Melatonin Flashcards
There are 3 theories as to how the photoperiod (day length) is measured. What is the first theory?
The first theory is that the photoperiod is measured by an internal timer. So when the day begins, a certain molecule also begins to synthesise. Increasing light causes levels of this molecule to rise and when the day ends/ there is no longer any light, the level of the molecule falls. It is thought that the level of the molecule tells the animal whether it is a long or short day.
The hourglass model assumes the gradual accumulation of a chemical product in the organism. A certain quantity of this chemical is necessary to trigger a physiological response (e.g., flowering in plants, or growth of gonads in mammals). The threshold is reached if the product is not first degraded. It may be degraded by dark and only accumulates during the light phase or it may accumulate during dark and be degraded by light. If the light (or the dark) is long enough threshold is reached and a physiological response, such as maturation of the reproductive system, is initiated. This model argues against the involvement of the circadian clock in the photoperiodic response because the hourglass lacks endogenous rhythmicity and must be reset or “turned over” by the light cycle each day
What is the second theory of measuring the photoperiod?
The second theory is called the external coincidence model, which was proposed by, Erwin Bunning, in 1936. This model proposed the existence of a circadian rhythm of photoperiodic photosensitivity (CRPP) in which most of the night-phase is sensitive to light, while the day-phase is photoinsensitive.
The model states that as the day gets longer in spring, light starts illuminating the photosensitive phase and triggers a physiological or behavioral response. As a result, in this model, light has a dual effect.
Light entrains the rhythm of photosensitivity and also acts as the stimulus (that is stimulates a photoperiodic response) if the light falls on a photosensitive phase of the rhythm of photoperiodic sensitivity. Therefore it makes sense why this model is termed an “external coincidence” model because it requires the coincidence of an external stimulus (light) with an internal rhythm of sensitivity to light.
What is the third theory of how the photoperiod is measured?
The third model is the internal coincidence model and was proposed when it was becoming apparent that multicellular organisms house more than one circadian pacemaker. In this model, the light’s only role is to entrain the circadian system. Each of the oscillators may behave differently under the influence of the light-dark cycles, and assume different phase-relationships with the entraining cycle.
Therefore it is thought that a biological/physiological response e.g. flowering is only induced under conditions that bring both rhythms in phase with each other.
Follet and Sharp, 1969?
Insects and some birds have been found to use external coincidence.
For instance, Follet and Sharp, (1969) studied Japanse quails. In their study it was found that brief light pulses interrupting the long nights (photosensitive period) of short-day conditions induced a photoperiodic response (gonads grew).
Why do animals migrate?
It is thought the benefits of migration outweigh the costs.
The costs include: increased energy expenditure, increased exposure, exhaustion and predation whilst the benefits include preventing starvation, successful reproduction and less competition for food.
How does a changing photoperiod affect activity in birds?
Zugunruhe (nocturnal restlessness) is observed in caged migratory birds during the migratory period, where there is a longer photoperiod. Furthemore, longer photoperiods are thought to cause birds to molt their feathers and fatten before they migrate.
What else do birds use for appropriate physiological responses?
In addition to photoperiod cues, some birds use an internal calendar, or a circannual clock. When these birds are kept under constant day length, they spontaneously exhibit Zugunruhe twice per year. Circannual rhythmicity is distinct from behaviors cued by day length because it does not require external stimuli such as light. However, photoperiod shapes the internal rhythm so that it accurately reflects the annual cycle.
What did Menaker and colleagues in the late 1960s show?
They showed that reproductive responses in birds in response to changing photoperiod (length of daylight) could be mediated entirely by photoreceptors outside of the retina and pineal. In his experiments he found a longer photoperiod stimulated testis growth. He also showed that removing the eyes and pineal gland didn’t stop this photoperiodic response. This reposnse was only prevented when he injected ink under the skin, which blocked light penetrating the skull.
What did Oliver and Bayle show in the late 1970s?
They placed radioluminous paint beads in the hypothalamus which induced testicular growth suggesting the existence of deep brain photoreceptors in this region.
How is the photoperiod detected in birds?
It has been found that in birds, photoperiod detection is not reliant on melatonin and is replaced by direct brain light detection, as birds have deep brain photoreceptors which can detect light and trigger reproduction.
All birds therefore need is one long summer day to trigger reproduction. It has been shown that the first wave to light occurs in the pars tuberalis of the pituitary gland and involves LH levels increasing which switches on EYA3 (a positive transcription factor) and TSH.
The second wave to light occurs in the ependymal cells lining the third ventricle of the hypothalamus (which express TSH receptors). The second wave occurs hours later and involves DIO2 levels increasing and DIO3 levels decreasing.
Using the info about the first wave and second wave in reponse to light, what occurs in short days and how does this change in long days? (birds)
In short days DIO2 is low and DIO3 is high. DIO2 converts (pro-thyroid hormone) T4 into (active thyroid hormone) T3 and DIO3 is a breakdown enzyme, so removes T4 and T3. Thus, T3 is maintained at low levels in short days.
In contrast, in the first long day, as previously explained DIO2 rapidly increases whereas DIO3 levels decrease. As a result T3 (the active thyroid hormone) increases in the hypothalamus, which is known to drive GnRH, LH and testicular growth.
Thus, it can be said that reproductive processes that occur seasonally are regulated by the switch of inactive thyroid hormone to active thyroid hormone.
How is the photoperiod detected in mammals?
The pineal gland, and its hormone melatonin, mediates photoperiodic time measurement in mammals.
Information about environmental light arrives to the brain via the lateral eyes in mammals. The retinohypothalamic tract, carries light information from melanopsin-expressing retinal ganglion cells directly to the SCN. The SCN are the primary mammalian biological clocks. From the SCN, the main pathway for photoperiod information to the pineal gland is relayed through the paraventricular nucleus of the hypothalamus.
Electrical activity from the SCN drives the rhythmic secretion of melatonin by the pineal gland. The melatonin produced can act on brain targets such as the hypothalamus, pituitary, to induce endocrine responses of increased LH (Karsch et al., 1984) and prolactin (Bartness et al., 1993), which causes hormonal enlargement of testes, and allows seasonal fertility.
How does the dark affect melatonin?
In dark, melatonin increases and is broken doen very quickly by light exposure. As a result short winter days have a longer melatonin window, longe summer days have a shorter melatonin window.
What did Karsh and Wayne (1972) show?
In their study they removed the pineal gland of sheep, which normally breed/are reproductively active on short days, where there is a long melatonin profile.
They then provided the sheep with different melatonin profiles on different L-D cycles and found that even when sheep on a long day, where they are normally reproductively inactive, giving them a winter long/short day melatonin signal drove reproduction status. Thus they demonstarted that the reproductive reponse follows the infused melatonin profile, conforming that melatonin is an essential mediator in mammals.
What is the refractory period in hamsters?
When an animals has gone into a winter state (switched off reproductive system), they can’t be left like that, so there is an automatic override mechanism, where animals become refractory to the photoperiodic signal.
It was shown in hamsters that when left in winter state for >25 weeks, physiology will ignore photoperiod and their testicles spontaneously regrow. Measuring measuring melatonin signal during this refractory period, it was found that these hamsters still have winter melatonin profile confirming that in the refractory period the short day melatonin signal is ignored/overrided.
