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The E and M Model is used in chronobiology, which is the study of circadian rhythms. Circadian rhythms are biological processes that oscillate with a stable phase and are synced to solar time. They are driven by a circadian clock. E and M is a type of dual oscillator model, which is the presence of two, coupled oscillator models. The model is composed of two components, the morning (M) and evening (E) oscillators. These oscillators are mutually coupled and control rhythm in a diurnal organism. The M oscillator tracks dawn and the E oscillator tracks dusk, with their outputs peaking at dawn and dusk respectively in an anti-phasic relationship, meaning that they have opposite phases.

Discovery
In 1966, Jürgen Aschoff, a German chronobiologist, was the first to observe that there were two activity bouts per day, one in the morning and one in the evening, in some animals. These activity bouts are defined by the anticipation of the lights turning on or off. In 1976, Colin Pittendrigh and Serge Daan, two chronobiologists, first proposed a dual-oscillator model specifically for nocturnal rodents as the mechanism for these E and M bouts of activity. The model assumed two separate oscillators (Evening and Morning) that have opposite dependence on light intensity. They found that the M oscillator is synchronized to dawn and experiences acceleration from light, meaning it takes less and less time for one cycle to occur. The E oscillator, on the other hand, is synchronized to dusk and experiences deceleration from light, meaning it takes more and more time for one cycle to occur. Pittendrigh and Daan postulated that this model had an enhanced ability to adjust the circadian rhythm to the season and changes in day length.

In 2004, Brigette Grima and Dan Stoleru, two chronobiologist researchers, renewed interest in the E and M model among the scientific community by publishing separate research papers on their work with the model in Drosophila melangoster (fruit flies). They separately investigated E and M activity peaks in fruit flies using different gene expression manipulations. They published two separate papers on their individual work, both demonstrating that two separate circadian neuron groups control the evening and morning peaks of activity in Drosophila melanogaster. Stoleru’s methods involved ablation of, or knocking-out, either the dorsal lateral neurons (LNd) or ventral lateral neurons (LNv) and observing the results. Grima used a non-ablative method involving the mutant per0, which eliminates the period of the circadian rhythm in the flies, and then reinstated the per expression in only LNd or LNv. The results were observed and were the same as Stoleru’s; these results are discussed further below.

In 2007, Stoleru performed further research on E and M in Drosophila melanogaster which involved discovering that the M cells dominate the circadian rhythm on short days and the E cells dominate the rhythm on long days; this is what keeps the circadian rhythm in animals running on short and long days, respectively. The main implications of this work are that it revealed the mechanism of how animals are able to adapt to environmental changes such as the changing day lengths that come with the change in seasons.

Dual oscillators in different organisms
E and M cells possess different capabilities to control behavior and respond to light through either accelerating or decelerating their individual, internal circadian clock speeds. The difference in the phase angle of entrainment, or the relationship between the timing of the biological clock and the timing of the external time cue, of each cell varies depending on the amount of light in the environment. Greater amounts of light lead to a greater phase angle of entrainment. Based on the amount of light, pigment dispersing factor (PDF), a neuropeptide, controls the acceleration and deceleration of the speed of M and E cells, respectively. Furthermore, the coupling of the E and M oscillators increases as the phase angle of entrainment decreases, displaying an inverse relationship between the duration of light and the coupling of the two oscillators. This phenomena also shows the importance of E and M cells for adapting the activity of an organism different photoperiods.

Mammals
Evidence for the dual oscillator model originated in mammals with the discovery of the splitting phenomena in nocturnal rodents. However, research focused on determining distinct morning and evening circadian oscillators is still ongoing. Evidence suggests that the suprachiasmatic nucleus (SCN) is the primary circadian pacemaker in mammals. Through splicing and cutting the SCN in different ways, separate morning and evening peaks in activity have been revealed. Also, two or more types of cell populations in the SCN that respond differently to short and long photoperiods have been determined. However, it still has not been determined what mechanisms these cells follow to track dawn and dusk.

Other hypotheses for the existence of E and M oscillators in mammals involve single cell dual-oscillator models. Within a mammalian cell, there exists a redundant copy of several clock genes (per1 and cry1; per2 and cry2). The hypothesis states that each set of these genes would be sufficient to produce endogenous oscillation in cell function; however, each set of genes responds differently to light and temporal cues. The per1/cry1 oscillator (morning oscillator) is energized by light and tracks dawn. The per2/cry2 oscillator is energized by darkness and tracks dusk.

Flies
Circadian oscillators in Drosophila melanogaster have been isolated to approximately 150 clock neurons in the fly brain. All of these neurons contain the transcription-translation feedback loop (TTFL) responsible for the daily oscillation of various biological processes. Distinct groups of clock neurons exist based on location in the Drosophila brain. The primary output pathways from these clock neurons lead to the Accessory Medulla, which acts as the central circadian pacemaker in insects, and to the hormonal center in the dorsal brain. Cells within the lateral neuron group act as the morning and evening oscillators, but contributions from the dorsal neuron group also affect morning and evening activity. Furthermore, the definition of morning and evening oscillator becomes increasingly more vague as more research is performed, with several lateral neurons acting as both morning and evening oscillators.

Clock neurons in the fly brain entrain to external light stimuli via a Cryptochrome (CRY) response pathway. In response to light exposure, CRY binds to the Timeless (TIM) protein, ultimately leading to the decomposition of TIM within the clock neuron and delaying the internal circadian oscillation of period (PER) and TIM proteins, meaning their onset and offset of activity occur later in the day.

Lateral neurons
The lateral neurons (LN) are the Drosophila 's main clock neurons. When circadian oscillation is inhibited in all clock neurons besides the LN, flies still maintain rhythmic activity in constant conditions. When this same inhibition is performed in LN cells, however, flies are not able to produce rhythmic activity, proving that LN cells are necessary for synchronized circadian rhythms in flies. LN neurons are split into three further subgroups (LNd, s-LNv, 1-LNv) which each perform different functions. Ablation of the s-LNv cells causes a loss in the morning peak of fly activity, providing evidence for this cell group to be the morning oscillator. Meanwhile, ablation of the LNd cells causes a loss in the evening peak of fly activity, which provides evidence for this cell group to be the evening oscillator. Furthermore, s-LNv cell outputs are inhibited by light whereas LNd cell outputs excited by light. This characteristic shows that these two cell types dominate circadian control under opposite conditions, providing further evidence for distinct morning and evening cells.

S-LNv cells play another vital role in synchronizing the circadian clock within flies. The majority of these cells produce pigment dispersing factor (pdf), a neurotransmitter that helps synchronize the various clock neurons in the fly brain. This creates a more significant role for s-LNv cells within the clock network because they are required to synchronize different clock neurons in the absence of light.

Dorsal neurons
Dorsal neurons (DN) are several other groups of clock neurons within the fly brain. While DN cells do contribute to circadian control in light-dark cycles, they are not sufficient to produce rhythmicity in constant conditions. Therefore, these cells are not the primary morning or evening circadian clocks with the fly brain. Research has shown, however, that several subsets of DN cells can contribute to morning and evening peaks in activity.

When in DD and manipulated to overexpress the shaggy (sgg) gene, the Drosophila ortholog of GSK3, M cells influenced the rate and rhythm of transcription of TIM in E cells. In LL conditions, drosophila overexpressing sgg in E cells remained rhythmic, while M cells became arrhythmic, like their WT counterparts. Drosophila’s clock is thought to consist of CRY-positive, s-LNv M cells, which cannot alone drive rhythmicity in LL, and the CRY-negative E cells that can drive rhythmicity in LL, but not DD, alone.

Other organisms
There is no substantial evidence for distinct morning and evening oscillator cells in plants, fungi, or cyanobacteria. However, several single cell dual-oscillator models exist, providing an alternative model for how these cells respond to changes in light stimuli.

Future research
Stoleru’s research in 2007 provided insights into the role the circadian clock plays in Seasonal Affective Disorder (SAD) and other related mood disorders that are responsive to light therapy, however, extensive research is lacking in this area. Many studies have also shown that the fruit fly brain is more complicated than the E and M model makes it out to be, as it consists of more than just the two clocks. It is also not yet clear how light and temperature affect the E and M oscillators. Recent research involving knocking out NPF-positive cells in Drosophila melanogaster has found opposing effects on the E oscillator, an advancement of the peak, meaning the onset and offset of activity occurs earlier in the day, but prolonging of the free-running period, which is difficult to make sense of given the simple E and M dual oscillator model.

Evidence against the E and M model
The E and M oscillator model is one of the most prominent models in chronobiology, however it does not work in all instances. In 2009, experiments on Drosophila tested period gene expression when restricted to the 5th s-LNv cells and 3 LNd cells, which were all thought to belong to the E oscillator. Under low light conditions, the Drosophila still expressed normal bimodal activity patterns only differing in the phase of the E and M peaks. 2 LNd advanced upon moonlight, acting as M, and 5th s-LNv and 1 LNd delayed upon moonlight, acting as E. This suggests that M and E characteristics are flexible and should not be interpreted strictly.