Prolactin Release in the Sheep

Abstract

The recent development of radioimmunoassay for prolactin has extended the depth and extent of studies of factors affecting prolactin levels in mammals and has permitted long-term studies to be made of changes in blood prolactin levels in individual animals. Although ovine prolactin has been widely studied in relation to its structure (Li, 1972) and biological activity (see Wolstenholme & Knight, 1972) and has been extensively used as a reference material, the serum prolactin levels of the sheep have not been widely investigated. Many of the early studies involved measurements of single samples obtained by venepuncture, a stressful procedure now known to affect prolactin release in the

Abrupt decrease in daylength and short-term changes in the plasma concentrations of FSH, LH and prolactin in anoestrous ewes

“…The release of prolactin observed at the start of the collection period in most of the ewes which had high prolactin levels was probably an initial response to stress (Lamming, Moseley & McNeilly, 1974), and was much less noticeable in the ewes on short daylengths which had low levels of prolactin. In ewes on long days, prolactin levels exhibited a marked diurnal variation: the concentrations increased during the dark period and fell during the beginning of the light period.…”

The Importance of Prolactin for Initiation. of Lactation in the Pregnant Ewe

“…In this connection, there is substantial evidence that placental lactogen is an important hormone in sheep (Martal andDjiane 1975, 1977;Kelly et al 1976) and goats (Kelly et al 1976; Buttle et al 1979). It is pertinent that the blood concentration of prolactin of pregnant sheep, goats and cows is low until shortly before parturition (Johke et al 1971;Fell et al 1972;Lamming et al 1974). In contrast, concentrations of placental lactogen are low during the first 6-7 weeks of pregnancy in the ewe, then increase to reach peak values 10-20 days prepartum (Kelly et al 1976;Chan et al 1978).…”

“…In the ewe, concentrations of plasma prolactin increase shortly before parturition to reach maximum values at term, then decrease during the post partum period (Davis et al 1971;Lamming et al 1974).…”

Differences in the reproductive endocrine status of ewes in the early post-partum period and during seasonal anoestrus

“…Before the ovulatory LH surge, during pro-oestrus, there is an increase in Gn-RH levels in the median eminence, and this is accompanied by an increase in pituitary LH content (Roche, Foster, Karsch, Cook & Dziuk, 1970 ;Crighton, Hartley & Lamming, 1973 ;Wheaton, 1979 (Chamley, Jonas & Parr, 1976 (Jenkins et al, 1977;Wright et al, 1980;Crowder et al, 1982) Prolactin secretion in post-partum anoestrous ewes was greater than that in seasonally anoestrous ewes, as found by others (Lamming et al, 1974;Kann & Denamur, 1974). Pituitary prolactin content was similar in both groups, indicating that the greater secretion postpartum is due to greater synthesis of prolactin.…”

“…For FSH and prolactin, animal mean values were calculated and used for between-group comparisons. For prolactin, the values obtained during the first hour of sampling were not included in the analysis because these may have been influenced by stress, known to occur at the start of such procedures (Lamming, Moseley & McNeilly, 1974 fig. 1).…”