Table 2 Summary of fertility and milk production figures for 45 pairs of cows that displayed increasing or decreasing
Ž .
social status based on dominance and submissiveness during the breeding period in three commercial dairy herds
Change in social status Increase
Decrease Ž
. Calving to conception days
97 143
Inseminations per conception 1.6
2.2 Ž
. Milk yield kgrday
q0.58 y1.03
Ž .
Somatic cell counts ’000rml y18
q371 Difference in lameness score
y0.21 q0.54
P - 0.05.
clearly hindering the genetic progress of one of the major domesticated species in the world. No doubt similar data can be compiled for other commercially important species.
If we are to avoid paying this price for domestication of any species, it is necessary to learn more about how animals respond to stressors, and how this affects the mechanisms
controlling reproductive efficiency.
3. Examination of control mechanisms
Studying the effects of stress on reproduction is beset with difficulties. The complex nature of some stressors in the modern farm environment simultaneously exposes
animals to several different stimuli. Furthermore, there is considerable variability between individuals in response to a given stimulus. Added to this, is the overriding
importance of the reproductive system to pass genes on to the next generation. This last issue means that animals have developed several strategies to cope with environmental
problems including alternative responses to compensate for failure of any part of the protection mechanism.
In brief, we hypothesise that there are several regulatory by which stressors regulate reproductive mechanisms. Endocrine systems appear to be an ideal way of coordinating
this regulation throughout the whole body. In order to unravel the complexity of stress-induced subfertility, it is necessary to study the reaction to stressors of repeatable
severity, firstly by examining responses to clearly defined stimuli, and then by investi- gating the influence on reproductive mechanisms.
The sheep is often used as a model for mammalian reproduction and this species provides a more manageable, less expensive, experimental animal than the cow. In
addition, mechanisms controlling normal reproductive endocrinology of the sheep are well characterised and are similar to those in the cow. Another species that might be
used for these studies, the rat, is too small for the simultaneous studies proposed. Furthermore, gonadotrophin secretion in the rat is controlled by a species-specific
reliance on circadian adrenal function that limits interpretation of observations concern- ing the effects of stress-induced adrenal activity on reproductive function.
4. Responses to specific stressful stimuli
The combined physical and psychological stimulus of 2 h transport in a vehicle Ž
. produces an immediate and constant increase in both arginine vasopressin AVP and
Ž .
corticotrophin-releasing hormone CRH concentrations in hypophyseal-portal blood of Ž
. ewes, but the adrenocorticotrophic hormone ACTH response reaches a maximum in
Ž the first hour while cortisol concentrations are highest during the second hour Fig. 1;
. Smith et al., 1997 . This suggests that the input into the hypothalamus is constant during
Ž .
this stimulus, even though the final output measured as plasma cortisol changes during the stimulus. Insulin-induced hypoglycaemia, a physiological stimulus that does not
require cognitive processing, produces a different response. After an insulin injection, Ž
. the hypothalamo–pituitary–adrenal HPA response only occurs after the blood glucose
Ž .
concentration decreases below a threshold approximately 3 mmolrl . Changes in AVP, CRH and ACTH each follow a similar time course, but eventually the secretion of AVP
Ž .
and CRH decreases while plasma glucose is still at a nadir Fig. 2 . Plasma cortisol
Fig. 1. HPA responses to 2 h transport. Mean and S.E.M. of data obtained from four ewes transported on four occasions at two week intervals.
Fig. 2. HPA responses to insulin-induced hypoglycaemia. Mean and S.E.M. of data obtained from three ewes treated on four occasions at two week intervals.
concentrations remain elevated due to the long half-life of this steroid. In this case, Ž
. during the prolonged presence of a stressful input low glucose concentration , there is a
decrease in hypothalamic AVPrCRH. To limit over-stimulation of the stress axis and its deleterious effects, there are
mechanisms to control the HPA including cortisol negative feedback effects at hypotha- lamic andror pituitary levels to restrict on-going responses to a stimulus. However, an
Ž .
opposite balancing mechanism facilitation also occurs within the HPA so that the Ž
. responses are not totally inhibited Dallman et al., 1992 .
From the above evidence comparing responses to transport and insulin, we suggest that negative feedback effects operate mainly at the pituitary level during transport and
at the hypothalamus during hypoglycaemia. However, these differences could be inherent in the pituitary responses to different concentrations and ratios of AVPrCRH
secreted in response to different stimuli.
Increasing the duration of transport or hypoglycaemia does not prolong the cortisol response at the same magnitude. Furthermore, if transport is repeated every week or at
longer intervals, there is no reduction in total cortisol response. However, if transport is Ž
repeated daily, some ewes have reduced responses after four trips Smith and Dobson, .
unpublished observation . Individual differences in response may be due to prenatal or Ž
. early life experiences
Lay et al., 1997; Liu et al., 1997 or genetic background
Ž .
Romeyer and Bouissou, 1992 . Our working hypothesis is that the time-course of a response varies at each level of
the system and depends on the nature of the stimulus. The corresponding time-course of the stress effect on the reproductive system may indicate which precise components of
Ž .
the HPA are important in the interaction with the hypothalamo–pituitary–ovarian HPO axis.
5. How do stressful stimuli affect reproduction?
