Biomedical Engineering Reference
In-Depth Information
responses (Greco
2002
) . The synthesis of cortisol
in the adrenal cortex is stimulated by adrenocor-
ticotrophic hormone (ACTH) from the anterior
lobe of the pituitary gland which is in turn stimu-
lated by corticotrophin-releasing hormone (CRH)
released from the hypothalamus. Cortisol inhibits
the secretion of CRH resulting in feedback inhi-
bition of ACTH secretion. Chronic stress is likely
to break down this normal feedback system
(Greco
2002
) .
day depending on the physiological status and
conditions. The absence of circadian variation in
cortisol levels was found in sheep (Kennaway
et al.
1981
; Lincoln et al.
1982
; Simonetta et al.
1991
), Eld's deer (Ingram et al.
1999
) and cattle
(Hudson et al.
1975
; Fulkerson et al.
1980
;
Lefcourt et al.
1993
) .
Serum cortisol levels have been reported to
increase either during the night or early morning
in goats (Kokkonen et al.
2001
) , sheep (Kennaway
et al.
1981
) and cattle (Fulkerson et al.
1980
;
Thun et al.
1981
; Lefcourt et al.
1993
; Lyimo
et al.
2000
). Mean plasma level of cortisol in
cattle and buffalo has been observed to be higher
early in the morning and lower during evening
hours. However, in general the increasing trend
in cortisol levels from evening to morning was
observed with a peak in early hours (Table
3
).
Lefcourt et al. (
1993
) found weak circadian
rhythms of cortisol in lactating cows acclimated
to a rigidly controlled environment and reported
that these could easily be obscured under less
rigidly controlled conditions.
The daily rhythm of adrenocorticoid activity
has been shown to be as a result of a diurnal
rhythm of corticotrophin-releasing factor secre-
tion from the hypothalamus (Takebe et al.
1972
) .
The timing of this cycle is dependent on the
sleep-wake activity of the subject rather than on
daily environmental variation. Lack of diurnal
rhythm or weak circadian rhythm in cows and
buffaloes may be because of the dependence of
cortisol on the sleep-wake schedule. Ruminants
do not enter the deep sleep similar to that observed
in man and other animals (Balch
1955
) . Sleep
periods in the ruminant have been related to the
digestive needs of the animals since rumination
requires both time and consciousness. This was
supported by Morag (
1967
) who found that sheep
fed only with finely chopped grass entered states of
deep sleep but that when their diet was changed to
unlimited amounts of hay, they did not sleep at all.
Alterations of cortisol levels have been
observed to be associated with the feeding times
in goats (Eriksson and Teravainen
1989
) , and a
circadian rhythm of plasma cortisol concentra-
tions has been observed in pregnant ewes fed
once a day, but not in ewes fed throughout the
7.1
Daily Rhythm of Cortisol
The diurnal variation of glucocorticoid levels in
blood occurs and may be regarded as a classical
circadian rhythm similar to melatonin as the con-
centrations are low during rest, and there is a
rapid rise just before the active period in many
mammalian species, performing activities during
the day or night. In the rats, the secretion of glu-
cocorticoids has been shown to exhibit rhythmic-
ity and is observed to be under the control of the
main body clock, that is, the SCN (Moore and
Eichler
1972
). The rhythm is maintained by other
mechanisms and is not exclusively under the
control of SCN at least in some mammals as is
evidenced in suprachiasmatic region-lesioned
primates that the daily cortisol rhythm does not
disappear (Reppert et al.
1981
) . The morning rise
of cortisol in animals active during daytime is
endogenous. In humans, the rise in cortisol may
be enhanced by bright light after awakening
(Scheer and Buijs
1999
) and also by sleep depri-
vation (Leproult et al.
2001
). In pigs, exposure to
supplementary artificial light in the morning after
sunrise increased the cortisol level (Andersson
et al.
2000
), and in male Creole goats (
Capra hir-
cus
), an abrupt exposure to sunlight in the middle
of the day resulted in enhanced cortisol concen-
trations (Sergent et al.
1985
) . There was no effect
of intensity of light on blood cortisol levels in
pigs (Griffith and Minton
1992
) or in bulls
(Leining et al.
1980
). The pattern of cortisol level
in blood of ruminants is inconsistent; the levels
fluctuate episodically or peaks and troughs do not
have any specific pattern indicative of rhythmicity
and have been observed at different times of the
