How do stressful stimuli affect reproduction?
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unpublished observation .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?
In the follicular phase of a normal oestrous cycle, the correct pattern of go- Ž
. nadotrophin-releasing hormone
GnRH secretion from the hypothalamus leads to
Ž .
Ž increased pulsatile release of luteinising hormone LH from the pituitary gland Moenter
. et al., 1990 . In concert with follicle stimulating hormone, this dictates the rate of
follicular growth and oestradiol production, ultimately leading to a preovulatory LH Ž
. surge and ovulation McNeilly et al., 1991 .
In order to achieve a perfectly timed LH surge, a series of closely controlled events must occur within the hypothalamus and pituitary gland. After removal of the suppres-
Ž .
sive effects of progesterone during luteolysis, GnRH and thus LH pulses are secreted with increasing frequency, to culminate eventually in continuous secretion at the onset
Ž of the LH surge in response to the positive-feedback effects of oestradiol Evans et al.,
. 1995 .
In view of the complications incurred with repeatability, habituation and duration of stressors as already high-lighted, these aspects have to be standardised as much as
possible when examining the influence of stress responses on physiological mechanisms such as reproduction. Furthermore, the effects of more than one stressor must be
investigated in order to avoid the dangers inherent with stressor-specific artefacts. However, in spite of some differences between stress responses discussed above, there
are some quite surprisingly consistent effects on reproductive endocrinology.
From a series of experiments conducted over the past 5 years, we suggest that stressors reduce fertility by interfering with the mechanisms that regulate the precise
Ž timings of events within the follicular phase. Acute stressors
either transport or .
hypoglycaemia imposed at precisely defined times have been investigated for effects on different parts of the reproductive control mechanism.
Transport for 4 or 8 h reduces the frequency and amplitude of LH pulses especially within the first few hours in ovariectomised ewes or intact animals in the late follicular
Ž .
phase Dobson et al., 1999b; Phogat et al., 1999b . Similar effects have been observed Ž
. during insulin-induced hypoglycaemia Fig. 3 — even though glucose concentrations
decrease after insulin but increase during transport. The reduction in LH pulse frequency suggests an effect of both these stressors on GnRH pulsatile secretion mediated through
effects at the hypothalamus or higher centres in the brain; whereas effects on LH pulse
Fig. 3. Mean LH concentration, LH pulse frequency and amplitude in 10 ovariectomised ewes for 4-h periods Ž
. Ž
. Ž
. Ž
. Ž
before open bars , during filled bars and after hatched bars insulin 2 IUrkg; top panels or transport 4 h; .
Ž .
lower panels . denotes significantly different P - 0.05 from pretreatment value.
amplitude could either be mediated by the hypothalamus, or at pituitary level. Direct proof of the suppressive effects of an acute stressor on GnRH secretion has been
Ž .
provided by Battaglia et al. 1997 after endotoxin administration. In addition, there is evidence from both in vitro perifusions and in vivo experiments
to show that exogenously increased ACTH concentrations or transport reduce the Ž
amount of LH released by challenges with small doses of GnRH Phogat et al., 1997, .
1999a,b . This provides support for additional effects at pituitary level. Clearly, activation of the hypothalamus–pituitary–adrenal axis by stressors reduces
the pulsatility of GnRHrLH by actions at both the hypothalamus and pituitary gland, ultimately depriving the ovarian follicle of adequate LH support. This will lead to
reduced oestradiol production by slower growing follicles. Such a hypothesis is sup- ported by the marked decrease in oestradiol secretion observed after reducing the
frequency of exogenous LH pulses driving follicular growth in an ovarian autotransplant
Ž .
model Dobson et al., 1999a . A combination of the above effects on LH pulsatility at hypothalamic and pituitary
levels no doubt contributes to the delay and reduced magnitude of the LH surge observed after transport or insulin administration in the follicular phase just prior to the
Ž .
expected LH surge Dobson et al., 1999c; Table 3 . This effect on the LH surge control mechanism could be exerted directly via an influence of GnRH on production of its own
receptors, or indirectly by the induced reduction in oestradiol which, in turn, will alter the balance of systems controlling LH surge release. Thus, another level of interference
Ž
. at the ovary has been revealed to play a part in the multi-centred effects of stress on
reproductive control mechanisms. One of the key sites for the changes exerted by stressful stimuli appears to be the
neuronal control of GnRH secretion. We have recently carried out studies on the effects
Table 3 Time of onset of the LH surge relative to progesterone withdrawal in intact control ewes and those treated with
insulin, with or without additional naloxone or RU486 Ž
. Treatment
Onset of LH surge hours after P withdrawal
a
Ž .
Control ns9 627
b
Ž .
Insulin alone 5 IUrkg at 38 and 40 h; ns13 761
Insulinqnaloxone infusion
a
Ž .
1 mg naloxonerkgrh for 12 h starting at 37 h; ns 5 615
b
Ž .
InsulinqRU486 1 injection of 100 mg at 37 h; ns 4 784
a
Ž .
Significantly different from insulin alone group P - 0.05 .
b
Ž .
Significantly different from control group P - 0.05 .
of two neurotransmitter regulators, and the preliminary results are very interesting. Infusion of the opioid antagonist, naloxone just before insulin administration prevented
Ž .
the delay in the onset of the LH surge observed after insulin alone Table 3 . This clearly implicates opioids in the mediation of stress-induced changes in LH secretion.
Furthermore, it would appear that progesteronerglucocorticoid receptors are not in- volved in the interaction between the stress and reproductive axes because the antagonist
Ž .
RU486 was unable to reverse the insulin-induced delays in the LH surge Table 3 .