Many species on earth have evolved a self-sustaining timing system, likely to facilitate robust 24-h rhythms in physiology and behavior despite non-24-h variations in the environment. This timing system, the circadian clock, has been studied in detail over the past decades, uncovering its cellular and molecular basis (1, 2). In addition to 24-h variations, there are also numerous accounts of cyclic changes in physiology and behavior with periods much shorter than 24 h, i.e., in the ultradian range. Ultradian rhythms with periods of 2–6 h have been reported in the context of locomotion, sleep, feeding, body temperature, and serum hormones levels, in species from the fruit fly to humans (3–13). However, in sharp contrast to circadian rhythms, the biological substrate and mechanistic basis of ultradian rhythm generation has remained elusive.

As DAT is only found in DA neurons and given that selective chemogenetic stimulation of DAT-expressing midbrain neurons leads to ultradian locomotor period lengthening (20), and because of the observation of striatal, extracellular dopamine fluctuating at ultradian periods, midbrain DA neurons could plausibly act as the site of ultradian locomotor rhythm generation. However, the current data are also consistent with an ultradian rhythm generator located elsewhere, which regulates extracellular dopamine levels by, for instance, rhythmic metabolic conversion, and whose period depends on dopamine tone. However, the DA degrading enzyme catechol-O-methyltransferase (COMT), which converts DA into 3-methoxytyramine and which is found in various brain regions including the striatum, seems not to have a significant role in clearing striatal extracellular DA upon evoked dopamine overflow based on the study of COMT deficient mice (40). As the striatum has been the site of detection of ultradian DA fluctuations (20), this finding argues against extracellular DA enzymatic conversion as a means to convey ultradian oscillator output. Interestingly, lesions to the retrochiasmatic, paraventricular, and/or arcuate nucleus regions greatly perturbs or even abolishes ultradian rhythm generation in the common voles, indicating that these brain areas either participate in rhythm generation or affect oscillator output (16, 41). Because DAT-expressing dopamine neurons are also found in the arcuate nucleus region (42, 43) and along the walls of the hypothalamic third ventricle (44), it is possible that these hypothalamic DA neurons contribute to rhythm generation as part of a network of DA neuronal populations that together make up the DUO oscillator (Figure 2A). However, selective and chronic in vivo activation of midbrain DA neurons using a chemogenetic strategy (20) led to a sustained lengthening of the ultradian period, suggesting that extra-midbrain DA neurons are not critical for ultradian rhythm generation/period determination.

Of note, gonadotropin-releasing hormone (GnRH) is released in a pulsatile fashion by GnRH neuron terminals at the portal vessels of the median eminence, a structure located at the base of the arcuate nucleus (45). Interestingly, serum levels of luteinizing hormone, whose release is controlled by GnRH, have been shown to fluctuate with an ultradian period of 2–3 h in male rhesus monkeys (12, 46) and luteinizing hormone levels were shown to rise about every 6 h in the mid luteal phase of the menstrual cycle in women (47). Given that the GnRH projections originating from the preoptic area traverse the retrochiasmatic area and arcuate nucleus to reach the median eminence, it is conceivable that the hypothalamic lesions affect ultradian rhythmicity in the vole by severing GnRH neuronal processes, and thus their ability to contribute to the ultradian locomotor rhythm generation by means of their role in pulse generation. However, the LH pulse frequency has been shown to differ substantially between female [1 pulse per 1 h (48)] and male [1 pulse per 2–3 h (49, 50)] mice. Because no such sexual dimorphism is reported for the ultradian locomotor periodicity, these findings argue against a key role of the GnRH pulse generator in ultradian locomotor rhythm generation. Pulsatory secretion is also a key characteristic of the hypothalamic–pituitary–adrenal axis (HPA) (51). Corticotrophin-releasing hormone (52, 53), adrenocorticotropic hormone (54, 55), as well as the glucocorticoids (CORT) (56, 57) are all rhythmically secreted into the circulation with pulse frequencies typically in the hourly range in rat (56–58) and man (59–61). Thus, as in case of GnRH/LH, also HPA axis pulse generation may not be involved in the production of ultradian locomotor rhythm which are characterized by multi-hour periodicities.

Several decades ago, it was found that treatment with the psychostimulant methamphetamine via the drinking water leads to the expression of a second rhythmic component in addition to the daily circadian component. Because this component exhibited periods in the circadian range (62) it was dubbed the MASCO (63). As SCN lesion (62) or genetic disruption of clock function (64) does not prevent the expression of methamphetamine-dependent rhythmicity, it was concluded that the MASCO rhythm expression does not require the known circadian clock machinery (64, 65). When methamphetamine-treated SCN-lesioned rats were given timed intraperitoneal injections with the antipsychotic haloperidol, which binds to the dopamine receptor 2 found on midbrain dopamine neurons, it shifted the rhythm phase, with the directionality of the shift depending upon the relative time point (with regard to activity onset) of haloperidol injection (66). Notably, this early finding already pointed to a critical role of dopamine in the oscillator process driving these methamphetamine-induced rhythms.

The observation that methamphetamine is not only capable of gradually lengthening the ultradian locomotor period of Bmal1^−/− mice from ~4 to ≥48 h (Figure 1E) (20), but to similarly affect the ultradian oscillator in circadian intact mice, causing the 3 night-time activity peaks to transition into 2 and then 1 single peak (20) now argues that the methamphetamine-induced rhythmicity described earlier in fact represents a long period manifestation of a highly tunable ultradian oscillator, the DUO.

Studies on the SCN-intact common vole specifically in constant darkness showed that the ultradian rhythms in LA and feeding are phase-locked with the circadian clock, indicating coupling of the two oscillator systems (67). It was suggested that the ultradian rhythm is reset daily by the circadian clock and that it is not directly sensitive to light cues, and that phase resetting by light is instead mediated through the circadian timer. Further support for interaction between the SCN and ultradian timer comes from the observation of a phase-dependent change in oscillator speed, which is also known as relative coordination if the speed change does not lead to stable entrainment between two oscillatory processes (68, 69). For instance, under conditions of methamphetamine treatment: the second (>24 h) locomotor component frequently seems to “slow down” when overlapping with the “primary,” SCN-driven bout in methamphetamine-treated animals (63, 70) (see Figure 2C,c for illustration). In addition to an influence of the circadian clock on the ultradian oscillator, there is also evidence for the inverse: the emergence of the second long period (>24 h), likely DUO-driven component, in DAT^−/− mice is associated with a simultaneous period lengthening of the SCN-driven (~24 h) component (Figure 1D, DD portion of the graph). Similar observations have been made in methamphetamine-treated animals where the SCN-component delays its phase in the presence of the second (methamphetamine dependent) component (63). Thus, it seems as if both the DUO and SCN clock produce signals for their reciprocal entrainment which may or may not lead to full entrainment between both oscillators. Of note, mice with reduced expression of DAT have been reported to exhibit a lengthened circadian LA period (71). While ultradian rhythmicity has not been explicitly probed, the authors did not rule out the possibility that the observed period lengthening could be due to the action of a dysregulated DUO as proposed by Blum et al. (20).

Genetic ablation of the orexins have been reported to attenuate the ultradian amplitude in daily locomotor behavior, heart rate, and body temperature (72), suggesting a modulating role of these peptides on DUO function. As with the monoaminergic systems, orexins and the orexin-expressing neurons are part of the ascending arousal pathway (26), receiving input from the SCN via the dorsomedial hypothalamic nucleus, and projecting to the midbrain area where the DA neurons reside (73). Orexins could thus serve as mediators of circadian clock control onto the DUO.

The data presented in Blum et al. (20) suggest that a second oscillator is operative in the mammalian brain (Figure 2B) which fundamentally differs from the circadian timer due to its high, frequency tunability. Figure 2C illustrates how this feature can explain the profoundly deviating patterns in daily LA that are observable upon manipulation of the dopamine system.

When unchallenged, the DUO cycles at an ultradian period of, e.g., 2–4 h alongside the circadian timer, producing activity bouts throughout the 24-h cycle in voles or infants, but accounts only for the three night-time activity peaks in mice, likely due to strong daytime inhibition of DUO locomotor output by the SCN timer (Figure 2C,a). Methamphetamine treatment or DAT disruption lengthens the DUO period. This lengthening may reach 24 h, a period at which the DUO can cycle harmoniously with the SCN timer/LD cycle (Figure 2C,b). The relative phasing between the SCN timer/LD cycle and the DUO will depend on the entrainment capacity of the SCN timer/LD cycle and the free-running period of the DUO, i.e., the period the DUO would adopt in the absence of the SCN timer, e.g., the longer the DUO free-running period, the more delayed the phase of entrainment with the SCN timer/LD cycle will be (Figure 2C,b). If the SCN/LD cycle is incapable to fully entrain a long-period (>24 h) DUO, the DUO will free-run in the presence of the SCN/LD cycle; however, as a consequence of partial entrainment, its speed will be altered in a phase-dependent manner, resulting in relative coordination (Figure 2C,c). Further DUO period lengthening may lead to entrainment at 48 h likely because this frequency is again harmonious with the SCN timer/LD cycle and thus 24-h entrainment cues cause a sufficient phase shift to stably entrain the DUO at the 48-h frequency (Figure 2C,d).

The finding that DAT removal has a profound period lengthening effect on ultradian LA rhythms together with the discovery of synchronous fluctuations in extracellular dopamine provides a first framework for the molecular underpinnings of the oscillatory process that underlies ultradian rhythmicity. The current data indicate a central role for DA neurons in the rhythm generating process; however, it remains to be seen if rhythm generation is cell autonomous, as in case of the circadian oscillator or instead requires one or more interconnected cell ensembles (Figure 2A). Intriguingly, at least some of the LA patterns observed in rodents upon dysregulation of the dopamine system show striking similarities to the aberrant sleep:wake behavior associated with psychopathologies such as bipolar disorder (74, 75) or schizophrenia (76, 77). Given the strong concordance of LA and wakefulness for both rodents and humans (37, 78) the pattern similarities between rodent models and these human subjects indicate that the study of the DUO may have important implications in understanding the etiology of these sleep abnormalities and perhaps the psychopathologies themselves.

CB and K-FS contributed equally to the writing of this review.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.