Summary. Plasma progesterone determinations were carried out on blood samples collected daily from Clun Forest ewes during the normal oestrous cycle and also after administration of LH-releasing hormone (LH-RH) during seasonal anoestrus.Levels of plasma progesterone at oestrus ranged from 0\m=.\1to 0\m=.\5 ng/ml and luteal phase levels from 3 to 6 ng/ml. Levels found during seasonal anoestrus were within the range of those observed at oestrus. Following treatment with LH-RH, there was an increase in the plasma LH level in all cases and ovulation occurred in twenty-three out of twenty-seven treated ewes. In the animals which ovulated, the plasma progesterone concentration either remained basal (eighteen animals) or rose to a lower level (<2 ng/ml) than that found during the luteal phase of the cycle.

Summary. Plasma

Administration of opioid agonists and antagonists and measurement of resulting hormone changes were used to study the possible effects of opioids on reproductive function in the ewe. Intravenous administration of the long-acting methionine-enkephalin analogue FK33-824 (250 micrograms/h for 12 h) to 3 ewes during the follicular phase of the oestrous cycle depressed episodic LH secretion. This effect was reversed by administration of the opiate antagonist naloxone (25 mg/h) in combination with the FK33-824 treatment; in fact LH secretion was enhanced by the combined regimen. Naloxone (25 mg/h for 12 h) administered alone to 3 ewes in the follicular phase also enhanced LH secretion. In 3 animals treated with FK33-824 during the follicular phase, progesterone remained basal for 14 days after treatment, suggesting that ovulation was blocked. Jugular venous infusion of naloxone (25, 50 or 100 mg/h for 8h) into 5 ewes during the early and mid-luteal phase of the cycle resulted overall in a significant increase in mean plasma LH concentrations and LH episode frequency. To investigate whether endogenous opioids suppress LH release in seasonally anoestrous sheep, naloxone was infused intravenously into mature (25, 50 or 100 mg/h for 8 h) and yearling ewes (12 . 5, 25 or 50 mg/h for 8 h) during early, mid- and late anoestrus and plasma LH concentrations were measured. In the mature ewes, there was a trend for naloxone to increase LH values during the early anoestrous period but naloxone was without effect during mid- and late anoestrus. In the yearlings, naloxone infusion consistently increased plasma LH concentrations as a result of a significant increase in LH episode frequency. These experiments indicate that endogenous opioid peptides probably modulate gonadotrophin secretion during both the follicular and luteal phases of the oestrous cycle. However, the follicular phase of the sheep cycle is of short duration, and there may be residual effects of luteal-phase progesterone during this period. Secondly, there may be an age-dependent effect of naloxone on LH secretion during seasonal anoestrus in the ewe, with opioids playing a part in the suppression of LH in young but not in mature animals.

Summary. Doses

Two groups of 6 rams were kept under constant photoperiod consisting of short days (8 h light (L): 16 h dark (D); Group S) and long days (16 h L:8 h D; Group L) from 4 to 38 months of age. Five other rams were reared under a photoperiod representative of that occurring naturally (Group N). Testis size and plasma prolactin concentrations were obtained weekly. These data were subjected to time series analysis. The results indicated that there were persistent periodic excursions in both parameters measured. In Group N, the average cycle length for both testis volume and plasma prolactin was about 1 year and the peaks in plasma prolactin preceded those in testis volume by about 18 weeks. Rams from Group L also showed rhythmical changes in these parameters with periodicities of around 35 weeks and it is suggested that these cyclic changes may constitute true endogenous circannual rhythms; again the prolactin peaks preceded those of testis volume by about 18 weeks. Overall, rams from Group S had excursions of testis growth of a similar magnitude to those of Group L but the changes were less regular than those of Group L. Plasma prolactin was significantly lower in Group S than in Group L and there was little evidence for rhythmicity. It is proposed on the basis of the temporal relationship between peaks of prolactin and testis volume in Groups N and L, that prolactin may play a role in the timing of the reproductive cycle in the ram.