336

EQUINE VETERINARY JOURNAL Eqrririe i'ef J . (1992) 24 (5)336-337

Readiness for birth; another piece of the puzzle In this issue Rossdale et a/. (p 347) report on treatments applied in urero to the foetus by an unconventional technique. They utilised transabdominal ultrasound to guide the placement of a long needle by which they were able to deliver subcutaneous or intramuscular injections of corticotrophin-releasing hormone (CRH), ACTH (Depot Synacthen) or the synthetic glucocorticoid, betamethasone. The ultimate goal was to test whether the foetal hypothalamic-pituitary-adrenal axis is responsible for the rise in maternal plasma progestagen concentrations near term. This minimally invasive approach, requiring simple sedation of the mare, allowed repeated treatment of the foetus (daily for 3 days in this study). The procedure had a variable effect on the subsequent length of gestation, curtailing it somewhat in half the number treated. Nevertheless, all but 3 of the foals were delivered live and viable. The simplicity of this approach, at least compared to intravascular catheterisation of the foetus, should give encouragement to other research workers as the technique is applicable to a range of studies, in addition to foetal/maternal endocrinology. The results reported by Rossdale et a/. (p 347) are an important addition to the relatively meagre body of knowledge relating to equine foetal maturation and preparation for birth, compared to other domestic species (see review by Silver 1990). It is becoming increasingly evident that gestational age alone is a poor indicator of readiness for birth in the mare. In this species developmental and functional dysmaturity of the neonate can and does occur at unpredictable times within the period of normal 'term' (ie 320-360 days); on the other hand, apparently normal, healthy foals may also be delivered before 320 days. It is unclear what controls this system of maturation and which endocrine mechanisms are involved. In studies on the chronically catheterised foetal foal at Cambridge (Silver and Fowden 1991) it is apparent that adrenal responsiveness to ACTH and, ultimately, increased foetal plasma cortisol concentrations, occur much later (relative to the total length of gestation) than in other species. This final pre-partum rise in foetal cortisol seems to be a prerequisite for full maturation of the foal, as it is in other species (Silver 1990). Indeed, a characteristic feature of premature or dysmature foals in the immediate postpartum period is the unresponsiveness of their adrenals to ACTH (Silver et al. 1984) and their abnormally high plasma progestagens (Houghton et a/. 199 1). The findings of Rossdale et al. (p 347) strongly suggest that the changes seen in maternal plasma progestagens near term are reflective of and possibly even driven by enhanced activity of the foetal pituitary-adrenal axis. In the mare, plasma 'total progestagens', monitored via

non-specific radioimmunoassay (RIA), increase gradually throughout gestation with a pronounced increase starting approximately 20-30 days pre pai'turn and reaching a maximum 1-2 days before birth, followed by a slight decline (Barnes el a/. 1975; Holtan et a/. 1975). These 'progestagens' are a series of pregnenes and pregnanes, precursors to and reduced products of progesterone. Three of these were identified in earlier studies (Holtan e f a/. 1975; Atkins et a / . 1976) and 6 more were identified in more recent, extensive studies (Holtan et a/. 1991) utilising gas chromatography/mass spectrophometry (GC/MS). Progesterone is detectable in maternal plasma only during the life span of ovarian luteal tissue (early pregnancy) and is undetectable (ie

Readiness for birth; another piece of the puzzle.

336 EQUINE VETERINARY JOURNAL Eqrririe i'ef J . (1992) 24 (5)336-337 Readiness for birth; another piece of the puzzle In this issue Rossdale et a/...
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