Biomedical Engineering Reference
In-Depth Information
day (Simonetta et al.
1991
) . This indicates that
ruminants exhibit only a weak and low-ampli-
tude intrinsic daily cortisol rhythm that is likely
to be masked by other external environmental
factors.
Increase in glucocorticoid secretion at the cir-
cadian peak depends on increased hypothalamic-
pituitary activity (HPA) and on increased
sensitivity of the adrenal cortex to ACTH
(Jacobson
2005
). The stress response is a classi-
cal example of rhythmic response. HPA feedback
loops occur at different time domains, referred as
slow that occurs in response to chronic exposure
to glucocorticoids; intermediate and fast feed-
back are observed both in response to stress and
to circadian events (Keller-Wood and Dallman
1984
). In response to the fast and intermediate
feedbacks, cortisol levels have been observed to
follow a circadian variability, in which maximum
levels are observed at 2-4 h around the time of
awakening and subsequently decrease to a mini-
mum or a nadir at 2-4 h around the time of slee-
ping. This activity is most likely driven by the
hypothalamic SCN (light) and by ventromedial
hypothalamus (food) which regulate the expres-
sion of the CRH gene. In a cycle of 24 h, there are
about 15-18 pulses of corticotrophin of various
amplitudes representing quiescent period and
an acrophase (Buckley and Schatzberg
2005
;
Veldhuis et al.
1989
) .
The role of the cortisol in awakening
response is characterised by a sharp increase of
cortisol release by about 38-75% of awakening
levels, reaching a maximum approximately
30 min after awakening (Pruessner et al.
1999
) .
The cortisol awakening response (CAR) is a
distinct phenomenon superimposing the circa-
dian rhythm of cortisol and adding a significant
incremental effect to the linear trend of increas-
ing cortisol concentrations in the early morn-
ing hours (Wilhelm et al.
2007
) . Stress is also
one of the important factors that influence
CAR. Other factors likely to have effect are
sex, health status and health behaviour (Fries
et al.
2008
). The CAR is associated with the
stress-related physical and mental symptoms,
particularly in anticipation of upcoming
demands (Fries et al.
2008
) .
7.2
Annual Rhythm of Cortisol
Long-term lighting conditions play an important
role in the seasonality displayed in behaviour and
physiology of many mammalian species
(Hastings et al.
1985
) . The cyclic hormonal
changes required for the reproduction and suc-
cessful production of offspring (Karsch et al.
1984
) and seasonal alterations in metabolism
result due to variation in environmental condi-
tions (temperature, humidity) and the availability
of feed resources and nutrients.
Seasonal changes in the activity and respon-
siveness of the hypothalamic-pituitary-adrenal
(HPA) axis have been reported in most mamma-
lian species particularly in rodents (Boswell et al.
1994
), primates (Schiml et al.
1996
) and humans
(Walker et al.
1997
) . These seasonal changes in
physiology and behaviour are associated with the
breeding season, for example, changes in the
secretion of hormones involved in the reproduc-
tion (Suttie et al.
1992
) and growth and metabolic
axes. Increased aggressive behaviour and reduced
voluntary food intake result in weight loss and an
acute rise in cortisol and testosterone levels in
male goats (Howland et al.
1985
) . HPA axis acti-
vity can be modulated by changes in reproductive
function (Verkerk and Macmillan
1997
) , meta-
bolic and growth demands and social factors.
Annual changes or photoperiodic modulations
of glucocorticoid levels have been observed in
ruminants including sheep (Brinklow and Forbes
1984
) and bulls (Leining et al.
1980
) . The vari-
ability of patterns depends on the age and sex of
animals or on the social structure of the herd
(Feher et al.
1994
) .
7.3
Altered Circadian HPA Activity
and Rhythmicity
In order to avoid the genesis of stress-associated
glucocorticoid excess, it is important that the cir-
cadian rhythm not be disrupted. Physiologically,
the increase in glucocorticoid after stressful
stimuli is beneficial to improve chances of sur-
vival. However, prolonged glucocorticoid excess
leads to myriad prejudicial effects. Changes in
