Effector mechanism mediating seasonal effects on the pituitary–gonadal axis

1. Introduction

The European wild boar is a true seasonal breeder with a breeding season occurring Ž . in early winter with piglets born once a year in late spring Mauget, 1982 . Despite being capable of producing piglets throughout the year, the domestic sow shows a Ž . reduction in fertility in late summer and early autumn Love, 1981 , at a time that the Ž European wild boar is in seasonal reproductive arrest Mauget, 1982; Claus and Weiler, . 1985 . The reduction in fertility commonly reported all around the world justifies the use of the term ‘‘seasonal breeder’’ for the domestic pig as well as for the European wild Ž . boar Love et al., 1993 . Manifestations of this seasonal infertility include a reduced Ž . farrowing rate Xue et al., 1994; Love et al., 1995; Peltoniemi et al., 1999b , delayed Ž puberty of gilts Ehnvall et al., 1981; Paterson and Pearce, 1989; Peltoniemi et al., . Ž 1999a,b , a prolonged weaning to oestrus interval Hurtgen and Leman, 1980; Benjamin- sen and Karlberg, 1981; Sterning et al., 1990; Prunier et al., 1996; Peltoniemi et al., . Ž 1999a and, possibly, a reduced litter size during late summer and early autumn Claus . and Weiler, 1985 . Although photoperiod is considered the primary environmental cue Ž . to seasonal infertility Love et al., 1993 , a whole variety of other environmental factors seem to interact with season either to exacerbate or to alleviate this infertility. These Ž environmental factors include housing group vs. individual; Hurtgen and Leman, 1980; . Ž . Ž Love et al., 1995 , feeding level Love et al., 1995 , light conditions Andersson et al., . Ž . Ž . 1998 , boar exposure Paterson et al., 1991 , group size Love et al., 1993 , ambient Ž . temperature Love, 1978; Stork, 1979; Hurtgen and Leman, 1980; Prunier et al., 1997 Ž . and interactions between females Wilson and Love, 1990 . The significance of seasonal infertility has increased in recent years as group housing of dry sows has become more Ž . common Peltoniemi et al., 1999b . In the present paper, recent advances in seasonal infertility-related research are reviewed. The focus of the present review is on the interaction of nutrition and season in the feed-restricted, early pregnant gilt and sow. A review on the effects of season, housing and nutrition on gonadotrophins is given Ž . elsewhere Peltoniemi et al., 1999c .

2. Effector mechanism mediating seasonal effects on the pituitary–gonadal axis

In seasonal breeders other than the pig, the overall mechanism by which seasonal changes in daylight are carried to the hypothalamic–gonadal axis is well-established, although some details of this mechanism still remain to be clarified at the cellular level Ž . Karsch et al., 1984; Lincoln, 1992; Malpaux et al., 1996 . These animals have a photoperiodic time-measuring system that converts information on day-length into a hormonal signal. In the sheep, for example, melatonin secretion is increased during the dark period meaning that under the influence of short days there is a longer period of high melatonin in each 24-h period. The increased melatonin secretion is recognised by the hypothalamus, which then stimulates the GnRH pulse-generator by a mechanism yet to be confirmed. Subsequently, gonadotrophin secretion from the pituitary gland occurs, which in turn results in the onset of ovarian activity and the commencement of the breeding season. Ž . Ž . Ž . Fig. 1. Pattern of light intensity lx s and mean SEM serum melatonin concentrations pgrml in European wild boars s l and in domestic gilts s B over a 48-h period. In the pig, many researchers have been unable to demonstrate the nocturnal rise in Ž melatonin concentration seen in other species Brandt et al, 1986; Peacock, 1991; . Ž . Diekman et al., 1992; Bollinger et al., 1997 . However, Paterson et al. 1992 reported a consistent nocturnal rise in melatonin concentration in pigs under varying photoperiodic Ž . circumstances and work by Klupiec et al. 1997 implied that the previous confusion had Ž . been due to methodological problems. We carried out an experiment Tast et al., 2000b , Ž where individual European wild boars were cannulated by intra-arterial catheters Tast et . al., 2000a for frequent blood sample collection. In the same experimental design, we used domestic YorkshirerLandrace crossbred gilts fitted non-surgically with intravenous Ž . jugular catheters Peacock, 1991 . Blood sampling was carried out at a 2-h interval for 48 h. The sampling was repeated in each season in Finland to establish seasonal variation in melatonin secretion for these experimental animals. As illustrated in Fig. 1 Ž . summer profiles , a consistent and repeatable melatonin profile was found in all of the experimental animals. Therefore, it is proposed that this aspect of the neuroendocrino- logical basis of seasonal reproduction in the pig is not different from that of other seasonal breeders.

3. Environmental factors involved in seasonal infertility