Use of GnRH to induce oestrus in seasonally anoestrous mares.

Abstract: THE majority of non-pregnant mares pass through winter and early spring in reproductive quiescence or anoestrus. True anoestrus begins when day length decreases to less than 10.5 h (Palmer, Driancourt and Ortavant 1982) and involves gradual change in behaviour, hormone secretion patterns and follicular dynamics. The length of anoestrus differs according to the breed, latitude and management and may extend to seven or eight months (Ginther 1974). It sometimes persists well into the normal breeding season, especially in mares in poor physical condition (Allen 1977). In mares, as in other mammals, it is not the actual concentra- tions of gonadotrophins in the blood at any particular phase of the oestrous cycle that are significant but rather the frequency and amplitude of the pulsatile releases of luteinising and folli- cular stimulating hormones (LH and FSH). Research on gonadotrophin releasing hormone (GnRH) has indicated that: pulsatile administration of GnRH causes an increase in blood levels of both LH and FSH (Evans and Irvine, 1977); the relative pulses in LH and FSH are related to ovarian activity and the stage of follicular growth (Thibier 1981); pulsatile administration of GnRH during the period of lutein- isation may induce the development of an inadequate corpus luteum (Fraser 1983); continuous infusion of GnRH initially causes a large increase in blood concentrations of FSH and LH followed by a steady decline to basal levels of both hormones (Fraser 1983); administration of high doses of long acting GnRH may suppress production and secretion of FSH and LH (Fraser 1983); LH and FSH secretion is suppressed by the administration of anti-CnRH. It is clear from the above findings that the biological properties, pharmacological effects and therapeutic uses of GnRH are multiple and apparently contradictory. Induction of gonadotrophin secretion in mammals, with hypothalamic inactivity, by means of long term intermittent administration of GnRH, has practical possibilities for inducing oestrus and ovulation in anoestrus animals (Minoia, Lacalandra and Zarrilli 1983a; Minoia, Zicarelli, Lacalandra and Mastronardi 1984a) and in birds in the non egg-laying period (Minoia et al 1983b, 1984b, c). Most experiments in this area have involved the administra- tion of GnRH every 2 h by repeated subcutaneous or intra- venous (iv) injection or by the use of a subcutaneous hormonal pump. Sheep have been the most commonly used experimental species (McNatty el a/ 1982, 1984; Wright, Geytenbeek, Clarke and Findlay 1983). Follicular development and ovulation have frequently been achieved although the lifespan of the resulting corpus luteum has usually been abnormal. Considerable research has been carried out into the effects of single or repeated injections of GnRH, or its synthetic analogues given to anoestrous mares (Arbeiter 1973; Heinze and Klug 1975; Evans and Irvine 1976, 1977, 1979; Bow, Waelchli-Suter and Vasey 1982; Alexander and Irvine 1984). The present report concerns a clinical trial undertaken in southern Italy during 1983 and 1984 that involved the daily administration of three injections of GnRH to seasonally anoestrous mares. Part of the results of this study have been published previously (Mastronardi, Minoia and Zarrilli 1984; Mastronardi and Minoia 1985.