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

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.

PACEMAKER PHASE CONTROL VERSUS MASKING BY LIGHT: SETTING THE CIRCADIAN CHRONOTYPE IN DUALOCTODON DEGUS

“…To date, partially based on studies that indicate no differences between the nocturnal and diurnal pacemaker (Schwartz et al, 1983), it has been generally accepted that the switch from diurnal to nocturnal probably occurs downstream from the circadian pacemaker (Vivanco et al, 2007). This circumstance may have an adaptive value, because after this inversion the master clock may go on keeping time for a number of functions, such as synchronization with conspecifics, photoperiodic responses, anticipation to prey and predators, etc., whereas the timing of some specific behaviors, such as WRA, might be altered to cope with a particular need (e.g., to avoid overheating) without affecting others (Mrosovsky, 2003).…”

Circadian organization of the diurnal Caviomorph rodent,Octodon degus

“…Lee episodes in the degus' daily activity patterns remained stable while the wheel-induced phase-switch occurred. We also note that, although the phase of core body temperature is altered by a wheel-induced phase switch, the timing of the nightly minimum in core temperature, prior to the morning crepuscular activity bout, appears to be unaffected (Kas & Edgar 1998, 1999Vivanco et al 2007). These data, in combination with evidence that the phase-response curves of animals with a diurnal and nocturnal entrained phase are the same (Kas & Edgar 2000), allow us to confidently argue that the circadian pacemaker of degus remains stably entrained to the LD cycle regardless of the instability of the phase of its activity output.…”

“…Although many experiments have examined degu circadian rhythms while the animals were displaying unimodal, day-active behavior (e.g., Refinetti 1996;Jechura et al 2000;Hummer et al 2007;Vivanco et al 2007), other studies have focused on conditions which induced bimodal, crepuscular, or even nocturnal activity patterns (Kas & Edgar 1998, 1999Garcia-Allegue et al 1999;Ocampo-Garces et al 2005;Ocampo-Garces et al 2006;Refinetti 2006). These studies are complementary for developing an understanding of the underlying mechanisms determining phase-preference in this species.…”

“…2. A recent study examining the masking effects of melatonin administration on body temperature in degus found that melatonin produced a hypothermic response in degus, regardless of diurnal or nocturnal chronotype (Vivanco et al 2007). Therefore, at least some masking responses are definitively the same, and diurnal-typical, in both chronotypes.…”

Sleep and Wake in Rhythmic versus Arrhythmic Chronotypes of a Microphthalmic Species of African Mole Rat (Fukomys mechowii)

“…The results of the present study support the notion of elevated SWA as it was observed that SWA in NREM of the arrhythmic group was consistently higher when compared to the rhythmic group and this was seen in both light and dark periods; however, the results disagree with the notion of an increase in sleep duration as it was observed that the average duration of an episode of NREM in the arrhythmic group was significantly lower when compared to the rhythmic group. Interestingly, investigation of circadian chronotypes in the O. degus showed that differential responses to melatonin do not induce distinct circadian chronotypes [Vivanco et al, 2007], yet scheduled feeding can induce differing patterns of rhythmicity [Vivanco et al, 2010], but assessment of locomotor activity yielded the most significant distinction of circadian chronotypes [Vivanco et al, 2009]. It has been shown that TST in the O. degus amounts to 37.6 8 3.7% per 24 h [Kas and Edgar, 1998], though it is unclear whether the patterns of sleep and wake are different between circadian chronotypes of the O. degus .…”