Ž .
any significant difference between feeding levels, whereas King 1989 observed a lower ovulation rate in underfed females. In cyclic gilts, imposing feed restriction only during
the luteal phase seems to have no effect on the number of corpora lutea at the Ž
. subsequent ovulation Table 2 . Contrarily, feed restriction starting during the luteal
phase or at luteolysis and maintained during the follicular phase reduces the ovulation rate at next oestrus. In this case, feed restriction occurs, at least, during recruitment and
selection of the preovulatory follicles.
In reproductive sows, most researchers have determined the effects of feeding level during lactation on ovulation rate after weaning. Therefore, they have imposed the
nutritional deficit before recruitment of the preovulatory follicles. This might explain Ž
. why, in most studies, feed restriction has no clear effect on ovulation rate Table 2 .
Ž .
Likewise, King and Williams 1984a showed similar ovulation rates in primiparous sows submitted or not to protein restriction during lactation. Few studies have evaluated
Ž the effects of feed intake between weaning and oestrus s during and after recruitment
. and selection . Out of the three studies reported, only one showed an altered ovulation
Ž .
rate in the restricted group Table 2 . Therefore, it seems that, in cyclic gilts, the critical period for the ovulation rate is at
luteolysis and during the follicular phase, i.e., during recruitment and selection of the preovulatory follicles. In reproductive sows, more studies are necessary to elucidate
whether nutrition may also influence the ovulation rate by focussing experiments around Ž
. weaning day of weaning and the following days .
3. Physiological mechanisms mediating the effects of nutrition directly at the ovarian level
Besides the effects of nutrition on the release of gonadotrophins and their conse- quences on folliculogenesis, metabolic mediators may act directly at the ovarian level. In
the pig as in other species, undernutrition leads to a decrease in circulating insulin, Ž
. insulin-like growth factor-I IGF-I and leptin and a rise in circulating growth hormone
Ž . Ž
. GH
Prunier and Quesnel, 2000 . Contrarily, undernutrition does not significantly reduce systemic glycemia, a tightly controlled parameter in the pig. Nutritional deficit
Ž .
may decrease plasma concentration of binding proteins for the IGFs IGFBPs but Ž
variations are less marked than for IGF-I Thissen et al., 1994; Louveau et al.,
. unpublished data .
Nutrition may also influence the ovarian activity via other pathways. For instance, feed intake may affect the hepatic clearance rate of steroid hormones and hence the
circulating concentrations of these hormones which, themselves, may act on folliculoge- nesis.
3.1. The influence of the IGF system The stimulating role of IGF-I on folliculogenesis, alone or in synergy with go-
Ž nadotrophins, has been well demonstrated by in vitro experiments for review, see
. Adashi et al., 1985; Giudice, 1992 and by the observation of alterations in the IGF
system during follicular maturation. In sows, concentrations of IGF-I and the ratio of IGFBP-3 to BP-2 and BP-4 increase during follicular growth, whereas concentrations of
IGFBP-2 and BP-4 increase during atresia. These changes in follicular IGFBPs, which could be explained by changes in their ovarian expression andror in the proteolytic
Ž .
activity degrading them, are likely to modulate the IGF-I effects Besnard et al., 1997 . Feed restriction decreases plasma IGF-I concentrations, by ‘‘uncoupling’’ systemic
GH and IGF-I secretions. Refeeding restricted gilts increases systemic concentrations of Ž
. IGF-I without changing follicular levels
Charlton et al., 1993 . In contrast, feed restriction during lactation simultaneously decreases systemic and intrafollicular levels
Ž .
of IGF-I Quesnel et al., 1998a,b . Low concentrations of IGF-I in plasma andror in follicular fluid have been associated with reduced ovulation rate or impaired folliculoge-
Ž .
nesis Cosgrove et al., 1992; Booth et al., 1994; Quesnel et al., 1998a . Indeed, as IGF-I amplifies FSH action, low levels of IGF-I in feed-restricted sows may alter follicular
recruitment. However, further studies are needed to check the influence of nutrition on ovarian IGFBPs and proteases to clearly estimate the bioavailability of IGF-I according
to the metabolic status.
3.2. The influence of insulin In mammals, insulin stimulates in vitro uptake and utilization of nutrients, granulosa
Ž cell proliferation and differentiation, alone or in synergy with gonadotrophins for
. review, see Poretsky and Kalin, 1987 . It also prevents in vitro apoptosis in pig
Ž .
granulosa cells Rein and Schomberg, 1982; Purvis et al., 1997 . Recently, binding sites Ž
. for insulin have been detected in pig ovaries Quesnel, 1999; Fig. 2 . In addition to these
in vitro studies, numerous in vivo experiments have provided evidence of a stimulating role of insulin in folliculogenesis. For instance, increasing plasma insulin, by treatment
Ž .
or overfeeding flushing starting in the late luteal phase or the early follicular phase, Ž
. increased ovulation rate irrespective of changes in plasma LH Cox et al., 1987 . This
may be related to the ability of insulin to decrease atresia in small- and medium-sized Ž
. follicles Matamoros et al., 1990, 1991 . Gilts with streptozotocin-induced diabetes
exhibited a high percentage of atretic follicles and low levels of IGF-I in large follicles, Ž
compared with normo-glycemic gilts Meurer et al., 1991; Cox et al., 1994; Edwards et .
al., 1996 . Moreover, withdrawal of insulin therapy in diabetic gilts resulted in an increased rate of atresia, lowered IGF-I levels in follicles and altered pattern of IGFBPs,
Ž .
without decrease in gonadotrophin secretion Cox et al., 1994; Edwards et al., 1996 . Taken together, these studies suggest that insulin may increase ovulation rate by
reducing atresia through enhanced levels of intrafollicular IGF-I. However, treating primiparous sows with long-acting insulin during the last days of
Ž lactation has been shown to be without effect on ovulation rate Quesnel and Prunier,
. 1998 , whereas treatment starting the day before weaning resulted in a lower ovulation
Ž .
rate Rojkittikhun et al., 1993b . Moreover, insulin treatment during the 5 days after weaning had no effect on the number of preovulatory follicles and on atresia in
Ž .
primiparous sows Whitley et al., 1998a,b . This treatment stimulated steroidogenesis Ž
. without altering ovarian IGF-I and IGFBPs in one experiment Whitley et al., 1998a ,
but resulted in lowered levels of oestradiol and IGF-I and increased levels of small Ž
. Ž
. IGFBPs - 40 kDa in the second one Whitley et al., 1998b . Insulin administration
Ž . Ž .
Fig. 2. Localization of binding sites for insulin A and GH B in swine ovary by in situ binding and Ž
. autoradiography. Binding of radiolabelled insulin white grains is observed in granulosa and thecal cells of
Ž .
Ž .
healthy antral follicles -1 mm: upper; 2 mm: lower, left and atretic antral follicle lower, right , and also in Ž
. stromal cells. Binding of radiolabelled GH white grains is observed in the oocyte, granulosa cells and theca
Ž . Ž .
interna cells of a preantral follicle and, although less intense, in stromal cells. =70 A , =120 B .
during 5 days starting at the induction of luteolysis in gilts had no effect on the number Ž
of preovulatory follicles and on their levels of IGF-I and oestradiol Quesnel et al., .
unpublished data . In summary, despite stimulatory effects of insulin on nutrition, growth and develop-
ment of follicular cells, the consequences of high circulating levels of insulin on follicular biochemistry and ovulation rate are controversial and vary between studies
from positive to negative. Variation in the metabolic and hormonal background may explain the discrepancies between studies. Finally, direct evidence is still missing to
demonstrate that low level of insulin in feed-restricted animals is responsible for low ovulation rate.
3.3. The influence of GH In vitro studies have shown that GH stimulates mitogenesis and amplifies the effects
of FSH on the induction of LH receptors and on steroidogenesis by granulosa cells in Ž
. numerous species for review, see Booth, 1990 . Binding sites for GH have been
Ž .
recently detected in pig ovaries Fig. 2 , being very abundant in preantral and small Ž
antral, less abundant in larger follicles and undetectable in atretic follicles Quesnel, .
1999 . In vivo manipulation of GH levels has led to contradictory results. Treatment of cyclic gilts with exogenous GH suppressed oestrus or increased ovulation rate in gilts
Ž .
that exhibited a second oestrus Kirkwood et al., 1988, 1989 . Implants of GH had
Ž .
positive effects on the number of medium-sized follicles 4–6.9 mm and on the Ž
concentration of IGF-I in plasma and follicular fluid of prepubertal gilts Echternkamp .
et al., 1994 . In contrast, over expression of GH in transgenic prepubertal gilts decreased Ž
. the number of oestrogenic follicles 5 mm and their ability to synthetize oestradiol
Ž .
Guthrie et al., 1993 , and GH treatment for 5 days after weaning did not influence the Ž
. number of oestrogenic follicles Whitley et al., 1998a . Most of these treatments induced
Ž .
an increase in plasma and probably follicular IGF-I, in one hand, and, in the other hand, a rise in both plasma insulin and glucose, suggesting an insulin resistance. It may
be hypothesized that stimulatory influence of GH on ovulation rate is mediated by high IGF-I, whereas inhibitory influence is related to insulin resistance at the ovarian level.
However, the mechanisms whereby one of these ways predominates are not known.
3.4. The inÕolÕement of leptin To our knowledge, data concerning the effects of leptin at the ovarian level are
lacking in the female pig. Leptin receptor mRNA has been identified in human ovary Ž
. tissue Cioffi et al., 1996 . However, this mRNA codes for the short form of the leptin
receptor and doubts exist concerning the efficiency of this truncated form to transduce Ž
. the hormonal signal for review, see Houseknecht et al., 1998 . Leptin inhibits in vitro
secretion of steroids by granulosa or theca cells stimulated by insulin, IGF-I or Ž
gonadotrophins human ovary: Zachow and Magoffin, 1997; bovine ovary: Spicer and .
Francisco, 1997, 1998 . Such in vitro inhibitory effects of leptin at the ovarian level contrast with the in vivo and in vitro stimulatory influence of leptin on gonadotrophin
Ž .
release demonstrated in rodents for review, see McCann et al., 1998 . 3.5. The progesterone hypothesis
It has been demonstrated that hepatic portal blood flow and metabolic clearance rate of progesterone are decreased in ovariectomized gilts submitted to feed restriction
Ž .
Prime and Symonds, 1993 . Moreover, higher concentrations of progesterone have been Ž
observed in feed-restricted than in well-fed gilts at the beginning of gestation Dyck et .
al., 1980; Dyck and Kennedy, 1995; Jindal et al., 1996 . Decline in plasma progesterone Ž
at prostaglandin-induced luteolysis is lengthened in feed-restricted gilts Prunier et al., .
1999 . High levels of progesterone may impair folliculogenesis through the inhibition of LH pulsatility, but also through inhibition of aromatase activity, and thus of follicular
Ž .
maturation, directly at the ovarian level Guthrie and Bolt, 1990; Flowers et al., 1991 .
4. Conclusion
