase must be located downstream from the clock, that is, in the humoral output molecules of the SCN, i.e. proki- neticin 2, transforming growth factor alpha [8–11] and, probably, melatonin [12]. In all vertebrate species, plasma melatonin peaks during the night, regardless of whether the animal shows nocturnal or diurnal behavior. This hormone is therefore considered to be the Ôchemical expression of darknessÕ [13]. In diurnal species, the nocturnal rise in melatonin coincides with the physiological rest period (increased sleepiness, decreased body temperature (BT) and loco- motor activity, and increase in the immune responses) [14]. By contrast, in nocturnal species, the stable endog- enous signal of nocturnal melatonin is associated with wakefulness (increased locomotor activity and BT and decreased sleepiness). According to Mendelson et al. [15], endogenous melatonin should promote those behaviors associated with night, in other words, resting in diurnal and activity in nocturnal species. The existence of a dual phasing behavior within the same animal species consti- tutes a unique animal model for determining this antag- onistic response to melatonin in nocturnal and diurnal animals. Abstract: 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. P. Vivanco, V. Ortiz, M. A. Rol and J. A. Madrid Department of Physiology, Faculty of Biology, University of Murcia, Murcia, Spain Key words: diurnality, exogenous melatonin, Octodon degus, phase shift, suprachiasmatic nucleus Address reprint requests to: J. A. Madrid, Department of Physiology, Faculty of Biology, University of Murcia, 30100-Murcia, Spain. E-mail: jamadrid@um.es Received October 16, 2006; accepted December 13, 2006. J. Pineal Res. 2007; 42:280–290 Doi:10.1111/j.1600-079X.2007.00418.x Ó 2007 The Authors Journal compilation Ó 2007 Blackwell Munksgaard Journal of Pineal Research 280