Biomedical Engineering Reference
In-Depth Information
Fig. 2
Circadian levels of
dopamine, norepinephrine,
epinephrine and melatonin in
crossbred cattle during summer
(Aggarwal et al.
2005
)
2500
2000
1500
1000
500
0
2000
1500
1000
500
0
2000
1500
1000
500
0
120
100
80
60
40
20
0
9.30
12.30
16.30
20.30
0.30
4.30
Light
Dark
melatonin and its synchronisation with the
external light-dark cycle. Light not only entrains
the circadian rhythm but also directly suppresses
nocturnal melatonin synthesis and levels in
plasma. Suppression in melatonin synthesis is
dependent on the intensity and dose of light. In
humans, a decrease in nocturnal melatonin levels
has been observed at relatively low light intensi-
ties, similar to normal indoor lighting (Lewy
et al.
1980
; Brainard et al.
2001
) . Light is also
able to phase shift the nocturnal melatonin
rhythm. In endogenous circadian mammalian
rhythms, melatonin acts as a synchroniser
(Armstrong
1989
) . The synchronising effect is
likely to occur at a particular circadian time and
may vary in different species. Application of
exogenous melatonin directly into the SCN has
been found to advance the endogenous melatonin
peak but also increases its peak (Bothorel et al.
2002
). Some in vitro studies have also demon-
strated a local effect of melatonin on SCN meta-
bolism, electrical activity and circadian rhythm
(Cassone et al.
1988
; McArthur et al.
1991
) .
Small doses of exogenous melatonin induce a
phase-shifting effect in rats (Warren et al.
1993
)
and humans (Lewy et al.
2005
) . Melatonin may
exert its synchronising properties indirectly on
circadian clock inputs and outputs or directly on
the clock via melatonin MEL-R receptors (Song
et al.
1999
) or other binding sites (Pévet et al.
2002
). This property of melatonin helps, along
with several circadian signals, in setting circadian
