Effects of pinealectomy, oestradiol and melatonin on plasma prolactin and LH secretion in ovariectomized sheep

Abstract

The study examines whether the pineal gland mediates in the seasonal rhythm in the response of the hypothalamo-pituitary axis to oestrogen in the sheep. Five groups of ewes, each comprised of five sham-pinealectomized ewes and five pinealectomized ewes, were maintained under field conditions over a 2-year period. Group I ewes were ovariectomized and treated with two oestradiol capsules in the first year and one oestradiol capsule in the second year; group 2 ewes were ovariectomized and treated with two oestradiol capsules and three melatonin sachets in the first year and one oestradiol capsule and three melatonin sachets in the second year; group 3 ewes were ovariectomized and treated with three melatonin sachets in the first year and one oestradiol capsule in the second year; group 4 ewes were ovariectomized and treated with empty implants; group 5 had intact ovaries. Blood samples were taken at weekly intervals for determination of plasma prolactin, LH and FSH levels. Cyclic ovarian activity, determined by rams fitted with a marking harness, occurred in both sham-pinealectomized and pinealectomized ewes in group 5 during December to May in both years. A circannual rhythm in plasma prolactin levels was apparent in all ten groups of animals. The levels were highest in spring (September to November) and decreased before the onset of breeding activity in midsummer (December). Ovariectomized ewes with intact pineal glands and treated with oestradiol capsules (group 1) exhibited marked changes in LH and FSH concentrations during the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Seasonal Modification of Ovine Pineal Function

“…The effect of OVX was abolished or prevented by (1) E2 im plants, presumably by restoring the regulatory influence of endogenous estrogens and (2) by ganglionectomy. These find ings differ from the conclusions of Kennaway et al [16], who were unable to detect an effect of pinealectomy, OVX, estra diol or melatonin treatment on the prolactin rhythm. The reason for the divergent findings is not known.…”

“…This is in keeping with observations from other laboratories in a variety of species [see e.g. 4,16], During anestrus, ovariectomy changed the pattern of cir culating prolactin levels compared to intact controls ( fig. 2a).…”

“…The reason for the divergent findings is not known. However, our findings are consistent with the existence of a physiological antagonism between gonadal steroids and the sympathetic in nervation of the pineal gland [5], in that OVX and SCGX elicit opposite effects on plasma prolactin levels (current re sults) as well as upon pineal [1-adrenoceptor variables [17], These observations may indicate a functional linkage between pineal hormone secretion (presumably melatonin or a related indoleamine) and pituitary prolactin secretion in the sheep, similarly to the findings of Mori and Okamura [18] in the goat, and of Kennaway et al [16] in the ewe. This is in keeping with and expands upon the link between photoperiod and prolactin release in rams [19].…”

Photoperiodic requirements for timing onset and duration of the breeding season of the ewe: Synchronization of an endogenous rhythm of reproduction

“…The persistence of these changes in reproductive neuroendocrine condition can be accounted for by the endogenous circannual rhythm which was not synchronized by photoperiod due to absence of the pineal. It is of interest in this regard that some studies suggest the neuroendocrine breeding season eventually becomes damped (Bittman et al 1983 a) or even disappears (Kennaway et al 1984) with time after pinealectomy, although there is inconsistency in this point as other studies suggest the LH swings can persist for many years after surgery .…”

The suprachiasmatic nuclei as a seasonal clock

“…This result has 656 been confirmed in hoofed animals with intact melatonin signaling. 657 Prolonging the melatonin signal by giving oral or intramuscular 658 melatonin to sheep and deer housed in long days triggers a short 659 day-like reproductive response (Bubenik, 1983;Nett and 660 Niswender, 1982;English et al, 2014;Kennaway et al, 1984;661 Poulton et al, 1987). Using radiolabeled melatonin as a tracer, sev-662 eral high-affinity binding sites for melatonin have been identified in 663 the Siberian hamster brain; these sites include the paraventricular 664 nucleus, the stria medullaris, the nucleus reuniens, the pars tuber-665 alis (PT), and the SCN Duncan et al, 1989;Weaver et al, 1989.…”