The development of sensitive and accurate radioimmunoassay (RIA) techniques for the determination of ovarian hormones has made possible the study of their circulating levels during the oestrous cycle and pregnancy of many animals. Results for the levels of oestrogens and progesterone in the cyclic cow using RIA techniques have been reported by Henricks, Dickey & Hill (1971), for oestrogens by Glencross, Pope & Munro (1972) and for progesterone by Sprague, Hopwood, Niswender & Wiltbank (1971). Levels of oestrone during pregnancy, estimated by a fluorometric procedure, have been reported by Robinson, Baker, Anastassiadis & Common (1970), and using a protein-binding method Donaldson, Bassett & Thorburn (1970) reported maximum levels of progesterone (7–8 ng/ml) between 20 and 40 days pre partum falling to less than 1 ng/ml 2–3 days post partum. The present paper reports the levels of oestrogens and progesterone during the last month of pregnancy in five cows from a
A. M. Symons, Josephine Arendt and C. A. Laud
Three groups of Suffolk-cross ewes were kept in (A) summer photoperiod plus melatonin feeding in such a way as to mimic the plasma levels found in winter photoperiod, (B) winter photoperiod or (C) natural light/dark from mid-June onwards. Prolactin levels remained high in group C throughout July and August but were dramatically reduced in both groups A and B. The rise in prolactin levels associated with dusk, however, was still apparent in all three groups. Appropriate administration of melatonin can thus influence prolactin secretion in the same way as an extension of the dark phase. This effect is associated with an early onset of the breeding season in the ewe.
A. L. Poulton, J. English, A. M. Symons and J. Arendt
Ewes were treated with s.c. implants of melatonin in mid-April, mid-May and mid-June. From mid-June, other animals were given oral doses of melatonin daily at 16.30 h and another group was maintained under a short (8 h light: 16 h darkness) artificial photoperiod (lights out 16.30 h). Serial blood samples were taken from all animals in June and July. Plasma prolactin concentrations were significantly reduced in ewes treated in May and June (implant, oral and photoperiod treatments) but not in those treated in April. After treatment in June, prolactin levels were significantly suppressed after 7 days of oral and implant melatonin therapy, and after 28 days of a short artificial photoperiod. Melatonin treatment appeared more efficient than an artificial photoperiod in reducing plasma prolactin concentrations.
J. Endocr. (1986) 108, 287–292
A. L. Poulton, J. English, A. M. Symons and J. Arendt
Breeding activity was similarly advanced in ewes given continuous (s.c. implant) or timed (oral dose at 15.30 h) melatonin treatments or subjected to a short (8 h light: 16 h darkness) artificial photoperiod. Treatments commenced in mid-June and were terminated in mid-November. Weekly and serial blood samples were collected before and after treatments commenced, to ascertain the effects on plasma prolactin, LH and FSH concentrations. In addition, serial blood samples were collected for 24 h plasma prolactin and melatonin estimations before and after cessation of the treatments.
Plasma prolactin levels were significantly reduced immediately following the start of the melatonin (implant and oral) and short-photoperiod treatments but 'rebounded' to levels greater than control values. The normal seasonal (spring) rise in plasma prolactin was noted in the following year. Before the onset of breeding activity, mean plasma LH and FSH concentrations and LH pulse frequency did not change following any of the treatments. The 24-h plasma melatonin profile accurately reflected the various applied treatments but had re-entrained to the prevailing (natural) photoperiod 1 week after termination of the treatments. There were no significant group differences in 24-h plasma prolactin levels 1 week before or 1 and 11 weeks after the treatments had ceased.
Such treatments, although successfully advancing the onset of breeding activity and modifying the seasonal plasma prolactin rhythm, were not manifested through any apparent change in peripheral LH or FSH.
J. Endocr. (1987) 112, 103–111
A. M. Symons, J. Arendt and S. J. Pryde
The effect of melatonin on the stimulated release of LH from prepubertal female rat pituitary cells in vitro was investigated. Significant inhibition of LH-releasing hormone and calcium ionophore-induced LH release was seen but not of potassium-induced release. These results suggest a specific interaction between melatonin and the endogenous events leading to LH release, and may implicate melatonin as an important neuroendocrine component of pubertal development in this species.
J. Endocr. (1985) 107, 107–112
Josephine Arendt, A. M. Symons, C. A. Laud and S. J. Pryde
Patterns of plasma melatonin, similar in the duration of high levels to those found in winter, were induced in Suffolk-cross ewes kept in summer light (16 h light: 8 h darkness) by daily oral administration of melatonin (3 mg/13 μmol). The onset of oestrous cycles in these sheep occurred in August, 2–8 weeks before the onset of oestrous cycles in untreated ewes kept in natural light. The onset of oestrous cycles in a further group of ewes kept in winter light (8 h light: 16 h darkness) from mid-June was indistinguishable from that of the melatonin-treated ewes. Rams were excluded from the premises. These data indicate that melatonin alone in physiological quantities is sufficient to induce early onset of the breeding season in the ewe, and provide strong evidence for a hormonal role of melatonin in a short-day breeder.